| Charge carrier photogeneration and transport mechanisms are the foundation of much of solid state physics as well as applications in photonics and optoelectronics. However, our understanding of the mechanisms of charge carrier photogeneration and transport in organic semiconductors is limited due to the presence of chemical impurities, disorder defects, and surface states.Pentacene thin films, one of the organic semiconductor materials, have attracted significant attention because of their promising applications in organic field-effect transistors, solar cells, and phototransistors. In this thesis, we use steady-state photoconductivity and sub-nanosecond transient photoconductivity techniques to study charge carrier photogeneration and transport mechanisms in pentacene thin films with interdigitated photodetector structures. We observed charge carrier hopping transport in localized band states and temperature-independent pre-trapping transport in extended band states. The carrier photogeneration quantum efficiency is independent of excitation photon energy, electric field and temperature, which indicates that exciton model is not valid in our case. Furthermore, we are the first to report bulk photoconductive gain as high as 40 in pentacene thin films on simple photodetector structures. The effect of film morphology and metal contact in the measured photocurrent yield is also examined.Thermally-activated hopping is thought to be one of the most important transport mechanisms, particularly in organic thin films. However, band-like transport, where carrier mobility increases with decreasing temperature, has been reported in polycrystalline pentacene thin films. For photogeneration, it is popular to believe that the primary photoexcitations in an organic semiconductor are tightly bound electron-hole pairs. i.e., excitons, and in order to generate mobile carriers, excitons can be dissociated by various mechanisms such as thermal activation, field assisted dissociation and higher energy photon excitation. Therefore, carrier photogeneration is believed to be a secondary process with a quantum efficiency that increases with temperature, external electrical field, and photon energy. However, quantum efficiency independent of photon energy and temperature has been reported in organic semiconductors. |
