Independently controllable stacked OLEDs with high efficiency by using semitransparent Al/oxide/Ag(Au intermediate connecting layer

Abstract

In stacked OLEDs, the intermediate connection layer plays critical role in modifying the device performance. For the independently controllable stacked OLEDs, the intermediate connecting structure should simultaneously function as an anode for one unit and as a cathode for another unit. To realize the simultaneous function, at present a semitransparent metal layer or a transparent indium-tin oxide (ITO), such as, Mg:Ag/ indium zinc oxide [1], CuPc /indium tin oxide (ITO) [2], and LiF/Ca/Ag [3], is generally used. However, the potential thin film damage due to sputtering of ITO and the transmission problem of the thin metal electrodes may limit the improvement of the device performance. Therefore, it is desirable to develop the intermediate controllable metal electrodes with the targets of not only improving the transmission but also enhancing the efficiency of the stacked OLEDs. A high efficiency OLED has been fabricated by using a vertical stack of two OLED units connected by Al/WO3/Ag. In the stacked OLEDs, each unit can be independently biased. It is found that the introduction of WO3 between Al and Ag (or Au) enhances the electroluminescent (EL) efficiency of top emissive unit. Meanwhile, the EL efficiency of the stacked OLED with Al/WO3/Ag and Al/WO3/Au interconnecting electrode is improved by 6 Cd/A and 8 Cd/A with respect to the case of using Al/Ag and Al/Au as the connecting electrode respectively. Our results indicate that Al/WO3/Ag(Au) is one of excellent connecting structures for fabrication of independently controlled high efficiency stacked OLEDs. [1] Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest, and M. E. Thompson, Science 276, (1997) 2009. [2] G. Gu, G. Parthasarathy, P. E. Burrows, P. Tian, I. G. Hill, A. Kahn, and S. R. Forrest J. Appl. Phys. 86 (1999) 4067. [3] J. X. Sun, X. L. Zhu, H. J. Peng, M. Wong, H. S. Kwok, Organic Electron., 8 (2007) 305.