A series of random conjugated copolymers (labeled PMLTQT, PMLT2T, and PMLT3T) consisting of 3,4-diphenyl-maleimide and various thiophene derivatives has been designed and synthesized via Stille cross-coupling for application in polymer solar cells. These copolymers were readily soluble in common organic solvents, thermally stable from 405 to 437 degrees C upon heating, and exhibited good absorption in the UV and visible regions from 300 to 650 nm. The intensities of the PL emission spectra of these copolymers in a solid film were dramatically quenched by the addition of 50 wt% [6,6]-phenyl C-61 butyric acid methyl ester (PC61BM). Their electrochemical properties indicated that the highest occupied molecular orbital levels of these copolymers were in the range of -5.63-5.73 eV, characteristic of better air stability and a high open-circuit voltage (V-oc) suitable for application to photovoltaic cells. Bulk heter-ojunction photovoltaic devices composed of an active layer of electron-donor copolymers blended with the electron acceptor PC61BM or [6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) at a weight ratio of 1:3 were investigated. The photovoltaic device containing PMLT3T and PC71BM (1:3, w/w) as the active layer afforded the best performance among these copolymers, with a V-oc of 0.74 V, J(sc) of 7.4 mA cm(-2) and a PCE of 1.20% under AM 1.5 G simulated solar light. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.