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Density Functional Theory Study of Organic Materials for Electronic Devices Applications

Density Functional Theory Study of Organic Materials for Electronic Devices Applications
Nguyen, Phuong Thao
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Recently, organic semiconductors (OSCs) have attracted a lot of attentions because of their remarkable advantages such as mechanical flexibility, lower cost, biodegradable and cheaper processing compared to the inorganic semiconductors. The current research strategy is to improve the charge mobility and air stability of OSCs in order to replace silicon as the benchmark material in electronic devices. Pentacene from the acene is reported to show one of the highest hole mobility among the OSCs. The value is as high as 3 cm2V-1s-1 in thin film transistors. Unfortunately, pentacene is known to degrade upon contacting to ambient environment. Its chemical instability leads to the reduction of charge transport mobility and device performance. In 2008, Kubozono et. al. reported picene – an isomer of pentacene possesses a good charge mobility and better air-stability than pentacene. It is suggested that picene can be a potential candidate to replace pentacene. However, there are not yet detailed explanations for the charge mobility mechanisms of picene have been reported. Thus, in the present work we investigated the charge transfer properties and the air stability of picene. We provide the angle-dependent anisotropy of hole mobility in picene and pentacne crystals which is a crucial information for better performance of organic field effect transistors (OFETs). The performance of p-type semiconductors are judged by two important factors: the hole mobility and air stability. The hole mobility has been calculated based on Marcus charge transfer theory within density function theory. We showed very anisotropic hole mobility within ab plane of crystal and the best performance along the herringbone arrangement. By choosing a correct conducting channel of picene, the hole mobility can reach to 7.21 cm2V-1s-1 which is high mobility among OSCs. In picene homologous series (phenacenes), if we continue to add phenyl rings, the mobility is predicted to increase significantly due to the dramatic reduction of reorganization energy. On the other hand, the HOMO levels still remain unchanged, so all phenacenes are expected to be stable under air environment. It is very contradict to pentacene homologous series (oligoacenes) where the HOMO levels become very unstable with increasing number of phenyl rings. Thus, our work predicts that picene and other phenacenes are promising candidates toward high hole mobility semiconductors with air stability.
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