Carbazole based bipolar host materials for organic electroluminescent devices -recent advances

Abstract

Organic light-emitting diode (OLED) is an emerging technology for flat panel displays and lighting
applications because they are highly efficient, ultrathin, lightweight, flexible and exhibit multicolor emission. A typical OLED is constructed by sandwiching an emissive layer between anode and cathode surrounded by additional layers for improved performance. These layers can employ host material doped with suitable guest material to accomplish the required effect of the layers such as hole transport effect, electron transport effect, or an emissive effect. The resulting guest-host system for phosphor-based OLEDs gives better performance over the traditional neat phosphor films because of low quenching concentration and triplet-triplet annihilation. Bipolar host materials containing both electron-donating and electron-accepting moieties in the
same molecule provides balance charge carrier mobility which results in higher quantum efficiency, pure emission of luminance layer, and high stability. The selection of appropriate host material is a challenging task as the non-emissive triplet excited state of host material must ordinarily be higher than the emissive triplet excited state of the guest phosphor. Carbazole based bipolar molecules are more important as hostmaterials for OLEDs due to advantageous properties of cabazole group to form stable radical cations (holes) along with high charge carrier mobilities. This mini review is a selection from recent advances in the design and synthesis of carbazole based bipolar host materials for OLEDs