Advanced Initialization
***********************

By default, devices are initialized without charge carriers. Polaron
injection is simulated at the electrode interface to model the
population of the charge density distribution. For studying bulk
materials, a minimum charge carrier density is specified to populate
the device without direct electrode contacts.

As a result, the initial state of the device can be quite far from
equilibrium. The first segment of the kMC simulation will be dominated
by carrier diffusion through the device, before photonic processes are
able to occur. In this tutorial, we will discuss charge carrier doping
as a method for pre-initialization of device distributions.


Charge Carrier Initialization
=============================

An additional set of charges can be distributed throughout the device
at the start of the simulation. This can be utilized to accelerate
device equilibration, to simulate specific off-equilibrium conditions
or to investigate dopants and trap states. Initial carrier
concentrations are configured in the stack editor by specifying the
fraction of sites that is occupied by a given type of polar or
exciton.


Import Materials
================

In this tutorial, we will showcase how to configure dopants during the
simulation of a single-layer phosphorescent OLED. We start by
importing the host and guest materials.

Open BBinput from the main SCM menu (**SCM → BBinput**). Select the
**File → Import → Material** option to access the material database.
Use the [image: Search][image] search option to find the **mCBP** host
and the **Ir(dmp)3** phosphorescent emitter. Select the checkbox next
to these materials and use the **Import** option to add the materials
to the project.

   [image]Import materials from the *Materials Database* in BBinput


Create a Host-Guest System
==========================

Navigate to the *Compositions* page. Pure compositions for our host
and guest have been created automatically. We will use the [image:
AddButton][image] button to create a new composition for a host-guest
blend.

In the composition editor, use the [image: AddButton][image] button in
the *Fractions* table to add the mCBP and Ir(dmp)3 materials. We set a
fraction of 0.9 for mCBP and 0.1 for Ir(dmp)3. Select *Save
composition* to add the new blend to the project.

   [image]Host-guest blend in the composition editor


Create a Single-layer Device
============================

Navigate to the *Stack* page. Use the [image: AddButton][image] button
in the *Layers* table to create a single-layer OLED device. We select
the host-guest mixture as the layer composition and change the
thickness to 20 nm.

   [image]Single-layer OLED device in the stack editor

Instead of starting with an empty device, we can add charges to the
system by choosing dopants. We navigate to the *Dopants* table in the
*Stack* page. Use the [image: AddButton][image] button to create 2 new
dopants. Both dopants will be added to the host mCBP material. We set
the type of the first dopant to *Electron* and the second dopant to
*Hole*. The fraction will be set to 0.005 for both types. This sets
the number of molecules that will contain charges.

   [image]Dopant configuration in the stack editor


Output Settings
===============

Navigate to the *Parameters* page. On the *Output* tab, we set the
*Report interval* to 10,000 and the *Output interval* to 100,000. On
the *Termination* tab, we set the *Convergence threshold* to 0.1. The
simulation will then end automatically once the current is uniform
across 90% of the device.


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

For this example, we will perform a single voltage point calculation,
which is configured by default. To reduce the resources required to
run this tutorial, we navigate to the *Simulation* page and set both
*first trajectory* and *last trajectory* to 1.

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


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

We can monitor the progress of the simulation using BBresults (**SCM →
BBresults**). Compared to the simulation for the undoped system,
charge carrier initialization enhances the rate of convergence as the
initial charge carrier distribution is closer to the equilibrium.

   [image]Convergence of the transient current for the doped device

   [image]Convergence of the transient current for an undoped device

In addition, by initializing carriers inside the device, we also see
that exciton generation at the emitter occurs earlier in the
simulation runtime. This provides faster access to the statistics on
the excitonic processes.

   [image]Transient exciton density inside the doped device

   [image]Transient exciton density inside the undoped device

Tip:

  To get the best performance, the initial charge carrier
  concentration should be as close as possible to the device
  equilibrium. Charge carrier distributions obtained from previous
  simulations can provide a suitable first estimate.Due to differences
  in electron and hole mobility in the device, the optimum starting
  conditions may require using a different dopant fraction for
  electron and holes.

See also:

  Charge carrier initialization can also be used to mimic exciton
  generation following a (near-instantaneous) illumination pulse. This
  can be used to investigate the transient response of a device or
  thin film.

  Consult the photoluminescence tutorial for more information on
  studying photoresponse properties.
