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

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 NiO_x. 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 NiO_x 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, CsPb_0.6Sn_0.4I₃) and WBG (1.74 eV, CsPb_0.7Sn_0.3IBr₂) 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%.