Guangjun Nan

Efficient hierarchical Liouville space propagator to quantum dissipative dynamics

We propose an efficient method to propagate the hierarchical quantum master equations based on a reformulation of the original formalism and the incorporation of a filtering algorithm that automatically truncates the hierarchy with a preselected tolerance. The new method is applied to calculate electron transfer dynamics in a spin-boson model and the absorption spectra of an excitonic dimmer. The proposed method significantly reduces the number of auxiliary density operators used in the hierarchical equation approach and thus provides an efficient way capable of studying real time dynamics of non-Markovian quantum dissipative systems in strong system-bath coupling and low temperature regimes.

Charge transfer rates in organic semiconductors beyond first-order perturbation: From weak to strong coupling regimes

Semiclassical Marcus electron transfer theory is often employed to investigate the charge transport properties of organic semiconductors. However, quite often the electronic couplings vary several orders of magnitude in organic crystals, which goes beyond the application scope of semiclassical Marcus theory with the first-order perturbative nature. In this work, we employ a generalized nonadiabatic transition state theory (GNTST) [Zhao et al., J. Phys. Chem. A 110, 8204 (2004)], which can evaluate the charge transfer rates from weak to strong couplings, to study charge transport properties in prototypical organic semiconductors: quaterthiophene and sexithiophene single crystals. By comparing with GNTST results, we find that the semiclassical Marcus theory is valid for the case of the coupling <10 meV for quaterthiophene and <5 meV for sexithiophene. It is shown that the present approach can be applied to design organic semiconductors with general electronic coupling terms. Taking oligothiophenes as examples, we find that our GNTST-calculated hole mobility is about three times as large as that from the semiclassical Marcus theory. The difference arises from the quantum nuclear tunneling and the nonperturbative effects.

Electron transfer dynamics: Zusman equation versus exact theory

The Zusman equation has been widely used to study the effect of solvent dynamics on electron transfer reactions. However, application of this equation is limited by the classical treatment of the nuclear degrees of freedom. In this paper, we revisit the Zusman equation in the framework of the exact hierarchical equations of motion formalism, and show that a high temperature approximation of the hierarchical theory is equivalent to the Zusman equation in describing electron transfer dynamics. Thus the exact hierarchical formalism naturally extends the Zusman equation to include quantum nuclear dynamics at low temperatures. This new finding has also inspired us to rescale the original hierarchical equations and incorporate a filtering algorithm to efficiently propagate the hierarchical equations. Numerical exact results are also presented for the electron transfer reaction dynamics and rate constant calculations.

Nonperturbative time-convolutionless quantum master equation from the path integral approach

The time-convolutionless quantum master equation is widely used to simulate reduced dynamics of a quantum system coupled to a bath. However, except for several special cases, applications of this equation are based on perturbative calculation of the dissipative tensor, and are limited to the weak system-bath coupling regime. In this paper, we derive an exact time-convolutionless quantum master equation from the path integral approach, which provides a new way to calculate the dissipative tensor nonperturbatively. Application of the new method is demonstrated in the case of an asymmetrical two-level system linearly coupled to a harmonic bath.

Crystal structure versus charge transport in organic single crystals of [1]benzothieno[3,2-b][1]benzothiophene derivatives from a multiscale theoretical study

[1]Benzothieno[3,2-b][1]benzothiophene derivatives with high air stability have recently displayed excellent charge transport properties in field-effect devices. In particular, the average charge mobilities can reach as high as 16.4 ± 6.1 cm2 V−1 s−1 in devices with a high quality semiconductor/insulator interface, which is comparable to the performance for a rubrene single-crystal device. To better understand these excellent charge transport properties, a multiscale approach combining molecular dynamics and quantum-chemical calculations was used in this work to assess the structure–property relationship for three of the [1]benzothieno[3,2-b][1]benzothiophene derivatives with different alkyl side chains. It is indicated that the extremely large electronic couplings along the a-axis direction are responsible for the excellent charge transport properties in these systems. While the molecular packings are centrosymmetrical in the ab plane, the lattice vibrations were found to hamper the charge transport in optimized crystal structures at the COMPASS molecular mechanics level which is opposite to the recent findings that the lattice dynamics should have a negligible effect on the charge mobility in the centrosymmetrical plane. The reason for such behavior was analyzed and the predicted order of the overall charge mobilities for the studied systems was consistent with the experiments. Meanwhile, how well the force field reproduces the observed crystal structures and dimer intermolecular separations and orientations is discussed in this work. In addition, it is shown that the present charge transport model can not only predict the magnitude of the charge mobility but also the measured “band-like” charge transport in experiments, so the nuclear tunneling effect is very important for charge transport in organic semiconductors as was demonstrated in recent theoretical work.

Theoretical comparative studies on transport properties of pentacene, pentathienoacene, and 6,13-dichloropentacene

Pentacene derivative 6,13‐dichloropentacene (DCP) is one of the latest additions to the family of organic semiconductors with a great potential for use in transistors. We carry out a detailed theoretical calculation for DCP, with systematical comparison to pentacene, pentathienoacene (PTA, the thiophene equivalent of pentacene), to gain insights in the theoretical design of organic transport materials. The charge transport parameters and carrier mobilities are investigated from the first‐principles calculations, based on the widely used Marcus electron transfer theory and quantum nuclear tunneling model, coupled with random walk simulation. Molecular structure and the crystal packing type are essential to understand the differences in their transport behaviors. With the effect of molecule modification, significant one‐dimensional π‐stacks are found within the molecular layer in PTA and DCP crystals. The charge transport along the a‐axis plays a dominant role for the carrier mobilities in the DCP crystal due to the strong transfer integrals within the a‐axis. Pentacene shows a relatively large 3D mobility. This is attributed to the relatively uniform electronic couplings, which thus provides more transport pathways. PTA has a much smaller 3D mobility than pentacene and DCP for the obvious increase of the reorganization energy with the introduction of thiophene. It is found that PTA and DCP exhibit lower HOMO (highest occupied molecular orbital) levels and better environmental stability, indicating the potential applications in organic electronics.

MOlecular MAterials Property Prediction Package (MOMAP) 1.0: a software package for predicting the luminescent properties and mobility of organic functional materials

ABSTRACT MOlecular MAterials Property Prediction Package (MOMAP) is a software toolkit for molecular materials property prediction. It focuses on luminescent properties and charge mobility properties. This article contains a brief descriptive introduction of key features, theoretical models and algorithms of the software, together with examples that illustrate the performance. First, we present the theoretical models and algorithms for molecular luminescent properties calculation, which includes the excited-state radiative/non-radiative decay rate constant and the optical spectra. Then, a multi-scale simulation approach and its algorithm for the molecular charge mobility are described. This approach is based on hopping model and combines with Kinetic Monte Carlo and molecular dynamics simulations, and it is especially applicable for describing a large category of organic semiconductors, whose inter-molecular electronic coupling is much smaller than intra-molecular charge reorganisation energy.

Crystal packing and charge transport in single crystals of chrysene derivatives: Impact of halogenation

Crystal packing has strong influence on the charge mobility for organic semiconductors, so the elucidation of the structure-property relationship is important for the design of high-performance organic semiconductors. Halogen substitution has been shown to be a promising strategy to alter the crystal structure without significantly changing the molecular size in previous reports. This paper studies the influence of halogenation on charge transport in single crystals of chrysene derivatives from a theoretical standpoint. The structure-property relationship is first rationalized by investigating the reorganization energy and electronic coupling from the density functional theory calculations. Based on the Marcus charge transfer theory, the mobilities in the molecular monolayer are then calculated with the random walk simulation technique from which the angular resolution anisotropic mobilities are obtained on the fly. It is shown that the mobilities become much larger for holes than those for electrons in the molecular monolayer when the halogenation occurs. Furthermore, the intra-layer charge transport is little influenced by the inter-layer pathways in the single crystals of the halogenated chrysene derivatives, while the opposite case is shown for the crystal of the nonhalogenated chrysene derivative. The reason for the variations of charge transport is discussed theoretically.