Improved Photovoltaic Properties of Donor-Acceptor Copolymers by Introducing Quinoxalino[2,3-b′]porphyrin as a Light-Harvesting Unit

Donoracceptor (DA) copolymerization is an effective approach to construct low bandgap polymers with tunable electronic energy levels for the application as donor materials in polymer solar cells (PSCs). Usually, DA copolymers possess an intramolecular charge transfer absorption band at long wavelength direction, so that the absorption of the polymers is broadened. However, absorption at short wavelength direction is also important and should be broadened and enhanced to increase the short-circuit current density (J(sc)) of the PSCs. In this study, a series of low bandgap conjugated polymers, P(QP4-BT-DPP1), P(QP1-BT-DPP1), and P(QP1-BT-DPP4), based on two acceptor units quinoxalino[2,3-b']porphyrin (QP) and diketopyrrolopyrrole (DPP) connected by oligothiophene donor units, were designed and synthesized by palladium-catalyzed Stille-coupling polymerization. As a complementary light-harvesting unit, QP was first introduced into the DA conjugated polymers for improving the photovoltaic performance of PSCs. The incorporation of QP broadened and enhanced the absorptions of short wavelength photons as well as kept the well-tuned electronic energy levels and bandgap of the pristine DA copolymer. Moreover, para-linked QP improved coplanarity and extended p-conjugation along the polymer backbone. As a result, P(QP1-BT-DPP4) with a proper feed ratio (10 mol %) of QP showed an increased Jsc of 11.85 mA/cm(2) without sacrificing open-circuit voltage (V-oc) or fill factor (FF) of the photovoltaic devices. Preliminary photovoltaic devices showed a highest power conversion efficiency of 5.07%, which was 3 times higher than that of the PSC fabricated from the pristine DA copolymer. This study provides a promising approach to circumvent the trade-off between light absorption and electronic energy levels so as to balance the J(sc), V-oc, and FF by introducing a third component into well-performed DA conjugated copolymers for achieving high performance PSCs.