Abstract: The transition of organic solar cells (OSCs) from laboratory-scale demonstrations to large-scale manufacturing is critically hindered by challenges associated with interfacial materials, including their poor compatibility with active layers and the complexity of multi-step processing techniques. While interfacial self-assembly has emerged as a promising strategy to mitigate these issues, the research focus has remained predominantly on the bottom electrode/active layer interface. Consequently, the development of effective and simple strategies for in situ constructing high-performance cathode interlayers (CIL) at the top interface, between the active layer and the metal top electrode, represents a significant and unresolved challenge in the field. This study successfully demonstrates a novel and effective surface energy-driven in situ self-assembly strategy to address this top-interface engineering challenge. We utilize the functional small molecule, 9-fluorenone-1-carboxylic acid (9F-1CA), as a highly efficient CIL material. When implemented as a conventional, spin-coated interlayer in an inverted device structure of Indium tin oxide (ITO)/9F-1CA/Poly [(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)benzo [c][1,2,5]thiadiazole)] (PM6):2,2'-((12,13-bis(2-butyloctyl)-12,13-dihydro-3,9-dinonylbisthieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo [3,2-e:2',3'-g][2,1,3]benzothiadiazole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BTP-eC9)/MoO₃/Ag, 9F-1CA delivers an impressive power conversion efficiency (PCE) of 16.40%, validating its excellent intrinsic electron-transport properties. The principal innovation of our work, however, lies in the application of 9F-1CA beyond this conventional role. By exploiting its low surface energy characteristic, we blended 9F-1CA directly with the non-fullerene acceptor material BTP-eC9. During the film-forming process, a pronounced surface energy difference between 9F-1CA and the host materials provides a strong thermodynamic driving force, prompting the spontaneous migration and preferential enrichment of 9F-1CA molecules at the upper surface of the active layer. This process results in the in situ formation of a functional CIL without requiring any additional processing steps. Based on this mechanism, we fabricated a simplified device with a structure of ITO/PM6:3,4,5-Trichlorobenzoic acid (3CBA)/BTP-eC9:9F-1CA/Ag via a one-step active layer deposition process. This simplified device achieved a high PCE of 15.45%, which corresponds to a remarkable 44.53% enhancement compared to the reference device fabricated without any interlayer. Extensive characterization, including transient photovoltage/photocurrent measurements and Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) analysis, confirmed that the self-assembled 9F-1CA layer effectively optimizes the interfacial energy level alignment and significantly suppresses charge recombination losses. Furthermore, morphological studies verified that this in situ self-assembly process does not disrupt the favorable bulk heterojunction morphology of the active layer. This work breaks the confinement of traditional self-assembly strategies limited to the bottom interface and provides a new paradigm for constructing high-quality electronic interfaces. Our surface energy-driven in situ self-assembly approach offers a feasible, simplified, and efficient technical pathway, holding great promise for advancing the development of high-performance and scalable solution-processed OSCs.

Key words: Organic solar cells, Cathode interface layer, 9-Fluorenone-1-carboxylic acid, Top interface, Surface energy, In situ self-assembly