Polymeric OLED
**************

For polymeric OLED materials, the charge transport along the polymer
backbone can differ from the transport between adjacent chains.
Polymeric morphologies can be created to account for this behavior
during Bumblebee simulations.


Create Materials
================

We will create a new project with BBinput, which can be opened through
the **SCM → BBinput** menu. On the *Materials* page, we start by
defining the layer materials.


Polymer
-------

In this tutorial, we will consider a SY-PPV PLED. We start by
generating a new material for the polymer. Use the [image:
AddButton][image] button to open the material editor for a new
material. We use the *Fluorescent Dye* template here.

We set a HOMO level of -5.4 eV and a LUMO level of -2.8 eV. For the
polymer, we will disable the energy level broadening by setting **DOS
type** to **Delta**. For the excitons, we use a singlet binding energy
of 1.3 eV and a triplet binding energy of 1.6 eV. A delta function is
again used as **Exciton DOS type** to disable energy level broadening.

An enhanced **Singlet fraction for exciton generation** of 0.4 will be
used. For singlets, the radiative decay rate is set at
10^{8}\,\textrm{s}^{-1} and the non-radiative decay rate is set at
5\cdot{}10^{7}\,\textrm{s}^{-1}. We leave both triplet decay rates at
the default values.

To describe the preferential carrier hopping along the conjugate
backbone, anisotropic hopping rates can be specified in the *Advanced*
tab of the materials editor.

   [image]Charge transport anisotropy settings for polymeric materials

We chose here to set an interchain prefactor of 0.1 to suppress charge
hopping between the chains.


Vacuum Level
------------

Due to the imperfect stacking of polymer chains in the emission layer,
voids will be present between the chains. To account for this, we
create a vacuum material to represent these voids using the *Advanced*
material template.

We set a HOMO level of 25 eV and a LUMO level of 50 eV. Energy level
broadening is disabled using **DOS type = Delta**. This choice of
energy levels creates a large barrier for transfer towards the vacuum,
preventing this material from participating in electron transport.

Because the vacuum should not carry any excitons either, the singlet
and triplet binding energies can be set to 0. Energy level broadening
is disabled by **Exciton DOS type = Delta**.

To inhibit transport, the hole mobility, electron mobility and both
Dexter prefactors are set to 0 on the Excitonic tab.


Transport Layer
---------------

PEDOT:PSS will be used as a hole transport layer. We select the
*Transport* template to create a new material.

We set a HOMO level of -5 eV and a LUMO level of -2.3 eV. A Gaussian
energy level broadening is enabled by default. For the excitons, we
use a singlet binding energy of 0.7 eV and a triplet binding energy of
1.2 eV.


Create a Polymer Network
========================

In order to include the morphology of the polymer network, we will
create an advanced composition.

Go the *Composition* page and select the [image: AddButton][image] to
create a new layer composition. In the composition editor, select the
**Morphology** option to access the advanced composition generators.
We will use these to create a 3D network of polymeric chains.

Select the vacuum as the background material and use the [image:
AddButton][image] button in the *Morphology* table to add a generator.
Change the material to SY-PPV in the table and select the *Polymer*
generator option by double-clicking the *Linear gradient* field. A new
editor for the polymer generator will appear below the table.

The polymer generator will attempt to fill the layer using polymeric
chains obtained through a self-avoiding walk. A polymer fraction is
specified to determine the portion of the grid that will be filled
with the polymeric material. The maximum size of the individual chains
is set using the chain length parameter. Note that not every polymer
will be able to reach the maximum chain length, either due to
confinement by neighboring chains or the finite size of the layer.

The behavior of the self-avoiding walker is set using an anisotropic
growth vector, which describes the relative probability that chains
grow in any given direction. The rigidity parameter restricts the
polymer chain from folding back onto itself within a set number of
backbone units.

For SY-PPV, we will use a polymer fraction of 0.8, a chain length of
25 and a backbone **Rigidity** of 4. The chain growth probability
towards the electrodes will be doubled using **Growth rate (x) = 2**
in order to generate a directed conductor.

   [image]Polymer generation settings


Create a Stack
==============

We will create a stack containing 2 layers. The electron transfer is
assumed to proceed through a metallic contact.

* Add a 20 nm layer of PEDOT:PSS

* Add a 60 nm layer of the polymer composition

We use the **Add default interactions** option in the *Förster
interactions* table to automatically configure the excitonic
mechanisms. Use the [image: DeleteButton][image] button to remove any
processes that involve the vacuum.

   [image]Förster interactions overview in the stack editor


Create a Parameter Set
======================

On the *Parameters* page, we use the *Load preset* button to select
the *Single Voltage Point* template.

The device voltage will be set to 5 V. Because the polymer layer
contains voids, we have to manually set the electrode energy levels.
We use an energy of -4.8 eV for the anode and -3.0 for the cathode.

Finally, we switch to the *Modules* tab and tick the **Excitonics**
checkbox to enable exciton modeling.


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

For this tutorial, we will set up a new simulation for a single
voltage point.

On the *Simulation* page, we select 5 trajectory instances to improve
the sampling of the polymer network. The **Sweep** is set to **None**
as we only want to run the simulation for a single set of parameters.

We can now use **File → Save** and **File → Run** to start the
simulation.


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

The morphology of the polymer layer can be viewed in the **Trajectory
→ Morphology → Device → Cross-section** tab of BBresults.

   [image]Layer cross-section for the PEDOT:PSS/SY-PPV stack (Red =
   PEDOT:PSS, Blue = SY-PPV, Green = Void Fraction)

The labeled tabs above the graph can be used to change between the
morphologies that were generated for the different trajectories. Each
graph shows a cross-section of the device.

The precise morphology differs per trajectory, which allows us to
account for the variations in the network structure. (Effectively, we
have increased the simulated surface area of our PLED.) From the shape
of the voids in the cross-sectional view, we can see the preferential
alignment of the polymer chains along the X-axis, in line with the
distribution specified for our network generator. Along with the
preferential intrachain charge transport along the polymer backbone,
this shortens the effective path length between the electrodes, which
is in turn reflected in the **Trajectory → Convergence → Overview**
reports.
