Building Next Generation Solar Cells with Nanotechnology

Prashant V. Kamat

http://www.nd.edu/~kamatlabOR Kamatlab.com

Department of Chemistry & Biochemistry and Radiation LaboratoryDepartment of Chemical & Bimolecular EngineeringUniversity of Notre Dame, Notre Dame, IN 46530

2018 Energy Outlookhttps://www.eia.gov/pressroom/presentations/Capuano_02052018.pdf

http://www.bloomberg.com/news/articles/2016-06-13/we-ve-almost-reached-peak-fossil-fuels-for-electricity

….. a Solar Wave

Progress in Photovoltaics: Research and Applications 2017, 25, 3-13.

Photovoltaic Advances

Can we address clean energy

challenge with Nanotechnolgy?Perovskite[22.7%]QDSC[13.4%]

How Nanotechnology came into play?

The Birth of Renewable Energy

1970’s

Oil crisis (OPEC oil embargo) of 1973 brings attention to Renewable Energy

Artificial Photosynthesis was coined to mimic photosynthesis-Photoinduced Electron Transfer Reactions

Semiconductor Photoelectrochemistry became a popular research topic

Photocatalytic properties of semiconductor particle systems were explored

Fujishima HondaBardGerischer Henglein

Energy (eV)2.0 2.5 3.0 3.5 4.0

Lu

min

esce

nce A

bso

rba

nce

10K 45Å

27Å

22Å

19Å

16Å

13Å

12Å

Colloidal Quantum Dots

CdSe

Murray, C.; Norris, D.; Bawendi, M., Synthesis and characterization of nearly monodisperse CdE (E=S, Se, Te) semiconductor. J. Amer. Chem. Soc. 1993,115, 8706-8715

Courtesy: M. Kuno

A Major Turning Point

Synthesis of QDs by Hot-Injection Method

"The technology works by harnessing nanocrystals that range in size from 2 to 10 nanometers. Each dot emits a different color depending on its size. By adding a film of quantum dots in front of the LCD backlight, picture color reproduction rate and overall brightness are significantly improved."

http://www.cnet.com/news/lg-leaps-quantum-dot-rivals-with-new-tv/

<$500

• The I-III-VI chalcopyrite structure is derived from the fact that the group II element in the II-VI zinc blende structure is substituted by group I and III elements.

Omata, T. et al. Size dependent optical band gap of ternary I-III-VI(2) semiconductor nanocrystals. J. Appl. Phys. 2009, 105, Art no 073106.

Ternary semiconductor nanocrystals

AgInSe2

CuGaSe2

CuInSe2

CuInS2

CuGaS2

AgGaS2

AgGaSe2

• Optical band gap of the QDs covers a wide wavelength range from near-infrared to ultraviolet.

• Bulk CuInS2 has a direct bandgap of 1.53 eV with size tunable absorption and emission (Bohr radius ~4.1 nm).

Nanostructured Hybrid Assemblies for Harvesting Light Energy

Quantum Dot Solar Cells

Tunable band edge Offers the possibility to harvest light energy over a wide range of visible-ir light with selectivity

Hot carrier injection from higher excited state (minimizing energy loss during thermalization of excited state)

Multiple carrier generation solar cells.Utilization of high energy photon to multiple electron-hole pairs

Polymer-SemiconductorHybrid Cell

Semiconductor Hetero-junction Solar Cell

Quantum Dot Senistized Solar Cell

PEDOT/PSS

P3HT/SC Nanocrystals

• Tuning properties of Nanomaterials

• Surface modification• Assembly on

Electrodes • Photon Capture (Light Absorption)

• Excited State Dynamics• Charge Separation

• ETL & HTL to capture charges

• Surface modification• Efficiency

Synthesis & Characterization

Photochemistry & Photophysics

Photovoltaic Performance

Deposition of QD Films

TiO2 CdSe

SILAR

ElectrophoreticdepositionChemical

bath

Drop cast/spin coat

Molecular linker

Cd2+

precursorSe2- or S2-

precursorOTE/TiO2/CdSe

or (CdS)OTE/TiO2

ea

bd

c

Experimental Approach

Synthesis of QDs by Hot-Injection Method

1-8 Cycles CdSe SILAR

15

Liquid Junction

Solid State

Optically Transparent Electrode

Substrate (Metal Oxide)

Sensitizer

Electrolyte / Hole Conductor

Counter Electrode

Anatomy of a Quantum Dot Solar Cell

350 400 450 500 550 600 650 7000

10

20

30

40

50 (A) 3.7 nm 3.0 nm 2.6 nm 2.3 nm

Wavelength (nm)

IPC

E (%

)

Quantum Dot Solar Cells

hν

e

OR

CdSeTiO2

VB

CB

ee

e

hh h

O

e

h

R

E

IPCE or Ext. Quantum Eff.= (1240/λ) x (Isc/Iinc) x 100

• Size selective deposition of CdSe QDs to TiO2 films

• Ability to tune the photoresponse of

QDSC

• Higher efficiency with smaller size QDs

1. Illuminate Solar Cell• 100 mW/cm2, Air Mass 1.5G solar

irradiation

2. Measure current at different voltage output

3. Simulates possible “real world” operating conditions

4. Gives us Power Conversion Efficiency

Current-Voltage Measuremnts (I-V curves)

Recombination and VOC

Pote

ntia

l

+

-

FTOTiO2

CdSe

S2-/Sn2-

EFermiCu2S/RGO

Desired ElectronTransfer

Recombination

Step by Step:1. Excitation2. Electron Transfer3. Recombination4. Build up e- in CB until

Rexcitation = Rrecombination

5. If recombination rate is increased, VOC is decreased

VOCVOC 1

2

345

Introduction 2-Electrode 3-Electrode Other Conclusions

In the Dark (at Equilibrium)FT

O

Cu2 S/

RGO

i

- +

Eappl

Pote

ntia

l

+

-

FTOTiO2

CdSeS2-/Sn

2- Cu2S/RGOEFermi

-+

Hodes, G., Gratzel, M. et. al, J. Phys. Chem. B, 2000, 104, 2053-2059

S2-/Sn2-

TiO2/CdSe

Let There Be Light!

Pote

ntia

l

+

-

FTOTiO2

CdSeS2-/Sn

2-

ΔV = Efermi - Ecounter

Eappl = - Voc i = 0Eappl > -Voc i < JscEappl= 0 V i = Jsc

Cu2S/RGO

EFermi ΔV

Hodes, G., Gratzel, M. et. al, J. Phys. Chem. B, 2000, 104, 2053-2059

FTO

Cu2 S/

RGO

i

- +

Eappl

-+

S2-/Sn2-

TiO2/CdSe

Short Circuit Current (ISC)The short circuit current ISC corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0.

I (at V=0) = ISC

ISC occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant. For an ideal cell, this maximum current value is the total current produced in the solar cell by photon excitation.

ISC = IMAX = Iℓ for forward-bias power quadrant

Open Circuit Voltage (VOC)The open circuit voltage (VOC) occurs when there is no current passing through the cell.

V (at I=0) = VOC

VOC is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant.

VOC= VMAX for forward-bias power quadrant

Solar Cell characteristics

Maximum Power (PMAX), Current at PMAX (IMP), Voltage at PMAX (VMP)The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV. At the ISC and VOC points, the power will be zero and the maximum value for power will occur between the two. The voltage and current at this maximum power point are denoted as VMP and IMP respectively.

The Fill Factor (FF) is essentially a measure of quality of the solar cell.

FF = = PMAX

PT

IMP ×VMP

ISC ×VOC

Overall Power Conversion Efficiency (η)Efficiency is the ratio of the electrical power output Pout, compared to the solar power input, Pin, into the PV cell. Pout can be taken to bePMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency.

Pin is taken as the product of the irradiance of the incident light, measured in mW/cm2 (or one sun =100 mW/cm2), with the surface area of the solar cell [cm2]. The maximum efficiency (ηMAX) found from a light test is not only an indication of the performance of the device under test, but, like all of the I-V parameters, can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light.

0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

Steady-state current @ 0.85 V (averaged over 30 s) Average of forward & reverse scans

Forward: Jsc = 22.4 mA/cm2 VOC = 1.01 Vff = 0.66η = 15.0%

Voltage (V)

Curre

nt D

ensi

ty ( m

A cm

-2)

Reverse: Jsc = 22.4 mA/cm2

VOC = 1.04 Vff = 0.73η = 17.0%

Area = 0.13 cm2

i

V

hν

Semiconductor bandgap determines maximum achievable efficiency

I-V Curve of PV Cell and Associated Electrical Diagram

where I0 is the saturation current of the diode, q is the elementary charge 1.6x10-19 Coulombs, k is a constant of value 1.38x10-23J/K, T is the cell temperature in Kelvin, and V is the measured cell voltage that is either produced (power quadrant) or applied (voltage bias).

The solar cell performance is further evaluated in terms of

1. External Quantum Efficiency (EQE) isthe ratio of the number of charge carrierscollected by the solar cell to the number ofphotons of a given energy shining on thesolar cell from outside (incident photons).

2.Internal Quantum Efficiency (IQE) isthe ratio of the number of charge carrierscollected by the solar cell to the number ofphotons of a given energy that shine onthe solar cell from outside and areabsorbed by the cell.

Spectral responsivity is how much current comes out of the device per incoming photon of a given energy and wavelength and expressed as : amperes per watt (A/W); Both the quantum efficiency and the responsivity are functions of the photon energy or excitation wavelength (indicated by the subscript λ).

ηEQE

Probing Photoinduced Electron Transfer Process

VB

CB

Ox

Redhν

et ht

VB

–

+

+

–CB

TiO2

CdSe

hν

e

OR

Photoelectrochemistry

pump probe

detector

Spectroscopy

GERISCHER H, LUBKE MA PARTICLE-SIZE EFFECT IN THE SENSITIZATION OF TIO2 ELECTRODES BY A CdS DEPOSITJOURNAL OF ELECTROANALYTICAL CHEMISTRY 204 (1-2): 225-227 1986

450 500 550 600 650

-0.08

-0.04

0.00B

CdSe-MPA∆A

Wavelength, nm

1 ps 35 ps 400 ps 1500 ps

Charge Separation in TiO2/CdSe (3 nm)

CdSe

VB

CB e

h

pump

probe

detector

450 500 550 600 650

-0.08

-0.04

0.00C

CdSe-MPA-TiO2

∆A

Wavelength, nm

1 ps 35 ps 400 ps 1500 ps

TiO2

k= 1.95x1011 s-1

CB

VB

Time, ps

ket= 107 s-1 ket= 1.2x1010 s-1

CdSe2.4 nm

CdSe7.5 nm

TiO2

ee

Size Dependent Electron transfer between CdSe and TiO2

The energy gap between donor and acceptor influences kinetics of electron injection.

2007; 129, 4136

Dependence of Electron Transfer Rate with CdSe Particle Size and Oxide Substrates

I. Robel; M. Kuno; P. V. Kamat, J. Am. Chem. Soc. 2007, 129, 4136-4137. K. Tvrdy; P. A. Frantsuzov; P. V Kamat, Proc. Natl. Acd. Sci. USA 2011, 108, 29-34.

• Electron transfer in QDSCs has been widely studied many research groups

• Not the limiting factor in solar cell performance in most cases

Follows Many-State Marcus Theory

Evolution of Thin Film Solar Cells

3623−3630

DOI: 10.1021/acs.jpclett.5b02524

Lead Halide “Perovskites”

• Prototypical compound is CH3NH3PbI3• Hybrid organic-inorganic materials• Adopt a perovskite (ABX3) crystal

structure• Possible substitution of A sites with Cs,

MA & FA

CH3NH3+

Pb2+

I-

M. Saliba et al., Science 10.1126/science.aah5557 (2016)

Late 1990s - Early 2000s: The Mitzi Era

David Mitzi - IBM: T. J. Watson Research Center

1. (C4H9NH3)2(CH3NH3)n-1SnnI3n+1

2. [NH2C(I)=NH2]2(CH3NH3)mSnmI3m+2

3. (C4H9NH3)2MI4 (M = Ge, Sn, Pb)

4. (C4H9NH3)2EuI4

5. (NH2CH=NH2)SnI3

1. D. B. Mitzi, C. A. Feild, W. T. A. Harrison, and A. M. Guloy, Nature, 1994, 369, 467–469.2. D. B. Mitzi, S. Wang, C. a Feild, C. a Chess, and a M. Guloy, Science, 1995, 267, 1473–6.3. D. B. Mitzi, Chem. Mater., 1996, 8, 791–800.4. D. B. Mitzi and K. Liang, Chem. Mater., 1997, 9, 2990–2995.5. D. B. Mitzi and K. Liang, J. Solid State Chem., 1997, 134, 376–381.6. D. B. Mitzi, K. Chondroudis, and C. R. Kagan, IBM J. Res. Dev., 2001, 45, 29–45

4

Explored Applications inOLED and Transistors

2012: Emergence of Perovskite Solar Cell

1. A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, 214th ECS Meeting 2008.2. H.-S. Kim, et al. Sci. Rep., 2012, 2, 591.3. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, and H. J. Snaith, Science, 2012, 338, 643–647.

• Initial attempt in 2008 with polypyrrole1

• Spiro-OMeTAD as HTM: A Landmark Development

Received May 31, 2012Received July 5, 2012

9.7% 10.9%

Published August 2012 Published October 2012

Nature 2013, 499, 316-319

Organic-Metal Halide Perovskite

1. Mix PbI2 and methyl ammonium iodide in chlorobenzene and stir for 2 hours at 70 °C –yellow colored solution

2. Spin coat the solution onto electrode surface kept on a hot plate (70-80 C)

3. The film turns black as it crystallizes into pervoskite structure

4. Make metal contact

Single Step Two Step

1. Spin PbI2

2. Dry

3. Dip in CH3NH3I

4. Anneal

36

Design of Perovskite Solar CellsDevice Components1. Optically Transparent Electrode2. Electron Transport Material3. Perovskite Absorber4. Hole Transport Material (HTM)5. Gold Counter Electrode

1

2 3

4 5

ETL

CH3NH3PbI3 Solar Cell with 16% Efficiency

37

1 μm

0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

Steady-state current @ 0.85 V (averaged over 30 s) Average of forward & reverse scans

Forward: Jsc = 22.4 mA/cm2 VOC = 1.01 Vff = 0.66η = 15.0%

Voltage (V)

C

urre

nt D

ensi

ty (m

A c

m-2)

Reverse: Jsc = 22.4 mA/cm2

VOC = 1.04 Vff = 0.73η = 17.0%

Area = 0.13 cm2

300 400 500 600 700 8000

102030405060708090

100

Wavelength (nm)

EQE

(%)

0

5

10

15

20

Inte

grat

ed C

urre

nt D

ensi

ty (m

A c

m-2)

2-step deposition of MAPbI3 onto mp-TiO2 (~200 nm thick)

Best Solar Cell Certified Efficiencies

The PSCs fabricated with LBSO and methylammonium lead iodide (MAPbI3) show a steady-state power conversion efficiency of 21.2%, versus 19.7% for a mp-TiO2 device.

6500+ Papers in 6y with>220,000 Citations

~1000 Highly Cited Papers>165000 Citations

2017, 2, 922−923

Source: Clarivate Analytics(Web of Science )

2012 2013 2014 2015 2016 2017

0

500

1000

1500

2000

2500

3000

Year

Publ

icat

ions

0

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100000

Cita

tions

1003002700/

Most Read/Most Cited Articles in Leading Journals are on Perovskites(JACS, ACS Nano, Nano Lett, ACE Energy Lett, Chem Mater………

Intriguing Optical Properties of Lead Halide Perovskites(Manser, Christians, Kamat Chem. Rev. 2016 10.1021/acs.chemrev.6b00136)

• Low exciton binding energy –complete charge separation at room temperature

• Nonthermalized carrier recombination • Excitation intensity dependent quantum efficiency of

emission• Two-photon amplified spontaneous emission• Lasing properties• Tuning of bandgap through halide composition ratio

J. Phys. Chem. Lett.,2015, 6, 5027–5033

J. Phys. Chem. Lett.,2014 5 1300–

ACSPhotonics, 2016DOI: 10.1021/acsphotonics.6b00209

Lasing Spontaneous emission Line Broadening

• Excitons are rapidly dissociated at room temperature, and that, under typical photovoltaic operating conditions, the recombination dynamics of CH3NH3PbI3are primarily governed by the interactions of free electrons and holes.

• Ultrafast evolution of the PL peak is attributed to emission from non-thermalized free carriers. (PL does not arise from excitonic states

EXCITED STATE DYNAMICS

Chen, et al. J. Phys. Chem. Lett. 2015, 6, 153–158.

Wu, X.; et al J. Am. Chem. Soc. 2015, 137, 2089–2096

• Primary emission band arising from charge carrier recombination becomes narrower and increases in intensity with decreasing temperature. At very low temperature (< 100K) a longer wavelength emission (800-950 nm) arising from the trap sites become dominant

Excited State Dynamics

43

1. b ~ 1.5 until 10 W/cm2 – trapping regime (single carrier)2. b ~ 1 above 10 W/cm2 for thin film3. Single crystal shows transitions at 1 order of magnitude lower intensity (lower

defect density)

Draguta, S.; Thakur, S.; Morozov, Y. V.; Wang, Y.; Manser, J. S.; Kamat, P. V.; Kuno, M. J. Phys. Chem. Lett. 2016, 7, 715–721.

Nt = 6.3 x 1017 cm-3Iem ∝ Iexcb

Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color GamutProtesescu et al. Nano Lett 2015, 15 (6), pp 3692–3696

Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap TuningDong Myung Jang et alNano Lett., 2015, 15, 5191

Mixed Halide Lead Perovskites

Transforming Hybrid Organic Inorganic Perovskites by Rapid HalideExchangeNorman Pellet, Joël Teuscher, Joachim Maier, and Michael GrätzelChem. Mater., 2015, 27, 2181–2188

Ion migration in a solid material is characterized by the activation energy (EA), and the migration rate (rm) is influenced by the EA as

where kBT represents thermal activationenergy. The EA is sensitive to the material’s crystal structure, ionic radius, ion-jumping distance, and the charge of ions.

Nat. Mater. 2014, 13, 897−903

Nat. Mater. 2015, 14, 193−198Acc. Chem. Res. 2016, 49, 286−293

Ion Migration and Hysteresis in Perovskite Solar Cells

Computational models suggest that I−

ions (EA ~ 0.33 -0.58 eV) are more readily mobile than MA+ and Pb2+ ions

schematics of ion migration in perovskite during positive and negative poling

Ion Migration in Organometal Trihalide Perovskites

Migration path of I− ions along the I−−I− edge of the PbI6

4− octahedron in the MAPbI3 crystal calculated from density functionaltheory (DFT) method.

FTIR images of the distribution of MA+

Optical images of the lateral MAPbI3 perovskite solar cell with a mobile PbI2

thread

Nat. Commun. 2015, 6, 7497.

Adv. Energy Mater. 2015, 5, 1500615

Adv. Energy Mater. 2016, 6, 1501803.

Acc. Chem. Res. 2016, 49, 286−293

CsPbX3 Perovskite Quantum Dots

• Highly luminescent QDs• CsPbX3 perovskites are seen as a

more stable alternative to MAPbX3 species.

• Ease of synthesis and control of surface chemistry

• Features a cubic perovskite structure and broad spectral control through size and composition.

Kulbak, M. et al. J. Phys. Chem. Lett. 2016, 7 (1), 167–172.

Guria et al., ACS Energy Lett. 2017, 2, 1014−1021.Eperon, G. E. et al. J. Mater. Chem. A 2015, 3 (39), 19688–19695.

47

Experimental Approach

QD Synthesis

Spectroscopy

Assembly

PbBr2, OctadeceneOleic acid, Olylamine

Cesium Oleate

120 °C

DOI: 10.1021/jacs.6b04661

2016,138, 8603–8611

Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3-x Through Halide Exchange

CsPbBr3 Film

DOI: 10.1021/jacs.6b04661

2016,138, 8603–8611

Tracking the Halide Exchange

(a) 0, (b) 1, (c) 3, (d) 7, (e) 15, and (f) 40 min

Compositional Depth Profile through EDX

• Using Energy Dispersive X-ray Spectroscopy an elemental composition depth profile was obtained.

• At film surface, Br-:I- ratio was ~1:1, but I- dropped to zero at film depths of 150 nm.

• Process likely governed by diffusion of halide in and out of the film.

52

The Thickness Dependence

The PCE of the solar cells increases with CsPbBr3 thickness until 250 nm140 nm 340 nm

The AX treatment strategy provides a general method for tuning the electronic properties of the CsPbI3 QD films.

FAI coating yields a doubling of the already-high mobility of CsPbI3QD films and results in a certified record PCE of 13.43% - above the best reported PCE for dye-sensitized solar cells, organic PVs, and CZTSSe PV technologies

Sci. Adv. 2017;3: eaao4204 27 October 2017

Moving Forward

Hybrid Metal Halide Perovskites

• Mixed halide (I/Br) systems –to further understand the role of charge transfer complex, halide ion mobility and establish the mechanism of phase segregation

• Surface treatment to modulate surface defects

• Multijunction Tandem Solar Cells(All Perovskite and Si-Perovskite double and triple junction can deliver 36-38% efficiency)

To minimize optical losses, it is necessary to develop routes to deposit the multilayers without destroying the underlying materials and with continuous charge extraction layers of minimal thickness (on the order of 5 nm) 2017, 2, 2506−2513

• Semiconductor quantum dotswith size and shape dependenttunability of bandgap offer newways to design energyconversion systems

• High Photoconversion Efficiencyof lead halide perovskites isattractive for developing thinfilm photovoltaics

• Challenges exist towardscommercialization ofsemiconductor nanostructurebased photovoltaic systems

Summary

Intriguing Optoelectronic Properties of Metal Halide PerovskitesChem. Rev. 2016, 116, 12956–13008

Making and Breaking of Lead Halide Perovskites Acc. Chem. Res. 2016, 49, 330–338

Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping (Perspective). J. Phys. Chem. Lett. 2015, 6, 2086-2095

Shift Happens.How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites ACS Energy Lett. 2017, 2, 1507-1514

What will the future hold?

Over the last twenty five years, the per-

kWh price of photovoltaics has

dropped from about $500 to ~ $0.50;

think of what the next twenty five years will bring.

It is Sun-Believable

Researchers who make it possible in our groupGraduate students

Jacob Hoffman (Chemistry)Danilo JaraQuinteros (Chemistry)Seog Joon Yoon (Chemistry)Steven Kobosko(Chem. Eng.) Victoria Bridewell (Chemistry)Rebecca Scheidt (Chemistry)Olivia Cracchiolo (Chemistry)Jeffrey Dubois (Chemistry)

Summer 2012

CollaboratorsHartland, Kuno, McGinn, Vinodgopal, George Thomas, Osman Bakr, K. O’Shea, G. Hodes, C. Janaky, E. Selli

Post-Docs/Visiting ScientistsSubila Balakrishnan, Gary Zaitas, M.Shanthil, Roxana Nicolaescu; Julie Peller, Geeta Balakrshna

Undergraduate studentsRebecca Radomsky, SavennahButler, Elisabeth Kerns

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