Publication date: 15th December 2025
Organic solar cells (OSCs) offer lightweight, flexible, and solution-processable photovoltaic platforms, yet their commercial viability remains hindered by limited operational stability. The inverted (n–i–p) device architecture demonstrates enhanced stability and superior compatibility with large-scale roll-to-roll fabrication, largely owing to the inorganic zinc oxide (ZnO) electron transport layer (ETL). However, intrinsic defects and photocatalytic activity in sol–gel-processed ZnO often trigger interfacial recombination and degradation of non-fullerene acceptors (NFAs), thereby constraining both power conversion efficiency (PCE) and longevity. Despite ongoing efforts to mitigate these issues, few strategies have simultaneously tackled the multiple intrinsic limitations of ZnO.
In this work, we introduce two new fullerene-based self-assembled monolayers (SAMs) engineered as multifunctional interfacial modifiers for ZnO. These SAMs effectively passivate surface defects, suppress photocatalytic activity, tune surface energy, and optimize the ZnO work function while maintaining the excellent electron extraction capability of fullerenes. Incorporating these SAMs enabled record-level PCE approaching 19.5% in ternary OSCs, ranking among the highest reported for inverted configurations. Moreover, devices employing modified ZnO demonstrated remarkable durability.
Using transient absorption spectroscopy (TAS), we further elucidate the underlying interfacial charge-transfer (CT) mechanism, showing that SAM-induced CT states promote efficient charge extraction and minimize non-geminate recombination. These insights establish a clear molecular-to-device correlation, providing a mechanistic foundation for the superior performance and stability observed. Altogether, this study introduces a generalizable molecular design strategy for multifunctional n-type SAMs, advancing interfacial engineering principles for next-generation OSCs.
H.-L.Y. acknowledges financial support from the Research Grant Council (RGC) of Hong Kong (GRF No. 11307323), the NSFC/RGC Collaborative Research Scheme (CRS_CityU104/23), and the ITF grant (GHP/394/22GD) from Innovation and Technology Commission of Hong Kong, and the Seed Collaborative Research Fund (No. SCRF/0069) provided by the State Key Laboratory of Marine Environmental Health at City University of Hong Kong. A.K.Y.J. thanks the sponsorship of the Lee Shau-Kee Chair Professor (Materials Science), and the support from the APRC Grants (9380086, 9610419, 9610440, 9610492, 9610508) of the City University of Hong Kong, the MHKJFS Grant (MHP/054/23), TCFS grant (GHP/121/22SZ) and MRP Grant (MRP/040/21X) from the Innovation and Technology Commission of Hong Kong, and the GRF grants (11307621, 11316422, 11308625) and CRS grants (CRS_CityU104/23, CRS_HKUST203/23) from the Research Grants Council of Hong Kong. This work was partially financially supported by City University of Hong Kong (9610739) for the project “Fostering Innovation for Resilience and Sustainable Transformation,” officially endorsed by the United Nations Educational, Scientific and Cultural Organization under the International Decade of Sciences for Sustainable Development (2024–2033).
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