In this contribution the effect of changes in the active material system and modification of contact properties in organic solar cells is investigated at the example of ZnPc:C60 small molecule solar cells. Devices with different blend mixing ratios and a variation of hole transport layer are prepared by vacuum deposition and compared to drift-diffusion simulation data. It is shown that the open circuit voltage is mainly defined by the mixing ratio, whereas the fill factor is strongly influenced by the choice of hole transport layer. Extraction barriers for photogenerated holes lead to S-shaped IV curves. The strength of the S-shape scales with the height of the extraction barrier. The slope of the IV curves at open circuit suggests that the observed increase in open circuit voltage with a higher amount of C60 in the blend might be due to a downshift of the highest occupied molecular orbital of ZnPc.