Asymmetric carbazole-based self-assembled monolayers enable simultaneous efficiency and stability enhancement in organic solar cells

Self-assembled monolayers (SAMs) have emerged as powerful interfacial modifiers for high-performance organic solar cells. Currently reported asymmetric substitution strategies have primarily focused on tuning molecular dipole moments and work functions or enhancing π-π stacking to improve interfacial quality. In contrast, our work reports an asymmetric carbazole-based SAM molecule, P-4PACz, featuring a unilateral phenyl substituent at the 3-position of the carbazole core. This asymmetric design alters the π-π stacking mode to a tightly packed yet slipped configuration, which enables ordered solid-state assembly while suppressing excessive pre-aggregation in solution. Such an approach enables a favorable balance between solution processability and interfacial ordering. Compared with its symmetric analogue 4PACz, P-4PACz exhibits reduced surface energy on indium tin oxide, improved energy-level alignment, and suppressed molecular aggregation, resulting in enhanced active-layer wetting and interfacial contact. This optimized interface promotes efficient hole extraction while mitigating interfacial recombination losses. Consequently, P-4PACz-based devices achieve a champion power conversion efficiency (PCE) of 19.03%, outperforming 4PACz (champion 18.28%) and PEDOT:PSS (champion 18.22%) controls. The superiority of P-4PACz is further validated across multiple representative systems, including PM6:L8-BO (champion 18.16%), PM6:PY-DT (champion 16.35%), PM6:Y6 (champion 16.73%), demonstrating its broad applicability. In addition to enhanced efficiency, P-4PACz-based devices exhibit improved operational stability, retaining 80% of their initial PCE after 782 h of continuous illumination.