Electromodulation and Transient Absorption Spectroscopy Suggest Conduction Band Electron Lifetime, Electron Trapping Parameters, and CH(3)NH(3)Pbl(3) Solar Cell Fill Factor Are Correlated

摘要

While lead halide perovskite solar cells have shown a steady increase in power conversion efficiencies over the past several years, it is well-known that minor variations in film fabrication conditions have a major impact on solar performance. In comparing devices assembled with either a 1-step or 2-step CH3NH3PbI3 film fabrication, we observe an increased fill factor and decreased subgap photocurrent in devices made with the 1-step method. These subgap states correlate with bleaching in NIR transient absorption measurements. We also observe induced absorptions in this region which we attribute to overlapping signals from photoexcited conduction band (E-1/2u -> F-1/2u) and valence band (F-3/2g -> E-1/2g) carrier transitions. This assignment is consistent with electromodulation spectroscopy under cathodic and anodic charge accumulation conditions, where we observed an absorption onset at a higher energy under electron accumulation (850 nm) compared to that of hole accumulation (1050 nm). Global target analysis of our time-resolved spectra suggests that the temporal evolution of photogenerated conduction band electrons and trap state bleaching dynamics are correlated. We modeled the kinetics of the species-associated spectra based on free charge extraction, bimolecular recombination, charge trapping, and Auger recombination processes and found that longer electron lifetimes correlate with decreased trap state density and improved device efficiency resulting from higher fill factors. The AC nature of our spectroelectrochemical detection also allows us to virtually eliminate the film degradation that has plagued DC electrochemical studies of halide perovskite materials. Our results provide fundamental insight into the impact of charge carrier dynamics on photovoltaic device performance.