High-Performance\nTernary Polymer Solar Cells Enabled\nby Using a Branched Polymer as the Guest Donor

The\nternary strategy has been regarded as the most convenient and\nefficient method to enhance the photovoltaic performance of organic\nsolar cells (OSCs). Here, a slightly branched polymer PM6-B is developed\nby introducing a 1,3,5-tribromobenzene unit (0.02 equiv) into the\npreparation process of PM6. The resulting PM6-B exhibits a downshifted\nHOMO energy level and demonstrates excellent compatibility with PM6.\nTernary devices are constructed based on the PM6:N3 host system, in\nwhich the branched PM6-B is applied as the guest electron donor. By\nadding 20 wt % of PM6-B, the ternary device obtains the optimal photovoltaic\nperformance with simultaneously improved open-circuit voltage (<i>V</i><sub>oc</sub>) of 0.844 V and short-circuit current density\n(<i>J</i><sub>sc</sub>) of 27.83 mA cm<sup>–2</sup>, yielding a significantly increased power conversion efficiency\n(PCE) of 17.94%. The enhanced performance of the PM6:PM6-B:N3 device\nis mainly ascribed to facilitated exciton dissociation, improved charge\ncollection, and suppressed charge recombination after the integration\nof PM6-B. This work demonstrates the potential of using a branched\npolymer as an isogenous third component to construct high-efficiency\nternary OSCs.