Virtual device measurements by charge transport simulations
Lightforge KMC, an efficient multi-purpose kinetic Monte-Carlo (KMC) package, simulates how charges and excitons move and interact in digital twins of multi-layer devices which the user can either assemble from the digital models created with Deposit and QuantumPatch, or even create materials and devices with fictitious properties. This allows to calculate I-V-curves, mobilities, charge distributions, quantum efficiencies of OE devices etc. in order to analyse microscopic bottlenecks in device performance. Parameters required to construct layer morphologies and hopping rates between molecules can be supplied explicitly in form of input files or generated given a selection of different models. In terms of our virtual lab approach, lightforge therefore allows you to operate and analyze complete devices. The conditions of the device operation, i.e. the “experimental parameters” like external electric fields, temperature or time-dependent light irradiation, can be controlled by the user. Virtual experiments that can be conducted are, amongst others, the measurement of: time of flight (TOF) electron and hole mobilities, conductivities of materials connected to electrodes, transient or steady photocurrents in sensors or OPV, internal quantum efficiency of light generation in an OLED, doping efficiency and Fermi-level alignment in doped injection layers. Feel free to contact us if you are interested in a customized virtual measurement. A typical use case is the analysis of where and why exciton quenching occurs in an OLED device. Once identified, the user could then eliminate this performance bottleneck by changing some property of the virtual material (e.g. the electron affinity) or device (e.g. emitter concentration) and rerun the experiment to see whether this would improve device performance. After finding the sweet spot for a specific material parameter, the user can screen molecular candidates with Deposit and QuantumPatch for previously identified criteria.
- Atomistic morphologies and hopping rates (e.g. from Deposit and QuantumPatch) OR
- Input generated by a selection of different models
- Charge carrier mobility
- Charge distributions
- Exciton distribution
- Quantum efficiency (roll-off)
- Detailed analysis of quenching mechanisms for a certain setup
Microscopic processes and rates
In any simulation the positions and dynamics of all individual particles (charge carriers and excitons) are tracked with molecular resolution, allowing for detailed analysis of what is going on in a given device. This way, lightforge can be used as a powerful virtual microscope, which gives access to information that is hard to obtain experimentally. In a lightforge simulation, the device or material is represented on molecular resolution, where charge carriers and excitons are located on sites representing individual molecules. Charge or exciton transport occurs as hopping transport between these sites. For the hop between each pair of molecules an individual hopping rate determines the speed of the process. These rates account for the interaction (e.g. coulomb) between the different charge carriers and excitons in the system and are recalculated in every different state of the system. Important processes which are taken into account in a lightforge simulation are:
- Charge injection
- Dopant-host charge transfer / dopant activation
- Charge hopping transport
- Exciton hopping Dexter and Förster transport
- Triplet and singlet excitons, spin-flips
- Charge recombination <-> charge separation
- Phosphorescent and fluorescent radiative decay of excitons
- Hyperfluorescence ( “Förster spin-flips” )
- TTA, TPQ, SPQ, STA, TSA, SSA
- Creation of excitons by light absorption
Further details on the lightforge method are provided at our documentation page, or in the references below:
The lightforge preprocessor allows you to set up single or multilayer stacks of fully customizable small molecule layers e.g. bulk heterojunctions or guest host systems. It is possible to attach electrodes to the system for realistic device characterization or to perfrom transport simulations in periodic bulk-systems. Morpholgies of individual layers can consist of amorphous user supplied morphologies or generated regular lattices, or a mix of both. Relevant microscopic parameters as ionzation potentials, electron affinities, excitation energies, transfer integrals etc. can be generated automatically from models, read in explicitly or extrapolated for any system size from limited input e.g. from quantum chemical calculations or experimental data.
- Compute charge carrier mobility of various materials following this tutorial.
- Parametric simulation of a small OLED, as presented here.
- Computation of the vacuum level shift induced by an interface charge transfer (ICT), either parametric or with material specific input from QP.
- Simulation of IVs of OPV devices under illumination with this tutorial.
- Analysis of photoluminescence quenching in OLED layers following this tutorial.
- Analysis of dopant activation in doped injection layers, as presented here.
- Ab-initio simulation of a full multilayer OLED stack, with input derived solely from first principles.
Citations and references
- Details on methodology implemented in LightForge: https://publikationen.bibliothek.kit.edu/1000080716
- PLQ simulations (TTA): https://onlinelibrary.wiley.com/doi/abs/10.1002/adts.201900222
- Role of electronic disorder for doping: https://www.nature.com/articles/s41467-019-12526-6
- Influence of impurities on charge carrier mobility: https://arxiv.org/abs/1908.11854
- Ab-initio modeling of roll-off and quenching: https://onlinelibrary.wiley.com/doi/abs/10.1002/sdtp.12905
- Simulation of multilayer OLED devices: https://onlinelibrary.wiley.com/doi/abs/10.1002/sdtp.12556
- Superexchange charge transport: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.276803
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