Low nonradiative energy losses within 0.2 eV in efficient non-fullerene all-small-molecule organic solar cells

Abstract

Despite the remarkable progress achieved in the field of non-fullerene acceptor (NFA)-based organic photovoltaics (OPVs) in recent years, the large energy loss remains a major factor limiting the power conversion efficiency (PCE) of OPVs. Although many studies on polymer OPVs have been reported, low energy loss systems with high efficiency are rarely reported with all-small-molecule organic-solar-cells (ASM-OSCs). This is partially because a low energetic offset between donors and acceptors is usually required to have a small energy loss, which could result in an insufficient exciton dissociation driving force, making the trade-off between the device performance and energy loss an important research focus. In this article, we report ASM-OSCs based on three small molecule donors ZR1-C2, ZR1-C3 and ZR1-C4 with small molecule NFAs BTP-C11-N2Cl, BTP-C9-N2F and BTP-C9-N4F. Charge carrier mobilities, exciton dissociation efficiency and charge collection efficiency were measured and calculated for blends ZR1-C3:TP-C11-N4Cl, ZR1-C3:BTP-C9-N2F and ZR1-C3:BTP-C9-N4F, revealing good charge transport properties. Morphology characterization methods such as AFM, TEM and GIWAXS have been employed, demonstrating excellent molecular compatibility and suitable phase separation with fibrous structures that provides efficient charge transport channels. By morphology and charge carrier optimization, the highest PCE of 14.29% was achieved with a small nonradiative energy loss of 0.2 eV which, to the best of our knowledge, is the highest PCE value for ASM-OSCs with a non-radiative voltage loss ≤ 0.2 eV, reaching a balance between high performance and low energy loss.