Exciton Simulation
******************

The basic working of an OLED is to excite molecules with a current.
The desired effect is that the molecules return to the ground state
via radiation. Therefore excitonics are the heart of the OLED. In this
tutorial we show some design principles and calculate the luminance
and efficiency of the proposed OLED.

   [image]The stack has three layers, abbreviated as HTL/EML/ETL. Here
   the layout is vertical, but in **BBinput** a horizontal layout is
   used, i.e. the above picture rotated clockwise by 90 degrees.

In this tutorial, a phosphorescent emitter is considered. Loss
processes are included using both Dexter (short range) and Förster
(long range ) mechanisms.

Open "BBinput" and set the project name to Exciton Simulation.

Note:

  Entering correctly the parameters can be a daunting task. You may
  prefer to download the "project file"


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

To construct the OLED device stack, we will create an electron
transport layer (ETL), a hole transport layer (HTL) and a host-guest
emissive layer (EML). This requires the definition of 4 materials.

Tip:

  If you have never created materials before, take a look at the start
  of the bulk tutorial.


Phosphorescent Dye
------------------

Ir(ppy)3 is used as the phosphorescent dye. We will select the
corresponding template when creating a new material on the *Materials*
page.

On the *Electronic* tab, we specify a HOMO level of -5.27 eV and a
LUMO level of -1.86 eV. A Gaussian broadening is enabled by default.

Several material-specific parameters are specified to describe exciton
generation and emission. These parameters are set on the *Excitonic*
tab of the material editor.

We will set a singlet binding energy of 0.75 eV and a triplet binding
energy of 1 eV. By enabling the option to link the singlet and triplet
binding energies, the exciton energy levels will be computed
automatically based on the exciton binding energy and the HOMO/LUMO
levels.

   [image]Exciton binding energies and energy levels can be linked
   automatically

An energy level broadening is defined for the exciton levels, just as
we did for the polaron levels, accounting for the variations in
molecular parameters due to the inhomogeneous environment of the
layer. A Gaussian broadening is used, this time with a width of 0.05
eV.

To describe Dexter-type exciton diffusion, Dexter transfer parameters
are specified. We choose a prefactor of 1, with a decay length of 0.3
nm.

   [image]Rate constants for Dexter-type exciton transfer

Note:

  The rates of transfer processes is specified in normalized units.
  I.e. the true prefactor is multiplied by a normalization factor.
  This time unit is specified in the parameter set.This decomposition
  allows us to write the material parameters using convenient
  factors.In contrast, the radiative processes are provided in natural
  units, without normalization. These parameters specify the real
  frequency in the input, with normalization applied internally by
  Bumblebee.

By selecting the phosphorescent material template, the singlet
fractions will have been set to 0, such that the exciton generation
products will exclusively be triplets.

   [image]Förster prefactors, intersystem crossing frequencies and
   singlet-triplet distributions. The PLQY and ELQY are reported
   automatically based on the provided rates

An intersystem crossing rate of 10^{10}\,\textrm{s}^{-1} will be
specified. The reverse intersystem crossing rate is set to 0. This
allows any singlets obtained through e.g. exciton transport to be
irreversibly converted to triplets.

The radiative decay rate of the triplet excitons is set to
6.1\cdot{}10^{5}\,\textrm{s}^{-1}. The non-radiative decay rate is set
to 1.9\cdot{}10^{4}\,\textrm{s}^{-1}. The photoluminescent and
electroluminescent quantum yields of the dye are now reported to
provide an indication of the molecular emitter efficiency.

Note:

  Förster processes will be configured in the stack editor. Because
  Förster transfer describes a dipolar process, the rate parameters
  exhibit strong variations with molecular environment. The stack
  editor allows definition of custom rates for inter-layer transfer
  processes, and allows definition of multiple intra-layer Förster
  processes to account for more complex rate expressions.


Host
----

CBP is used as a host material. Select the appropriate template when
creating a new material entry.

We use a HOMO level of -6.08 eV and a LUMO level of -1.75 eV. A
Gaussian broadening is enabled by default. For the excitons, we use a
singlet binding energy of 1 eV and a triplet binding energy of 1.7 eV.
For Dexter-type exciton transfer, a prefactor of 0.95 is used along
with a decay length of 0.3.

The singlet-triplet generation ratio will be set to 0.25
(corresponding to a statistical 1:3 distribution of singlet and
triplet excitons).

Thermalization losses during exciton transport from the dye through
the host are included by setting the non-radiative decay rates to
10^{5}\,\textrm{s}^{-1} for singlets and 10^{4}\,\textrm{s}^{-1} for
triplets. The radiative decay rates are set to 0.


Electron Transport Layer
------------------------

TPBi is used as an electron transport layer. Select the *Transport*
layer when creating a new material.

We use a HOMO level of -6.2 eV and a LUMO level of -1.7 eV. For the
excitons, we use a singlet binding energy of 0.75 eV and a triplet
binding energy of 1 eV. For Dexter-type exciton transfer, a prefactor
of 1 is used along with a decay length of 0.3.

To mimic the effect of an exciton diffusion barrier in the stack, a
non-radiative decay rate of 10^{8}\,\textrm{s}^{-1} is specified for
both excitons.


Hole Transport Layer
--------------------

TAPC is used as the hole transport layer. We use a HOMO level of -5.5
eV and a LUMO level of -0.96 eV. For the excitons, we use a singlet
binding energy of 1 eV and a triplet binding energy of 1.59 eV. For
Dexter-type exciton transfer, a prefactor of 1 is used along with a
decay length of 0.3. A non-radiative decay rate of
10^{8}\,\textrm{s}^{-1} is specified for both excitons.


Create Compositions
===================

We will create a host-guest mixture containing 0.9 CBP and 0.1
Ir(ppy)3.


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

We start by composing the stack layers. We use a 20 nm TAPC hole
transport layer, a 30 nm host-guest layer in the center and a 20 nm
TPBi electron transport layer.

   [image]The three layers HTL/EML/ETL. Why could this make a good
   OLED? The holes will go from left to right and for those you need
   to look at the HOMO levels, i.e. the bottom of the blocks. Holes
   can go from the HTL to the emissive layer when the target HOMO is
   higher. This is true for the 10% phosphorescent material, but not
   for the 90% CBP host. Holes cannot pass from the EML to the ETL, as
   the latter has a lower HOMO. For electrons to cross a layer
   boundary the target LUMO (top of the block) needs to be lower. In
   going from the ETL to the EML this is true for both compounds in
   the EML. Based on this simplified argumentation we expect: 1) a
   slower penetration of holes from the HTL to the EML, and no
   penetration of holes to the ETL. 2) Fast penetration of electrons
   from the ETL to the EML, but no further penetration into the HTL.
   This design ensures that there can only be both electron and holes
   in the EML where they can combine to make excitations. If these
   decay with radiation there is light.

The stack editor now allows us to define the Förster radii. Förster
rates are defined for various processes, including diffusion,
quenching and annihilation. Because the Förster rates depend on the
molecular environment, separate reactions have to be specified for
each pair of materials. To streamline this process, the GUI provides
the option to automatically configure the most common processes for
the current stack. The mechanisms that are included are based on the
material templates.

Enable the *Auto Förster* option. This will include the triplet
diffusion, triplet quenching and exciton annihilation reactions.

   [image]When using *Auto Förster*, the corresponding table is
   disabled.


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

Press the *Load preset* button on the *Parameters* page and select the
*Single voltage point* template. In the *Main* tab, we set the default
device voltage to 5 V. In the *Termination* tab, the maximum number of
simulation steps is set to 1,000,000,000.

Because carrier mobilities are enhanced at higher voltages, the
required number of steps typically changes during a voltage sweep. To
account for this, we can have the simulation terminate automatically a
stable current has been reached. On the *Termination* tab, we set a
convergence threshold of 0.1. This will automatically stop the
simulation once the current is uniform over 90% of the device.

Excitonic processes are included in the simulation by enabling the
*excitonics* module. The single voltage point template includes this
option by default. You can check the module configuration by
navigating to the *Modules* tab.


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

A voltage sweep can be performed to investigate the roll-off in device
efficiency at higher voltages. We select a voltage range from 3 to 6 V
and select 7 voltage points. A single disorder instance can be
selected to decrease the simulation runtime.

If you wish to limit the computational time required for this
tutorial, you can perform the single voltage point simulation instead.
This will use the 5 V default chosen in the parameter set.


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

We can use BBresults to analyze the output of our simulation (**SCM →
BBresults**). BBresults can also be used to monitor ongoing
simulations, so we can see how the statistics evolve as time
progresses.

The distribution of electrons and holes over the stack layers can be
viewed in the **Device → Profiles** section.

   [image]Voltage-dependent electron density in the OLED stack

   [image]Voltage-dependent hole density in the OLED stack

A good sign is that there are (almost) no electrons in the HTL and no
holes in the ETL: we want them to only combine in the emissive layer.
The graphs show that no hole or electron blocking layers are needed.

Because we are using a phosphorescent emitter, the triplet
distribution is a key indicator for the quantum yield.

   [image]Voltage-dependent triplet density in the OLED stack. Clearly
   most are generated on the left of the EML (mind the log scale).
   This is consistent with the idea that holes penetrate the EML from
   the left more slowly than electrons from the right. There is some
   "leakage" into the HTL, due to Förster triplet diffusion.

Triplets are primarily located at the HTL/EML interface, with hole
injection appearing to limit the exciton density.

Device losses can be analyzed in the **Device → Overview → Performance
→ Event Distribution**. The desired process is radiative decay, with
other processes reducing the OLED efficiency through a decrease in the
exciton density.

   [image]Distribution of processes as a function of voltage. At
   higher voltages there is rather a lot of quenching going on,
   suppressing the radiative decay.

Finally you can consider the important trade-off between luminance and
efficiency in the **Device → Luminance** panel.

   [image]The luminance increases with voltage

   [image]The efficiency decreases rather strong with voltage
