Synthesis and characterization of organic electroluminescent materials

Abstract

Generally, OLEDs are thin-film multi-layer structures composed of a hole-transporting, an emitting, and an electron-transporting material sandwiched between two electrodes, which result in a good charge balance of electrons and holes. Moreover, the buffer layer was inserted between ITO and hole-transporting layer to lower the operating voltage. In this thesis, the fluorene-based poly(iminoarylene)s with triarylamine unit were simply synthesized from palladium-catalyzed polycondensation of 2,7-dibromo-9,9-di-n-alkyllfluorene with primary amines such as aniline and p-toluidine. The polymers with high molecular weight were obtained and were thermally stable. The HOMO levels of the polymers (~ -5.1 eV) were close to the work function of ITO. Organic light-emitting diodes (OLEDs) of the form ITO/polymer/TPD/$Alq3_$/LiF/Al showed lower turn-on voltage ($V_T$ = 2.2V), the enhanced efficiency, and the higher maximum luminance at the higher current density (PFA1: 12370cd/㎡ at 427mA/㎠) than those of the device without polymer ($V_T$ = 3.6V, 5790cd/㎡ at 233mA/㎠). It is expected that these polymers can be used as a buffer layer in OLEDs. For full color displays, red, green and blue OLEDs should have high luminance, high efficiency, appropriate chromaticity, and long lifetimes. The performances of green and blue OLEDs have been improved up to the requirements of practical applications, but red OLEDs continue to exhibit poor color purity and low efficiency. First, the bis-condensed 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM) derivatives are introduced as a new class of red dye for OLEDs. They showed more red-shifted emission than the mono-substituted DCM derivatives and the emission maxima increased as the electron-donating ability of the aromatic donor group increased. On the basis of these results, red OLEDs were fabricated with bis-condensed DCM derivatives as red dopants. For a device of configuration with ITO/TPD/$Alq_3$ :DADB (5.2wt%)/$Alq...