Surface characterisation and functionalisation of indium tin oxide anodes for improvement of charge injection in organic light emitting diodes

• Published in: Thin solid films 2008-02, Vol.516 (7), p.1341-1344
• Main Authors: Davenas, J., Besbes, S., Abderrahmen, A., Jaffrezic, N., Ben Ouada, H.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical Sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Contact angle; Electrical properties of specific thin films; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronics; Exact sciences and technology; Indium tin oxide; Mechanical and acoustical properties; Optoelectronic devices; Physical properties of thin films, nonelectronic; Physics; Polymer; Polymers; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Solid-fluid interfaces; Surface energy; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Wettability; Wetting
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2007.03.163

Wettability studies have been performed to probe the surface properties of ITO substrates, aimed to be used as hole injecting electrode in OLEDs. The elimination of organic contaminants upon the cleaning treatment (ultrasonic bath in organic solvents) leads to an increase of the free energy of the ITO surface becoming hydrophilic. The surface energy components calculated from the Van Oss model show the appearance of a basic component upon the cleaning treatment. A thermal treatment at 100 °C for 3 h leads to a decrease of the surface free energy due to surface dehydration. These properties are attributed to the hydroxides formed at the ITO surface inducing improved adhesion at the ITO/polymer interface. The ITO surfaces have been functionalised with a chloroethylphosphonic acid mono-layer to increase their stability. The appearance of an acid–base component leads to a dipolar character of the ITO surface. The formation of a compact layer of a spin coated poly(phenylenevinylene) derivative induces the shielding of the ITO basic character. The weakening of the near infrared absorption associated to ITO free carriers confirms the formation of a dipole layer at the interface with the molecular layer in contact with ITO. Improved injection properties, shown by the current/voltage characteristics, result from the interface modifications.