Electronic Structure of Disordered Conjugated Polymers: Polythiophenes

Abstract

Electronic structure of disordered semiconducting conjugated polymers was studied. Atomic structure was found from a classical molecular dynamics simulation and the charge patching method was used to calculate the electronic structure with the accuracy similar to the one of density functional theory in local density approximation. The total density of states, the local density of states at different points in the system and the wavefunctions of several states around the gap were calculated in the case of poly(3-hexylthiophene) (P3HT) and polythiophene (PT) systems to gain insight into the origin of disorder in the system, the degree of carrier localization and the role of chain interactions. The results indicated that disorder in the electronic structure of alkyl substituted polythiophenes comes from disorder in the conformation of individual chains, while in the case of polythiophene there is an additional contribution due to disorder in the electronic coupling between the chains. Each of the first several wavefunctions in the conduction and valence band of P3HT is localized over several rings of a single chain.It was shown that the localization can be caused in principle both by ring torsions and chain bending, however the effect of ring torsions is much stronger. PT wavefunctions are more complicated due to larger interchain electronic coupling and are not necessarily localized on a single chain.

Developing accurate molecular mechanics force fields for conjugated molecular systems

“…it is increased (decreased) if the acceptance rate is higher (lower) than 25%. The optimization is deemed to be converged and is terminated when the change in the objective function is lower than a threshold, here set to 10 10  kcal/mol/Å. For all studied systems, the optimized objective function is always very similar if the procedure is repeated with a different random seed, indicating that there is a single minimum or a number of equivalent minima.…”

“…Large-scale electronic structure calculations are then used to correlate the local structure with the observable electronic structure properties. 6,10 Another new class of applications of classical MD is the elucidation of experiments in ultrafast electronic spectroscopy of biomolecules. It was recently shown that long quantum coherences can be observed in biological molecules [11][12][13] and the aim of atomistic simulations is to explain how the (classical) environment can influence the (quantum) evolution of the electronic states.…”

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…It is important to note that many studies predict a more broad density-of-states for the chromophores than is expected experimentally [80,[107][108][109][110]. Therefore, some studies have successfully utilized Koopman's approximation, which effectively maps the density-of-states to a single narrow peak by assuming that each chromophore is identical with ∆E ij = 0 [104,111,112].…”

Narrower Bands with Better Charge Transport: The Counterintuitive Behavior of Semiconducting Copolymers

“…Calculations of long polymer chains in conformations obtained from molecular dynamics simulations suggest that the main cause of localization of the orbital in single polymer chains is the variation (disorder) in the dihedral angles of the conjugated backbone, in the 15°-35° range for different polymers, which causes a variation of the electronic coupling between monomers corresponding to 5-20% of the average coupling. [12][13][14] Therefore, we start considering a purely electronic disordered Hamiltonian (the effect of electronphonon coupling will be discussed later) with one state i per site i (representing a monomer) and nearest neighbor coupling: consider only the disorder in the off-diagonal terms, following the indications from our atomistic studies [13]  contain states in two separate energy regions. The wavefunctions in the low energy band have their probability density concentrated on the sites corresponding to the low energy monomer and viceversa.…”