Reduced Energy Loss in Non-Fullerene Organic Solar Cells with Isomeric Donor Polymers Containing Thiazole π-Spacers

Large energy loss is one of the key factors that limit the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we report reduced energy losses of OSCs via introducing thiazole π-spacers with different orientations to replace the thiophene π-spacers of the prototype polymer PBDB-T. The newly formed thiazole-containing isomeric polymers, PBDBTz-2 and PBDBTz-5, exhibited blue-shifted absorption and deeper lying energy levels compared to PBDB-T. When blended with IT-4F, the two polymers realized PCEs of 10.4% for PBDBTz-2 and 9.6% for PBDBTz-5, respectively, which were higher than that of PBDB-T (PCE = 9.3%). More critically, considerable open-circuit voltage (V_oc) enhancements were achieved by PBDBTz-2 and PBDBTz-5, which were 0.14 and 0.21 V higher than that of PBDB-T. A detailed analysis showed that the reduced energy loss resulted from the lower radiative recombination below the band gap and nonradiative recombination loss. This study demonstrated that the introduction of thiazole π-spacers with different orientations is effective to reduce the energy losses of OSCs, which provided valuable inspirations for the development of new conjugated polymers to the efficiency breakthrough of OSCs in future.