The process of charge transfer at an interface is key to the understanding of the function of organic solar cells. In order to fully understand how one might utilize interface materials for improved solar cell devices, it is important to first consider a few basic principles regarding the processes of charge carriers at the interface. One can consider in most devices two kinds of interactions, organic to metal and organic to organic interactions. The understanding of the interfacial electronic structure and energy level alignment at the electrode-bulk heterojunction (BHJ) contact is important for designing better contacts to improve charge extraction for organic solar devices. In a solar cell with an organic material as the light-absorbing medium, photon absorption leads initially to the production of a strongly bound electron-hole pair, which is termed an exciton. The excitons must diffuse to an interface with another organic material to dissociate into free charges.