Electronic states of vapor deposited electron and hole transport agents and luminescent materials for light-emitting diodes

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Abstract
The electronic states of vapor-deposited materials used in electroluminescent devices were measured by ultraviolet and x-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence spectroscopy. The combination of these measurements on ultrathin films of these materials allows (1) the determination of the energy (with respect to vacuum) of the highest occupied molecular orbital (HO) and the ionization potential (IP), and (2) the estimation of the lowest unoccupied molecular orbital (LU) energy and an approximation of the electron affinity, (EA). The knowledge of the binding energies of these states is important for the understanding of light-emitting diode properties and the potential optimization of such devices. The luminescent material tris(8-hydroxy-quinoline) aluminum has an IP of 5.9 eV and an apparent EA smaller than 3.5 eV. The IP of both hole transport agents, tri-p-tolylamine and 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane, is 5.4 eV and their EA is estimated to be smaller than 1.8 eV. The electron transport agents 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 2-t-butyl-9,10-n,n’-dicyano-anthraquinonediimine and dicyano-diphenylsulfone differ in IP from 7.1 to 7.6 eV and the EA for these materials are estimated to be smaller than 3.5, 4.9 and 5.5 eV, respectively.