Zwitterion‐Regulated Carbazole Phosphonic Acid Self‐Assembled Monolayers via Proton Transfer and Electrostatic Interactions for 21% Organic Solar Cells

The efficiency and stability of high‐performance organic solar cells (OSCs) are increasingly determined by the interfacial quality of self‐assembled monolayer (SAM)‐based hole‐selective layers. Carbazole phosphonic‐acid SAMs such as 2PACz are prone to aggregation in alcoholic precursor solutions because of phosphonic‐acid hydrogen bonding and carbazole π–π interactions, leading to incomplete coverage, nonuniform anchoring, and local electrostatic heterogeneity. Here, we incorporate trimethylglycine (TMG) into 2PACz SAMs to induce proton‐transfer‐coupled charge‐state regulation. Through acid–base association, partial proton transfer from 2PACz to TMG alters the protonation state and local electronic environment of 2PACz. At the molecular‐assembly level, this regulation, together with zwitterion‐derived electrostatic interactions, suppresses 2PACz self‐association and improves solution‐state dispersion. At the interfacial‐anchoring level, the increased phosphonate character favors charge‐assisted P–O–M coordination with ITO, enhancing anchoring stability. The resulting 2PACz:TMG layer exhibits improved coverage, surface‐potential homogeneity, and interfacial robustness, reducing recombination and leakage while facilitating charge extraction. D18:L8‐BO binary OSCs achieve a PCE of 20.52%, while ternary devices reach 21.07%. The strategy also shows broad applicability, enables 18.02% efficiency in 1 cm 2 devices, and improves thermal and light stability. These results establish proton‐transfer‐coupled charge‐state regulation as an effective route to simultaneously control molecular assembly, SAM formation uniformity, and interfacial anchoring in phosphonic‐acid SAMs.