Enhanced open-circuit voltage in methoxyl substituted benzodithiophene-based polymer solar cells

The open-circuit voltage (V_oc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high V_oc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)-T1) exhibited a HOMO value of–5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)-T1). As a result, POBDT(S)-T1-based solar cells show a high V_oc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)-T1-based devices show a relatively lower V_oc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.