Photo-generated carrier dynamics in the vicinity of the donor/acceptor interface of organic solar cells

Change in electronic states before and after photo-irradiation at a Donor/Acceptor (D/A) interface in organic solar cells consisting of zinc-porphyrin and fullerene films has been examined using in situ impedance spectroscopy (IS), which makes it possible to non-destructively measure a built-in potential (V_bi) of both D and A films at the D/A interface. The accumulation of photo-generated carriers in the vicinity of the D/A interface was found to increase the V_bi of both D [Zn(OEP)] and A [C₆₀] films at the interface. In addition, it was noted that the sum of V_bi obtained on photo-irradiation was in good agreement with the open-circuit voltage (V_OC), of which the origin is still unsolved for organic solar cells. Thus, the charge accumulation of photo-generated carriers in the vicinity of the D/A interface plays a key role in determining V _OC.