Transparent electrode for top emission organiclight-emitting diode

Abstract

Organic light-emitting diode (OLED) has been successfully used in mobile display (small area) and television display (large area). For small area display screens, the top emission structure is mostly adopted because of the limitation of display area. Top emission OLED (TEOLED) can get higher power efficiency at the same current density than traditional bottom emission devices because of its higher aperture-opening ratio. However, the most important part of top emission device is the preparation of transparent electrode. In summary, according to the method and material, TEOLED transparent electrode is divided into the following categories: (1) Transparent conductive oxide electrode (TCO); (2) ultrathin composite metal electrode; (3) dielectric/metal/dielectric (DMD) composite electrode; (4) nanomaterial electrodes, etc. The general oxide has great optical transmittance in the visible light range, but the thermal evaporation temperature is very high. It needs to be fabricated by sputtering, which damages the organic layer. The composite metal electrode can be made by thermal evaporation, which is simple and easy. It is adopted by most commercial products by now, however its transparency is relatively low. Nano materials have high transparency and can be fabricated for flexible electrodes, but because of the solution processing, it is easier to destroy the organic layer. DMD composite electrode not only has high transmittance, but also can use thermal evaporation method to form film, it is expected to become better choice of top electrode for TEOLED. In order to achieve the objective of producing top emitting OLED with excellent properties, it is most important to find enough transparent, high conductive, ohmic contact, film forming process without destroying organic layer and stable electrode material. Up to now, the researchers have proposed several methods for preparing barrier layer-ITO, pulsed laser deposition transparent conducting oxide, thermal evaporated ultrathin composite metal, DMD composite, nano-materials, etc. In this paper, the applications of these materials in the TEOLED transparent electrode are reviewed. By comparing the advantages and disadvantages of thin film electrode, the research progress of TEOLED transparent electrode is reviewed.