Energy Levels and Charge Transport in Host-dopant Organic Semiconductors

The organic light-emitting diode (OLED) generates light through charge carrier recombination in the emissive organic layer which has a host-dopant structure and, thus, the OLED device performance is impacted by the electronic properties of this host-dopant organic semiconductor. In this thesis, the energy level alignment and charge transport process in the host-dopant organic semiconductor are investigated by photoelectron spectroscopy and by analysis of the current-voltage behavior, respectively.First, the energy level alignment of host-dopant organic semiconductors at electrode-organic interface is studied by photoelectron spectroscopy. The interface energy disorder is found for molecules close to an electrode surface. This energy disorder has significant impact on energy level alignment. Second, the energy level alignment of bulk host-dopant organic semiconductors is investigated. A host/dopant hetero-interface formed by a layer-by-layer method is demonstrated to correlate well the host-dopant energy level alignment in mixed form. It is found that the host-dopant energy level alignment falls into the elastic region of the universal energy level alignment rule, involving negligible charge transfer. Third, charge transport in the host-dopant organic semiconductors is studied using single carrier devices. Charge detrapping and percolation are found to be two major mechanisms for transport in the host-dopant materials. A theoretical formula is proposed and validated to describe the mobility dependence on the energy level alignment, and an empirical formula is demonstrated to describe the mobility dependence on the dopant concentration. Lastly, the impact of dopants on charge transport across an organic-organic interface is investigated. This charge injection from the transport layer into the emissive layer is critical for device efficiency. It is found that charge transport characteristics at an organic-organic junction varies depending on the energy level alignments of the dopant to that of the host and the hole transport layer. In addition, the dopant concentration is found to dictate the device electrical characteristics. Detailed numerical studies show that carrier transport across the junction is charge injection-limited at low dopant concentration (≤5 wt%), while an Ohmic junction is obtained at high dopant concentration.Ph.D