1. Materials and Technologies for Making Perovskite-based Solar Cell DENG Sunbin 3/12/2014 1

2. 2 Outline 1. Introduction 2. Materials for PSC Fabrication 3. Processes for PSC Fabrication 4. Potential Trend in the Future 5. Conclusion

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

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.

5. 5 Progress of Perovskite Solar Cell Fabrication  First stage: Material leading  Second stage: Process leading

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.

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. Grätzel M. Nature materials, 2014, 13(9): 838-842.

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

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.

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.

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 p–i–n 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.

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 μm); (c) tapping-mode AFM height images (5 × 5 μm); and (d) cross-sectional SEM image. Chen Q, et al. J. Am. Chem. Soc., 2013, 136(2): 622-625.

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.

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.

15. 15 Thank you!