Abstract
Open-circuit voltage (VOC) plays a key role of determining the power conversion efficiency for solar cells, as well as short-circuit current. However, the origins of VOC have still remained unsolved for organic solar cells. Although VOC has been reported to depend on the electronic states at the Donor/Acceptor (D/A) interface such as the energy difference (ΔEHL) between HOMO (the highest occupied molecular orbital) of donor and LUMO (the lowest unoccupied molecular orbital) of acceptor, some organic solar cells did not exhibit such the dependence. This is because previous discussion has been based on the D/A interfacial electronic states under “dark condition”. Recently, we have performed in situ impedance spectroscopy to examine the correlation between VOC and the electronic states (built-in potential: Vbi) in the vicinity of the D/A interface upon photo-irradiation, and found an excellent agreement between VOC and the sum of Vbi estimated from capacitance-voltage (C-V) characteristics.
