Abstract
Halide perovskites are known to be photoconductive for more than half a century, and their efficient photocarrier generation gives rise to positive photoconductivity (PPC). In this work, the discovery of negative photoconductivity (NPC) in hybrid perovskite CH3NH3PbBr3 after Bi doping is reported. Transient photoconductivity measurements reveal a surprising bipolar behavior with a fast positive response followed by exponential negative photocurrent decay, resulting in an equilibrium photocurrent even below the dark level. The NPC effect in Bi-doped CH3NH3PbBr3 is among the largest ones reported so far for semiconductors. It is proposed that the transition to negative photoconductance is related to the presence of DX-like centers in Bi-doped halide perovskites, similar to doped III-V and chalcopyrite semiconductors. Such photogenerated DX-like centers in the Bi-doped CH3NH3PbBr3 can trap mobile band electrons and enhance charge recombination, thus reducing the conductivity. This mechanism is consistent with the observations of crossover from PPC to NPC as functions of temperature, composition, and illumination. The results underscore the importance of defect engineering for tuning the optoelectronic properties of halide perovskites.