Björn Baumeier

Microscopic Simulations of Charge Transport in Disordered Organic Semiconductors

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.

Impact of mesoscale order on open-circuit voltage in organic solar cells

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

Comparative Study of Microscopic Charge Dynamics in Crystalline Acceptor-Substituted Oligothiophenes

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole-Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.

Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent Organic Light-Emitting Diodes

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.

Excited States of Dicyanovinyl-Substituted Oligothiophenes from Many-Body Green's Functions Theory.

Excited states of dicyanovinyl-substituted oligothiophenes are studied using many-body Greens functions theory within the GW approximation and the Bethe-Salpeter equation. By varying the number of oligomer repeat units, we investigate the effects of resonant-antiresonant transition coupling, dynamical screening, and molecular conformations on calculated excitations. We find that the full dynamically screened Bethe-Salpeter equation yields absorption and emission energies in good agreement with experimental data. The effect of resonant-antiresonant coupling on the first singlet π → π* excitation monotonically decreases with increasing size of the molecule, while dynamical screening effects uniformly lower the excitation energies.

Frenkel and Charge-Transfer Excitations in Donor−acceptor Complexes from Many-Body Green's Functions Theory

Excited states of donor-acceptor dimers are studied using many-body Greens functions theory within the GW approximation and the Bethe-Salpeter equation. For a series of prototypical small-molecule based pairs, this method predicts energies of local Frenkel and intermolecular charge-transfer excitations with the accuracy of tens of meV. Application to larger systems is possible and allowed us to analyze energy levels and binding energies of excitons in representative dimers of dicyanovinyl-substituted quarterthiophene and fullerene, a donor-acceptor pair used in state of the art organic solar cells. In these dimers, the transition from Frenkel to charge transfer excitons is endothermic and the binding energy of charge transfer excitons is still of the order of 1.5-2 eV. Hence, even such an accurate dimer-based description does not yield internal energetics favorable for the generation of free charges either by thermal energy or an external electric field. These results confirm that, for qualitative predictions of solar cell functionality, accounting for the explicit molecular environment is as important as the accurate knowledge of internal dimer energies.

Challenges for in silico design of organic semiconductors

We outline the objectives of microscopic simulations of charge and energy transport processes in amorphous organic semiconductors, describe the current status of techniques used to achieve them, and list the challenges such methods face when aiming at quantitative predictions.

Electronic Excitations in Push-Pull Oligomers and Their Complexes with Fullerene from Many-Body Green's Functions Theory with Polarizable Embedding

We present a comparative study of excited states in push-pull oligomers of PCPDTBT and PSBTBT and prototypical complexes with a C60 acceptor using many-body Greens functions theory within the GW approximation and the Bethe-Salpeter equation. We analyze excitations in oligomers up to a length of 5 nm and find that for both materials the absorption energy practically saturates for structures larger than two repeat units due to the localized nature of the excitation. In the bimolecular complexes with C60, the transition from Frenkel to charge transfer excitons is generally exothermic and strongly influenced by the acceptors position and orientation. The high CT binding energy of the order of 2 eV results from the lack of an explicit molecular environment. External polarization effects are then modeled in a GW-BSE based QM/MM approach by embedding the donor-acceptor complex into a polarizable lattice. The lowest charge transfer exciton is energetically stabilized by about 0.5 eV, while its binding energy is reduced to about 0.3 eV. We also identify a globally unbound charge transfer state with a more delocalized hole at higher energy while still within the absorption spectrum, which opens another potential pathway for charge separation. For both PCPDTBT and PSBTBT, the energetics are largely similar with respect to absorption and the driving force to form intermediate charge transfer excitations for free charge generation. These results support that the higher power conversion efficiency observed for solar cells using PSBTBT as donor material is a result of molecular packing rather than of the electronic structure of the polymer.

Parametrization of Extended Gaussian Disorder Models from Microscopic Charge Transport Simulations.

Simulations of organic semiconducting devices using drift-diffusion equations are vital for the understanding of their functionality as well as for the optimization of their performance. Input parameters for these equations are usually determined from experiments and do not provide a direct link to the chemical structures and material morphology. Here we demonstrate how such a parametrization can be performed by using atomic-scale (microscopic) simulations. To do this, a stochastic network model, parametrized on atomistic simulations, is used to tabulate charge mobility in a wide density range. After accounting for finite-size effects at small charge densities, the data is fitted to the uncorrelated and correlated extended Gaussian disorder models. Surprisingly, the uncorrelated model reproduces the results of microscopic simulations better than the correlated one, compensating for spatial correlations present in a microscopic system by a large lattice constant. The proposed method retains the link to the material morphology and the underlying chemistry and can be used to formulate structure-property relationships or optimize devices prior to compound synthesis.

Charge transport in columnar mesophases of carbazole macrocycles

Charge transport properties of a columnar mesophase of carbazole macrocycles are analyzed. Realistic morphologies are sampled using all-atom molecular dynamics simulations while charge transport is simulated using the kinetic Monte Carlo method with transfer rates obtained from the high temperature nonadiabatic limit of Marcus theory. It is shown that the molecular design with side chains pointing inside the macrocycle allows close approach between molecules of neighboring columns, thus enabling three-dimensional transport and helping to circumvent charge trapping on structural defects.