Interfacial properties of the K-doped C60: CuPc heterojunctions

We report on the interfacial properties of the K-doped C₆₀: copper phthalocyanine (CuPc) heterojunctions, studied via synchrotron-radiation photoemission. The K-doped heterointerfaces were obtained by means of C60 on K_1.5CuPc and CuPc on K₃C₆₀. The valence-band and K 3p core-level spectra show that the K diffuses and transfers negative charge into C₆₀ overlayer at the C₆₀/K_1.5CuPc interface, while only physisorption could be found at the CuPc/K₃C₆₀ interface. A significant shift of the vacuum level is observed in both cases, caused by the charge transfer for the C₆₀/K_1.5CuPc, while by the pillow effect for the CuPc/K₃C₆₀. The creation of gap states and an increased separation between the lowest unoccupied molecular orbital of C60 and the highest occupied molecular orbital of CuPc at the C₆₀/K_1.5CuPc interface reveal that the K-doped film could improve the photovoltaic effect. However, the advantage has not been found at the CuPc/K₃C₆₀ interface.