Transient Switching
*******************

Bumblebee simulations can be used to investigate the response of
organic electronic devices to external perturbations.

Here, we consider the effect of switching the device illumination on
the current produced by an organic photovoltaic (OPV).

Note:

  A pre-made "input file" is available for this tutorial.


Load the OPV Device
===================

A key benefit of Bumblebee projects is the ability to re-use
materials, stacks or simulation settings. For this tutorial, we will
be adding pulsed illumination to a photoluminescence simulation that
was created in an earlier tutorial.

We open BBinput and use the **File → Open** option to load our
project.

Note:

  The photoluminescence project file can be downloaded "here".


Create a Bulk Heterojunction
============================

The OPV layer materials are already available. For this tutorial, we
will now be considering the photoresponse of a bulk heterojunction
morphology for the OPV. (Compared to the bilayer device used
previously.)

We navigate to the *Compositions* page and use the [image:
AddButton][image] button to create a new composition. By selecting the
**Morphology** option, the composition editor will change to the
*Advanced Composition* mode. While basic compositions are used to
create ideal mixtures, advanced composition allow specification of
gradients, inhomogeneous distributions or 3D systems such as polymer
networks.

We will create an advanced composition using the PCBM and P3HT
components. Select PCBM as the background material. Then use the
[image: AddButton][image] button in the *Morphology* table to add a
gradient. A linear gradient will be selected by default. The material
can be changed to P3HT in the table.

When we select the linear gradient, an editor will appear below the
table. Here, the material fractions at the layer boundaries can be
set. We will have the gradient range from 0.8 at the start of the
layer, to 0.2 near the end of the layer.

The morphology view includes a figure that previews the material
distribution in the layer. We can enable the *Normalize* and
*Background* options to view both components.

   [image]Bulk heterojunction morphology preview in the advanced
   composition editor

By using the linear gradient, we have created a graded system
corresponding to the interpenetrating phases of a bulk heterojunction.
Save the new composition.


Update the Stack
================

The OPV stack current contains separate donor and acceptor layers. We
will now replace these with the heterojunction structure.

We need to remove the *Acceptor* layer from the *Layers* table shown
below, but before we can do this, we first need to remove all
references to that layer.

   [image]

The first step is to uncheck the **Automatic Förster Interactions**
checkbox at the top of the window depicted above. Then, we remove the
existing absorption processes using the [image: DeleteButton][image]
button in the *Absorption* table.

   [image]

Then, we click the **Delete all** option in the *Försters
interactions* table.

   [image]

Now, the *Acceptor* layer can be removed from the *Layers* table.

Next, we change the composition of the *Donor* layer to the PCBM/P3HT
blend, and change the name of the layer to *Junction*.. The thickness
of the layer is changed to 20 nm. The diagram will update to now show
a mixed donor-acceptor system.

   [image]Bulk heterojunction device in the stack editor

Recheck the **Automatic Förster interactions** option, to process the
changes to the device structure when configuring the allowed Förster
mechanisms.

We add new absorption processes to both P3HT and PCBM materials in the
heterojunction layer. (A new absorption process can be created using
the [image: AddButton][image] button.) We use an absorption
probability of 0.8 for both materials to account for optical loss
processes. The singlet state is selected as the absorption product.

   [image]Absorption configurations in the stack editor

Save the resulting stack.

Tip:

  For this tutorial, we focus on excitonic processes. In bulk
  heterojunction devices, geminate pair formation can also occur,
  wherein the positive and negative charge of the excited state become
  delocalized between donor and acceptor molecules. This affects the
  rate of charge separation in the junction. For donor-acceptor
  systems that feature strong delocalization character (e.g.
  fullerenes), the **exciplex** absorption product can be enabled to
  model the spatially-dependent field dissociation. Consideration of
  this effect becomes relevant when studying devices with space-charge
  limited photocurrent.


Transient Illumination
======================

The *Parameters* have already been configured for photo-absorption
simulations of an OPV device. We will now change to a pulsed
illumination using the **checkpoints** feature.


Transient Switches
------------------

Perturbations to the device operation can be added in the *Transient*
tab.

Checkpoints can be provided for various parameters, such as the
voltage, current or fluence. Checkpoints occur at a designated time
after the simulation has commenced. Multiple checkpoints of different
types can also be combined to replicate complex interactions.

Device perturbations must first be activated by enabling checkpoints.
Individual perturbations can then be added to the simulation by
selecting the [image: AddButton][image] button.

   [image]Transient configuration for pulsed illumination

Light switching will be investigated by simulating 2 illumination
pulses. We create a **Fluence** checkpoint at 10 microseconds, with a
value of 300 photons/s/nm^3. Additional checkpoints are added every 10
microsecond to alternate the fluence.

The starting fluence is set to 0 photons/s/nm^3 in the
*Photoluminescence* tab (tab labeled **Fluence**). A **target
simulation time** of 10 milliseconds is specified in the *Termination*
tab, at which point the simulation concludes. Save the parameter
settings.

Tip:

  When performing continuous pulsing measurements, the **pulses**
  module can be used to apply a continuously varying signal, instead
  of manually entering checkpoints. This also allows specification of
  different pulse shapes, to more accurately replicate the signals
  encountered in measurement setups. Consult the manual for more
  information.


Starting the Simulation
=======================

Because we are simulating a fluence signal with a fixed illumination
intensity, we will change the simulation to a voltage sweep. This will
allow us to measure the field-dependence of the OPV current.

Navigate to the *Simulation* tab and set **Sweep → Voltage**. We run
the sweep from 1 to 5 V in 5 steps.

   [image]Voltage sweep setup in the simulation settings

Use **File → Save** and **File → Run** to start the simulation.

Tip:

  If you wish to limit the computational time required for this
  tutorial, you can perform a single-point calculation (**Sweep →
  None**) instead. This will use the default voltage of 0.5 V defined
  in the parameter set.


Simulation Output
=================

After the simulation has concluded, the transient response of the OPV
current to the pulsed illumination can be viewed in the **Sweep → Time
Progress** section of BBresults. The **Electronic → Transient Electron
Density** tab shows the increase in charge carriers due to photo-
absorption, which coincides with the pulse intervals.

   [image]Transient electron density during pulsed illumination at 1 V

The photo-generation of excitons is the dominant excitation mechanism
at low voltages. The pulses are therefore similarly reflected in the
transient IQE(x). The **Convergence → Overview → Transient IQE-IQEx**
tab shows that some of the excitons are long-lived compared to the
interval between the pulses, resulting in a smearing of the photo-
response due to the presence of residual singlets.

   [image]Transient IQE during pulsed illumination at 1 V

This effect diminishes at higher voltages as a stronger field
facilitates the dissociation of the excitons.

   [image]Transient IQE during pulsed illumination at 3 V

These fluctuations in the charge carrier densities are also reflected
in the **Sweep → Convergence → Overview → Current Density**. Although,
for a single trajectory, the noise in the transient signal will
probably be too high to distinguish any appreciable change in the
current. Part of the reason for this is because the dark current has
not yet stabilized at the start of the simulation. By adding a pre-
equilibration stage, along with an increase in the number of
trajectories, we can improve the signal clarity in our JV response
measurements.
