Tetrabromination versus Tetrachlorination: A Molecular Terminal Engineering of Nonfluorinated Acceptors to Control Aggregation for Highly Efficient Polymer Solar Cells with Increased V_oc and Higher J_sc Simultaneously

Herein, a pair of tetrahalogenated nonfullerene small molecular acceptors (NF-SMAs) (TSeIC4Cl and TSeIC4Br) are designed and synthesized, with the same indacenodithieno[3,2-b]selenophene central unit and two different dihalogenation terminal groups, respectively. The systematic investigation is achieved to reveal the impact of two different nondifluorinated terminal groups on the device performance of the resultant ITIC series NF-SMAs. TSeIC4Br shows red-shifted absorption range and higher frontier energy levels compared with that of TSeIC4Cl. Moreover, PM6:TSeIC4Br blend film exhibits more suitable phase-separated morphology with more ordered molecular packing and improved miscibility compared with that of the tetrachlorinated counterpart. Importantly, PM6:TSeIC4Br-based device exhibits a better power conversion efficiency (PCE) of 11.92%, with a higher open-circuit voltage (V_oc) and an enhanced short-circuit current density (J_sc) when compared with that of PM6:TSeIC4Cl-based device (11.13%). Furthermore, the energy loss can be reduced by replacing the disubstituents of end group from chlorine to bromine atoms. The results demonstrate that incorporation of indacenodithieno[3,2-b]selenophene and replacement of tetrachlorination by tetrabromination on the end group contributes to elevating the J_sc, reduce energy loss, and enhancing the PCE for its relevant ITIC series device simultaneously, which may give a new avenue for achieving high-performance multihalogenated ITIC series NF-SMAs.