Photoelectronic properties of novel materials such as conjugated polymers, fullerenes, and insulating cuprates: Subnanosecond transient and steady-state photoconductivity studies.

摘要

The development of {dollar}{bsol}pi{dollar}-conjugated polymers as electronic materials has attracted increasing scientific and technological interests. Many potential applications have been foreseen based on the combination of their electronic and optical properties of semiconductors with the attractive mechanical properties and the processing advantages of polymers. Among conjugated polymers, poly(p-phenylenevinylene), PPV, has been extensively studied in relation to the role of polarons, excitonic polarons, and bipolarons in understanding the mechanism of the intense electroluminescence observed in PPV light-emitting diodes.; Still, a general picture of electronic excitations of PPV upon photoexcitation remains a subject of intense debate. This dissertation studies the photoconductivity of PPV and its derivatives with primary emphasis on the relevance of these studies to the fundamental question of the nature of primary photoexcitation. The study is extended to the case where an electron acceptor, C{dollar}{bsol}sb{lcub}60{rcub}{dollar} is added to the {dollar}{bsol}pi{dollar}-conjugated polymers. In such systems, a new channel is created for carrier photogeneration mechanism--photoinduced electron transfer from the excited states of the conjugated polymer to the acceptor molecule C{dollar}{bsol}sb{lcub}60{rcub}.{dollar}; The results of photoconductivity measurement on PPV and its soluble derivatives are consistent with a description of the electronic structure of the PPVs in terms of a semiconductor band model (rather than an exciton model). Especially, the fact that the measured photoconductivity spectrum follows the theoretical curve calculated using DeVore's theory from the measured absorption spectrum proves that charged carriers are photogenerated via a direct {dollar}{bsol}pi{dollar}-{dollar}{bsol}pi{bsol}sp*{dollar} interband transition. This conclusion is supported by a variety of experimental results which are briefly summarized.; We also present comprehensive studies of photoconductive and photovoltaic properties of the pristine C{dollar}{bsol}sb{lcub}60{rcub}{dollar} thin films, including the systematic study of effects of oxygen exposure. As will be demonstrated, oxygen in C{dollar}{bsol}sb{lcub}60{rcub}{dollar} film drastically reduces the photocarrier lifetime and effectively quenches the multiple trapping transport mechanism.; Finally, the photoelectronic properties of insulating cuprates are presented in Chapter 8. The results show that metallic-superconducting state can be achieved through photoexcitation (photodoping).