Highly efficient polymer solar cells based on low-temperature processed ZnO: application of a bifunctional Au@CNTs nanocomposite

Although ZnO electron transport layers (ETL) contribute to the world record for single-junction polymer solar cells, its sintering temperature, which is higher than 200 degrees C, imposes limits on the application of low-cost flexible devices. In this study, we report a modified low-temperature processed ZnO ETL by incorporating a composited nanostructure (Au@CNTs). Compared to the doping-free ZnO, the introduction of Au@CNTs improves the device performance in various aspects, through the initial charge generation until the following dissociation and transport processes. Furthermore, visible light soaking of the cells triggers the electric trap filling effect within the ZnO matrix, enabling the ZnO ETL to perform better in the electron extraction and transport and then reduces the interfacial recombination rate. Photoluminescence spectra together with Kelvin probes indicate that the visible light arouses a hot charge injection from Au nanoparticles (on CNTs) into ZnO, which populates the ZnO trap states and is responsible for the observed performance enhancement. As a result, a promising power conversion efficiency of 10.67% was achieved based on a state-of-the-art PTB7-Th:PC71BM photoactive system. Thus, we demonstrate that Au@CNTs nanocomposites have the potential to contribute toward promising ETL design for efficient polymer solar cells.