Understanding the physics of organic solar cells (OSCs) is a must for their rapid development and successful implementation in energy production. Along with high efficiency, the stability and reliability in different environmental conditions are highly important issues that should be well understood for the success of this technology. The photovoltaic effect, generation of photocurrent and photovoltage, and calculation of power conversion efficiency (PCE) in OSCs were discussed in previous chapters. In this chapter, I shall present an in-depth analysis of device functioning, the charge transport mechanism, and effects of surrounding conditions on the cell performance. Energy conversion by an OSC can be broken down into four important processes: (1) exciton generation via light absorption, (2) exciton diffusion, (3) exciton dissociation, and (4) charge carrier transportation and collection. The overall PCE of a solar cell is decided by individual efficiencies of these processes and the product of these efficiencies gives the external quantum efficiency (EQE). Therefore, a clear fundamental understanding of these processes is necessary for the design and development of efficient and stable OSCs. Device modeling is an important tool to interpret the device behavior and find out the optimum conditions for high performance; therefore some of the models developed to interpret the behavior of OSCs will also be discussed here.