The electronic and optical properties of amorphous organic semiconductors

Organic semiconductors are beginning to find technological applications, initially in electroluminescent devices, but increasingly also in electronic devices such as transistors. This thesis considers aspects of the electronic and optical physics occurring within the amorphous films that often comprise organic devices. After analysis of the process of organic electroluminescence, we demonstrate that the quantum efficiency of organic light emitting devices can be improved using phosphorescent materials. We also employ phosphors to sensitize fluorescent materials, thereby increasing the efficiency of fluorescence.; Harnessing phosphorescence enables us to probe fundamental properties of organic electroluminescence such as the ratio of electrically-generated singlets and triplet excitons. We study non-radiative processes to determine the desirable characteristics of phosphorescent molecules and quantify the requirements for an electrically-pumped organic laser. Significant obstacles are identified, possibly preventing the realization of an electrically pumped organic laser fabricated from amorphous materials.; In the second part, two aspects of electrical transport in amorphous organic films are considered. We examine electron transport in the archetype amorphous organic material tris(8-hydroxyquinoline) aluminum (Alq₃). It is established that injection processes at the metal/organic contact dominate the current-voltage characteristics. At low temperatures, the cathode dependence of current-voltage characteristics is substantially eliminated, raising doubts over metal-to-organic injection models that depend on the cathode work function. We investigate the impact of interfacial dipoles on the adjacent molecules in the organic film. Consequently, it is proposed that injection is limited by charge hopping out of interfacial molecular sites whose energy distribution is broadened by local disorder in the interfacial dipole field. We derive a general analytic model of injection from interfacial states. The model is extended to other amorphous organic semiconductors and is found to be applicable to both polymers and small molecular weight organic compounds. Finally, we examine the influence of strong permanent dipole moments on electronic transport in a molecular solid. It is shown that the fluorescence spectrum of polar molecules may shift due to the formation of ordered polar domains within an otherwise amorphous film. A model is derived to explain this shift and the associated quenching of luminescence as domains are formed.