Electroluminescence (EL) is a non-thermal light emitting process resulting from the application of an electric field to a solid material. A simple alternating current thin-film electroluminescent (ACTFEL) device consists of a metal-insulator-semiconductor-insulator- metal (MISIM) structure deposited on a substrate, usually being glass. (Fig. 1)
Fig. 1. Illustration of a monochrome ACTFEL display. Transparent conducting oxide (TCO) electrodes such as indium tin oxide (ITO) are used together with a glass substrate that allows the information to be viewed.
Fig. 2. Energy-band diagram of a ZnS:Mn based EL divice.
In a typical ZnS:Mn based EL device, high electric field (on the order of 108 V/m) causes tunnel injection of electrons from the interface states at one of the phosphor/insulating layer interfaces into the phosphor conduction band. The injected electrons are accelerated in the high field, where they gain enough kinetic energy to impact excite the luminescence center of the phosphor. Light emission is realized at the luminescence center via the relaxation of electrons from the excited state to the ground state. The high-energy electrons that pass through the phosphor layer are trapped at the other phosphor/insulator interface causing an internal polarization. An energy-band diagram of the EL mechanism is illustrated in Fig. 2. The same process takes place in the opposite direction of the EL device when the polarity of the applied ac field is reversed.
Electroluminescence was first demonstrated by Destriau in 1936,(1) who put copper doped zinc sulfide crystals into castor oil and applied alternating current to produce light. The voltage he used was as high as 15 kV but the luminance was extremely poor. Nevertheless, the Destriau cell triggered great interest in EL as the "source de lumière du futur". The early stage of EL development during the 50s and 60s was mainly concentrated with dc and ac powder cells. It ended without much success, however, owing to the poor performance and reliability of the devices. The situation of today's powder EL has not changed much though improvements have been made in efficiency and stability. A few applications based on powder EL, such as backlight for LCDs, have proved attractive.
The advent of TFEL was marked by a paper by Inoguchi et al. in 1974,(2) reporting a TFEL device of ZnS:Mn sandwiched between two insulators which showed surprisingly high luminance and extremely long operating life under an ac electric field. During the past 25 years, ACTFEL has progressed from the commercialization of monochromic flat panel displays in the early 80s to the development of multi-color and full-color displays in the 90s. Current TFEL displays provide high brightness and contrast, wide viewing angle, fast response time, ruggedness to shock and vibration, wide temperature range, and long operating life. These features meet the requirements for demanding applications such as industrial process control and instrumentation, medical equipment, civil and military avionics, and transportation. The rapid progress has turned the fiction of "light source of the future" into reality.
Lack of true full-color TFEL materials, nevertheless, has prevented EL displays from penetrating further into an already highly competitive display market. While efficient green and red light can be achieved by filtering the yellow emission from ZnS:Mn, the biggest and the most challenging problem is to find a satisfactory blue phosphor. Strontium sulfide based blue phosphors, such as SrS:Ce, SrS:Cu, and SrS:Ag,Cu, are the most promising candidates so far.
Bluish-green emitting SrS:Ce was already studied as a blue TFEL phosphor in 1984,(3) but the blue portion in its emission spectrum has been too small to meet the requirements for a full-color TFEL display. Significant efforts have been devoted to improving its EL performance. One frequently studied method is to modify the SrS:Ce phosphor by adding codopants so that either emission intensity can be enhanced or the spectrum can be shifted towards blue. Only recently, improved blue emission and excellent EL performances have been obtained with codoped SrS:Ce,Ag,Mn,Cl (CIEx=0.26, CIEy=0.47, L50=142 cd/m2, h=2 lm/W, 60 Hz).(4) Meanwhile, newly developed SrS:Cu shows a good blue-color gamut (CIEx=0.15, CIEy=0.23);(5) in particular, SrS:Ag,Cu has true blue color (CIEx=0.17, CIEy=0.13) that almost matches the hue of the CRT blue phosphor.(6) The EL performances of SrS:Cu and SrS:Ag,Cu are satisfactory (L40»34 cd/m2, h»0.24 lm/W, 60 Hz).(7) Nevertheless, the luminescence mechanisms of these new phosphors are poorly understood.
The aim of the study was to better understand and to improve state-of-the-art SrS based blue TFEL materials, namely SrS:Ce, SrS:Cu, and SrS:Ag,Cu phosphors. An extensive review of all materials used in TFEL displays is included in the list of publications [I]. Therefore, the present summary only briefly describes the concept and current status of TFEL that is related to this work. The experiments that were carried out focused on the effects of various impurity ions on the luminescence of the SrS based phosphors. Both intrinsic impurities in the phosphors and intentionally added impurities (codopants) were studied by combined ion beam analysis techniques, ion beam implantation and PL (photoluminescence) and EL measurements. Possible luminescence mechanisms for the new SrS:Cu and SrS:Ag,Cu blue phosphors are proposed.
2. T. Inoguchi, M. Takeda, Y. Kakahara, Y. Nakata, and M. Yoshida, SID 1974 Digest 5, pp. 84.
3. W.A. Barrow, R.E. Coovert, and C.N. King, SID 1984 Digest 15, pp. 249.
4. K.O. Velthaus, B. Hüttl, U. Troppenz, R. Hermann, and R.H. Mauch, SID 1997 Digest 28, pp.411.
5. S.-S. Sun, E. Dickey, J. Kane, and P.N. Yocom, Conf. Rec. 1997 Inter. Display Res. Conf., Toronto, pp. 301.
6. S.-S. Sun, 4th Inter. Conf. Sci. Tech. Display Phosphors, Extended Abstracts, Bend, OR, 1998, pp. 183.
7. S.-S. Sun, Conf. Rec. 1998 Inter. Display Research Conf., Asia Display'98, Seoul, CD-ROM.