Atmospheric pressure chemical vapor deposition of methylammonium bismuth iodide thin films

We demonstrate the atmospheric pressure chemical vapor deposition of methyl ammonium bismuth iodide ((CH₃NH₃)₃Bi₂I₉ or MA₃Bi₂I₉) films. MA₃Bi₂I₉ possesses an indirect optical bandgap of 1.80 eV and a room temperature excitonic peak at 511 nm. In contrast to recent reports, the films are n-type semiconductors with a room temperature carrier concentration of 3.36 × 10¹⁸ cm^-3 and a Hall mobility of 18 cm² V^-1 s^-1, which are superior to those of solution-processed, undoped films. The precursors used for the deposition are methylammonium iodide and bismuth iodide which are co-sublimated at 199 °C and 230 °C, respectively, in an Ar flow inside a tube furnace with a variable temperature profile. The substrate temperature is set at 160 °C, and dense polycrystalline films (∼775 nm thick) are deposited. Extensive characterization combined with first-principles density functional theory calculations unravels the synthesis-structure-property relationship in these films. The degradation of properties under ambient conditions results from film oxidation with a characteristic bi-exponential decay in resistivity, signifying a fast surface oxidation followed by a slower oxidation of the bulk.