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Despite the widespread research on organic–inorganic hybrid perovskites, the ambient air instability and ion migration‐induced hysteresis in the current–voltage characteristics of their devices remain unsolved. Here, it is shown that stable ambient air operation of methylammonium lead iodide (MAPbI3) thin‐film transistors can be achieved by solution processing of the MAPbI3 film in ambient air via solvent engineering. N,N‐dimethylformamide (DMF), mixed with dimethyl sulfoxide (DMSO) and hydroiodic acid (HI), is found to be the most suitable solvent for one‐step deposition in ambient air. While the HI promotes homogeneous nucleation, the low boiling point of DMF and the high vapor pressure and strong Lewis base property of DMSO lead to fast crystallization and consequent large grain size. The oxygen in air passivates grain boundary defects, thereby improving lateral conduction and minimizing grain boundary defect‐mediated ion migration. This approach paves the way for the simple fabrication of hybrid perovskites by eliminating the need for well‐controlled inert environments and provides a solution for the problem of ambient air stability in perovskite devices.

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We report clear room temperature ambipolar transport in ambient-air processed methylammonium lead iodide (MAPbI3) thin-film transistors (TFTs) with aluminum oxide gate-insulators and indium-zinc-oxide source/drain electrodes. The high ionicity of the MAPbI3 leads to p-type and n-type self-doping, and depending on the applied bias we show that simultaneous or selective transport of electrons and/or holes is possible in a single MAPbI3 TFT. The electron transport (n-type), however, is slightly more pronounced than the hole transport (p-type), and the respective channel resistances range from 5–11 and 44–55 MΩ/μm. Both p-type and n-type TFTs show good on-state characteristics for low driving voltages. It is also shown here that the on-state current of the n-type and p-type TFTs is highest in the slightly PbI2-rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by varying precursor ratio.