Development of printed ITO coatings on PET and PEN foil for flexible organic photodiodes

Abstract

ITO (tin doped indium oxide) coatings with a sheet resistance of 2 to 3 kΩ(square) were produced by gravure printing process on PET and PEN foil. The printing paste consisted of ITO nanoparticles which were dispersed in a solvent by using a surfactant. The dispersion was mixed with a binder and a photo initiator before printing. The printed films were hardened under UV-irradiation at low temperatures (< 130°C). The sheet resistance could be decreased by heat treatment at 120°C under forming gas atmosphere (N₂/H₂) to 1.5 kΩ(square). The transmission of the ITO coated PET and PEN foils is more than 80 % in the visible range. The ITO films were directly used as the bottom electrode in an organic photodiode (OPD). The setup of the OPD originates from the well known Tang photodiode, consisting of a stacked layer of copper phthalocyanine (p-type material) and perylene tetracarboxylic bisbenzimidazole (n-type material). The photodiodes are characterised via current-voltage (I-V) characteristics. The performance of the photodiodes with printed ITO on plastic substrates could be improved by the deposition of a PEDOT/PSS layer (Baytron^(R) P) on the ITO coated foils and was then comparable to the performance of photodiodes with semi-transparent gold as anode on PET substrates. These results demonstrate the suitability of the printed ITO layers as anode for organic photodiodes.