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Hybrid lead halide perovskites are widely studied in photodetectors due to their excellent optoelectronic properties. However, most of the perovskite photodetectors require an external power source, which adds additional costs and limits their application. Here, a self‐powered photodetector based on the heterojunction formed by methylammonium lead tribromide perovskite nanocrystals (MAPbBr3 NCs) and mesoporous TiO2 (m‐TiO2) is demonstrated by a space‐confined growth method. The NCs are uniformly grown on the m‐TiO2 framework and mirco‐heterojunctions are formed. Systematic research shows that the built‐in electric field of the mirco‐heterojunctions increases the exciton dissociation and carrier transport. In the absence of an external power supply, the photodetectors display an impressive on/off ratio (>105) and high detectivity (>1011 Jones) under 100 mW cm−2 solar irradiation. In addition, the winning device shows excellent long‐term stability (photocurrent maintains over 92% after exposure of 102 days to the air), benefiting from the enhanced binding energy and reduced defect density of NCs. It is believed that this study sheds light on the further development of self‐powered perovskite‐based photodetectors with high performance. [ABSTRACT FROM AUTHOR]