Excitons are electron-hole pairs bound by Coulomb interaction which can be generated in semiconductors or insulators by interaction with light. In the simplest case, they involve excitations of electrons from the (highest) occupied to the (lowest) unoccupied molecular orbital, or from valence to conduction band. Regarded as quasi-particles in solid-state materials, excitons can transport energy without transporting net electric charge. Eventually they release their energy by recombination, coupling to lattice vibrations, or by dissociation into separate charges. Efficient excitonic energy transport is of paramount importance in a variety of opto-electronic applications. For example, in photovoltaic solar cells, excitons have to migrate from "antenna" sites of efficient light absorption to active interfaces such as electrodes or embedded catalytic sites in order for charge separation to occur.
Exciton dynamics in organic semiconductors
Rupert Klein with Burkhard Schmidt
Cooperations with Patrick Gelss, Felix Henneke, Sebastian Matera
Support by ECMath (Einstein Center for Mathematics Berlin) through project SE 20 (2017/18)
In organic semiconductors such as molecular crystals or conjugated polymer chains, excitons are typically localized (Frenkel excitons), and their transport is normally modeled in terms of excitons diffusively hopping between sites. An improved understanding of excitonic energy transport has to account for the role of electron-phonon coupling (EPC). We limit ourselves to the use of rather simple models of quantum dynamics of excitons, i.e., only two electronic states with nearest-neighbor interactions, only harmonic lattice vibrations, and only linear EPC (known as Frenkel, Holstein, Fröhlich, Davydov, and/or Peierls Hamiltonians).
Despite of these models being under investigation for several decades already, and despite of their apparent simplicity, solving the corresponding quantum-mechanical Schrödinger equation still represents a major challenge. Analytic solutions are elusive, and numeric approaches suffer from the curse of dimensionality, i.e. the exponential growth of computational effort with the number of sites involved. To cope with that problem, we employ a hierarchy of different approaches detailed in the following.
Fully quantum-mechanical approaches
Our work on a fully quantum-mechanical approach to coupled excitons and phonons focuses on the use of efficient low-rank tensor decomposition techniques to beat the curse of dimensionality. The limitation to chain structures with nearest neighbor interactions in the electron-phonon Hamiltonians mentioned above suggests the use of tensor train formats, also known as matrix product states, representing a good compromise between storage consumption and computational robustness. The time-independent Schrödinger equation is solved using an alternating linear scheme (ALS), and higher quantum states are obtained by an approach that directly incorporates the Wielandt deflation technique into the ALS for the solution of eigenproblems. In test calculations for homogeneous systems, we find that the tensor-train ranks of the state vectors only marginally depend on the chain length, which results in a linear growth of the storage consumption. However, the CPU time increases slightly faster with the chain length because the ALS requires more iterations to achieve convergence for longer chains and a given rank .
As a first test, tensor train approaches based on a SLIM decomposition of the Hamiltonian have been used to investigate the phenomenon of self-trapping, i.e., the formation of localized excitons "dressed" with deformations of the ionic scaffold. Within a certain range of the parameters involved, our calculations exactly reproduce the predictions by Davydov's soliton theory of excitonic energy transport, but we are also able to explore cases where the rigorous assumptions of that approximate analytic theory do not apply .
Mixed quantum-classical approaches
In cases where the space and time scales governing the dynamics of excitons and phonons are well separated, mixed quantum-classical molecular dynamics (QCMD) provides a suitable approximation for exciton-phonon coupling. There, the electronic degrees of freedom (excitons) are treated quantum-mechanically while the ionic motions (molecular vibrations and/or lattice vibrations, aka phonons) are treated classically. In Ehrenfeld (mean field) approaches, the latter ones are subject to forces averaged over the quantum states of the former ones. An alternative is the widely used concept of surface hopping trajectories (SHT) algorithms, where the ionic positions are modeled by classical trajectories that may stochastically switch between electronic states thus resembling non-adiabatic transitions. In cooperation with L. Cancissu Araujo and C. Lasser at TU Munich, we implemented and evaluated various non-standard SHT variants for the case of Holstein-type Hamiltonians typically used to describe the dynamics of excitons coupled to phonon modes , see also our page on quantum-classical dynamics.
In another line of activities, we combine advanced semi-classical simulation techniques for the ionic degrees of freedom with multiscale asymptotics to take advantage of a systematic scaling behavior of exciton-nuclear couplings in models of conjugated polymer chains. Our work rests on work by Hagedorn who extended the well-established theory of approximate Gaussian wave packet solutions to the time-dependent Schrödinger equation toward moving and deforming complex Gaussian packets multiplied by Hermite polynomials, yielding semi-classical approximations which are valid on (at least) the Ehrenfest time scale, i.e., the characteristic time scale of the motion of the ions. Lubich and Lasser, see their 2020 review article, developed numerical approximations based on those ideas. Their variational approaches rely on approximations to wave functions by linear combinations of (frozen or thawed) Gauss or Hagedorn functions. In principle, error bounds of any prescribed order in the semi-classical smallness parameter can be obtained, and also estimators for both the temporal and spatial discretization can be obtained efficiently, thus paving the way for fully adaptive propagation.
While the techniques sketched above will serve to overcome (at least the worst of) the curse of dimensionality, we aim at a further reduction of complexity by employing multi-scale analysis. We will utilize the fact that the expected displacements are small and that exciton-phonon coupling is much slower than the exciton transfer rate along the chain, so that we have a fast spreading of excitations that are only weakly coupled to the lattice degrees of freedom. This justifies, on sufficiently long time scales, an asymptotic WKB-like ansatz involving weak variation, or long-wave behavior. We expect to obtain the desired further complexity reduction by focusing on the associated low wave number modes. This will be particularly relevant for a large number of sites, in which case the numerical calculation of will become very expensive otherwise. In summary, building on multiscale analysis and semi-classical asymptotics, the present project aims at providing analytical insight, and hence understanding, at least in some interesting limit regimes of the relevant parameter space.