Alkoxy Modification of the Terminal Group in Nonfullerene Acceptors to Achieve Efficient Ternary Organic Solar Cells With a High Open-Circuit Voltage

Y-series nonfullerene acceptors (NFAs) usually bear halogenated end groups to achieve narrow bandgaps and tunable molecules crystallinity; however, it results in small open-circuit voltage (V_oc) of 0.8–0.9 V. Here, three Y-series NFAs BTP-eC9-G51, BTP-eC9-G52, and BTP-eC9-G53 are synthesized by introducing both an electron-withdrawing fluoro group and electron-donating alkoxy group to commonly used 2-(3-oxo-2,3-dihydroinden1-ylidene)-malononitrile (IC) terminal groups. These compounds demonstrate a high V_oc larger than 0.9 V when employed as acceptors in organic solar cells (0.91 V for BTP-eC9-G51 and 0.95 V for the others). The effect of alkoxy chain length of the molecules on the photoelectric properties is systematically studied. The results show that the dipole moments and aggregation behaviors of these molecules changes obviously with the increase of alkoxy chain length. The active layer based on BTP-eC9-G51 shows suitable phase separation structure and good charge transport. Devices based on BTP-eC9-G51 achieve a device efficiency of 16.65%, higher than those of BTP-eC9-G52 and BTP-eC9-G53 based devices (14.33% and 13.24%, respectively). Furthermore, BTP-eC9-G51 is introduced into D18:L8-BO devices as a third component, which improves the J_sc and V_oc, reduces the nonradiation energy loss, and the device efficiency is increased to 19.03% with a high V_oc of 0.92 V.