A chemically bonded monolayer interface enables enhanced thermal stability and efficiency in Pb-Sn perovskite solar cells

Advances in narrow-band-gap (NBG) mixed lead-tin (Pb-Sn) perovskites have enabled increasingly efficient all-perovskite tandem solar cells, yet device stability remains limited by acidic poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) hole transport materials (HTMs). Although carbazole-based self-assembled monolayers (SAMs) were considered as alternatives, they also degrade rapidly (T80 < 200 h) under external stresses. We identified weak chemical interaction at the transparent conductive oxide:SAM:perovskite interface and hypothesized that stronger binding could enhance stability. Introducing bifunctional SAMs with thiol groups established robust S-Pb chemical coordination, improving fracture energy by 30%. Replacing acidic phosphonic groups with milder carboxylic groups and optimizing SAM chain length led to selecting 16-mercaptohexadecanoic acid (16-MHDA), balancing coverage, energy alignment, and series resistance. This approach doubled photocarrier lifetime and increased thermal degradation resistance by 1.3×. Single-junction Pb-Sn cells achieved 24% power conversion efficiency (PCE) and encapsulated devices retained 80% efficiency after 680 h under 1-sun illumination at a heatsink temperature of 50°C.