The Evaluation of the Fracture Strain of ITO Films on Polymeric Substrates

Abstract

One of the mechanical issues concerning flexible organic light emitting device (OLED) is the flexibility, which is controlled by the fracture strength of the brittle films in the device. For example, the integrity of the anode material, the ITO film in the device directly controls the functioning of the device. Understanding the behaviour of these films under flexed condition will help maximize the flexibility of the device. Experiments have been devised to achieve this goal by bending the films of interests to gradually increased curvature over the point of film cracking, both under tension and compression. Fracture mechanisms under tension is found to be parallel channelling crack. Under compression the film fails by tunnelled buckling delamination prior to film cracking, which superficially looks very similar to the tensile cracking if observed under an optical microscope or lower-resolution SEM. Based on the understanding of the thin film mechanics on the above phenomena, we are able to calculate the fracture toughness of the ITO film, as well as to have an estimation of the film adhesion toughness. These parameters serve as the design values for flexible devices. Ways to improve the flexibility are also discussed from device design point of view.