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Polaron Structure and Transport in Fullerene Materials: Insights from First-Principles Calculations

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posted on 25.09.2014, 00:00 by Kenley M. Pelzer, Maria K. Y. Chan, Stephen K. Gray, Seth B. Darling
Organic semiconductors offer a low-cost alternative to inorganic semiconductors. However, their usefulness is limited by a relatively low mobility of polaron charge carriers. Past research indicates a positive correlation between charge density and charge mobility in organic semiconductors. This relationship is usually attributed to the phenomenon of excess charges filling traps. Here, we explore whether charge density may also affect mobility via influence on intermolecular couplings. Density functional theory (DFT) with a long-range corrected (LC-BLYP) functional is used to calculate charge densities and electronic couplings of negative charges on C70 fullerenes in the presence of nearby negative point charges, which provides an upper limit calculation of the influence of nearby polarons. We find that in C70 systems with relatively low couplings, the presence of additional charges has an effect of maximizing intermolecular couplings and hence transport. This effect drops off quickly with distance, suggesting that it is relevant only at extremely high charge densities that are an unlikely event in current C70 devices. The effect of charge density on couplings may be useful in understanding transport in very limited regions of C70 materials where the local charge density is high; however, it is unlikely to affect overall device performance.