Effect Of Graphene/Metal Oxide Composite Transparent Electrode On Optical,Thermal And Electrical Properties Of LED

As a new generation of solid-state light source,GaN-based light emitting diode(LED)has been widely used due to its low energy consumption,long life and environmental protection.However,there are still some problems to be solved in the application of LED,such as current crowding,low external quantum efficiency,and concentrated thermal stress and so on.A transparent conductive layer(TCL)can promote current spreading and uniform illumination.However,when the LED is developed to a short wavelength,the commonly used transparent indium tin oxide(ITO)material in the visible light band can not meet the transparency requirements in the ultraviolet(UV)or deep ultraviolet(DUV)band,and new transparent electrode materials need to be developed urgently.Graphene(Gr)is expected to be applied as a transparent electrode to UV or deep ultraviolet GaN-based LEDs,but the work function between Gr and p-GaN is very different.When Gr is directly grown on the p-GaN layer,it will result in higher The Schottky barrier and the large contact resistance affect the performance of the LED.Therefore,it is necessary to insert a conductive and work-matching metal or metal oxide between Gr and p-GaN as a transparent conductive intermediate layer.Gr-based composite transparent electrode.Therefore,it is necessary to study the influence of Gr composite transparent electrode on LED performance to obtain the best combination type and thickness matching and optimize the composite structure.Due to the expensive and time-consuming manufacturing of prototype samples of LED chips,the design method based on software simulation is much faster and more economical than that of redesigning prototype samples and continuous testing and updating samples.In this paper,based on chip current diffusion theory,the effect of three composite transparent electrodes of Gr/zinc oxide(Gr/ZnO),Gr/ indium tin oxide(Gr/ITO),Gr/fluorine-doped tin oxide(Gr/FTO)on the thermal and electrical performances of LED were simulated by using COMSOL Multiphysics simulation software.Meanwhile,Gr quantum dots(GQDs)with yellow fluorescence properties were synthesized,and a preliminary attempt to obtain white light by encapsulating blue LED instead of conventional yellow fluorescent powder was carried out.The main results obtained are as follows:(1)Three kinds of ZnO films doped with different elements(Al-,Ga-or In)-doped ZnO(AZO,GZO,IZO)were introduced as intermediate layers between Gr and p-GaN and their effect on the optical and thermal properties of GaN-LEDs were investigated.It is found that both the transmittance of TCL and the surface temperature of the LED chip decrease with the increase of the thicknesses of Gr and ZnO.In order to make the transmittance of the Gr/ZnO hybrid TCL higher than 80%,the appropriate electrode combination should be a single layer of Gr with ZnO not thicker than 400 nm(1L Gr / 400-nm ZnO),or 2L Gr /300-nm ZnO,or 3L Gr/200-nm ZnO or 4L-Gr/100-nm ZnO.Through the comparison of the performance of LEDs with different composite combination of Gr-ZnO TCLs,it is found that the thermal performance of 1L Gr/300-nm GZO LED is the best,and its maximum surface temperature drops about 7K compared with that with only Gr without ZnO as TCL.(2)The optical and thermal properties of LEDs with Gr/FTO or Gr/ITO composite transparent electrodes were investigated.It shows that the best combination for these two composite transparent electrode was two layers of graphene with 300 nm FTO or 400 nm ITO,respectively.The maximum surface temperature of LEDs with 2 L Gr/300 nm FTO or 2L Gr/300 nm ITO decrease about 10 K compared with that with only Gr as TCL.The performances of LEDs with Gr/ZnO,Gr/FTO and Gr/ITO composite transparent electrodes were compared and analyzed.It is found that Gr/ITO composite transparent electrodes is the best.(3)GQD with yellow fluorescence properties was successfully prepared by chemical oxidation.Without any surface treatment,GQD and UV glue were evenly mixed,and white light was obtained by packaging them on a blue LED chip.As the concentration of GQD increases,the intensity of the yellow light peak increases,and the color coordinate gradually increases.When the concentration of GQD phosphor is 0.15mg/mL,the performance of white LED is optimal.