Abstract
A robust understanding of the mechanoelectric response of organic semiconductors is crucial for the development of materials for flexible electronics. In particular, the prospect of using external mechanical strain to induce a controlled modulation in the charge mobility of the material is appealing. Here we develop an accurate computational protocol for the prediction of the mechanical strain dependence of charge mobility. Ab initio molecular dynamics simulations with a van der Waals density functional are carried out to quantify the off-diagonal electronic disorder in the system as a function of strain by the explicit calculation of the thermal distributions of electronic coupling matrix elements. The approach is applied to a representative molecular organic semiconductor, single-crystal rubrene. We find that charge mobility along the high-mobility direction a⃗ increases with compressive strain, as one might expect. However, the increase is larger when compressive strain is applied in the perpendicular direction than in the parallel direction with respect to a⃗, in agreement with experimental reports. We show that this seemingly counterintuitive result is a consequence of a significantly greater suppression of electronic coupling fluctuations in the range of 50–150 cm–1, when strain is applied in the perpendicular direction. Thus our study highlights the importance of considering off-diagonal electron–phonon coupling in understanding the mechanoelectric response of organic semiconducting crystals. The computational approach developed here is well suited for the accurate prediction of strain–charge mobility relations and should provide a useful tool for the emerging field of molecular strain engineering.
Highlights
A robust understanding of the mechanoelectric response of organic semiconductors is crucial for the development of materials for flexible electronics
A few theoretical studies on the mechanoelectric properties of rubrene have been carried out as well,[10−12] broadly agreeing with the experimental studies of Morf et al and Matta et al but in contrast with those of Choi et al Gali et al used standard band theory, which is known to be problematic for the estimation of charge mobility in molecular organic crystals,[10] as is small polaron hopping theory.[13]
We employ ab initio molecular dynamics with a van der Waals density functional to sample the room-temperature thermal distribution of electronic couplings between rubrene molecules in the strained and unstrained crystal, and we use them in the framework of transient localization theory (TLT) to compute charge mobilities
Summary
A robust understanding of the mechanoelectric response of organic semiconductors is crucial for the development of materials for flexible electronics. A few theoretical studies on the mechanoelectric properties of rubrene have been carried out as well,[10−12] broadly agreeing with the experimental studies of Morf et al and Matta et al but in contrast with those of Choi et al Gali et al used standard band theory, which is known to be problematic for the estimation of charge mobility in molecular organic crystals,[10] as is small polaron hopping theory.[13] Ruggiero et al and Landi et al used the more suitable transient localization theory (TLT)[14,15] and treated the off-diagonal electron−phonon coupling and the lattice dynamics in the linear and harmonic approximation, respectively.[11,12,16,17] in the latter study, a semiempirical electronic structure method was used for the lattice dynamics.
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