Transport properties of indium tin oxide on anisotropic flexible transparent cellulosic substrates

Hydroxypropylcellulose (HPC) systems have been actively investigated as flexible transparent substrates for use in LCD technology. HPC solid films (20-60 gm) obtained from aqueous liquid crystalline solutions present tunable topography. The need for transparent contacts in working electronic devices leads to,the study of the properties of Indium Tin Oxide (ITO) deposited on these flexible substrates. Substrates-obtained from a 30% w/w solution of HPC in water (HPC30) were found to have an isotropic topography with a roughness (Ra) of 18.4 nin as measured by atomic force microscopy (AFM). Substrates with an anisotropic topography were obtained from a 65% w/w HPC solution (HPC65). AFM measurements show a Ra = 29.1 nm. Two types of peridiocities can be observed in HPC65 substrates. A primary set of large bands, perpendicular to the shear direction is characterized by a periodicity of 1.8-2.1 mum and an average peak-to-valley height of 74-87 nm. This first set of bands lie's under a secondary set of smoother band texture with a periodicity of 0.35 - 0.40 mum and an average peak-to-valley height of 2.9-4.4 nm. Transport properties of ITO deposited on HPC30 substrates (ITO+HPC30) show a resistivity around 3x10(-2) Omega.cm independent of orientation. ITO films deposited onto HPC65 substrates (ITO+HPC65) are conductive and show a resistivity which depends strongly on current direction about two orders of magnitude. The resistivity measured along the shearing direction is 3.8x10(-2) Omega.cm and along a perpendicular direction is 2.7 Omega.cm. The anisotropic resistivity of ITO+HPC65 cannot be a direct consequence of the current path difference between both directions due to surface topography. The optical microscope observation of the ITO+HPC65 surface reveals the presence of cracks in the ITO film along the shear direction, which could be induced by the anisotropic mechanical properties of HPC65 substrates. The resistivity difference in ITO+HPC65 could be attributed to these cracks.