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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]
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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.
©2014, Wenchun Feng
Schematic
2
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
©2014, Wenchun Feng
Perovskite Solar Cells Made
Simply – C&EN Video
3
For a C&EN video, please see
https://www.youtube.com/watch?v=oQ2bz6jlbz0
©2014, Wenchun Feng
Perovskite Solar Cell is the
Poster Child for Solar Energy Conversion
Source: Web of Science
4
Henry Snaith
©2014, Wenchun Feng
5
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
Crystallinity
Tetragonal Structure
Lattice Parameters:
a = 8.825 Å, b = 8.835 Å, c = 11.24 Å.
Science. 2012, 338, 643.
11
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©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
20
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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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©2014, Wenchun Feng
New Structures
Current Perovskite Structure: CH3NH3PbX
Possible modifications:
Cation (some success)
Metal (challenge)
Halogen (some success, with lingering questions)
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©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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)
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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
©2014, Wenchun Feng
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