This study presents how the polymer molecular weight (M-w) of PBDTTT-C affects the PBDTTT-C:PC71BM nanoscale morphology and device performances. In particular, this work demonstrates that different M-w, values of the PBDTTT-C are optimized for different processing solvents, leading to various morphological characteristics, as thoroughly examined by atomic force microscopy, transmission electron microscopy and X-ray diffraction. Thus, when PBDTTT-C with a relatively low molecular weight is used, an optimal blend texture that supports efficient charge transport to ideally connected crystalline structures can be obtained through a treatment with chloroform as a processing solvent. As a result, significantly enhanced power conversion efficiency (PCE, over 5.5%) is achieved. These findings highlight the important role of the molecular weight with regard to device performance capabilities and may help in the further development of novel, efficient donor polymers with lower bandgaps for photovoltaic applications. (C) 2016 Elsevier Ltd. All rights reserved.