New architecture for thermally stable high efficiency polymer solar cells

Abstract

By applying the specific fabrication conditions such as postproduction annealing at 150oC for 30 minutes, polymer solar cells with 5% power conversion efficiency are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells. Also new architectural polymer solar cells with 5% power conversion efficiency have been fabricated using titanium oxide (TiOx) as an optical spacer. Solar cells with a TiOx layer (deposited by a sol-gel process) between the active layer and the electron collecting aluminum electrode exhibit approximately 50% enhancement in power conversion efficiency compared to similar devices without the optical spacer. The TiOx layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.