Electronic structure and localized states in starburst trisphenylquinoxaline

Abstract

Quinoxaline model compounds are interesting materials for Organic Light Emitting Devices (OLEDs) because of their thermal stability and their higher ionization potential in comparison to other electron transporting/hole blocking materials. In this work we studied a starburst trisphenylquinoxaline by means of Ultraviolet Photoelectron Spectroscopy (UPS) and Thermally Stimulated Luminescence (TSL). UPS provided not only the characterization of the valence band structure but also parameters like ionization potential, 6 eV, and, combined with absorption spectroscopy, electron affinity, that are of crucial importance in designing optimized OLED configurations. On the other hand, a wide distribution of localized states occurs in organic layers due to several factors and TSL can investigate these states. The combination of the used techniques together with semi-empirical quantum chemical calculation, gave a detailed description not only of the valence and conduction band of the studied materials, i.e. the energy position of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), but also of the trap distributions localized in the band gap: a shallow one at 0.06 eV and a deeper state centered at 0.24 eV. The full spectroscopic and electrical characterization of the material formed the background for understanding its behavior in heterolayer devices.