Quantitative Prediction of the Electro‐Mechanical Response in Organic Crystals

Abstract

Organic semiconductors’ inherent flexibility makes them appealing for advanced applications such as wearable electronics, e‐skins, or pressure sensors, and can even be used to enhance their intrinsic electronic properties. Unfortunately, these applications for organic materials are currently hindered by the lack of a quantitative understanding of the interplay between their electrical and mechanical properties. In this work, this gap is filled by presenting an accurate methodology able to predict quantitatively the effects of external deformation on the charge transport properties of any organic semiconductors. Three prototypical materials are investigated, showing that the experimental variation of charge carrier mobility with strain is fully reproduced, even in a wide range of deformations applied along different crystal axes. The results indicate that the intrinsic electro‐mechanical response of the materials varies by orders of magnitude within the class of organic semiconductors, a difference rationalized observing that the mobility trend is primarily influenced by the transfer integrals’ variation, rather than by a modification of the crystal phonons. In light of its robustness, accuracy, and low computational cost, this protocol represents an ideal tool to quantify the electro‐mechanical response in new organic compounds, thus establishing a reliable route for a full exploitation of strain engineering in advanced technologies.

Mechanoelectric Response of Single-Crystal Rubrene from Ab Initio Molecular Dynamics

“…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.…”

“… 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 Moreover, in the latter study, a semiempirical electronic structure method was used for the lattice dynamics. Because the electronic parameters that govern charge mobilities are very sensitive to fine details of the intermolecular structure and dynamics, the above approximations could tip the balance in favor of one set of experimental results 5 , 6 or the other.…”

“…However, this also means that applying gentle external compressive strain as a means to boost the charge mobility is not very effective for this material. Yet, recent theoretical evidence suggests that the mechanoelectric response may strongly depend on the organic molecule under consideration, 12 more specifically, on the nodal shape and the relative orientation of the charge-mediating molecular frontier orbitals in the crystal. Hence it might be possible in the future to develop organic semiconducting materials that show either weak or strong dependence on the external strain, as desired for a given electronic application.…”

High-throughput virtual screening for organic electronics: a comparative study of alternative strategies

“…265,288 Furthermore, as shown in a recent study, this theory is also able to reproduce electromechanical responses. 291 The fragment-orbital based surface hopping method, relying on explicit time propagation of the electron-nuclear dynamics, is another viable approach that is used to capture the impact of thermal fluctuations on charge transport. 259 A recent study, through applying this theory to a set of eight crystalline structures, shows that the excess charge carrier leads to a polaron which is delocalised over 10-20 molecules in highly conductive crystals.…”

Is a Single Molecule Sufficient to Determine the Internal Charge Trapping Energy in Crystalline Organic Semiconductors?

“…Understanding the charge-transfer properties in organic semiconductors with refined theoretical models or descriptors constitutes a vibrant research field. In parallel to the recent advances on the computation of the transfer integral that well explains the electromechanical response and the diversity of measured charge mobilities for typical organic semiconductors, increasing attention has also been paid to the assessment of the internal charge trapping energy (λ), − which influences the computed charge mobility and is widely used for material design. − λ (aka the internal charge reorganization energy) is the energy (local charge–phonon coupling term) that stems from structural relaxation of a single molecule in the condensed state to accommodate an electron or a hole. A large λ will result in a low charge mobility. − In most literature, λ is computed based on an isolated molecule via either Nelsen’s four-point (NFP) method , or normal-mode (NM) analysis .…”