Researchers from the University of Stuttgart, Forschungszentrum Jülich, and German company Solarzentrum Stuttgart have modified a low-cost consumer digital camera to detect infrared radiation emitted by electrically biased silicon solar modules and measure their external quantum efficiency (EQE).
In PV devices, EQE describes how efficiently electrons passing through a solar cell are converted into emitted photons during electroluminescence. The parameter can be used to assess the radiative quality of a solar cell and determine its open-circuit voltage.
“Our approach shows that even a relatively inexpensive consumer camera can provide quantitative results when its physical response is properly modeled and calibrated,” lead researcher Jürgen Werner said. “An electroluminescence image contains much more quantitative information than simply showing bright and dark regions. With a suitable physical camera model and calibration, it can provide absolute luminescent quantum efficiency and, therefore, information about the local quality of a solar cell or module.”
The scientists explained that conventional digital cameras employ silicon-based complementary metal-oxide-semiconductor (CMOS) sensors that can detect both visible and near-infrared radiation, including part of the spectral range associated with electroluminescence (EL) from silicon PV devices. Consumer cameras, however, typically incorporate an infrared-cut filter that suppresses near-infrared radiation. This filter strongly attenuates the weak EL emission from silicon solar cells, limiting the use of unmodified consumer cameras for EL imaging and quantitative PV characterization.
The researchers used a Canon EOS 4000D camera and removed its infrared-blocking filter to increase its sensitivity to infrared radiation. They also used a Heliopan ES RG850 long-pass filter to block shorter-wavelength visible background radiation from reaching the sensor. The camera, originally designed for visible-light imaging between approximately 400 nm and 800 nm, uses a silicon CMOS detector. Its Bayer filters are sufficiently transparent around 1,120 nm to detect the luminescence signal emitted by silicon solar cells.
In addition to modifying the camera hardware, the researchers developed a calibration model that relates recorded image brightness to the absolute luminescence emission of the PV device. The model accounts for both the linear and nonlinear response regimes of the camera, enabling the researchers to convert measured brightness into absolute electroluminescent EQE.
The team tested the camera in EL measurements performed in darkness to minimize background radiation. The calibration established a conversion factor between the measured brightness response and absolute external EQE and required a reference cell or module with known electroluminescent EQE and open-circuit voltage.
According to the research team, the experiments revealed a clear relationship between the measured luminescence response, EQE, and open-circuit voltage. At room temperature, a tenfold increase in the luminescence-related response corresponded to an increase of approximately 60 mV in open-circuit voltage. For the higher-efficiency modules, results obtained from the camera’s linear and saturation regimes were reportedly in good agreement. Deviations observed for older, lower-efficiency modules were attributed primarily to differences in light trapping and photon escape probability.
“An electroluminescence image contains much more quantitative information than simply showing bright and dark regions,” said Werner. “With a suitable physical camera model and calibration, it can provide absolute luminescent quantum efficiency and, therefore, information about the local quality of a solar cell or module.”
The researchers presented the camera and calibration method in “New camera model for absolute quantum efficiency measurements from electroluminescence of solar cells or modules,” published in the Journal of Applied Physics.
“Our next step is to use the calibrated camera to determine quantum efficiencies and open-circuit voltages of further, previously uncharacterized solar cells and modules,” Werner added. “The same model should also be applicable to photoluminescence measurements and potentially to measurements performed in daylight.”
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