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

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.