Doping for Controlling Open-Circuit Voltage in Organic Solar Cells

Doping, addition of trace amount of p-type and n-type impurities, to form the pn junction is the central technology in inorganic solar cells. However, the doping effect on the energy-level alignment and the performance in organic solar cells (OSCs) are still unclear. Here, we report that the addition of p-type (MoO3) and n-type (Cs2CO3) dopants into a donor layer in phthalocyanine/fullerene planar heterojunction OSCs controls the open-circuit voltage (VOC). The VOC decreased to 0.36 V when a p-type dopant was added to the donor layer, whereas it increased to 0.52 V with an n-type dopant. In contrast to the previous reports where p-type dopants were usually added to the donor layer, the n-type dopant was found to increase the VOC. Energy-level mapping revealed that the origin of the VOC change was the vacuum level shifts occurring near the donor/acceptor (D/A) interface because of the Fermi-level alignment. The results demonstrated that the VOCs in OSCs are largely affected by the energy-level shift near t...