Vacuum Ultraviolet Photochemical Sol-Gel Processing of Zn, Sn, Zn-Sn Oxide Thin Films for Encapsulation of Organic Light Emitting Diodes

Abstract

Room temperature solution processing of highly compact, glass-like thin films of ZnO, SnO2 and Zn-Sn oxide was achieved for the purpose to use them as thin film encapsulations (TFEs) for organic light emitting diodes (OLEDs), by employing photochemical decomposition of metallorganic precursors under vacuum ultraviolet irradiation in dry N2. While hydration water in the source chemical strongly promoted granular crystal growth, anhydrous precursors achieved highly flat thin films without pinholes and cracks. The analysis by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ellipsometry revealed fast decomposition of organic ligand and polycondensation into metal oxide/hydroxide thin films with high densities within 5 minutes. Transmission electron microscopic observation revealed crystallization in about 5 nm size in case of ZnO, whereas SnO2 remained totally amorphous. Mixing of the two precursors to yield Zn2SnO4 and ZnSnO3 was able to control the degree of crystallization vs. compactness of the thin film to maximize their ability as TFEs to extend the lifetime of OLEDs for about 4 times for operation under ambient air.