Materials and Technologies for Making Perovskite-based Solar
Cell DENG Sunbin 3/12/2014 1
Slide 2
2 Outline 1. Introduction 2. Materials for PSC Fabrication 3.
Processes for PSC Fabrication 4. Potential Trend in the Future 5.
Conclusion
Slide 3
3 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
Slide 4
4 Perovskite Materials in PSCs 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. Formula:
ABX 3 Organometal halide (for photovoltaics) A Organic cations (CH
3 NH 3 +, CH 3 CH 2 NH 3 +, NH 2 CH=NH 2 + ) B Metal cations (Pb
2+, Sn 2+ ) X Halides (I -, Br -, Cl - ) CH 3 NH 3 MX 3 (M=Pb, Sn;
X=Cl, Br or I) Some key attributes: Ease of fabrication Strong
solar absorption Low non-radiative carrier recombination etc. Green
M A, et al. Nature Photonics, 2014, 8(7): 506-514.
Slide 5
5 Progress of Perovskite Solar Cell Fabrication First stage:
Material leading Second stage: Process leading
Slide 6
6 Milestones Dye: Others Perovskite 3.8%, CH 3 NH 3 PbI 3 /CH 3
NH 3 PbBr 3 ( 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%, CH 3 NH 3 PbI 3 /
Spiro-MeOTAD ( Park N G, Gra tzel M, et al. Scientific reports,
2012, 2. ) 9.7%, CH 3 NH 3 PbI 3-x Cl x / Spiro-MeOTAD ( Snaith H
J, et al. Science, 2012, 338(6107): 643-647. ) Enhanced stability,
record-breaking efficiency, thinner Mesoscopic scaffold layer: TiO
2 Al 2 O 3 10.9%, CH 3 NH 3 PbI 3 ( Park N G, Gra tzel M, et al.
Scientific reports, 2012, 2. ) Electron transport property HTM
elimination 5.5%, TiO 2 /CH 3 NH 3 PbI 3 heterojunction ( Etgar L,
et al. J. Am. Chem. Soc., 2012, 134(42): 17396-17399. ) 12.8%, TiO
2 /ZrO/(5-AVA) x (MA) 1-x PbI 3 ( Mei A, et al. Science, 2014,
345(6194): 295-298. ) Hole transport property Ambipolar
semiconductor Planar p-i-n heterojunction PSC: 15.4%, CH 3 NH 3 PbI
3-x Cl x ( Snaith H J, et al. Nature, 2013, 501(7467): 395-398. )
Figure 3: Several notable milestones led by materials in the
progress of PSC fabrication, resulting in the evolution of device
structure.
Slide 7
7 (a) (b) (c) (d) PSC Structure 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 Al 2 O 3
scaffold layer with a conformal overlayer of the perovskite which
plays as a light harvester and electron conductor; (c) PSC with
mesoscopic TiO 2 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. Grtzel M. Nature materials, 2014, 13(9):
838-842.
Slide 8
8 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
Slide 9
9 One-step Spin Coating A mixture of PbX 2 and CH 3 NH 3 X
(X=Cl, Br, I) in a common solvent (DMF, GBL, DMSO, etc.)
Uncontrolled precipitation of the perovskite Shapeless morphology
Poor reproducibility of photovoltaic performance Figure 5:
Schematic illustration of one-step spin coating method.
Slide 10
10 Two-step (Sequential) Solution-Based Deposition i.Spin coat
PbX 2 solution ii.Dip into CH 3 NH 3 X solution iii.CH 3 NH 3 PbX 3
film Better morphology and interfaces Increased light scattering
due to large crystal size Boosted photovoltaic performance (15%)
and reproducibility Figure 6: Schematic illustration of sequential
solution-based deposition method. 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.
Slide 11
11 Dual-Source High-Vacuum Thermal Evaporation (Planar) Better
morphology and uniformity of perovskite film Better thickness
control 15.4% (for planar CH 3 NH 3 PbI 3-x Cl x solar cell)
Compatible with traditional technologies (high vacuum) Inorganic
source PbX 2 + Organic source CH 3 NH 3 X Co-evaporation at 10 -5
mbar Annealing for crystallization Figure 8: (a) Scheme of
dual-source thermal evaporation system. (b) Generic structure of a
planar heterojunction pin perovskite solar cell. (c) Current-
density/voltage curves of vapor-deposited and solution-processed
PSCs. Figure 9: Comparison of the perovskite film uniformity
between vapor-deposition and solution-process methods. Snaith H J,
et al. Nature, 2013, 501(7467): 395-398.
Slide 12
12 Sequential Liquid-Vapor Phase Deposition (Planar) Solution
process (Inorganic PbX 2 ) Annealing at 150 C Vapor treatment
(Organic CH 3 NH 3 X) In situ reaction Overcome high vacuum issue
Kinetic reactivity of CH 3 NH 3 X 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 CH 3 NH 3 PbI solar cell) Figure 10: Schematic
illustration of perovskite film formation in the sequential
liquid-vapor phase deposition. Figure 11: Perovskite film on the
FTO/c-TiO 2 substrate, obtained by reacting PbI 2 film and CH 3 NH
3 I vapor at 150 C for 2 h in N 2 atmosphere: (a) XRD pattern; (b)
top-view SEM image (inset image with higher resolution, scale bar 1
m); (c) tapping-mode AFM height images (5 5 m); and (d)
cross-sectional SEM image. Chen Q, et al. J. Am. Chem. Soc., 2013,
136(2): 622-625.
Slide 13
13 Future Potential Technologies for PSC Fabrication
Low-temperature process New ETM: TiO 2 ZnO Figure 12: (a) Schematic
illustration of the hole- conductor-free, fully printable
mesoscopic PSC. (b) Energy band diagram of this triple-layer PSC.
Mei A, et al. Science, 2014, 345(6194): 295-298. Extra HTM free
Printing technology Figure 13: (a) Device architecture of the
ITO/ZnO/CH 3 NH 3 PbI 3 /spiro-OMeTAD/Ag PSC. (b) Energy band
diagram of the various device components. Liu D, et al. Nature
Photonics, 2014, 8(2): 133-138. Interface Engineering 19.3% ! Zhou
H, et al. Science, 2014, 345(6196): 542-546. Figure 14: (a) SEM
cross-sectional image of the device. The layers from the bottom
are: (i) ITO/PEIE, (ii) Y-TiO 2, (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.
Slide 14
14 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.