Improved OPV efficiency of fluorene-thiophene-based copolymers with hole- and electron-transporting units in the main chain

In this paper, we have synthesized a series of fluorene-thiophene-based copolymers by Suzuki coupling polymerization for the use of donor materials in organic photovoltaic cells (OPVs). New conjugated polymers have an electron-deficient moiety such as benzothiadiazole (BT), oxadiazole (OX) and triazole (TZ) or an electron-rich triphenylamine (TPA) moiety or both in the main chains as third or fourth comonomers. The photovoltaic properties of the polymers were varied by introduced electron- or hole-transporting units. The order in the efficiency of the photovoltaic cells was P1 (BT) <P2 (TPA) <P3 (TPA, BT) <P4 (TPA, OX) <P5 (TPA, TZ). The highest power conversion efficiency (PCE) of 0.33% (P5) with an open-circuit voltage (V_OC) of 0.76V and a short circuit current (J_SC) of 1.43mA/cm² was achieved by device annealing at 80°C. The relationship between molecular structures and photovoltaic properties was systematically established herein.