Enhanced Charge Transfer, Transport and Photovoltaic Efficiency in All-Polymer Organic Solar Cells by Polymer Backbone Fluorination

We successfully designed and synthesized two BDT-BT-T (BDT=benzo[1,2-b:4,5-b'] dithiophene, BT-T=4,7-dithien-2-yl-2,1,3-benzothiadiazole) based polymers as the electron donor for application in all-polymer solar cells (all-PSCs). By adopting N2200 as the electron acceptor, we systematically investigated the impact of fluorination on the charge transfer, transport, blend morphology and photovoltaic properties of the relevant all-PSCs. A best power conversion efficiency (PCE) of 3.4% was obtained for fluorinated PT-BT2F/N2200 (BT2F=difluorobenzo[c][1,2,5] thiadiazole) all-PSCs in comparison with that of 2.7% in non-fluorinated PT-BT/N2200 (BT=benzothiadiazole) based device. Herein, all-polymers blends adopting either non-fluorinated PT-BT or fluorinated PT-BT2F exhibit similar morphology features. In depth optical spectrum measurements demonstrate that molecular fluorination can further enhance charge transfer between donor and acceptor polymer. Moreover, all-polymer blends exhibit improved hole mobilities and more balanced carriers transport when adopting fluorinated donor polymer PT-BT2F. Therefore, although the PCE is relatively low, our findings may become important in understanding how subtle changes in molecular structure impact relevant optoelectronic properties and further improve the performance of all-PSCSs.