tudy on the electric structure and ferroelectricity in epitaxial CsSnI 3 fi lms †

Ferroelectricity is a potentially crucial issue in inorganic halide perovskites, which are breakthrough materials in photovoltaic research. However, to date, conclusive evidence for ferroelectricity in allinorganic halide perovskites is still lacking. Herein, using density functional theory simulations and symmetry analysis, it is found that two stable ferroelectric phases, P4bm and Pmc21-(I), can be induced by compressive and tensile strain in all-inorganic halide perovskite CsSnI3 films. More importantly, the calculated polarization value is as high as several mC cm 2 which is desirable for the separation of photo-excited carriers in these materials. In addition, it is found that the P4bm phase has an indirect band gap and the Pmc21-(I) phase has a suitable direct band gap for the absorption of visible light. In particular, tensile strain can alter the band gaps by several tenths of an electronvolt in the Pmc21-(I) phase. Overall, these results give useful insight for strained CsSnI3 films and hopefully provide a new route to design ferroelectric semiconductors for photovoltaic materials in the less-explored inorganic halide perovskites.