Utilizing Zinc Oxide and Fluorescent Agent as a Versatile Electron Transport Layer for Highly Efficient and Stable Inverted Polymer Solar Cells

The development of excellent electron transport layers (ETLs) is crucial for high-performance organic solar cells (OSCs). In this work, we have developed a novel, versatile ETL composed of zinc oxide (ZnO) and a fluorescent agent to enhance the photovoltaic performance and photostability of OSCs. Unlike bulk doping of ZnO interlayer, we use a conjugated small-molecule fluorescent agent, sodium 2,2'-([1,1'-biphenyl]-4,4'-diyldivinylene)-bis(benzenesulfonate) (CBS), to modify the surface of ZnO interlayer, and thus construct a ZnO/CBS bilayer structure. The ZnO/CBS bilayer shows a lower work function, which is beneficial for electron extraction. Moreover, the photoinduced electron transfer from CBS to ZnO increases the conductivity of ZnO. Notably, the fluorescence generated by CBS can also be quenched by the active layer, indicating the existence of exciton or charge transfer between CBS and the active layer. The bidirectional charge transfer from CBS to ZnO and the active layer synergistically improves charge transport and enhances photovoltaic performance. Consequently, the PM6:eC9 and PM6:L8-BO based OSCs with ZnO/CBS bilayer as ETL achieve power conversion efficiencies of 17.42 and 18.16%, respectively, which are among the highest levels in inverted OSCs. Moreover, the ZnO/CBS ETL shows excellent thickness insensitivity and the PM6:eC9-based OSCs still exhibit a high PCE of 15.66% at the thick-film ETL with 130 nm ZnO and 20 nm CBS. In addition, the CBS modification efficiently blocks the ultraviolet light and reduces the catalytic activity of ZnO to the organic active layer, thereby enhancing the photostability of OSCs.