Y‐Type Non‐Fullerene Acceptors with Outer Branched Side Chains and Inner Cyclohexane Side Chains for 19.36% Efficiency Polymer Solar Cells

Abstract Raising the lowest unoccupied molecular orbital (LUMO) energy level of Y‐type non‐fullerene acceptors can increase the open‐circuit voltage ( V oc ) and thus the photovoltaic performance of the current top performing polymer solar cells (PSCs). One of the viable routes is demonstrated by the successful Y6 derivative of L8‐BO with the branched alkyl chains at the outer side. This will introduce steric hindrance and reduce intermolecular aggregation, thus open up the bandgap and raise the LUMO energy level. To take further advantages of the steric hindrance influence on optoelectronic properties of Y6 derivatives, two Y‐type non‐fullerene acceptors of BTP‐Cy‐4F and BTP‐Cy‐4Cl are designed and synthesized by adopting outer branched side chains and inner cyclohexane side chains. An outstanding V oc of 0.937 V is achieved in the D18:BTP‐Cy‐4F binary blend devices along with a power conversion efficiency (PCE) of 18.52%. With the addition of BTP‐eC9 to extend the absorption spectral coverage, a remarkable PCE of 19.36% is realized finally in the related ternary blend devices, which is one of the highest values for single‐junction PSCs at present. The results illustrate the great potential of cyclohexane side chains in constructing high‐performance non‐fullerene acceptors and their PSCs.