perovskite solar cells present scenario and future prospects · a perovskite solar cell is a type...

Perovskite solar cellsPresent scenario and future prospects

ANURADHA ASHOK Ph .D

PSG Institute of Advanced Studies

[email protected]

A perovskite solar cell is a type of solar

cell which includes a perovskite structured

compound, most commonly a hybrid

organic-inorganic lead or tin-halide based

material, as the light-harvesting active

layer. Perovskite materials such

as methylammonium lead halides are cheap

to produce and simple to manufacture.

What is a perovskite solar cell?

19 Aug 2016 SOLAR SEMINAR-PSG IAS 2

Web of Science search for “hybrid perovskite OR MAPI OR CH3NH3PbI3 solar cell”

Perovskite solar cells

Efficiency as reported in 2016: 20.1%


Perovskite materials

Usually have stoichiometry ABX3

X is an oxide or halide anion such as Cl, Br and I.

B refers to a metal cation with a coordination number of 6.

The BX6 octahedra share corners and A is usually a large cation that fills the

cuboctahedral holes with coordination number of 12.

A can be Ca, K, Na, Pb, Sr, other rare earth metals.

B


First three-dimensional organic–inorganic hybrid perovskite, discovered by replacing

caesium in CsPbX3 (X = Cl, Br or I) with methylammonium cations (MA =

CH3NH3+) by Dieter Weber, in 1978.

CH3NH3PbI3 is most commonly used material for making high efficiency perovskite

solar cells.

CH3NH3PbI3 is a semiconducting pigment with a direct bandgap of 1.55 eV with

absorption coefficient as high as 104–105 cm−1

JACS, 136, 622, 2014Methylammonium (MA)

519 Aug 2016 SOLAR SEMINAR-PSG IAS

Organic-Inprganic Hybrid perovskites


Phys. Rev. B 90, 045207

Organic-Inprganic Hybrid perovskites


Variation in band structure according to molecular orientation

(111)(011)

(111) (011)

Motta et al, Nature communications, DOI: 10.1038/ncomms8026


Methyl ammonium lead iodide-MAPI (CH3NH3PbI3)

Semiconducting material with direct bandgap 1.55eVAbsorption onset 800nmGood light absorber in the whole visible solar emission

spectrum

Weak binding energy of Excitons produced by light absorption ~ 0.030 eVMost of the excitons dissociate rapidly into electrons and

holes at room temperature

Small effective mass with high carrier mobility

Recombination time hundreds of nanoseconds Longer carrier diffusion lengths (100-1000nm) before they

recombine Michael Grätzel , Nature materials vol 13, 2014


Evolution of Perovskite solar cells

Incorporation of MAPI in liquid based DSSCs as a sensitizer improved the power conversion efficiency significantly (3.8%). Poor device stability due to rapid dissolution of the perovskite in the organic solvent

Liquid electrolyte replaced by solid state hole conductor (HTM) CsSnI3-xFx or spiro-MeOTAD

Michael Grätzel , Nature materials vol 13, 2014



Mesoporous TiO2 replaced by Al2O3, electrons percolate alomg the surface of alumina NPs through ultrathin MAPI coating—efficiency -5%

MAPI can also act as hole conductor no need of additional HTM TiO2 scaffold is fully infiltrated with the perovskite light harvesterreduction in thickness and efficiency-12%

Michael Grätzel , Nature materials vol 13, 2014


1. High efficiency; with an efficiency of 20% after only several years work.

2. Facile low temperature solution-based fabrication method.

3. High absorption coefficient.

4. High diffusion length, high charge-carrier mobilities.

it means that the light-generated electrons and holes can move large

enough distances to be extracted as current, instead of losing their energy

as heat within the cell

5. very high values of open-circuit voltages (VOC) typically obtained.

12

Advantages of Perovskite solar cells


High absorption coefficient

Optical absorption as a function of the metal halide

Band- tuning

M. A. Green, Nature Photonics 8, 50-514 (2014) Peng Gao Energy Environ. Sci., 2014, 7,2448

Optical properties


Bandgap tuning is required to extend the absorption to longer wavelengths without

sacrificing the absorption coefficient.

Changing in any of A, M and X in AMX3 changes the bandgap

The bandgap also can be tuned in between 1.55 eV and 1.17 eV by varying the ratio

of lead to tin

SmallVolume 11, Issue 1, pages 10-25, 30 OCT 2014 DOI: 10.1002/smll.201402767http://onlinelibrary.wiley.com/doi/10.1002/smll.201402767/full#smll201402767-fig-0002

FA: formamidinium HC(NH2)2+

Band gap tuning

http://onlinelibrary.wiley.com/doi/10.1002/smll.v11.1/issuetoc

http://onlinelibrary.wiley.com/doi/10.1002/smll.201402767/full


Energy levels for different materials acting as electron transporting material (left), absorbers (middle) and hole transporting materials (right) in solar cells.

Journal of materials chemistry A, DOI: 10.1039/C5TA00873E

Device fabrication based on energy levels

Pablo et al Materials Today Volume 17, Number 1


The device structure, related materials, and interfacial

modification are key factors in performance of solar cells.

Two typical structures can be constructed: a) mesoscopic

nanostructure and b) planar structure.

Mesoporous TiO2 layer usually is used to collect the

electrons

Organic Hole transporting material (HTM) collects the

holes

Planar structure has simpler structure and higher efficiency


Device structure




Pablo et al, Journal of Physical Chemistry Letters DOI: 10.1021/jz502547f, February 2015

Beyond MAPI


There are two common methods:

1) one step coating: spin-coating a mixed CH3NH3I and PbI2solution

2) two-step coating: spin-coating CH3NH3I after coating with PbI2


Preparation method




One-StepSequential Deposition

Dual-Source Vapor Deposition

Vapor-Assisted Solution Process

Peng Gao Energy Environ. Sci., 2014, 7,2448

Other deposition methods


All deposition process happens at a low temperature (below 150 °C), which is

suitable for the fabrication of flexible solar cells based on PET substrates.

The concentration of the CH3NH3I solution affects the crystal size from about 90 nm

to about 700 nm.

Photovoltaic performance was strongly influenced by the CH3NH3I concentration,

i.e., the crystal size of CH3NH3PbI3

CH3NH3PbI3 degrades in humid conditions and forms PbI2 at higher temperatures

due to the loss of CH3NH3I

Lead (Pb) compounds are very toxic and harmful to the environment.

Highlights


Niu et al, J. Mater. Chem. A, 2015,3, 8970-8980

CH3NH3PbI3 (s) ↔ PbI2 (s) + CH3NH3I (aq)

CH3NH3I (aq) ↔ CH3NH2 (aq) + HI (aq)

4HI (aq) + O2 (g) ↔ 2I2 (s) + 2H2O (l)

2HI (aq) ↔ H2 (g) + I2 (s)

21

Degrades instantaneouly when measured under ambient conditions due to reaction with oxygen, moisture and UV light

Stability issues


Shin et al, ACS Nano, 2013, 7 (2), pp 1027–1035Kim, et al, ChemSusChem 2013, 6, 449 – 454

Inorganic perovskites

Inorganic perovskites for solar cells


Perovskite and silicon heterojunction solar cells

APPLIED PHYSICS LETTERS 106, 013506 (2015)

Efficiency 24.1%


Data from OXFORD PV

A comparison…


• Perovskite stack is printed on top of conventional silicon PV cells•Will add 3–5% absolute cell efficiency•Is easily integrated into existing PV manufacturing lines•Adds minimal additional cost

Expected efficiency > 28%

Tandem solar cells


Data from OXFORD PV

PSC performance roadmap


Improving efficiency to exceed thin film solar cells

By understanding their material properties and optimal cell

designs

Increasing air and temperature stability

Replacing toxic Pb with a greener element

Is AMX3 (perovskite structure) the best stoichiometry? Have we

tried other structures?

Future challenges for PSCs

Thank you


perovskite solar cells present scenario and future prospects · a perovskite solar cell is a type...

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