Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.

A versatile method for the preparation of mesoporous tin doped indium oxide (ITO) thin films via dip coating is presented. Two different PIB/PEO copolymers are used as structure directing agent. Transparent conductive coatings were obtained after thermal treatment at 500 °C and conductivity could be improved by annealing under reducing atmosphere. In situ conductivity measurements during annealing were carried out and lead to specific resistances of < 1 Ωcm for films of ~100 nm in thickness. Further characterisation included common techniques like SEM, TOF-SIMS and XPS. Prussian Blue was deposited electrochemically to confirm accessibility of the porous system and electrode functionality.