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.

有机半导体为无机半导体提供了一种低成本替代方案，但其应用受限于极化子载流子相对较低的迁移率。过往研究表明，有机半导体中的电荷密度与电荷迁移率呈正相关关系，这一关联通常被归因于过剩电荷填充电荷陷阱的现象。本研究旨在探究电荷密度是否也可通过影响分子间耦合来改变迁移率。我们采用带有长程校正（LC-BLYP）泛函的密度泛函理论（DFT），计算了存在邻近负点电荷时，C70富勒烯上负电荷的电荷密度与电子耦合强度，该计算为邻近极化子的影响提供了上限估算。我们发现，在耦合强度相对较低的C70体系中，额外电荷的存在可最大化分子间耦合强度，进而提升电荷传输性能。但该效应随距离快速衰减，表明其仅在极高电荷密度下才具有相关性，而这在当前的C70器件中几乎不可能发生。电荷密度对分子间耦合的影响，或可用于理解局部电荷密度较高的有限区域内C70材料的电荷传输行为，但不太可能对器件的整体性能产生影响。