Abstract

Electroluminescent (EL) organic thin films have attracted a lot of attention in recent years due to their great potential for use in large-area low-cost displays. Bright electroluminescence has been demonstrated in thin films fabricated by either vacuum vapor-deposition¹ or by spin-coating semiconducting polymers such as PPV and derivatives.^2,3 The two approaches can also be combined and devices based on molecularly-doped polymers have been fabricated and characterized recently.⁴ The typical geometry of an organic light-emitting device is shown in Fig. 1(a). It consists of a transparent electrode, usually an ITO-(Indium Tin Oxide) coated glass slide (anode), a hole transport layer (HTL), a light-emission layer (EML), an electron transport layer (ETL), and an upper electrode (cathode). In this geometry the light is emitted through the transparent glass slide but because of reflection losses at the different interfaces, only less than 20% of the emitted light can be collected. In order to collect more light by stimulated emission, we propose the waveguide geometry shown in Fig. 1(b). In this geometry, the light-emission layer forms the core of an index-guided waveguide and the electroluminescence is excited into waveguide modes. Thus, if the material shows gain more light can be collected at the output of the waveguide and, furthermore, the device can act as an optical amplifier because the effective length of the active region is much larger than that of a surface- emitting device. Furthermore, the thickness of the EML has to be sufficient to support a waveguide mode and requirements on the optical properties of the HTL and ETL layers, especially lower refractive index than the EML and transparency, need to be met.

© 1996 Optical Society of America