A bi-functional structure with tunable electrical and optical properties for organic photovoltaic cells

We have successfully developed a C60:LiF/BCP (bathocuproine) bilayer-buffer structure to optimize both the light absorption enhancement and the exciton-blocking at the cathode interface of a small molecular photovoltaic cell based on the archetypical CuPc/C60 structure. The function of the C60:LiF layer is to serve as an optical spacer and is found to yield a peak power conversion efficiency (PCE) with a 50 wt. % LiF at a thickness of 30 nm. A BCP layer is added between the optimized C60:LiF layer and the Al metal electrode to function as a barrier for excitons. This combined bilayer-buffer structure yields an optimal performance in short circuit current (Jsc) and PCE. A detailed study using a single carrier electron-only structure and a numerical simulation of optical electric-field distribution suggests that C60:LiF layer enhances light absorption at long wavelengths.