Interface Design to Improve the Performance and Stability of Solution-Processed Small-Molecule Conventional Solar Cells

A systematic study on the effect of various cathode buffer layers on the performance and stability of solution-processed small-molecule organic solar cells (SMOSCs) based on tris{4-[5-(1,1-dicyanobut-1-en-2-yl)-2,2-bithiophen-5-yl]phenyl}amine (N(Ph-2T-DCN-Et)(3)):6,6-phenyl-C71-butyric acid methyl ester (N(Ph-2T-DCN-Et)(3):PC70BM) is presented. The power conversion efficiency (PCE) in these systems can be significantly improved from approximately 4% to 5.16% by inserting a metal oxide (ZnO) layer between the active layer and the Al cathode instead of an air-sensitive Ba or Ca layer. However, the low work-function Al cathode is susceptible to chemical oxidation in the atmosphere. Here, an amine group functionalized fullerene complex (DMAPA-C-60) is inserted as a cathode buffer layer to successfully modify the interface towards ZnO/Ag and active layer/Ag functionality. For devices with ZnO/DMAPA-C-60/Ag and DMAPA-C-60/Ag cathodes the PCEs are improved from 2.75% to 4.31% and to 5.40%, respectively, compared to a ZnO/Ag device. Recombination mechanisms and stability aspects of devices with various cathodes are also investigated. The significant improvement in device performance and stability and the simplicity of fabrication by solution processing suggest this DMAPA-C-60-based interface as a promising and practical pathway for developing efficient, stable, and roll-to-roll processable SMOSCs.