19.35%-Efficiency organic solar cells and reduced non-radiative recombination energy loss by a ternary copolymerization strategy†

Abstract

The low open-circuit voltage (V_OC) imposed by the large energy loss, especially non-radiative recombination energy loss (ΔE_nr), accounts for the behindhand power conversion efficiency (PCE) of organic solar cells (OSCs), compared to those of silicon/perovskite solar cells. Hence, it is vital to reduce ΔE_nr to remedy the gap and further improve the PCEs. Herein, two terpolymer donors, DQ20 and DQ40, are developed via introducing a dimethyl dithieno[3,2-f:2′,3′-h]quinoxaline-2,3-dicarboxylate unit (TQC) into the backbone of D18 in consideration of the features of TQC. The introduction of TQC endows DQ20 and DQ40 with down-shifted energy levels and improved miscibility with the electron acceptor, L8-BO. As a result, the V_OC increases from D18:L8-BO (0.895 V) to DQ20:L8-BO (0.906 V) to DQ40:L8-BO (0.920 V)-based OSCs, mainly ascribed to the gradually decreased ΔE_nr. Moreover, the DQ20:L8-BO blend film exhibits fine phase separation with ordered molecular stacking, and thus achieves the highest charge carrier mobility and weakest charge recombination in devices for the best J_SC (27.11 mA cm⁻²) and FF (78.73%). Consequently, DQ20:L8-BO based OSCs afford a higher PCE of 19.35%, compared with D18:L8-BO and DQ40:L8-BO counterparts. This work demonstrates that ternary copolymerization is an effective strategy to realize suppressed ΔE_nr and high efficiency via finely tuning the energy level offset and miscibility between the donor and acceptor.