Robust Stability of Efficient Lead-Free Formamidinium Tin Iodide Perovskite Solar Cells Realized by Structural Regulation.

The instability issue of Pb-free Sn-based perovskite is one of the biggest challenges for its application in optoelectronic devices. Herein, a structural regulation strategy is demonstrated to regulate the geometric symmetry of formamidiniumtin iodide (FASnI3) perovskite. Experimental and theoretical works show that the introduction of cesium cation (Cs+) could improve the geometric symmetry, suppress the oxidation of Sn2+, and enhance the thermodynamical structural stability of FASnI3. As a result, the inverted planar Cs-doped FASnI3-based perovskite solar cell (PSC) is shown to maintain 90% of its initial power-conversion efficiency (PCE) after 2000 h stored in N2, which is the best durability to date for 3D Sn-based PSCs. Most importantly, the air, thermal, and illumination stabilities of the PSCs are all improved after Cs doping. The PCE of the Cs-doped PSC shows a 63% increase compared to that of the control device (from 3.74% to 6.08%) due to the improved quality of the Cs-doped FASnI3 film.