Fluorine substituted thiophene-quinoxaline copolymer to reduce the HOMO level and increase the dielectric constant for high open-circuit voltage organic solar cells

This study reported a novel fluorinated copolymer (FTQ) and shown it to exhibit a significantly higher open circuit voltage (V-OC) in bulk heterojunction solar cells with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) compared to the low band-gap polymer Thiophene-Quinoxaline (TQ). Fluorination lowers the polymer HOMO level effectively which pushes down the highest occupied molecular orbital (HOMO) level of the TQ from -5.36 eV to -5.51 eV and increases the relative dielectric constant from 4.2 to 5.5, resulting in a high V-OC. The highest V-OC of 950 mV was achieved in the FTQ/PCBM solar cell device. For these optimized blends, the device made of FTQ:PC71BM with a 1:1 weight ratio yielded a high power conversion efficiency of 5.3% after a very short time thermal annealing process. These findings will be of importance for achieving high-performance of polymer solar cells by functional group substitution in low band gap polymers.