Mixed Solvent Engineering for Morphology Optimization of the Electron Transport Layer in Perovskite Photovoltaics

The interfacial charge transport dynamics in perovskite solar cells (PSCs) play a significant role for achieving improved device efficiency. We have shown here the morphology optimization of the electron transport layer (ETL), i.e., [6,6]-phenyl-C61-butyrate methyl ester (PCBM), through solvent engineering by introducing a mixed solvent system (chlorobenzene (CB) and dichlorobenzene (DCB)) in PCBM layer fabrication. The results showed an improvement in morphology and better film coverage with reduced surface roughness and pinholes in ETL by replacing half of CB with a high boiling point solvent in the CB0.5-DCB0.5 system. Strong quenching in steady-state photoluminescence (PL) indicates that an improved interaction in the perovskite/ETL interface leads to efficient charge transfer, which has been further realized through time-resolved PL (TR-PL) measurement with reduced lifetime tau(avg) similar to 2.35 ns. The power conversion efficiency (PCE) in PSCs improved from 17.2% for CB1.0-DCB0 to 18.46% for CB0.5-DCB0.5 mixed solvent treated ETL. Furthermore, this mixed solvent processing for ETL strengthened the perovskite/ETL interface and minimized the charge carrier recombination, which seems to be a convenient mixed solvent strategy for high-performance PSCs.