Solar cells based on blends of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, P3HT/PCMB, constitute one of the most efficient polymer photovoltaic cell types. One of the main factors that determine the efficiency of the solar cells is the open-circuit voltage, V-OC. In this work, we provide an analysis of the parameters affecting the V-OC in a P3HT/PCBM complex. Electronic transitions, excited states, and electron transfer parameters are evaluated under the classical Marcus formalism via the time-dependent and unrestricted CAM-B3LYP/6-31G* methods. The charge-recombination driving force is found to mainly affect the charge-recombination rate constant and, in turn, V-OC. Even though other parameters also determine the value of V-OC like density of states, dimensions of the cell, and microstructure of the donor/acceptor interface, the current work highlights the understanding attained by modeling charge-transfer parameters. The analysis reported here encourage further quantum-chemical investigations in organic photovoltaics with the aim of estimating and improving V-OC, such that more efficient organic solar cells may be predicted.