Defect trapping states and charge carrier recombination in organic-inorganic halide perovskites

Organic-inorganic perovskite solar cells have attracted huge research interest due to rapid improvement in device performance showing great potential to be the next generation flexible solar cells. Unique defect properties in perovskite have been considered as the possible mechanism for the superior performance, and closely relevant to the effects of hysteresis and light soaking. To date, the quantitative correlation and in-depth understanding of defects in organic-inorganic perovskite are still lacking although extensive investigation have been undertaken. Here we study defect trapping states and carrier recombination dynamics in organic-inorganic halide perovskites. At low excitation the photoluminescence (PL) intensity exhibits a super-linear increase with increasing excitation, due to the slow depopulation rate of the defect states. The steady state and time-resolved photoluminescence (PL) carried out in this work reveal that the carrier recombination dynamics is ultimately correlated with both the defect density and the relaxation rate of the carriers in defects. A model is established for the relationship between the properties of the defect trapping state and steady state PL intensity. Two key parameters, (i) the ratio of the trap-state density to the depopulation rate of trapped states and (ii) ratio of the maximum density of covalence band electrons to the trapping rate, can be extracted from the model based on the excitation dependent steady state PL. This work demonstrates that the properties of defect trapping states are closely related to the fabrication technique, and suggests that the organic-inorganic halide perovskite is partly defect-tolerant.