Photovoltaic Small Molecules of TPA(FxBT-T-Cz)3: Tuning Open-Circuit Voltage over 1.0 V for Their Organic Solar Cells by Increasing Fluorine Substitution

To simultaneously improve both open-circuit voltage (V-oc) and short-circuit current density (J(sc)) for organic solar cells, a novel D(A-pi-Ar)(3) type of photovoltaic small molecules of TPA(FxBT-T-3Cz)(3) was designed and synthesized, which contain central triphenylamine (TPA), terminal carbazole (Cz), armed fluorine-substituted benzothiadiazole (FxBT, where x = 1 or 2), and bridged thiophene (T) units. A narrowed ultraviolet visible absorption and a decreasing highest occupied molecular orbital energy level were observed from TPA(F1BT-T-3Cz)(3) to TPA(F2BT-T-3Cz)(3) with increasing fluorine substitution. However, the TPA(F2BT-T-3Cz)(3)/PC71BM-based solar devices showed a rising V-oc of 1.01 V and an enhanced J(sc) of 10.84 mA cm(-2) as well as a comparable power conversion efficiency of 4.81% in comparison to the TPA(F1BT-T-3Cz)(3)/PC71BM-based devices. Furthermore, in comparison to the parent TPA(BT-T-3Cz)(3) molecule without fluorine substitution, the fluorine-substituted TPA(FxBT-T-3Cz)(3) molecules exhibited significantly incremental V-oc and J(sc) values in their bulk heterojunction organic solar cells, owing to fluorine incorporation in the electron-deficient benzothiadiazole unit.