Nonfullerene acceptors have caused a step change in organic optoelectronics research but little is known about the mechanism and factors limiting charge transport in these molecular materials. Here a joint computational-experimental investigation is presented to understand the impact of various sources of disorder on the electron transport in the nonfullerene acceptor O-IDTBR. We find that in single crystals of this material, electron transport occurs in the transient quantum delocalization regime with the excess charge delocalized over about three molecules on average, according to quantum-classical nonadiabatic molecular-dynamics simulations. In this regime, carrier delocalization and charge mobility (${\mu }_a=7{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$) are limited by dynamical disorder of off-diagonal and diagonal electron-phonon coupling. In molecular assemblies representing disordered thin films, the additional static disorder of off-diagonal electron-phonon coupling is sufficient to fully localize the excess electron on single molecules, concomitant with a transition of transport mechanism from transient quantum delocalization to small polaron hopping and a drop in electron mobility by about 1 order of magnitude. Yet, inclusion of static diagonal disorder resulting from electrostatic interactions arising from the acceptor-donor-acceptor (A-D-A) structure of O-IDTBR, are found to have the most dramatic impact on carrier mobility, resulting in a further drop of electron mobility by about 4–5 orders of magnitude to ${10}^{-5}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, in good agreement with thin-film electron mobility estimated from space-charge-limited-current measurements. Limitations due to diagonal disorder caused by electrostatic interactions are likely to apply to most nonfullerene acceptors. They imply that while A-D-A or A-DAD-A motifs are beneficial for photoabsorption and exciton transport, the electrostatic disorder they create can limit carrier transport in thin-film optoelectronic applications. This work shows the value of computational methods, in particular, nonadiabatic molecular-dynamics propagation of charge carriers, to distinguish different regimes of transport for different types of molecular packing.

中文翻译：

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非富勒烯受体材料中无序诱导的从瞬态量子离域到电荷载流子跳跃传导的转变

非富勒烯受体引起了有机光电子学研究的阶跃变化，但人们对这些分子材料中限制电荷传输的机制和因素知之甚少。这里提出了联合计算实验研究，以了解各种无序来源对非富勒烯受体 O-IDTBR 中电子传输的影响。我们发现，根据量子经典非绝热分子动力学模拟，在这种材料的单晶中，电子传输发生在瞬态量子离域区域，多余的电荷平均在大约三个分子上离域。在这种情况下，载流子离域和电荷迁移率（${\mu }_A=7{\mathrm{厘米}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$）受到非对角线和对角线电子声子耦合动力学无序的限制。在代表无序薄膜的分子组装体中，非对角电子-声子耦合的额外静态无序足以将多余电子完全局域化在单个分子上，同时传输机制从瞬态量子离域到小极化子跳跃和下降电子迁移率提高约1个数量级。然而，O-IDTBR 的受体-供体-受体 (ADA) 结构产生的静电相互作用导致的静态对角无序的包含，被发现对载流子迁移率具有最显着的影响，导致电子迁移率进一步下降约4-5 个数量级${10}^{-5}{\mathrm{厘米}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$，与通过空间电荷限制电流测量估计的薄膜电子迁移率非常一致。由于静电相互作用引起的对角无序造成的限制可能适用于大多数非富勒烯受体。他们暗示，虽然 ADA 或 A-DAD-A 基序有利于光吸收和激子传输，但它们产生的静电无序会限制薄膜光电应用中的载流子传输。这项工作展示了计算方法的价值，特别是电荷载流子的非绝热分子动力学传播，以区分不同类型分子堆积的不同传输机制。