Growth of ZnO/Bi2S3 electron transport layer films to improve the efficiency and stability of organic solar cells

Zinc oxide (ZnO) grown on indium tin oxide (ITO) glass substrates were decorated with bismuth sulfide (Bi2S3) nanorods by thermal decomposition of bismuth ethylxanthate precursor method. Different thicknesses of ZnO/Bi2S3 bilayer films were prepared by controlling the spin speed of bismuth xanthate precursors (5000, 6000 and 7000 rpm, respectively) and applied as the electron transport layer (ETL) in organic solar cells (OSCs) with the structure of ITO/ZnO/Bi2S3/PM6:Y6/MoO3/Ag. The effects of different thickness of ZnO/Bi2S3 as ETLs on device performance and stability were investigated in detail. Firstly, the device based on ZnO/Bi2S3 6000 rpm as ETL confirmed excellent performance with the power conversion efficiency (PCE) of 12.50%, which was improved by 28.60% of the reference device without Bi2S3 nanorods coated. XRD and SEM results showed that Bi2S3 nanorods covered the ZnO surface well. Electrical conductivity and the current density-voltage (J-V) curve results under dark conditions showed that when Bi2S3 nanorods were added between the active layer and ZnO, the interfacial conductivity can be effectively increased and improved the interfacial carrier transport efficiency. Secondly, the stability results of the ZnO/Bi2S3 6000 rpm device demonstrated that the PCE of ZnO/Bi2S3 6000 rpm device was maintained about 91.54%, while that of pure ZnO as ETL was maintained about 75.26% of the original value after 33 h in air. This means that the ZnO/Bi2S3 prepared by thermal decomposition of the bismuth xanthate precursor on ZnO is a promising electron transport material in OSCs.