Comparative study of charge carrier dynamics in bismuth-based dimer and double perovskites

Abstract

From the environmental and economical aspects, the development of less-toxic lead-free perovskite is an imperative challenge for next generation photovoltaics. Although bismuth-based dimer perovskite (A3B2X9) and double perovskite (A2BB’X6) (A = cation, B = Bi, B’ = Ag, X = halogen) have been considered as the plausible candidates, their power conversion efficiencies (PCE) are still low (~2.2%). In this work, we comparatively evaluated the photoconductivity and charge carrier transfer to electron/hole transport layer (ETL/HTL) in A3B2X9 and A2BB’X6 by time-resolved microwave conductivity (TRMC). In contrast to A3B2X9, A2BB’X6 showed much higher photoconductivity and charge transfer efficiency to ETL/HTL. This result is consistent with the PCEs of our photovoltaic devices, in which the double perovskite of Cs2AgBiBr6 exhibited greatly improved fill factor and one order larger photocurrent density than dimer perovskites (Cs3Bi2I9, MA3Bi2I9, and FA3Bi2I9; MA = methlyammonium cation, FA = formamidinium cation). Our results provide an insight into the key role of photoactive layer and charge transfer process.