Sixth International Workshop on Electroluminescence

Sixth International Workshop on Electroluminescence

Conferences Sixth International Workshop on Eleetrolumineseenee El Paso, Texas, USA, 13-15 May 1992 More than 80 participants from ten countries share...

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Conferences Sixth International Workshop on Eleetrolumineseenee El Paso, Texas, USA, 13-15 May 1992 More than 80 participants from ten countries shared their experience during the 6th International Workshop on Electroluminescence at El Paso, Texas in May 1992. The official programme of the workshop, which was organized by the University of Texas at El Paso, comprised 45 posters, 20 invited and 11 other contributed presentations. A wide range of different topics related to electroluminescence (EL) was covered, from recent advances in thin-film electroluminescent (TFEL) multicolour displays to the energy distribution of the hot electrons in the light-emitting phosphor films. Among the invited papers a particularly interesting presentation was given by Dr A. Mikami from Sharp Corporation who reported on the recently developed low-pressure hydrogen transport CVD process for making EL films. Ageing characteristics of these device structures can be explained in terms of interfacedominated and bulk-dominated electron transport in the phosphor films. As a result the ageing behaviour cannot only be controlled but also kept very small, which is a most significant result. The development of colour phosphors for multicolour EL displays was very much in focus during the workshop. The status of TFEL eolour phosphors was covered in an invited talk by Dr D. Tuenge from Planar. Best luminance values measured at 60 Hz driving voltage frequency are: Red (ZnS:Mn with red filter) 75 cdm -2 Green (ZnS :Tb, O, F without coupling layer) 125 cd m - 2 Blue (proprietary phosphor, Planar) 10 cd m -2 The red and green colours are sufficiently bright for multicolour displays. The blue luminance of Vol 13 No 4 1992

10 cdm -2 at 60 Hz driving frequency indicates very important progress in the blue, which has been considered a problem. This luminance level is in fact already close to that required in a full-colour display. The colour shade is also very good (x = 0.15, y = 0.20). A VGA size display panel with all primary colour has been fabricated at Planar. Another impressive result on the blue phosphor was reported by Dr R. Mooch from the University of Stuttgart in Germany. A ZnS/SrS: Ce/ZnS/SrS :Ce/ZnS... multilayer phosphor film was developed in collaboration with the group from Tottori University in Japan. The high vacuum deposition process in Stuttgart gave structures with a blue luminance of 10cdm -2 at 60Hz (x = 0.10, y - 0.26). Several other studies of multilayer phosphor films were presented at the workshop. In an invited talk Professor H. Kobayashi from Tottori University gave an interesting overview of work in this direction until now. These approaches give advantages as the composition can be 'tailored' to provide both good electron transport and an appropriate environment for the lightemitting dopants. One may still ask why sophisticated epitaxial techniques such as MBE are not more explored in this research where important, novel work still remains to be done. Phosphor films based on fluorine compounds have aroused new interest. CaF2:Eu has been presented as one promising candidate for the blue phosphor. These compounds are much more stable than the rare earth alkaline sulphides that have been considered as potential materials for EL phosphors, the blue in particular. Interesting results using oxide phosphors were reported. In particular, the work done at the Kanazawa

Institute of Technology in Japan on Zn2 SiO4: Mn should be mentioned. A luminous efficiency of 0.8 lm W -~ in the green has been achieved. The advances in meltieolour TFEL display development was illustrated by multicolour displays from Planar and Komatsu Ltd. Both multicolour displays were made using an inverted TFEL display structure. In the monochrome TFEL displays on the market at present the light emission is observed through the substrate. In the inverted structure the light emission is observed from the thin-film side of the substrate through a transparent electrode on top of the TFEL film structure. By using a phosphor with a broad emission spectrum and patterned colour filters on the top, a multicolour device structure almost as simple as the monochrome one is achieved. In addition to wide viewing angle and fast response time, excellent contrast without any contrast enhancement filters is a further benefit of this display structure. Planar demonstrated a red/green/ yellow 9 in diagonal display based on the well known ZnS:Mn phosphor. Luminance, contrast and power consumption were comparable to those of monochrome TFEL displays. This display concept, which is expected to be a cost-effective alternative in demanding multicolour flat-panel applications, is now being developed towards production. Komatsu Ltd demonstrated a 5 in diagonal display with all primary colours. A broad emission spectrum extending from the blue to the red was obtained using a

ZnS: Mn/SrS: Ce/ZnS: Mn phosphor. A white areal luminance of 10cdm -2 was reported. Although this display does not yet meet all requirements for practical applications, it clearly shows how simple the structure of a full-colour TFEL can be. On an academic level a deeper understanding of the electron transport in the phosphor films ac TFEL devices has been gained. The simple model according to which electrons are trapped only at the phosphor193

Conferences insulator interfaces was challenged. Evidence for bulk trapping and space charge in the phosphor films was given in several presentations. The energy distribution of the hot electrons in the light-emitting phosphor films was widely discussed. An illustrative presentation of the energy distribution was given in an invited talk by Dr E. Bringuier from Universit6 Pierre et Marie Curie in Paris. He questioned some earlier experimental results indicating that the energy may be even 10eV in ZnS and pointed out that intervalley scattering is expected to limit the

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energy to 2-3eV. The counterargument from Professor G. O. Miiller from Berlin was the validity of the band structure in electric fields in excess of 10SVm -1. Compared with earlier workshops, a considerable number of novel approaches were presented in El Paso. Young newcomers in the field contributed to the discussions in a fruitful way. The first multicolour TFEL displays will appear on the market in 1993. The gap that to some extent existed earlier between academic and industrial research is getting narrower. As Jim

Hurd, president of Planar, pointed out in his introduction, collaboration could be even closer between different groups to promote progress in this very exciting flat-panel technology. The papers presented at the Workshop are published in the Proceedings (Cinco Puntos Press in conjunction with the University of Texas at E1 Paso). The Seventh International Workshop will be held in Tianjin, China, in 1994. Runar Tiirnqvist Planar International Ltd Finland

Displays