Deterioration mechanism of perovskite solar cells by operando observation of spin states

Abstract

Perovskite solar cells are attractive because of their remarkably improved power conversion efficiency. In view of their application, however, it is important not only to increase the power conversion efficiency, but also to elucidate the deterioration mechanism. Here, we show operando direct observation of spin states in the cells using electron spin resonance, thereby investigating the operation and deterioration mechanisms from a microscopic viewpoint. By simultaneous measurements of solar cell characteristics and electron spin resonance, the spin states in the hole transport material spiro-OMeTAD are demonstrated to change in accordance with the device performance variation under operation. These variations are ascribed to the change of hole transport and to interfacial electric dipole layers. Reverse electron transfer from TiO2 to the hole transport material layer is demonstrated under ultraviolet light irradiation, which decreases hole doping. Conducting such operando microscopic investigation will be useful to obtain further guidelines for improving the device performance and durability.