Research On Optical Characteristics Of Organic Light-emitting Devices With Microcavity: Theory And Experiment

Organic light-emitting device(OLED) has been attracting much attention for their potential application in flat panel display.The organic electroluminescence(EL) device with microcavity(MOLED) has some new unique characteristics,such as narrowed spectrum,intensity enhancement,stongly-directioned emission and so on. These features in emission quality are extraordinary valuable.Moreover,microcavity top-emitting organic light-emitting devices(TEOLED) now are focused,since they are capable of achieving monolithic integration of an OLED displays on a silicon chip, fabricating OLED displays for higher display quality without sacrificing aperture ratios of pixels.There are mainly two kinds of partially reflecting mirror of microcavity OLED (MOLED) as follows:1.one-dimensional photonic crystal(PC) DBR partially reflecting mirror;2.partially metal reflecting mirror.In the first case,ITO films are prepared by the sputtering method.Due to high energy sputtering process,the organics would be destroyed.Besides,ITO cathode goes against electron injection.All these disadvantages limit the application of transparent ITO electrode.In the second case, although metal mirror can improve the reflectivity,and the performance of cavity,but it has high adsorption and poor transmission.In 1987,Yablonovitch and John introduced the concept of PC and photonic band gap structure respectively.Therefore,the theories of PC,the application of PC and related experiment have been developed rapidly.Compared with the two-dimensional and three-dimensional PC,one-dimensional PC was easier to prepare,so it plays an important role in practical application.Using one-dimensional metallic-organic photonic crystal(MOPC) as partially reflecting mirror electrode,the microcavity OLED has two kinds of advantages.On one hand,MOPC has filtering performance,the same as ITO/BBR。On the other hand, MOPC has better transmission performance and some other performance than partially metal reflecting mirror.To date,there is little report on the microcavity OLED,which was based on MOPC electrode.In this thesis,I analyze the transmission characteristics of Ag and MOPC mirrors respectively by transfer matrix method based on theory of optical thin film using Matlab software.As for Ag mirror,I investigated the effect of thicknesses of Ag film on the reflectance spectrum and reflection phase shift spectrum with the structure of organic matter/Ag/SiO₂(1600nm)/Si and organic matter as medium.As for MOPC mirror(the structure was(Ag/NPB)_n,I have investigated the effect of thicknesses of Ag film(d₁),thicknesses of NPB film(d₂) and periodicity(n) on the reflectance spectrum, transmission spectrum and reflection phase shift spectrum.The obtained result provided theoretical guidance for the actual design of the microcavity formed by reflective mirrors in TEOLED.I designed a novel kind of TEOLED,which composed of Alq₃(emitting layer), metal electrode and MOPC electrode.The structures of the devices were (Ag/NPB)_n/Alq₃/NPB/Ag/ SiO₂(1600nm)/Si.The green,red,greenish blue,bluish green,yellow,orange,and pink devices were simulated by Matlab based on the micro-cavity theory.Furthermore,white device was obtained,which has a good CIE coordinate.1.Green device:The maximum proportion of enhancement of EL intensity was 3.061,when the structure was Si/SiO₂(1600nm)/Ag(100nm)/NPB(51nm)/ Alq₃(196nm)/(Ag(10nm)/NPB(150nm))_2.5.The results also exhibited the peak at (λ=515,I=2.840),with the CIE coordinates of(0.136,0.704) and the narrowest FWHM of 14nm,94nm less than that of the non-cavity device.The thickness of organic layers was thinnest,while the structure was Si/SiO₂(1600nm)/Ag(100nm)/ NPB(49nm)/Alq₃(178nm)/(Ag(10nm)/NPB(170nm))2.5.The results exhibited peak at(λ=509,I=1.407),with the proportion of enhancement of EL intensity of 1.618,the CIE coordinates of(0.106,0.577) and the FWHM of 21nm,87nm less than that of the non-cavity device.2.Red device:The maximum proportion of enhancement of EL intensity was 1.791,when the structure was Si/ SiO₂(1600nm)/ Ag(100nm)/ NPB(67nm)/ Alq₃(255nm)/(Ag(10nm)/ NPB(170nm))_2.5.Meanwhile,the thickness of organic layers was thinnest and the CIE coordinates was(0.554,0.339).3.Yellow device:The maximum proportion of enhancement of EL intensity was 3.476,when the structure was Si/SiO₂(1600nm)/Ag(100nm)/NPB(60nm)/ Alq₃(236nm)/(Ag(10nm)/NPB(150nm))_2.5.The results exhibited the peak at(λ=575, I=3.058) with the CIE coordinates of(0.467,0.518) and the FWHM of 13nm,95nm less than that of the non-cavity device.The thickness of organic layers was thinnest, while the structure was Si/ SiO₂(1600nm)/Ag(100nm)/NPB(60nm)/ Alq₃(224nm)/ (Ag(10nm)/NPB(170nm))_2.5.The results exhibited peak at(λ=575,I=2.986) with the proportion of enhancement of EL intensity of 3.020,the CIE coordinates of (0.448,0.517) and the FWHM of 12nm,96nm less than nonmicrocavity device.The most narrow FWHM was 1 lnm,97nm less than that of the non-cavity device,when two kinds of structures were designed:①Si/SiO₂(1600nm)/Ag(100nm)/NPB(60nm)/ Alq₃(229nm)/(Ag(10nm)/NPB(160nm))_2.5.The results exhibited the peak at(λ=575, I=3.058),with the proportion of enhancement of EL intensity of 3.221,the CIE coordinates of(0.457,0.521).②Si/ SiO₂(1600nm)/ Ag(100nm)/ NPB(61nm)/ Alq₃(228nm)/(Ag(10nm)/ NPB(170nm))_2.5.The results exhibited the peak at (λ=582,I=2.837) with the proportion of enhancement of EL intensity of 2.869 and the CIE coordinates of(0.483,0.481).4.Orange device:The maximum proportion of enhancement of EL intensity was 2.828,while the structure was Si/SiO₂(1600nm)/ Ag(100nm)/ NPB(63nm)/ Alq₃(247nm)/(Ag(10nm)/NPB(150nm))_2.5.The results exhibited the peak at(λ=591, I=2.175),with the CIE coordinates of(0.544,0.434) and the FWHM of 15nm,93nm less than that of the non-cavity device.The thickness of organic layers was thinnest, when the structure was Si/SiO₂(1600nm)/ Ag(100um)/ NPB(18nm)/ Alq₃(121nm)/ (Ag(10nm)/NPB(150nm))_2.5.The results exhibited the peak at(λ=591,I=2.557),with the CIE coordinates of(0.543,0.436),the proportion of enhancement of EL intensity of 1.768 and FWHM of 23nm,85nm less than that of the non-cavity device.The narrowest FWHM was 11nm,97nm less than that of the non-cavity device,while the structure was Si/SiO₂(1600nm/Ag(100nm)/ NPB(63nm)/Alq₃(233nm)/(Ag(10nm)/ NPB(170nm))_2.5.The results exhibited the peak at(λ=591,I=2.557) with the CIE coordinates of(0.526,0.435) and the proportion of enhancement of EL intensity of 2.626.5.Pink device:The maximum proportion of enhancement of EL intensity was 1.614.while the structure was Si/ SiO₂(1600nm)/ Ag(100nm)/ NPB(69nm)/ Alq₃(261nm)/(Ag(10nm)/ NPB(170nm))_2.5.Meanwhile,the thickness of organic layers was thinnest and the CIE coordinates was(0.506,0.333)。6.Greenish blue device and bluish green device:The maximum proportion of enhancement of EL intensity of greenish blue light was 0.891,while the structure was SiO₂(1600nm)/Ag(100nm)/NPB(46nm)/Alq₃(170nm)/(Ag(10nm)/NPB(150nm))_2.5. The results also exhibited the CIE coordinates of(0.197,0.0302).The maximum proportion of enhancement of EL intensity of bluish green device was 1.535,while the structure was Si/SiO₂(1600nm)/Ag(100nm)/NPB(47nm)/Alq₃(179nm)/(Ag(10nm)/ NPB(150nm))_2.5.The CIE coordinates was(0.155,0.397).7.White device:The maximum proportion of enhancement of EL intensity was 1.510,while the structure was Si/ SiO₂(1600nm)/Ag(100nm)/ NPB(70nm)/ Alq₃(296nm)/(Ag(10nm)/ NPB(150nm))_2.5.The results also exhibited the CIE coordinates of(0.335,0.346).The thickness of organic layers was thinnest,while the structure was Si/SiO₂(1600nm)/Ag(100nm)/NPB(70nm)/Alq₃(276nm)/(Ag(10nm)/ NPB(170nm))_2.5.The results showed that the proportion of enhancement of EL imensity was 1.478 and the CIE coordinates was(0.333,0.352).The CIE coordinates was closest to the equal energy point of white light at(0.332,0.334),while the structure was Si/ SiO₂(1600nm)/ Ag(100nm)/ NPB(72nm)/ Alq₃(284nm)/(Ag(10nm)/ NPB(160nm))_2.5.The results showed that the proportion of enhancement of EL intensity was 1.419.The introduction of the MOPC electrode not only enhanced the practicality, but also extends the application of the MOPCs.Additionally,the performance of the down-conversion layer capping on top of the electrode was also investigated.I have analyzed the performance of the white TEOLED based on down-conversion system with both experimental and theoretical issues.The structure of this device was Si / SiO₂(1600nm)/Ag(100 nm)/MoO_x(1.0 nm)/ 4,4', 4"-tris(3-methylphenyl-phenylamino)-tripheny-lamine(m-MTDATA,25 um)/ N, N'-bis-(1-naphthyl)-N,N'diphenyl-1,1-biphenyl-4,4'diamine(NPB,5 nm)/ 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl(DPVBi,20 nm)/tris-(8-hydroxyquinoline) aluminum(Alq₃,20 um)/ LiF(1 nm)/Al(1 nm)/Ag(20 nm)/ 3-(4-(diphenylamino)phenyl)-1 -phenylprop-2-en-1-one(DPPO,220 nm).The measured spectra were in excellent agreement with calculated ones.The results showed that this kind of TEOLED can emit white color light.The CIE coordinates of th device are(0.218,0.279),(0.231,0.290) and(0.267,0.405) at viewing angle of 0°,30°,and 60°,respectively.Besides,the CIE coordinates of the device was slight voltage-independent.The maximum brightness reached 5887 cd/m² and the maximum current efficiency was 1.1 cd/A,which was two times as that of the corresponding blue top-emitting organic light-emitting device,as the voltage was less than 10V.This present work provides a guidance towards the fabrication of the white TEOLED based on down-conversion system.