All‐Inorganic Perovskite CsSnBr 3 as a Thermally Stable, Free‐Carrier Semiconductor

Hybrid organic‐inorganic perovskites, especially methylammonium lead triiodide (MAPbI 3 ), are intensely studied for their optoelectronic properties. The organic MA + cation is held responsible for the superior performance of MAPbI 3 but also its instability toward moisture and heat. To explore compositions beyond MAPbI 3 , we performed experiments and calculations on two isomorphous perovskites CsSnBr 3 and MASnBr 3 . CsSnBr 3 is slightly smaller than MASnBr 3 in cell dimension, but outperforms MASnBr 3 in band gap energy, charge‐carrier reduced effective mass, and optical dielectric constant all by ≈19 %. These merits accumulate to drastically cut the exciton binding energy from 33 meV for MASnBr 3 to 19.6 meV for CsSnBr 3 , making CsSnBr 3 a black, free‐carrier semiconductor. CsSnBr 3 also exhibits distinctly higher stability toward moisture and heat than its organic counterparts. These advantages suggest ecofriendly applications for CsSnBr 3 , such as tandem solar cells and direct X‐ray detectors.