New –(D–A1–D–A2)n– type conjugated polymers for photovoltaic applications: consensus between low band-gap and low HOMO energy level

To gain ideal conjugated polymers with a low band-gap (Eg) and a deep HOMO (highest occupied molecular orbital) energy level for photovoltaic application, both strong and weak electron withdrawing units were introduced into polymer backbone. A series of –(D–A1–D–A2)n– type diketopyrrolopyrrole (DPP) containing polymers P1–P7 were designed and synthesized. The resulting copolymers exhibited both low Eg of 1.27–1.48 eV and deep HOMO energy levels of −5.25to −5.44 eV. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated. All photovoltaic devices featured high open circuit voltages (Voc, 0.72–0.91 V). Among the seven copolymers, P5 exhibited the best performance with a Voc of 0.80 V, a short circuit current density (Isc) of 3.48 mA/cm2, a fill factor (FF) of 43.2%, and a power conversion efficiency (PCE) of 1.33%.