Solution-Processed Ternary Tin (II) Alloy as Hole-Transport Layer of Sn–Pb Perovskite Solar Cells for Enhanced Efficiency and Stability

Tin–lead (Sn–Pb) narrow-bandgap (NBG) perovskites show great potential in both single-junction and all-perovskite tandem solar cells. Sn–Pb perovskite solar cells (PSCs) are still limited by low charge collection efficiency and poor stability. Here, a ternary Sn (II) alloy of SnOCl is reported as the hole-transport material (HTM) with a work function of 4.95 eV for Sn–Pb PSCs. The solution-processed SnOCl layer has a texture structure that not only reduces the optical loss of the devices, but also changes grain growth of Sn–Pb perovskites and boosts the carrier diffusion length to 3.63 µm. The formation of small perovskite grains at the HTM/perovskite interface is suppressed. These result in an almost constant internal quantum efficiency (IQE) of 96 ± 2% across the absorption spectrum of Sn–Pb perovskites. The SnOCl HTM significantly enhances the stability of Sn–Pb PSCs with 87% of its initial efficiency retained after 1-sun illumination for 1200 h, and keeps 85% efficiency under 85 °C thermal stress for 1500 h. The hybrid HTM further improves the stabilized efficiencies of single-junction Sn–Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multicomponent metal alloys as HTM in PSCs.