Poly(ethyl thieno[3,4-b]thiophene-2-carboxylate) (PTTEt), showing near infrared (NIR) absorption was employed as a polymer sensitizer in dye-sensitized solar cell (DSSC). The photovoltaic performance of DSSC was investigated with PTTEt of different molecular weights. Low molecular weight aliphatic diacids were also employed as coadsorbents along with PTTEt. It was revealed that the reduction of polymer molecular weights caused only a slight enhancement in overall power conversion efficiency (eta). The species and concentration of coadsorbents, on the other hand, greatly affects eta. The best performance with eta of ca. 1.5% was obtained when 3,3'-dithiodipropionic acid was adsorbed along with PTTEt, indicative of significant improvement of eta (ca. 50% increase). (C) 2010 Elsevier B. V. All rights reserved.