Enormous efforts have been put into different aspects of perovskite solar cells (PSCs) and the progress has been incredibly fast on all fronts. The background, on-going R&Ds, and future direction of PSC research have been reported by our group as a comprehensive review.¹ Despite the current record efficiency of 25.5%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites using modulator molecules and mixing 2D and 3D structures has improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.² We have conducted the device fabrication in this direction using CsPbI₃ and CsPbI₂Br as solution-processed perovskite films. The perovskite absorbers were made into junction with dopant-free HTMs to develop thermally stable PSCs. By chemical engineering to improve the quality of interfacial structure at the hetero-junction, all-inorganic and dopant-free PSCs yielded power conversion efficiency up to 15%. Further we could enhance the open-circuit voltage (Voc) of this PSC up to 1.42V, which is the highest Voc ever achieved with a single perevskite cell.³ In preparation of all-inorganic perovskites, a big challenge should also be directed to development of lead-free materials for environmental safety in practical applications.²

   Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to high energy particle irradiations (proton and electron beams).⁴ Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current progress in making perovskite photovoltaics and future perspectives will be discussed.

1. A. K. Jena, A. Kulkarni, and T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
2. T. Miyasaka, A. Kulkarni, G. M. Kim, S. Oez and A. K. Jena, Adv. Energy Mat., 2019, 1902500.
3. G. Zhanlin, T. Miyasaka, et al. J. Am. Chem. Soc. 2020, 142, 21, 9725–9734.
4. Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, and T. Miyasaka, iScience 2018, 2, 148-155.