A Coassembled PTAA:BPDT Molecular Blend Hole-Transport Layer Enables Efficient and Stable Inverted Perovskite Single‑Crystal Solar Cells

Perovskite single-crystal (SC) solar cells promise superior charge transport and lower bulk defect densities than their polycrystalline counterparts, yet they suffer from poor interfacial contact, high surface trap densities, and limited operational stability. The interfacial delamination remains a critical barrier to the operational stability of SC photovoltaics. This is primarily due to the hydrophobic and weakly interacting nature of conventional hole-transport layers (HTLs) such as PTAA in inverted solar cells. Here, we overcome these limitations by blending PTAA with a dual-functional small molecule, biphenyl-4,4-dithiol (BPDT). Spectroscopic and theoretical studies reveal that BPDT forms noncovalent secondary interactions, thereby enabling π-electron delocalization with the PTAA's aromatic rings, rendering the HTL more hydrophilic and shifting its Fermi level closer to the valence band (enhanced p-type character). The exposed thiol groups of BPDT strongly coordinate to Pb2+ at the perovskite SC surface, establishing a chemically anchored, mechanically robust interface. Using FA0.95Cs0.05PbI3 single crystals grown by inverse temperature crystallization on the PTAA:BPDT blend, the HTL exhibits markedly enhanced interfacial adhesion and long-term durability. The strong interfacial bonding suppresses delamination, enabling devices to retain 90% of their initial efficiency after 1000 h of maximum power point tracking under 50-60% relative humidity. This stability enhancement is accompanied by improved photovoltaic performance, with a champion power conversion efficiency of 24.85%, compared with 22.95% for pristine PTAA. This molecular blending strategy resolves the long-standing trade-off between efficiency and stability in perovskite single-crystal photovoltaics.