Formic acid-engineered PEDOT:PSS for enhanced crystallinity and performance in tin–lead perovskite solar cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have shown tremendous promise due to their excellent optoelectronic properties and cost-efficient fabrication. However, the efficiency of traditional lead halide PSCs is approaching the Shockley–Queisser limit, prompting interest in tin-lead perovskite solar cells (Eg ≈ 1.25 eV) as a candidate for tandem configurations with the potential to surpass this limit. A key challenge lies in optimizing the hole transport layer (HTL), as widely used PEDOT:PSS suffers from high acidity and poor crystallinity, hindering device performance. In this work, we used a formic acid modification of PEDOT:PSS to enhance its conductivity, energy band alignment, and crystallinity. Acid treatment promotes proton transfer, reducing insulating PSS chains and improving phase separation, thereby facilitating efficient hole transport. Tin–lead perovskite films fabricated on formic acid-treated PEDOT:PSS (Fa-PEDOT:PSS) exhibit improved crystallinity, larger grain size, and reduced defect density. Devices incorporating Fa-PEDOT:PSS demonstrate enhanced photovoltaic performance, achieving a power conversion efficiency (PCE) of 21.87% with reduced hysteresis and excellent stability, retaining ∼90% of initial efficiency after 1600 h in an inert atmosphere. These findings highlight the potential of acid-treated PEDOT:PSS as an optimized HTL for tin–lead PSCs, paving the way for high-efficiency, environmentally friendly photovoltaic technologies.