Efficient Light-emitting Diodes Based On Modified Graphene Electrode

Graphene has been widely applied in optoelectronic devices as one of the most promising transparent electrodes due to its excellent optoelectronic and mechanical properties.When graphene is used in light-emitting diodes,there is usually a large injection barrier between graphene anode and hole transporting layer,due to the mismatched work function??4.5 e V?between them,which results in undesirable device performances.In this thesis,we adopted chemical modification methods for single layer graphene prepared by chemical vapor deposition?CVD?to adjust its work function and reduce the sheet resistance,and aimed to improve the device performance of organic light-emitting diodes?OLEDs?and perovskite light-emitting diodes?Pe LEDs?based on the modified graphene electrodes.The main results are summarized as follows:?1?We introduced a thin layer of high dielectric constant polymer between graphene anode and the hole injection layer PEDOT:PSS?poly?3,4-ethylenedioxythiophene?:poly?styrene sulfonate??,which can produce an interfacial dipole to improve the hole injection ability of graphene anode.The two high dielectric constant polymers we used are polyvinylidene chloride?PVDC?and polyvinylidene fluoride?PVDF?.The results show that the utilization of high dielectric polymers on graphene can effectively reduce its surface roughness without transmittance deterioration.Ultraviolet photoelectron spectroscopy?UPS?results confirm the formation of interfacial dipoles induced by the high dielectric polymers.The work function is increased from 4.60 e V of pristine graphene to 4.72 and 4.94 e V for PVDF and PVDC modified graphene electrodes,respectively.Thus,the hole injection barrier between graphene anode and PEDOT:PSS is reduced from 0.54 e V to 0.42 and 0.20 e V,respectively.The OLED based on PVDC-modified graphene anode achieved the highest current efficiency and power efficiency of 80.0 cd/A and 41.6 lm/W,which were improved by25.4%and 44.9%compared with the pristine graphene anode-based OLED,respectively.?2?We proposed to adopt the cluster compound phosphomolybdic acid(H₃[P(Mo₃O₁₀)₄],abbreviated as PMo₁₂)as a novel dopant for graphene,and optimized its doping concentration.As a result of the PMo₁₂ doping,the sheet resistance of graphene decreased from 694?/sq to235?/sq with a long-time stability for over 30 days and the surface roughness was significantly reduced,while maintaining high light transmittance.The shifted G and 2D band of the Raman spectrum proved the p-type doping effect of PMo₁₂ on graphene,which was also discovered in the UPS results from the increased work function.Furthermore,highly efficient OLEDs using the PMo₁₂-doped graphene electrode was obtained,which were 129.3cd/A,77.6 lm/W and 36.5%with an increase of 50.8%,66.9%and 50.8%compared with the pristine counterpart.?3?We applied PMo₁₂-doped graphene electrode in perovskite light-emitting diodes based on the synthesized quasi-two-dimensional perovskite and perovskite nanocrystals.Thanks to the p-type doping effect of PMo₁₂ on graphene,the turn-on voltage of the device is reduced from 4.5 to 4.2 V,and the maximum luminance is increased from 10470 cd/m² to17608 cd/m².The current efficiency and power efficiency are increased from 7.7 cd/A and 2.0lm/W to 11.8 cd/A and 4.5 lm/W,respectively.In this thesis,the work function are adjusted and sheet resistance are effectively reduced for CVD graphene electrode by chemical modification using high dielectric polymers and cluster compound phosphomolybdic acid,in order to enhance the hole injection ability of the graphene anode and the device performance of OLEDs and Pe LEDs based on graphene anode.Our work confirms that interfacial dipole and p-doping are promising chemical modification methods to realize ideal graphene electrodes,and can provide some references for the research on graphene electrode light-emitting devices.