Reduced Nonradiative Recombination Energy Loss Enabled Efficient Polymer Solar Cells via Tuning Alkyl Chain Positions on Pendent Benzene Units of Polymers

Nonradiative recombination energy loss (Î"E₃) plays a key role in enhancing device efficiencies for polymer solar cells (PSCs). Until now, there is no clear resolution for reducing Î"E₃ via molecular design. Herein, we report two conjugated polymers, PBDB-P-p and PBDB-P-m, which are integrated from benzo[1,2-b:4,5-b′]dithiophene with alkylthio chain substituted at para-or meta-position on pendent benzene and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione. Both the polymers have different temperature-dependent aggregation properties but similar molecular energy levels. When BO-4Cl was used as an acceptor to fabricate PSCs, the device of PBDB-P-p:BO-4Cl displayed a maximal power conversion efficiency (PCE) of 13.83%, while the best device of PBDB-P-m:BO-4Cl exhibited a higher PCE of 14.12%. The close J_SCs and fill factors in both PSCs are attributed to their formation of effective nanoscale phase separation as confirmed by atomic force microscopy measurements. We find that the PBDB-P-m-based device has 1 order of magnitude higher electroluminescence quantum efficiency (EQE_EL) than in the PBDB-P-p-based one, which could arise from the relatively weak aggregation in the PBDB-P-m-based film. Thus, the PBDB-P-m-based device has a remarkably enhanced V_OC of 0.86 V in contrast to 0.80 V in the PBDB-P-p-based device. This study offers a feasible structural optimization way on the alkylthio side chain substitute position on the conjugated polymer to enhance V_OC by reducing nonradiative recombination energy loss in the resulting PSCs.