Advantage of perovskite-based solar cells in manufacture process is applicability of rapid solution process without use of high temperature treatments, which leads to high throughput in large area cell production.¹ Low process temperatures in perovskite cell making enables fabrication of lightweight flexible devices on plastic substrates without losing their high performance. Among metal oxide materials, ZnO and SnO_x are good electron collectors which can be prepared with sinter-less solution process at temperatures below 150^oC. Dehydration condensation reaction on the surface of nanocrystalline SnO₂ and ZnO results in interparticle necking to form dense compact layer and mesoporous layer by controlling the condition of solution process. Recently thin SnO₂ layer has been found to function as excellent electron collector for MA and FA perovskites exhibiting high voltage (>1.1V), interestingly, without reflecting low conduction band level of the bulk SnO₂ semiconductor. We have been fabricating high performance perovskite devices using ZnO, SnO_x, and ZnO/SnOx/Al₂O₃ composite compact layers. These cell have high stability in long term preservation, maintaining high Voc >1.0V and power conversion efficiency of 13-15%.^2-4 Here, FA perovskite was found to have higher stability than MA perovskite in combination with low temperature ZnO. Bookite TiO₂ (bk-TiO_²) mesoporous layer prepared at low process temperature achieves stronger interparticle necking than anatase TiO₂ to form void-less uniform scaffolds for perovskite.⁵ This leads to give defect-less continuous junction between TiO₂ and perovskite, which minimize I-V hysteresis. Flexible plastic film device comprising SnO_x/bk-TiO₂ and MA perovskite shows hysteresis-less I-V performance and mechanical robustness against cyclic bending test.
   We also applied low temperature solution coating methods to lead-free hybrid materials. Bi-based organic halide perovskites (absorbing < 620 nm) were prepared on anatase and bk TiO₂ scaffolds of different porosities. Our experiments corroborated strong influence of the morphology of metal oxide collectors on the Bi perovskite photovoltaic devices.⁶ 

References
1. T. Miyasaka, Chem. Lett., 2015, 44, 720-729.
2. J. Song, T. Miyasaka, et al., J. Mater. Chem. A, 2015, 3, 10837-10844.
3. J. Song, T. Miyasaka, et al., Chem. Lett., 2015, 44, 610-612.
4. J. Song, E. Zheng, X.-F. Wang, W. Tian, T. Miyasaka, Solar Ener. Mater. Solar Cells, 2016, 144, 623-630.
5. A. Kogo, Y. Sanehira, M. Ikegami, and T. Miyasaka, J. Mater. Chem. A., 2015, 3, 20952-20957.
6. T. Singh1, A. Kulkarni, M. Ikegami, and T. Miyasaka,2016, ACS Appl. Mat. Interfaces, submitted.