Fibril ZnO as an Electron Transporting Layer for Enhancing the Photovoltaic Performance of Inverted Polymer Solar Cells Based on Nonfullerene Acceptors

Despite the rapid progress of organic solar cell research due to nonfullerene acceptors, additional research efforts are still required to achieve competitive power conversion efficiency. Here, we demonstrate the effectiveness of fibrous ZnO as an electron transport layer for enhancing the performance of nonfullerene-based solar cells with the feature ITO/ZnO/PBDBT: ITIC/MoO3/Ag. To obtain fibrous morphology, ZnO was synthesized using the sol-gel method on ITO substrates by varying precursor concentrations, namely, 0.1 0.5, 0.75, and 1 M. The fibrous morphology of ZnO emerged at 0.5 M; beyond this threshold concentration, the roughness and size of ZnO fibers increased without altering the basic nature of the morphology. However, the best-performing inverted polymer solar cells were obtained from fibril ZnO films with 0.5 M used as an ETL. The champion cell based on 0.5 M exhibited a power conversion efficiency of 11.76%, exceeding cells based on ZnO films prepared from the 1 M precursor by over 49% and superseding the cells based on planar ZnO films by similar to 31%. The superior performance of 0.5 M ZnO-based cells is attributed to efficient charge collection and transportation due to the intimate contact between ZnO and the active layer, facilitated by the optimal roughness of the fibrous morphology.