Researchers at Germany's Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have developed new imaging methods to measure losses in individual sub-cells of perovskite-silicon tandem and perovskite-perovskite-silicon triple junction devices.
The research team noted the growing need to be able to measure the electrical and thermal effects of new functional multi-junction solar cell materials and deposition methods at the individual sub-cell level without necessarily having direct electrical access.
It was especially in demand for emerging perovskite-silicon dual junction and perovskite-perovskite-silicon triple junction solar cells.
“We found existing characterization methods to quantify electric losses not satisfactory since they either compromise on measurement speed, are optimized for single-junction solar cells only, or cannot cope with the metastability of perovskites,” Oliver Fischer, corresponding author of two recent papers on the subject, told pv magazine.
In “Revealing charge carrier transport and selectivity losses in perovskite silicon tandem solar cells,” published in Matter, an international research group led by a Fraunhofer ISE team reported the Suns open-circuit voltage (Suns-Voc) and intensity-dependent photoluminescence imaging (Suns-PLI) methods, which were specifically adapted to perovskite-silicon tandem solar cells.
A more recent paper, “Imaging-based loss-analysis for perovskite/perovskite/silicon triple-junction solar cells,” published in Solar Energy Materials and Solar Cells, detailed the measurement methods for individual sub-cells within triple-junction solar cells.
“We investigated electrical losses of individual sub-cells in several ways. Illuminated lock-in thermography (ILIT) allowed us to attribute the origin of shunts to individual sub-cells. Electroluminescence imaging (EL), which is sub-cell selective by using appropriate optical filters in front of the camera, tells us a lot about charge carrier injection and extraction. This allows us to examine the quality of electron and hole transport layers,” explained Fischer.
In the earlier study, the Suns-Voc and the Suns-PL imaging methods adapted to perovskite silicon tandem solar cells were detailed. The implied IV curves measurement based on the Suns-PLI method pinpoints the local origin of electric losses more precisely than other spatially averaged luminescence-based measurements, according to the paper. The implied fill factor for each sub-cell and for the tandem itself could be determined as either an average, or spatially resolved, by taking advantage of the imaging-based method.
In combination with the sub-cell resolved Suns-Voc measurements, the two methods are suitable to determine selectivity losses as well as resistive losses, according to the paper.
“Both losses will be important to track also in a production line. Hence, we see the potential that this method will be applied for quality assurance in inline production as well as in R&D laboratories,” said Fischer.
For the triple-junction solar cells analysis, a combination of luminescence imaging (EL/PL) and lock-in thermography was used to determine the “lateral homogeneity of the various layers, the internal voltage of the sub-cells, and the location of shunts,” according to the related paper.
“Most important was the capability to detect shunts, evaluate the homogeneity of the deposited layers, and quantify the implied open-circuit voltage of each sub-cell,” said Fischer.
The EL imaging used optical filters matching the sub-cell transmissive range to establish charge carrier injection and extraction data, enabling the examination of the quality of electron and hole transport layers.
“Quantitative photoluminescence imaging in turn allows to access the implied open-circuit voltage of each sub-cell,” said Fischer.
Illuminated lock-in thermography (ILIT) was used to attribute the origin of shunts to individual sub-cells. The ILIT and dark illuminated lock-in thermography (DLIT) typically detect far infrared (IR) radiation emitted from solar cell sites with increased non-radiative recombination. When there are three instead of two sub-cells, a third light source is added to the ILIT measurement system, according to the research.
To demonstrate the effectiveness of the methods, the team measured the effect of a passivation agent on iVoc in the wide bandgap (WBG) perovskite top cell and subsequent light soaking effects on triple-junction solar cells with an active area of 1 cm2. Based on the iVoc for the individual sub-cells, small variations in the bottom cell and middle cell were determined.
The team noted that the WBG perovskite top cell showed an improvement of 42 mV in iVoc upon passivation. Since it was significantly more than the total gain in iVoc, the remaining Voc improvement could be attributed to the reduction of selectivity losses upon passivation of the top cell. Further analysis could provide insights on issues with the passivation process.
In addition, the temporal evolution of Voc as well as of iVoc of the sub-cells was investigated during a light-soaking procedure, which provided insights into temporal changes of the selectivity losses, according to the researchers.
These new imaging methods are applicable to other tandem and triple junction cell technologies with no restrictions, according to Fischer, including III-V compound solar cells for space applications.
The lock-in thermography measurements were made using equipment from Germany-based Ircam GmbH. The luminescence imaging measurements were made with Tandem Modulum, supplied by Germany-based Intego GmbH, with excitation light sources adapted to match peak wavelengths and optical filters chosen to match the transmissive range. Fischer noted that the instrument was originally developed at Fraunhofer ISE.
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