Materials and Technologies for Making
Perovskite-based Solar Cell
DENG Sunbin
3/12/2014
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Outline
1. Introduction
2. Materials for PSC Fabrication
3. Processes for PSC Fabrication
4. Potential Trend in the Future
5. Conclusion
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Perovskite Solar Cell (PSC) —— A New Era
Figure 1: Research cell efficiency records. This plot is courtesy of the National Renewable
Energy Laboratory, Golden, CO.
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
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Perovskite Materials in PSCs
 Formula: ABX3
 Organometal halide (for photovoltaics)
A  Organic cations (CH3NH3+, CH3CH2NH3+, NH2CH=NH2+)
B  Metal cations (Pb2+, Sn2+)
X  Halides (I-, Br-, Cl-)
—— CH3NH3MX3 (M=Pb, Sn; X=Cl, Br or I)
 Some key attributes:




Ease of fabrication
Strong solar absorption
Low non-radiative carrier recombination
etc.
Figure 2: Cubic pervovskite crystal structure. For
photovoltaically interesting perovskites, the larger organic
cations occupy position A whereas metal cations and
halides occupy the B and X positions, respectively.
Green M A, et al. Nature Photonics, 2014, 8(7): 506-514.
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Progress of Perovskite Solar Cell Fabrication
 First stage: Material leading
 Second stage: Process leading
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Milestones
 Dye: Others  Perovskite

3.8%, CH3NH3PbI3/CH3NH3PbBr3
(Kojima A, et al. J. Am. Chem. Soc., 2009, 131(17): 6050-6051.)


Thinner and stronger sensitizer
Rapid degradation
 HTM: Liquid electrolyte  Solid state

10.9%, CH3NH3PbI3/ Spiro-MeOTAD
(Park N G, Grätzel M, et al. Scientific reports, 2012, 2.)

9.7%, CH3NH3PbI3-xClx/ Spiro-MeOTAD
(Snaith H J, et al. Science, 2012, 338(6107): 643-647.)

Enhanced stability, record-breaking efficiency, thinner
 Mesoscopic scaffold layer: TiO2  Al2O3

10.9%, CH3NH3PbI3
(Park N G, Grätzel M, et al. Scientific reports, 2012, 2.)

Electron transport property
 HTM elimination

5.5%, TiO2 /CH3NH3PbI3 heterojunction
(Etgar L, et al. J. Am. Chem. Soc., 2012, 134(42): 17396-17399.)

12.8%, TiO2 /ZrO/(5-AVA)x(MA)1-xPbI3
(Mei A, et al. Science, 2014, 345(6194): 295-298.)
Figure 3: Several notable milestones led by materials in the progress of PSC fabrication,
resulting in the evolution of device structure.
 Hole transport property
 Ambipolar semiconductor
 Planar “p-i-n” heterojunction PSC: 15.4%, CH3NH3PbI3-xClx (Snaith H J, et al. Nature, 2013, 501(7467): 395-398.)
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PSC Structure
(b)
(a)
(c)
(d)
Figure 4: Historic evolution of PSC structure, starting from (a) original mesoscopic DSSC, using the perovskite dye as a sensitizer, to currently (b) Meso-superstructured
PSC (MSSC), employing a mesoscopic Al2O3 scaffold layer with a conformal overlayer of the perovskite which plays as a light harvester and electron conductor; (c) PSC
with mesoscopic TiO2 scaffold infiltrated by the perovskite. The perovskite is a light harvester as well as hole conductor; (c) Planar p-i-n heterojunction PSC without
mesoscopic metal oxide scaffold. The perovskite behaves as both ambipolar semiconductor and light harvester.
Grätzel M. Nature materials, 2014, 13(9): 838-842.
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Deposition of the Perovskite
 Solution process
 One-step spin coating
 Two-step (Sequential) deposition
 Vapor process (for planar PSCs particularly)
 Dual-source thermal evaporation
 Sequential liquid-vapor phase deposition
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One-step Spin Coating
 A mixture of PbX2 and CH3NH3X
(X=Cl, Br, I) in a common solvent
(DMF, GBL, DMSO, etc.)
 Uncontrolled precipitation of the perovskite
Figure 5: Schematic illustration of one-step spin coating method.
 Shapeless morphology
 Poor reproducibility of photovoltaic performance
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Two-step (Sequential) Solution-Based Deposition
i. Spin coat PbX2 solution
ii. Dip into CH3NH3X solution
iii. CH3NH3PbX3 film
Figure 6: Schematic illustration of sequential solution-based deposition method.
 Better morphology and interfaces
 Increased light scattering due to large
crystal size
 Boosted photovoltaic performance (15%)
and reproducibility
Figure 7: The cross-section images of PSC fabricated by (a) the sequential spin
coating process and (b) the conventional single-step spin coating process.
Burschka J, et al. Nature, 2013, 499(7458): 316-319.
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Dual-Source High-Vacuum Thermal Evaporation (Planar)
 Inorganic source PbX2 +
Organic source CH3NH3X
 Co-evaporation at 10-5 mbar
 Annealing for crystallization




Better morphology and uniformity of perovskite film
Better thickness control
15.4% (for planar CH3NH3PbI3-xClx solar cell)
Compatible with traditional technologies (high vacuum)
Figure 9: Comparison of the
perovskite film uniformity
between vapor-deposition and
solution-process methods.
Figure 8: (a) Scheme of dual-source thermal evaporation system. (b) Generic
structure of a planar heterojunction p–i–n perovskite solar cell. (c) Currentdensity/voltage curves of vapor-deposited and solution-processed PSCs.
Snaith H J, et al. Nature, 2013, 501(7467): 395-398.
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Sequential Liquid-Vapor Phase Deposition (Planar)




Solution process (Inorganic PbX2)
Annealing at 150 °C
Vapor treatment (Organic CH3NH3X)
In situ reaction
Figure 10: Schematic illustration of perovskite film formation in the sequential
liquid-vapor phase deposition.
 Overcome high vacuum issue
 Kinetic reactivity of CH3NH3X and
thermodynamic stability of perovskite
 Well-defined grain structure with grain sizes up
to microscale
 Full surface coverage & small surface roughness
 12.1% (for planar CH3NH3PbI solar cell)
Chen Q, et al. J. Am. Chem. Soc., 2013, 136(2): 622-625.
Figure 11: Perovskite film on the FTO/c-TiO2 substrate, obtained by reacting PbI2 film and CH3NH3I vapor at
150 °C for 2 h in N2 atmosphere: (a) XRD pattern; (b) top-view SEM image (inset image with higher resolution,
scale bar 1 μm); (c) tapping-mode AFM height images (5 × 5 μm); and (d) cross-sectional SEM 12
image.
Future Potential Technologies for PSC Fabrication
 Extra HTM free
 Printing technology
 Low-temperature process
 New ETM: TiO2  ZnO
Figure 13: (a) Device architecture of the
ITO/ZnO/CH3NH3PbI3/spiro-OMeTAD/Ag PSC.
(b) Energy band diagram of the various device
components.
Figure 12: (a) Schematic illustration of the holeconductor-free, fully printable mesoscopic PSC.
(b) Energy band diagram of this triple-layer PSC.
Mei A, et al. Science, 2014, 345(6194): 295-298.
Liu D, et al. Nature Photonics, 2014, 8(2): 133-138.
 Interface Engineering
 19.3% !
Figure 14: (a) SEM cross-sectional image of the
device. The layers from the bottom are: (i) ITO/PEIE,
(ii) Y-TiO2, (iii) perovskite, (iv) spiro-OMeTAD, and
(v) Au. (b) Diagram of energy levels (relative to the
vacuum level) of each functional layer in the device.
Zhou H, et al. Science, 2014, 345(6196): 542-546.
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Conclusion
 Four material-leading milestones and three possible device structures for PSC
fabrication are concluded.
 In PSC fabrication, there are solution processes and vapor processes (for planar
PSCs).
 Sequential (two-step) solution-processed deposition could form better
morphological perovskite layer than one-step spin coating deposition, resulting
in better photovoltaic performance and reproducibility.
 High-quality and controllable perovskite film could be deposited by vapor
processes in planar PSC fabrication.
 Emerging technologies such as low-temperature process and interface
engineering may represent potential trend for PSC fabrication in the future.
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Thank you!
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