Studies On Top-emitting White Organic Light-emitting Devices

Since top-emitting organic light-emitting devices(TEOLEDs) were introduced, it had attracted more and more attentions due to its advantages on resolving the competition between pixel driving circuits and active emissive area in active matrix displays and fabricating OLED displays with improved aperture ratios of pixels. So this technology of organic light emitting was thinked the most promising nest generation flat display technology to replace LCD. TEOLEDs now are being a focus in the field of OLEDs.Besides TEOLEDs on silicon can achieve monolithic integration of OLED displays on a silicon chip and are also suitable for making microdisplay on silicon.To apply to display, top-emitting white organic light-emitting devices(TEWOLEDs) are widely researched.But building TEWOLEDs is difficult due to the microcavity effects of the devices with double metallic electrodes.This paper is focus on how to achieve silicon-based top-emitting white organic light emitting devices with improved performance by adjusting the optical performance of the devices.With microcavity effects,we find two methods to adjust the color purity of devices and discuss the issue of contrast ratio and encapsulation.Because of microcavity effects, the TEOLEDs employing Ag as anode and cathode hardly achieve multi-mode emission. So we build TEWOLEDs with down-convertion phosphors.White light emitting are obtained by adjusting the thickness of the down-convertion layer and the blue device.The results for experiments are in excellent agreement with calculated one.The current efficiency of TEWOLED is 1.1cd/A, which is twice over the corresponding blue TEOLED. The color of TEWOLEDs built with this method will not change with time,which is a problem for WOLEDs with multi-emitting layer due to different aging of the emissive species. The relative low efficiency of the white TEOLED is attributed to the low efficiency of the blue emission material and low quantum yield of the down-conversion material. The performance of the white TEOLED can be improved by using high efficiency down-conversion materials and corresponding blue device.The phase shift on reflection of one-dimensional metallic-dieletric photonic crystal(1D-MDPC) is different from single metal layer.Reports showed that the phase shift on reflection of periodic or quasi-periodic MDPC was approximately periodic change with wavelength.Thus,multi-mode emission and white light emission can be achieved in TEOLED by adjusting the parameter of MDPC.Firstly,we fabricate TEWOLEDs with periodic 1D-MDPC as cathode.The cathode with a structure of 1D-MDPC Ag(20 nm)/Alq(x nm)/Ag(20 nm)/Alq(x nm) are built.Two -peak TEWOLEDs are obtained by changing the value of x. MDPC. The efficiency of the TEWOLEDs is comparable to the corresponding bottom WOLED, which are 9.51 (two-peak device) and 9.23 cd/A.The emission spectra of TEWOLEDs can be adjusted by changing the value of x.When x is changed from 75 to 85,the resonant wavelengths (RWs) shift about 10nm to longer wavelength.In additional, encapsulation is important for OLED to prevent oxidation of light-emitting materials and electrodes by blocking permeation of water vapor and ambient oxygen, and to protect devices from external shocks.Buffer layer is indispensable in thin-film encapsulation. In the cathode of Ag(20 nm)/Alq(x nm)/Ag(20 nm)/Alq(x nm),the effective contact is the first Ag layer,so the rest layers of /Alq(x nm)/Ag(20 nm)/Alq(x nm) can be used as butter layer,which would make the process of encapsulation simpler.Theory calculated results show that the spectra and CIE coodinates are almost the same between with and without encapsulation.secondly,we fabricate TEWOLEDs with quasi-periodic 1D-MDPC as cathode. The anodes with a structure of 1D-MDPC Ag(150 nm)/Alq(x nm)/Ag(20 nm)/Alq(y nm) /Ag(20 nm) are built. A two-peak(x=75,y=45) or three-peak(x=95,y=85) TEWOLED was obtained by simply adjusting the thickness of the organic layers in the MDPC.The efficiency of the TEWOLEDs is comparable to the corresponding bottom WOLED(11.1 cd/A), which are 9.9 cd/A (two-peak device) and 9.4 cd/A (three-peak device).In principle,we can obtain desired multi-peak device through changing the period number and the thickness of dielectric layers of 1D-MOPC.Thus,we can fabricate devices that meet the practical needs.In addition,the devices fabricated with this method possess higher pixel contrast ratio(PCR) than bottom ones,which is 52:1 and 77:1 under an ambient illumination of 140lx, corresponding to an enhancement of 0.86 and 1.75 over that of bottom one. The efficiency of three-peak device is lower than that of the two-peak one because of only two emitting units(blue and yellow) in the TEWOLED.The efficiency of the three-peak device is lower by 5% but the PCR is higher by 48% than the bottom one.So The three-peak device is superior to the two-peak one for application of display.Finally,we investigated the performance of 1D-MOPC and found the effects of 150 nm thick metal on the phase shift on reflection of anode is dominating,especially affecting the resonance wavelength(RW) in short wavelength.In additional,in order to be compatible with the driving circuit[such as organic thin film transistor (OTFT) or complementary metal oxide semiconductor (CMOS)],we utilized Au and Al to build anodes with structure:Au(150 nm)/Alq(75 nm)/Ag(20 nm)/Alq(45 nm)/Ag(20 nm) and Al(150 nm)/Alq(75 nm)/Ag(20 nm)/Alq(45 nm)/Ag(20 nm). We calculated the effective refractive index and investigated the relation between RWs and different metal materials.In conclusion,only the RW in short wavelength is changed markedly ,but no effects on the yellow emission when utilizing different metal materials in anodes,Finally,we fabricated the TEWOLEDs with three different anodes and were in agreement with the calculated results,which demonstrated our prediction.