Versatile p‐Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes

Abstract

Abstract:We report effective solution-processed chemical ptype doping of graphene using trifluoromethanesulfonic acid (CF 3 SO 3 H, TFMS), that can provide essential requirements to approach an ideal flexible graphene anode for practical applications:i )h igh optical transmittance,i i) lows heet resistance (70 %d ecrease), iii)h igh work function (0.83 eV increase), iv) smooth surface,a nd iv) air-stability at the same time.The TFMS-doped graphene formed nearly ohmic contact with ac onventional organic hole transporting layer,a nd ag reen phosphorescent organic light-emitting diode with the TFMS-doped graphene anode showed lower operating voltage, and higher device efficiencies (104. The widely used indium tin oxide (ITO) transparent electrode is not appropriate for use in flexible electronics, because it is brittle,i ncreasingly expensive,a nd causes diffusion of impurities into devices. [1][2][3] Therefore,f lexible transparent electrodes must be developed before the use of flexible electronics is practical. Graphene is aone atom thick sheet of sp 2 hybridized carbon atoms that has unique electrical properties and mechanical robustness.[2, 4-11] Therefore,m any researchers have explored ways to use graphene electrodes in flexible organic optoelectronics. [2,[12][13][14][15][16][17][18][19][20][21][22] However, the pristine graphene has suffered from two inherent major problems:1)The first disadvantage comes from its low charge carrier density.T herefore,t he pristine graphene has much higher sheet resistance (200-300 W sq À1 )t han does ITO ( % 10 W sq À1 ), which resulted in high operating voltage and poor luminous power efficiency (PE)inorganic light-emitting diodes (OLEDs), [15,16] and thus graphene requires an additional doping process to improve the charge carrier concentration and the electrical conductivity. [2,[20][21][22][23][24][25][26][27] 2) Thep ristine graphenes work function (WF = 4.4 eV) is much lower than that of ITO( 4.8 eV).[2] Thel ow WF of the pristine graphene significantly limits the hole injection from the graphene anode to an overlying organic layers in OLEDs. [2,15,16] Thenitric acid (HNO 3 )and the gold (III) chloride (AuCl 3 )have been widely used as chemical p-type dopants for graphene to reduce sheet resistance (R s ). [2,6,[21][22][23] However,these conventional chemical dopants cause instability problems:t he HNO 3 gradually evaporates from the graphene in the ambient condition due to its volatility,thereby causing gradual decrease of electrical conductivity of the p-doped graphene and its device; [2a, 22-24] TheA uCl 3 leaves 50-100 nm Au particles on the graphene surface,w hich degrades device efficiencya nd stability increasing current leakage in the thin film devices.[2a] Therefore,tofabricate ideal graphene anodes for organic optoelectronics needs aversatile p-type doping method that can meet essential requirements at the same time:i )h igh optical transmittance (OT), ii)the low R s comparable to that of ITO, iii)t he high surface WF that can effectively reduce the hole i...

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