Near-Infrared Absorbing Thienoisoindigo-Based Copolymers for Organic Photovoltaics

We present bulk heterojunction organic photovoltaic (OPV) cells of novel electron accepting moiety, dithienoketopyrrole-based pi-conjugated copolymers. An analog of isoindigo, thus so-called thienoisoindigo, was coupled to benzodithiophene, cyclopentadithiophene, and fluorene, affording a low bandgap and an appropriate LUMO level (-3.8 eV) for n-type methanofullerene (PCBM). A laser and Xe-flash-time-resolved microwave conductivity was extensively applied for guiding the processing conditions (p/n blend ratio, solvent, and thermal annealing), giving clearly defined optimized parameters. OPV performances are discussed in-depth in conjunction with atomic force microscope, grazing incidence X-ray diffraction, modulation of light intensity, field-effect transistor, and femtosecond transient absorption spectroscopy. We found that excessively shortened singlet exciton lifetime of polymer (<1 ps) competes with the photocurrent generation, which may arise from its near-infrared absorbing nature. The maximized power conversion efficiency was 1.4%, and there is still room for boosting the performance by rational design of counter donor unit and solubilizing substituents.