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 (iVOC) 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-VOC and current–voltage analysis. Maximum power point tracking demonstrated enhanced stabilized efficiencies. All-perovskite tandem solar cells incorporating HTL4 exhibited both increased iVOC and overall performance, outperforming tandem devices employing the standard HTL.