Amorphous organic semiconductors are used in many applications such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). The almost unlimited variety of chemical compounds and material combinations makes experimental material and device improvement difficult.
The challenge in modeling charge transport in amorphous materials is twofold. Models must incorporate effects on vastly different length scales ranging from the electronic structure of single molecules in the sub-nanometer scale to percolation effects on distances up to the micrometer scale. Charge mobility and energy dissipation during transport depend exponentially on the energy disorder in amorphous systems, making accurate quantum mechanical analysis of large molecular systems necessary.
This challenge can be tackled using a multiscale approach for the simulation of charge transport in disordered organic materials. The methodology of this approach consists of several successive steps building upon one another. For the generation of atomistic morphologies, molecule specific force field parameters are required. These are determined using a quantum mechanical analysis of single molecules. The parameters obtained in this first step are then used in classical molecular dynamics or Monte Carlo methods for the generation of amorphous thin film morphologies. These systems are analyzed using quantum mechanical methods in order to extract electronic properties relevant for charge transport. In the last step, the master equation of charge transport in the amorphous system is solved using an analytical or numerical approaches such as kinetic Monte Carlo simulations.
Charge Transport by Superexchange in Molecular Host-Guest Systems
Franz Symalla, Pascal Friederich, Andrea Massé, Velimir Meded, Reinder Coehoorn, Peter Bobbert, Wolfgang Wenzel
Charge Transport by Superexchange
Charge transport in disordered organic semiconductors is generally described as a result of incoherent hopping between localized states. In this work, we focus on multicomponent emissive host-guest layers as used in organic light-emitting diodes (OLEDs), and show using multiscale ab initio based modeling that charge transport can be significantly enhanced by the coherent process of molecular superexchange. Superexchange increases the rate of emitter-to-emitter hopping, in particular if the emitter molecules act as relatively deep trap states, and allows for percolation path formation in charge transport at low guest concentrations.
Phys. Rev. Lett. 117, 276803
Molecular origin of the charge carrier mobility in small molecule organic semiconductors
Pascal Friederich, Velimir Meded, Angela Poschlad, Tobias Neumann, Vadim Rodin, Vera Stehr, Franz Symalla, Denis Danilov, Gesa Lüdemann, Reinhold F. Fink, Ivan Kondov, Florian von Wrochem, Wolfgang Wenzel
Simulation protocol for calculation of charge carrier mobility
A first principles based multiscale model is presented, which accurately predicts charge carrier mobility of different materials varying over ten orders of magnitude. The model allows for the decomposition of the carrier mobility into molecule-specific quantities. The availability of fast and parameter-free screening tools to compute macroscopic materials properties may enable in-silico screening of the chemical compound space for the development of highly efficient opto-electronic devices.
Adv. Funct. Mater., 26: 5757–5763
Influence of meso and nanoscale structure on the properties of highly efficient small molecule solar cells
Tobias Moench, Pascal Friederich, Felix Holzmueller, Bogdan Rutkowski, Johannes Benduhn, Timo Strunk, Christian Koerner, Koen Vandewal, Aleksandra Czyrska-Filemonowicz, Wolfgang Wenzel, and Karl Leo
Influence of meso and nanoscale structure
The nanoscale morphology of the bulk heterojunction absorber layer in an organic solar cell (OSC) is of key importance for its efficiency. The morphology of high performance vacuum-processed, small molecule OSCs based on oligothiophene derivatives (DCV5T-Me) blended with C60 on various length scales is studied. The analytical electron microscopic techniques such as scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, highly sensitive external quantum efficiency measurements, and meso and nanoscale simulations are employed. Unique insights into the relation between processing, morphology, and efficiency of the final devices are obtained. It is shown that the connectivity of the oligothiophene-C60 network is independent of the material domain size. The decisive quantity controlling the internal quantum efficiency is the energetic disorder induced by material mixing, strongly limiting charge and exciton transport in the OSCs.
Adv. Energy Mater., 6: 1501280
Ab Initio Treatment of Disorder Effects in Amorphous Organic Materials: Toward Parameter Free Materials Simulation
Pascal Friederich, Franz Symalla, Velimir Meded, Tobias Neumann, Wolfgang Wenzel
Disordered organic materials have a wide range of interesting applications, such as organic light emitting diodes, organic photovoltaics, and thin film electronics. To model electronic transport through such materials it is essential to describe the energy distribution of the available electronic states of the carriers in the material. Here, we present a self-consistent, linear-scaling first-principles approach to model environmental effects on the electronic properties of disordered molecular systems. We apply our parameter free approach to calculate the energy disorder distribution of localized charge states in a full polaron model for two widely used benchmark-systems (tris(8-hydroxyquinolinato)aluminum (Alq3) and N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD)) and accurately reproduce the experimental charge carrier mobility over a range of 4 orders of magnitude. The method can be generalized to determine electronic and optical properties of more complex systems, e.g. guest-host morphologies, organic–organic interfaces, and thus offers the potential to significantly contribute to de novo materials design.
J. Chem. Theory Comput., 2014, 10 (9), pp 3720–3725