Light-emitting characteristics of top-emitting organic light-emitting diodes based on the interfacial electronic feature of Cs-on-Alq₃

This study used ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) to examine the interfacial electronic structures of Cs deposited on tris(8-quinolinolato)aluminum (III) (Alq ₃). In the valence band spectra, the low work function of Cs reduces the barrier height for electron injection (Φ^B_n) in the Cs-on-Alq₃ interfaces to 0.2 eV when a 1- ̃ 3-nm-thick layer of Cs is deposited on Alq₃. This decrease in Φ^B _n to 0.6 eV means more efficient electron injection than with a Φ^B_n of 0.8 eV for the pure Alq₃ layer. The XPS O 1s peak revealed a two-step interface reaction, which showed the formation of a stable radical anion for low Cs coverage (Θ_Cs ≤ 0.4 nm) and the appearance of a metallic component through the decomposition of the highly reactive Cs at higher coverage (Θ_Cs ≥ 1.0 nm). Top-emitting organic light-emitting diodes with Θ_Cs (1-0 nm) based on these interfacial electronic structures showed much higher device performance with a luminous efficiency of 2.0 % and a maximum luminance of 48000 cd/m² compared to other devices with Θ_Cs > 1.0 nm.