Manipulation of work function and surface free energy of tungsten oxide hole injection layer modified with a self-assembled monolayer

As a hole injection layer for organic devices, a tungsten oxide (WO _x ) 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 WO_x substantially increased the ionization potential (I_p) of the substrate surface, which was effective in enhancing hole injection. The formation of SAM on WO_x reduced Ip, 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,N0-diphenyl-N,N0-bis(3- methylphenyl)-(1,10-biphenyl)-4,40-diamine (TPD). In the space-charge-limited region, a high current was drawn by using an anode that has a high Ip. At low driving voltages, however, the current flow was considerably influenced by the surface free energy. It was found that the PTMS-SAM on WO_x gives a satisfactory accommodation of both the work function and the surface energy.