Surface and interface studies of organic semiconductors

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2010.In recent decades, research and development of organic based semiconductor devices have attracted intense interest. One of the most essential elements is the understanding of the electronic structures at various interfaces involved in these devices, as the interface properties control many of the critical electronic processes. However, the conventional theories developed for inorganic semiconductors are often adopted without further experimental confirmation in the design of innovative organic electronic devices. It is, thus, necessary to study the electronic properties of organic semiconductors with surface analytical tools, in order to improve our understanding of the fundamental mechanism involved in the interface formation. This thesis covers experimental investigations on some of the most interesting topics raised in the recent development of organic electronic devices. The intent of this thesis is to reveal the physical processes at the interface and their contributions to the device performance with photoemission and inverse photoemission investigations on the evolution of the occupied and unoccupied electronic structures. The topics include alkali metal doping, insertion layers, spin injection and organic single crystal studies. The electronic structure modification induced by alkali metal doping in tris-(8-hydroxyquinoline) aluminum (Alq) and copper phthalocyanine (CuPc) will be discussed. Based on the experimental observations, I propose a two-stage model to describe the doping effect in organic materials, which differs significantly from the classical theories used for inorganic semiconductors. I investigate the electronic structure of a number of insertion layers used in organic electronic devices, and the mechanisms of the induced performance improvement are discussed based on the observed interface properties. Next, I examine the spin injection and dynamics for organic thin films. Efficient spin injection for the hot electrons across the interface is demonstrated with spin and time-resolved two photon photoemission (STR-2PPE). Finally, I describe my studies about a particularly interesting organic material -- rubrene. The band structure measurement of rubrene single crystal samples is presented with angle-resolved photoemission spectroscopy (AR-PES). The energy level alignment at the interfaces between the rubrene thin film and various metal substrates and the morphology of the amorphous films prepared under various growth conditions are also discussed