Bulk heterojunction solar cells comprising a new low band gap polymer (PBEHTB) based on alternating electron-donating 3,4,3',4'-tetrakis[2-ethylhexyloxy]-2,2'-bithiophene and electron-deficient 2,1,3-benzothidiazole units along the chain as electron donor and [6,6]-phenyl-C-61 butyric acid methyl ester (PCBM) as electron acceptor have been prepared. The polymer absorbs light up to 800 nm and thus covers a substantial part of the solar spectrum. Solar cells based on PBEHTB:PCBM exhibit excellent behavior giving an external quantum efficiency in the near infrared of 13%. Under simulated AM 1.5 conditions an open circuit voltage of 0.77 V, a short circuit current of 2.8 mA/cm(2), and a power conversion efficiency of 0.9% have been achieved, which is among the best achieved for low band gap polymer solar cells. (c) 2006 Elsevier B.V. All rights reserved.