Enhancing the crystallinity and perfecting the orientation of formamidinium tin iodide for highly efficient Sn-based perovskite solar cells

Low power conversion efficiency (PCE) and poor reproducibility are among the main challenges for tin-based perovskite solar cells (HPSCs). The facile formation of tin vacancies and oxidation of the divalent tin cation during the thin film fabrication process are among the causes of these problems, because the tin perovskite layer then becomes p-doped, resulting in significant trap-assisted recombination losses in devices. In this paper, we demonstrate that increasing the crystallinity of the tin perovskite film is an effective way to address the open issues with Sn-based perovskites. We succeed in improving the crystallinity of the 3D formamidinium tin iodide (FASnI(3)) grains, increasing their size, and perfecting their orientation in the out-of-plane direction by incorporating ethylammonium iodide (EAI) into a 2D/3D tin perovskite film (where 2D is PEA(2)FASn(2)I(7), PEA = phenylethylammonium). This leads to a decrease of traps and background charge carrier density, and therefore to decreased charge recombination losses in EA(x)2D/3D based devices, as compared not only to devices based on FASnI(3) but also to those based on 2D/3D mixtures. As a consequence, devices using a perovskite layer with composition EA(0.08)2D/3D exhibit much higher PCE (8.4%) and better reproducibility compared to devices based on mixed 2D/3D perovskites (7.7%) and 3D perovskite (4.7%).