The modification of ZnO surface with natural antioxidants to fabricate highly efficient and stable inverted organic solar cells

Zinc oxide (ZnO) has proved to be an effective electron transport layer (ETL) in organic solar cells (OSCs). However, a variety of defects on ZnO surface and improperly matched work function (WF) greatly restrict the ability of charge transportation in the OSCs. Surface modification is an effective strategy for solving these problems. In this work, the ZnO film is modified with three environment-friendly natural antioxidants (NAOs): caffeic acid (CA), ferulic acid (FA) and 3, 4-dimethoxy cinnamic acid (DMCA), respectively. The results show that the modification of NAOs not only effectively passivates the defects of ZnO surface, but also lowers the WF of ZnO by forming interface dipoles and upward shift of Fermi level of ZnO. Moreover, the inverted organic solar cells (IOSCs) with ZnO modified by NAOs as ETLs are firstly fabricated. As the result, the IOSC based on PBDB-T: ITIC obtains the increased PCE of 11.75% (ZnO/CA), 11.25% (ZnO/FA) and 10.90% (ZnO/DMCA), which is higher than those of the reference 10.15% (ZnO). Meanwhile, the PCE of IOSC based on PM6:L8-BO with ZnO/CA ETL is also obviously increased from 16.41% to 18.22%. The ambient stability of the IOSCs new ETLs is also significantly improved due to the increased hydrophobicity of ETL and the enhanced crystallinity of active layer. The novel ETLs provide a facile, eco-friendly, low-cost method to realize the efficient and stable photovoltaic devices.