Substituted thiourea as versatile ligands for crystallization control and surface passivation of tin-based perovskite

Tin-based perovskite solar cells could replace their toxic, lead-containing counterparts and have been developed rapidly to exceed a power conversion efficiency (PCE) of 14%. However, further improvement is limited by poor film quality, largely because of fast crystallization and the tendency of tin-based perovskites toward defects. Here, we report that substituted thiourea can effectively slow the crystallization of formamidinium tin triiodide (FASnI3) and inhibit its surface defects. Thus, pinhole-free films comprising tiled FASnI3 crystallites are fabricated reproducibly. Annealed films retain a small percentage of the residual ligand, which can account for the observed carrier lifetimes of as long as 43 ns. By tuning the ligand structure, we progressively enhance the open-circuit voltage and short-circuit current density of our solar cells and attain PCEs of over 12%. Our simple solution simultaneously addresses two key issues of FASnI3 and paves the way for more efficient and robust tin-based perovskite solar cells.