Manipulating the charge transfer at CuPc/graphene interface by O₂ plasma treatments

The manipulation of charge transfer at CuPc/graphene interface has been demonstrated by treating pristine graphene with O₂ plasma. As revealed by in situ ultraviolet photoelectron spectroscopy measurements, a much stronger interfacial charge transfer occurs when the pristine graphene is exposed to O₂ plasma prior to the growth of CuPc films, which is attributed to the increased work function of graphene after O₂ plasma treatment. Moreover, the highest occupied molecular orbital leading edge of CuPc locates at ∼0.80 eV below substrate Fermi level on O₂ plasma treated graphene, whereas it locates at ∼1.10 eV on pristine graphene. Our findings provide detailed information regarding the electronic structure at CuPc/graphene and CuPc/O₂ plasma treated graphene interfaces. The increased work function in combination with the relatively smaller energy offset between the highest occupied molecular orbital of CuPc and Fermi level of O₂ plasma treated graphene facilitates the extraction of holes at the interface, and hence paves the way for improving the performance of graphene-based organic photovoltaic cells.