Manipulation of Work Function and Surface Free Energy of Tungsten Oxide Hole Injection Layer Modified with a Self-Assembled Monolayer

概要

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As a hole injection layer for organic devices, a tungsten oxide (WOx) thin film was vapor-deposited on an indium--tin oxide (ITO) substrate, on which a self-assembled monolayer (SAM) of either 3-aminopropyltrimethoxysilane (APS), phenyltrimethoxysilane (PTMS), or octadecyltrimethoxysilane (ODS) was prepared to modify the surface characteristics. The deposition of WOx substantially increased the ionization potential ($I_{\text{p}}$) of the substrate surface, which was effective in enhancing hole injection. The formation of SAM on WOx reduced $I_{\text{p}}$, but enabled the control of the surface free energy so as to modify the growth morphology of an organic film deposited on its surface. A hole-only device was prepared using a hole transport material of N,N$\ake '$-diphenyl-N,N$\ake '$-bis(3-methylphenyl)-(1,1$'$-biphenyl)-4,4$'$-diamine (TPD). In the space-charge-limited region, a high current was drawn by using an anode that has a high $I_{\text{p}}$. At low driving voltages, however, the current flow was considerably influenced by the surface free energy. It was found that the PTMS-SAM on WOx gives a satisfactory accommodation of both the work function and the surface energy.
2011-01-25