Study On Organic Optoelectronic Devices Based On Patterned Graphene Electrodes

In the past few decades,graphene material has been widely applied in various optoelectronic devices as transparent and conductive electrodes due to its ultra-high optical transmittance,good electrical conductivity,excellent mechanical flexibility and chemical stability.Among them,organic optoelectronic devices with graphene electrodes are drawn much attention according to their application prospect in flexible and stretchable domains.Up to now,large area,high quality and layer-controlled graphene can be synthesized on catalytic metal substrates by chemical vapor deposition.However,graphene as transparent and conductive electrodes applied in organic optoelectronic devices requires further transfer and pattern techniques.Traditional graphene wet transfer technique based on high-molecular polymer always causes problems of material damage and residual polymer pollution,and complicated techniques are needed to obtain graphene patterns after the transfer process.Beyond that,pristine graphene possesses low conductivity and mismatched work function with adjacent organic materials.In this doctoral dissertation,we focus on graphene transfer and pattern techniques as well as its application in organic optoelectronic devices.Furthermore,pristine graphene is integrated with ultra-thin metal film and conductive polymer material as composite electrodes to optimize electrode parameters,thus improve the performance of organic optoelectronic devices based on patterned graphene electrodes.The main research results of our dissertation can be divided into four parts as follows:(1)Thermally vacuum evaporated Cu pattern film as the masking layer combined with plasma etching technique is adopted to fabricate consistent patterns on graphene transferred onto desired substrate.The vacuum thermal annealed treatment can remove polymethyl methacrylate(PMMA)polymer residue induced during the transfer process thus improve the surface morphology of graphene.Further HNO3 doping treatment of graphene enhances its electrical conductivity,work function and wettability with PEDOT:PSS hole transport material.The best power conversion efficiency(PCE)of small molecule organic solar cells based on patterned graphene electrodes with two-step treatment reaches to 2.43 %,which is comparable to that of devices based on traditional ITO electrodes.These results have demonstrated that two-step treatment could efficiently improve the properties of graphene electrode and corresponding device performance.(2)A novel graphene transfer and pattern strategy based on positive photoresist(S1805G)has been approved.In this technique,S1805 G simultaneously severs as the supporting layer and masking layer during graphene transfer and pattern processes,respectively.As-prepared patterned graphene electrode exhibits smooth surface morphology and high-precision graphic,corresponding largest patterns reach up to 2 inch.Patterned graphene electrodes with different layers are obtained via multiple transfer process and Mo O3 is applied to increase the conductivity and work function of patterned graphene electrodes.The performance of organic light-emitting devices(OLEDs)based on Mo O3 doped bilayer patterned graphene electrodes is comparable to that of ITO electrode,showing the highest luminance of 63700 cd/m2 and current efficiency of 33 cd/A.These patterned graphene electrodes are also compatible to flexible OLEDs.More important,OLEDs with microscale patterned graphene electrode achieve small lighting line width of 25 um with uniform light area and ideal contrast,demonstrating that patterned graphene electrodes prepared by the transfer and pattern technique possess excellent application potential for future display devices.(3)Patterned graphene electrodes are modified by vacuum thermally evaporated ultra-thin Au film,and corresponding composite electrodes exhibit high transmittance,excellent conductivity and proper work function.The bonding effects between Au atom and regular hexagonal arranged C atom of graphene suppress the “island” growth mode of Au film,thus ultra-thin Au film grown on patterned graphene is uniform and continuous.The highest current density and luminance of OLEDs based on graphene and 6 nm Au composite electrodes is 525 m A/cm2 and 49200 cd/m2,which exhibits around 1633 % and 634 % performance enhancement compared to devices with pristine graphene electrodes.(4)PH1000 conductive polymer and graphene composite electrodes with continuous wrinkles are achieved on SU-8 polymer film due to the tensile strain caused by their different thermal expansion coefficients.The height of wrinkled composite electrode structure is 200-300 nm,possessing high transmittance of 88.5 %,low sheet resistance of 147.5 Ω/□ and work function of 5.0 e V.Polymer organic solar cells(POSCs)based on graphene and PH1000 composite electrode could transmit the bottom wrinkled structures to top metal cathodes,and the structure height still maintains 100-150 nm.The scattering effect of graphene and PH1000 composite electrode enables stronger light absorption of POSCs active layer,and the enlarged contact area between active layer and composite electrode would facilitate the charge collection.Hence,POSCs with graphene and PH1000 composite electrode show the highest short circuit current density(JSC)of 12.38 m A/cm2 and PCE of 4.67 %.Corresponding PCE exhibits 60 % and 35 % enhancement compared to devices with individual graphene and PH1000 electrodes,respectively.