Introduction and output structure overview
LightForge offers a variety of outputs to analyze charge transport and exciton dynamics. This page offers an overview over all (or at least most) outputs. Depending on the setup of your simulation, some files may or may not be generated during your LightForge run.
Output can be found in the "results" folder in the runtime directory or alternatively in the downloaded "results.zip" archive.
- results/experiments/: This subdirectory contains output of the "virtual measurements", e.g. I-V, IQE, charge-carrier and exciton densities and information on excitonic processes.
- results/material/: Here you can find analysis of material related properties such as energy levels distributions (band diagrams) or rates for Foerster and Dexter energy transport between molecular species.
The following documentation is roughly structured by physical categories instead of file directory. However, file directories are clearly indicated.
Macroscopic device or layer characteristics, namely I-V, IQE and charge carrier mobility are located in the directory "results/experiments/current_characteristics". This folder contains pre-plotted figures labeled "IQE.png", "IV.png" and "mobility.png", displayed below, as well as raw-data in the following files:
- current_density_*.dat: For each applied field, one file is generated, containing three rows: field in V/nm; current density in A/m2; standard deviation of the current density in A/m2
- IQE_*.dat: contains IQE for one field, where IQE is computed by averaging over the IQE of all individual runs at this field. First column is the field in V/nm, second column the IQE.
- IQE2_*.dat: Contains similar content to IQE_*.dat, however here the total number of generated excitons and emitted photons is averaged over all individual runs to compute the IQE. This data is plotted in "IQE.png"
- IQE_all_currents.dat and IQE_all_fields.dat: IQE-compilation, where first column is current density in A/m2 or field in V/nm and the second column the IQE as computed in IQE2_*.dat.
Further, you can find particle trajectories of each single run (one randomly generated set of energy levels, one field) in trajec_*.dat. These trajectories can be used to analyze convergence, as each single simulation should result in a straight line for typical LightForge runs. The trajectories are also visualized in the file "trajecs_all.png", as illustrated below. Different colores correspond to different fields, and different lines of a single color to single runs with a different set of energy levels.
Charge carrier and exciton profiles
To analyze charge carrier balance and investigate in which part of your device excitons are generated, quenched or decay radiatively, check out the following files in the folder "results/experiments/particle_densities/":
- charge_density_average_*.png: Number of charges (electrons in blue (top) and holes in red (bottom)) over the device cross section at one field, averaged over individual LightForge runs. These values are in fact absolute numbers and not densities.
- photon_creation_density_average_*.png: Photon emission (top, yellow) and exciton creation (bottom, green) rates over the device cross section. Again, one file per field is generated.
- quenching_density_average_*.png: compilation of exciton quenching profile (black) and charge densities.
These files are exemplified below for our TTF model OLED. Respective raw data can be found in files with the ending ".dat" instead of ".png".
Excitonic processes over device cross section
The directory "results/experiments/particle_densities/" further contains information on excitonic processes over the device cross section to analyze which events (generation by recombination, radiative decay, TTF, exciton-exciton-annihilation, polaron-quenching, etc.) occur in which part of the device and on which material:
- exciton_decay_density_average_*_species_total.png: For each field (*) one of these files is generated displaying the excitonic events on all materials.
- exciton_decay_density_average_*species*.png: Of these files you can find one per field and material. The second index corresponds to the materials in the order as defined in the materials tab of the LightForge GUI. Please note that color code is different in each figure due to technical reasons.
You can find plots for the TTF model OLED below. In this setup, material index 1 correspods to the host material, index 2 to the emitter. Here we see that recombination (i.e. exciton generation) occurs mainly on the host material, whereas polaron-quenching and singlet-triplet-quenching happens mainly on the emitter. TTF occurs on both molecular species.
|Total decay density (all materials)||Decay density on the host material||Decay density on the emitter|
To track the transitions, excitons undergo before they are quenched or decay radiatively, we provide an exciton life cycle analysis. This analysis is available for each field either as image (exciton_lifecycle_*.svg) or as interactive file “exciton_lifecycle_*.html” (download and open with a web browser, or open directly via the simstack file browser). These pie-charts, illustrated below for a medium field for the TTF model OLED, show states and “conversion” processes an exciton undergoes until gone from the system. Howevering over each section displays details such as occurrence and distribution of molecular species on which these events occur. These charts are read from the inside out: The innermost fields are the “birth” of an exciton, consisting of ~75% recombination into a triplet and 25% recombination into a triplet. The field “start_tracking” contains excitons (mainly triplets) already exist at the beginning of the tracking analysis. Fields at the edge indicate that excitons are destroyed, e.g. by polaron quenching (“move_charge” in light orange).
For further analysis, raw data of excitonic processes can be found in the files "exciton_lifecycle_*.yml" in the field "lifecycle", where the star denotes a single KMC run. The event type ids in the exciton lifecycle files and the labels in the exciton lifecycle plot above correspond to the following events:
|ID in "exciton_lifecycle_*.yml"||Label in exciton lifecycle plot||Description|
|0||move chg||Heuristic CT based exciton quenching by charge moving to exciton site|
|4||move exc Foerster||Exciton transfer by Foerster energy transfer|
|-4||move exc Dexter||Exciton transfer by Dexter energy transfer|
|5||absorb phot||Creation of exciton due to irradiation source|
|6||rad decay||Radiative decay of exciton (emission of photon)|
|61||thermal decay||Monomolecular thermal dexcitation|
|7||separate he||hole electron separation, where an electron remains on the exciton site and a hole hops to another molecule|
|8||separate eh||electron hole separation, where a hole remains on the exciton site and an electron hops to another molecule|
|9||TTA||Triplet exciton is deexcited by transfering energy to another triplet exciton|
|-9||TPA||Triplet exciton is deexcited by transfering energy to a charge|
|10||spin flip exc||exciton undergoes a spinflip due to spin-orbit coupling|
|11||move+flip exc Foerster||Foerster energy transfer together with a spinflip. This has to happen prior to hyperfluorescence|
|-11||move+flip exc Dexter||Dexter energy transfer together with spinflip.|
|41||SPQ||Singlet exciton is deexcitated by tranferring energy to a charge|
|42||SSA||Singlet exciton is deexcitated by tranferring energy to another singlet exciton|
|43||STA||Singlet exciton is deexcitated by tranferring energy to another triplet exciton|
|44||TSA||Triplet exciton is deexcitated by tranferring energy to another singlet exction|
|100||start_tracking||Excitons existing at the beginning of the tracking analysis|
|101||end_tracking||Excitons existing when the simulation/tracking terminates|
|102||recombination dop||Electron-hole recombination to deactivate a dopant|
|103||recombination S1||Electron-hole recombination to form a singlet exciton|
|104||recombination T1||Electron-hole recombination to form a triplet exciton|
|105||TTF||triplet of TTF accepting energy|
|106||TTF_donor||triplet of TTF donating energy, thereby being destroyed|
|107||Dexter EPT||Heuristic dexted based exciton-polaron-quenching|
Exciton transport analysis
Analysis of exciton transport processes resolved by molecular species are available in the directory "results/experiments/particle_densities/exciton_molpairs". For each pair of molecule types you find occurence of foerster and dexter transport events in dependence of pair distance in files labeled "Type0_Type1_transport_count.png", where "Type0" and "Type1" are names of the materials as defined in the materials tab of the LightForge GUI. If for example a certain process was identified as microscopic bottleneck in the exciton lifecycle analysis, these plots are useful to gather further information on involved moltypes and details of this process. The figure below illustrates exciton transfer processes in the TTF model OLED between TTF-host and fluorescent emitter.
Foerster and Dexter rates in materials/
As microscopic rates for Foerster and Dexter processes are not directly set as parameter, but are computed from different material properties, distance dependent distributions of the resulting rates are provided in the directory "results/material/Foerster". In case of unexpected occurence of an excitonic process (e.g. in the exciton lifecycle or transport analysis), you can view the plots in this folder to check if the computed rates are in a reasonable range. Available files are:
- Dexter_X_Y.png: rates for Dexter processes between species X and Y (ids according to the order of materials defined in the materials tab) without spinflip
- Dexter_S1T1_X_Y.png and Dexter_T1S1_X_Y.png: rates for Dexter processes between species X and Y with spin flip from singlet to triplet and vice versa.
- S1S1_X_Y.png, S1T1_X_Y.png, : rates for Foerster processes between species X and Y with (S1T1, T1S1) and without (S1S1, T1T1) spin flip.
In case your excitons undergo frequent separation into electrons and holes (identified e.g. in the spatial distribution or the lifecycle analysis), figures in the "results/material/exciton_separation" directory may generate useful insight. For all applied fields you can find histograms of the exciton separation energy resolved by molecular species. In each plot you find distributions labeled "hole/elec_sep_X_to_Y", indicating the separation of an exciton by transfer or a hole/electron from X to Y. If any process shows negative exciton separation energy, this process can occur frequently. The example below shows the exciton separation between HTL and TTF-Host in the TTF model OLED. Here we see that an exciton on the HTL can separate easily into a hole remaining on the HTL, while the electron is transfered to the TTF-Host. Energy levels can be tuned to enable or suppress these processes. Details on separation rates can be found here
Band diagrams of OLED layer or device simulations are displayed in "results/material/energy_levels" in files named "energy_crosssection_*_x.png", where * is the respective field and x is the direction of transport. These band diagrams are a useful double check if your energy levels were supplied correctly and if layer thickness was set as intended. Further, band diagrams can be used to analyze fermi-level alignment at electrodes for doped injection layers band diagrams. This is illustrated below for alpha-NPD, where the first 15nm are doped with F4TCNQ: Before activation of dopants (all sites neutral, left figue), there is an injection barrier of approx. 0.3 eV between electrode and alpha-NPD. Upon dopant activation (right figure), fermi-level alignment eliminates this injection barrier and leads to an increase in energy disorder.
|Energy cross section before doping activation and equilibration||Energy cross section after doping activation and equilibration|
The results of the search are