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

In order to simultaneously improve both open-circuit voltage (Voc) and short-circuit current density (Jsc) of organic solar cells, two novel D(A-π-Ar)3 type small molecules of TPA(FxBT-T-3Cz)3 were designed and synthesized, which contain central triphenyl- amine (TPA), terminal carbazole (Cz), armed fluorine-substituted benzothiadiazole (FxBT, x=1 or 2) and bridged thiophene (T) units. A narrowing UV-vis absorption and a decreasing HOMO 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 devices showed a higher open-circuit voltage (Voc) of 1.01 V and an increasing short-circuit current density (Jsc) of 10.84 mAcm-2, as well as a comparable power conversion efficiency of 4.81% compared with the TPA(F1BT-T-3Cz)3/PC71BM based devices under a simulated AM 1.5 G irradiation with an intensity of 100 mW cm-2. Furthermore, compared to the parent TPA(BT-T-3Cz)3 molecule without fluorine substi- tution, the TPA(FxBT-T-3Cz)3 molecules exhibited incremental Voc and Jsc values in their bulk-heterojunction organic solar cells owing to fluorine incorporation into the electron-deficient benzothiadiazole unit.