Dipole-engineered co-adsorbed self-assembled monolayers boost Cs-based inorganic Pb-Sn perovskite tandem efficiency beyond 20%

Self-assembled monolayers (SAMs) have emerged as pivotal components for achieving high-performance perovskite solar cells (PSCs). However, most high-efficiency PSCs employing SAMs utilize Pb-based wide-bandgap (WBG) perovskites, whereas their application in tandem-compatible Pb-Sn mixed narrow-bandgap (NBG) perovskites remains scarcely documented. Herein, high-performance PSCs with Cs-based inorganic Pb-Sn absorbers are fabricated through dipole engineering via co-assembling 4-[(4-aminophenyl)diazenyl]benzenesulfonic acid (4-ASA) with Me-4PACz on NiOx. We demonstrated that 4-ASA induces intermolecular interactions with Me-4-PACz, inhibiting SAM aggregation and micelle formation. This enables the formation of co-SAMs with oriented dipole moments directed from NiOx to the perovskite, thereby effectively optimizing the energy-level alignment at the buried interface. Moreover, the co-SAMs interlayer exhibits a superior hydrophilic surface, serving as an ideal nucleation template for perovskite deposition, contributing to perovskite films with increased grain boundary groove (GBG) angles, suppressed interfacial Sn2+ oxidation, and mitigated residual tensile stress. As a result, the NBG Pb-Sn-based inorganic PSCs achieve a scanned power conversion efficiency (PCE) of 17.82% with outstanding ultraviolet irradiation and operational stability. Furthermore, by integrating NBG (1.36 eV, CsPb0.6Sn0.4I3) and WBG (1.74 eV, CsPb0.7Sn0.3IBr2) subcells, we demonstrate the first four-terminal (4 T) perovskite tandem device in which both subcells are based on Cs-based Pb-Sn mixed inorganic absorbers, achieving a total scanned PCE of 20.36%. Self-assembled monolayers can improve perovskite solar cells, but their use in lead and tin devices for tandem cells is limited. Zhang et al. designed a mixed layer that improved interfaces and crystal growth, producing stable cells and a tandem efficiency of 20.36%.