The low toxicity and a near-ideal choice of bandgap make tin perovskite an attractive alternative to lead perovskite in low cost solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low-dimensional Sn perovskites that exhibit markedly enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly oriented low-dimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 h and found no appreciable decay in efficiency. These findings raise the prospects of pure Sn perovskites for solar cells application.

Hybrid organic-inorganic perovskites, especially methylammonium lead triiodide (MAPbI ), are intensely studied for their optoelectronic properties. The organic MA cation is held responsible for the superior performance of MAPbI but also its instability toward moisture and heat. To explore compositions beyond MAPbI , we performed experiments and calculations on two isomorphous perovskites CsSnBr and MASnBr . CsSnBr is slightly smaller than MASnBr in cell dimension, but outperforms MASnBr 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 to 19.6 meV for CsSnBr , making CsSnBr a black, free-carrier semiconductor. CsSnBr also exhibits distinctly higher stability toward moisture and heat than its organic counterparts. These advantages suggest ecofriendly applications for CsSnBr , such as tandem solar cells and direct X-ray detectors.