Microscopic Simulations of Charge Transport in Disordered Organic Semiconductors

Abstract

Charge carrier dynamics in an organic semiconductor can often be described in terms of charge hopping between localized states. The hopping rates depend on electronic coupling elements, reorganization energies, and driving forces, which vary as a function of position and orientation of the molecules. The exact evaluation of these contributions in a molecular assembly is computationally prohibitive. Various, often semiempirical, approximations are employed instead. In this work, we review some of these approaches and introduce a software toolkit which implements them. The purpose of the toolkit is to simplify the workflow for charge transport simulations, provide a uniform error control for the methods and a flexible platform for their development, and eventually allow in silico prescreening of organic semiconductors for specific applications. All implemented methods are illustrated by studying charge transport in amorphous films of tris-(8-hydroxyquinoline)aluminum, a common organic semiconductor.

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

“…densities of states from first principles, and ultimately to establish their connection to device characteristics [18,28,50,[97][98][99]. In Sec.…”

“…In the following, we will detail the theoretical foundation of the embedding technique developed as part of this thesis and implemented in the VOTCA suite [50]. Its application to both atomistic and lattice models of organic bilayers and trilayers, ordered and disordered systems will follow in Chapters 4 to 5, where we will investigate structure-energy relationships through the comparison of different molecular architectures, packing modes and ordering, indicating how a long-range treatment not only proves crucial in quantifying long-range effects in mesoscopi- , z 0 and z 1 refer to a molecular ion positioned at the centre and surface of the film, respectively.…”

“…Already at this stage, structureproperty relationships that relate morphological features to the energy landscape (as the focal point of this work) can be established. Finally, dynamics on the network can be simulated to evaluate charge-carrier or exciton mobilities, which can be further used in device simulations built on larger stochastically generated transport networks or continuous device models.The outlined simulation protocol, which is by now routinely used for the study of charge transport in organic semiconductors, is implemented in the open-source simulation package VOTCA (Versatile Object-oriented Toolkit for Coarse-graining Applications) [50]. The modular structure of this toolkit -whose extension was an important aspect of this work -includes four sub-libraries, each devoted to a specific task in the workflow from Fig.…”

“…A more rigorous approach therefore builds on a Hamiltonian which includes a quantum-mechanical description of both the electronic and nuclear degrees of freedom: and U out aB are the potentials that describe the coupling with a (still classical) outer-sphere mode. Assuming that the initial state from which the charge transfer reaction proceeds is the vibrational ground state with k = l = 0, we obtain for the charge transfer rate [50,77] This expression, which ultimately simplifies to the Levich-Jortner [16, 78] rate expression, demonstrates explicitly how the charge transfer between the initial and final states is mediated by the nuclear degrees of freedom. This extended theory incorporates nuclear tunnelling important at low temperatures, where the charge transfer proceeds via tunnelling through the energy barrier in the PES [79].…”

“…A more rigorous approach therefore builds on a Hamiltonian which includes a quantum-mechanical description of both the electronic and nuclear degrees of freedom: and U out aB are the potentials that describe the coupling with a (still classical) outer-sphere mode. Assuming that the initial state from which the charge transfer reaction proceeds is the vibrational ground state with k = l = 0, we obtain for the charge transfer rate [50,77] …”