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Recognizing and correcting for errors in frequency-dependent modulation spectroscopy

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Physica Scripta
Authors:
Nora Marita Wilson at Åbo Akademi University
Nora Marita Wilson
Harri Aarnio at Centria University of Applied Sciences
Harri Aarnio
Ronald Österbacka at Åbo Akademi University
Ronald Österbacka
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This work shows how to acquire reliable data from frequency dependent continuous-wave modulation spectroscopy. We demonstrate this through the example of continuous-wave photoinduced absorption (cwPA), a characterization technique useful for studying long-lived photoexcitations in thin-film solar cell materials. Experimental errors arising at moderate frequencies in modulation spectroscopy are identified and corrected for. Limitations of the detectors and electronics are seen to cause both signal loss and phase shifts. Imperfect charge collection in the detector leads to wavelength-dependent correction factors, while induced phase shifts in the experimental setup cause a need for frequency dependent corrections. The methods outlined in this work act as a guide to avoid pitfalls in setting up modulation spectroscopy measurements and correcting for limitations.
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Phys. Scr. 98 (2023)115042 https://doi.org/10.1088/1402-4896/ad03bf
PAPER
Recognizing and correcting for errors in frequency-dependent
modulation spectroscopy
N M Wilson
1
, H Aarnio
2
and R Österbacka
1
1
Physics, Faculty of Science and Engineering, Åbo Akademi University, Henriksgatan 2, 20500 Turku, Finland
2
Mathematics and physics, Centria University of Applied Sciences, Bondegatan 2, 67100 Karleby, Finland
E-mail: Ronald.Osterbacka@abo.fi
Keywords: photomodulation spectroscopy, modulation spectroscopy, photoinduced absorption, continuous-wave photoinduced
absorption, phase-sensitive measurements, pump-prope spectroscopy, organic semiconductors
Supplementary material for this article is available online
Abstract
This work shows how to acquire reliable data from frequency dependent continuous-wave
modulation spectroscopy. We demonstrate this through the example of continuous-wave photo-
induced absorption (cwPA), a characterization technique useful for studying long-lived photoexcita-
tions in thin-film solar cell materials. Experimental errors arising at moderate frequencies in
modulation spectroscopy are identified and corrected for. Limitations of the detectors and electronics
are seen to cause both signal loss and phase shifts. Imperfect charge collection in the detector leads to
wavelength-dependent correction factors, while phase shifts caused by the experimental setup call for
frequency-dependent corrections. The methods outlined in this work act as a guide to avoid pitfalls in
setting up modulation spectroscopy measurements and correcting for limitations.
1. Introduction
Continuous-wave modulation spectroscopies can be used to observe reflectance and absorbance to gain insight into
material properties and the dynamics of long-lived photoexcitations [1–3]. These spectroscopic techniques all use a
periodically varying modulation, such as light or electric field, to probe the properties of a material while measuring a
spectroscopic response. The modulation will lead to a periodicity in the measured signal. This enables the use of lock-in
techniques, which have the capacity to single out minimal signals as long as they repeat at a specified frequency. Thereby
we can eliminate random noise and get a very sensitive measurement. Changing the modulation frequency can be used
to probe material dynamics but care must be taken when working with higher frequencies, as made evident in this work.
Continuous-wave photoinduced absorption (cwPA)is a measurement technique that provides information
about the recombination and generation of long-lived excitations in thin films of organic semiconductors [4–7].
The measurement is relatively simple to its setup and provides information about processes in the bulk of the
material without being overshadowed by contact effects. The measurement only requires a film of the active
material to enable characterization of the material’s photophysical properties.
Here we discuss problems arising in modulation spectroscopies, exemplified by using cwPA, and provide
tools for correcting them. The setup is described in section 2. Section 3discusses handling background
disturbances in the setup. In section 4we discuss problems caused by the limitations of detectors and how these
can be identified. Section 5explores the impact of phase shifts. Lastly, section 6demonstrates how all the
previously discussed effects impact two example data sets.
2. Continuous-wave photoinduced absorption
The cwPA setup consists of a pump light, which generates photoexcitations in the sample, and a probe light, for
which the change in transmission due to the photoexcitations is measured. Figure 1shows a sketch of the setup.
RECEIVED
9 May 2023
ACCEPTED FOR PUBLICATION
16 October 2023
PUBLISHED
30 October 2023
© 2023 IOP Publishing Ltd
... The physical conditions are similar to those of the ideal V OC measurement, as all carriers that are generated will also recombine. The cwPA-setup is discussed in more detail in the supplemental material and the preceding article [13]. ...
... The probe light was a continuous spectrum lamp, the absorption of which depends on the number of charge carriers in the sample. To acquire reliable results from cwPA it is crucial to recognize limitations of the setup and correct them, as discussed in the preceding article [13]. ...
... 2 , to avoid errors resulting from unknown phase shifts in the detector [13]. For PTB7: ...
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