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 O-2 plasma. As revealed by in situ ultraviolet photoelectron spectroscopy measurements, a much stronger interfacial charge transfer occurs when the pristine graphene is exposed to O-2 plasma prior to the growth of CuPc films, which is attributed to the increased work function of graphene after O-2 plasma treatment. Moreover, the highest occupied molecular orbital leading edge of CuPc locates at similar to 0.80 eV below substrate Fermi level on O-2 plasma treated graphene, whereas it locates at similar to 1.10 eV on pristine graphene. Our findings provide detailed information regarding the electronic structure at CuPc/graphene and CuPc/O-2 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 O-2 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.