The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Abstract

Charge transport in crystalline organic semiconductors is intrinsically limited by the presence of large thermal molecular motions, which are a direct consequence of the weak van der Waals inter-molecular interactions. These lead to an original regime of transport called transient localization, sharing features of both localized and itinerant electron systems. After a brief review of experimental observations that pose a challenge to the theory, we concentrate on a commonly studied model which describes the interaction of the charge carriers with inter-molecular vibrations. We present different theoretical approaches that have been applied to the problem in the past, and then turn to more modern approaches that are able to capture the key microscopic phenomenon at the origin of the puzzling experimental observations, i.e. the quantum localization of the electronic wavefuntion at timescales shorter than the typical molecular motions. We describe in particular a relaxation time approximation which clarifies how the transient localization due to dynamical molecular motions relates to the Anderson localization realized for static disorder, and allows us to devise strategies to improve the mobility of actual compounds. The relevance of the transient localization scenario to other classes of systems is briefly discussed.

Charge transport network dynamics in molecular aggregates

“…To accurately incorporate dynamic disorder into future charge transport calculations, semiclassical dynamics or dynamic kinetic Monte Carlo may be useful (19,28), although the issues with these methods have been described in a recent review (16).…”

“…Operating under the assumption that charge transport in noncrystalline organic semiconductors occurs in the perturbative limit, the timescales of significant changes in the intermolecular coupling appear to be competitive with the rates derived from a Marcus-like approach. This consideration warrants a reevaluation of the dominant mechanism of charge transport, as suggested by other authors (11,12,14,16).…”

“…Whereas the static picture provides many insights into the nature of charge transport in soft materials, recent work has emerged indicating the importance of time-dependent fluctuations. Throughout the rest of this article we refer to these fluctuations as "dynamic disorder" (11)(12)(13)(14)(15)(16).In this work we use network analysis to examine the impact of dynamic disorder on charge transport networks of aggregates of two representative organic semiconducting molecules, perylenediimide (PDI, Fig. 1) (17) and bBDTðTDPPÞ 2 (18).…”

Unraveling Unprecedented Charge Carrier Mobility through Structure Property Relationship of Four Isomers of Didodecyl[1]benzothieno[3,2‐b][1]benzothiophene

“…[ 3,18 ] Moreover, as recently highlighted by Fratini et al, the strong dynamical disorder present in OSCs at room temperature imposes a transient localization of the charges which results in an intermediate regime of charge transport where the carriers exhibit both localized and extended characters. [ 19 ] We report here on the structural and electronic properties of four isomers of didodecyl[1]benzothieno [3,2-b ][1]benzothiophene (C12-BTBT-C12) varying by the isomerism of the alkyl side-chains. The choice of the BTBT core is motivated by the previously reported high mobility values, its high chemical stability, and the ease of derivatization.…”

Crossover from band-like to thermally activated charge transport in organic transistors due to strain-induced traps

“…Although the band-like transport proposed for high-mobility organic semiconductors has many common features with the classical band transport found in many inorganic semiconductors, it is fundamentally different due to the fact that, in organic crystals, the charge carriers are delocalized only over a few molecules (unit cells). The transient localization (dynamic disorder) model, which relates charge localization with the lattice vibrations, captures a more accurate picture of charge transport in high-mobility organic crystals (6). A thermally activated mobility, described by an Arrhenius-like relation, μ ≈ exp[-E A /k B T], where E A stands for the activation energy and k B is the Boltzmann constant, is generally a signature of disorder, and is described in terms of the multiple trapping and release model (7,8) or variable range hopping (9,10).…”