Investigation of top-emitting organic light-emitting devices with highly saturated color employing metal-mirror structure
Introduction
Organic light-emitting devices (OLEDs) have attracted extensive interests owing to their enormous potential applications in full-color flat panel displays since the first report by Tang and Van Slyke [1]. In the bottom-emitting OLEDs, the organic layers are deposited on glass with transparent anodes (such as indium tin oxide (ITO) or indium zinc oxide (IZO)) and capped with reflective cathodes [2], [3]. However, there has been increasing interests in fabrication of top-emitting organic light-emitting devices (TEOLEDs) that are capable of emitting light from the top surface. For example, in the design of active-matrix organic light-emitting devices (AMOLEDs) which have been demonstrated with high information content, a TEOLED structure is preferred because in a TEOLED the thin-film transistors can be placed under the OLED [4]. Thus, complicated pixels can be fabricated without diminishing the aperture ratio. TEOLEDs are also eminently suitable for making microdisplay on silicon because of the high level of integration of necessary driven circuits. A TEOLED can have anode in contact with the substrate as in a bottom-emitting OLED, or it can have the cathode in contact with the substrate in an invert structure. Most of the reported TEOLEDs adopt the former structure with indium-tin oxide (ITO) as anode on highly reflective metals, such as ITO/Ag [5], ITO/Al [6]. From the processing point of view, for such devices, it is better to have the metal electrodes in a TEOLED. For the top cathode, LiF/Al/Ag is commonly used because of its low cost, high electrical conductivity and easier deposition on organic materials without damage [7]. Ultrathin layers of Lif (1 nm)/Al (1 nm) which will not affect the optical properties of cathode are sandwiched between the organic layer and 20-nm-thick Ag film to improve the electron injection. For the bottom anode, the metal Ag with highest reflectivity in visible region is a good choice. The hole injection efficiency from Ag to most of the hole-transporting materials will be greatly improved by UV ozone treatment while the characteristics of low resistance, stable performance in air are retained [8]. Au is the other suitable choice for its high work-function [9]. In this paper, we report the influence of different metal anode on the overall efficiency of the TEOLEDs. Therefore, three devices with the same cathode structure of LiF/Al/Ag had been fabricated. Device one and device two using the same emitting material of Alq adopt Au and Ag as anode, respectively. For achieving high luminance efficiency and less angle dependence, the characteristics of device three employing totally the same electrode structure as that of the second one and making use of C545T-doped Alq instead of Alq as the emitting layer had also been studied.
Section snippets
Experimental details
The devices were built on the 1600-nm SiO2-coated silicon substrates precoated with 70-nm-thick Ag and Au. Then a thin silver oxide (Ag2O) was induced at the surface of Ag by the UV ozone treatment for 30 s, which greatly enhances the hole injection from Ag to the organic hole-injection layer. For all the devices, the organic multilayer structures sequentially consisted of 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamin}triphenylamine [m-MTDATA (45 nm)], N,N′-bis-(1-naphthyl)-N,N
Results and discussion
The normal direction (L–V) characteristics of device one and device two are displayed in Fig. 1. The plots clearly confirm that although the work-function of metal Au is higher than that of the metal Ag, device one still has the rather higher turn-on voltage. The turn on voltage of device one is 6 V with luminance of 4.6 cd/m2, however, device two turns on at 3 V with luminance of 2.4 cd/m2 reaches to 1543 cd/m2 while the increasing voltage from 3 V to 6 V. Such phenomena can be illustrated from the
Conclusion
In summary, the characteristics of TEOLEDs employing the same cathode and different anode had been studied. It revealed that the performance TEOLED using UV-ozone treatment Ag as anode is better than that of the device using Au as anode. Finally, by doping the appropriate dopant to Alq, a TEOLED with high power efficiency (8.7 lm/w with luminance of 640 cd/m2), almost independent on viewing angle had been reported.
Acknowledgements
This work was supported by the National key research and development program of China under Grant No. 2003cb314703 and the Natural Science Foundation of Fujian Province of China (No. T0750007).
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Cited by (1)
Enhancing hole-injection efficiency of top-emitting organic light-emitting diodes based on Al/MoN<inf>x</inf> anodes
2023, Materials LettersCitation Excerpt :In Fig. 4(a), the driving voltage of TOLEDs at the same current density is found to decrease with an increase in the growth time of the MoNx layers, which could be due to the increase in the work function of the MoNx layers. Importantly, compared to the TOLED with the common Ag anodes [8], all TOLEDs with Al/MoNx anodes show much lower driving voltages at the same current density due to their higher work functions. Remarkably, the TOLED with Al/MoNx (8 s) shows a lower driving voltage of 3.1 eV than that (3.4 eV) of the BOLED at 20 mA/cm2, indicating that the Al/MoNx (8 s) has a stronger hole-injection due to its higher work function (4.95 eV) than that of ITO (4.84 eV).