Modification Of Hole Injection Layer Of Organic Light-Emitting Diodes With Graphene Oxide

Organic light-emitting diodes (OLEDs) have attracted much attention forapplication in display and lighting field because of their excellent characteristics,and lots of efforts have been made to optimize their performance. Among thefuncional layers of OLEDs, the hole injection layer (HIL) plays an importantrole for hole injection and anode modification, and further for the deviceperformance improvement. For the devices prepared by spin-coating, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) is the mostcommon hole injecting layer. However, the transmittance and conductivity ofPEDOT:PSS are limited, and PSS may corrode the ITO layer because of thestrong acidity. In this thesis, the effect of graphene oxide (GO) on the holeinjection ability of PEDOT:PSS, and further the device performance as well asthe stability were investigated systemically by adjusting the fabrication methodof HIL.1. GO aqueous solution was prepared by improved Hummer method andGO film was prepared by spin-coating. The influences of anneling temperature,the rotate speed as well as the spin-coating times on the thickness of the GO filmwere investigated. In addition, the influence factors of PEDOT:PSS film wereresearched and the film thickness-rotate speed was drawed. 2. PEDOT:PSS:GO film was used as the HIL, and OLED devices werefabricated with the configuration of ITO/PEDOT:PSS:GO(x)/NPB/Alq3/LiF/Al.High current efficiency and power efficiency at a low driving voltage wereobtained because of the enhanced conductivity and transparency of thePEDOT:PSS:GO HIL compared to the tranditional PEDOT:PSS HIL. Theoptimized OLED containing PEDOT:PSS:GO (0.8%, mass ratio) HIL shows atransmission value of96.8%, a maximum current efficiency of3.74cd A-1and amaximum power efficiency of2.12lm W-1, respectively, which were67.6%and35.7%higher than the reference OLED containing a PEDOT:PSS HIL. Wealso fabricated green, red and blue light-emitting organic electrophosphorescentdevices (PHOLEDs) using PEDOT:PSS:GO HILs. The luminance, the currentefficiency and the power efficiency of the devices were improved and theelectroluminescent (EL) spectra remained compared to the reference OLEDs.The hole injection ability and transmission value of PHOLEDs usingPEDOT:PSS:GO HILs were also enhanced.3. Considering the highly acidic PSS corroding the ITO layer, and furtherdecreasing the stability of devices, GO/PEDOT:PSS was used as the hole-injection layer. With the optimal thickness of GO (2.8nm), the power efficiencyand the stability of the prepared fluorescence and phosphorescence OLEDs wereimproved remarkably respect to the reference device. The maximum currentefficiency and the maximum power efficiency were59.1%and128%higherthan the reference OLED containing a PEDOT:PSS HIL. The improvement ofthe power efficiency was attributable to the reduction of driving voltage byenhancing the hole-injection ability, while the enhancement of the stabiliy canbe ascribed to the reduced corrosion of ITO by separating PSS from the ITOlayer.