Successful fabrication and operation of highly stable organic photovoltaic (OPV) cells demand improved durability of the semiconductor materials used in the active layer. In this study, we performed photo oxidation studies on DTS(FBTTh2)(2):fullerene bulk-heterojunction (BHJ) films, one of the promising small molecule materials for OPV cells, by using two-dimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS), absorption spectroscopy, and atomic force microscopy (AFM). The results of 2D-GIWAXS and absorption spectroscopic analyses indicate that increasing the crystallinity of DTS(FBTTh2)(2): fullerene BHJ films improved their photostability. The AFM results suggest that the roughness of the original DTS(FBTTh2)(2):fullerene films was also related to their photooxidation rates. The results obtained in this work show that changing the fullerene derivative species and controlling the nanostructures of the thin films are both important criteria for improving the photostability of OPV cells. (C) 2016 Elsevier B.V. All rights reserved.