Characterization and synthesis of organic semiconductor materials for light-emitting diodes: Structure-property relationships

摘要

Light generation in organic light emitting diodes (OLEDs) requires the recombination of electron-hole pairs at a p-n junction. Radiative recombination has been investigated with solution electrochemical studies of the principal lumophores, dopants, and hole-transport agents of small molecule OLEDs. We have found that solution electrogenerated chemiluminescent (ECL) reactions between radical anion and cation states of OLED components model the spectral output and efficiency of solid-state electroluminescence. These studies have shown that the molecular structures of commonly used materials in OLEDs are not optimized. ECL has been used to develop and optimize new materials that dramatically improve device performance. The results of the ECL experiments can be explained using commonly accepted electron transfer theories. Nonaqueous electrochemistry and spectroelectrochemistry have been used to study the energetics and stability of radical cation states of new and existing biphenyl-bis-triarylamines (TPDs), triphenylamines, and carbazoles, common hole-transport materials in OLEDs. We have established that these triarylamines, upon anodic oxidation, cation radicals with widely variable stability that react via coupling-deprotonation to form a neutral dimer. We have quantified the decay of these reactions by following the visible absorption decay of the cation radicals. The stability of these states to dimerization reactions is critically dependent on molecular structure. We speculate that these dimerization reactions could occur in the solid state, and that this reaction could lead to OLED degradation, since it involves the loss of a proton. Derivatives of 9,10-diphenylanthracene were synthesized for use as Forster energy transfer dopants and stable charge traps in OLEDs. Cyclic voltarnmetry shows that these compounds form stable radical cation and anion states in nonaqueous electrolytes. Introduction of substituents to the anthracene ring can modify the HOMO - LUMO gap of these structures. When these molecules are doped into poly(N-vinylcarbazole), they act as Foster energy acceptors. Photoluminescent and electroluminescent spectra of these doped polymer composite films show that the luminescence originates from the singlet excited state of the diphenylanthracenes. Preliminary results of single layer OLEDs made from these materials are promising; device external quantum efficiencies attained up to ca. 1.2 % and brightness up to 800 cd/m² at 12 volts.