X. Chen , Y. Myung , A. S. Thind , Z. Gao , B. Yin , M. Shen , S. B. Cho , P. Cheng ,B. Sadtler , R. Mishra and P. Banerjee, J. Mater. Chem. A. (2017)
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 x 1018 cm-3 and Hall mobility of 18 cm2/V·s; values superior to 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 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 unravel the synthesis-structure-property relationship in these films. Degradation of properties in ambient 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.