Perovskite Solar Cells

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©2014, Wenchun Feng Perovskite Solar Cells Wenchun Feng Garfunkel Group Department of Chemistry and Chemical Biology Rutgers University 3/7/2014 Image credit: B. Zhang, W.C.L. Glenn & M. Liu.

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This presentation summarizes history and recent development of perovskite solar cells. If you have any questions or comments, you can reach me at [email protected]

Transcript of Perovskite Solar Cells

Page 1: Perovskite Solar Cells

©2014, Wenchun Feng

Perovskite Solar Cells

Wenchun Feng

Garfunkel Group

Department of Chemistry and Chemical Biology

Rutgers University

3/7/2014 Image credit: B. Zhang,

W.C.L. Glenn & M. Liu.

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©2014, Wenchun Feng

Schematic

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Perovskite

Precursor

Solution

CH3NH3I PbI2

fluorine-doped

tin oxide substrate

spin coat

then anneal FTO FTO

TiO2

FTO

TiO2

Perovskite

spin coat

perovskite

then

anneal

spin coat

spiro-OMeTAD

FTO

TiO2

Perovskite

spiro-OMeTAD

FTO

TiO2

Perovskite

spiro-OMeTAD

Electrode

Deposition

leave in air to

dope via

oxidation

Au Au Au

-3.9

-5.4

-4.0

-5.2 -5.1

hv

-

+

-

+ +

E

(eV) LUMO

HOMO

CB

TiO2 (CH3NH3)PbI3

spiro-OMeTAD Au

HOMO

Adv. Funct. Mater. 2014, 24, 151.

Sci. Rep. 2012, 2.

DOI: 10.1038/srep00591.

Image Credit: M.B. Johnston

(CH3NH3)PbI3

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©2014, Wenchun Feng

Perovskite Solar Cells Made

Simply – C&EN Video

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For a C&EN video, please see

https://www.youtube.com/watch?v=oQ2bz6jlbz0

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©2014, Wenchun Feng

Perovskite Solar Cell is the

Poster Child for Solar Energy Conversion

Source: Web of Science

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Henry Snaith

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Science. 2013, 342, 794

Chem. Mater. 1999, 11, 3028

Science. 1999, 286, 945

J. Chem. Soc., Dalton Trans., 2001, 1, 1

In the late 1990s, David Mitzi (IBM) made TFT and LED with organometal halide perovskites.

In general, Good light emitters = Good light absorbers = Potentially Good PV materials.

However, due to its instability, he didn’t pursue them as viable PV material.

OPV DSSC

CIGS

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What is Perovskite?

A perovskite structure is any material with the same type of crystal

structure as calcium titanium oxide (CaTiO3), known as the perovskite

structure ABX3.

First discovered by Gustav Rose in 1839 and named after Russian

mineralogist L. A. Perovski.

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CH3NH3PbX

methylammonium lead halide

Nature. 2013, 12, 1087

J. Mater. Chem. A, 2013, 1, 15628

Phase Transition (CH3NH3PbI3):

Orthorhombic Tetragonal Cubic

162 K 327 K (54 C)

The organic ligand is disordered in Tetragonal and Cubic phase.

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Material Properties:

Good for Photovoltaics, but with Caution

Cheap Manufacturing: Lower manufacturing costs expected: directly deposited from solution

Caution: Encapsulation needed, which may increase cost

Material Properties for High Efficiency Photovoltaics: 1. High Optical Absorption Coefficient

2. Excellent Charge Carrier Transport (crystallinity, diffusion length, carrier mobility)

3. Promising Device Parameter: High Voc of >1.1 V is reported

Stability: Study shows it can maintain more than 80% of its initial efficiency after 500

hours.

Caution: More studies needed. Lifetime of 15 years has not been demonstrated. The ultimate goal of 15-year-lifetime not demonstrated.

Other Real World Concerns (equally important but omitted here): Toxicity from Pb

Scaling Problem Nature. 2013, 12, 1087

Nature Comm. 2013, DOI: 10.1038/ncomms3885

Science. 2013, 342, 317

Nature. 2013, 499, 316

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High Optical Absorption Coefficient

Perovskite Absorption Coefficient: 1.5X104 cm-1 at 550 nm,

which is one order of magnitude higher than conventional

ruthenium-based organometallic N719 dye

Nanoscale, 2011, 3, 4088

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Crystallinity

Tetragonal Structure

Lattice Parameters:

a = 8.825 Å, b = 8.835 Å, c = 11.24 Å.

Science. 2012, 338, 643.

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Carrier Diffusion Length

quartz

CH3NH3PbI3

PCBM

quartz

CH3NH3PbI3

Spiro-OMeTAD

electron-extracting layer hole-extracting layer

Science. 2013, 342, 341

Science. 2013, 342, 344

e h

PL quenching originates from the charge-carrier

extraction across the interface.

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Remaining Question:

Is the Solar Cell Excitonic?

Similar electron and hole diffusion lengths (130 nm and 100 nm in average),

either indicate similar mobilities for both electrons and holes,

or indicate that the predominant diffusion species is the weakly bound exciton.

Based on the work so far, we cannot tell whether the photoexcited species are excitons or free charges

Conventional Solar Cell

(p-n junction Si)

Excitonic Solar Cell

(DSSC, OPV)

+ - + - + -

+ -

interface

exciton

free charges

hv hv

J. Phys. Chem. B 2003, 107, 4688

Science. 2013, 342, 341

Science. 2013, 342, 344

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Carrier Mobility

High carrier mobility is important:

Because, coupled with high charge carrier lifetimes, it allows for the light-

generated carriers move large enough distances to be extracted as

current, instead of energy loss by recombination.

Inorg. Chem. 2013, 52, 9019

Science. 2013, 342, 317

Adv. Funct. Mater. 2005,15, 77

Combining resistivity and Hall effect data, the electron mobility was calculated to

be ~ 66 cm2/V·s for CH3NH3PbI3. This is remarkably high (in comparison,

PCBM, a common electron acceptor in OPV, has electron mobility of

0.21 cm2/V·s)

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High Voc Perovskite solar cells are quite effective

in generating a high electric voltage, which is represented by a high open circuit voltage (Voc):

CH3NH3PbI2Cl:

optical bandgap of 1.55 eV

Voc of 1.1 V

A voltage drop of only 0.45 eV, competitive with the best thin-film technologies (CIGS: 1.15 eV 0.7 V; Si: 1.1 eV 0.7 V)

Compares favorably to DSSC or OPV, which has a larger 0.7-0.8 V voltage drop, resulting in a small portion of the bandgap being extracted as Voc.

Other optimization predictions: Higher FF is possible (60-70% over 80%), current can also be higher.

Science. 2013, 342, 794

Science. 2012, 338, 643

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Getting the Best of Both Worlds

It has been agreed on that the organic-inorganic

hybrid nature is what makes it so successful:

The organic component renders good solubility to

the perovskite and facilitates self-assembly,

effectively enabling its precipitation/deposition from

solution.

The inorganic component produces an extended

network by covalent and/or ionic interactions

(instead of weaker forces such as Van der Waals or

π-π interactions). Such strong interaction allows for

precise crystalline structure in the deposited films.”

Nature. 2013, 12, 1087

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DSSC:

Predecessor to Perovskite Solar Cells

Current Perovskite Solar Cells are

built upon the architectural basis

for DSSCs

Pioneering work by Grätzel (EPFL,

Switzerland) that has garnered ~

17000 citations

Nature. 1991, 353, 737

www.sigmaaldrich.com technical documents

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Replacing Dye with Perovskite

Dyes do not absorb all the incident light, reducing DSSC efficiency.

In 2009, Miyasaka (Toin U. of Yokohama, Japan) turns to perovskite

as possible replacement of the dye and achieved 3.8% efficiency.

Problem: Liquid electrolyte dissolved away the perovskite within

minutes.

Liquid Electrolyte

0.15 M LiI and 0.075 M

I2 in methoxyacetonitrile

JACS. 2009, 131, 6050.

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Replace Liquid Electrolyte with a Solid

Hole Transporting Layer (HTL)

2012, Nam-Gyu Park (Sungkyunkwan U., South Korea) teamed

up with Grätzel, over 9% efficiency.

HTL layer: spiro-OMeTAD

Sci. Rep. 2012, 2, DOI: 10.1038/srep00591

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©2014, Wenchun Feng TiO2

Al2O3 Scaffold and Single Layer

2012, Henry Snaith (Oxford U.): Is TiO2 essential for high

efficiency?

Switched to an insulating Al2O3 scaffold, expected to see a

decrease in efficiency. Surprisingly, the device with Al2O3 has a

higher efficiency than that with TiO2 (11% vs. 7.6%).

If all that Al2O3 does is scaffolding,

what if we get rid of it as well?

2013 15 %

Al2O3

solution

vacuum

deposition

Perovskite

Precursor

Solution

CH3NH3I PbI2

2013 11 % Science. 2012, 338, 643.

Nature. 2013, 501, 395.

Adv. Funct. Mater. 2014, 24, 151

Do not need artificial

interfaces for efficient

charge separaton!

These two results are

remarkable, in that they

prove that these

perovskites work as

conventional

semiconductors (Si, GaAs,

etc).

single layer (thin film) device

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Sequential Deposition

2013, Grätzel sticks with the TiO2 structure and tinkered with the deposition

step.

mesoporous

TiO2

PbI2

CH3NH3I

Efficiency: 15%

Nature. 2013, 499, 316

perovskite

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Vapor-Assisted Deposition Drawbacks of the above two deposition techniques:

Despite success in labs, both are challenging for large-scale industrial production.

The all-solution process results in decreased film quality, and the vacuum process requires expensive equipment and uses a great deal of energy.

Alternative: Vapor-assisted solution process (Yang Yang, UCLA):

The organic material infiltrates the inorganic matter and forms a compact perovskite film that is significantly more uniform than the films produced by the wet technique.

JACS. 2014, 136, 622

surface roughness 23.2 nm

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New Structures

Current Perovskite Structure: CH3NH3PbX

Possible modifications:

Cation (some success)

Metal (challenge)

Halogen (some success, with lingering questions)

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Cation:

CH3NH3 vs. HC(NH2)2

Formamidinium cation slightly larger

Bandgap shrunk from 1.55 eV to

1.48 eV

Retained favorable transport property

Best solution-based thin film

perovskite solar cell (efficiency:

14.2%).

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N

C N N

C

Energy Environ. Sci. 2014, 7, 982

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Metal

In principle, perovskite is a flexible structure type: Many elements in the periodic table (such as Co2+ , Fe2+ , Mn2+ ,

Pd2+ , and Ge2+) can be incorporated

Sn: Earlier work by Mitzi showed that perovskites with Sn show metallic

character with small bandgap (not the semiconductors ideal for PV)

Recent work confirmed this notion.

It was also found that the facile oxidation of Sn2+ to Sn4+ gives a metal-like behavior in the semiconductor which lowers the photovoltaic performance.

Cu: At 2013 Fall MRS meeting, a group from Hokkaido U., Japan, is

studying Cu-based materials with a general formula of R2CuBr4. Field effect transistor by inkjet deposition showed weak n-type properties.

In general, replacing Pb with other less or non toxic metals remains a challenge.

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Science. 1995, 267, 1473

Materials Today. 2014, 17, 16

Dalton Trans., 2011, 40, 5563

Inorg. Chem. 2013, 52, 9019

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Halogen:

MAPbI3, MAPbBr3, MAPbCl3, MAPbI3-xClx

CH3NH3PbI3 bandgap 1.5 eV, CH3NH3PbBr3 2.3 eV, bandgap tailoring can

be simply achieved by alloying of both.

MAPb(I1-xBrx)3

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Nano Lett. 2013, 13, 1764

J. Mater. Sci., 2002, 37, 3585

Chem. Mater. 2013, 25, 4613

Adv. Mater. 2013, DOI: 10.1002/adma.201304803

Science. 2012, 338, 643

Why don’t researchers use CH3NH3PbCl3? Bandgap (3.1 eV) too large for good absorption, appearance is colorless.

Mixed Halide CH3NH3PbI3-xClx: Currently the best perovskite material. The extent of incorporation of Cl into the perovskite is in debate. Recent results indicate that the

Cl incorporation is very low (3-4%), due to the large difference in the ionic radii of Cl- and I- anions. Another XPS study shows Cl:I exactly at 1:2.

This mixed halide have similar bandgaps as CH3NH3PbI3 (another indication that the Cl incorporation is low)

This mixed halide has superior charge transport property than iodide one (diffusion length >1000 nm)

Better stability in air (reason unclear)

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Snaith Cell Stability

Delivers Stable Photocurrent for 1000 hrs under continuous illumination of full

spectrum simulated sunlight.

However, efficiency suffers from a 50% loss at 200 hr period.

Nature Comm. 2013, DOI: 10.1038/ncomms3885

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Grätzel Cell Stability

This structure has TiO2 mesoporous structure.

less than 20% loss of its initial efficiency after 500 hours. Nature. 2013, 499, 316

Note:

Both Snaith and Gratzel cells

require encapsulation.

Takeaway message:

Stability testing shows different

results for different devices.

Carefully calibrated testing on

the same system is needed.

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Summary and Outlook

Perovskite solar cells made strides in efficiency gains over the past 5 years. Further efficiency improvement is predicted to lie in FF and current (Voc is probably saturated at a mere 0.4 V drop).

Photophysical studies show extremely long electron and hole diffusion length (>100 nm; for mixed halides >1000 nm). However, it is unclear whether excitons or free charges are the predominant diffusion species.

Although electron mobility has been studied by one paper, mobility studies in general are lacking.

Preliminary stability testing shows promise. Long term stability testing (which should guarantee a lifetime of 15 years or more) is essential to successful commercialization.

New structures emerging with varying degree of success: Organic Cation

Toxicity associated with Pb should be mitigated by exploring other metal cations (e.g. Sn, Cu. Unfortunately, they currently do not make good devices).

Halogen (probably the most mature understanding of all three)

Scaling remains a challenge. Best performing devices generally have a low surface area of < 0.1 cm2 . Efficiency dropped ~ 30% from ~ 0.1 cm2 to 1 cm2 cell area. Mostly by FF

decrease. Reason unclear.

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Nature Photon. 2014, 8, 128