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Quite different from organic semiconductors, the ambipolar carrier diffusion lengths of perovskite semiconductors are large enough allowing to utilize the planar heterojunction with charge selective interfaces.[9–12] One interesting aspect of the planar architecture is the potential lower processing temperature as compared to the mesoporous metal-oxide-based electrodes.[13] However, the planar structure perovskite devices generally suffer from a series of deficiencies in the first place the hysteresis effect, which induces inconsistent device performance between the two different scan directions.[14–16] Although the origin of such hysteresis effect is still under debate, one of the potential explanations is the ion migration process in the device during measuring current–voltage characteristics. Grätzel group reported that charge carrier collection efficiencies strongly depend on the built-in potential.[17] They believed that …