Singlet Excitation Energy Transfer

Tutorial Created with Software Release: 2024-3
Topics: Organic Electronics
Methodology: Molecular Quantum Mechanics
Products Used: Jaguar, MS Maestro

Tutorial files

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Words found in the Glossary of Terms are shown like this: Workspacethe 3D display area in the center of the main window, where molecular structures are displayed

 

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Abstract:

 

In this tutorial, we will learn how to compute the rate of singlet excitation energy transfer occurring in a system of organic optoelectronics.

 

Tutorial Content

  1. Introduction to Singlet Excitation Energy Transfer

  1. Creating Projects and Importing Structures

  1. Calculating and Analyzing the Singlet Excitation Energy Transfer

  1. Conclusion and References

  1. Glossary of Terms

     

1. Introduction to Singlet Excitation Energy Transfer

Singlet excitation energy transfer (SEET) is a fundamental process in the operation of organic light emitting diodes (OLEDs). In OLEDs, light emission occurs through the recombination of electrons and holes to form excitons, which are bound states of an electron and a hole. These excitons can exist in either singlet or triplet states, depending on the spins of the electron and hole.

Singlet excitons, where the spins are opposite and thus paired, are particularly important for the efficiency and brightness of OLEDs. When a singlet exciton transfers its energy to a neighboring molecule in the emissive layer, this process is known as SEET. Efficient SEET ensures that the energy is effectively channeled to the emissive molecules, leading to the emission of photons and, consequently, visible light.

A cutting-edge advancement in OLED technology that leverages SEET is hyperfluorescence. Hyperfluorescence combines the high efficiency of thermally activated delayed fluorescence (TADF) with the high color purity of fluorescent emitters. In hyperfluorescence, a TADF sensitizer transfers its excitation energy to a fluorescent emitter via SEET. This process harnesses the efficiency of triplet exciton harvesting by the TADF molecule and the high emission intensity of the fluorescent molecule.

Incorporating hyperfluorescence into OLEDs offers several benefits:

  1. Efficiency: Hyperfluorescence maximizes the utilization of both singlet and triplet excitons, significantly enhancing the overall efficiency of the OLED device.
  2. Color Purity: By transferring energy to highly pure fluorescent emitters, hyperfluorescence achieves superior color purity and brightness.
  3. Device Lifetime: The combination of TADF and fluorescent materials can lead to more stable exciton dynamics, potentially extending the operational lifespan of OLED devices.  

While efficient SEET is generally desirable for optimizing OLED performance, there are scenarios where a low SEET rate can be beneficial. For example, when a neat film of the emitter is used for the emissive layer, a lower SEET rate helps to avoid concentration quenching, a phenomenon where excitons transfer their energy to non-emissive states or annihilate each other due to close proximity. By reducing concentration quenching, the quantum yield can be improved, leading to a more efficient and brighter OLED.

Understanding and optimizing SEET and hyperfluorescence is crucial for advancing OLED performance. Researchers focus on designing materials and molecular systems that facilitate efficient energy transfer and exciton management while also considering scenarios where lower SEET rates can be advantageous. These innovations are paving the way for the next generation of high-performance, energy-efficient OLED displays and lighting solutions.

In this tutorial, the SEET rate for a neat film of TADF emitters, oBFCzTrz (5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole) will be calculated using the Optoelectronic Film Properties panel and analyzed using the Optoelectronic Film Properties Viewer.

This tutorial does not cover building the oBFCzTrz system. See the Disordered System Building and Molecular Dynamics Multistage Workflows tutorial for practice building an optoelectronic film morphology system. If interested in using the Optoelectronic Film Properties panel to calculate other optoelectronic film properties, see the Calculating Transition Dipole Moments (TDM), TDM Distributions, and Order Parameter, the Singlet-Triplet Intersystem Crossing Rate, or the Computational Ellipsometry tutorial.

2. Creating Projects and Importing Structures

At the start of the session, change the file path to your chosen Working Directorythe location where files are saved in MS Maestro to make file navigation easier. Each session in MS Maestro begins with a default Scratch Project, which is not saved. A MS Maestro project stores all your data and has a .prj extension. A project may contain numerous entries corresponding to imported structures, as well as the output of modeling-related tasks. Once a project is saved, the project is automatically saved each time a change is made.

Structures can be built in MS Maestro and are added to the Entry List and Project Table. The Entry List is located to the left of the Workspace. The Project Table can be accessed by Ctrl+T (Cmd+T) or Window > Project Table if you would like to see an expanded view of your project data.

  1. Double-click the Materials Science icon

Figure 2-1. Change Working Directory option.

  1. Go to File > Change Working Directory
  2. Find your directory, and click Choose
  3. Pre-generated input files are included for running jobs. Download the zip file here: schrodinger.com/sites/default/files/s3/release/current/Tutorials/zip/seet.zip
  4. After downloading the zip file, unzip the contents in your Working Directory for ease of access throughout the tutorial

Figure 2-2. Save Project panel.

  1. Go to File > Save Project As
  2. Change the File name to seet_tutorial, click Save
    • The project is now named seet_tutorial.prj

Figure 2-3. The input structure entry.

  1. Go to File > Import Structures
  2. Navigate to where you downloaded the provided tutorial files, choose input.maegz and click Open

The imported entry oBFCzTrz is now available in the entry list. This amorphous system is composed of 250 molecules of oBFCzTrz. This system is initially built using the Disordered System Builder panel. To equilibrate the morphology, energy minimization followed by a 5 ns molecular dynamics (MD) simulation using NPT ensemble at 1 atm and 300 K were first conducted. The resulting system was next treated by a 5 ns simulated annealing, increasing the temperature from 300 K to 550 K, then cooled down to 300 K through a 25 ns MD. Finally, an MD simulation was carried out for 20 ns at 300 K.

3. Calculating and Analyzing the Singlet Excitation Energy Transfer

TADF emitters are typically embedded at low concentrations in a host matrix through host–guest co-evaporation to suppress emission quenching and exciton annihilation caused by molecular aggregation. However, recent studies have shown that some TADF emitters exhibit negligible concentration quenching, presenting an opportunity to simplify OLED structure and fabrication while reducing manufacturing costs. For example, OLEDs with an active layer containing a high concentration of oBFCzTrz, a blue TADF emitter, have demonstrated high external quantum efficiency (EQE) with low roll-off during operation and minimal concentration quenching. In order to provide a

molecular insight into the low concentration quenching of the oBFCzTrz emissive layer, we will calculate the SEET rates in a neat film composed of the TADF emitters.

Specifically, we will calculate the SEET rates between oBFCzTrz molecules in a MD equilibrated box using the Optoelectronic Film Properties panel and visualize the results with the Optoelectronic Film Properties Viewer panel.

We note the SEET rates are impacted by intermolecular orientations and hence the film morphology. Therefore, for practical application and a better agreement with experimental measurements, we recommend preparing the film's morphology according to the experimental condition, e.g., vacuum deposition or solution processing. See the Molecular Deposition and Evaporation tutorials for more information.         

Figure 3-1. Opening the Optoelectronic Film Properties panel.

  1. In the entry list, includethe entry is represented in the Workspace, the circle in the In column is blue and selectthe entry is chosen in the Entry List (and Project Table), the row is highlighted; project operations are performed on all selected entries the oBFCzTrz entry
  2. Go to Tasks > Materials > Quantum Mechanics > Optoelectronic Film Properties > Optoelectronic Film Properties

This panel allows you to determine various optoelectronic film properties, such as order parameter (emitting dipole orientation) and singlet exciton energy transfer rate. The former allows users to estimate emitter orientation in thin films which impacts the outcoupling efficiency of OLED devices, see the Calculating Transition Dipole Moments (TDM), TDM Distributions, and Order Parameter tutorial. Here, the singlet exciton energy transfer rate tab options are explored.

Singlet Exciton Energy Transfer Rate tab: Specifies the dimer and corresponding parameters for the singlet exciton energy transfer rate calculation.

Donor and acceptor molecules option menu: Specifies the molecule type to be used as a donor or acceptor in the energy transfer.

Donor or acceptor singlet excitation energy correction text box: Specifies the correction to the donor or acceptor S1 excitation energy in electronvolts (eV).

Reorganization energy text box: Specifies the S1 reorganization energy.

For detailed information about the various parameters in the panel, see the help documentation.

Figure 3-2. Setting up the Optoelectronic Film Properties panel for a SEET calculation.

  1. Ensure that Use structures from shows Workspace (included entry)
  2. Uncheck the Transition dipole moment order parameter option
  3. Check the Singlet excitation energy transfer option
  4. Click Jaguar Options to open the Jaguar Options panel

Figure 3-3. Setting the Jaguar Options.

  1. Change the Theory to wB97X-D
  2. Change the Basis set to 6-31G**
  3. Add lrc-omega=0.0217 to the Additional keywords
  4. Click OK to close the panel

Figure 3-4. Running the SEET calculation.

  1. Change the Reorganization energy to 0.89 eV
  2. Change the Job name to opto_seet
  3. Adjust the job settings () as needed
    • This job requires a CPU host and can be completed in about 8 hours on 4 processors
  4. If running the job, proceed to click Run. If you would prefer to proceed with imported files, please proceed to the next steps.
  5. Close the Optoelectronic Film Properties

Figure 3-5. The calculation output.

Let’s import the results:

  1. Go to File > Import Structures
  2. Navigate to where you downloaded the provided tutorial files, choose Section_03 > opto_seet > opto_seet-out.maegz
  3. Selectthe entry is chosen in the Entry List (and Project Table), the row is highlighted; project operations are performed on all selected entries and includethe entry is represented in the Workspace, the circle in the In column is blue the output in the entry list and workspace, respectively.
    • Note that the structure will resemble the input structure, but now also contains the data from the Optoelectronic Film Properties calculation
  4. Use the WAM (workflow action menu) button () to open the Optoelectronic Film Properties Viewer
    • Alternatively, access the panel via Tasks > Materials > Quantum Mechanics > Optoelectronic Film Properties > Optoelectronic Film Properties Viewer

Figure 3-6. Viewing the Optoelectronic Film Properties result.

  1. Ensure that the Singlet excitation energy transfer tab is selected

The log of the maximum singlet exciton energy transfer rate and its corresponding intermolecular distance is shown on top of the plot. The plot displays the singlet exciton energy transfer rate in seconds-1 as a function of intermolecular distance in angstroms.

In this approach, kSEET is calculated for all possible dimers, which allows for a good sampling of kSEET rates and also kSEET for dimers with large intermolecular distances. The kSEET rates are estimated for the TADF system and show good agreement with previous calculations (see References).

The rates of SEET can reach up to 1.8 ×108 s−1 with intermolecular COM (center of mass) at around 6 Å. Considering only energy-transfer processes, the exciton diffusion coefficient for SEET is approximately 1.08×107 nm2 s−1. This coefficient is significantly lower than those previously reported for thin films based on conventional organic semiconductors, which have values around 1010−1011 nm2 s−1, see References.

The diffusion length for the oBFCzTrz excitons is within the time τ=1/kRISC=0.45 μs (see References), which is considered the upper limit of the TADF process. The singlet exciton diffusion length is calculated to be approximately 2.2 nm, which is about an order of magnitude smaller than those found for conventional organic semiconductors such as NPD (N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine), which have a diffusion length of 5.1 nm, see References.

Considering the average distance between two excitons is about 9.3 nm under normal device operation conditions (see References), the SEET results suggest that the singlet excitons are able to perform only about 3-4 intermolecular hops before decaying to their ground state. These findings underline that the exciton energy-transfer processes in oBFCzTrz films are too slow to lead to any significant luminescence quenching via exciton-exciton annihilation or exciton decay at device interfaces.

4. Conclusion and References

This tutorial demonstrated how to compute the singlet excitation energy transfer rate for optoelectronic materials using MS Maestro.

For further learning:

For introductory content, focused on navigating the Schrödinger Materials Science interface, an Introduction to Materials Science Maestro tutorial is available. Please visit the materials science training website for access to 70+ tutorials. For scientific inquiries or technical troubleshooting, submit a ticket to our Technical Support Scientists at help@schrodinger.com. 

For self-paced, asynchronous, online courses in Materials Science modeling, including access to Schrödinger software, please visit the Schrödinger Online Learning portal on our website.

If you are interested in running Jaguar calculations from the command line, please visit the documentation for example files and guidance.

For some related practice, proceed to explore other relevant tutorials:

For further reading:
  • Suppression of concentration quenching in ortho-substituted thermally activated delayed fluorescence emitters. Solution-processed multilayer small-molecule light-emitting devices with high efficiency white-light emission. DOI:10.1002/adts.201900185
  • Long-range resonant energy transfer for enhanced exciton harvesting for organic solar cells. DOI: 10.1002/adma.200700917
  • Triplet exciton diffusion in fac-tris(2-phenylpyridine) iridium(III)-cored electroluminescent dendrimers. DOI: 10.1063/1.1867571
  • Trap-limited exciton diffusion in organic semiconductors. DOI: 10.1002/adma.201304162
  • Investigation of energy transfer in organic photovoltaic cells and impact on exciton diffusion length measurements. DOI: 10.1002/adfm.201001928
  • Transient analysis of triplet exciton dynamics in amorphous organic semiconductor thin films. DOI: 10.1016/j.orgel.2006.04.007
  • Organic solar cells: understanding the role of forster resonance energy transfer. DOI: 10.3390/ijms131217019
  • Exciton transport in an organic semiconductor exhibiting thermally activated delayed fluorescence. DOI: 10.1021/acs.jpcc.6b01679
  • Exciton diffusion in organic semiconductors. DOI: 10.1039/C5EE00925A
  • Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching. DOI: 10.1063/1.3079797

5. Glossary of Terms

Entry List - a simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion

Included - the entry is represented in the Workspace, the circle in the In column is blue

Incorporated - once a job is finished, output files from the working directory are added to the project and shown in the Entry List and Project Table

Project Table - displays the contents of a project and is also an interface for performing operations on selected entries, viewing properties, and organizing structures and data

Selected - the entry is chosen in the Entry List (and Project Table), the row is highlighted; project operations are performed on all selected entries

Working Directory - the location where files are saved

Workspace - the 3D display area in the center of the main window, where molecular structures are displayed