Open-Circuit Voltage in Organic Solar Cells

Organic solar cells (OSCs) based on blends of electron donor (D) and acceptor (A) semiconducting materials now have a power conversion efficiency exceeding 17%, and 100% internal quantum efficiency of free charge generation has already been realized. Therefore, reducing the energy loss in the open-circuit voltage (VOC) is critically important for further enhancing the efficiency of OSCs. In this chapter, the background and recent findings concerning VOC in OSCs are reviewed, beginning with an explanation of the photoconversion mechanism in OSCs in relation to VOC. Following this, the empirical understanding of VOC is outlined, and the models used for explaining the determination of VOC in OSCs are described. In the next section, recent progress for increasing VOC in OSCs by modifying the D/A interface is summarized. The topics discussed are energy-level alignment, doping, the pn homojunction, energy-level control at the monolayer scale, crystallinity, and functionalization of acceptor molecules. Finally, ways in which the VOC in OSCs can be maximized are proposed with a view to the future.