Perovskite Solar Cells

All rights reserved. ©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.
Current Address:
Kotov Group,
Department of Chemical Engineering,
University of Michigan, Ann Arbor
Email: wefeng@umich.edu

Schematic
2
Perovskite
Precursor
Solution
CH3NH3I PbI2
fluorine-doped
tin oxide substrate
spin coat
then annealFTO 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

Perovskite Solar Cells Made
Simply – C&EN Video
3
For a C&EN video, please see
https://www.youtube.com/watch?v=oQ2bz6jlbz0

Perovskite Solar Cell is the
Poster Child for Solar Energy Conversion
Source: Web of Science
4
Henry Snaith

5

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
6

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.
7

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.
8

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
9

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
10

Crystallinity
Tetragonal Structure
Lattice Parameters:
a = 8.825 Å, b = 8.835 Å, c = 11.24 Å.
Science. 2012, 338, 643.
11

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.
12

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
13

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)
14

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
15

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
16

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
17

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.
18

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
19

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
20

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
21

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
22

New Structures
 Current Perovskite Structure: CH3NH3PbX
 Possible modifications:
 Cation (some success)
 Metal (challenge)
 Halogen (some success, with lingering questions)
23

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%).
24
N
C
N N
C
Energy Environ. Sci. 2014, 7, 982

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.
25
Science. 1995, 267, 1473
Materials Today. 2014, 17, 16
Dalton Trans., 2011, 40, 5563
Inorg. Chem. 2013, 52, 9019

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
26
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)

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
27

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.
28

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.
29
Nature Photon. 2014, 8, 128

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