Multi‐Functional Silole Hole Transport Layer for Efficient and Stable Lead–Tin Perovskite and Tandem Solar Cells

Abstract Despite high theoretical efficiencies and rapid improvements in performance, high‐efficiency ≈1.2 eV mixed Sn–Pb perovskite solar cells (PSCs) generally rely on poly(3,4‐ethylenedioxythiophene) polystyrenesulfonate (PEDOT: PSS) as the hole transport layer (HTL); a material that is considered to be a bottleneck for long‐term stability due to its acidity and hygroscopic nature. Seeking to replace PEDOT: PSS with an alternative HTL with improved atmospheric and thermal stability, herein, a silole derivative (Silole‐COOH) tuned with optimal electronic properties and efficient carrier transport by incorporating a carboxyl functional group is designed, which results in an optimal band alignment for hole extraction from Sn–Pb perovskites and robust air and thermal stability. Thin films composed of the Silole‐COOH exhibit superior conductivity and carrier mobility compared to PEDOT: PSS, in addition to reduced nonradiative quasi‐Fermi‐level splitting losses at the HTL/perovskite interface and improved quality of Sn–Pb perovskite. Replacement of PEDOT: PSS with Silole‐COOH leads to 23.2%‐efficient single‐junction Sn–Pb PSCs, 25.8%‐efficient all‐perovskite tandems, and long operating stability in ambient air.