Study On The High Efficiency And Superior Color-stability Of White Organic Light-emitting Diodes

Organic light emitting diodes?OLEDs?have huge potential applications in solid state lighting and flat panel display due to their small size,light-weight,quick-response,large angle of view,environmental protection,soft lighting,and so on.During the past two decades,the performance of OLEDs has been intensively developed and some OLED products have already entered the market.However,there are still some problems existed in OLEDs like complex structure,high fabricating cost,low efficiency,which hampers the application of OLEDs.In this work,we present two novel structures based on white OLEDs,achieving high performance white light emitting.The main contents are as follows:?1?We carried out a series of research to solve low color stability and high efficiency roll-off of multiple EMLs device structure.In general,multiple EMLs devices display high efficiency due to wide range of exciton recombination,but shift of exciton recombination will cause unstable light emitting.Some researchers insert spacer or interlayer in the devices to confine the exciton recombination to keep stable light emitting.However,this kind of method complicates the fabrication process and increases the fabrication cost,and the driving voltage always increases and the power efficiency generally decreases due to the presence of heterojunction interfaces by the additional layer,leading to bad device performance.To alleviate the issue,we designed new device structure containing double blue mixed-host EMLs with different mixed-host ratios and an ultrathin non-doped orange EML.We achieved high performance white light emitting without any spacer or interlayer.Firstly,we fabricated four blue devices with different structure.4,4',4''-tri?N-car-bazolyl?-triphenylamine?TCTA?with high hole-transport mobility was used as HTL,and 2,2',2''-?1,3,5-Benzinetriyl?-tris?1-phenyl-1-H-benzimidazole??TPBi?with high electron-transport mobility was used as ETL.The phosphorescent blue emitter is iridium???Bis?3,5-diflouro?-2-?2-pyridyl?phenyl-?2-carboxypyridyl??Firpic?,the doping concentration is 10%.We found that device with structure of ITO/MoO₃?2 nm?/TCTA?50 nm?/TCTA: TPBi: Firpic?2:1,10%,15 nm?/TCTA: TPBi: Firpic?1:2,10%,15 nm?/TPBi?30 nm?/Liq?1 nm?/Al?100 nm?showed best carrier injection ability and transport ability.Secondly,we inserted an ultrathin sensing layer at different positions of double mixed-host layers to investigate the main exciton recombination region.Finally,we fabricated four white devices by inserting an ultrathin orange?Iridium?III?bis?4-phenylthieno[3,2-c]pyridinato-N,C2??acetylacetonate,PO-01?layer with different thickness between double mixed-host blue EMLs.By optimizing the ultra-thin layer thickness,we achieved stable white light emitting.Peak efficiencies of 39.8 cd/A and 40.8 lm/W,low turn-on voltage of 2.71 V and negligible CIE coordinates change of?0.005,0.008?in a large range of luminance are realized.Moreover,the current efficiency of optimized white device still retains 35.9 cd/A,31.4 cd/A and 29.0 cd/A at the luminance of 1000 cd/m2,3000 cd/m2 and 5000 cd/m2,respectively.We concluded that high color stability and efficiency of white device is attributed to blue/orange/blue structure and balanced carrier injection and transport ability.?2?We solved the problem of low efficiency of simplified white OLEDs.Simplified device contains one organic layer which displays unbalanced electron and hole transport characteristics and the region of exciton recombination is close to electrode.The two above phenomena lead to simplified device low efficiency.To alleviate the issue,we provided new doping profile to improve device performance,named step-graded p-/ n-type doping profile.Firstly,we fabricated four orange devices with different doping structures.A common host 4,4'-Bis?carbazol-9-yl?biphenyl?CBP?with ambipolar transport characteristic used as the only active organic material.The results indicated that orange device with step-graded p-/ n-type doping profile showed better performance than that with uniformed doping profile.One orange device displays high efficiency of 30.0 cd/A.Secondly,we fabricated a series of single electron and hole devices to investigate the working mechanism.The results indicated that devices with step-graded p-/ n-type doping profile showed high current density than those with uniformed doping profile.Next,we investigated the exciton recombination width,the main exciton recombination region was located at the middle area of the device.Finally,we fabricated four white devices based complementary colors.By optimizing the thicknesses of EMLs,we achieved current efficiency of 15.4 cd/A,excellent CIE coordinates change of?0.008,0.002?in a wide range of luminance and low efficiency roll-off.We concluded that step-graded p-/ n-type doping profile could evidently improve carrier injection and transport ability,the high color stability of device was attributed to wide exciton recombination region and effective control on movement of main exciton recombination region.