The HZB research team led by Dr. Roland Mainz and Dr. Christian Kaufmann conducted the first real-time observation of high-efficiency chalcopyrite thin-film solar cells worldwide to study the formation and degradation of defects that affect their efficiency.
Scientists set up an anomaly laboratory in Berlin's electronic storage ring BESSYII to use multiple different kinds of measurement techniques.
The results of the study confirm that at what stage the growth can accelerate and reduce the time required for the defect to take additional time. Their research has now been published online.
At present, the efficiency of chalcopyrite thin film solar cells based on copper indium gallium selenide has reached more than 20%.
For the development of extremely thin poly layers, the co-evaporation process has now achieved the best results: In the co-evaporation process, two separate elements evaporate simultaneously, first with indium (or antimony) and selenium, followed by copper and selenium, and finally Again indium (or antimony) and selenium. In this way, a thin film of crystal is formed and only a few defects are displayed.
"It was only recently that we really understood what really happened during the co-evaporation process," said Dr. Roland Mainz, an HZB technical agency. To find the answer to this question, the team of physicists used field live measurement for three years.
Abnormal laboratory built
To measure, they set up a new laboratory that can analyze the formation of polycrystalline chalcopyrite in co-evaporation under synchrotron radiation. In addition to the element's evaporation source, the vacuum chamber also contains heating and cooling equipment to control the evaporation process.
Mainz said, "The biggest challenger is to adjust the laboratory weighing 250 kilograms with a precision of 10 microns." Due to thermal expansion during evaporation, the height of the laboratory is automatically adjusted every few seconds.
Combining X-ray diffraction and fluorescence analysis
With the completion of the laboratory, they can perform real-time analysis of X-ray diffraction and fluorescence in the co-evaporation process by observing the formation of polycrystalline thin films for the first time worldwide.
"We can also determine when these defects disappear." This occurs during the second stage of the process of evaporation of copper and selenium. The remaining copper adheres to the surface of copper selenide and can help remove defects.
"People have learned this phenomenon from previous experiments. But now, using fluorescent signals and numerical model calculations, we can show how copper selenide penetrates the copper indium selenide layer," Mainz explained.
Therefore, the difference between the copper indium selenide layer and the copper tellurium selenide layer is obvious: Since copper can penetrate the copper indium selenide layer, the same is true for the copper germanium selenide layer, and the remaining copper will adhere to surface. This may be one of the reasons why high-performance solar cells cannot be produced with pure copper selenium.
Consolidate optimization steps
"We now know that in order to further optimize the process, it is important to focus on the turning point in the copper-rich phase. At present, the process is proceeding very slowly so that the defects have enough time to disappear." Mainz explained, "Our findings show that certain stages of the process It can be accelerated, but it must slow down during the time when the defect disappears."
Mainz is already looking forward to the future EMIL project, which is located at BESSYII and is currently under construction. By then, there will be more powerful tools to assist researchers in real-time research on the complex formation process of new solar cells.
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