Incorporation of a Guaiacol‐Based Small Molecule Guest Donor Enables Efficient Nonfullerene Acceptor‐Based Ternary Organic Solar Cells

To improve the power conversion efficiency (PCE) of the organic solar cells (OSCs), it is necessary to widen the absorption profile of the active layer. It is possible by using a ternary active layer consisting of either two donors (D) and one acceptor (A) or two acceptors and one donor having complementary absorption and appropriate frontier energy levels for efficient exciton generation and their dissociation into free charge carriers and subsequent charge/energy transfer. Herein, a large bandgap guaiacol‐based small molecule (SMD) is used as guest donor in ternary OSCs to improve the PCE and suppress the energy loss. SMD exhibits a lager bandgap and deeper highest occupied molecular orbital (HOMO) energy level compared with conjugated polymer donor (P). Therefore, the HOMO energy level is effectively down‐shifted when P is mixed with SMD, which is beneficial for attaining high open circuit voltage (VOC). OSCs based on the optimized ternary blend P:SMD:Y6 (0.8:0.2:1.2, w/w) after solvent vapor annealing attained a higher VOC of 0.85 V and low energy loss of 0.51 eV compared with the binary device P:Y6 (1:1.2, w/w) with VOC of 0.81 V and energy loss of 0.55 eV, delivering an overall PCE of 15.37%. After the incorporation of guaiacol‐based small molecule donor (SMD) in the single binary bulk heterojunction (BHJ) active layer, the ternary (P:SMD:Y6) organic solar cell shows overall power conversion efficiency of 15.37% with low energy loss of 0.51 eV, which is higher than that for the binary counterpart P:Y6, i.e., 11.98% with energy loss of 0.55 eV.