Efficient ternary organic solar cells with BT-rhodanine-based nonfullerene acceptors in a PM6:Y6-BO blend

A series of benzothiadiazole (BT)–rhodanine-based nonfullerene acceptors (NFAs) were designed and synthesized. These BT–rhodanine-based NFAs were composed of BT as a central core with a 3-octylthienothiohene π-bridge and two different rhodanine end groups (BT-rho for octylrhodanine and BT-rhoCN for dicyano-octylrhodanine end groups). The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels were −5.53/−3.78 eV for a BT-rho film and −5.63/−3.90 eV for a BT-rhoCN film. The strong electron-withdrawing dicyano-octylrhodanine end group of BT-rhoCN downshifted the HOMO and LUMO levels by ∼0.1 eV compared with those of BT-rho. The 10% BT-rho in a PM6:Y6-BO ternary organic solar cell exhibit a maximum power conversion efficiency (PCE) of 16.17% with a short-circuit current density (JSC) of 25.29 mA cm−2, open-circuit voltage (VOC) of 0.84 V, and fill factor (FF) of 76.13%. The hole/electron mobilities of the PM6:Y6-BO, 10% BT-rho, and 10% BT-rhoCN ternary devices were 2.20 × 10−4/1.85 × 10−4 cm2 V−1 s−1, 3.66 × 10−4/3.77 × 10−4 cm2 V−1 s−1, and 1.34 × 10−4/0.94 × 10−4 cm2 V−1 s−1, respectively. The crystal coherence lengths (LCs) of the (010) peaks for the PM6:Y6-BO, 10% BT-rho, and 10% BT-rhoCN ternary films are 43.2, 60.8, and 33.7 Å, respectively. The superior PCE of the 10% BT-rho ternary devices compared with that of the PM6:Y6-BO binary device is attributed to the enhanced hole and electron mobilities and the high crystallinity of the blend films, which contribute to the enhancement of the JSC and PCE.