Novel Low Bandgap EDOT-Naphthalene Bisimides Conjugated Polymers: Synthesis, Redox, and Optical Properties

Low band-gap conjugated polymers based on naphthalene bisimide (NBI) and 3,4-ethylenedioxythiophene (EDOT) were synthesized by Stille cross-coupling reaction. The alternating conjugated poly(EDOT-NBI) (P1) and random poly(EDOT-NBI) (P2) are both solution-processable due to the existence of bulky 2,6-diisopropylphenyl substituent. Their optical and electrochemical properties were characterized. P1 and P2 films show optical band gaps of 1.75 and 1.38 eV estimated from UV-Vis absorption spectra. Cyclic voltammograms of both polymers display reversible reduction peaks with onset reduction potentials at -0.55 V for P1 and -0.61 V for P2, which correspond to the electron affinity (EA) values (LUMO energy level) of 3.85 and 3.79 eV, respectively. The ionization potential (IP, HOMO level) values of 5.60 eV for P1 and 5.17 eV for P2 were also calculated by combining solid-state optical and electrochemical data. A double heterojunction device was fabricated. It exhibits an open circuit voltage of 0.30 V and average power conversion efficiency of 0.15%.