Low Band-Gap D–A Conjugated Copolymers Based on Anthradithiophene and Diketopyrrolopyrrole for Polymer Solar Cells and Field-Effect Transistors

摘要

Two conjugated copolymers PADT-DPP and PADTFDPP based on anthradithiophene and diketopyrrolopyrrole, with thiophene and furan as the p-conjugated bridge, respectively, were successfully synthesized and characterized. The number-averaged molecular weights of the two polymers are 38.7 and 30.2 kg/mol, respectively. Polymers PADT-DPP and PADT-FDPP exhibit broad absorption bands and their optical band gaps are 1.44 and 1.50 eV, respectively. The highest occupied molecular orbital energy level of PADT-DPP is located at 25.03 eV while that of PADT-FDPP is at 25.16 eV. In field-effect transistors, PADT-DPP and PADT-FDPP displayed hole mobilities of 4.7 3 1023 and 2.7 3 1023 cm2/(V s), respectively. In polymer solar cells, PADT-DPP and PADT-FDPP showed power conversion efficiency (PCE) of 3.44% and 0.29%, respectively. Atomic force microscopy revealed that the poor efficiency of PADT-FDPP should be related to the large two-phase separation in its active layer. If 1,8-diiodooctane (DIO) was used as the solvent additive, the PCE of PADT-DPP remained almost unchanged due to very limited morphology variation. However, the addition of DIO could remarkably elevate the PCE of PADT-FDPP to 2.62% because of the greatly improved morphology. Our results suggest that the anthradithiophene as an electron-donating polycyclic system is useful to construct new D–A alternating copolymers for efficient polymer solar cells.