Polymer solar cells (PSCs) based on alkyl-substituted thienylenevinylene and phthalimide-based D-A copolymer (PTVPhI-Eh) as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor were investigated with the intent of gaining high performance photovoltaic properties. PTVPhI-Eh had a deep highest-occupied molecular orbital (HOMO) level, similar to 0.3 eV lower than rr-P3HT. As a result, PTVPhI-Eh:PC71BM devices showed a high open circuit voltage (V-oc) of 0.87 V, which is over 0.2 V higher compared to that of P3HT:PCBM-based PSCs. The maximum power conversion efficiency (PCE) of PSCs was 4.14% with V-oc of 0.87 V, short circuit current (J(sc)) of 7.32 mA/cm(2), and fill factor (FF) of 65%, under optimized conditions of a 1:1 wt/wt ratio and treatment with a 2 vol% 1-chloronaphthalene solvent additive. The V-oc and consequent photovoltaic performance of this PSC was significantly affected by the concentration of the solvent additive. To determine the relationship between V-oc and concentration of solvent additive, PTVPhI-Eh:PC71BM devices were investigated with various analyses such as optical spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). (C) 2014 Elsevier B.V. All rights reserved.