A long photoluminescence decay lifetime has been regarded as a generic indication of long charge carrier recombination lifetime in semiconductors such as metal halide perovskites (MHPs), which have shown tremendous success in solar cells. Here, we report that MHP polycrystalline films with extrinsic metal ions have a very long charge recombination lifetime, but a much shorter photoluminescence decay lifetime, and this huge difference can be explained by a model of lateral homojunction within each individual grain. The lateral homojunction is formed due to the doping along grain boundaries by metal ions, and then verified by nanoscale potential mapping and transient photo-response mapping. The built-in electric field within each grain reduces the recombination of free charge carriers within the perovskite grain and along grain boundaries, while the free electrons and holes are collected to cathode and anode through the grain boundaries and grain interiors, respectively. Then, the efficiencies of MHP polycrystalline solar cells are increased.