Biexciton Generation and Dissociation Dynamics in Formamidinium- and Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals.

Recent studies show that perovskites (ABX-type) comprising mixed A or B cation and/or mixed halide (X) are more stable and efficient materials for photovoltaic applications than their respective pure forms. Herein we report how doping of a small quantity of formamidinium and/or chloride ion influences the single and multiexciton dynamics of CsPbI nanocrystals (NCs). With the help of ultrafast pump-probe spectroscopic measurements, we show that chloride doping can enhance the biexciton lifetime of the system significantly by slowing down the Auger recombination (AR) process. The measured biexciton AR time scale (∼195-205 ps) in some of these NCs is the longest among those reported to date for any similar size perovskites. We further demonstrate that suppression of the AR rate and consequent lengthening of biexciton lifetime allow harvest of these species for their utilization through rapid (18-45 ps) electron transfer to fullerene. The insights obtained from this study are expected to help design more efficient doped perovskites for energy conversion purposes.