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 (E-g) 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-A(1)-D-A(2))(n)- type diketopyrrolopyrrole (DPP) containing polymers P1-P7 were designed and synthesized. The resulting copolymers exhibited both low E-g 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-C-61-butyric acid methyl ester (PCBM) were investigated. All photovoltaic devices featured high open circuit voltages (V-oc, 0.72-0.91 V). Among the seven copolymers, P5 exhibited the best performance with a V-oc of 0.80 V, a short circuit current density (I-sc) of 3.48 mA/cm(2), a fill factor (FF) of 43.2%, and a power conversion efficiency (PCE) of 1.33%. (C) 2013 Elsevier Ltd. All rights reserved.