We have performed X-ray Photoelectron Spectroscopy (XPS) and N-K Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy on PSiF-DBT:PC71BM, which is a low bandgap polymer blended with a C-70 fullerene derivative, aiming at understanding the change in electronic and structural properties caused by thermal annealing, since these are crucial parameters for the efficiency of bulk-heterojunction organic photovoltaic devices. We observe polymer segregation and a preferred lying-down orientation of the benzothiadazole unit for blends that underwent annealing. An optimization of the film orientational order is essential for a better charge and exciton mobility, thus, enhancing the efficiency of the device. In addition, theoretical calculations at Configuration Interaction with Singles and Doubles excitations (CISD) level, including the relaxation of the core orbitals due to the hole formation are used to interpret the photoabsorption spectra. (C) 2016 Elsevier B.V. All rights reserved.