Photoluminescence
*****************

Absorption processes can be included to replicate photoresponse
measurements. Device models can be constructed to simulate
photovoltaics and photodetectors.

Note:

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


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

In this tutorial, we will model the photoluminescent behavior of an
organic photovoltaic (OPV).


Donor
-----

P3HT is used as the donor material. Excitation processes are assumed
to be singlet-dominant. We therefore choose to use the *Fluorescent
Dye* template.

We set a HOMO level of -5.2 eV and a LUMO level of -3.3 eV. For the
excitons, we use a singlet binding energy of 1.2 eV and a triplet
binding energy of 1.4 eV. Default Gaussian broadening is used for both
polaron and exciton energy levels. The Dexter prefactor for excitonic
transfer is set to 1.9.

The fluorescent material template will have automatically configured a
radiative singlet decay process. We will set this rate to
10^{7}\,\textrm{s}^{-1}. We will simulate a singlet-mediated device by
setting both singlet fractions to 1. ISC rates remain at zero.


Acceptor
--------

PCBM is used as the acceptor material. We will use the *Fluorescent
Dye* template to describe the singlet-dominant system.

We set a HOMO level of -6.1 eV and a LUMO level of -3.9 eV. For the
excitons, we use a singlet binding energy of 1.6 eV and a triplet
binding energy of 1.9 eV. Default Gaussian broadening is used for both
polaron and exciton energy levels.

The Dexter prefactor for excitonic transfer is set to 3.1. The singlet
radiative decay rate is set to 10^{7}\,\textrm{s}^{-1}. Both singlet
fractions are set to 1.


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

BCP will be used as the electron transport layer. We select the
*Transport* template to create a new material.

We set a HOMO level of -6.3 eV and a LUMO level of -2.9 eV. A Gaussian
energy level broadening is enabled by default. For the excitons, we
use a singlet binding energy of 1.5 eV and a triplet binding energy of
2.6 eV.


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

PEDOT:PSS will be used as the hole transport layer.

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 Stack
==============

We will utilize pure compositions to construct the OPV stack. We start
with a 5 nm PEDOT:PSS hole transport layer. We then add 10 nm layers
for both the P3HT donor and PCBM acceptor. A 5 nm BCP electron
transport layer is added to complete the device.

Enable the **automatic Förster processes** option to include inter-
layer singlet diffusion.

   [image]OPV bilayer device in the stack editor

Photo-absorption processes can be added in the stack editor. Select
the [image: AddButton][image] button in the *Absorption* table to
define a new absorption process. We select the P3HT material in the
donor layer. Absorption is described as a fixed excitation probability
per incident photon. We will assume a factor of 0.8, thereby
accounting for a 20% loss due to optical processes. The singlet state
is selected as the absorption product. We then repeat these steps to
add the same reaction to the PCBM material in the acceptor layer.

   [image]Absorption configurations in the stack editor


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

Bumblebee offers multiple parameter templates for photoluminescent
processes:

* The *Photoluminescence* template configures a periodic system for
  measuring the bulk properties of the layer materials. In addition to
  configuring photo-absorption processes, higher resolutions are
  obtained compared to the default *Bulk Simulation* template

* The *Photovoltaic/Photodetector* templates are used for regular
  (bipolar) device simulations

We will use the *Photovoltaic* template to model the photovoltaic
device.


Device Settings
---------------

In the *Main* setting tab, we will set the electrode levels to -4.6
for a silver anode and -4.7 for an ITO cathode contact. The external
device voltage will be set to 0.5 V.

Having chosen a PV template, the photoluminescence module should have
been enabled automatically in the *Modules* tab.


Fluence
-------

Photo-absorption settings are configured in the *Photoluminescence*
tab.

   [image]Photoluminescence settings in the parameter set editor

The incident irradiation is set by defining a device fluence. We use a
value of 300 photons/s/nm^3, in line with ambient solar lighting
conditions.

The absorption product is configured to be material-specific by
default. This will use the absorption products defined earlier in the
*Stack* editor. This selection can also be overwritten to force a
specific absorption product for the whole device. We can keep the
default value during this tutorial.

When simulating bulk material properties, a minimum exciton density
can also be added in this tab. For the current device simulation, we
will keep this value at 0.


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

For this tutorial, we will set up a new simulation using a single
trajectory instance.

A fluence sweep can be performed to investigate how the device current
changes as a function of the irradiation. We choose to vary the
fluence from 300 to 1500 photons/s/nm^3 in 5 steps.

   [image]Fluence sweep setup in the simulation settings

Once you are done, 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 300 photons/s/nm^3 default
  fluence defined in the parameter set.


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

We can view the transient photoluminescence rate from the **Sweep →
Photoluminescence → Transient Luminescent Decay Rate** section of
BBresults. As the simulation progresses, the rate of luminescence is
seen to stabilize, with the steady-state singlet density equilibrating
absorption, emission and quenching processes.

   [image]Transient singlet luminescence at an incident fluence of
   1500 photons/s/nm^3

   [image]Transient singlet density at an incident fluence of 1500
   photons/s/nm^3

Singlets are generated uniformly in the center of the device, in line
with the dictated absorption processes. The distributions can be found
under **Sweep → Profiles per Material → Occupations**. A slight
depression is noted at the interface between the donor and acceptor
layers. Charge separation occurs at the interface, resulting in a
higher quenching rate (**Sweep → Profiles per Material → Rates →
Quenching Rate**) and thus a lower local concentration of excitons.

   [image]Steady-state singlet distribution at an incident fluence of
   300 photons/s/nm^3

   [image]Steady-state electron distribution at an incident fluence of
   300 photons/s/nm^3

   [image]Steady-state hole distribution at an incident fluence of 300
   photons/s/nm^3

   [image]Steady-state quenching rates at an incident fluence of 300
   photons/s/nm^3

With an increased fluence, the singlet density and luminescent
emission rates are seen to increase. This is reflected in the internal
electrostatic field gradient, which increases in proportion to the
free carrier density.

   [image]Transient singlet luminescence at an incident fluence of 600
   photons/s/nm^3

The objective of this tutorial was to monitor the steady-state PL
response of an OPV device. If we wanted to model the photocurrent
generation process, we would run the simulation at a higher voltage to
achieve faster separation of charges. A voltage sweep can then be
employed to determine the V_{\textrm{OC}}, J_{\textrm{DC}} and fill
factor.

See also:

  For dynamic PL measurements, one would not use a constant
  illumination intensity. Pulsed illumination is commonly used to
  inject excitons into the OPV device, before subsequently monitoring
  JV and re-emission profiles to characterize device efficiencies.
  These pulsed illumination experiments can be replicated using the
  **pulse** or **checkpoint** settings found in the *Transient* tab of
  the *Parameters* page.

  Check out the transient response tutorial for more details.

See also:

  We have configured a uniform generation of excitons over the dual-
  layer device. A bulk heterojunction morphology could also be created
  through the use of *Advanced Compositions*. This would provide a
  more realistic picture of charge distribution between donor and
  acceptor species at the interface.

  Consult the morphology tutorial for more details.
