The sun provides many times the energy we need, making it the most promising energy source. Conversion of sunlight to electrical energy is obtained in photovoltaic (PV) devices. Organic Solar Cells represent a new class of PV devices in which organic molecules serve as semiconductor materials. Although there has been huge progress in the performance of these devices, the physical fundamentals of charge transport still require a complete description. The author developed a powerful and novel method to investigate the cross section of electronic devices using in-situ focused ion beam preparation and scanning Kelvin probe microscopy. Using this method it was possible for the first time to spatially resolve the open circuit voltage in operating organic solar cells. Furthermore, charge transport loss mechanisms in bulk heterojunction solar cells and the origin of S-shaped current-voltage characteristics were revealed. This dissertation describes preparation techniques, challenges, and results in an unconcealed way complementing the publications "Imaging the electric potential within organic solar cells" and "Understanding S-shaped current-voltage characteristics of organic solar cells."