The modification of ZnO with 2D g-C3N5 can effectively fill surface oxygen vacancies and eliminate free radicals (OH), significantly improving the work function (WF) of ZnO. First-principles calculations reveal that a small number of surface defects in ZnO facilitates its surface modification.

ZnO has been traditionally applied in organic solar cells (OSCs) as an electron transport layer (ETL). However, inevitable vacancy defects on the surface of ZnO result in trap-assisted recombination centers and thus low-efficiency electron transport in OSCs. Herein, an effective and facile method has been developed to modify the ZnO surface with 2D g-C₃N₅ for high-performance and stable OSCs. The results show that 2D g-C₃N₅ can effectively passivate various defects on the surface of ZnO, such as oxygen vacancies and OH, leading to the reduction of the work function of the ZnO layer. The combination of theoretical calculations and experimental characterizations reveals a charge transfer mechanism between g-C₃N₅ and the ZnO surface and a physical mechanism of oxygen vacancy filling in ZnO. Furthermore, with 1 wt% g-C₃N₅-modified ZnO as the ETL, inverted OSCs based on 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[methylidyne(5,6-chloro-3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (BTP-eC9) and PM6:L8-BO:BTP-eC9 exhibit the highest the power conversion efficiency (PCE) of 18.01% and 18.84%, respectively, which is much higher than that for the corresponding reference devices without the modified ETL (16.32% and 17.67%). Therefore, this study provides an effective and facile way for the defect modification of ZnO by 2D materials, and offers a deep understanding of the passivation mechanism of ZnO defects.