Improvement of work function and hole injection efficiency of graphene anode using CHF3 plasma treatment

We report improvement of hole injection efficiency of a graphene anode by tuning its work function (WF) via surface fluorination. We used chemical vapor deposition to synthesize high-quality graphene sheets and then treated them with CHF3 plasma to induce fluorination. We used x-ray photoelectron spectroscopy to examine the fluorine coverage and the kind of chemical bonds in fluorinated graphene (FG). Also, we used ultraviolet photoelectron spectroscopy to systematically study the changes in the WF and sheet resistance of the FG sheets with varying plasma exposure time (0, 10, 30, 60, 90 s) to find an optimum fluorination condition for hole injection. The WF of graphene sheets was increased by up to 0.74 eV, as a result of the formation of carbon-fluorine bonds that function as negative surface dipoles. We fabricated hole-only devices and conducted dark injection space-charge-limited-current transient measurement; the fluorination greatly increased the hole injection efficiency of graphene anodes (from 0.237 to 0.652). The enhanced hole injection efficiency of FG anodes in our study provides wide opportunities for applications in graphene-based flexible/stretchable organic optoelectronics.