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New CIGS Solar Cell Sets Efficiency Benchmark

The new world record holder for producing electricity from CIGS solar cells is Uppsala University, with an efficiency of 23.64%. An impartial institute conducted the measurement, and the findings were published in the journal Nature Energy.

Marika Edoff, Professor of Solar Cell Technology at Uppsala University, responsible for the study. Image Credit: Uppsala University
Marika Edoff, Professor of Solar Cell Technology at Uppsala University, responsible for the study. Image Credit: Uppsala University

The record is the outcome of a partnership between First Solar European Technology Center, formerly known as Evolar, and Uppsala University solar cell researchers

The measurements that we have made ourselves for this solar cell and other solar cells produced recently are within the margin of error for the independent measurement. That measurement will also be used for an internal calibration of our own measurement methods.

Marika Edoff, Study Lead Author and Professor, Department of Solar Cell Technology, Uppsala University

The previous global record was 23.35% (Solar Frontier, Japan) and 22.9% (ZSW, Germany). The record was first held by Uppsala University in the 1990s during the Euro-CIS research collaboration.

At one time we also held the record for a series-connected prototype. Even though it is quite a long time since we held the cell record, we have often been just behind the best results and of course there are many relevant aspects to consider, such as the potential for scaling up to a large-scale process, where we have always been at the forefront.

Marika Edoff, Study Lead Author and Professor, Department of Solar Cell Technology, Uppsala University

According to the International Energy Agency (IEA), solar power generated slightly more than 6% of the world's electricity in 2022. Solar cell technology is expanding quickly on a global scale. Currently, about 22% of sunlight can be converted into electric power by the best solar modules made of crystalline silicon, the most common material used in solar cells. Additionally, modern solar cells are long-lasting and inexpensive.

A goal in the field of solar cell research is to achieve above 30% efficiency while maintaining affordable production costs. Tandem solar cells are frequently highlighted as being more efficient, but up until now, their cost has prevented widespread deployment.

The independent German institute Fraunhofer ISE measured the global record, which stands at 23.64%. The academic study includes a detailed material and electrical examination of the solar cell along with a comparison with other research institutes' records for the same kind of solar cell.

The capacity to transfer energy to an electrical load and absorb light are two of a solar cell's most crucial characteristics. For this to work, the material needs to be able to take in the maximum amount of solar radiation without squandering it by turning it into heat inside the solar cell.

The glass sheet used in CIGS solar cells is formed of regular window glass that has been covered in many layers, each of which is responsible for a certain function. With the addition of silver and sodium, the substance that absorbs sunlight is composed of copper, indium, gallium, and selenide, which suggests the name CIGS.

This layer is positioned within the solar cell itself, in between a transparent front contact and a metallic molybdenum back contact. The CIGS layer is treated with rubidium fluoride to maximize the solar cell's electron-separation efficiency.

The composition of the CIGS layer and the ratio of the two alkali metals, sodium and rubidium, are crucial for determining the conversion efficiency, or the portion of the whole solar spectrum that is converted to electric power in the solar cell.

Measurement institutes use filtered light that closely resembles the sun in terms of both intensity and spectrum to assess the performance of solar cells. The sun cell is maintained at a specific temperature during the measurement process, and the independent institutes routinely exchange calibration solar cells with one another.

An independent measurement is necessary for a record to be officially recognized as a world record. In this instance, the measurement institute Fraunhofer ISE conducted the measurement.

Edoff said, “Our study demonstrates that CIGS thin-film technology is a competitive alternative as a stand-alone solar cell. The technology also has properties that can function in other contexts, such as the bottom cell of a tandem solar cell.”

To gain a deeper understanding of the relationship between efficiency and solar cell structure, a number of sophisticated measurement techniques have been employed. At the MAX IV facility in Lund, material from the solar cell was characterized using nano-XRF (X-Ray fluorescence spectroscopy), which allowed for a thorough compositional analysis.

High-resolution transmission electron microscopy (TEM) has been used to examine solar cell cross sections, including the interfaces between layers, composition as a function of depth, and the structure of the crystal grains. The spectrum of light released by the solar cell following laser stimulation has been examined using photoluminescence as a way to gain insight into how efficiently the solar cell manages its internal electron population.

Compared to a solar cell that glows weakly, a brighter solar cell loses less heat internally. Ultimately, the doping of the CIGS material has been analyzed using electrical testing techniques.

The fact that we now hold the world record means a lot for both Uppsala University and First Solar European Technology Center. For the CIGS technology, which is known for high reliability, a world record also means that it may offer a viable alternative for new applications in e.g. tandem solar cells. This is important for our research colleagues around the world. We hope that the analyses of the material and electric properties will provide a basis for further improvements in performance.

Marika Edoff, Study Lead Author and Professor, Department of Solar Cell Technology, Uppsala University

Journal Reference:

Keller, J., et al. (2024) High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency. Nature Energy.


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