Unified Bulk and Interface Control of Tin‐Lead Perovskite via a Molecular Bridge for Efficient All‐Perovskite Tandem Solar Cells

ABSTRACT Mixed tin‐lead perovskite solar cells (Sn‐Pb PSCs), as a key candidate for next‐generation photovoltaics owing to their narrow bandgap enabling efficient near‐infrared photon harvesting and suitability for tandem devices. However, their performance is hindered by challenges such as the acidity and hygroscopicity of commonly used hole‐transport layers like PEDOT:PSS, which degrade the perovskite, as well as energy level misalignment at the hole transport layer (HTL)/perovskite interface that aggravates defect‐mediated recombination. To overcome these issues, we introduce (R)‐2‐amino‐2‐phenylacetic acid hydrochloride (APHH) as a modifier for PEDOT:PSS. Featuring a chiral amino group, a carboxyl moiety, and a hydrochloride substituent, APHH neutralizes the acidic nature of PEDOT:PSS, passivates undercoordinated ions to suppress defects, optimizes energy level alignment, and enhances charge transport through π‐πstacking interactions. This modification improves the nucleation, crystallinity, and interfacial properties of the Sn–Pb perovskite, elevating the power conversion efficiency from 21.90% to 23.30%. The optimized devices also demonstrate enhanced stability, retaining 85% of their initial efficiency after 1,500 h in a nitrogen atmosphere. Furthermore, integration of this tailored narrow‐bandgap Sn‐Pb subcell enables all‐perovskite tandem solar cells with a remarkable 28.42% efficiency. This work highlights how optimizing NBG PSCs via PEDOT:PSS interface engineering advances high‐performance all‐perovskite tandem photovoltaics.