Water‐Free PEDOT:PSS Formulation for Pb–Sn Mixed Perovskite Single‐Junction and All‐Perovskite Tandem Solar Cells

Single‐junction Pb–Sn perovskite solar cells with a 1.24 eV bandgap have recently achieved power conversion efficiencies exceeding 23%, driven by advances in absorber passivation and interface engineering. Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is widely used as a hole transport layer (HTL) due to its favorable conductivity and hole selectivity. However, its water‐based processing is incompatible with inert glovebox conditions and can degrade the underlying perovskite in all‐perovskite tandem architectures. Furthermore, the acidic and hygroscopic nature of PSS compromises long‐term device stability, particularly for narrow‐bandgap absorbers. To address these issues, an anisole‐based PEDOT:PSS formulation (HTL4) with reduced PSS content was employed. X‐ray photoelectron spectroscopy revealed modified surface composition, enhancing hydrophobicity and perovskite film formation. Steady‐state photoluminescence measurements of narrow‐bandgap perovskite films deposited on the modified HTL exhibited a 60 mV increase in implied open‐circuit voltage (i V _OC ) compared to reference devices. Single‐junction solar cells utilizing the modified HTL showed up to a 6% absolute efficiency gain, attributed to reduced series resistance and improved fill factor, as confirmed by Suns‐ V _OC and current–voltage analysis. Maximum power point tracking demonstrated enhanced stabilized efficiencies. All‐perovskite tandem solar cells incorporating HTL4 exhibited both increased i V _OC and overall performance, outperforming tandem devices employing the standard HTL.