Studies On Top-emitting Organic Light-emitting Devices And White Light Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) have been attracting much attention due to their potential applications in flat-panel displays. Since Tang demonstrated the high brightness OLEDs at low operating voltage for the first time in 1987, materials and devices for OLEDs have developed rapidly,and now, OLEDs are on the way to industrialization.Nowadays, white organic light-emitting devices (WOLEDs) are considered as low-cost alternatives for backlight in liquid-crystal displays, full-color displays and illumination purposes. Greater success has been achieved in development of the WOLEDs since the first high-efficiency WOLED reported by Kido. High efficiency WOLEDs fabricated using different structures have been reported, most of which were the bottom-emitting WOLEDs. Report about top-emitting WOLEDs was relatively few.Our research work is on WOLEDs. The significative work accomplished in this dissertation are summarized as follows:(Ⅰ) Multilayer structures are commonly used in WOLEDs due to its easy fabrication processing, convenient chromaticity-tuning and high luminance and efficiency. We fabricated a new multiple emissive layer (MEML) white organic light emitting devices. In the MEML, yellow phosphorescent emitter bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2′)iridium(acetylacetonate) [(F-BT)2Ir(acac)] is introduced to codoped with the red one bis(2,4-diphenyl-quinoline) irdium acetylanetonate [Ir(piq)2acac] into 4,4'-N,N'-dicarbazole-biphenyl (CBP), fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped CBP is used for green emitting, N,N'-di(naphthalene-1-yl)- N,N'-diphenyl-benzidine (NPB) is used for blue emission. In addition, we insert a thin CBP film to restrain the energy transfer from green emission of Ir(ppy)3 to yellow of (F-BT)2Ir(acac) or red emission of Ir(piq)2acac. By adjusting devices'structure, we get a WOLED with a maximum luminance of 50030cd/m2, maximum efficiencies of 21.6cd/A (at 7V), 10.3 lm/W (at 5V) and 8.4% (at 7V) and color rendering index (CRI) of 85. In addition, its Commission International De L'Eclairage (CIE) coordinates slightly changes from (0.3867, 0.4075) to (0.3638, 0.4070) when drive voltage shifts from 7V (1862cd/m2) to 12V (39270cd/m2).(Ⅱ) In this paper, performances of TOLEDs have been improved by two methods as follows:Firstly, we fabricated the high-transmissivity cathode to reduce microcavity effects. Though Ag is a stable metal in air and has a high conductivity and a low absorption in visible light region, the thin Ag layer has a poor transparency. Samarium (Sm) is a metal with a high transmissivity in visible light region, the transmissivity can be improved by introuducing a combined Sm/Ag electrode. And the key merit of Sm is its relatively low work function (~2.8 eV) which can enhance electron injection ability in top-emitting OLEDs. Therefore, we adopted Sm/Ag as semi-transparent cathode for our TOLEDs, which ensured both the injection of electron and the conductivity and transmission of the cathode.Secondly, the high reflectivity of bottom anode is essential for achieving high efficiency in TOLEDs based on silicon. Although Ag has a high reflectivity among metals, it is not considered as ideal anode for TOLEDs, because of its low work function (~4.3eV). UV-ozone treatment is a simple way to induce a thin Ag2O film on Ag surface, with which hole injection from anode can be improved.Finally, the deposition of BCP layer on the semitransparent cathode as an outcoupling layer is an effective way to improve the efficiency of the TOLEDs. A BCP layer is chosen as the output layer for the merits of easily evaporation and no absorption in visible area. The performances of our device was the best when BCP thickness had been adjusted to 80~85nm. However, the spectra with a wide full width at half maximum (FWHM) can be obtained when the BCP thickness is around 40~50nm which is benefical to the fabrication of a top-emitting WOLED.The research work of this paper is not only an integrated research about a new MEML WOLEDs, but also the fabrication of TOLEDs with a wide FWHM, all of which above is the experimental foundation for the research of white TOLEDs in the future. I hope that we can use all of above results to fabricate a new sort of high-performance white TOLED.