Modulating Binding Strength and Acidity of Benzene‐Derivative Ligands Enables Efficient and Hysteresis‐Free Perovskite/Silicon Tandem Solar Cells

Passivating defects at the wide-bandgap perovskite/C₆₀ interface without impeding interfacial charge transport can effectively enhance the efficiency of perovskite/silicon tandem solar cells (TSCs). Herein, we study the impact of benzene-derivative ligands with elaborately modulated binding strength and acidity on wide-bandgap perovskites for high-performance perovskite/silicon TSCs. Specifically, the acidity/alkalinity and binding strength are preliminarily tuned using different functional groups of -PO₃H₂, -COOH, and -NH₂, and further finely adjusted by altering the chain lengths between the benzene ring and the functional groups. The results show that strong binding is indispensable for effectively suppressing voltage loss. However, the commonly used benzylphosphonic acid (BPPA) for firm surface binding exhibits too strong acidity that can etch the perovskite surface, resulting in halide-vacancy defects and pronounced hysteresis. Increasing the side chain length of BPPA to (2-phenylethyl)phosphonic acid not only enables a suitable acid dissociation constant (pKa) to avoid acid-induced etching but also achieves robust anchoring to the perovskite surface with a parallel adsorption orientation, which reduces the charge transport barrier at the interface. These properties enable strong-adsorption surface termination (SAST) of the perovskite surface while preventing acid-induced etching. As a result, the SAST strategy achieves a remarkable efficiency of 32.13% (certified 31.72%) for hysteresis-free perovskite/silicon TSCs.