Multi-Mode Emission from an ALQ Based Optical Microcavity
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INTRODUCTION Electroluminescent (EL) devices based on the thin films of molecular or polymer materials have attracted a great deal of attention due to the promising applications in flat panel displays[1-5]. One advantage of organic light emitting devices (OLEDs) is that the EL emission could cover a wide range of colors over the full visible region. However, the emission spectra of most organic materials are very broad. In the past few years, optical microcavity has been introduced into the OLEDs to control the emission properties of the organic materials[6-9]. It had been shown that not only three primary colour, but also a multicolour emission could be achieved in the OLED with amicrocavity[10]. In this paper, white light emission from an OLED with a microcavity was demonstrated. 2.
EXPERIMENTAL The molecular structures of organic materials and the microcavity structure used in the experiment are shown in Fig.l. The microcavity is sandwiched between a DBR and a metal Al reflective mirror. The ITO and Al film was used as the anode and cathode, respectively. The TPD was used as hole transport layer. The Tris(8quinolinolato) aluminum (Alq) was used as the lightemitting layer. DBR consists of three quarter-wavelength stacks, which is made according to the prescription: G[H1L1]6[H2L2]6[H3L3]4H3. There the G label represents the glass substrate, the notation [HL] 6 implies a quarter-wave of high-index material, H. followed by a quarter-wave of low-index material, L, 6 times. The high-index material is ZrO2 with a refractive indices nH of 2.0. The low-index material used here is SiO 2 with a refractive indices nL of 1.46. The quarter wavelengths of the three stacks are 430nm, 570nm and 630nm, respectively. The DBR was electron-beam evaporated on the polished K9 glass substrates. The ITO film was electron-beam evaporated on the DBR reflector. Then TPD, Alq and Al layer was deposited on the ITO layer subsequently by thermal evaporation in a vacuum of 1x103 Pa. For comparison an OLED without the DBR reflector was also fabricated on an ITO coated glass substrate. The transmittance spectrum was measured on a Shimadzu
Dual-Wavelength/Double-Beam Recording Spectrophotometer UV-3000. The EL emission spectra were recorded for the normal direction on a Hitachi Spectrophotometer F4500.
Fig. l Microcavity structure and the molecular structures of the organic materials
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RESULTS AND DISCUSSION DBR used in an organic microcavity usually consists of a stack of quarter wavelength thick slabs of dielectric with alternating high and low refractive index. The high reflectance of this kind of DBR is obtained over a limited range of wavelengths. In this paper, the DBR was made by overlapping of the three stacks. The transmittance spectrum of it was shown in Fig.2.
Fig.2 Transmittance spectrum of DBR It showed that the stopband range of the DBR was extended from 410nm to 650nm. The reflectivity at the stopband of the DBR was estimated fi om the transmittance spectrum to be about 99%. Fig.3 shows the EL spectra measured at current
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