Thermal treatment and chemical doping of semi-transparent graphene films

• We investigated thermal and chemical treatments of graphene transparent films.
• We increased the conductivity of the films and tuned their work function.
• Sheet resistance was reduced by up to 3 orders of magnitude via thermal annealing.
• We chemically p-doped thin films of graphene via physisorption of TFSA.
• Work function was increased by up to 0.5 eV to about 5.3 eV.

Graphene transparent conductive films have been proposed as indium tin oxide (ITO) modifiers for application in optoelectronic devices. Here thermal and chemical treatments of graphene-based transparent films are systematically investigated with a view to increasing their conductivity and tuning their work function. We look at two different types of graphene, obtained either via spin-coating of graphene oxide (GO) aqueous dispersions, or vacuum-filtration of liquid-phase exfoliated (LPE) graphene in highly volatile, non-toxic solvents such as isopropanol and ethanol. As-deposited films are relatively resistive, but we are able to lower their sheet resistance by up to three orders of magnitude for both LPE and GO, so as to reach values of ∼105 Ω/□ via thermal treatments at temperatures of ∼350/400 °C at pressures of ∼10−6/10−4 mbar. Most importantly, the physisorption of the molecule (CF3SO2)2NH (trifluoromethanesulfonimide) results in an increase of the films work function by up to 0.5 eV, to yield a value of ∼5.3 eV. This is comparable or slightly better than what can be achieved with poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), depending on PSS concentration, thereby confirming the potentially beneficial role of chemical doping of liquid dispersions of graphene-derivatives for application to organic electronics.

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