Phelos Measurements

Our purpose is to characterize a thin film of a green organic emitter deposited on a glass substrate with our angular resolved photoluminescence (PL) setup, Phelos. We measured the p-polarized and s-polarized emission spectra of this material over a wide range of angles by exciting at 275 nm (or at 405 nm). Figure 1 shows the s-polarized PL spectrum recorded a 0 deg, as an example. Phelos can also perform angular electroluminescence (EL) measurements to characterize a full OLED stack.

 
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Figure 1: PL measurements with Phelos. The integrated driving software is capable of both data collection and analysis.

With the integrated software Characterization Suite, we can define the device and the experiment to start the measurement:

  • We define the experimental setting, as shown in Figure 2 (defined in the tab "Measurement Procedure").

  • In the tab "Acquire and Manage Data" we create a new device and select Measure Device to start the measurement.

 
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Figure 2: PL experimental settings

After the measurement, we export all measured data to Setfos to run a fitting algorithm :

  • We open the export dialogue from the menu Phelos => Export Phelos Meas. for Setfos

  • We select a device and a measurement and then click on the Export button


Setfos fitting

To perform the data fitting with Setfos, we need the emission module and the optimization mode.

Figure 3 shows the Setfos GUI with highlighted:

  • a section (red line) to define the layer structure, such as the layer thickness, the nk files of layers..

  • a section (green line) to define the wavelength range, the angle of the emission and the emission mode (spectral, dissipation power analysis, mode analysis).

  • a section (yellow line) to define the emitting layer, emission spectrum, dipole distribution, position and orientation.

 

 
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Figure 3: Setfos Graphical User Interface (GUI)

Then, we have to define the target of the fitting ( Simulation settings => Optimize settings => Targets). We use Spectral Sweep as target mode, and the s and p experimental spectral intensities obtained by Phelos. During the fitting, we can also easily extract the luminescence spectrum (select extract luminescence spectrum below the target definition, and choose white instead of file for luminance in the main window).

 
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Figure 4: Definition of the fitting parameters/targets

We also have to choose the optimization parameters, such as the emission layer thickness, the dipole position, orientation and the emission intensity. This is done with Simulation settings => Optimize settings => Parameters.

We need to define reasonable min and max values of the fitting parameters, in order to find the global minimum of the error function in the optimization.

Finally, we need to define the method to get the best starting point for each parameter.

  • InitVal: the optimization will start at the given value

  • Elements: Setfos will take linearly N points between min and max to choose the best starting point.

  • Elements log: is in the same principle as the method elements by taking points logarithmically.

  • Delta: will take a point with a given step.

More about setfos fitting


Results and comparison between measurement and simulation

 
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After the optimization, we obtain values for all optimization parameters.

 

Figure 5: Results obtained by fitting the experimental results with Setfos.

We can compare the simulated radiance vs angle with the experimental data. Also, the simulated spectra can be compared to the experimental data to qualify the simulation results.

 
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Figure 6: Comparison between measured data and fitted data for the radiance and the normalized spectrum

Figure 7 shows the comparison between simulation and experiment at five different angles. We can see that there is a good matching between measurement and simulation for both p- and s-polarization at each angle. This is showing the potential of Setfos to understand the main mechanisms regulating light emission from an organic material.

 
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Figure 7: Comparison between measured data and fitted data for spectrum at different angles

In conclusion, the combination of Phelos and Setfos can be used to study light emitting materials by extracting important parameters, such as the orientation of the dipoles and the width of the emission zone. This allows us to further optimize OLEDs that are using these materials, to reach higher EQEs.

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