Bulk Simulation¶
Periodic boundary conditions can be enabled in order to study the behavior of the bulk layer material under an active current.
Create Materials¶
We start by creating the materials that make up the OLED layers. For this tutorial, we will investigate a bulk TAPC material.
Navigate to the Materials tab in the GUI and select the New Material option. Choose the Transport template to set up a material without excitation processes.
Fig. 15 Materials template prompt. Alternatively, a dropdown menu is available next to the New Material button¶
You can provide a name for the material on the Main tab. When creating a new material, the Create a pure composition option will be enabled by default. Compositions are used to create blends of multiple materials for use as layers in the OLED. Pure compositions only contain a single material, and allow you to directly use the new materials to create OLED stacks.
Fig. 16 Main settings page for new materials¶
The Electronic tab specifies the parameters for the polarons (electrons and holes). For this tutorial, we will use a HOMO level of -5.5 eV and a LUMO level of -0.96 eV.
Fig. 17 Polaron settings page for new materials¶
The DOS type is used to introduce variations in the energy levels between gridpoints. Due to the amorphous nature of typical OLED materials, the molecular environment differs throughout the layers. These environmental differences affect the inter-molecular interactions, resulting in a distribution of energy levels.
Here, a Gaussian distribution will be used to model this effect. Set both the standard deviations to 0.1 eV. We will leave the transport parameters at the default for now.
Fig. 18 Exciton settings page for new materials¶
The Excitonic tab specifies parameters for the excitons (singlets, triplets) and the excitation processes. As we will not be modeling excitons in this tutorial yet, simply set the singlet and triplet energy levels to 0. You can now save the material.
The material should now be visible in the Materials tab. A pure composition has also been created in the Compositions tab. If you select the pure TAPC composition, you will see that it has a mole fraction of 1 for the TAPC material that we just created.
Fig. 19 Pure compositions are automatically created for new materials¶
Create a Stack¶
After defining the materials, we can now create an OLED stack by defining the layers. Here, we will be using only a single layer in order to model the bulk material behavior.
Navigate to the Stacks tab in the GUI and select the New Stack option. As before, provide a name for the stack and select Save. This will bring you to the stack editor.
Fig. 20 Stack editor layout. Hovering over the stack diagram shows the material properties¶
In the stack editor, we can create a new layer. We will use a layer thickness of 50 nm. Select the Pure TAPC composition created earlier and select the Create layer button to add the new layer to the stack. A diagram of the current stack layout will be shown. Hovering over the diagram will show a summary of the material properties.
The remaining sections of the stack editor relate to the excitonic processes, so we can leave these alone for now.
Create a Parameter Set¶
Having created an OLED stack, we will now configure the simulation settings. Navigate to the Parameter Sets tab and select the New Parameter Set option.
You will be prompted to select a template. As we are modeling a bulk material, select the Periodic Box. This will automatically configure some of the presets required for the bulk simulation.
Fig. 21 Parameter template prompt¶
Device Settings¶
In the Main tab, provide a name for this parameter set. Select the stack that we created in the previous step to add it to the parameter set.
In the Physical Parameters section of the Main tab, we can set the device voltage. We will use 1 V for this simulation. Keep the temperature at 300 K. The relative permittivity is used to set the effective permittivity of the device (i.e. for all the layers in series). For now, we will use a default value of 3.
Fig. 22 Physical parameter settings for a new parameter set¶
Simulation Settings¶
Initialization settings are provided in the Box and Energy Landscape tab. The periodic boundary conditions should have been enabled by default when using the Periodic Box template.
Fig. 23 Initialization settings for a new parameter set¶
The number of sites determines the simulation grid and sets the size of the modeled surface area of the cell. We will use the default of 50 nm in both directions.
Because we are modeling the bulk of the device using periodic boundary conditions, this means that the electrode contacts are not included in the simulation grid. A fixed number of polarons will therefore be used to investigate charge transport. We will use a single polaron type for this tutorial by setting the number of electrons to 30 and the number of holes to 0.
Simulation Duration¶
The Termination Criteria tab specifies the duration of the simulation.
As Bumblebee determines the device performance through stochastic sampling, it is important that a sufficient number of steps is allowed in order to provide accurate statistics. This can be achieved in 2 ways:
Increase the number of steps in the simulation
Perform multiple simulations in parallel and collect the results (this can be enabled during the job submission step)
For this tutorial, we will set the number of simulation steps to 1.000.000.000. This determines the maximum number of steps that the simulation will go through. Additional convergence criteria can be specified in this tab to allow early termination. These can be left off for now.
Output Settings¶
The Output tab allows specifying the frequency with which simulation results are reported. This frequency is set separately for 2 types of files:
The report interval determines how often the simulation summary and log files are updated
The output interval determines updates to the remaining output files
Writing output to a large number of files can slow down the simulation significantly. For this reason, the output interval is often taken to be larger than the report interval. Individual output files can also be enabled or disabled manually in the Output tab to reduce the cost of writing files.
For this tutorial, we will set a report interval of 5000 steps and an output interval of 1.000.000 steps.
Finally, select the Save option to create the parameter set.
Starting the Simulation¶
The Simulations tab allows submitting simulation jobs using the previously specified parameter set.
Select New Simulation to set up a submission. Specify a name for the simulation and check that the correct server is selected for running the job. Then, select the parameter set for the TAPC periodic box.
Fig. 24 Simulation submission form. Note that your server name may differ¶
The disorder instance can be used to determine the number of parallel simulations that will be conducted. Bumblebee will automatically collect sample data from each simulation and include this data in the device statistics.
We will select 2 simulation boxes in order to illustrate this process (without using too many resources). To obtain better statistics, you can increase the number of boxes here, or the simulation time in the parameter set. Note however, that this will also increase the computational cost of the simulation.
The sweep parameters will not be used for this tutorial. Select Submit Simulation to submit the job to the server.
Monitoring the Simulation¶
You can check the progress of the simulation by selecting the Jobs tab in the taskbar.
Fig. 25 Job overview in the web interface¶
Selecting the simulation will show jobs for the 2 parallel boxes. Clicking on a running job will bring you to an overview menu showing the current simulation status.
Fig. 26 Job monitor in the web interface¶
The Overview tab shows the simulation convergence and transient device parameters. The Time Progress tab provides the latest summary of the simulation statistics.
If you wish to terminate a simulation before it has reached the total number of steps, choose the Select Jobs option. You can now highlight specific boxes or entire simulations. Selecting the Stop Selected Jobs option will send a signal to the job to label the current iteration as the end of the simulation. The job will continue until the next output step has been reached and then finalize the simulation statistics.
The Kill Selected Jobs option will immediately terminate jobs, without recovering the output, and should only be used if there are issues with the job itself.
Simulation Output¶
After the simulation has finished, you can view the results in the Simulations tab. Selecting a simulation will bring you to the Overview tab. The Sweeps tab lists the associated jobs. The job progress can be viewed in the Jobs tab.
The simulation output is collected in the Reports tab. The web interface handles automatic visualization of various results. An overview of the visualization options is collected in the manual.
The Single Box tab shows the results for each job. This allows you to analyze the individual trajectories. The Multibox tab provides the aggregate results obtained from collecting the data from multiple boxes. Navigate to the Files tab if you want access to the raw simulation output.
Tip
It is possible to add additional boxes to the simulation in order to improve the statistical estimates, even if the simulation has already been completed. Select the simulation and navigate to the Sweeps tab. Select the Add Sweep option to increase the disorder instance range. This will submit additional jobs to the server. Simulation reports are updated as these jobs complete.