Determination of Quasi-Fermi Levels across Illuminated Organic Donor/Acceptor Heterojunctions by Kelvin Probe Force Microscopy

Using Kelvin probe force microscopy (KFM), we have measured the electrochemical potentials across indium tin oxide/organic and donor/acceptor heterojunctions in the dark and under illumination with white light. We have found that the photovoltage generated across these heterojunctions is strongly correlated with the difference between the respective HOMO and LUMO levels of the donor and acceptor and also very closely approximates measured open-circuit voltages in completed solar cells. These results imply that KFM tracks the Fermi level positions within the donor and acceptor layers under photoexcitation. Overall, these results demonstrate the utility of KFM for understanding potential profiles across active layers in planar-heterojunction organic solar cells.