Higher open circuit voltage caused by chlorinated polymers endows improved efficiency of binary organic solar cell

Organic solar cell (OSC) has achieved great progress in the past few years. Power conversion efficiency (PCE) has stepped into a new stage due to the evolution of non-fullerene acceptors (NFAs). The gap between lowest unoccupied molecular orbit of acceptor and highest occupied molecular orbit of donor (|E_LUMOA-E_HOMOD|) is proportional to the value of open circuit voltage (V_OC). Applying two similar polymeric donors with different energy levels offers possibility of changing V_OC without significantly impacting short circuit current (J_SC) and fill factor (FF). Here we chose halogenated polymers PM6 and PM7 as donors to cope with a newly design asymmetric molecule TPIC-4Cl derived from IT-4Cl. As a result, the PM7:TPIC-4Cl device achieved a PCE of 15.1% than its PM6:TPIC-4Cl counterparts (14.4%), wherein the improvement of V_OC from 0.855 V to 0.885 V contributed most. Our work proves the feasibility of improving photovoltaic performance of NFA OSCs by utilizing polymeric donors with similar structure yet different HOMOs. Besides, the PCE over 15% of OSCs involving no Y6 or its derivatives sheds light on another direction of OSC research.