This thesis discusses the accuracy of a number of analytical models commonly used to analyse current density-voltage curves from space-charge-limited current measurements, such as Ohm's law and the Mott-Gurney law. We studied the accuracy of the models using a numerical drift-diffusion solver which can calculate single-carrier devices containing either an intrinsic semiconductor, a doped semiconductor or a semiconductor with traps. This allowed us to calculate the current density-voltage curves of a single-carrier device using known input values for the mobility and trap characteristics, and subsequent fitting with the analytical models yielded insight into their accuracy. We found that Ohm's law only correctly describes the current when the probed semiconductor is highly doped, in which case the Mott-Gurney law is no longer valid. Numerical drift-diffusion solvers can also be used for direct fitting of experimental current density-voltage curves, allowing for the inclusion of traps but also injection barriers at the contact interfaces. We used this approach to fit to data obtained from single-carrier devices based on Spiro-OMeTAD. We then compared the charge-carrier mobility and trap characteristics obtained using the numerical approach to characteristics obtained from the analytical models, and we found that charge-transport could be described with a temperature-independent charge-carrier mobility when traps and injection limitation was included explicitly. Finally, we examined the effect of oxidation of fullerenes on organic solar cell effi ciency. We studied charge-transport when a small fraction of oxidized species was added to the film, and we found that the oxidized species act like charge traps. First principle calculations were used to calculate the energetics of the oxidized fullerene species, with results matching those of the charge-transport analysis. With the obtained knowledge we could qualitatively reproduce the observed reduction in solar cell efficiency.