Two-dimensional perovskite capping layer for stable and efficient tin-lead perovskite solar cells

Mixed tin-lead iodide perovskites exhibit the characteristics of low toxicity and improved light harvesting ability up to nearinfrared (NIR) spectral region, making them as an attractive alternative for traditional lead based perovskites. However, the performance of lead-based perovskites solar cells is still far inferior to their lead analogues owing to the unavoidable oxidation of Sn²⁺ to Sn⁴⁺. Here we introduced two-dimensional (2D) perovskite on the top of three dimensional (3D) perovskite film as a capping layer to reduce the self-oxidation, and thus improved the device stability. 2D capping layer was then confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The existence of the 2D protecting thin layer significantly reduce the spontaneous Sn²⁺ oxidation, thus improve the device performance and reduce the hysteresis. The phenomena could be ascribed to the improved charge extraction efficiency causing by prohibited nonradiative recombination. On top of this, the photovoltaic devices based on conventional-structure configuration were fabricated. Taking advantage of the 2D capping layer, 2D/3D hybrid perovskite photovoltaic devices achieve a open-circuit voltage (V_oc) of 0.77 V with short circuit current density (J_sc) of 26.60 mA cm⁻², delivering the best-performing power conversion efficiency of 15.5%. Moreover, the 2D/3D perovskite devices maintained 60% its initial efficiency after 40 h exposed in air (humidity around 30%, temperature 22 °C), while 3D perovskite-based devices completely failed.