Fused thiophene/quinoxaline low band gap polymers for photovoltaic's with increased photochemical stability

We investigate a family of low band-gap polymers based on the common acceptor moiety 2,3-bis-(3-octyloxyphenyl)quinoxaline (Q) combined with thiophene (T) or the fused thiophene systems: benzo[2,1-b:3,4-b′]-dithiophene (BDT) or dithieno[3,2-b,2′,3′-d]-thiophene (DTT). The photochemical stability of the three polymers was examined and compared to P3HT. They were found to be substantially more robust than P3HT with a ranking of DTTQ>BDTQ>TQ1P3HT, indicating that the fused ring systems of DTT and BDT impart a large degree of photochemical stability than thiophene. Furthermore devices with normal and inverted geometry were prepared and tested in air. The normal geometry devices showed the highest efficiencies compared to the inverted, in particular owing to a higher Voc, with TQ1 being the most efficient with a power conversion efficiency (PCE) of 1.5% (1000 W m−2, AM1.5 G). For the inverted devices TQ1 and DTTQ showed the best PCEs of 0.9%.