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 (Δ
EHL) 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.
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