Polymer solar cells made from poly(2,6-bis(3-dodecylthiophen-2-yl)-N-alkyl-dithieno[3,2-b:2',3'-d]pyrrole) (PDTPBT) with different side chain lengths as donors and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor were investigated. After optimization with thermal treatments, the device made from PDTPBT with a longer alkyl side chain (P1) delivers a power conversion efficiency (PCE) of 1.06%, which is much higher than that made from PDTPBT with a shorter side chain (0.26%). Atomic force microscopy (AFM) and modeling results show that the polymer with longer side chain produces much better uniform nanostructure with less pinhole than the polymer with a short side chain, resulting in lower interfacial resistance for higher short circuit current and reduced cathode penetration into the active layer for lower charge recombination rate, and further leading to higher power conversion efficiency. (C) 2010 Elsevier By. All rights reserved.