Polymer solar cells (PSC) have become a considerable competitor to traditional silicon-based inorganic solar cells. PSCs are in a great interest due to their light-weight, flexibility and low-cost fabrication techniques. The ongoing research pursues a great deal of effort to develop more effective materials in the active layer of a solar cell. The active layer consists of a donor and an acceptor that together participate in transforming light energy into electrical energy. Nowadays, the most interesting donors are polymers that pair up with non-fullerene acceptors in the active layer of a PSC. A derivative of benzodithiophene, PDTB-EF-T, as a donor and a small-molecule as an acceptor in the active layer of a PSC gave one of the highest power conversion efficiencies so far, as much as 14.2%. The goal of this thesis was to examine the less thoroughly examined photovoltaic properties of the donor polymer PDTB-EF-T with computational calculations. The properties of the polymer studied in this work were the geometry, the electronic structure, the charge-transport properties and the excited states. These characteristics are often studied for the π-conjugated systems of the donor and acceptor materials in PSCs. The thesis being the first theoretical study of PDTB-EF-T gives new information on the properties of the polymer. New information of the localization of the electronic density and the delocalization of the molecular orbitals of the periodical model that was illustrated with pictures, was provided. The thesis created a great base of the future studies that focus on the electron transport properties and the theoretical studies of the coupling of this donor molecule and the small-molecule acceptor. The results of the calculations followed the known theories. As the backbone chain length of the model of the polymer increases, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) get closer to each other, and thus, the HOMO‒LUMO gap narrows. In addition, the increasing backbone chain length decreases the bond length alternation (BLA) values. The planarity of the radical cation resulted proper charge-carrier transport properties. As for the calculations of the excited states, the vertical transitions of the first two singlet and the first triplet excited state of the trimer of PDTB-EF-T correspond to the visible and near-infrared wavelengths.