Device Lifetime (Degradation)
*****************************

Lifetime simulations can be performed to understand the role of
degradation processes on the OLED performance.

Note:

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


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

Degradation events involve a change in molecular properties of the
layer materials. Create both healthy and degraded variants of the dye
molecule.


Healthy Phosphorescent Ir(ppy)₃ Dye
-----------------------------------

Ir(ppy)₃ is used as the phosphorescent dye.

[image]

Create a new material with the *Phosphorescent Dye* template, then
set:

* **Electronic**

  * **Energy levels**

    * HOMO = -5.27 eV

    * LUMO = -1.86 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 0.75 eV

    * Triplet binding energy = 1 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 1

    * Dexter prefactor (triplet) = 1

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Intersystem crossing rate = 10^{10}\,\textrm{s}^{-1}

    * Reverse intersystem crossing rate = 0\,\textrm{s}^{-1}

    * Triplet radiative decay rate = 6.1\cdot{}10^{5}\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate =
      1.9\cdot{}10^{4}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


Degraded Ir(ppy)₃ Dye
---------------------

The deactivated Ir(ppy)₃ dye molecule loses access to its radiative
decay pathway. Create a new *Phosphorescent Dye* material to store
these properties

The radiative decay rate of the triplet excitons is set to 0 following
deactivation. The non-radiative decay rate is kept at
1.9\cdot{}10^{4}\,\textrm{s}^{-1}.

Create a new material with the *Phosphorescent Dye* template, then
set:

* **Electronic**

  * **Energy levels**

    * HOMO = -5.3 eV

    * LUMO = -2.25 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 0.85 eV

    * Triplet binding energy = 1.25 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 1

    * Dexter prefactor (triplet) = 1

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Intersystem crossing rate = 10^{10}\,\textrm{s}^{-1}

    * Reverse intersystem crossing rate = 0\,\textrm{s}^{-1}

    * **(!)** Triplet radiative decay rate = 0\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate =
      1.9\cdot{}10^{4}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


Host: CBP
---------

CBP is used as the host material. 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.

[image]

Create a new material with the *Host* template, then set:

* **Electronic**

  * **Energy levels**

    * HOMO = -6.08 eV

    * LUMO = -1.75 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 1 eV

    * Triplet binding energy = 1.7 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 0.95

    * Dexter prefactor (triplet) = 0.95

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Singlet fraction for exciton generation = 0.25

    * Singlet radiative decay rate = 0\,\textrm{s}^{-1}

    * Singlet non-radiative decay rate = 10^{5}\,\textrm{s}^{-1}

    * Triplet radiative decay rate = 0\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate = 10^{4}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


Electron Transport Layer: TPBi
------------------------------

TPBi is used as an electron transport layer.

[image]

Create a new material with the *Transport* template, then set:

* **Electronic**

  * **Energy levels**

    * HOMO = -6.2 eV

    * LUMO = -1.7 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 0.75 eV

    * Triplet binding energy = 1 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 1

    * Dexter prefactor (triplet) = 1

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Singlet non-radiative decay rate = 10^{8}\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate = 10^{8}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


Hole Transport Layer: TAPC
--------------------------

TAPC is used as the hole transport layer.

[image]

Create a new material with the *Transport* template, then set:

* **Electronic**

  * **Energy levels**

    * HOMO = -5.5 eV

    * LUMO = -0.96 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 1 eV

    * Triplet binding energy = 1.59 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 1

    * Dexter prefactor (triplet) = 1

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Singlet non-radiative decay rate = 10^{8}\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate = 10^{8}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


Electron Blocking Layer: fac-Ir(pmb)₃
-------------------------------------

fac-Ir(pmb)₃ is used as an electron blocking layer.

Create a new material with the *Advanced* template, then set:

* **Electronic**

  * **Energy levels**

    * HOMO = -5.2 eV

    * LUMO = -1 eV

* **Excitonic**

  * **Energy levels**

    * Singlet binding energy = 0.8 eV

    * Triplet binding energy = 1.4 eV

  * **Transfer**

    * Dexter prefactor (singlet) = 0.9

    * Dexter prefactor (triplet) = 0.9

    * Singlet hopping decay length = 0.3 nm

    * Triplet hopping decay length = 0.3 nm

  * **Photophysics**

    * Singlet radiative decay rate = 0\,\textrm{s}^{-1}

    * Singlet non-radiative decay rate = 0\,\textrm{s}^{-1}

    * Triplet radiative decay rate = 3.4\cdot{}10^{5}\,\textrm{s}^{-1}

    * Triplet non-radiative decay rate =
      5.7\cdot{}10^{5}\,\textrm{s}^{-1}

Click the **Save Material** button at the top.


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

Create a host-guest system for the emission layer.

* Set the fraction of the CBP host material to 0.9

* Set the fraction of the Ir(ppy)3 dye to 0.1

* Add the degraded Ir(ppy)3 material with a fraction of 0 to make it
  accessible for degradation simulations

Click the **Save New Composition** button at the top.


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


Stack Layers
------------

On the 'Stack' page, add the different OLED layers to the device.

* Hole transport layer (HTL): 20 nm of TAPC

* Electron blocking layer (EBL): 5 nm of fac-Ir(pmb)3

* Emissive layer (EML): 40 nm of the CBP/Ir(ppy)3 composite

* Electron transport layer (ETL): 20 nm of TPBi

   [image]Layers configuration of the OLED device


Förster interactions
--------------------

After adding the layers, go to the *Förster interactions* table and
click the *Add default interactions* button to include the relevant
excitonic processes.


Degradation processes
---------------------

In the *Degradation processes* table of the stack editor, specify the
excitonic events that can trigger degradation of the materials.

Add the following degradation processes with the [image:
AddButton][image] button:

* **Cause: Exciton annihilation**

  * Layer = EML,

  * Material = Ir(ppy)3,

  * Degradation product = degraded Ir(ppy)3,

  * Probability = 1.0

* **Cause: Polaron quenching**

  * Layer = EML,

  * Material = Ir(ppy)3,

  * Degradation product = degraded Ir(ppy)3,

  * Probability = 0.8

In this setup, all annihilation events in the Ir(ppy)₃ phase cause
degradation, while only 80% of polaron-quenching events do so.

   [image]Degradation processes in the stack editor


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

On the *Parameters* page, use *Load preset* and select the *Lifetime
simulation* template. This configures the simulation to include
degradation events and enables the **Degradation** option in the
*Modules* tab.

[image]

Only one setting needs to be changed manually: **Main → Physical
Parameters → Device voltage** = 6 V.

Leave all remaining parameter settings at their default values.


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

Set up a new simulation using a single trajectory instance. Then use
**File → Save** and **File → Run** to start the simulation.


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

Monitor the progress of the simulation using BBresults (**SCM →
BBresults**). Device degradation statistics are found in the **Sweep →
Degradation** section.

The number of degraded dye molecules (**Sweep → Degradation → Overview
→ Degradation Events**) increases as the simulation progresses.

   [image]Number of degradation events at 6 V

The exponential decay in the number of degradation events is mirrored
by the decline in the average degradation rate (**Sweep → Degradation
→ Overview → Transient Degradation Rate**) as the available number of
active dye molecules decreases.

   [image]Transient degradation rate at 6 V

Inspect the distribution of degradation events across the device in
the **Sweep → Degradation → Profiles → Degradation Rate** tab.

Reminder:

* The EBL is at 20-25 nm from the anode

* The EML is at 25-65 nm from the anode,

The degradation events localize at the interface between the EBL and
the EML, where the exciton density is highest.

   [image]Distribution of degradation events across the device at 6 V

The dominant degradation mechanism is triplet-polaron quenching (TPQ),
as seen by comparing the **Sweep → Profiles per Material → Rates →
Quenching** and **Annihilation** profiles. This is attributed to the
high density of polarons compared to the exciton density required for
triplet-triplet annihilation (TTA).

   [image]Distribution of quenching events across the device at 6 V

After degradation, TPQ processes continue to occur for triplets that
are formed on the degraded dye molecules. This lowers the efficiency
of the device as degradation advances.

The emission rate (**Sweep → Degradation → Degradation Phase →
Transient Radiative Decay Rate**) reduces exponentially, roughly in
line with the rate of degradation.

   [image]Transient emission rate at 6 V

The quenching processes reduce the efficiency of the device, but also
limit the rate of degradation by reducing the exciton density inside
the device, resulting in an extended lifetime. (At the cost of a low
luminosity.)

Finally, inspect the molecular distribution in the device after
degradation processes have occurred. In **Trajectory → Morphology →
Device → Volume Fractions**, the degraded material shows a localized
build-up at the EML-EBL interface.

   [image]Material distribution after operating for 100 microseconds
   at 6 V

The **Trajectory → Morphology → Device → Cross-section** view can also
be used to visualize the exact distribution of materials inside the
stack.

   [image]Layer cross-section of the device after operating for 100
   microseconds at 6 V

Tip:

  Increasing the number of trajectories helps denoise the transient
  profiles. Particularly for slower processes (such as radiative
  decay), this reduces the sensitivity to rare events. It also
  accounts for variations in the distribution of materials and
  material parameters inside the layers.An illustration is provided as
  part of the bulk simulation tutorial.
