Gyu Min Kim1 Trilok Singh1 Yoshitaka Sanehira1 Tsutomu Miyasaka1

1, Toin University of Yokohama, Yokohama, , Japan

Perovskite solar cells (PVSCs) have attracted great attention recently due to their high power conversion efficiency (PCE) with low cost despite their short history. Development of engineering method of perovskite crystallization has boosted the PCE of PVSCs up to 22.7% recently. Among the several fabrication methods, dripping anti-solvents to perovskite precursors in DMF:DMSO mixed solvents is one of the promising way to high quality perovskite films, leading to superior performances. It is known that lewis base and acid interaction by using DMSO forming intermediate states prevents abrupt crystallization. On the other hand, anti-solvents which are casted to perovskite precursors such as chlorobenzene and ethyl ether accelerate crystallization of perovskites by super-saturation. It is considered that kinetics of perovskite precursor during the formation of perovskite crystals directly result in optical and electrical properties of PVSCs. In this sense, we tried to focus on the kinetic control of Cecium (Cs)-mixed intermediate states during the formation of perovskite crystals. By putting the petri-dish on the perovskite substrates right after spin-coating with lower temperature for certain time (stage 1) and further annealing without petri-dish for 1h at 100 celsius (stage 2) for complete conversion to black phase of perovskite, evaporation kinetics of intermediate states can be easily controlled. The remaining DMF and DMSO vapors inside petri-dish decelerate decomposition of intermediate states at stage 1. The optimum temperature at stage 1 depends on types of perovskite compositions. The addition of cation dopants such as Cs, Rubidium (Rb) and Potassium (K) decreases intermediate state binding energy. Thus, for adequate control of intermediate states, lower temperature at stage 1 is required especially for cations-doped perovskite precursors. The mirror-like color, which is indicative of intermediate states, is retained for several seconds or minutes depending on the annealing temperature with petri-dish at stage 1. From the measurement of AFM and SEM, the grain sizes and the surface roughness are largely affected by the time at which intermediate states are retained at stage 1. Photoluminescence (PL) measurement also reveals that adequate control of vaporization kinetics of intermediates enhances the PL intensity, indicating that non-radiative recombination is suppressed. The engineering of kinetic control at stage 1 has resulted in high PCE recording 21.3% with negligible hysteresis for Cs-mixed PVSCs.