Plastic Multilayered Molecular Organic Light Emitting Diodes
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George M. Daly,t Hideyuki Murata,ý Charles D. Merritt and Zakya H. Kafafi* United States Naval Research Laboratory Washington, DC 20375 Hiroshi Inada and Yasuhiko Shirota Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan
ABSTRACT Enhanced performance has been observed for plastic molecular organic light emitting diodes (MOLEDs) consisting of two to four organic layers sequentially vacuum vapor deposited onto patterned indium-tin oxide (ITO) on polyester films. For all device structures studied, the performance of plastic diodes is comparable to or better than their analogs on glass substrates. At 100 A/m 2, a luminous power efficiency of 4.4 lm/W and external quantum yield of 2.7% are measured for a device structure consisting of two hole transport layers, a doped emitting layer and an electron transport layer on a polyester substrate. The same device made on a silica substrate has a luminous power efficiency of 3.5 lm/W and external quantum yield of 2.3%. Electrical and optical performance for comparable device structures has been characterized by current-voltage-luminance measurements and electroluminescence spectra collected normal to the emitting surface. In addition, an integrating sphere was used to collect the total light emitted and to determine the optical output coupling on glass versus plastic substrates. INTRODUCTION Emissive flat panel displays (FPDs) fabricated on a plastic rather than a glass back-plane represent the ultimate achievement in thin, lightweight devices with the added advantage that they are flexible and less costly to manufacture than their glass counterparts. Plastic FPDs are particularly suited to markets where flexibility and/or safety issues are important such as military and commercial head mounted and hand-held gear, and automobile and entertainment displays. Recently, it has been demonstrated that light emitting diodes (LEDs) made from polymer"'2 (PLEDs) and molecular organic3-5 (MOLEDs) materials can be fabricated on plastic substrates. The realization of flexible displays is possible through low temperature deposition processes for transparent indium-tin-oxide (ITO) electrodes 6 and the use of 'soft' organic materials as active device components. Amorphous organic films can be deposited onto any substrate, with no requirement for lattice matching or substrate orientation. Light emitting diodes made from molecular organic thin films have shown higher efficiencies compared to PLEDs. This is due largely to more efficient luminescent molecules and in some cases better carrier transport American Society for Engineering Education Postdoctoral Fellow *Sachs Freeman Associates, Inc., Largo MD 20774 Author to whom correspondence should be addressed. 33
Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
properties. In the present study, we evaluate the performance of MOLEDs fabricated on ITO/polyester (PET) and compare them to their analogs on ITO/silica (glass) substrates. EXPERIMENTAL SECTION Four MOLED
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