Transparent Conductive Graphene Electrodes For High Efficient Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs)show the great potential in the field of flat panel display due to their excellent properties such as light weight,high saturation,fast response and simple fabrication technology.Graphene is one of the most promising electrode materials for realizing flexible OLEDs due to its excellent photoelectric and mechanical properties.In this thesis,we used graphene as the transparent electrode of OLEDs,and a series of research work were carried out in order to solve the high sheet resistance,mismatch work function,large surface roughness of pristine graphene.Firstly,rhodium chloride(Rh Cl3)was used to modify graphene in order to decrease the sheet resistance and increase the work function of intrinsic graphene,and OLEDs based on the Rh Cl3 modified graphene were fabricated successfully.The sheet resistance of graphene was decreased from 1200 to 523 ?/sq with the Rh Cl3 doping concentration of 1×10-2 m M,and the work function was increased from 4.4 to 4.61 e V simultaneously.The performance of graphene-based OLEDs modified with Rh Cl3 was better than that based on the intrinsic graphene,with the maximum current efficiency(CE)and power efficiency(PE)increased from 19.9 cd/A and 3.9 lm/W to 28.2 cd/A and 5.9 lm/W.Secondly,in order to decrease the roughness and improve the hydrophilicity of the graphene surface induced by redox rhodium particles and poly(methyl methacrylate)(PMMA)residue,we doped graphene with chloroauric acid(HAu Cl4)and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)modified with isopropyl alcohol(IPA),and the yield rate of OLEDs based on graphene electrodes was enhanced successfully.The sheet resistance of graphene was decreased from 1200 to 500 ?/sq with the HAu Cl4 doping concentration of 2×10-2 m M,and the work function was increased from 4.4 to 4.62 e V.The resulting maximum CE and PE of OLEDs based on this composite structure were increased from 26.33 cd/A and 7.88 lm/W to 38.25 cd/A and 13.59 lm/W.Finally,in order to eliminate the PMMA residue,we developed a facile and efficient two-in-one method for graphene transfer,which not only obtained clean graphene,but also simplified the device preparation process.Different from the traditional transfer method,we inserted the poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-(4-sec-butylphenyl)-imino)-1,4-phenylene)(TFB)between the graphene and PMMA both as a protector during the graphene transfer and a hole-injection layer in OLEDs.Devices based on the two-in-one method transferred graphene exhibited a much higher device performance than those control ones with a traditional approach,such as the turn-on voltage was decreased from 5.3 to 3.9 V,while the maximum CE and PE were improved from 64.8 cd/A and 18.5 lm/W to 74.5 cd/A and 26.6 lm/W.In this thesis,the high sheet resistance,low work function and large surface roughness of graphene were improved via doping and surface modification.The graphene was doped by Rh Cl3,HAu Cl4 and IPA modified PEDOT:PSS.Furthermore,a two-in-one method was developed for graphene transfer.Finally,the sheet resistance was decreased,work function was increased,PMMA residue on the graphene surface was eliminated,and the graphene-based OLEDs were obtained with an increased device efficiency.