31. Chemical vapor deposition of methylammonium bismuth iodide

Atmospheric pressure chemical vapor deposition of methylammonium bismuth iodide thin films

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.