Benzodithiophene-based low band-gap polymers with deep HOMO levels: synthesis, characterization, and photovoltaic performance

A series of benzo[1,2-b:4,5-b'] dithiophene-based low band-gap polymers were synthesized using Stille coupling reactions. The polymers with electron-withdrawing side chains (P1, P2, P4 and P5) exhibited wide absorption due to the bathochromic effect derived from the electron-withdrawing side chains compared with P3 and P6, which possessed hexyl side chains. Furthermore, as donor materials, P1, P2, P4 and P5 exhibited deep HOMO levels (-5.53 to -5.65 eV) due to the incorporation of electron-withdrawing side chains. An organic photovoltaic (OPV) device with a P1/[6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) active layer achieved a power conversion efficiency (PCE) of 2.68% with an open-circuit voltage (V-oc) of 0.71 V, a short-circuit current density (J(sc)) of 10.64 mA cm(-2) and a fill factor (FF) of 0.35. By contrast, the OPV device with the P3/PC61BM active layer exhibited a PCE of 0.86% with a V-oc of 0.59 V, J(sc) of 4.93 mA cm(-2) and FF of 0.30. This comparison demonstrates that the deep HOMO level derived from the incorporation of the electron-withdrawing side chains contributed to the larger Voc and higher PCE values for P1 than those of P3.