Defect/interface recombination limited quasi-Fermi level splitting and open-circuit voltage in mono- and triple cation perovskite solar cells.

Multi cation metal-halide perovskites exhibit desirable performance and stability compared to their mono cation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanism in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskites solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than in triple cation perovskite absorbers which defines the device open-circuit voltage. PL measurements at low temperatures (~50 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the non-radiative recombination loss at room temperature. We conclude that whilst triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states in MAPI films is the primary consideration for device optimization.