The formation of an optically functional nano-lens array by substrate surface treatment

Recently, a nano-lens array (NLA) technology has been developed to greatly enhance light extraction efficiencies and color stabilities of red, green and blue organic light-emitting diodes. The fabrications of N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (C₄₄H₃₂N₂, NPB) NLAs on oxide layers were accomplished by an organic vapor phase deposition during which surface tensions of NPB were increased by its crystallization. To emphasize the importance of the fabrication technology, the kinds of substrate materials on which NLAs are formed should be extended to high surface energy nitrides. Herein, to understand the role of the surface condition of a silicon nitride substrate in the formation of NLA, three types of nitride substrates are prepared by using surface treatment, while one NPB deposition recipe is used. Very interestingly, even though nano-lens is not observed in an untreated substrate with high surface energy, nano-lens is formed by surface treatment and the resulting decrease in surface energy. This indicates that NLA formation is not limited to an oxide substrate, but is determined by the difference in surface energy values between a substrate and a deposited material on the substrate. Furthermore, the morphological change of NPB by surface treatment is extensively discussed using contact angle experiments.