Vacuum Ultraviolet-assisted Sol-gel Processing of Zn, Sn, Zn-Sn Oxide Thin Films for OLED Encapsulation

摘要

Room temperature solution processing of highly compact, glasslike thin films of ZnO, SnO_2 and Zn-Sn oxide was achieved for the purpose to use them as thin film encapsulation (TFE) for flexible film organic light emitting diodes (OLEDs), by employing photochemical decomposition of metalorganic precursors under vacuum ultraviolet (VUV) irradiation. Care had to be taken to control the amount of water in the coating solution since hydration water in the precursor compound already gave a strong influence to promote granular crystal growth. With anhydrous precursors, highly flat thin films without pinholes and cracks could be obtained. The analysis by FTIR and ellipsometry revealed fast decomposition of organic ligand and polycondensation to metal oxide thin films with high densities within 5 minutes. TEM observation in fact revealed crystallization into ZnO in about 5 nm size, despite the reaction was carried out in the absence of water and under N_2. Since the process does not require a high temperature, O_2 condition and H_2O to harm OLED devices, these metal oxides could be applied to OLEDs as TFEs to extend their lifetime for 3-4 times.