Charge transport network dynamics in molecular aggregates

Abstract

Due to the nonperiodic nature of charge transport in disordered systems, generating insight into static charge transport networks, as well as analyzing the network dynamics, can be challenging. Here, we apply time-dependent network analysis to scrutinize the charge transport networks of two representative molecular semiconductors: a rigid n-type molecule, perylenediimide, and a flexible p-type molecule, bBDT(TDPP) 2 . Simulations reveal the relevant timescale for local transfer integral decorrelation to be ∼ 100 fs, which is shown to be faster than that of a crystalline morphology of the same molecule. Using a simple graph metric, global network changes are observed over timescales competitive with charge carrier lifetimes. These insights demonstrate that static charge transport networks are qualitatively inadequate, whereas average networks often overestimate network connectivity. Finally, a simple methodology for tracking dynamic charge transport properties is proposed.organic semiconductors | charge transport | network analysis | dynamic disorder | molecular semiconductors C omputational tools using periodic boundary conditions are integral to understanding charge and excitation transport in crystalline semiconducting materials (1, 2). In noncrystalline systems, where Bloch's theorem is inapplicable and inclusion of structural disorder is required for an accurate description of charge transport, comparable computational strategies are rare. Typically, one uses an approximate solution of the master equation with semiclassical rates derived from a combination of atomistic molecular dynamics (MD) and quantum chemistry. While this strategy has proven effective (3, 4), it possesses obvious deficiencies: it lacks both an explicit inclusion of structural dynamics and an understanding of the topology of charge transport networks.Work in this group and others has proposed a view of charge transport based in network analysis (5-10). Network analysis represents a powerful means of analyzing structurally disordered charge transport networks, with the unique ability to place different mechanisms of charge transport on the same footing via the selection of a suitable graph metric. Recently, network views of charge transport in structurally disordered systems have provided useful insights: notably, the relationship between a molecule's topology and the percolation threshold of its charge transport networks (5, 6), and that charge mobility can be independent of the global morphology, being primarily determined by local molecular packing (7).Although a useful framework, previous applications of network analysis to structurally disordered charge transport networks have neglected the role of dynamic structural disorder: static snapshots of the charge transport network were used for the network analysis. Transport was assumed to occur on a charge transport network defined by time-independent site energies and electronic couplings. Whereas the static picture provides many insights into the nature of charge transport in soft mater...

Unexpectedly Large Couplings Between Orthogonal Units in Anthraquinone Polymers

“…This confirms that the independence of electronic couplings from nuclear coordinates (Condon approximation) is not valid in this case, as previously noted for the coupling between phenyl rings . The impact of thermal disorder on charge transport is difficult to predict without calculations on a larger scale, and the thermally averaged coupling may lead to an artificial overestimation of mobility . Should the high sensitivity to nuclear displacements determine a similar distribution of couplings along a disordered polymer chain (i.e., a similar amount of static disorder), the subsequent localization of the transport states would be expected to negatively impact hole transport with respect to electron transport …”

Unexpectedly Large Couplings Between Orthogonal Units in Anthraquinone Polymers

“…[12] The impact of thermal disorder on charget ransport is difficult to predict withoutcalculations on a larger scale, [25,27] and the thermally averagedc oupling may lead to an artificial overestimation of mobility. [61] Should the high sensitivity to nuclear displacements determine as imilar distribution of couplings along ad isordered polymer chain (i.e.,asimilara mounto fs tatic disorder), the subsequentl ocalization of the transport states would be expectedt on egatively impact hole transport with respect to electron transport. [34] The magnitude and distribution of the computed chargetransfer integrals is consistentw ith the experimental observation of an on-negligible conductivity of 14PAQ in the undoped state (10 À8 Scm À1 ), [50] which is comparable to the undoped conductivity of redox polymers specifically designed to be conducting, such as poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)).…”

New frontiers for the materials genome initiative

“…2c), further resulting in efficient screening models 26 and new design strategies. 27,28 Graph-based algorithms have been developed for rapidly screening the charge percolation properties of molecular networks, thus accelerating characterization of multiscale charge transport and enabling MGI data-driven screening techniques [29][30][31][32] ( Fig. 2d) The tight integration of computation and experiment has also greatly improved understanding of the behavior of charged polymer complexes, solutions, and brushes mimicking biological functionality [33][34][35] .…”