Organic LEDs: from limits analysis to device performances improvement

Abstract

This thesis is centred on the study of opto-electronic devices and their characteristics to give a right understanding on the injection and transport mechanisms of charge carriers. Temperature dependence and high field limits are analyzed and the failure mechanisms during the glass transition phenomena in simple OLED structure are investigated by using variable temperature and AC impedance measurements. By these data, for the archetypal OLED structure ITO/TPD/Alq3/Al discussed in this thesis, temperatures around 328-329 K appear as a separation point between two different working regimes. Moreover, different possible approaches in the improvement of the photons extraction are analyzed to pick up the device performances. A modelling of the output coupling and some methods, like adopting microlenses or light scattering layers, to increase the external efficiency are discussed. Several kind of polystyrene-ZnO nanocomposites have been characterized to study the optical parameters of this mixture to be used as scattering film in lighting application. The optical parameters have been investigated and the anisotropy factor evaluated. We have found that increasing the thickness or the percentage in weight of ZnO nanoparticles there is an improvement of device efficiency. In particular, with a thickness of around 12μm is possible to reach 1.7 times the initial OLED efficiency value.