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

Abstract

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 cm2 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.