Chain conformations dictate multiscale charge transport phenomena in disordered semiconducting polymers

Abstract

Existing models for the electronic properties of conjugated polymers do not capture the spatial arrangement of the disordered macromolecular chains over which charge transport occurs. Here, we present an analytical and computational description in which the morphology of individual polymer chains is dictated by well-known statistical models and the electronic coupling between units is determined using Marcus theory. The multiscale transport of charges in these materials (high mobility at short length scales, low mobility at long length scales) is naturally described with our framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational variability and spatial correlation. Our model offers a predictive approach to connecting processing conditions with transport behavior.organic electronics | charge mobility | computational model T he performance of organic semiconductors has improved substantially in recent years through careful molecular engineering and processing (1, 2), and solution-processed systems are now competitive with amorphous silicon. A path to constant improvement is best guided by rational materials design, which requires that the relationship between processing, structure, and ensuing properties be well understood. In complex macromolecular systems, such as conjugated polymers, there is no generally accepted model for charge transport that explicitly takes into account the macromolecular nature of the material. Such difficulty is partially due to their complicated microstructural behavior and strongly disordered intramolecular conformation and intermolecular packing.Several properties of flexible macromolecules are understood in terms of the statistics of their chain conformations. For example, the viscoelasticity of polymers in solution has been shown to depend on chain entanglements (3-5), and liquid-crystalline microstructures result from nematic ordering of individual chains (6). We present a model of electrical charge transport in conjugated polymers that places this important property on the same theoretical footing as many other macromolecular properties. To achieve this goal, it is necessary to build the model by starting from the polymer and its disordered conformations.The disordered nature of semiconducting polymers is rationalized to be the cause of many observations in these systems, most notably thermally activated charge transport attributed to disorder-induced traps (7, 8). On-site energetic disorder and positional disorder are invoked in phenomenological models to explain the dispersive nature of charge transport and the electric-field dependence of charge mobility (i.e., the Poole-Frenkel effect), often including intersite correlations (8,9).Gaussian disorder models (GDMs) are based on describing a 3D material as a grid of sites. The on-site energy is selected from a Gaussian probability distribution to reflect the energetic disorder in the material. Structural disorder is introduced by varying the localization radi...

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

“…Recent work suggests that interchain transport and intrachain transport operate at different time-scales, and that this distinction can be used to qualitatively explain a large body of experimental work. 54 In ref 54, by distinguishing between (slow) interchain and (fast) intrachain transport, the authors were able to explain the multiscale nature of mobility that has been reported in a variety of materials. 55,56 We use this as a motivation to distinguish between interchain and intrachain regions in hole pathways.…”

How Many Parameters Actually Affect the Mobility of Conjugated Polymers?

“…So, rather than assuming a density of states (DOS) and an independent constant localization length we generate the oneelectron states involved in the transport by computing the eigenstates of a random electronic Hamiltonian [13]. This is defined for a polymer chain as el 0…”

“…We will compare with experimental data extrapolated to the same limit, while generalizations, including to non-equilibrium situations, would be possible within the model but are not considered here. We ignore the role of inter-chain hopping, which has been shown to be correct for polymers with very long persistence length [21], with means to extend the results to the general case recently proposed in [13] at the cost of additional parameters in the model. These corrections could become more significant if the inter-chain hopping had substantially different transport characteristics from the intra-chain one.…”

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes

“…orphology and excitonic coupling are among the most important factors dictating the functions and performance of conjugated polymers (CPs) in a variety of optoelectronic applications (1)(2)(3)(4). To tune and optimize the morphological and optoelectronic properties of CPs, chemical structure modification of the backbone and side chains is one of the main and most effective approaches (5)(6)(7).…”