polycrystalline tandem photovoltaics - phd dissertation - colin bailie

7/23/2019 Polycrystalline Tandem Photovoltaics - PhD Dissertation - Colin Bailie

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POLYCRYSTALLINE TANDEM PHOTOVOLTAICS

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND

ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTSFOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Colin David Bailie

December 2015



http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/kx438zm4205

© 2015 by Colin David Bailie. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-

Noncommercial 3.0 United States License.

ii


http://purl.stanford.edu/kx438zm4205

http://purl.stanford.edu/kx438zm4205





I certify that I have read this dissertation and that, in my opinion, it is fully adequate

in scope and quality as a dissertation for the degree of Doctor of Philosophy.

Michael McGehee, Primary Adviser



Yi Cui



Hemamala Karunadasa

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost for Graduate Education

This signature page was generated electronically upon submission of this dissertation in

electronic format. An original signed hard copy of the signature page is on file in

University Archives.

iii



Acknowledgments

I would like to thank everyone who helped me reach the finish line.

First, I thank my family, who have supported me throughout my life in all pursuits. Mymother, who has seen her efforts in helping with grade school science fair projects blossom into

science and engineering projects far beyond such world-changing efforts as ”Which paper airplane

design flies the furthest”. My father, who has always provided the counterpoint by playing sports

with me. My sister, who has always been my greatest champion.

Second, I thank my groupmates and collaborators without whom I would not have man-

aged to publish anything of consequence. Science and engineering today is truly a team effort. I also

thank my groupmates for distracting me from being in the laboratory 24/7. Without them, I may

never have purchased (and used) a surfboard, climbing gear, trail running shoes, a squash racquet,

backpacking gear, and a road bicycle.

Finally, I thank my advisor Michael McGehee. His advice has proved invaluable on count-

less occasions. I am eternally grateful for the time he has dedicated to helping me grow as a scientist

and as a person and am glad for the friendship that has developed.

iv



Published Works

Dye-sensitized solar cell works

C. D. Bailie, W. H. Nguyen, J. Burschka, T. Moehl, M. Gratzel, M. D. McGehee, and A. Sell-inger, ”Molecular engineering of organic dyes for improved recombination lifetime in solid-state

dye-sensitized solar cells,” Chemistry of Materials, vol. 25, pp. 15191525, apr 2013.

C. D. Bailie, E. L. Unger, S. M. Zakeeruddin, M. Gratzel, and M. D. McGehee, ”Melt-infiltration of

spiro-OMeTAD and thermal instability of solid-state dye-sensitized solar cells,” Physical Chemistry

Chemical Physics, vol. 16, pp. 4864, jan 2014.

Co-authored

T. P. Brennan, J. R. Bakke, I.-K Ding, B. E. Hardin, W. H. Nguyen, R. Mondal, C. D. Bailie,G. Y. Margulis, E. T. Hoke, A. Sellinger, M. D. McGehee, and S. F. Bent, ”The importance of

dye chemistry and TiCl 4 surface treatment in the behavior of Al 2O 3 recombination barrier layers

deposited by atomic layer deposition in solid-state dye-sensitized solar cells,” Physical Chemistry

Chemical Physics, vol. 14, pp. 12130, jul 2012.

K. E. Roelofs, T. P. Brennan, J. C. Dominguez, C. D. Bailie, G. Y. Margulis, E. T Hoke, M. D.

McGehee, and S. F. Bent, ”Effect of Al 2O 3 recombination barrier layers deposited by atomic layer

deposition in solid-state CdS quantum dot-sensitized solar cells,” Journal of Physical Chemistry C,

vol. 117, pp. 5584-5592, feb 2013.

G. Y. Margulis, M. G. Christoforo, D. Lam, Z. M. Beiley, A. R. Bowring, C. D. Bailie, A. Salleo,

and M. D. McGehee, ”Spray Deposition of Silver Nanowire Electrodes for Semitransparent Solid-

State Dye-Sensitized Solar Cells,” Advanced Energy Materials, vol. 3, pp. 1657-1663, dec 2013.

v



W. H. Nguyen, C. D. Bailie, E. L. Unger, and M. D. McGehee, ”Enhancing the Hole-Conductivity

of Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI)2 in Perovskite and Dye-

Sensitized Solar Cells,” Journal of the American Chemical Society, vol. 136, pp. 10996-11001, aug

2014.

Perovskite solar cell and tandem works

C. D. Bailie, M. G. Christoforo, J. P. Mailoa, A. R. Bowring, E. L. Unger, W. H. Nguyen, J.

Burschka, N. Pellet, J. Z. Lee, M. Gratzel, R. Noufi, T. Buonassisi, A. Salleo, and M. D. McGehee,

”Semi-transparent Perovskite Solar Cells for Tandems with Silicon and CIGS,” Energy & Environ-

mental Science, vol. 8, pp. 956-963, dec 2015.

C. D. Bailie, J. P. Mailoa, E. C. Johlin, E. T. Hoke, A. J. Akey, W. H.Nguyen, M. D. McGehee, and

T. Buonassisi, ”A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel

junction,” Applied Physics Letters, vol. 106, p. 121105, 2015.

C. D. Bailie, and M. D. McGehee, ”High-efficiency tandem perovskite solar cells,” MRS Bulletin,

vol. 40, pp. 681-686, aug 2015.

Co-authored

E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A. R. Bowring, T. Heumuller, M. G. Christo-foro, and M. D. McGehee, ”Hysteresis and transient behavior in current-voltage measurements of

hybrid-perovskite absorber solar cells,” Energy Environ. Sci., vol. 7, pp. 3690-3698, aug 2014.

vi



Contents

Acknowledgments v

Published Works vi

1 Introduction 1

1.1 State of the Photovoltaic Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Physics of Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Tandem Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Tandem Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Monolithic Perovskite/Silicon Tandems 15

2.1 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Limitations of Monolithic Tandems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Mechanically-Stacked Tandems using Silver Nanowires 26

3.1 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.1 Semi-Transparent Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.2 Mechanically-Stacked Tandems . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4 Mechanically-Stacked Tandems using ITO 36

4.1 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1.1 Deposition Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.1.2 Semi-Transparent Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.1.3 Tandems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.1.4 Stability Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.2 Future of Mechanically-Stacked Tandems . . . . . . . . . . . . . . . . . . . . . . . . 42

vii



5 Cost-Modeling of Perovskite Solar Cells 44

5.1 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.1.1 Model Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.1.2 Single-Junction Perovskite Cost Model . . . . . . . . . . . . . . . . . . . . . . 47

5.1.3 Mechanically-Stacked Tandem Perovskite Cost Model . . . . . . . . . . . . . 49

5.1.4 LCOE Comparison Across Technologies . . . . . . . . . . . . . . . . . . . . . 51

6 Conclusions 53

7 Appendices 55

7.1 Monolithic Tandem Experimental Information . . . . . . . . . . . . . . . . . . . . . . 55

7.1.1 Silicon Sub-cell Fabrication Procedure . . . . . . . . . . . . . . . . . . . . . . 55

7.1.2 Perovskite Sub-Cell Fabrication Procedure . . . . . . . . . . . . . . . . . . . . 567.1.3 Multijunction Cell Testing Protocols . . . . . . . . . . . . . . . . . . . . . . . 58

7.1.4 Supplemental Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7.2 Silver Nanowire Deposition Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7.2.1 Lamination By Point Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7.2.2 Lamination By Distributed Force . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.3 Mechanically-Stacked Tandems Using Silver Nanowires Experimental Information . . 65

7.3.1 Perovskite Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7.3.2 Silicon Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.3.3 Measurement Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.3.4 Supplemental Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697.4 Mechanically-Stacked Tandems Using ITO Experimental Information . . . . . . . . . 71

7.4.1 Perovskite Device Fabrication Method . . . . . . . . . . . . . . . . . . . . . . 71

7.4.2 J-V Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

7.4.3 EQE Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

7.4.4 Stability Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

7.4.5 Supplementary Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

7.5 Cost-Modeling of Perovskite Solar Cells Model Information . . . . . . . . . . . . . . 76

7.5.1 Process flow for single-junction standard architecture . . . . . . . . . . . . . . 76

7.5.2 Tandem Performance Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Bibliography 78

viii



List of Tables

1.1 LCOE of comparable electricity technologies. Data acquired from [1]. . . . . . . . . . 3

2.1 Photovoltaic parameters from 2-terminal perovskite/Si multijunction cell (hero device). 19

3.1 Performance metrics of semi-transparent and opaque perovskite devices. . . . . . . . 29

3.2 Performance metrics of mechanically-stacked tandems . . . . . . . . . . . . . . . . . 32

4.1 Photovoltaic parameters of a semi-transparent perovskite solar cell with ITO rear

electrode compared to an opaque perovskite solar cell with Al/Ag rear electrode. . . 39

4.2 Photovoltaic parameters of semi-transparent perovskite and mono-crystalline silicon

cells and the resulting tandem efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.3 Efficiency as a function of temperature extracted by averaging the data in Figure

4.4 over the entire time period. The temperature coefficient is extracted as a linear

fit of the data and normalized against the expected efficiency at 25

C. . . . . . . . . 42

ix



List of Figures

1.1 2015 utility scale benchmark system price from [2]. Module price is 0.65 $/W, balance

of systems price is 1.12 $/W for a total system price of 1.77 $/W. . . . . . . . . . . . 2

1.2 Average commercial efficiency over time. Figure from [3]. . . . . . . . . . . . . . . . 21.3 Absorption process in a semiconductor. Photons of energy greater or equal to the

bandgap energy are absorbed and create electron-hole pairs that thermalize down to

the bandgap energy. Photons of energy less than the bandgap energy are not absorbed

by the semiconductor. Figure adapted from Stanford University MatSci 302 lecture

slides taught by Prof. Michael McGehee. . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 The terrestrial solar spectrum, AM1.5G, spans ultraviolet (UV), visible (VIS), and

infrared (NIR) light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Theoretical efficiency of single-junction solar cells. Input spectrum is 6000 K black-

body. AM1.5G spectrum yields slightly different values with a theoretical maximum

of 33.7 %. Current silicon record from NREL record photovoltaics chart [4]. . . . . . 51.6 Essential function of single-junction vs. double-junction solar cells. Double-junction

(tandem) solar cells split the solar spectrum between two absorbers, resulting in a

higher potential efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.7 Theoretical efficiency of double-junction solar cells. Input spectrum is 6000 K black-

body. AM1.5G spectrum yields slightly different values with a theoretical maximum

of 46.1 %. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.8 Scalable tandem architectures. (Left) A mechanically-stacked tandem combines two

separately-fabricated solar cells. (Right) A monolithic tandem integrates two junc-

tions into a single solar cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

x



1.9 Perovskite is the name of the mineral CaTiO3 but this term is being used for all

compounds with the general formula ABX3 that have the same crystal structure as

CaTiO3 or are derived from this structure. These materials consist of two cations,

the cation A is 12 fold coordinated by the anions X and the cation B 6-fold where

X can either be oxygen or a halide. The most prominent perovskite in solar cells

right now is methylammonium lead iodide. It is worth noting that the structure of

methylammonium lead iodide deviates from the ideal cubic perovskite structure as

the octrahedra become slightly tilted and the structure is consequently tetragonal. . 10

1.10 Perovskite solar cell architectures. (Left) an n-i-p architecture using a mesoporous

TiO2 scaffold. (Middle) an n-i-p architecture in a planar configuration. (Right) a

p-i-n architecture in a planar configuration . . . . . . . . . . . . . . . . . . . . . . . 11

1.11 J-V curve of a perovskite solar cell. The relevant points of JSC , VOC , and PMAX are

shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.12 External quantum efficiency of a perovskite solar cell as a function of wavelength.

Unity represents perfect quantum efficiency. . . . . . . . . . . . . . . . . . . . . . . . 13

1.13 Schematic examples depicting the processes of transmission, reflection, and absorption

of light. Image from www.chroma.com. . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1 (a) The device structure of a 2-terminal monolithically grown perovskite/Si multi-

junction solar cell with an n-type Si base. The polished SEM image is taken at 45

tilt to show the Ag nanowire mesh (500 nm scale bar). (b) Band diagram of the

perovskite/silicon cell interface showing the charge-transport mechanism around the

Si tunnel junction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 (a) TEM image of the n++/p++ silicon tunnel junction interface after the dopant

activation annealing (left: 30 nm scale bar) and high-resolution TEM image of the

n++ layer, showing the partially crystalline nature of this layer (right: 5 nm scale

bar). (b) SIMS profile of the Si emitter and tunnel junction layer showing the sharp

doping profile at the tunnel junction interface. (c) Comparison of J-V profile for

identical Si cells with and without a tunnel junction, showing negligible effect of the

tunnel junction to the single-junction Si cell performance. . . . . . . . . . . . . . . . 17

xi



2.3 (a) J-V curve of the 2-terminal perovskite/silicon multijunction solar cell under AM1.5G

illumination. Forward and reverse-bias scan directions are shown with 5 s measure-

ment delay per data point. Steady-state values for JSC , VOC , and MPP are depicted

as blue circles and are measured by averaging over 30 s after reaching steady state.

The VOC of 1.58 V is approximately the sum of the perovskite and Si cell VOC . (b)

Time-dependent output current of the multijunction cell near maximum power point

(1.20 V forward bias) showing that the output reaches steady state after a measure-

ment delay of 30 s. (c) Total device reflection and EQE of the perovskite and Si

sub-cells of a typical perovskite/Si multijunction cell. The EQE spectra exhibit a low

blue EQE in the top perovskite sub-cell due to spiro-OMeTAD parasitic absorption,

and low red EQE in the bottom Si sub-cell due to both spiro-OMeTAD parasitic

absorption and the lack of good Si back surface passivation scheme. The perovskite

sub-cell EQE is corrected to match the measured JSC and the silicon sub-cell EQE is

reported as measured . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Perovskite is solution-processed onto a textured silicon wafer. The nominal thickness

of the perovskite layer is 250-500 nm while the thickness of silicon pyramids is 5-12

µm. More perovskite is deposited in the valleys between the pyramids while the peaks

are left completely bare of perovskite . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.5 Result of a PC1D simulation. Blue curve represents the EQE of a silicon cell with

full texturing and an ideal double layer anti-reflection coating. Black curve represents

the EQE of a silicon cell without front surface texturing and the silicon surface in

direct contact with air. Red curve is the EQE of a silicon solar cell with a planarfront surface with a thin film layer stack with thicknesses and real refractive indices

similar to a perovskite solar cell showing an intermediate performance between the

two extremes. Numbers in the legend refer to the refractive index and thickness of

the thin film layers on top of the silicon. . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.6 Representative spectra of the sun’s illumination of Denver, CO over a year. Figure

reproduced from [5]. In this plot, all light to the left of ∼700 nm is absorbed by the

perovskite and all light to the right of ∼700 nm is absorbed by the silicon (assuming

a modified perovskite bandgap). Note that for many of these spectra, a current-

mismatch will occur between the perovskite and silicon. . . . . . . . . . . . . . . . . 25

xii



3.1 Mechanically-stacked tandem wiring configurations that result in two terminals exit-

ing the module. (Left) The perovskite and silicon cells are wired in series to enforce

a current-matching condition. The total current in each cell can be matched by ad-

justing the size of the perovskite solar cell (the perovskite and silicon do not need to

be current-density matched). (Right) The perovskite cells are wired in series and the

silicon cells are separately wired in series. These two strings are wired together in

parallel to enforce a voltage-matching condition. A third alternative is to use a power

electronic circuit in either configuration to correct for any mismatches in current or

voltage between the strings of cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2 Perovskite device results. a) Current-voltage curves comparing best opaque vs. semi-

transparent perovskite devices. b) EQE of semi-transparent device and opaque device.

Note that the opaque device does not have AR coatings. c) Transmission through

semi-transparent perovskite with AR coatings. Peak transmission is 77 % around 800

nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.3 Perovskite and CIGS/Si tandem results. a) Current-voltage and b) EQE of semi-

transparent perovskite cell, unfiltered CIGS cell, and CIGS cell filtered by the per-

ovskite cell. c) IV curves and d) EQE of semi-transparent perovskite cell, unfiltered

TI-Si cell, and TI-Si cell with an infrared-optimized anti-reflection coating filtered by

the perovskite cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.4 NREL certification of a mechanically-stacked tandem. The tandem efficiency is the

sum of the individually measured cells, 17.9%. . . . . . . . . . . . . . . . . . . . . . . 33

4.1 Semi-transparent inverted perovskite device architecture. a) Energy level diagram.

b/c) Cross-sectional SEM and illustrative schematic of device architecture showing

ITO electrode encapsulation layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2 Performance comparison of the opaque and semi-transparent devices showing a) J-V

curve of devices showing comparable FF and VOC and b) max power point tracking. 38

4.3 Mechanically-stacked perovskite/silicon tandem performance. a) J-V curves of tan-

dem with the max power of the tandem calculated from the addition of the perovskite

and silicon cells. b) EQE of original mono-Si, perovskite, and filtered silicon solar cells. 40

4.4 Thermal stability of ITO-capped perovskite solar cells at 60 and 100 C compared to

opaque device with ZnO and Al/Ag. . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.5 J-V, EQE, and Transmission of a possible semi-transparent perovskite solar cell based

on the best-in-class 20.1 % opaque solar cell. Semi-transparent perovskite cell effi-

ciency is 17.5 % . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.6 J-V and EQE of the 25.6 % record silicon solar cell if filtered by the transmission

curve in Figure 4.5. Filtered silicon efficiency is 8.6 % . . . . . . . . . . . . . . . . 43

xiii



5.1 Single-junction perovskite module scaled process flow. . . . . . . . . . . . . . . . . . 46

5.2 Mechanically-stacked tandem module scaled process flow. . . . . . . . . . . . . . . . 47

5.3 Single-junction perovskite module step costs. . . . . . . . . . . . . . . . . . . . . . . 48

5.4 Single-junction perovskite compared to CdTe. . . . . . . . . . . . . . . . . . . . . . . 49

5.5 Mechanically-stacked tandem module step costs. . . . . . . . . . . . . . . . . . . . . 50

5.6 LCOE calculations for different perovskite scenarios assuming an operational lifetime

of 30 years, a 140 $/m2 BoS cost, and a 7 % internal rate of return. . . . . . . . . . 52

7.1 EQE curve of single-junction n-type silicon cells with and without a tunnel junction.

This curve shows negligible parasitic absorption in the tunnel junction for wavelength

λ > 500 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7.2 Transient (a) JSC and (b) VOC of the perovskite/silicon tandem cell as they stabilize

over the time. The measured steady-state values are JSC = 11.5 mA/cm2 and VOC

= 1.58 V, respectively. The 30 s settling time for the VOC is not shown as the cell

was stabilized at VOC for > 30 s prior to starting the measurement. . . . . . . . . . 59

7.3 EQE of a semi-transparent perovskite solar cell illuminated through either the n-side

(glass/TiO2 side) or through the p-side (AgNW/spiro-OMeTAD side). The glass

side EQE integrates to 17.3 mA/cm2 while the AgNW side EQE integrates to 11.4

mA/cm2. Neither side has anti-reflection coatings. . . . . . . . . . . . . . . . . . . . 60

7.4 The light transmission through a 470-nm-thick doped spiro-OMeTAD film on glass.

The contribution of the glass is removed from this plot. The absorption features from

300-400 and 450-550 nm are readily visible in the EQE plot above. . . . . . . . . . . 60

xiv



7.5 Process for vacuum lamination of silver nanowires onto a solar cell. 1) The baseplate

of the transfer chamber designed to withstand the pressures of the process. At the

bottom of this baseplate is a connection to a vacuum source used to remove all air

pockets that can disrupt the uniformity of the applied pressure. 2) A porous metal

(aluminum) baseplate is used to distribute the vacuum evenly across the surface of the

porous plate to avoid plugging the vacuum source and prevent removal of air pockets.

3) An O-ring is used to provide an edge seal between the top and bottom halves of

the chamber to ensure proper pressure buildup. 4) The solar cell is placed in the

middle of the porous metal plate. 5) The silver nanowires on a plastic film are placed

face-down onto the solar cell. 6) A tape seal is placed in-between the O-ring and the

porous metal plate to allow the application of a diaphragm. 7) A plastic diaphragm

is attached to the tape and provides a barrier between the two halves of the transfer

chamber. Vacuum is pulled on the lower half and the diaphragm deforms around

the solar cell, providing a flat bubble-free surface onto which a uniform pressure can

be applied. 8) The top half of the transfer chamber is placed on top and bolted to

the bottom half through radial bolt holes in both chambers. The o-ring provides a

seal between the top half and ambient. A fitting on the side of the top half connects

to a positive pressure source such as a compressed gas canister. Positive pressure is

applied to the diaphragm, uniformly pressing the nanowires into the solar cell. . . . 64

7.6 Optical density over time of perovskite film in dipping solution. The signal at 700 nm

was used to determine the growth rate of the perovskite, while the signal at 850 nm

was used to detect the presence of other optical phenomena (changes in scattering,reflection, incident light intensity). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.7 Transmission of the 12.4 Ω/ AgNW film with the PET substrate subtracted. . . . 69

7.8 Distribution of semi-transparent cell performance gathered from 3 batches where de-

vice procedures were largely kept constant between batches. Low-efficiency devices

exhibited shorting likely caused by too much pressure applied manually during the

AgNW electrode transfer. Medium-efficiency devices generally exhibited low pho-

tocurrent likely caused by incomplete transfer of the AgNW electrode due to too

little pressure applied manually during the electrode transfer. . . . . . . . . . . . . . 70

7.9 Example of difference in spectral response depending on AR coating applied to Si cell.

The regular TI-Si cell has a broadband AR coating. . . . . . . . . . . . . . . . . . . 707.10 J-V Curves showing problems with the ZnO/ITO interface. At room temperature,

a large extraction barrier is present which greatly limits fill factor. This barrier is

alleviated as temperature is raised; however the barrier reappears upon cooling. . . . 73

7.11 A 50 nm film of AZO nanoparticles displays high transmission. The AZO nanoparti-

cles are annealed at either 75 or 150 C after deposition. . . . . . . . . . . . . . . . . 73

xv



7.12 Transmission through an ITO/Glass reference sample from the same sputter deposi-

tion batch as the best devices. The 500 nm layer of ITO was annealed at 100 C for

15 minutes to match the thermal processing profile of the perovskite samples and the

resistivity dropped from 11 to 10 Ω/. . . . . . . . . . . . . . . . . . . . . . . . . . . 74

7.13 Comparison of the EQE of semi-transparent and opaque perovskite solar cells. The

lack of a metal back reflector in the semi-transparent solar cells results in lower pho-

tocurrent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

7.14 Transmission through the semi-transparent perovskite solar cell. Transmission is lim-

ited at longer wavelengths due to coherent reflections. . . . . . . . . . . . . . . . . . 75

7.15 Reflection of semi-transparent and opaque perovskite solar cells. Mitigating reflection

is an area of future work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

7.16 Modeled EQE of perovskite and HIT-Si cells. . . . . . . . . . . . . . . . . . . . . . . 77

xvi



Chapter 1

Introduction

This thesis details the initial efforts on perovskite/Silicon and perovskite/CIGS tandem solar cells

by Colin Bailie and his advisor Prof. Michael McGehee at Stanford University in collaboration with

numerous academic and industrial partners.

1.1 State of the Photovoltaic Industry

The history of the photovoltaic industry can be divided generally into four generations.

In the first generation, silicon solar cells were primarily used for space applications (1955-1975). [6]

Then from 1975-1985, the initial large purchases of terrestrial solar cells placed the emphasis on

designing modules that could be deployed on land and reaching today’s warranty specifications. [6]

The third generation of the industry ended just 3-4 years ago, where prior to 2012 the total system

cost for utility scale installations was dominated by the cost of the module itself and primary focus

was on reducing module costs. [2] Enter the fourth generation, where the module cost represents a

decreasing portion of the total system cost (Figure 1.1). The remainder of the system cost, termed

the balance of systems (BoS), consists of inverters, labor, land, racking, and developmental costs

among others.

1



CHAPTER 1. INTRODUCTION 2

Figure 1.1: 2015 utility scale benchmark system price from [2]. Module price is 0.65 $/W, balanceof systems price is 1.12 $/W for a total system price of 1.77 $/W.

Even with the recent precipitous cost reductions in silicon modules [7], electricity from

solar power is not yet on par with comparable electricity technologies (Table 1.1). [1] With the

module no longer the driver of the total system cost, reducing the BoS cost is increasingly important.

Efforts in this area are multi-faceted including streamlining labor and installation, developing novel

racking systems, and improving inverters among others. A large opportunity also exists for the

module to greatly affect the overall system price as well. Because this system price is given in $/W,

increasing the performance of the panel (i.e power output) allows the module to affect the BoS cost

indirectly. The recent history of the average commercial module efficiency (Figure 1.2) shows aslowly increasing efficiency with time with the average efficiency in 2015 ∼16 %.

Figure 1.2: Average commercial efficiency over time. Figure from [3].




Table 1.1: LCOE of comparable electricity technologies. Data acquired from [1].

Technology LCOE (¢/kWh)

Coal 9.5-14Natural Gas 7.5-10Nuclear 10Wind (Offshore) 7.4 (19.7)Solar 12.5

In order to reach costs competitive with other electricity technologies by improving the

efficiency of the panel, the efficiency must be dramatically improved from the typical 16 % efficiency

today to 25-30 %. In this thesis, I will outline one potential path for reaching these targets of efficiency and commercial viability.

1.2 Physics of Solar Cells

The goal of a solar cell has been articulated most simply by Prof. Dick Swanson.

• First, a solar cell should generate as many electron-hole pairs as possible from the incident

sunlight (i.e. absorb as much of the solar spectrum as possible and turn it into electrical

current).

• Second, coax as many electrons and holes as possible to go to the correct electrode (i.e extract

as much of the current as possible).

• Finally, do this at as high a voltage as possible (as high a difference in electromotive force

between the leads as possible)

A property in all semiconducting materials called a bandgap governs both the current and

voltage potential of a solar cell. The bandgap is defined by the top of the valence band (or ionization

potential, Φ) to the bottom of the conduction band (or electron affinity). Only photons with an

energy greater than or equal to the bandgap energy can be absorbed by the semiconductor (Figure

1.3). Because the solar spectrum consists of photons of a wide range of energy, mostly situated

between 1 and 4 electron-volts (eV) (Figure 1.4), the solar cell can generate more current (goal

#1) by decreasing the bandgap of the absorbing semiconductor. However, the bandgap also restricts

the voltage that the current can be extracted at. In practice it is impossible to extract the current

at a voltage higher than the bandgap, meaning that the bandgap should be as large as possible to

maximize goal #3. An optimization occurs because power (or efficiency) is the product of current

and voltage.




Figure 1.3: Absorption process in a semiconductor. Photons of energy greater or equal to thebandgap energy are absorbed and create electron-hole pairs that thermalize down to the bandgap

energy. Photons of energy less than the bandgap energy are not absorbed by the semiconductor.Figure adapted from Stanford University MatSci 302 lecture slides taught by Prof. Michael McGehee.

Figure 1.4: The terrestrial solar spectrum, AM1.5G, spans ultraviolet (UV), visible (VIS), andinfrared (NIR) light.




Single-junction solar cells use one absorbing semiconductor material. Every unconcentrated

commercial module uses single-junction solar cells. Because of the wide energy range of the solar

spectrum, these types of solar cells are theoretically limited to 33.7 % efficiency (Figure 1.5).

[8, 9] Any photon energy in excess of the bandgap energy is lost almost immediately as waste heat

in a process called thermalization (Figure 1.4). The practical efficiency limit for silicon, which

dominates the commercial market, is ∼25 % (depicted as a star in Figure 1.5). One way to push

beyond the limits of silicon solar cells is to use tandem solar cells.

Figure 1.5: Theoretical efficiency of single-junction solar cells. Input spectrum is 6000 K blackbody.AM1.5G spectrum yields slightly different values with a theoretical maximum of 33.7 %. Currentsilicon record from NREL record photovoltaics chart [4].

1.2.1 Tandem Solar Cells

Tandem (or multijunction) solar cells use multiple absorbing semiconductors to harvest the

solar spectrum. Using multiple absorbers allows each absorber to specialize in a portion of the solar

spectrum, as illustrated in Figure 1.6 instead of a single absorber that is responsible for absorbing

the entire solar spectrum and therefore has a fairly small bandgap and extracts current at a low

voltage. In a tandem, a large-bandgap solar cell first absorbs the high-energy portion of the solar

spectrum and extracts that current at a high voltage. This large-bandgap solar cell is transparent

to the low-energy portion of the solar spectrum which is absorbed by a small-bandgap solar cell

below that extracts the current at a lower voltage. Using two absorbers in this fashion pushes the

theoretical efficiency of 46.1 % with the correct choice of bandgaps ( Figure 1.7).




Figure 1.6: Essential function of single-junction vs. double-junction solar cells. Double-junction(tandem) solar cells split the solar spectrum between two absorbers, resulting in a higher potentialefficiency.

Figure 1.7: Theoretical efficiency of double-junction solar cells. Input spectrum is 6000 K black-body. AM1.5G spectrum yields slightly different values with a theoretical maximum of 46.1 %.




1.3 Tandem Architectures

There are two scalable methods of making tandem solar cells, each with their own advan-tages and disadvantages (Figure 1.8). A mechanically-stacked tandem combines two separately-

fabricated solar cells by simply putting them on top of each other. From a high-level practical view,

these types of solar cells are easy to make since both solar cells can be independently optimized for

efficiency then combined at the end to form a tandem cell, which accelerates prototyping. Because

the cells are independent of each other, there are few restrictions on the design (e.g. no tunnel

junction or recombination layer required, no current matching condition). Monolithic tandems are

the classic structure employed in most tandem designs from III-V tandems to organic tandems. In

a monolithic tandem, junctions are deposited on top of one another in order, generating a single

solar cell with multiple junctions contained within. To make this architecture function, a tunnel

junction or recombination layer is deposited between each junction to allow the flow of current

between junctions. Compared with mechanically-stacked tandems, which have three transparent

electrodes, monolithic tandems require only one transparent electrode. Because transparent elec-

trodes are not perfectly transparent, this leads to a higher practical efficiency potential for monolithic

tandems than mechanically-stacked tandems. Monolithic tandems have drawbacks as well, requiring

a current-matching restriction on the system (the same amount of current must pass through both

junctions) as well as requiring more manufacturing steps to be processed serially, which affects yield

and makes prototyping more difficult.

A tandem solar cell works best with an ideal division of the solar spectrum between the

two absorbers (Figure 1.7). For the bottom absorber in the tandem, a commercially available

technology such as silicon (Si) or copper indium gallium diselenide (CIGS) provides a number of

advantages for the tandem. Both of these technologies have a bandgap around 1.1 eV, which is

smaller than the desired bandgap for a single-junction solar cell, around 1.3-1.4 eV. The fact that

these absorbers are not ideal in single-junction solar cells is what makes them perfect absorbers

for tandems. For the standard AM1.5G solar spectrum, the ideal distribution is to have a 1.1 eV

absorber and a 1.7-1.8 eV absorber [9]. Small-bandgap solar cells (with bandgaps <1.5 eV) are

much more developed than large-bandgap solar cells (bandgap >1.5 eV). Choosing a commercial

bottom-cell technology allows for a simplification of the problem and focus on the top cell alone.




Figure 1.8: Scalable tandem architectures. (Left) A mechanically-stacked tandem combines twoseparately-fabricated solar cells. (Right) A monolithic tandem integrates two junctions into a singlesolar cell.

1.4 Perovskite Solar Cells

The top cell technology used in this thesis is based on the metal-halide perovskite absorber.

First discovered in 1978 [10], this absorber was originally developed for solar cells as a sensitizer in

liquid dye-sensitized solar cells without much acclaim due to the tendency for the perovskite material

to dissolve in the liquid. [11] In 2012, researchers at Oxford University in the UK and at EPFL in

Switzerland developed this material for use in solid-state dye-sensitized solar cells. [12, 13]

The use of these materials in a solid state architecture shocked the academic community

of photovoltaics researchers. First, these cells were 10-12 % efficient, far beyond the previous solid-

state dye-sensitized solar cell records (7.2-8.5 %) [14, 15]. Second, the devices exhibited extremely

high voltages (0.9-1.1 V) uncommon for a material deposited onto a high surface area scaffold bysolution-processing (and therefore amorphous, semi-crystalline, or polycrystalline in nature). Third,

the bandgap of the initial metal-halide perovskite composition used was around 1.6 eV, making

it a new large-bandgap solar technology (which is a category that lacks many high-performance

options) with a bandgap nearly ideal for the top cell in a tandem combination on top of either

Si or CIGS. This initial metal-halide composition was called ”methylammonium lead iodide” with




the chemical formula CH3NH3PbI3 (sometimes written as MAPbI3 with ”MA” as a shorthand for

”methylammonium”).

A perovskite describes a crystal structure (discovered first for the material calcium titanate

- CaTiO3 - and named after mineralogist Lev Perovski) that has the basic stoichiometry ABX3. A is

a cation, which in this case is methylammonium. B is a metal, in this case lead. X is an anion, which

in this case is the halide element iodine. This metal-halide perovskite is formed by the combination

of two salts, PbI2 and MAI.

Discovery of this material set off a flurry of research activity in the area. It was discovered

that this material could be deposited an a vast variety of methods from single-step deposition meth-

ods from both stoichiometric [13] and non-stoichiometric solutions [15], two-step solution deposition

methods where the lead salt was deposited first from solution then the organic salt was introduced

by either dipping [16] or spinning [17], two-step mixed deposition methods where the lead salt was

deposited first from solution then the organic salt was introduced by vapor phase [18], evaporation

methods where both salts were deposited by vapor phase [19], and later, conversion methods where

formation of the full perovskite required a solvent rinse to activate the crystallization [20]. It was

also (re)discovered that the composition of the perovskite could change its optoelectronic proper-

ties. Specifically, while the MAPbI3 perovskite had a bandgap of 1.6 eV, substituting the iodine for

bromine (another halide element) allowed the bandgap to be continuously tuned up to 2.3 eV (the

bandgap of MAPbBr3). [21] In addition, switching the A site material could affect the bandgap.

Substituting the methylammonium for a slightly larger molecule formamidinium (chemical formula

HC(NH2)2) shifts the tunable bandgap range by bromine substitution to 1.5-2.2 eV. [22] Substitut-

ing the methylammonium for the inorganic element cesium shifts the bandgap in the other direction,to 1.7 eV for CsPbI3 [23] and 2.4 eV for CsPbBr3 [24]. The possibility exists for the metal site to

be replaced with elements other than lead (MASnI3 has a 1.2 eV bandgap for example) [25], though

these materials have severe instability to air exposure. So far these efforts have culminated in the

optimization of the perovskite cell to 20.1 % efficiency. [26]




Figure 1.9: Perovskite is the name of the mineral CaTiO3 but this term is being used for allcompounds with the general formula ABX3 that have the same crystal structure as CaTiO3 orare derived from this structure. These materials consist of two cations, the cation A is 12 foldcoordinated by the anions X and the cation B 6-fold where X can either be oxygen or a halide.The most prominent perovskite in solar cells right now is methylammonium lead iodide. It is worthnoting that the structure of methylammonium lead iodide deviates from the ideal cubic perovskitestructure as the octrahedra become slightly tilted and the structure is consequently tetragonal.

The perovskite can be considered an intrinsic material as deposited and no experimental

evidence exists that the perovskite can be controllably doped with either carrier type, making p-n

homojunctions and heterojunctions unavailable architectures at this time. Instead, the perovskite

solar cell consists of the perovskite material sandwiched between two heterojunctions, one p-type and

one n-type (Figure 1.10). This sandwich configuration can be either an n-i-p architecture where the

n-type heterojunction is deposited before the perovskite and the p-type heterojunction is deposited

after the perovskite or a p-i-n architecture where the p-type heterojunction is deposited before and

the n-type heterojunction is deposited after. The n-i-p architecture was the first to be developed

(a direct transition from dye-sensitized solar cells) in both mesoporous configurations (where the

n-type heterojunction forms a porous layer that the perovskite fills in) and planar configurations

(where each layer is solid and has a smooth surface). The mesoporous n-i-p configuration generally

uses titanium dioxide (TiO2) nanoparticles to form the porous layer [13]. This is a holdover from

dye-sensitized solar cells, yet the record 20.1 % efficient perovskite cell still uses this mesoporous

architecture. [26] The planar n-i-p structure has been demonstrated using a number of different n-

type materials, including TiO2, ZnO, SnO2, and C60. The list of p-type heterojunction layers is too

vast to list fully - the most common materials being the organic small molecule spiro-OMeTAD and

the polymer PTAA. The p-i-n architecture is a more recent development, using either a polymeric

layer (e.g. PEDOT:PSS or PTAA) or an inorganic layer (e.g. NiO) as the p-type layer upon which




the perovskite is deposited then the n-type layer from the n-type materials list above.

Figure 1.10: Perovskite solar cell architectures. (Left) an n-i-p architecture using a mesoporousTiO2 scaffold. (Middle) an n-i-p architecture in a planar configuration. (Right) a p-i-n architecturein a planar configuration

For the purposes of the remainder of this thesis, solar cells can be fully described by three

main experiments: Solar simulator I-V or J-V curves, external quantum efficiency, and basic optical

measurements of reflection, absorption, and transmission.

A solar simulator is a xenon arc lamp with filters between the lamp and the solar cell to help

the lamp closely resemble the sun within a small degree of error. The sun, by the AM1.5G standard,

illuminates the earth with an intensity of 100 mW/cm2 (or 1000 W/m2). The solar cell is attached

to a load (resistance) in series to complete the solar cell’s circuit. By changing this resistance, the

voltage drop over the solar cell can be modified. At different loads, the voltage drop across the

cell can be measured and the current running through the system can be measured, generating an

I-V curve. There are three important points on an I-V curve. At zero resistance, the solar cell is

in a short circuit condition and the current measured in this condition is called the ”short-circuit

current” (ISC ). At infinite resistance, the solar cell is in an open circuit condition and the voltage

measured across the solar cell in this condition is called the ”open-circuit voltage” (VOC ). At some

intermediate resistance, the product of current and voltage is maximized and this represents the

most power that can be extracted from the solar cell and is therefore called the ”maximum power

point” (MPP or PMAX). To extract an efficiency value, the I-V curve must be converted to a J-V

curve by dividing the current by the area of the solar cell to obtain a current density (in units of

current/area) (Figure 1.11). This turns the ISC into a ”short-circuit current density” (JSC ) and

the power into a power density. The power density, when divided by the input power density of the

sun (100 mW/cm2) yields the efficiency of the solar cell (η), given in %. A fourth metric called the

fill factor (FF) is the ratio of the measured PMAX over the product of the JSC and VOC .




Figure 1.11: J-V curve of a perovskite solar cell. The relevant points of JSC , VOC , and PMAX areshown.

The short-circuit current density is a product of the solar cell’s ability to absorb light,

extract that current under zero external resistance, and how much light is available from the sun to

absorb at each wavelength (energy) integrated over all wavelengths (energies). The product of the

amount of light absorbed at each wavelength and the probability of extracting the current generated

by that light is called the external quantum efficiency (EQE) (Figure 1.12). Integrating the EQE

with the solar spectrum generates a solar cell’s JSC . EQE is generally measured by connecting the

solar cell to an ammeter, placing it in a dark space, illuminating it with monochromatic light, and

measuring the amount of current generated compared to the amount of light shining on the cell.The monochromatic light is generated by splitting a white light source into separate wavelengths

(similar to a prism) and selecting only one wavelength at a time.




Figure 1.12: External quantum efficiency of a perovskite solar cell as a function of wavelength.Unity represents perfect quantum efficiency.

The interaction of light with the solar cell is extremely important in determining its ef-

ficiency. At all times, light has three options when interacting with matter. It can be reflected,

absorbed, or transmitted (Figure 1.13). For solar cells, it is ideal to have no external reflections

for light above the bandgap of the absorber (but internal reflections can be good), and to absorb

all of the light above the bandgap. For a tandem, these rules extend to the bandgap of the bottom

absorber in the tandem, meaning that the top absorber must be able to transmit the light of an

energy between the top and bottom absorber bandgaps so that it can be absorbed below. These

measurements are conducted in a spectrophotometer.




Figure 1.13: Schematic examples depicting the processes of transmission, reflection, and absorptionof light. Image from www.chroma.com.

With this knowledge of solar cells, tandems, and the metal-halide perovskite, we can con-

struct the first prototypes of perovskite/silicon and perovskite/CIGS solar cells to learn the limita-

tions of these architectures and the practical iterations necessary to make tandems a viable commer-

cial photovoltaic technology and help photovoltaics reach cost competitiveness with other sources of

electricity. In this thesis I will discuss the initial prototypes of monolithic and mechanically-stacked

tandems, the practical advantages and limitations of both based on these prototypes, and model thecost of scaled manufacturing.



Chapter 2

Monolithic Perovskite/Silicon

Tandems

Monolithic tandems are the traditional architecture for tandems and are a good place

to start to develop a perovskite/silicon tandem. This chapter is predominantly work published

in Applied Physics Letters in 2015 [27] followed by an unpublished section on the limitations of

monolithic tandem solar cells.

2.1 Published Work

In this prototype, we develop a 2-terminal perovskite/Si multijunction architecture on an n-type

Si solar cell, as shown in Figure 2.1a. We adopt a device area of 1 cm2. We first start with a

double-side polished <100> n-type float zone Si wafers (15 Ω-cm, 300 µm thickness). The front

side of the wafer is then coated with a silicon nitride (SiNX) film, which protects the planarity of

the Si front surface during the subsequent random pyramidal texturing step on the back surface of

the wafer. [28] After removing the SiNX protective layer using hydrofluoric acid (HF), we implant

boron on the planar front surface and phosphorus on the textured back surface of the wafer. We

then simultaneously form the p-type B emitter and n-type P back surface field (BSF) by drive-in

annealing in an N2 ambient.

15



CHAPTER 2. MONOLITHIC PEROVSKITE/SILICON TANDEMS 16

Figure 2.1: (a) The device structure of a 2-terminal monolithically grown perovskite/Si multijunc-tion solar cell with an n-type Si base. The polished SEM image is taken at 45 tilt to show the Agnanowire mesh (500 nm scale bar). (b) Band diagram of the perovskite/silicon cell interface showingthe charge-transport mechanism around the Si tunnel junction.

After the emitter and BSF formation, we form the silicon-based band-to-band tunnel junc-

tion on top of the p-type emitter. This interlayer is necessary to facilitate carrier recombination

(holes from the n-type Si base passing through the p-type emitter and electrons from the perovskite

layer passing through its TiO2 electron transport layer respectively, as shown in Figure 2.1b). We

form an n++/p++ tunnel junction by depositing heavily doped n++ hydrogenated amorphous sil-

icon (a-Si:H) using plasma-enhanced chemical vapor deposition (PECVD). The 30 nm-thick a-Si:Hlayer is deposited at 250C at a pressure of 200 mTorr (55 sccm of SiH4 gas and 50 sccm of 1%

PH3 in H2 gas) and plasma power density of 0.13 W/cm2, and subsequently annealed in N2 ambient

at 680C for 15 minutes to activate the dopants. [29–31] It is known that interdiffusion of dopant

species during the device fabrication process (such as the dopant activation anneal) may degrade the

tunnel junction conductivity. Accordingly, a 2-3 nm-thick intrinsic a-Si layer is inserted between the

p++ emitter and the n++ amorphous Si layer during the PECVD process (pressure of 150 mTorr,

55 sccm of SiH4 gas with a plasma power density of 0.16 W/cm2) to mitigate possible dopant inter-

diffusion. [31] After the dopant-activation anneal, the amorphous layers are partially crystallized as

shown in the transmission electron microscopy (TEM) image in Figure 2.2a. [29] Using secondary

ion mass spectrometry (SIMS), we then show that the dopant concentration on the n++/p++ Siinterface after the dopant activation anneal is in the order of 1019-1020 cm−3, which is suitable to

form a high-quality interband tunnel junction (Figure 2.2b). [31]




Figure 2.2: (a) TEM image of the n++/p++ silicon tunnel junction interface after the dopantactivation annealing (left: 30 nm scale bar) and high-resolution TEM image of the n++ layer,showing the partially crystalline nature of this layer (right: 5 nm scale bar). (b) SIMS profile of the Si emitter and tunnel junction layer showing the sharp doping profile at the tunnel junctioninterface. (c) Comparison of J-V profile for identical Si cells with and without a tunnel junction,showing negligible effect of the tunnel junction to the single-junction Si cell performance.

We further confirm the functionality of the Si-based interband tunnel junction by fabri-

cating single-junction n-type Si solar cells out of the tunnel-junction substrates. We apply an 80

nm-thick SiNX anti-reflection coating (ARC) on the planar front surface of the Si cell, as well as front

finger and back metalization using a Ti/Pd/Ag stack. We show in Figure 2.2c that the addition of

the tunnel junction on top of the n-type solar cell slightly reduces the short-circuit current (JSC ),

but the interband tunnel junction has a negligible effect on the series resistance. The measured

series resistances (RS ) of a cell with and without a tunnel junction are 1.03 Ω-cm2 and 1.08 Ω-cm2,

respectively. The RS

of the cell without a tunnel junction is larger than the cell with a tunnel junc-tion because the RS addition from the tunnel junction itself is smaller than the sample-to-sample

RSE variability of our c-Si cell fabrication process.

The efficiency of the planar single-junction Si cells are 13.8 % (without tunnel junction),

and 13.2 % (with tunnel junction). This efficiency is lower than commercial averages in part because

we designed our device to be bottom sub-cell compatible with a top perovskite sub-cell (function

under perovskite-filtered light), not a single-junction device functioning under an AM1.5G spectrum.

Some of these intentional design considerations are: (1) No surface texturing for light trapping

is applied because a planar front surface simplifies deposition of the top perovskite sub-cell; (2)

No p-type front surface passivation scheme is applied on the emitter because the same technique

cannot be implemented on the n-type portion of the tunnel junction. In addition to these intentionaldesign choices, the full-area back surface field (BSF) passivation only provides moderate back surface

passivation and should be upgraded in the future. A front surface passivation scheme which can be

decoupled from the tunnel junction formation needs to be developed. Dedicated clean furnaces for

emitter formation and back surface passivation are also necessary to make more efficient bottom Si

sub-cells.




While the thin layer of Si composing the interband tunnel junction on the top of our cell

degrades the blue response (300-400 nm light) of our device and reduces JSC from 31.6 mA/cm2 to

31.0 mA/cm2, in practice this effect is negligible, as only perovskite-filtered light with wavelength λ

> 500 nm is incident on the bottom Si sub-cell in the tandem. [32] The external quantum efficiency

(EQE) of our bottom Si sub-cells is shown in Figure 7.1, confirming the negligible parasitic ab-

sorption of the tunnel junction λ > 500 nm. It is worth noting that our interband tunnel junction

has low parasitic absorption because it is made from partially crystallized Si, which is an indirect

band gap material. This is in contrast with III-V-based interband tunnel junctions where the tunnel

junction layer thicknesses need to be minimized to reduce the parasitic absorption. [33]

Based on the knowledge that our interband tunnel junction has negligible impact on the

bottom Si sub-cells series resistance, we next fabricate the full monolithic perovskite/Si multijunction

solar cell. We first form 1.1x1.1 cm2 square patterns spaced 1.4x1.4 cm2 apart using photolithogra-

phy. Afterwards, we form a 1.1x1.1 cm2 mesa on the silicon tunnel junction substrate by etching away

a 300 nm-thick Si layer (the entire n++ tunnel junction and most of the p++ emitter) using reactive

ion etching (RIE) to reduce the dark current in the final perovskite/Si multijunction device. We met-

alize the textured back side of our bottom Si sub-cell with a Ti/Pd/Ag/Pt metal stack to create the

negative contact for our multijunction cell. Then, we deposit a 30 nm-thick TiO2 layer on the planar

n++ c-Si front surface using atomic layer deposition (ALD) of a tetrakis(dimethylamido)titanium

(TDMAT) precursor (Cambridge NanoTech Savannah ALD tool, 150 C substrate temperature, 80C precursor temperature, 440 mTorr base pressure, and 20 sccm N2 carrier gas). To achieve the

desired TiO2 thickness, we do 604 cycles of pulsing H2O vapor for 0.02 s, waiting for 7 s, pulsing

TDMAT vapor for 0.2 s, and waiting for 7 s. This TiO2 layer serves as the n-type heterojunctioncontact for the top perovskite sub-cell. It is known that TiO2 is a good electron-selective contact for

c-Si because of its conduction-band alignment, which also eases electron transport from the TiO2

layer into the n++ Si tunnel junction layer. [34, 35] After removing the TiO2 that remains on the

back metal (deposited during ALD) using a dilute (10%) HF solution, we cut the substrate into

1.4x1.4 cm2 pieces with the 1.1x1.1 cm2 tunnel junction mesa in the middle.

To build the perovskite sub-cell, an approximately 300-nm-thick mesoporous TiO2 layer

is first deposited by spin coating and then sintered at 500 C. The perovskite is then deposited in

a two-step conversion method modified from the procedure developed by Burschka et al. [16] An

organic p-type heterojunction contact, spiro-OMeTAD, is deposited by spin-coating on top of the

perovskite. This layer is chemically doped with 8 mol% spiro-(TSFI)2 as developed by Nguyenet al. [36] and additionally includes the organic additives tert-butylpyridine (tBP) and lithium

bis(trifluoromethanesulfonyl)imide (LiTFSI). The top silver nanowire (AgNW) electrode is deposited

using the procedure developed by Bailie et al. [32] AgNWs are sprayed from solution onto a plastic

(PET) film, and then transferred from the PET to the spiro-OMeTAD layer via mechanical transfer

through the application of pressure via a ball bearing. The silver nanowire electrode before transfer




has a sheet resistance of 9 Ω/ with a peak transmission of 89.5%. A 300-nm-thick silver contact

pad is deposited around the edge of the AgNW area by thermal evaporation to increase the geometry

of current collection as well as provide a means of making mechanical contact to the AgNW mesh,

resulting in active device area of 1x1 cm2. To reduce incident-light reflection, a 111-nm-thick lithium

fluoride (LiF) layer is deposited by thermal evaporation.

The J-V curve of our 2-terminal perovskite/Si multijunction solar cell under AM1.5G illu-

mination is shown in Figure 2.3a. Due to the hysteresis often observed in organic-inorganic halide

perovskite-based solar cells, it is important to be rigorous with the J-V characterization. [37] For

our device, we use a 5 s delay after each 100 mV voltage step before measuring the current and show

the performance of the device under both forward and reverse bias. However, we still find hysteresis

to be evident at this slow scan rate and further find that the current requires up to 30 s at a given

voltage to settle to a steady-state value. We therefore choose instead to measure the steady-state

values of the three critical points on the J-V curve: open circuit, short circuit, and the maximum

power point, depicted as blue circles in Figure 2.3a. We find the steady-state photocurrent at

short circuit (JSC ) to be 11.5 mA/cm2, the steady-state voltage at open circuit (VOC ) to be 1.58

V (Figure 7.2), and the steady-state maximum power under load (MPP) to be 13.7 % at 1.20 V

bias as shown in Figure 2.3b. These three values result in a fill factor (FF) of 0.75. It is important

to note that the transient output current (and hence pseudo-efficiency) can be significantly over-

estimated relative to its steady-state output at MPP without a sufficient settling time. [37] The 1

cm2 cell was aperture-masked to ensure a correct illumination area. These results are summarized

in Table 2.1.

Table 2.1: Photovoltaic parameters from 2-terminal perovskite/Si multijunction cell (hero device).

VOC 1.58 VJSC 11.5 mA/cm2

FF 0.75Efficiency 13.7 %Aperture Area 1 cm2




Figure 2.3: (a) J-V curve of the 2-terminal perovskite/silicon multijunction solar cell underAM1.5G illumination. Forward and reverse-bias scan directions are shown with 5 s measurementdelay per data point. Steady-state values for JSC , VOC , and MPP are depicted as blue circles andare measured by averaging over 30 s after reaching steady state. The VOC of 1.58 V is approximatelythe sum of the perovskite and Si cell VOC . (b) Time-dependent output current of the multijunction

cell near maximum power point (1.20 V forward bias) showing that the output reaches steady stateafter a measurement delay of 30 s. (c) Total device reflection and EQE of the perovskite and Sisub-cells of a typical perovskite/Si multijunction cell. The EQE spectra exhibit a low blue EQEin the top perovskite sub-cell due to spiro-OMeTAD parasitic absorption, and low red EQE in thebottom Si sub-cell due to both spiro-OMeTAD parasitic absorption and the lack of good Si backsurface passivation scheme. The perovskite sub-cell EQE is corrected to match the measured JSC and the silicon sub-cell EQE is reported as measured

The VOC of our 2-terminal perovskite/Si multijunction has been measured as high as 1.65

V in some of our devices. This result is encouraging, especially because the VOC is approximately

the sum of the VOC for the perovskite top sub-cell and the bottom Si sub-cell illuminated through

a separate semi-transparent perovskite device on FTO [32] (approximately 1.05 V and 0.55 V,respectively). We speculate that the tandem perovskite sub-cell’s VOC benefits from being in contact

with silicon rather than FTO. Mechanical lamination of the AgNW electrode was previously found

to be highly dependent on pressure, with too much pressure causing shorting. [32] However, we did

not observe shorting of the AgNW electrode in the tandem. This may be because the silicon emitter

is not as conductive as FTO, and therefore local shorts do not affect the full device area.

The slow current-dynamics and corresponding hysteresis observed in the tandem resemble

the sluggish dynamics of our perovskite solar cells and suggest that the perovskite sub-cell limits the

current of the tandem. [37] To investigate further, we illuminate the tandem with a white light LED,

which emits only in the visible spectrum, placing the silicon sub-cell in a current-limiting regime. In

this regime, we observe the current to settle within milliseconds as expected for silicon solar cells and

do not observe hysteresis. These findings are substantiated by EQE measurements (Figure 2.3c) of

the individual sub-cells. Notably, the perovskite junction EQE suffers at wavelengths shorter than

550 nm.

Our low tandem JSC of 11.5 mA/cm2 is caused by the fact that the perovskite illumina-

tion in our multijunction configuration comes from the p-side of the device, the opposite direction




from conventional perovskite devices. To understand the directional dependence of illumination

on the perovskite sub-cell, we illuminate a semi-transparent single-junction perovskite solar cell as

developed by Bailie et al. [32] from both the standard illumination direction - through the TiO2

heterojunction, and the reverse illumination direction - through the spiro-OMeTAD heterojunction,

corresponding to the illumination direction of the top perovskite sub-cell in the monolithic tandem.

When illuminated through the TiO2 heterojunction the EQE of the semi-transparent cell integrates

to 17.3 mA/cm2, whereas when illuminated through the spiro-OMeTAD heterojunction the EQE

integrates to 11.4 mA/cm2 (Figure 7.3). This device did not have anti-reflection coatings to sim-

plify analysis, therefore the photocurrent values are 0.5 to 1.0 mA/cm2 lower than if anti-reflection

coatings were used. We attribute the lower current under illumination from the spiro-OMeTAD-side

to parasitic absorption by the doped spiro-OMeTAD layer. From absorption measurements of doped

spiro-OMeTAD on glass (Figure 7.4) we estimate that the absorbed flux of the AM1.5G spectrum

in this layer is 6.4 mA/cm2 from 300-750 nm, though this value may change slightly in a full device

stack due to optical interference between the device layers. We find that parasitic absorption by

spiro-OMeTAD also reduces the photocurrent available to the bottom Si sub-cell, absorbing the

equivalent of 2.0 mA/cm2 of infrared photons from 750-1200 nm.

The performance of our perovskite/Si multijunction cell is current-limited by the pho-

tocurrent of the top perovskite sub-cell to 13.7 % efficiency at steady state. In part, the limited

performance of the perovskite sub-cell is due to parasitic absorption in the spiro-OMeTAD layer.

The parasitic absorption can be reduced with a thinner spiro-OMeTAD layer. At present, the spiro-

OMeTAD layer thickness is optimized to planarize a rough perovskite top surface. With smoother

perovskite films, the spiro-OMeTAD layer can achieve planarization with a thinner layer. Theparasitic absorption may be completely removed by replacement of spiro-OMeTAD with an alter-

nate p-type heterojunction contact that simultaneously exhibits both good conductivity and low

parasitic absorption. The perovskite sub-cell is additionally limited by quality of the perovskite

absorber. Our single-junction perovskite cells with gold back contacts fabricated as control devices

in a similar fashion to the perovskite sub-cell achieve at best 13.5 % efficiency. Optimization of

deposition conditions, precursor materials, and annealing protocols along with replacement or re-

duction of the spiro-OMeTAD layer is expected to yield a perovskite top sub-cell equivalent to the

record single-junction perovskite cell, which currently stands at 20.1 %. [38]

For the silicon bottom sub-cell, three improvements are envisioned: first, apply a back-

surface field and excellent surface passivation to the back of the bottom Si sub-cell; second, fabricatethe silicon sub-cell in dedicated furnaces; third, decouple the front surface passivation scheme from

the tunnel junction formation. These three improvements are expected to yield a bottom Si sub-cell

with a VOC of 660-720 mV and a matched tandem JSC of 18-19 mA/cm2 upon illumination through

the top perovskite sub-cell.

We expect, with these changes designed to improve the sub-cells to the match the highest




quality devices available today, the monolithic tandem would have a VOC of 1.84 V, a JSC of 19

mA/cm2, a FF of 0.83, and a corresponding efficiency of 29.0 %. Ultimately, it has been suggested

that these monolithic tandems can surpass 35% efficiency through careful photon management. [39]

In summary, we have demonstrated a 1 cm2, 2-terminal organic-inorganic halide per-

ovskite/Si multijunction solar cell. This monolithic integration for n-type bottom Si sub-cell ar-

chitecture is enabled by a Si-based tunnel junction fabricated directly on top of the bottom Si

sub-cell emitter, and by incorporating a semi-transparent silver nanowire-based electrode for the

top perovskite sub-cell. We obtained a multijunction device VOC as high as 1.65 V, which is the

expected sum of the perovskite and filtered silicon single-junction VOC s, demonstrating the po-

tential of this approach. The best 2-terminal multijunction prototype device efficiency is 13.7 %.

This value is low compared to the record efficiency for perovskite or Si cells, in part because this

tandem prototype does not yet have best-in-class perovskite and Si layers. The parasitic absorption

in the spiro-OMeTAD hole-transport layer on the top of the perovskite sub-cell results in a tandem

current-limited by the top sub-cell. Further improvements of the multijunction perovskite/Si device

can be achieved by replacing the spiro-OMeTAD layer with wider band gap hole transport mate-

rial, improving the quality of the perovskite absorber, use of dedicated furnaces for the Si sub-cell

fabrication, and by implementing better surface passivation scheme on the front and back side of

the bottom Si sub-cell. These improvements can yield a 29.0 % efficient tandem, with the ultimate

efficiency potential of these monolithic tandems surpassing 35 %.

2.2 Limitations of Monolithic Tandems

In the previous section, a 13.7 % efficient perovskite/silicon monolithic tandem had a high

VOC and FF but was highly limited by a large current mismatch between the perovskite (11.5

mA/cm2) and the silicon (14.7 mA/cm2). Parasitic absorption in the spiro-OMeTAD caused the

mismatch, and replacing the spiro-OMeTAD with a more appropriate window layer material with

no absorptions below 3 eV would result in a current-matched system generating ∼15 mA/cm2.

Assuming no other changes to the system, this new tandem with a better window layer would be

∼18 % efficient.

As in the earlier section, we believe with the best possible silicon and perovskite materials

available today the tandem could be improved electronically to a VOC of 1.84 V and a FF of 0.83.

With a JSC of 15.0 mA/cm2, this would produce a tandem efficiency of 22.9 %. This is still a far cryfrom the 30 % that we would like the tandem to be. To get to 30 %, a matched photocurrent of 19

mA/cm2 is necessary (along with some minor improvements in VOC or FF). 19 mA/cm2 represents

an efficient division and collection of the solar spectrum between the two sub-cells.

Reaching such a high matched photocurrent may be impossible to reach in practice due to

a fundamental and potentially incompatible difference between silicon solar cells and the solution




deposition of the perovskite. Through a half-century of device optimization, the best silicon solar

cells have texturing on the front and back surfaces of the silicon wafer. Silicon, an indirect-bandgap

semiconductor, absorbs the infrared light at energies close to its bandgap very weakly. Texturing the

surfaces of the wafer promotes internal reflection of the infrared light, allowing it to bounce around

inside the wafer until it has a high probability of being absorbed by the silicon.

The perovskite is solution-processed, which is an inherently planarizing method of depo-

sition. This means that if the perovskite is deposited on a rough surface, more perovskite will be

deposited in the valleys and less perovskite will be deposited on peaks. In the case of silicon, the

texturing of the wafer produces pyramids that have a height around 5-12 µm. The perovskite, on

the other hand, is ideally between 250 and 500 nm thick. With a full order of magnitude difference

between the perovskite thickness and the roughness of the surface, the consequence is that the peaks

of the silicon pyramids are left completely bare of perovskite (Figure 2.4). Exposed silicon, when

the tandem is finished with an electrode on top, represents a shorting pathway that bypasses the

perovskite cell. This shunt reduces the effectiveness of the perovskite top cell, and will limit the

tandem’s efficiency. One possible way around this limitation is to vacuum-deposit the perovskite

and heterojunction layers (vacuum deposition is a conformal method of deposition) - but this may

offset the low tooling cost and high throughput that were major advantages of the solution-processed

metal-halide perovskite.

Figure 2.4: Perovskite is solution-processed onto a textured silicon wafer. The nominal thicknessof the perovskite layer is 250-500 nm while the thickness of silicon pyramids is 5-12 µm. Moreperovskite is deposited in the valleys between the pyramids while the peaks are left completely bareof perovskite

If the perovskite cannot be deposited onto a textured silicon surface, the silicon wafer must




be polished on top. Losing some of this light-trapping in the silicon will hurt the infrared response

in the silicon solar cell, making current-matching more difficult.

A further complication is the optical effect of depositing thin film layers onto a silicon

solar cell (Figure 2.5). With a high refractive index ∼3.9, silicon requires an anti-reflection coating

in order to prevent reflections from the air-silicon interface (a silicon solar cell without an anti-

reflection coating is shown as the black curve in Figure 2.5, and a silicon solar cell with an ideal

double-layer anti-reflection coating is shown as the blue curve in Figure 2.5). In the monolithic

tandem, the perovskite must be an anti-reflection coating for the silicon as well as optimized for

optical and electrical properties of the layers to make the perovskite cell efficient. As optimized for

a single-junction perovskite solar cell, if these layers are deposited onto a silicon solar cell, the result

is the red curve in Figure 2.5 which shows an intermediate performance between the two extremes.

Changes in the perovskite layers can improve the anti-reflection properties of the stack, but will

more than likely come as a sacrifice in the performance of the perovskite sub-cell.

Figure 2.5: Result of a PC1D simulation. Blue curve represents the EQE of a silicon cell withfull texturing and an ideal double layer anti-reflection coating. Black curve represents the EQEof a silicon cell without front surface texturing and the silicon surface in direct contact with air.Red curve is the EQE of a silicon solar cell with a planar front surface with a thin film layer stackwith thicknesses and real refractive indices similar to a perovskite solar cell showing an intermediateperformance between the two extremes. Numbers in the legend refer to the refractive index andthickness of the thin film layers on top of the silicon.

If the processing issues with the monolithic tandem can be overcome to manufacture a 30

% efficient solar cell, it must be put in a module and operated outside in the field. In the lab, a solar

cell is designed to operate under a standardized spectrum AM1.5G (Figure 1.4). Unlike laboratory

conditions, the solar spectrum changes constantly, with a different shape depending on the time

of day, time of year, latitudinal position, elevation, and whether the sun is obscured by clouds or

the sky is clear. These changes produce solar spectra with a wide variety of shapes - an example




of Denver, CO shown in Figure 2.6. In Figure 2.6, light to the left of ∼700 nm is absorbed

by the perovskite and light to the right of ∼700 nm is absorbed by the silicon. With a tandem

current-matched according to the AM1.5G spectrum, the tandem will also be current-matched in

the field with an average photon energy (APE) ∼1.9 eV. This implies a current-mismatch between

the perovskite and silicon for all other spectra. Therefore, the monolithic tandem, while 30 % in a

laboratory condition, is unlikely to be near this efficiency for much of the time in the field.

Figure 2.6: Representative spectra of the sun’s illumination of Denver, CO over a year. Figure

reproduced from [5]. In this plot, all light to the left of ∼700 nm is absorbed by the perovskite and alllight to the right of ∼700 nm is absorbed by the silicon (assuming a modified perovskite bandgap).Note that for many of these spectra, a current-mismatch will occur between the perovskite andsilicon.



Chapter 3

Mechanically-Stacked Tandems

using Silver Nanowires

The other scalable tandem architecture as illustrated in Figure 1.8 is the mechanically-

stacked tandem. This architecture overcomes many of the limitations of the monolithic tandem

architecture. First, because the perovskite and silicon cells are fabricated independently of each

other, many of the original design optimizations can be maintained. For the silicon cell, this specifi-

cally means the silicon wafer can be textured on both sides, recovering the long-wavelength EQE lost

when switching to a planar front surface. For the perovskite solar cell, it is generally designed to be

deposited onto a smooth sheet of TCO-coated glass rather than a rough silicon surface. Second, theperovskite is no longer coherently connected to the silicon, and anti-reflection/index-matching coat-

ings on the perovskite cell and the silicon cell as well as an appropriate intermediate layer in-between

the cells can effectively minimize reflections. Third, the mechanically-stacked tandem has versatility

in wiring allowing it to recover potential loss in performance from changes in the environment that

the monolithic tandem cannot.

A common critique of the mechanically-stacked tandem is that the cells are often tested in

the laboratory in a four-terminal configuration. This means that the perovskite and silicon/CIGS

solar cells are tested (electrically) independently of each other. If no changes to this design are

made, then the module will also have four terminals (wires) exiting the junction box. Since a

single-junction module has two wires, the extra wires in the tandem will require extra invertersand complicate the installation process to a degree that installing the tandem is too expensive for

utility-scale deployment, regardless of its efficiency. Fortunately, the mechanically-stacked tandem

can be wired in multiple ways that result in two terminals exiting the module.

Figure 3.1 shows two potential wiring configuration that result in two terminal exiting

the module. In Figure 3.1(Left), The perovskite and silicon cells are wired in series to enforce

26



CHAPTER 3. MECHANICALLY-STACKED TANDEMS USING SILVER NANOWIRES 27

a current-matching condition. The total current in each cell can be matched by adjusting the size

of the perovskite solar cell (the perovskite and silicon do not need to be current- density matched).

However, this current-matching condition is one of the limitations of the monolithic tandem and

should be avoided if possible. In Figure 3.1(Right), the perovskite cells are wired in series and

the silicon cells are separately wired in series. These two strings are wired together in parallel to

enforce a voltage-matching condition. A voltage-matching condition is much less sensitive to the

shape of the solar spectrum. However, the voltage of solar cells is sensitive to the temperature of the

module and the temperature-dependence of the cells are highly dependent upon the bandgap of the

absorber. A third alternative is to use a power electronic circuit in either configuration to correct

for any mismatches in current or voltage between the strings of cells.

Figure 3.1: Mechanically-stacked tandem wiring configurations that result in two terminals exitingthe module. (Left) The perovskite and silicon cells are wired in series to enforce a current-matchingcondition. The total current in each cell can be matched by adjusting the size of the perovskite solarcell (the perovskite and silicon do not need to be current-density matched). (Right) The perovskitecells are wired in series and the silicon cells are separately wired in series. These two strings arewired together in parallel to enforce a voltage-matching condition. A third alternative is to use apower electronic circuit in either configuration to correct for any mismatches in current or voltagebetween the strings of cells.

When making a semi-transparent perovskite solar cell, two transparent electrodes are re-

quired. The first is deposited onto the glass sheet, which is already a highly-optimized process, and

the second is deposited onto the perovskite cell. The perovskite cell has a number of design require-ments for the transparent electrode that make depositing the electrode onto the perovskite difficult.

First, the perovskite is thermally sensitive and if left uncovered the methylammonium iodide salt

with decompose and evaporate at temperatures as low as 60 C given enough time. Second, the per-

ovskite is solvent-sensitive. Because the perovskite is formed from two salts and deposited from polar

solvents, it is sensitive to the solution-deposition of layers on top of it that are deposited from either

water or polar organic solvents. Third, the heterojunction layers on either side of the perovskite are




often organic layers and with the methylammonium molecule, it is possible that the perovskite itself

can be considered as an organic in some situations. The typical transparent electrode is formed by

sputtering of a transparent conducting oxide (TCO). Sputtering is well known to damage organic

layers, and so TCOs are likely incompatible with direct deposition onto the perovskite or organic

heterojunction layers.

The first solution that we developed for compatibility with the perovskite was to use silver

nanowires that were initially sprayed onto a plastic film then mechanically transferred at room

temperature without exposing the perovskite to raised temperature or solvent. The next section is

work published in Energy and Environmental Science in 2015 [32], followed by a section concerning

the stability of these cells. Specifics of the nanowire lamination procedure can be found in the

Appendix.

3.1 Published Work

3.1.1 Semi-Transparent Cells

Mechanically-stacked tandems require a semi-transparent top cell, as illustrated in Figure

3.2. The perovskite solar cell architecture used in this study is similar to that developed by Burschka

et al [16]. We use a mesoporous titanium dioxide (TiO2) layer infiltrated with the perovskite and con-

tacted on either side by electron-selective (compact TiO2) and hole-selective (2,2,7,7-tetrakis(N,N-di-

p-methoxyphenylamine)-9,9-spirobifluorene, spiro-OMeTAD) contacts (see Perovskite Methodology

subsection). For compatibility with these existing electron- and hole-selective contacts, we use the

MAPbI3 perovskite rather than the optically ideal MAPbBrI2. MAPbBrI2 was also not chosen due

to a photo-instability observed in this material [40]. The transparent front electrode is fluorine-doped

tin oxide (FTO) coated glass. Typically, a perovskite solar cell is opaque with an approximately

100-nm-thick metal back electrode of either Au or Ag. This metal back electrode provides a low-

resistance electrical contact and a reflective surface, giving the perovskite a second chance to absorb

any light that was not absorbed on the first pass. To enable the transparency required to make a

mechanically-stacked tandem, we needed a transparent top electrode.

The technical constraints that the top transparent electrode must meet are stringent. The

electrode must be highly transparent in the critical 600-1000 nm window where the perovskite

is not absorbing all of the light and the bottom cell has significant external quantum efficiency

(EQE). The sheet resistance of the transparent electrode should be at most 10 Ω/ [41] because the

transparent electrode must have high lateral conductivity to minimize resistive loss when carrying

the large current density generated in the perovskite cell. Perhaps most importantly, this electrode

must be applied after deposition of the spiro-OMeTAD layer onto a temperature- and solvent-

sensitive perovskite solar cell without damaging it. For these reasons, high-performance transparent

conductive oxides widely used in industry cannot be directly sputtered onto a perovskite solar cell




without a buffer layer. An electrode meeting these criteria has not been demonstrated before now.

We use a silver nanowire (AgNW) mesh electrode which has been shown in other cases to have a low

sheet resistance and high optical transmission [42–44], and develop a new method of depositing this

electrode onto our perovskite cell in a room-temperature solvent-free process. This AgNW electrode

serves as the linchpin for our mechanically-stacked tandem architecture.

The current-voltage curves and metrics of the semi-transparent perovskite cells and opaque

control devices are shown in Figure 3.2 and Table 3.1. The loss in absorption in the perovskite

due to the removal of the opaque metal back electrode was offset by reduced reflection from the glass

surface by the AR coating, yielding comparable JSC between the semi-transparent and opaque cells.

We note that if the opaque cell had an AR coating, it would have approximately 0.5 mA/cm2 higher

photocurrent. We control our measurements for hysteresis in accordance with a paper by Unger et

al [37]. We found a 5 s delay time between stepping the voltage and measuring current necessary to

achieve steady state and remove any semblance of hysteresis. This procedure for removing hysteresis

was corroborated and confirmed by NREL when a device was sent for certification. Shadow masks

were used to define the illuminated area of a device. Opaque devices were illuminated through a

0.12 cm2 mask and semi-transparent devices were illuminated through a 0.39 cm2 mask.

Table 3.1: Performance metrics of semi-transparent and opaque perovskite devices.

JSC (mA/cm2) VOC (mV) FF (-) Efficiency (%)Semi-Transparent Perovskite 17.5 1025 0.710 12.7Opaque Electrode Perovskite 17.5 982 0.740 12.7




Figure 3.2: Perovskite device results. a) Current-voltage curves comparing best opaque vs. semi-transparent perovskite devices. b) EQE of semi-transparent device and opaque device. Note thatthe opaque device does not have AR coatings. c) Transmission through semi-transparent perovskitewith AR coatings. Peak transmission is 77 % around 800 nm.




Figure 3.2c shows that the transmission through the semi-transparent device peaks at 77%

around 800 nm, the center of the 600-1000 nm transmission window that is critical for tandems.

Much of the transmission loss is due to parasitic absorption in the FTO electrode, AgNW electrode,

and spiro-OMeTAD layer. Uniquely, our semi-transparent device has both a high below-bandgap

transmission and a high efficiency. Previous semi-transparent devices have had to sacrifice one of

these metrics to achieve the other. [43, 45] There remains significant room for improvement in the

transmission. Low-temperature processes would allow for fabrication of the perovskite cell on ITO,

which is more transparent than FTO. A more transparent hole-transporter than spiro-OMeTAD,

which in its oxidized form absorbs light throughout the visible and infrared [36], would also improve

transmission.

3.1.2 Mechanically-Stacked Tandems

We have made tandems with both Si and CIGS as bottom cells. Both have a bandgap

around 1.1 eV, which is sub-optimal for a single-junction solar cell but optimal for a double-junction

tandem [46], and are commercially successful solar technologies. We use a 17.0 % laboratory-scale

CIGS device made using previously reported procedures. [47–50] Although CIGS cells with 21 %

efficiency can be made, we chose a cell with a more modest efficiency for this demonstration to

illustrate how cells that can be made at scale could be enhanced with a perovskite top cell. The

current-voltage curves and external quantum efficiency of the semi-transparent perovskite solar cell,

the CIGS solar cell and the CIGS solar cell underneath the perovskite solar cell are shown in Figure

3.3. To arrive at the efficiency of the 4-terminal tandem, the efficiency of the semi-transparent

perovskite cell is added to the efficiency of the CIGS solar cell when underneath the perovskite cell.

With our 12.7 % semi-transparent perovskite cell, we improve the 17.0 % CIGS cell to 18.6 % in a

tandem (Figure 3.3 a,b/Table 3.2) as measured in-house.




Figure 3.3: Perovskite and CIGS/Si tandem results. a) Current-voltage and b) EQE of semi-transparent perovskite cell, unfiltered CIGS cell, and CIGS cell filtered by the perovskite cell. c) IV

curves and d) EQE of semi-transparent perovskite cell, unfiltered TI-Si cell, and TI-Si cell with aninfrared-optimized anti-reflection coating filtered by the perovskite cell.

Table 3.2: Performance metrics of mechanically-stacked tandems

JSC (mA/cm2) VOC (mV) FF (-) Efficiency (%)Semi-Transparent Perovskite 17.5 1025 0.710 12.7

TI-Si - Unfiltered 29.3 582 0.667 11.4TI-Si w/ IR-ARC - Filtered 11.1 547 0.704 4.3Tandem w/ Perovskite + Ti-Si 17.0

UMG-Si - Unfiltered 27.9 590 0.705 11.6UMG-Si w/ IR-ARC - Filtered 9.4 553 0.698 3.6Tandem w/ Perovskite + UMG-Si 16.3

CIGS - Unfiltered 31.2 711 0.768 17.0CIGS - Filtered 10.9 711 0.788 5.9Tandem w/ Perovskite + CIGS 18.6




Figure 3.4: NREL certification of a mechanically-stacked tandem. The tandem efficiency is thesum of the individually measured cells, 17.9%.




Perovskite solar cells are already efficient enough to upgrade the performance of silicon so-

lar cells made with low-quality silicon using the polycrystalline tandem approach. Here, we explore

lower-quality sources of Si including cast multicrystalline silicon (mc-Si) wafers made from feedstock

with high impurity content recycled from the top 10 % of other cast multicrystalline ingots (TI-Si)

(Figure 3.3 c,d/Table 3.2) and cast mc-Si wafers grown using 4.5 N (99.995 % pure) upgraded

metallurgical-grade Si (UMG-Si) (Table 3.2) instead of the more expensive Siemens-grade polysil-

icon (9 N, or 99.9999999 % pure). Low-quality Si sources generally are not commercially viable

today in single-junction devices because the material cost advantage of low-quality Si is offset by the

reduction in performance due to impurities and crystal defects. We improve an 11.4 % low-quality

Si cell to 17.0 % as a tandem, a remarkable relative efficiency increase of nearly 50 %. Such a dras-

tic improvement in efficiency has the potential to redefine the commercial viability of low-quality

Si. Another tandem cell was sent to NREL for certification using a 17 % silicon bottom cell. The

tandem was certified as 17.9 % efficient (Figure 3.4).

When making tandems as opposed to single-junction devices, some design parameters

change for the bottom cell. The tandem relaxes the design constraints for both the Si and CIGS

top layers. For example, the 20-50 nm CdS window layer used in commercial CIGS devices results

in a photocurrent loss of 0.5 mA/cm2 due to reduced EQE from 400-550 nm caused by parasitic

light absorption in the CdS layer. However, this does not affect the EQE of a tandem because the

400-550 nm light is already absorbed in the top cell, decoupling the optimization of the electronic

and optical properties of the CdS layer. In single-junction Si cells, there is a strict tradeoff of the

series resistance vs. EQE from 400-550 nm due to minority carrier recombination in the emitter

layer. As the bottom cell in a tandem, the emitter thickness or doping can be increased without anEQE penalty. In this work, the Si has a 35 Ω/ phosphorus-diffused emitter as opposed to 100 Ω/

in industry [51]. Lower sheet resistance in the emitter layer means bus bar spacing can be increased,

reducing shading losses. The design parameters also change for the optimal anti-reflection coatings

used in a tandem. All commercial solar cells use AR coatings to improve the transmission into the

solar cell. For a single-junction cell, the AR coating on top of the cell is optimized for a broad

spectral range from 400-1100 nm and necessarily suffers in performance at the edges of this range.

However, for the bottom cell in a tandem, the AR coating is optimized for a much narrower spectral

range between 800 and 1100 nm, and can maintain a much higher performance through this narrower

spectral range. Full consideration of the different design parameters between single-junctions and

tandems such as these examples could yield further improvements in the future.

3.2 Stability

With the silver nanowire lamination method, we successfully developed a transparent elec-

trode deposition process that did not damage the perovskite. While this allowed for the prototyping




of efficient mechanically-stacked tandems, this solution did not provide sufficient stability of the

perovskite cell. The silver nanowire electrode is a mesh of metal on top of the solar cell, implying

that much of the perovskite cell is uncovered by metal. These gaps between the silver nanowires

allowed raised temperatures to cause evaporation of the methylammonium iodide as well as ingress

of moisture from ambient air that degraded the perovskite. Additionally, direct contact of silver

(also true of most metals) with the perovskite (spiro-OMeTAD is not a good ionic diffusion barrier)

corrodes the silver to silver iodide [52], which is an insulator. The combination of these issues re-

sulted in a lifetime for these semi-transparent cells in ambient air under illumination of only a few

hours. These short lifetimes motivated the search for a new transparent electrode solution.



Chapter 4

Mechanically-Stacked Tandems

using ITO

The application of a mesh transparent electrode on the perovskite created a solar cell with a

limited lifetime due to holes in the mesh as well as corrosion of the metal by halides in the perovskite.

The solution to these problems is to deposit a transparent electrode that is a solid layer and is not

composed of metal. Indium tin oxide (ITO) is the industry standard transparent electrode [53] and

is already known to have good moisture barrier properties [54].

Sputtering ITO onto perovskite cells has been attempted before [55] but with limited

success. The solution to the sputtering damage to organics during deposition of the ITO so far hasbeen to thermally evaporate a layer of MoOX . Even so, these devices have suffered from a lower VOC

than their opaque metal electrode counterparts. The alternative solution we came up with was to

coat the top of the perovskite cell with oxide nanoparticles. These oxide nanoparticles were deposited

from an alcohol solution that was compatible with the perovskite and then provided a buffer layer to

the sputtering of the ITO on top. By using doped oxide nanoparticles, ohmic contact could be made

between the heterojunction layer, the doped oxide nanoparticles, and the ITO, ensuring efficient

operation of the semi-transparent solar cell. The ITO provided a solid capping layer, stabilizing

the perovskite solar cell at high temperatures and because it was not a metal, was not corroded by

halides in the perovskite.

The next section is work submitted for publication to Advanced Materials [56] followed bya section on the future of mechanically-stacked tandems.

36



CHAPTER 4. MECHANICALLY-STACKED TANDEMS USING ITO 37

4.1 Published Work

4.1.1 Deposition Method

Inverted perovskite solar cells, in which the perovskite is fabricated on top of a hole-selective

contact, can achieve a high power conversion efficiency >18 % [57]. Briefly, we spin poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) onto ITO-coated glass for a smooth,

hydrophilic p-type heterojunction layer. The perovskite is then deposited using a technique devel-

oped by Zhang et al. [58], followed by phenyl-C61-butyric acid methyl ester (PCBM) as an n-type

heterojunction layer. See the Experimental section for more details. We solution deposit nanoparti-

cles of the wide bandgap semiconductor ZnO to serve as an electron-selective buffer layer, protecting

the underlying organic PCBM and perovskite layers. The deep valence level of ZnO enables its use

as a selective contact, Figure 4.1a. ZnO has been demonstrated by Guo et al. [59] and later shownby You et al. [60] to favorably affect the ambient stability of a perovskite solar cell. A 50 nm layer

of ZnO nps on the PCBM layer and an Al rear electrode enables a stabilized power output of 13.5

% (Figure 4.2b).

Figure 4.1: Semi-transparent inverted perovskite device architecture. a) Energy level diagram.b/c) Cross-sectional SEM and illustrative schematic of device architecture showing ITO electrodeencapsulation layer.




Figure 4.2: Performance comparison of the opaque and semi-transparent devices showing a) J-V

curve of devices showing comparable FF and VOC

and b) max power point tracking.

We sputter 500 nm of ITO directly onto the ZnO-np capped films to achieve a low sheet

resistance of 9.9 Ω/ after annealing at 100 C. The transmission spectrum of identically processed

ITO on a glass is shown in Figure 7.12. Figure 4.1b shows an SEM cross-section of the ITO

electrode on top of the perovskite. The open-circuit voltage (VOC ) is greater than 0.9 V, indicating

that ZnO successfully acts as a buffer layer to prevent sputter damage of the organic PCBM layer.

However, a large interfacial barrier exists between the ZnO and ITO layers, preventing carrier ex-

traction. This barrier is temporarily removed when operating the device at an elevated temperature

of 70 C, as shown in Figure 7.10. However, the barrier returns as the device cools. We speculate

that the barrier arises from the misaligned work functions of ZnO and ITO. We introduce aluminumdoped (2 mol %) zinc oxide (AZO) nanoparticles to eliminate the extraction barrier, allowing the

device to operate optimally at 25 C.

4.1.2 Semi-Transparent Cells

The J-V curves of the opaque (Al/Ag electrode) and semi-transparent (ITO electrode)

devices are shown in Figure 4.2a. The AZO nanoparticles allow operation of the semi-transparent

devices at room temperature with stabilized power efficiency of 12.3% (Figure 4.2b). The semi-

transparent device displays a high fill factor of 0.77 with a VOC of 0.95 V and JSC of 16.5 mA/cm2

(Table 4.1). The semi-transparent device with a top electrode area of 6 mm x 11 mm is illuminated

through a 4.5 mm x 8.75 mm aperture mask and the opaque device with a top electrode area of

4 mm x 5 mm is illuminated through a 3 mm x 4 mm aperture mask. We scan the devices in a

positive to negative voltage direction with a step size of 50 mV and a delay time between points of

0.2 s (Figure 4.2a). We validate the J-V measurements by operating the devices at their maximum

power point (Figure 4.2b). The performance of the sputtered ITO electrode is validated by the

comparable FF and voltage in the opaque and semi-transparent devices. The drop in current density




in the semi-transparent device can be explained by the lack of a metal electrode back-reflector to

increase light absorption. This decrease primarily occurs at longer wavelengths as shown by EQE

in Figure 7.13.

Table 4.1: Photovoltaic parameters of a semi-transparent perovskite solar cell with ITO rearelectrode compared to an opaque perovskite solar cell with Al/Ag rear electrode.

JSC * (mA/cm2) VOC (mV) FF (-) Efficiency** (%)Semi-Transparent Perovskite 16.5 952 0.77 12.3Opaque Electrode Perovskite 18.8 938 0.77 13.5*obtained from EQE integration of AM1.5G spectrum**obtained from maximum power point tracking

4.1.3 Tandems

We test the efficacy of our semi-transparent device in a mechanically-stacked tandem con-

figuration with a mono-crystalline silicon (mono-Si) bottom cell. The results are summarized in

Figure 4.3 and Table 4.2. Alone, the efficiency of the mono-Si is 17.0 %. The silicon cell (6 mm x

13 mm) is limited in VOC due to excess shaded area from the aperture mask and the top electrode.

With the semi-transparent perovskite solar cell stacked in front of the mono-Si cell, the efficiency

improves to 18.0 % (12.3 % + 5.7 %) with a JSC of 13.3 mA/cm2 from the filtered Si bottom cell.

Strong coherent reflections (see Figure 7.15) limit the transmission through the perovskite cell and

the JSC of the filtered silicon cell.

Table 4.2: Photovoltaic parameters of semi-transparent perovskite and mono-crystalline siliconcells and the resulting tandem efficiency.

JSC (mA/cm2) VOC (mV) FF (-) Efficiency* (%)Perovskite 16.5 952 0.774 12.3Silicon 38.3 587 0.754 17.0Filtered Silicon 13.3 562 0.762 5.7Mechanically-Stacked Tandem 18.0*obtained from maximum power point tracking




Figure 4.3: Mechanically-stacked perovskite/silicon tandem performance. a) J-V curves of tandem

with the max power of the tandem calculated from the addition of the perovskite and silicon cells.b) EQE of original mono-Si, perovskite, and filtered silicon solar cells.

4.1.4 Stability Measurements

Recent work has demonstrated that exposure to moisture results in the formation of both

mono- and di-hydrates of the CH3NH3PbI3 material, which slowly degrades into PbI2 [61–63]. This

process is accelerated by heat, which induces egress of methylammonium iodide, and electric fields,

which induce drift of methylammonium and iodine. [64] It is thus crucial to exclude water from reach-

ing the perovskite layer. Other recent work has suggested that the material is thermally unstable

even in inert conditions, and that heating to just 85 C in a nitrogen atmosphere results in methylam-

monium decomposition and egress. [65] We examine the thermal stability of both semi-transparent

and opaque devices in ambient atmosphere under AM1.5G illumination using a maximum power

point tracking program. The perovskite in both cases is capped with PCBM then AZO nanoparti-

cles followed by either Al/Ag (opaque) or ITO (semi-transparent). The opaque device has an AZO

nanoparticle barrier similar to the recently published study by You et al. [60] that demonstrated

improved ambient stability compared to devices without a nanoparticle layer. At 100C, we find

that the nanoparticle layer is an insufficient barrier to prevent degradation, despite its reported

effectiveness as a moisture barrier. This result agrees with Conings et al. [65], suggesting that the

methylammonium iodide evolves as a gas (likely as CH3NH2 and HI), degrades the remaining ma-

terial into PbI2 and corrodes the metal electrode with the evolved HI vapor. The results make it

clear that storage (in dark conditions) or operation at room temperature is an insufficient metric

to represent the stability of devices operated in the field; exposure to increased temperatures is

necessary to fully evaluate layers used to improve the stability of perovskite cells.

The semi-transparent device, with an amorphous ITO capping layer, is operated at the

maximum power point under full AM1.5G illumination at 60 C and 100 C overnight in an ambient

atmosphere without additional encapsulation and shows no measurable degradation (Figure 4.4).




We use a AAA class solar simulator without any UV filtration. Because the cells do not degrade, we

conclude the ITO capping layer has prevented moisture ingress, methylammonium egress, and halide

corrosion of the metal electrode. We should also note that the lack of any deterioration in device

performance is strong support for the notion that the ionic motion responsible for hysteresis does

not necessarily induce degradation in operating devices as long as they are well encapsulated [64,66]

Additionally, the organic charge extraction layers are also protected by the ITO electrode barrier.

Figure 4.4: Thermal stability of ITO-capped perovskite solar cells at 60 and 100 C compared toopaque device with ZnO and Al/Ag.

In addition to improved thermal stability, we extract efficiency values at multiple temper-

atures to estimate the efficiency temperature coefficient of the semi-transparent solar cell (Figure

4.3). We measure the temperature coefficient as -0.22 %(rel)/C (Table 4.3), a significant improve-

ment over the -0.34 %(rel)/C for a commercial CdTe module[38].




Table 4.3: Efficiency as a function of temperature extracted by averaging the data in Figure 4.4over the entire time period. The temperature coefficient is extracted as a linear fit of the data and

normalized against the expected efficiency at 25

C.

Temperature (C) Efficiency (%)35 12.2060 11.67100 10.46

Temperature coefficient -0.22 %(rel)/C

4.2 Future of Mechanically-Stacked Tandems

Through the use of silver nanowire and ITO transparent electrodes, we have shown the abil-

ity to make 12-13 % efficient semi-transparent perovskite solar cells and 18-19 % efficient mechanically-

stacked tandems using commercial-grade CIGS and silicon solar cells that were 17 % efficient alone.

The semi-transparent perovskite solar cells had 70-80 % peak transmission of sub-bandgap light.

With the ITO electrode, the solar cell was made perfectly stable to operation at 100 C over the

course of 10 h.

While a strong starting point from which to develop, these demonstrated mechanically-

stacked tandems are far from the 30 % efficiency that we would like. Part of the difference can be

made up by using the best available silicon and perovskite solar cells available today. If we were to

take the metrics from the record 20.1 % perovskite solar cell and record 25.6 % silicon solar cell [38]

and make the perovskite solar cell semi-transparent, and not change the transmission of our semi-

transparent perovskite solar cell made in the n-i-p architecture except to flatten the transmission

curve, we would have a much more efficient tandem.

The 20.1 % record perovskite cell has a bandgap ∼1.5 eV and the JSC corresponds to a

perovskite thickness around 400 nm using perovskite n&k values modified from [67]. In a single-pass

solar cell (semi-transparent), the JSC would drop to 21.72 mA/cm2. From the J-V curve of the 20.1

% cell [38], I extract the ideal diode parameters I0 = 0.65 nA, RS = 0.35 Ω, RSH = 2000 Ω, and n

= 2.364. Using the new JSC of 21.72 mA/cm2, the semi-transparent efficiency is 17.5 % (Figure

4.5). We take the transmission curve of the cell to be 77 % broadband of the light not absorbed

by the perovskite. This flattens out the transmission curve of Figure 3.2, which is possible if the

parasitic absorption of the spiro-OMeTAD layer in the infrared has been corrected.




Figure 4.5: J-V, EQE, and Transmission of a possible semi-transparent perovskite solar cell basedon the best-in-class 20.1 % opaque solar cell. Semi-transparent perovskite cell efficiency is 17.5 %

The 25.6 % record silicon cell from [68] has a fitted ideal diode with parameters I 0 =

15.0 fA/cm2, RS = 0.364 Ω, RS H = 9782.9 Ω, n = 1.019. Unfiltered, the silicon solar cell has an

integrated EQE of 41.8 mA/cm2. When filtered by the perovskite cell in Figure 4.5 (with 77 %

transmission), the integrated EQE of the silicon cell drops to 14.58 mA/cm2. Using this new JSC ,

the filtered silicon cell efficiency is 8.6 % (Figure 4.6)

Figure 4.6: J-V and EQE of the 25.6 % record silicon solar cell if filtered by the transmission curvein Figure 4.5. Filtered silicon efficiency is 8.6 %

Adding the best-in-class perovskite and silicon solar cells, a mechanically-stacked tandem

cell with 26.1 % efficiency could be made. From this point, 30 % efficiency is in reach through two

parallel efforts: raising the bandgap of the perovskite to 1.7 eV (with similar improvements in VOC )

and improving the transmission of the semi-transparent perovskite solar cell to 85-90 % from the 77

% today.



Chapter 5

Cost-Modeling of Perovskite Solar

Cells

With the prospect of efficient and stable perovskite solar cells and mechanically-stacked

perovskite/silicon tandems, cost modeling can be used to determine the opportunity for commercial-

ization of these technologies. Further, we can estimate whether this new material, especially in the

mechanically-stacked tandem architecture, can make photovoltaics cost-competitive with the other

technologies described in the introduction Table 1.1.

One of the most thorough publicly-available analyses of the requirements to reach cost

competitiveness with other electricity technologies is the SunShot Vision Study technical report [69]commissioned and published by the U.S. Department of Energy in 2012. The technical and cost

goals set forth by the study were to reach 1.00 $/W installed cost for utility scale solar by the

year 2020 with the suggestions that reaching this target would require a module efficiency > 25 %

and a module cost < 0.50 $/W. These requirements have been updated recently in an article by

Rebecca Jones-Albertus [70] and refined to be defined as a goal of 6 ¢/kWh for the levelized cost

of electricity (LCOE), which is a metric more directly comparable across electricity technologies.

These targets are likely to be reached with additional development over the next few years, with

the International Technology Roadmap for Photovoltaics [7] projecting these efficiencies and LCOE

targets will be met before 2020. This target of 6 ¢/kWh can be viewed as a minimum to reach cost-

competitiveness for moderate grid penetration in areas with moderate solar insolation over the year.Further reductions in the LCOE for solar enable deeper grid penetration in areas with moderate

and high solar insolation as well as open up markets with low solar insolation, making continued

development beyond the SunShot target highly attractive.

To model the cost of the perovskite solar cells, we develop a scalable process flow and make

the assumption that the perovskite can be scaled at high yield. We then account for all material

44



CHAPTER 5. COST-MODELING OF PEROVSKITE SOLAR CELLS 45

input costs, tooling depreciation, and labor and utilities costs in order to arrive at a bottom-up cost

for the perovskite and perovskite/silicon tandem.

The next section is work being prepared for submission to Energy and Environmental

Science [71].

5.1 Published Work

5.1.1 Model Assumptions

To model the single junction perovskite solar cell, we adapt the most common perovskite

cell architecture (Figure 5.1) termed the standard [13] architecture for large area deposition (see

supplemental information for more detail including all layer thicknesses and material costs). The

standard architecture for perovskite cells involves deposition on a glass substrate beginning with

an n-type heterojunction, followed by the perovskite, then a p-type heterojunction layer. 20 nm

of TiO2 is employed as the n-type heterojunction deposited via printing onto a fluorine-doped tin

oxide (FTO) glass sheet followed by crystallization at 500 C. The perovskite is printed from a

stoichiometric solution of lead(II) iodide (PbI2) and methylammonium iodide (MAI) followed by a

100 C anneal to form 500 nm of the MAPbI3 perovskite. We assume a thicker perovskite layer

than is currently optimized in literature with the expectation that this layer will grow in thickness

as the ability to deposit a high quality perovskite crystal improves. We choose to forego the use of

a porous TiO2 architecture in the model as the literature is methodically moving away from this

architecture and towards thin film architectures more typically utilized in commercial photovoltaics.

50 nm of spiro-OMeTAD, is printed on top of the device followed by the bottom contact. During

operation, the glass sheet is illuminated in a superstrate configuration, implying illumination through

the heterojunction layer deposited directly onto the glass. We model a typical serial-connected thin

film solar module architecture by employing laser scribes and insulator fill to isolate individual cells

and connect them serially. We choose aluminum as the back metal, cognizant of the problem of

metal corrosion by the halogen in the perovskite layer with the expectation that an appropriate

ion barrier will be developed and to estimate what are believed to be a reasonable best-case cost

scenario for a perovskites cell. We choose to seal the perovskite with both a front and back glass

sheet as a likely solution to the environmental instability of the perovskite layer. See appendix for

more details on process flow.




Figure 5.1: Single-junction perovskite module scaled process flow.

To model the mechanically-stacked tandem we deviate from a bottom-up cost model to

simplify the model. We choose to use the silicon module target set by the Sunshot program which

is 80 $/m2 at 20 % efficiency [70]. We follow the same perovskite process as Figure 5.1 with the

exception of step 10 where ITO is used as a transparent electrode instead of aluminum ( Figure

5.2). The complete semi-transparent perovskite half-module replaces the top glass sheet in thesilicon module. We ignore the step of wiring the two half-modules together or any power electronics

required for field deployment at this time.




Figure 5.2: Mechanically-stacked tandem module scaled process flow.

For the tandem performance, we consider the case where the perovskite has reached 20 %

efficiency. HIT-Si bottom cells are modeled after industry standard cells (22 % efficiency). We use

publicly available commercial data and representative quantum efficiency for the silicon technology,

make simple optical assumptions (no interfacial reflections, 3 % reflection broadband from top glass

surface, 5 % absorption loss broadband in each transparent electrode, no sub-bandgap absorption

in the top cell - for a total sub-bandgap transmission of 87 % - and single-pass absorption in theperovskite), and ignore any deviations from the ideal diode equation for the sub-cells as a function of

light intensity (see supplemental information for details). We assume the bandgap of the perovskite

is tunable, finding a bandgap of 1.74 eV is ideal for the HIT-Si bottom cell. Given a 20 % perovskite

top cell, a perovskite/HIT-Si module would be 30 %.

5.1.2 Single-Junction Perovskite Cost Model

The specific breakdown of perovskite manufacture represented in $/m2 is depicted in Fig-

ure 5.3. The total cost is 53.28 $/m2 for the standard architecture. We can aggregate the specific

costs in three categories: active layers, cell integration, and module assembly. The active layers

consist of the heterojunction layers and perovskite absorber layer. The cell integration category

consists of the TCO layer, P1, P2, and P3 laser scribes, insulator fill, and aluminum electrode.

The module assembly category is defined as the front and back glass, bus bars, EVA and edge seal,

junction box, inspection, testing, and binning. For the standard architecture, the active layers cost

5.58 $/m2 (10.5 % of total), the cell integration costs 15.65 $/m2 (29.4 %), and the module assembly

costs 32.02 $/m2 (60.2 %).




Figure 5.3: Single-junction perovskite module step costs.

For the perovskite module, the cost is dominated by module assembly costs that define

a significant cost floor for the perovskite module despite the use of relatively inexpensive absorber

layers. These module assembly costs are largely unavoidable unless flexible packaging is developedthat is compatible with the environmental instability of the perovskite. The cost is interpreted in

the common cost metric, $/W(DC ) for a range of efficiencies from 12 to 22 % yielding a potential

cost range from 0.44 $/W(DC ) to 0.24 $/W(DC ). As shown in Figure 5.4, there is a $/W(DC )

advantage for the perovskite compared to CdTe given a consistent module efficiency of 16 %.




Figure 5.4: Single-junction perovskite compared to CdTe.

5.1.3 Mechanically-Stacked Tandem Perovskite Cost Model

The specific breakdown of the tandem process flow in Figure 5.2 represented in $/m2 is

depicted in Figure 5.5. As a baseline, the silicon module is taken to be 80 $/m2 of which we assume

7 $/m2 is related to the front glass sheet. We remove the 7 $/m2 glass sheet from the 80 $/m2 silicon

module to result in a 73 $/m2 silicon half-module (includes EVA, edge seal, junction box, testing

costs). The perovskite half-module (steps 1-12 in Figure 5.2) totals 32 $/m2, bringing the full

mechanically-stacked tandem to 105 $/m2. This translates to 0.35 $/W if the module reaches 30 %

efficiency.




Figure 5.5: Mechanically-stacked tandem module step costs.




5.1.4 LCOE Comparison Across Technologies

While $/W(DC ) is easy to calculate and readily interpretable, it does not account for thedegradation rate of the solar cell. The degradation rate of the module determines its usefulness and

economic value as a solar technology both in terms of the total energy yield of the module deployed

in the field as well as how the module is financed by lending institutions. High degradation rates are

a leading obstacle to the commercialization of perovskite solar cells [64]. To account for degradation

we calculate the levelized cost of electricity (LCOE) over a 30 year operational lifetime. LCOE

is calculated in the same method as laid out by Jones-Albertus et al [70]. We assume a constant

140 $/m2 BoS cost (therefore the BoS cost in $/W is inversely proportional to efficiency) and a

7 % internal rate of return [70] using the Department of Energy’s System Advisor Model (SAM)

to perform the annual solar harvesting and financial calculations. The LCOE target for the solar

industry as set forth by the SunShot program is 6 ¢/kWh, depicted in Figure 5.6 as a horizontalblack line. Figure 5.6 shows the LCOE for technologies with a variety of initial $/W(DC ) values. A

horizontal line denotes the 6 ¢/kWh target. Depending on the efficiency of the perovskite cell, the

degradation rate of the a system employing a perovskite module to reach 6 ¢/kWh LCOE varies.

By this method of calculation, the minimum perovskite efficiency to reach 6 ¢/kWh is 16 % with 0

% annual degradation. An 18 % module can afford an annual degradation rate of 0.9 % to reach

6 ¢/kWh, 20 % module - a 1.8 % rate, 22 % module - a 2.7 % rate if the operational lifetime of

the system is 30 years regardless of degradation rate. These limits for the degradation rate are far

slower than what has been achieved to date. The mechanically-stacked tandem, despite being more

expensive from a $/m2 and $/W perspective, has a substantially lower LCOE as a full system than

the other technology scenarios considered. Namely, the high efficiency of the tandem has a largeeffect on the system BoS cost, resulting in a < 5 ¢/kWh LCOE for a degradation rate up to 1.4

%/yr and a < 6 ¢/kWh LCOE for a degradation rate up to 3.3 %/yr.




Figure 5.6: LCOE calculations for different perovskite scenarios assuming an operational lifetimeof 30 years, a 140 $/m2 BoS cost, and a 7 % internal rate of return.

The scenarios modeled in this work, in particular the mechanically-stacked perovskite/silicon

tandem, have the potential to drive the system cost for utility-scale photovoltaics to 4-6 ¢/kWh

LCOE. This cost range will make utility-scale photovoltaics directly competitive with other sources

of electricity outlined in Table 1.1.



Chapter 6

Conclusions

The future of the photovoltaic industry is unwritten. With worldwide shipments around

40 GW/year today and needing to reach 100-250 GW/year in the future [7], new innovations are

necessary to bridge this gap. In this thesis, I have discussed the initial experimental efforts in a new

class of photovoltaics: polycrystalline tandems using metal-halide perovskites.

The metal-halide perovskite is a unique material class that fills a hole in available semicon-

ductor materials - a high-bandgap, polycrystalline (solution-processable) semi-conductor with small

energetic losses. The combination of these properties has the potential to disrupt a solar industry

that is striving to improve the efficiency of their products while continuing to lower the cost of man-

ufacture. The perovskite bandgap, potentially tunable between 1.5 and 2.3 eV, can be beneficial to

technologies that are sub-optimal in single-junction configurations. These include silicon, the domi-

nant technology in the industry, as well as technologies needing help to reach mass-commercialization

such as CIGS and CZTS.

Tandems can be made successfully in both monolithic and mechanically stacked architec-

tures, as demonstrated by the proof-of-concept prototypes in this thesis. The initial learning while

making these prototypes has led to some insights into the benefits and limitations of these archi-

tectures. The monolithic tandem requires substantial re-optimization of both the silicon cell and

the perovskite cell and possibly the development of new layer materials for better window layer

or index-matching properties. Even if these issues are overcome, the current-matching condition

between the silicon and perovskite limits the field performance of these tandems since the solar

spectrum is not constant. Furthermore, the serialized manufacturing (by adding 4-8 steps onto the

silicon cell) may result in a low yield for the finished product. The low yield was certainly more

true in these initial prototypes for monolithic tandems than for the mechanically-stacked option.

The mechanically-stacked tandem has the flexibility necessary to allow the silicon and perovskite

cells to undergo only minimal (or no) re-optimization then the flexibility to wire them into a module

in a configuration that better maintains the laboratory performance when placed in the field. The

53



CHAPTER 6. CONCLUSIONS 54

lone drawback of the mechanically-stacked tandem is the extra transparent electrodes, which make

reaching efficiency targets more difficult.

The path forward for perovskite polycrystalline tandems is clear. With the realistic effi-

ciency potential of a mechanically-stacked tandem made with record perovskite and silicon materials

surpassing 26 % efficiency, efforts towards improving the transparency of the perovskite cell and rais-

ing the perovskite’s bandgap should push the tandem efficiency past 30 % within another year or

two of effort. Through bottom-up cost-modeling, the cost of these tandems can significantly reduce

the cost of solar if the module efficiency target of >25 % is reached. After this, the major barriers

to commercialization are scaling the perovskite while maintaining efficiency, doing so at high yield,

and demonstrating solutions to the stability problems of the perovskite. Efforts in all three phases

are already underway by multiple teams around the world and can hopefully be combined into the

tandem structure and processes to form a successful product that leads to ubiquitous solar energy.



Chapter 7

Appendices

7.1 Monolithic Tandem Experimental Information

7.1.1 Silicon Sub-cell Fabrication Procedure

We first start with a double-side polished <100> n-type float zone silicon (Si) wafer (15

Ω-cm, 300 µm thickness). The front side of the wafer is then coated with a 300 nm-thick silicon

nitride (SiNX) film, which protects the planarity of the Si front surface during the subsequent random

pyramidal texturing step (3% weight KOH solution in DI mixed with isopropanol (6:1 volume), 80 C

etch for 20 minutes) on the back side of the wafer. After removing the SiNX protective layer using

hydrofluoric acid (HF), we clean the sample using RCA cleaning procedure (RCA1 = 10 minute,80C dip in 5:1:1 NH4OH:H2O2:H2O, RCA2 = 10 minute, 80 C dip in 5:1:1 HCl:H2O2:H2O):

deionized (DI) water dip, HF dip, RCA1 clean, DI water dip, HF dip, DI water dip, RCA2 clean,

DI water dip, HF dip, DI water dip, N2 drying. We then implant boron on the planar front surface

(11B with 1.8x1015 cm−2 dose, 6 keV implantation energy) and phosphorus on the textured back

surface (31P with 41015 cm−2 dose, 10 keV implantation energy) of the wafer. After cleaning the

wafer again using RCA cleaning procedure, we dip the wafer in dilute HF solution for oxide removal,

clean it with DI water and dry it with N2. Then we simultaneously form the p-type B emitter and

n-type P back surface field (BSF) by drive-in annealing at 960 C in an N2 ambient for 30 minutes.

After the emitter and BSF formation, we clean the wafers again with RCA cleaning pro-

cedure, and then perform dilute HF oxide removal, DI water dip, and N2 drying. We then form

and create our n++/p++ tunnel junction interface by depositing heavily doped n++ hydrogenated

amorphous silicon (a-Si:H) using plasma-enhanced chemical vapor deposition (PECVD). We first

deposit a 2-3 nm-thick intrinsic a-Si layer on top of the p++ emitter using the PECVD process

(temperature of 250 C, pressure of 150 mTorr, 55 sccm of SiH4 gas with a plasma power density

of 0.16 W/cm2). Afterwards, a 30 nm-thick a-Si:H layer is deposited at 250 C at a pressure of 200

55



CHAPTER 7. APPENDICES 56

mTorr (55 sccm of SiH4 gas and 50 sccm of 1% PH3 in H2 gas) and plasma power density of 0.13

W/cm2. Subsequently, the Si sample is annealed in N2 ambient at 680 C for 15 minutes to activate

the dopants and partially crystallize the amorphous layer.

After the tunnel junction formation, we form 1.1x1.1 cm2 square-shaped mesa spaced

1.4x1.4 cm2 apart. This is done using photolithography. We deposit positive photoresist on both

sides of the wafer (Shipley 1813 photoresist spun at 4000 rpm for 40 s, baked at 115 C for 1 minute).

The front side undergoes mesa patterning and is exposed for 4.5 s, and developed in CD-26 developer

for approximately 1 minute), while the back side is left unexposed to protect the BSF on the back

side during the subsequent mesa formation step. The mesa formation is then done on the front

side of the Si wafer using reactive ion etching (RIE) to etch 300 nm of the Si layer, removing the

n++ tunnel junction and most of the p++ emitter outside the square-shaped mesa. The remaining

photoresist mask is then removed using solvent clean (3 minute sonication in acetone, 3 minute

sonication in IPA, rinsing with DI water and drying with N2).

We perform dilute HF oxide removal, DI water dip, and N2 drying. We then form the

back metal by electron-beam evaporation. A layer stack of Ti/Pd/Ag/Pt with layer thicknesses

of 20/20/300/30 nm is chosen (Ti for adhesion, Pd for metal diffusion barrier, Ag for electrical

conduction, and Pt for corrosion protection during perovskite sub-cell processing) followed by rapid

thermal anneal (400C in N2 for 5 minutes) to improve metal adhesion.

We again perform dilute HF oxide removal, DI water dip, and N2 drying before depositing

30 nm-thick TiO2 using atomic layer deposition (Cambridge NanoTech Savannah ALD tool, 150C substrate temperature, 80C precursor temperature, 440 mTorr base pressure, and 20 sccm N2

carrier gas. To achieve the desired TiO2 thickness, we do 604 cycles of pulsing H2O vapor for 0.02 s,waiting for 7 s, pulsing TDMAT vapor for 0.2 s, and waiting for 7 s. After the TiO2 ALD deposition,

we deposit photoresist on the top for TiO2 protection (Shipley 1818 photoresist spun at 4000 rpm

for 40 s, baked at 115 C for 1 minute). Dilute HF with 10% concentration in DI is then used to

dissolve the TiO2 layer which got deposited on the back metal during the ALD process. Finally, the

tunnel junction substrates are then laser-scribed from the back to form 1.4x1.4 cm2 substrates. After

mechanical cleaving, we have 1.4x1.4 cm2 tunnel junction substrates with 1.1x1.1 cm2 square-shaped

mesa (active junction area) in the middle and TiO2 layer on the planar front surface protected by

photoresist.

7.1.2 Perovskite Sub-Cell Fabrication ProcedureThe tunnel junction substrates were then shipped with a protective photoresist layer. The

substrates were prepared for further processing by sonication in acetone for 5 minutes, sonication

in IPA for 5 minutes, rinsing with DI water and drying with N2, 5 minutes of UV-ozone cleaning,

and sintering for 30 minutes at 450 C. After cooling down, mesoporous TiO2 films were spun onto

the TiO2 surface at 4000 rpm for 30 s and sintered at 450 C. The spin-coating solution was a 1:3




dilution of 18-NRT TiO2 paste (Dyesol) in ethanol.

All previous steps were performed in ambient atmosphere. The remainder of device fabri-

cation was performed in a N2 glovebox with <5 ppm O2 and H2O. The TiO2 substrates were dried

by heating to 500 C with a hot air gun for 30 minutes and immediately brought into the glovebox.

A 1.3 M PbI2 solution was prepared by dissolving PbI2 (Aldrich, 211168) into anhydrous DMF

(Acros, 32687) and stirring on a hotplate at 100 C. The DMF was filtered through a 200 nm PTFE

filter (Pall, 4552) prior to adding to the solution in order to remove particulates. Methylammonium

iodide (MAI) was purchased from Dyesol and used as received. A solution of 10 mg MAI per 1 mL

anhydrous IPA (Acros, 61043) was prepared and allowed to dissolve at room temperature. A pure

IPA rinse solution was prepared as well. The IPA was filtered through a 20 nm PTFE filter prior to

adding to the solutions in order to remove particulates.

After the TiO2 substrates were cooled to room temperature, 100 µL of the 100 C PbI2/DMF

solution was pipetted onto the substrate and spun at 6500 rpm for 90 s. The resulting film was dried

for 30 minutes on a 70C hotplate. After cooling, the films were dipped in the MAI/IPA solution

for 15 minutes. Films were then rinsed in IPA, dried by spinning at 4000 rpm for 30 s, and placed

back on the 70 C hotplate for 30 minutes. After cooling, 75 µL of a spiro-OMeTAD (Lumtec,

LT-S922) solution was spun on top at 4000 rpm for 30 s. The spiro-OMeTAD solution was 163

mM spiro-OMeTAD in anhydrous chlorobenzene (Sigma-Aldrich, 284513). The spiro-OMeTAD was

dissolved by placing on a hotplate at 70 C for more than 30 minutes. 534 mM of tert-butyl pyridine

(Aldrich, 142379) and 86 mM of Li-TFSI (Aldrich, 15224) dissolved as 520 mg/mL in anhydrous

acetonitrile (Acros, 61096) were added to the spiro-OMeTAD solution. In this study, 8 mol% of the

spiro-OMeTAD was spiro-OMeTAD(TFSI)2, resulting in 16 % of spiro-OMeTAD molecules beingchemically oxidized to ensure conductivity in the spiro-OMeTAD layer. The spiro-OMeTAD(TFSI)2

was synthesized as reported elsewhere in literature. After the spiro-OMeTAD solution was prepared,

it was filtered through a 20 nm Al2O3 filter (Whatman, 6809-3102) to remove any aggregates and

particulates. Films were then removed from the glovebox and stored overnight in a desiccator at

20% RH.

An AgNW film on PET was transferred on top of the spiro-OMeTAD film similar to

previously reported procedures. A change was made to use a spring-loaded ball bearing to better

control the applied pressure to the AgNW film. The spring was set to 500g force at full depression

of the ball into the housing. After transfer of the AgNW electrode, 300 nm Ag was thermally

evaporated through a patterned shadow mask around the edges of the device leaving 1x1 cm2

activearea in the middle of the 1.1x1.1 cm2 mesa. These bars of silver helped reduce the unnecessary

series resistance in the AgNW electrode by not limiting the current collection to one geometrical

direction. The nearly completed device was lights-soaked under visible illumination for 10 minutes

then stored in a desiccator for 12 hours before applying the anti-reflective coating. A 111 nm-thick

LiF anti-reflective coating was then added to the device. This thickness was optimized to provide




anti-reflection for the visible spectrum.

7.1.3 Multijunction Cell Testing Protocols

Current-voltage characteristics for the multijunction cells were measured using a Keithley

model 2400 digital source meter and a Newport Oriel model #94023A solar simulator. The so-

lar simulator irradiance was characterized and compared to the AM1.5G spectral standard. The

perovskite sub-cell’s spectral mismatch factor was calculated as 0.990 using the EQE data from

Figure 2.3c. The silicon sub-cell’s spectral mismatch factor was calculated as 0.965 using the EQE

data from Figure 2.3c. Neither the solar simulator intensity nor the calculated efficiencies were

increased to account for this small spectral mismatch factor. Consequently, the reported currents

and power conversion efficiencies in this manuscript are likely slightly conservative. Samples were

illuminated through a 1 cm2 aperture area. A 5 s delay time at each voltage step was used to tryand minimize hysteresis. Longer delay times were impractical due to the limitations of the testing

software. Because transient hysteretic behavior was observed for up to 30 s after setting a voltage,

steady-state values of JSC , VOC , and MPP were determined by setting the voltage condition for 30

s then averaging data for the next 30 s. The cell was illuminated through 1 cm2 aperture mask to

ensure a correct illumination area.

External quantum efficiency (EQE) for the tandem was recorded as a function of the

wavelength using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). A 100 W

tungsten lamp (Newport) was used to provide an excitation beam, which was focused through a

Princeton Instruments SpectraPro 150 monochromator. To measure the EQE of the perovskite sub-

cell, an 870 nm infrared LED light source (Enfis) illuminated the sample to force the top perovskite

sub-cell into a current-limiting regime. The monochromated signal was chopped slowly at 1.2 Hz

to allow for settling of the signal. At each wavelength, the program waited for a delay time of 30 s

then data was collected for 5 s. The time constant on the lock-in amplifier was 1 s. We found that

the infrared chromatic bias caused an approximately 500 mV forward bias in the silicon sub-cell,

applying a 500 mV reverse bias to the perovskite sub-cell to maintain short circuit conditions. It

has been shown that applying a reverse bias to many perovskite solar cells prior to and during

photocurrent measurements results in a uniform decrease in the measured EQE spectrum, possibly

due to a temporarily reduced current collection efficiency resulting from ion drift. To estimate the

EQE spectrum of the perovskite sub-cell under broadband AM 1.5 illumination where these non-

ideal operating conditions are not present, the perovskite sub-cell EQE was scaled by a constant

factor to match the measured JSC . To measure the EQE of the bottom Si sub-cell, a 465 nm visible

LED light source (Enfis, 7 mW/cm2) illuminated the sample to force the bottom Si sub-cell into a

current-limiting regime. The monochromated signal was chopped quickly at 500 Hz to overcome the

capacitive impedance of the perovskite sub-cell. The Si EQE is reported as measured.




7.1.4 Supplemental Figures

Figure 7.1: EQE curve of single-junction n-type silicon cells with and without a tunnel junction.This curve shows negligible parasitic absorption in the tunnel junction for wavelength λ > 500 nm.

Figure 7.2: Transient (a) JSC and (b) VOC of the perovskite/silicon tandem cell as they stabilizeover the time. The measured steady-state values are JSC = 11.5 mA/cm2 and VOC = 1.58 V,respectively. The 30 s settling time for the VOC is not shown as the cell was stabilized at VOC for

> 30 s prior to starting the measurement.




Figure 7.3: EQE of a semi-transparent perovskite solar cell illuminated through either the n-side (glass/TiO2 side) or through the p-side (AgNW/spiro-OMeTAD side). The glass side EQEintegrates to 17.3 mA/cm2 while the AgNW side EQE integrates to 11.4 mA/cm2. Neither side hasanti-reflection coatings.

Figure 7.4: The light transmission through a 470-nm-thick doped spiro-OMeTAD film on glass.The contribution of the glass is removed from this plot. The absorption features from 300-400 and450-550 nm are readily visible in the EQE plot above.




7.2 Silver Nanowire Deposition Methods

We first form our AgNW transparent electrode on a flexible polyethylene terephthalate (PET) filmby spray deposition. AgNWs were deposited using a custom built spray deposition system. 4.5 mg

of silver nanowires (Blue Nano BL35L - 35 nm diameter, 15 µm length or ACS Agnws-60 - 60 nm

diameter - 30µm length) were dispersed in 40 mL of methanol, mixed in a vortex mixer for 30 s, and

placed in an ultrasonic bath for 10 s. This value was chosen such that the transmission/conductivity

tradeoff of the AgNW electrode maximizes the power conversion efficiency of the tandem. The

dispersion was delivered at a rate of 4 mL/min by a syringe pump to an atomizing nozzle (Spray

Systems Co. 1/4JN-SS+SU11DF-SS). The nozzle was positioned 72 mm above the substrate surface

and was supplied with nitrogen gas at 50 psi. The PET was taped to a computer controlled X-Y

motion stage under the nozzle and moved at 20 cm/s in a pattern such that the nozzle above

uniformly covered a 4.5 cm x 25 cm rectangular area during deposition. . The deposition was

carried out on 1-5 mil thick Polyethylene terephthalate (PET). The choice of PET as a substrate for

the AgNW film is important for the transfer lamination technique for reasons described below. The

spray deposition was carried out at the elevated temperature of 60 C to increase the conductivity

of the AgNW electrode. PET has a glass transition temperature of about 70 C and so higher

deposition temperatures and further annealing, although desirable, could not be used here.

This spray deposition process yielded a 50 x 300 mm of AgNW transparent electrode on

PET. This was then cut with scissors into pieces approximately matching the perovskite substrate

size. The AgNW film on PET was stored at ambient laboratory conditions for two weeks before

transfer lamination.

7.2.1 Lamination By Point Force

The patterned AgNW film on PET was placed facedown onto the nearly completed per-

ovskite device so that the AgNWs were in contact with the top spiro-OMeTAD layer. A 0.17 mm

thick glass coverslip was placed on top of the PET substrate. The AgNWs were transfer laminated

from the PET to the perovskite solar cell by applying approximately 500 g of downward force onto

the coverslip through a single 1/4-inch diameter ball bearing.

The 500 g force was applied by one of three methods. First, A metal ball bearing was

controlled by hand to apply the correct pressure. Manual control of pressure was found to often

result in overpressure, causing shorts in the solar cell by pressing the nanowires too far into the film.Second, a metal ball bearing on a spring was controlled by hand to apply the correct pressure. The

spring was set to apply 500g of force when fully compressed. As manual control over position is

much better than manual control over force, this method proved much more reproducible. Third,

the metal ball bearing was enclosed in a housing on rails where the height of the ball and therefore

the compression on the spring could be set without relying on manual control.




The bearing was selectively rolled over the active area of the perovskite device so that the

AgNW film was completely and uniformly donated from the PET to the top spiro-OMeTAD layer

of the perovskite solar cell. The rolling action of the bearing reduces lateral shear force on the PET

preventing any movement of the donor PET substrate relative to the acceptor perovskite solar cell.

Lateral movement here causes discontinuities in the laminated AgNW film severely degrading its

conductivity. The flexibility and softness of the PET substrate allows the AgNW film to conform

to the surface of the perovskite device during transfer despite any dust or other imperfections that

may be present on either the surface of the AgNW film or the surface of the spiro-OMeTAD layer.

This, coupled with the relatively small contact point of the ball bearing ensures complete transfer

lamination of the AgNW film to the perovskite device without damaging the mechanically sensitive

nanostructured AgNW film in the presence of dust or other imperfections.

The 500 g transfer force was found to be sufficient to ensure the AgNWs were completely

donated from the PET but not too much that they were forced through the spiro-OMeTAD layer

causing bridges/shunts across it. AgNW bridges across the spiro-OMeTAD layer lead to increased

recombination since the spiro-OMeTAD layer can no longer effectively block electrons and in extreme

cases shunting of the device if the AgNWs bridge through the TiO2 as well. The coverslip serves two

purposes. First, it isolates the lateral movement of the ball bearing from the PET, which prevents

cracks, or discontinuities in the transferred AgNW film as described above. Second, it serves to

increase the area over which the force from the ball bearing is applied to the PET, thus reducing the

pressure felt by the AgNWs during the transfer process. This reduced pressure is a further safeguard

against AgNWs bridging through the spiro-OMeTAD layer.

7.2.2 Lamination By Distributed Force

Lamination of the silver nanowires by point force was highly variable. The glass coverslip

was necessary to the process in order to distribute the force appropriately from the point source.

While this was often fine, the rigid glass coverslip did not conform well to dust particles and other

sources of macroscopic roughness on the surface of the perovskite device. These particles caused

”keep-out” zones to form around the particles where no silver nanowires were deposited. While these

”keep-out” zones could be reduced in size by pressing harder, the increased pressure often caused

shorting of the solar cell.

The solution to the ”keep-out” zones, aside from removing all particulates from the system,

was to remove the need for the glass coverslip. Applying pressure directly rather than applying a

force would bypass the need for distribution of an applied force. To this end, I designed and built a

system to apply a gas pressure uniformly onto the perovskite solar cells to laminate the nanowires

(Figure 7.5)

Vacuum/pressure lamination is a strategy common to the manufacture of composite parts,

especially with non-standard shapes such as airplane wings or boat hulls. The idea is to use the




vacuum to remove air bubbles from the system and pressure (sometimes) to press parts together

then heat is generally applied to cure a resin and adhere the composite layers together. A similar

system was used here, using vacuum to remove air bubbles and a positive pressure source to press

the nanowires into the solar cell uniformly. Heat was not necessary for this application. A pressure

∼3000 psi was applied to the top half of the chamber as the transfer pressure.

This method proved much more reproducible than the point force lamination and was much

less prone to shorting. However, the diaphragms used were not well designed for this system and

often broke under the high pressures and the sharp edges of the glass.

While this is a batch process and therefore would likely have been expensive to scale, a

compromise between the 0-dimensional (point source) application of force and the 2-dimensional

(pressure source) application of force could be the use of a 1-dimensional application of force such

as using a roller. Here, dust and other particulates would be an issue still, but by using a soft roller

the applied pressure may still be uniform without the use of a pressure distribution layer (such as

the glass coverslip).




1. 2. 3.

4. 5. 6.

7. 8.

Figure 7.5: Process for vacuum lamination of silver nanowires onto a solar cell. 1) The baseplateof the transfer chamber designed to withstand the pressures of the process. At the bottom of thisbaseplate is a connection to a vacuum source used to remove all air pockets that can disrupt the

uniformity of the applied pressure. 2) A porous metal (aluminum) baseplate is used to distributethe vacuum evenly across the surface of the porous plate to avoid plugging the vacuum source andprevent removal of air pockets. 3) An O-ring is used to provide an edge seal between the top andbottom halves of the chamber to ensure proper pressure buildup. 4) The solar cell is placed in themiddle of the porous metal plate. 5) The silver nanowires on a plastic film are placed face-downonto the solar cell. 6) A tape seal is placed in-between the O-ring and the porous metal plate toallow the application of a diaphragm. 7) A plastic diaphragm is attached to the tape and provides abarrier between the two halves of the transfer chamber. Vacuum is pulled on the lower half and thediaphragm deforms around the solar cell, providing a flat bubble-free surface onto which a uniformpressure can be applied. 8) The top half of the transfer chamber is placed on top and bolted to thebottom half through radial bolt holes in both chambers. The o-ring provides a seal between the tophalf and ambient. A fitting on the side of the top half connects to a positive pressure source suchas a compressed gas canister. Positive pressure is applied to the diaphragm, uniformly pressing the

nanowires into the solar cell.




7.3 Mechanically-Stacked Tandems Using Silver Nanowires

Experimental Information7.3.1 Perovskite Methodology

Pilkington TEC15 FTO glass (1.6 and 2.2 mm) was patterned by selective etching with Zn

powder (J.T. Baker, 4282-01), 4 M HCl (Fisher, A144-212), and mechanical abrasion with a cotton

swab. The glass was cleaned by sonication in a diluted Extran solution (EMD, EX0996-1), acetone

(EMD, AX0115-1), and isopropanol (IPA) (EMD, PX1835P-4). After 20 min of UV-ozone, the glass

was heated to 500 C on a hotplate. A 1:10 dilution of titanium diisopropoxide bis(acetylacetonate)

(Aldrich - 325252) in ethanol (Sigma-Aldrich, 187380) was repeatedly sprayed from an airbrush

nozzle to achieve a ∼50 nm thick films of TiO2 on top of the FTO. After cooling down, the glass

was immersed in a 70 C batch of 40 mM TiCl4 (Sigma-Aldrich, 208566) in ultrapure water (J.T.

Baker, 6906-02) for 30 min. The glass was then rinsed in DI water and dried on a 70 C hotplate

for 15 min. After cooling down, mesoporous TiO2 films were spun onto the TiO2/FTO surface at

4000 rpm for 30 s and sintered at 450 C. The spin-coating solution was a 1:3 dilution of 18-NRT

TiO2 paste (Dyesol) in ethanol.

All previous steps were performed in ambient atmosphere. The remainder of device fabri-

cation was performed in a nitrogen glovebox with < 5 ppm O2 and H2O. The TiO2 substrates were

dried by heating to 500 C with a hot air gun for 30 min and immediately brought into the glove-

box. A 1.3 M PbI2 solution was prepared by dissolving PbI2 (Aldrich, 211168) into anhydrous DMF

(Acros, 32687) and stirring on a hotplate at 100 C. The DMF was filtered through a 200 nm PTFE

filter (Pall, 4552) prior to adding to the solution in order to remove particulates. Methylammonium

iodide (MAI) was synthesized according to a previously reported procedure. [12] A solution of 10

mg MAI per 1 mL anhydrous IPA (Acros, 61043) was prepared and allowed to dissolve at room

temperature. A pure IPA rinse solution was prepared as well. The IPA was filtered through a 20

nm PTFE filter prior to adding to the solutions in order to remove particulates.

After the TiO2 substrates were cooled to room temperature, 100 µL of the 100 C PbI2/DMF

solution was pipetted onto the substrate and spun at 6500 rpm for 90 s. The resulting film was

translucent yellow and dried for 30 min on a 70 C hotplate. After cooling, the films were dipped

in the MAI/IPA solution. The films were monitored optically for the formation of the perovskite

(Figure 7.6). The signal at 700 nm was used to determine the growth rate of the perovskite,

while the signal at 850 nm was used to detect the presence of other optical phenomena (changes

in scattering, reflection, incident light intensity). When the derivative of both signals matched, the

formation was considered complete and the perovskite film was removed, rinsed in IPA, dried by

spinning at 4000 rpm for 30 s, and placed back on the 70 C hotplate for 30 min. At this point,

the films were a translucent brown. After cooling, 75 µL of a spiro-OMeTAD (Lumtec, LT-S922)

solution was spun on top at 4000 rpm for 30 s. The spiro-OMeTAD solution was 59 mM (for opaque




devices) and 163 mM spiro-OMeTAD (for semi-transparent devices) in anhydrous chlorobenzene

(Sigma-Aldrich, 284513). The spiro-OMeTAD was dissolved by placing on a hotplate at 70 C for

>30 min. 193 mM of tert-butyl pyridine (Aldrich, 142379) and 31 mM of Li-TFSI (Aldrich, 15224)

dissolved as 520 mg/mL in anhydrous acetonitrile (Acros, 61096) were added to the 59 mM spiro-

OMeTAD solution. For the 163 mM spiro-OMeTAD solution, the amount of tert-butyl pyridine and

Li-TFSI was kept consistent with respect to the concentration of the spiro-OMeTAD. In this study,

8 mol% of the spiro-OMeTAD was spiro-OMeTAD(TFSI)2 [36], resulting in 16% of spiro-OMeTAD

molecules being chemically oxidized to ensure conductivity in the spiro-OMeTAD layer. After the

spiro-OMeTAD solution was prepared, it was filtered through a 20 nm Al 2O3 filter (Whatman,

6809-3102) to remove any aggregates and particulates. Films were then removed from the glovebox

and stored overnight in a desiccator at 20% RH.

Figure 7.6: Optical density over time of perovskite film in dipping solution. The signal at 700nm was used to determine the growth rate of the perovskite, while the signal at 850 nm was usedto detect the presence of other optical phenomena (changes in scattering, reflection, incident lightintensity).

For the opaque electrode devices, 100 nm Au was thermally evaporated through a patterned

shadow mask to form the back electrode. For the semi-transparent devices, an AgNW film on PET

(details in following section). After transfer of the AgNW electrode, 100 nm Ag was thermally

evaporated through a patterned shadow mask around 3 edges of the devices. These bars of silver

helped reduce the unnecessary series resistance in the AgNW electrode by not limiting the currentcollection to one geometrical direction. The nearly completed device was stored in a desiccator for

12 hours before applying the anti-reflective coatings. LiF anti-reflective coatings were then added to

the semi-transparent devices. 133 nm LiF was deposited onto the glass surface. This was optimized

to provide anti-reflection for the broad solar spectrum from 400-1100 nm. 176 nm LiF was deposited

onto the AgNW mesh. This thickness was optimized to provide anti-reflection for the infrared




spectrum from 800-1100 nm.

7.3.1.1 AgNW Stability

To make these devices more stable, we suggest developing a surface coating on the wires

to make them more stable. Our laboratory environment has normal oxygen levels as well as a small

partial pressure of sulfur gas added to the environment. While we have no direct proof of a coating

on the silver nanowires, we believe either an oxide or a sulfide layer was formed on the surface of the

wires. Silver oxide and sulfide are much more stable than metallic silver in the presence of iodine

(which readily forms silver iodide) and improve the resistance of the electrode to corrosion. We see

that electrodes stored in the laboratory environment prior to transfer onto a device are much more

stable than electrodes stored in an inert nitrogen atmosphere. We set aside one of these devices for

long-term testing and it showed no change in performance over a month of storing in the dark indesiccated air. There is certainly room to improve on this procedure by intentionally introducing

an oxide or sulfide layer onto the nanowires or by coating the surface with other materials (e.g.,.

ligands, a gold coating, etc.)

7.3.2 Silicon Methodology

We fabricate our silicon cells from multicrystalline silicon wafers with high impurity content.

Instead of using expensive polysilicon made using the Siemens process, the feedstock for our cast

multicrystalline Si wafers are 4.5 N (99.995% pure) upgraded metallurgical silicon (UMG-Si) or recy-

cled silicon from the top 10% of a cast multicrystalline industrial ingot (TI-Si). These ∼200 µm-thick

wafers are subjected to saw-damage removal wet etch in CP4 solution (15:5:2 HNO3:CH3COOH:HF)

for 2 cycles of 2 minute etch, resulting in ∼180 µm thick wafers. The wafers are then cleaned us-

ing RCA1 (5:1:1 H2O:NH4OH:H2O2) and RCA2 (5:1:1 H2O:HCl:H2O2) solutions at 70 C, 10 min

each, to remove organic and metallic surface contaminants. The wafers are then loaded into a Tystar

POCl3 diffusion furnace at 700 C for phosphorus emitter formation. After ramping up the furnace

temperature to 865 C, POCl3 gas is flowed into the furnace for 12 minutes, followed by 6 minutes

of N2 purge. After waiting for an extra time of 10 minutes, we purge the furnace chamber with O2

for 7 minutes followed by temperature ramp down of ∼3 C/minute. The samples are then unloaded

at 500 C.

At the end of the diffusion process, phosphorus emitter with diffusion depth of 200-300

nm and emitter sheet resistance of 35 Ω/ are formed on both sides of the wafer. We remove the

phosphosilicate glass (PSG) layer formed on the wafers surface by dipping them in buffered oxide

etch (BOE 5:1) solution for approximately 30 s, then deposit SiNx anti-reflection coating (ARC)

using plasma-enhanced chemical vapor deposition (PECVD). In this intermediate step, the standard

Si cells have an ARC thickness of 82 nm, while the cells optimized for infrared light (800-1100 nm)

have an ARC thickness of ∼125 nm. We remove the backside emitter by SF6 reactive ion etch (RIE)




with a rate of ∼1 µm/min to etch ∼1 µm deep. After, we deposit 1 µm thick Al on the backside

of the wafer using e-beam evaporation, followed by rapid thermal annealing (RTA) at 900 C for

30 s in N2 atmosphere to form aluminum back surface field (Al BSF) at the back of the wafer.

After this annealing step, the thickness of the ARC layers shrink down to 78 nm (optimized for

standard AM1.5 illumination) and 115 nm (optimized for IR response). We defined our finger area

of the cells using photolithography, and etched SiNX on the corresponding area using RIE. We then

deposit 20/20/300 nm Ti/Pd/Ag metal stack as the finger, for which Ti acts as the adhesion layer,

Pd acts as the diffusion barrier, and Ag acts as the conductive layer. Lift-off process in acetone

is used to form the metal fingers, followed by RTA at 400 C for 5 minutes in Ar atmosphere for

metal adhesion. The cells are finally cut out of the wafers by laser scribing followed by mechanical

cleaving.

7.3.3 Measurement Procedures

7.3.3.1 IV Measurements

Current-voltage characteristics of the perovskite cells were measured using a Keithley model

2400 digital source meter. The irradiation source was a 300 W xenon lamp (Oriel). The lamp was

calibrated against the integrated photocurrent obtained by EQE. The voltage was swept in the

direction of open circuit to short circuit. A 5 s delay time at each voltage step before taking data

removed any transient hysteretic behavior of the perovskite devices. [37] For consistency, Si and

CIGS cells were measured with the same sweep parameters.

The semi-transparent perovskite was illuminated through a 0.39 cm2 aperture area. The

total area of the CIGS and Si is also 0.39 cm2, to minimize leakage current. The opaque perovskite

cell was illuminated through an 0.12 cm2 aperture area.

7.3.3.2 Tandem Measurement Method

The EQE of the top cell, bottom cell, and bottom cell filtered by the top cell are individually

determined. The solar simulator is calibrated against the expected photocurrent of the cell from

the EQE measurement. A J-V curve returns the essential metrics of the cell’s performance. The

procedure is repeated for all three cases. In a 4-terminal configuration, the performance of the

individual cells is added together.

7.3.3.3 EQE Measurements

For the perovskites, the external quantum efficiency (EQE) was recorded as a function of the

wavelength using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems) without light

bias. A 100 W tungsten lamp (Newport) was used to provide an excitation beam, which was focused

through a Princeton Instruments SpectraPro 150 monochromator and chopped at approximately 2




Hz. At each wavelength, a delay time of 3 s was used to allow the signal to settle, and afterwards

data was collected for 4 s. The time constant on the lock-in amplifier was 1 s.

For the Si and CIGS cells, the EQE measurement was sped up because these cells have

much faster settling times than the perovskites. The excitation beam was chopped at 300 Hz, the

delay time was reduced to 0.1 s, data was collected for 1 s, and the time constant of the lock-in

amplifier was 30 ms. To measure the EQE of the bottom cell in the tandem, the above procedure

for the Si and CIGS cells was repeated with the perovskite top cell placed in front of the Si or CIGS

cell to filter the incoming light. The EQE and IV measurements were made immediately following

the evaporation of the LiF anti-reflective coatings.

7.3.4 Supplemental Figures

Figure 7.7: Transmission of the 12.4 Ω/ AgNW film with the PET substrate subtracted.




Figure 7.8: Distribution of semi-transparent cell performance gathered from 3 batches where deviceprocedures were largely kept constant between batches. Low-efficiency devices exhibited shortinglikely caused by too much pressure applied manually during the AgNW electrode transfer. Medium-efficiency devices generally exhibited low photocurrent likely caused by incomplete transfer of theAgNW electrode due to too little pressure applied manually during the electrode transfer.

Figure 7.9: Example of difference in spectral response depending on AR coating applied to Si cell.The regular TI-Si cell has a broadband AR coating.




7.4 Mechanically-Stacked Tandems Using ITO Experimental

Information7.4.1 Perovskite Device Fabrication Method

We acquired 10 Ω/ ITO coated glass from Xin Yan Technology. Prior to device fabrica-

tion, 105 nm of MgF2 are thermally evaporated as an anti-reflection coating onto the glass. We find

low deposition rate to be important to prevent islanding. After cleaning, Al 4083 PEDOT:PSS from

Clevios is spun onto the ITO surface at 4000 rpm for 30 seconds and subsequently dried at 135 C

for 15 minutes. Next, a 40 wt% solution of 3:1 molar ratio methylammonium iodide (MAI, Dyesol)

to lead(ii) acetate (PbAc2, Sigma) is dissolved in anhydrous n,n-dimethylformamide (DMF, Acros).

Hypophosphorous acid (HPA) from Sigma Aldrich is added to the solution with a molar ratio of

0.75 % HPA/PbAc2. The solution of MAI and PbAc2 is spun at 2000 rpm in a dry air box with

>25 ppm water until a color change occurs from clear to light brown, at approximately 60 seconds.

The films are dried at room temperature until they are uniformly brown and then annealed at 100C for 5 minutes in dry air. A 2 wt% solution of PCBM (Solenne bv) in anhydrous DCB is spun at

3000 rpm for 45 seconds and annealed for 5 minutes at 90 C. The samples are then taken out of the

glovebox and either AZO (Sigma) or ZnO (Sigma) nanoparticle inks with 15 nm average particle

diameter dispersed in IPA (2.5 wt%) are spun on at 4000 rpm. Two layers of AZO nanoparticles

are spun sequentially to produce an approximately 50 nm thick layer. The ZnO and AZO films are

dried at 75 C for 5 minutes.

For the opaque devices, 100 nm of Al and 125 nm of Ag were thermally evaporated through

a patterned shadow mask with a 0.2 cm2 aperture size as the back electrode. We observe an

improvement in air stability with the addition of the Ag layer.

For the semi-transparent devices, 500 nm of ITO is sputtered onto the device on the day

following perovskite deposition. For sputtering, we use a base pressure < 5x10−6 torr, deposition

pressure of 2x10−3, a power density of 8 W/in2, and 5 % oxygen partial pressure. The devices are

post-annealed at 100 C for 15 minutes. 300 nm of Ag was evaporated around the perimeter of the

device, leaving a 0.6 cm2 aperture size open. Finally, 150 nm of MgF2 is thermally evaporated as

an anti-reflection coating.

7.4.2 J-V MeasurementsCurrent-voltage measurements were performed using a Keithley model 2400 digital source

meter and 300 W xenon lamp (Oriel) solar simulator was used for irradiation. Due to the coherent

reflections in the perovskite causing a significant mismatch between a KG5 reference diode and the

spectral response of the perovskite cell, the lamp was calibrated against the integrated photocurrent

calculated by EQE. J-V curves are taken from forward to reverse bias. Reported values for efficiency




were obtained by operating the cells at steady state max power and averaging over a minimum of

60 seconds.

The opaque cells and semi-transparent cells were illuminated through a 0.12 and 0.39 cm2

aperture area, respectively.

7.4.3 EQE Measurements

The external quantum efficiency (EQE) was recorded as a function of the wavelength using

a Keithley model 236 without light bias. A 100 W tungsten lamp (Newport) was used to provide an

excitation beam, which was focused through a Princeton Instruments SpectraPro 150 monochroma-

tor and chopped at approximately 0.6 Hz. At each wavelength, data was collected for and averaged

for 5 s.

To measure the EQE of the bottom cell in the tandem, the above procedure for the Si wasrepeated with the perovskite top cell placed in front of the Si or CIGS cell to filter the incoming

light.

7.4.4 Stability Measurements

To test the thermal stability of the semi-transparent devices, the cells were placed on a

hot plate, glass side down, and illuminated from the top with a Newport Oriel model # 94023A

solar simulator and monitored using a Keithley model 2400 digital source meter. For opaque device

thermal stability, the devices were sandwiched between a second glass slide before being placed on

the hot plate to prevent scratching of the perovskite device, while still allowing light to enter through

the glass side.

7.4.5 Supplementary Figures




Figure 7.10: J-V Curves showing problems with the ZnO/ITO interface. At room temperature,a large extraction barrier is present which greatly limits fill factor. This barrier is alleviated astemperature is raised; however the barrier reappears upon cooling.

Figure 7.11: A 50 nm film of AZO nanoparticles displays high transmission. The AZO nanopar-ticles are annealed at either 75 or 150 C after deposition.




Figure 7.12: Transmission through an ITO/Glass reference sample from the same sputter deposi-tion batch as the best devices. The 500 nm layer of ITO was annealed at 100 C for 15 minutes tomatch the thermal processing profile of the perovskite samples and the resistivity dropped from 11to 10 Ω/.

Figure 7.13: Comparison of the EQE of semi-transparent and opaque perovskite solar cells. Thelack of a metal back reflector in the semi-transparent solar cells results in lower photocurrent.




Figure 7.14: Transmission through the semi-transparent perovskite solar cell. Transmission islimited at longer wavelengths due to coherent reflections.

Figure 7.15: Reflection of semi-transparent and opaque perovskite solar cells. Mitigating reflectionis an area of future work.




7.5 Cost-Modeling of Perovskite Solar Cells Model Informa-

tion7.5.1 Process flow for single-junction standard architecture

1. APCVD of 250 nm SnO2:F

2. Wash front glass

3. Print 20 nm TiO2 compact layer. Print a mixture of ethanol and titanium isopropoxide using a

slot die coater. Uses 0.27 g titanium isopropoxide and 3.33 g ethanol per m2 of glass. Catalog

price for titanium isopropoxide is $0.0724 /g and catalog price for denatured ethanol is $0.0085

/g

4. Anneal TiO2 layer. 5 min anneal at 500 C

5. Print 500 nm perovskite layer. Print a 1 M stoichiometric solution of lead(ii) iodide (PbI2)

and methylammonium iodide (MAI) in n,n-dimethylformamide (DMF) using a slot die coater.

Uses 2.94 g PbI2, 1.02 g MAI, and 6 mL DMF per m2 of glass. Catalog price for PbI2 is $0.952

/g, catalog price for MAI is $0.892 /g, and catalog price for DMF is $0.02 /mL.

6. P1 laser scribe through SnO2:F, TiO2 and perovskite layers to define cells

7. Insulator fill for cell isolation

8. Print 50 nm spiro-OMeTAD hole transport layer. Print a 72 mg/mL solution of spiro-OMeTADin chlorobenzene using a slot die coater. Uses 0.092 g of spiro-OMeTAD and 1.3 mL chloroben-

zene per m2 of glass. Futures quote for bulk spiro-OMeTAD is $10 /g, catalog price for

chlorobenzene is $0.08/mL.

9. P2 laser scribe down to SnO2:F to enable series connection

10. Deposit 1 µm aluminum.

11. P3 mechanical scribe down to spiro-OMeTAD and edge isolation for cell isolation

12. Electrically connect cells by solder-welding metal ribbon busbars and conductive adhesive tape

13. Feed bus bar ribbons through pre-drilled hole in tempered back glass. Bond the cells and the

busbar assembly to the back class with EVA and edge seal.

14. Solder the busbar ribbons to the junction-box leads, and bond the J-box to the back glass

with potting agent and tape.




15. Light soaking followed by high voltage isolation (Hi-Pot), ground continuity, and solar simu-

lator JV testing.

16. Visual inspection and module binning and packing.

7.5.2 Tandem Performance Modeling

HIT-Si initial data acquired from Panasonic HIT VBHN285SJ40 285W panel datasheet. Quantum

efficiency represented from published wafer results [72], scaled to match the photocurrent density of

the panel. To determine the filtered performance, cell represented by the ideal diode equation with

the following parameters: J0 = 7.5E-11 A/cm2, JPH = 38.54E-3 A/cm2, RS = 0.36 Ω-cm, RSH =

1.4E10 Ω-cm, n = 1.4.

Perovskite absorption represented by published complex refractive index data [67]. Optical

effects include no interfacial reflections, 3 % reflection from top glass surface, 5 % absorption loss

broadband in each transparent electrode, no sub-bandgap absorption in the top cell, and single-pass

absorption in the perovskite. The published data on the 1.6 eV MAPbI 3 perovskite is compressed in

energy space to approximate higher-bandgap perovskite materials. A perovskite thickness of 1000

nm is chosen to achieve high optical density in single-pass absorption. A perovskite bandgap of 1.74

eV is current matched with the HIT-Si cell. Perovskite sub-cell represented as 20 % efficiency and

is assumed to not suffer losses upon integration with the silicon sub-cells.

Figure 7.16: Modeled EQE of perovskite and HIT-Si cells.



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