Publications
On the Determination of Ionization Potentials
Authors: Symalla, Franz, Fediai, Artem, Neumann, Tobias, Strunk, Timo
Abstract: OLED device performance is intricately linked to microscopic processes influenced by molecular properties, creating a complex interdependence. Two key material properties in the design of balanced devices are ionization potential (IP) and electron affinities (EA). In this work we discuss how measured quantities relate to relevant energy levels in OLEDs. We present a simulation approach to compute IP and EA, benchmarked against experimental data, to shed light on the complexity of these properties. The thereby derived understanding has the potential to improve material alignment in OLED devices, ultimately improving efficiency.
DOI: 0.1002/sdtp.17597
Controlling doping efficiency in organic semiconductors by tuning short-range overscreening
Authors: Armleder, Jonas, Neumann, Tobias, Symalla, Franz, Strunk, Timo, Olivares Peña, Jorge Enrique, Wenzel, Wolfgang, Fediai, Artem
Abstract: Conductivity doping has emerged as an indispensable method to overcome the inherently low conductivity of amorphous organic semiconductors, which presents a great challenge in organic electronics applications. While tuning ionization potential and electron affinity of dopant and matrix is a common approach to control the doping efficiency, many other effects also play an important role. Here, we show that the quadrupole moment of the dopant anion in conjunction with the mutual near-field host-dopant orientation have a crucial impact on the conductivity. In particular, a large positive quadrupole moment of a dopant leads to an overscreening in host-dopant integer charge transfer complexes. Exploitation of this effect may enhance the conductivity by several orders of magnitude. This finding paves the way to a computer-aided systematic and efficient design of highly conducting amorphous small molecule doped organic semiconductors.
DOI: 10.1038/s41467-023-36748-x
In silico studies of OLED device architectures regarding their efficiency
Authors: Özdemir, Ali Deniz, Li, Fabian, Symalla, Franz, Wenzel, Wolfgang
Abstract: Simulations have become increasingly important to understand and design organic optoelectronic devices, such as organic light emitting diodes (OLEDs) and to optimize their performance by selecting appropriate materials and layer arrangements. To achieve accurate device simulations, it is crucial to consider the interplay between material properties, device architecture, and operating conditions and to incorporate physical processes such as charge injection, transport, recombination, and exciton decay. Simulations can provide insights into device bottlenecks and streamline optimization cycles, eliminating the need for physical prototyping and rationalizing OLED design. In this study, we investigated three heuristic OLED architectures with a 3D kinetic Monte Carlo (kMC) model and compared their quantum efficiency at different operation voltages. Our investigation focused on examining the effects of various layer arrangements on charge and exciton dynamics in OLED devices and establishing design principles for achieving high efficiency, which are consistent with experimental observations. Notably, we find that increasing the thickness of the emissive layer (EML) led to higher luminance efficiency, and that an emitter concentration of approximately 5% results in optimal performance. By using this model, it is possible to rapidly study the influence of many device parameters and explore a broad range of parameter and architecture space within a reasonable time-frame.
DOI: 10.3389/fphy.2023.1222589
Link: https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1222589
From Molecule to Device: Prediction and Validation of the Optical Orientation of Iridium Phosphors in Organic Light-Emitting Diodes
Authors: Degitz, Carl, Schmid, Markus, May, Falk, Pfister, Jochen, Auch, Armin, Brütting, Wolfgang, Wenzel, Wolfgang
Abstract: Due to their thin amourphous structure, unique electrical properties, and the associated variety of possible applications, OLEDs can now be found in smartphones, TVs, laptops, and wearables. While already big steps have been made in optimizing and understanding the properties influencing the external quantum efficiency (EQE), there is still room for improvement, especially when it comes to finding design principles for new emitter complexes. One contributer to the EQE here is the molecular orientation of the emitter in a given host matrix. In this work we study the viability of using molecular modeling approaches in sampling these emitter orientations for a set of already published homoleptic Ir carbene emitters and a set of emitter materials synthesized at Merck KGaA, Darmstadt, Germany, comprising both homoleptic and heteroleptic Ir(ppy)3 derivatives. We combine these simulations with different measurements for the orientation parameter and EQE, all performed with the same material stack under the same conditions. We observe a good agreement between simulation and experiment and find that the horizontal orientation of emitter molecules seems to be the main factor contributing to a higher EQE.
DOI: 10.1021/acs.chemmater.2c03177
Systematic kMC Study of Doped Hole Injection Layers in Organic Electronics
Authors: Özdemir , Ali Deniz, Kaiser , Simon, Neumann , Tobias, Symalla , Franz, Wenzel , Wolfgang
Abstract: Organic light emitting diodes (OLED) play an important role in commercial displays and are promising candidates for energy-efficient lighting applications. Although they have been continuously developed since their discovery in 1987, some unresolved challenges remain. The performance of OLEDs is determined by a multifaceted interplay of materials and device architectures. A commonly used technique to overcome the charge injection barrier from the electrodes to the organic layers, are doped injection layers. The optimization of doped injection layers is critical for high-efficiency OLED devices, but has been driven mainly by chemical intuition and experimental experience, slowing down the progress in this field. Therefore, computer-aided methods for material and device modeling are promising tools to accelerate the device development process. In this work, we studied the effect of doped hole injection layers on the injection barrier in dependence on material and layer properties by using a parametric kinetic Monte Carlo model. We were able to quantitatively elucidate the influence of doping concentration, material properties, and layer thickness on the injection barrier and device conductivity, leading to the conclusion that our kMC model is suitable for virtual device design.
DOI: 10.3389/fchem.2021.809415
Link: https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2021.809415
Bottom-Up OLED Development by Virtual Design: Systematic Elimination of Performance Bottlenecks Using a Microscopic Simulation Approach
Authors: Neumann, Tobias, Symalla, Franz, Strunk, Timo, Feidai, Artem, Kaiser, Simon, Friederich, Pascal, Wenzel, Wolfgang
Abstract: The gap between computational chemistry and parametric device simulations limits the potentially immense impact of computer models on OLED R&D. We present a review on a bottom-up multiscale modeling approach to bridge this gap and systematically eliminate performance bottlenecks by virtual design. In several case studies we demonstrate how microscopic simulations can support experimental R&D by identifying fundamental reasons for performance bottlenecks, and by deriving strategies for their elimination.
DOI: 10.1002/sdtp.15485
De Novo Calculation of the Charge Carrier Mobility in Amorphous Small Molecule Organic Semiconductors
Authors: Kaiser, Simon, Neumann, Tobias, Symalla, Franz, Schlöder, Tobias, Fediai, Artem, Friederich, Pascal, Wenzel, Wolfgang
Abstract: Organic semiconductors (OSC) are key components in applications such as organic photovoltaics, organic sensors, transistors and organic light emitting diodes (OLED). OSC devices, especially OLEDs, often consist of multiple layers comprising one or more species of organic molecules. The unique properties of each molecular species and their interaction determine charge transport in OSCs—a key factor for device performance. The small charge carrier mobility of OSCs compared to inorganic semiconductors remains a major limitation of OSC device performance. Virtual design can support experimental R&D towards accelerated R&D of OSC compounds with improved charge transport. Here we benchmark a <italic>de novo</italic> multiscale workflow to compute the charge carrier mobility solely on the basis of the molecular structure: We generate virtual models of OSC thin films with atomistic resolution, compute the electronic structure of molecules in the thin films using a quantum embedding procedure and simulate charge transport with kinetic Monte-Carlo protocol. We show that for 15 common amorphous OSC the computed zero-field and field-dependent mobility are in good agreement with experimental data, proving this approach to be an effective virtual design tool for OSC materials and devices.
DOI: 10.3389/fchem.2021.801589
Link: https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2021.801589
Computing Charging and Polarization Energies of Small Organic Molecules Embedded into Amorphous Materials with Quantum Accuracy
Authors: Armleder, Jonas, Strunk, Timo, Symalla, Franz, Friederich, Pascal, Enrique Olivares Peña, Jorge, Neumann, Tobias, Wenzel, Wolfgang, Fediai, Artem
Abstract: The ionization potential, electron affinity, and cation/anion polarization energies (IP, EA, P(+), P(−)) of organic molecules determine injection barriers, charge carriers balance, doping efficiency, and light outcoupling in organic electronics devices, such as organic light-emitting diodes (OLEDs). Computing IP and EA of isolated molecules is a common task for quantum chemistry methods. However, once molecules are embedded in an amorphous organic matrix, IP and EA values change, and accurate predictions become challenging. Here, we present a revised quantum embedding method [Friederich et al. J. Chem. Theory Comput. 2014, 10 (9), 3720−3725] that accurately predicts the dielectric permittivity and ionization potentials in three test materials, NPB, TCTA, and C60, and allows straightforward interpretation of their nature. The method paves the way toward reliable virtual screening of amorphous organic semiconductors with targeted IP/EA, polarization energies, and relative dielectric permittivity.
DOI: 10.1021/acs.jctc.1c00036
De Novo Simulation of Charge Transport through Organic Single-Carrier Devices
Authors: Kaiser, Simon, Kotadiya, Naresh B., Rohloff, Roland, Fediai, Artem, Symalla, Franz, Neumann, Tobias, Wetzelaer, Gert-Jan A. H., Blom, Paul W. M., Wenzel, Wolfgang
Abstract: In amorphous organic semiconductor devices, electrons and holes are transported through layers of small organic molecules or polymers. The overall performance of the device depends both on the material and the device configuration. Measuring a single device configuration requires a large effort of synthesizing the molecules and fabricating the device, rendering the search for promising materials in the vast molecular space both nontrivial and time-consuming. This effort could be greatly reduced by computing the device characteristics from the first principles. Here, we compute transport characteristics of unipolar single-layer devices of prototypical hole- and electron-transporting materials, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) using a first-principles multiscale approach that requires only the molecular constituents and the device geometry. This approach of generating a digital twin of the entire device can be extended to multilayer stacks and enables the computer design of materials and devices to facilitate systematic improvement of organic light-emitting diode (OLED) devices.
DOI: 10.1021/acs.jctc.1c00584
22-3: Tuning ETL Mobility by Disorder Passivation
Authors: Kaiser, Simon, Symalla, Franz, Wehl, Daniel, Neumann, Tobias, Wenzel, Wolfgang
Abstract: In organic electronics (OE) application a high degree of control of material parameters such as transport levels and charge carrier mobilities is required to build a balanced device. We demonstrate that electron mobility of the prototypical electron transport material TPBi can be tuned freely by mixing it with a secondary electron transport inert material. We can in particular increase electron mobility in TPBi by up to a factor of 10 by diluting TPBi. This increase is due to a reduced electrostatic disorder in the mixed morphology. Using our predictive ab-initio based modeling tools we can find the optimal mixture to maximize or pinpoint electron mobility in TPBi.
DOI: 10.1002/sdtp.14666
Disorder-driven doping activation in organic semiconductors
Authors: Fediai, Artem, Emering, Anne, Symalla, Franz, Wenzel, Wolfgang
Abstract: Conductivity doping of organic semiconductors is an essential prerequisite for many organic devices, but the specifics of dopant activation are still not well understood. Using many-body simulations that include Coulomb interactions and dopant ionization/de-ionization events explicitly we here show significant doping efficiency even before the electron affinity of the dopant exceeds the ionization potential of the organic matrix (p-doping), similar to organic salts. We explicitly demonstrate that the ionization of weak molecular dopants in organic semiconductors is a disorder-, rather than thermally induced process. Practical implications of this finding are a weak dependence of the ionized dopant fraction on the electron affinity of the dopant, and an enhanced ionization of the weak dopants upon increasing dopant molar fraction. As a result, strategies towards dopant optimization should aim for presently neglected goals, such as the binding energy in host-dopant charge-transfer states being responsible for the number of mobile charge carriers. Insights into reported effects are provided from the analysis of the density of states, where two novel features appear upon partial dopant ionization. The findings in this work can be used in the rational design of dopant molecules and devices.
DOI: 10.1039/D0CP01333A
Ab-initio Simulation of Doped Injection Layers.
Authors: Symalla, Franz, Fediai, Artem, Armleder, Jonas, Kaiser, Simon, Strunk, Timo, Neumann, Tobias, Wenzel, Wolfgang
Abstract: Optimization of doped injection layers in state-of-the-art OLEDs via experimental trial&error by tuning host-dopant combinations/concentrations is time-consuming and costly. We present a multiscale-simulation approach to investigate doping on microscopic level, i.e. the impact of microscopic properties on doping performance, and illustrate how to apply simulations towards materials design.
DOI: 10.1002/sdtp.13946
Multiscale Simulation of Photoluminescence Quenching in Phosphorescent OLED Materials
Authors: Symalla, Franz, Heidrich, Shahriar, Friederich, Pascal, Strunk, Timo, Neumann, Tobias, Minami, Daiki, Jeong, Daun, Wenzel, Wolfgang
Abstract: Abstract Bimolecular exciton-quenching processes such as triplet?triplet annihilation (TTA) and triplet?polaron quenching play a central role in phosphorescent organic light-emitting diode (PhOLED) device performance and are, therefore, an essential component in computational models. However, the experiments necessary to determine microscopic parameters underlying such processes are complex and the interpretation of their results is not straightforward. Here, a multiscale simulation protocol to treat TTA is presented, in which microscopic parameters are computed with ab initio electronic structure methods. With this protocol, virtual photoluminescence experiments are performed on a prototypical PhOLED emission material consisting of 93 wt% of 4,4?,4?-tris(N-carbazolyl)triphenylamine and 7 wt% of the green phosphorescent dye fac-tris(2-phenylpyridine)iridium. A phenomenological TTA quenching rate of 8.5 ? 10?12 cm3 s?1, independent of illumination intensity, is obtained. This value is comparable to experimental results in the low-intensity limit but differs from experimental rates at higher intensities. This discrepancy is attributed to the difficulties in accounting for fast bimolecular quenching during exciton generation in the interpretation of experimental data. This protocol may aid in the experimental determination of TTA rates, as well as provide an order-of-magnitude estimate for device models containing materials for which no experimental data are available.
DOI: 10.1002/adts.201900222
The influence of impurities on the charge carrier mobility of small molecule organic semiconductors
Authors: Friederich, Pascal, Fediai, Artem, Li, Jing, Mondal, Anirban, Kotadiya, Naresh B., Symalla, Franz, Wetzelaer, Gert-Jan A. H., Andrienko, Denis, Blase, Xavier, Beljonne, David, Blom, Paul W. M., Brédas, Jean-Luc, Wenzel, Wolfgang
Abstract: Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors. We use the model to quantitatively describe the influence of water molecules and water-oxygen complexes on the electron and hole mobilities. These species are seen to impact the shape of the density of states and to act as explicit charge traps within the energy gap. Our results show that trap states introduced by molecular oxygen can be deep enough to limit the electron mobility in widely used materials.
DOI: 10.48550/arXiv.1908.11854
Disorder compensation controls doping efficiency in organic semiconductors
Authors: Fediai, Artem, Symalla, Franz, Friederich, Pascal, Wenzel, Wolfgang
Abstract: Conductivity doping of inorganic and organic semiconductors enables a fantastic variety of highly-efficient electronic devices. While well understood for inorganic materials, the mechanism of doping-induced conductivity and Fermi level shift in organic semiconductors remains elusive. In microscopic simulations with full treatment of many-body Coulomb effects, we reproduce the Fermi level shift in agreement with experimental observations. We find that the additional disorder introduced by doping can actually compensate the intrinsic disorder of the material, such that the total disorder remains constant or is even reduced at doping molar ratios relevant to experiment. In addition to the established dependence of the doping-induced states on the Coulomb interaction in the ionized host-dopant pair, we find that the position of the Fermi level and electrical conductivity is controlled by disorder compensation. By providing a quantitative model for doping in organic semiconductors we enable the predictive design of more efficient redox pairs.
DOI: 10.1038/s41467-019-12526-6
Concentration dependent energy levels shifts in donor-acceptor mixtures due to intermolecular electrostatic interaction
Authors: Bag, Saientan, Friederich, Pascal, Kondov, Ivan, Wenzel, Wolfgang
Abstract: Recent progress in the improvement of organic solar cells lead to a power conversion efficiency to over 16%. One of the key factors for this improvement is a more favorable energy level alignment between donor and acceptor materials, which demonstrates that the properties of interfaces between donor and acceptor regions are of paramount importance. Recent investigations showed a significant dependence of the energy levels of organic semiconductors upon admixture of different materials, but its origin is presently not well understood. Here, we use multiscale simulation protocols to investigate the molecular origin of the mixing induced energy level shifts and show that electrostatic properties, in particular higher-order multipole moments and polarizability determine the strength of the effect. The findings of this study may guide future material-design efforts in order to improve device performance by systematic modification of molecular properties.
DOI: 10.1038/s41598-019-48877-9
Host dependence of the electron affinity of molecular dopants
Authors: Li, Jing, Duchemin, Ivan, Roscioni, Otello Maria, Friederich, Pascal, Anderson, Marie, Da Como, Enrico, Kociok-Köhn, Gabriele, Wenzel, Wolfgang, Zannoni, Claudio, Beljonne, David, Blase, Xavier, D’Avino, Gabriele
Abstract: Charge carriers energetics is key in electron transfer processes such as those that enable the electrical doping of organic semiconductors. In this study, we take advantage of the quantitative accuracy of embedded GW calculations to perform a series of virtual experiments that allow measuring the electron affinity of p-type dopants in different host solids. Our calculations show that the energy levels of a molecular impurity strongly depend on the host environment as a result of electrostatic intermolecular interactions. In particular, the electron affinity of a dopant impurity in a given semiconductor is found to be up to 1 eV lower than that of the pure dopant crystal. This result questions the pertinence of the electron affinity measured for pure dopants in order to predict doping efficiency in a specific host. The role of the Coulomb electron–hole interaction for the dopant-to-semiconductor charge transfer and for the release of doping-induced charges is discussed.
DOI: 10.1039/C8MH00921J
Organic Semiconductors: Toward Design of Novel Materials for Organic Electronics (Adv. Mater. 26/2019)
Authors: Friederich, Pascal, Fediai, Artem, Kaiser, Simon, Konrad, Manuel, Jung, Nicole, Wenzel, Wolfgang
Abstract: In article number 1808256, Wolfgang Wenzel and co-workers discuss the state of the art of predictive simulation methods, including machine learning, to complement experimental research in the identification of novel materials for organic electronics. Their potential is illustrated by highlighting some prominent recent applications.
DOI: 10.1002/adma.201970188
19-4: Boosting OLED Performance with Ab-initio Modeling of Roll-off and Quenching Processes
Authors: Symalla, Franz, Heidrich, Shahriar, Kubillus, Maximilian, Strunk, Timo, Neumann, Tobias, Wenzel, Wolfgang
Abstract: Device-scale computer simulations support experimental R&D in the identification of microscopic bottlenecks in device performance. We present full ab-initio computation of the parameters required for simulation of roll-off and quenching in OLED stacks and illustrate how strategies to improve device design can be derived from computer simulations without relying on experimental input.
DOI: 10.1002/sdtp.12905
Built-In Potentials Induced by Molecular Order in Amorphous Organic Thin Films
Authors: Friederich, Pascal, Rodin, Vadim, von Wrochem, Florian, Wenzel, Wolfgang
Abstract: Many molecules used to fabricate organic semiconductor devices carry an intrinsic dipole moment. Anisotropic orientation of such molecules in amorphous organic thin films during the deposition process can lead to the spontaneous buildup of an electrostatic potential perpendicular to the film. This so-called giant surface potential (GSP) effect can be exploited in organic electronics applications and was extensively studied in experiment. However, presently, an understanding of the molecular mechanism driving the orientation is lacking. Here, we model the physical vapor deposition process of seven small organic molecules employed in organic light-emitting diode applications with atomistic simulations. We are able to reproduce experimental results for a wide range of strength of the GSP effect. We find that the electrostatic interaction between the dipole moments of the molecules limits the GSP strength and identify short-range van der Waals interactions between the molecule and the surface during deposition as the driving force behind the anisotropic orientation. We furthermore show how the GSP effect influences the energy levels responsible for charge transport, which is important for the design of organic semiconductors and devices.
DOI: 10.1021/acsami.7b11762
26-4: Computer-Aided Optimization of Multilayer OLED Devices
Authors: Symalla, Franz, Friederich, Pascal, Kaiser, Simon, Strunk, Timo, Neumann, Tobias, Wenzel, Wolfgang
Abstract: Development of efficient OLED devices is presently driven by experimental trial&error R&D. We developed a bottom-up multiscale modeling approach enabling the computation of device properties without the use of experimentally determined parameters. Researchers can identify bottlenecks, develop new materials and optimize devices using computer aided design.
DOI: 10.1002/sdtp.12556
Molecular Origin of the Anisotropic Dye Orientation in Emissive Layers of Organic Light Emitting Diodes
Authors: Friederich, Pascal, Coehoorn, Reinder, Wenzel, Wolfgang
Abstract: Molecular orientation anisotropy of the emitter molecules used in organic light emitting diodes (OLEDs) can give rise to an enhanced light-outcoupling efficiency, when their transition dipole moments are oriented preferentially parallel to the substrate, and to a modified internal quantum efficiency, when their static dipole moments give rise to a locally modified internal electric field. Here, the orientation anisotropy of state-of-the-art phosphorescent dye molecules is investigated using a simulation approach which mimics the physical vapor deposition process of amorphous thin films. The simulations reveal for all studied systems significant orientation anisotropy. Various types are found, including a preference of the static dipole moments to a certain direction or axis. However, only few systems show an improved outcoupling efficiency. The outcoupling efficiency predicted by the simulations agrees with experimentally reported values. The simulations reveal in some cases a significant effect of the host molecules, and suggest that the driving force of molecular orientation lies in the molecule-specific van der Waals interactions of the dye molecule within the thin film surface. The electrostatic dipole–dipole interaction slightly reduces the anisotropy. These findings can be used for the future design of improved dye molecules.
DOI: 10.1021/acs.chemmater.7b03742
Rational In Silico Design of an Organic Semiconductor with Improved Electron Mobility
Authors: Friederich, Pascal, Gómez, Verónica, Sprau, Christian, Meded, Velimir, Strunk, Timo, Jenne, Michael, Magri, Andrea, Symalla, Franz, Colsmann, Alexander, Ruben, Mario, Wenzel, Wolfgang
Abstract: Abstract Organic semiconductors find a wide range of applications, such as in organic light emitting diodes, organic solar cells, and organic field effect transistors. One of their most striking disadvantages in comparison to crystalline inorganic semiconductors is their low charge-carrier mobility, which manifests itself in major device constraints such as limited photoactive layer thicknesses. Trial-and-error attempts to increase charge-carrier mobility are impeded by the complex interplay of the molecular and electronic structure of the material with its morphology. Here, the viability of a multiscale simulation approach to rationally design materials with improved electron mobility is demonstrated. Starting from one of the most widely used electron conducting materials (Alq3), novel organic semiconductors with tailored electronic properties are designed for which an improvement of the electron mobility by three orders of magnitude is predicted and experimentally confirmed.
DOI: 10.1002/adma.201703505
Molecular Origin of the Charge Carrier Mobility in Small Molecule Organic Semiconductors
Authors: Friederich, Pascal, Meded, Velimir, Poschlad, Angela, Neumann, Tobias, Rodin, Vadim, Stehr, Vera, Symalla, Franz, Danilov, Denis, Lüdemann, Gesa, Fink, Reinhold F., Kondov, Ivan, von Wrochem, Florian, Wenzel, Wolfgang
Abstract: Small-molecule organic semiconductors are used in a wide spectrum of applications, ranging from organic light emitting diodes to organic photovoltaics. However, the low carrier mobility severely limits their potential, e.g., for large area devices. A number of factors determine mobility, such as molecular packing, electronic structure, dipole moment, and polarizability. Presently, quantitative ab initio models to assess the influence of these molecule-dependent properties are lacking. Here, a multiscale model is presented, which provides an accurate prediction of experimental data over ten orders of magnitude in mobility, and allows for the decomposition of the carrier mobility into molecule-specific quantities. Molecule-specific quantitative measures are provided how two single molecule properties, the dependence of the orbital energy on conformation, and the dipole-induced polarization determine mobility for hole-transport materials. The availability of first-principles based models to compute key performance characteristics of organic semiconductors may enable in silico screening of numerous chemical compounds for the development of highly efficient optoelectronic devices.
DOI: 10.1002/adfm.201601807
Link: https://doi.org/https://doi.org/10.1002/adfm.201601807
Multiscale Simulation of Organic Electronics Via Smart Scheduling of Quantum Mechanics Computations
Authors: Friederich, Pascal, Strunk, Timo, Wenzel, Wolfgang, Kondov, Ivan
Abstract: Simulation of charge transport in disordered organic materials requires a huge number of quantum mechanical calculations and becomes particularly challenging when the polaron effect is explicitly included, i.e. the influence of the electrostatic environment of the molecules on the energy disorder. The polaron model gives rise to tasks of varying resource footprints and to dependencies between a large number of tasks. We solve the resulting tightly coupled multiscale model using the quantum patch approach by accounting for the dependencies arising from the self-consistency loops for constructing the workflow and applying a specific scheduling strategy for different task types. Our implementation of the method fully exploits the parallelism of the multiscale model alleviating the effects of load imbalance and dependencies so that it can be efficiently used on high performance computing machines.
DOI: 10.1016/j.procs.2016.05.495
Link: https://doi.org/https://doi.org/10.1016/j.procs.2016.05.495
A self-consistent first-principle based approach to model carrier mobility in organic materials
Authors: Meded, Velimir, Friederich, Pascal, Symalla, Franz, Neumann, Tobias, Danilov, Denis, Wenzel, Wolfgang
Abstract: Transport through thin organic amorphous films, utilized in OLEDs and OPVs, has been a challenge to model by using ab-initio methods. Charge carrier mobility depends strongly on the disorder strength and reorganization energy, both of which are significantly affected by the details in environment of each molecule. Here we present a multi-scale approach to describe carrier mobility in which the materials morphology is generated using DEPOSIT, a Monte Carlo based atomistic simulation approach, or, alternatively by molecular dynamics calculations performed with GROMACS. From this morphology we extract the material specific hopping rates, as well as the on-site energies using a fully self-consistent embedding approach to compute the electronic structure parameters, which are then used in an analytic expression for the carrier mobility. We apply this strategy to compute the carrier mobility for a set of widely studied molecules and obtain good agreement between experiment and theory varying over several orders of magnitude in the mobility without any freely adjustable parameters. The work focuses on the quantum mechanical step of the multi-scale workflow, explains the concept along with the recently published workflow optimization, which combines density functional with semi-empirical tight binding approaches. This is followed by discussion on the analytic formula and its agreement with established percolation fits as well as kinetic Monte Carlo numerical approaches. Finally, we skatch an unified multi-disciplinary approach that integrates materials science simulation and high performance computing, developed within EU project MMM@HPC.
DOI: 10.1063/1.4938835
QM/QM Approach to Model Energy Disorder in Amorphous Organic Semiconductors
Authors: Friederich, Pascal, Meded, Velimir, Symalla, Franz, Elstner, Marcus, Wenzel, Wolfgang
Abstract: It is an outstanding challenge to model the electronic properties of organic amorphous materials utilized in organic electronics. Computation of the charge carrier mobility is a challenging problem as it requires integration of morphological and electronic degrees of freedom in a coherent methodology and depends strongly on the distribution of polaron energies in the system. Here we represent a QM/QM model to compute the polaron energies combining density functional methods for molecules in the vicinity of the polaron with computationally efficient density functional based tight binding methods in the rest of the environment. For seven widely used amorphous organic semiconductor materials, we show that the calculations are accelerated up to 1 order of magnitude without any loss in accuracy. Considering that the quantum chemical step is the efficiency bottleneck of a workflow to model the carrier mobility, these results are an important step toward accurate and efficient disordered organic semiconductors simulations, a prerequisite for accelerated materials screening and consequent component optimization in the organic electronics industry.
DOI: 10.1021/ct501023n
Ab Initio Treatment of Disorder Effects in Amorphous Organic Materials: Toward Parameter Free Materials Simulation
Authors: Friederich, Pascal, Symalla, Franz, Meded, Velimir, Neumann, Tobias, Wenzel, Wolfgang
Abstract: 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.
DOI: 10.1021/ct500418f
Modeling disordered morphologies in organic semiconductors
Authors: Neumann, Tobias, Danilov, Denis, Lennartz, Christian, Wenzel, Wolfgang
Abstract: Organic thin film devices are investigated for many diverse applications, including light emitting diodes, organic photovoltaic and organic field effect transistors. Modeling of their properties on the basis of their detailed molecular structure requires generation of representative morphologies, many of which are amorphous. Because time-scales for the formation of the molecular structure are slow, we have developed a linear-scaling single molecule deposition protocol which generates morphologies by simulation of vapor deposition of molecular films. We have applied this protocol to systems comprising argon, buckminsterfullerene, N,N-Di(naphthalene-1-yl)-N,N’-diphenyl-benzidine, mer-tris(8-hydroxy-quinoline)aluminum(III), and phenyl-C61-butyric acid methyl ester, with and without postdeposition relaxation of the individually deposited molecules. The proposed single molecule deposition protocol leads to formation of highly ordered morphologies in argon and buckminsterfullerene systems when postdeposition relaxation is used to locally anneal the configuration in the vicinity of the newly deposited molecule. The other systems formed disordered amorphous morphologies and the postdeposition local relaxation step has only a small effect on the characteristics of the disordered morphology in comparison to the materials forming crystals.
DOI: 10.1002/jcc.23445
Generalized effective-medium model for the carrier mobility in amorphous organic semiconductors
Authors: Symalla, Franz, Meded, Velimir, Friederich, Pascal, Danilov, Denis, Poschlad, Angela, Nelles, Gabriele, von Wrochem, Florian, Wenzel, Wolfgang
Abstract: Electronic transport through disordered organic materials is relevant in many applications, including organic light-emitting diodes and organic photovoltaics. The charge-carrier mobility is one of the most important material characteristics that must be optimized to make organic devices competitive. Here we introduce a general effective-medium model for the analytic calculation of zero-field mobilities on the basis of material-specific parameters that are obtained from extensive ab initio simulations. By means of kinetic Monte Carlo simulations, we generalize the model to also include the strong disorder limit. As a proof of concept the model is applied to two different disordered organic materials exhibiting medium and strong disorder, respectively. Surprisingly, even at strong disorder the hole mobilities computed with the effective-medium model in its original form are found to agree best with the experimental data. Seeking a possible explanation for this result, we investigate the strong dependence of the mobility on the connectivity of the model topology and show that the distribution of hopping matrix elements in the material is indeed much broader than assumed in simple lattice models. As the input parameters of the model can be computed on the basis of relatively small samples, this model may be used for materials’ screening without adjustable parameters.
DOI: 10.1103/PhysRevB.91.155203
Effects of energy correlations and superexchange on charge transport and exciton formation in amorphous molecular semiconductors: An ab initio study
Authors: Friederich, Pascal, Symalla, Franz, Liu, Feilong, Meded, Velimir, Coehoorn, Reinder, Wenzel, Wolfgang, Bobbert, Peter A.
Abstract: In this study, we investigate on the basis of ab initio calculations how the morphology, molecular on-site energies, reorganization energies, and charge transfer integral distribution affect the hopping charge transport and the exciton formation process in disordered organic semiconductors. We focus on three materials applied frequently in organic light-emitting diodes: 𝛼−NPD, TCTA, and Spiro-DPVBi. Spatially correlated disorder and, more importantly, superexchange contributions to the transfer integrals, are found to give rise to a significant increase of the electric field dependence of the electron and hole mobility. Furthermore, a material-specific correlation is found between the HOMO and LUMO energy on each specific molecular site. For 𝛼−NPD and TCTA, we find a positive correlation between the HOMO and LUMO energies, dominated by a Coulombic contribution to the energies. In contrast, Spiro-DPVBi shows a negative correlation, dominated by a conformational contribution. The size and sign of this correlation have a strong influence on the exciton formation rate.
DOI: 10.1103/PhysRevB.95.115204
Charge Transport by Superexchange in Molecular Host-Guest Systems
Authors: Friederich, Pascal, Massé, Andrea, Meded, Velimir, Coehoorn, Reinder, Bobbert, Peter, Wenzel, Wolfgang
Abstract: 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.
DOI: 10.1103/PhysRevLett.117.276803