Extremely Efficient Organic Light-Emitting Devices Based On Graphene Anode

In recent years,organic light-emitting devices(OLEDs)have attracted much attention due to the advantages of high luminous efficiency,fast responding speed,ultra-thin thickness,light weight,high color purity,and flexibility is the biggest advantage of OLEDs that differ from other display and lighting technologies.At present,graphene has become one of the most interesting transparent electrodes in flexible OLEDs with its excellent photoelectric and mechanical properties.In this thesis,we used graphene as the anode of OLED to start a series of research work,as belows:We first discoveried the negative differential resistance(NDR)phenomenon in OLED based on graphene as electrode and found that it had a negative effect on the electroluminescence performance,which led to the increase in the device's turn on voltage and the decrease in brightness and efficiency.Based on the analysis on single carrier device,low-temperature electrical performance and film's composition,we demonstrated that the supporting material polymethyl methacrylate(PMMA)used in the process of graphene transfer was the main cause of NDR,clarifying its working mechanism.We invented the electric field annealing method to eliminate the NDR and made the device performance greatly improved.The turn-on voltage of device drops 1 V,and efficiency of device increased by 66.4%.The brightness and current efficiency of yellow phosphorescent OLED based on solution method were 59548 cd m-2(14 V)and 41.6 cd A-1(6 V),respectively.The above work is particularly suitable for abnormal electrical properties in those OLEDs due to occasional PMMA residue onto the graphene's surface.In most cases,the graphene has been thoroughly cleaned before the employment as an electrode.In the follow-up part of this paper,the process of transfering and cleaning of graphene was improved.The hot acetone was used to replace a room-temperature acetone to clean graphene together with a prolonged cleaning time,thus a more clean graphene was obtained,accompanied by normal electrical properties in those as-prepared OLEDs.In order to solve the problem of low work function of graphene,we further used poly(4-styrenesulfonate)(PSS)to modify the graphene electrode and obtained a stable p-type doping with an increase in work function from 4.51 eV to 4.79 eV.With a gradient hole injection structure of PSS/PEDOT:PSS/PEDOT:PSS:tetrafluoroethylene-3,6-dioxa-4-methyl-7-octene sulfone acid copolymer(PFI),we obtained extremely efficient blue and white OLEDs.The luminous efficiency of blue and white phosphorescent OLEDs reached 199.3 cd A-1(69.6 lm W-1)and 346.1 cd A-1(135.9 lm W-1)at high brightness of 2981 and 5270 cd m-2,respectively.The OLED in this work has a comparable performance with the most effective display and lighting technology.In this thesis,we put forward two methods to improve the performances of graphene-based OLEDs.The proposed interface modification technique is beneficial for the acquirement of high luminous performances,and once it occurs abnormal electrical properties in these graphene-based OLEDs,we can compensate the device performance with an electric field annealing method.As a result,this work provides two important technical means for obtaining high performance graphene OLEDs.