Atmospheric pressure chemical vapor deposition of methylammonium bismuth iodide thin films

摘要

We demonstrate the atmospheric pressure chemical vapor deposition of methyl ammonium bismuth iodide ((CH3NH3)(3)Bi2I9 or MA(3)Bi(2)I(9)) films. MA(3)Bi(2)I(9) 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 x 10(18) cm(-3) and a Hall mobility of 18 cm(2) 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 degrees C and 230 degrees C, respectively, in an Ar flow inside a tube furnace with a variable temperature profile. The substrate temperature is set at 160 degrees C, and dense polycrystalline films (similar to 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.