Another world record has been established in the field of solar efficiency. This time it involves a “two-terminal” tandem cell consisting of perovskite and CIGS.
A research team led by Dr. Steve Albrecht from Helmholtz Zentrum Berlin (HZB) set the new certified efficiency record of 24.16 percent, recently published in the renowned journal JOULE.
Tandem cells convert different areas of the light spectrum into energy by using unique semiconductors. The visible parts of the spectrum are conducted from metal halide perovskite compounds, and infrared light is converted using the CIGS semiconductors. CIGS is a combination of copper, indium, gallium, and selenium.
The cells are stacked on top of one another. The perovskite layers are very thin at 0.5 micrometers and the CIGS have a thickness of 3 to 4 micrometers, which makes the tandem solar cells very flexible at just 4.38 micrometers combined.
Dr. Christian Kaufmann from PVcomB at HZB developed the new CIGS sub-cell with his team, he explains how they improved the efficiency:
This time we connected the lower cell from CIGS directly to the upper cell from perovskite, so that the tandem cell only has two electrical contacts, so-called” terminals. The introduction of rubidium, in particular, has significantly improved the CIGS absorber material.
A former postdoc from Dr. Albrecht’s group, Dr. Marko Jo št who is currently researching at the University of Ljubljana, Slovenia, said they “used a trick” they had previously developed. They used so-called “SAM molecules” on the CIGS layer that self-organize “in close proximity to a monomolecular layer,” which helps the bonding process.
Once this was accomplished, Dr. Albrecht and his team at the HySPRINT lab at HZB placed the perovskite layer onto the rough CIGS layer.
With the perovskite/CIGS tandem solar cells being so thin it also makes them very lightweight. This could make them especially useful for space applications as Dr. Albrecht points out:
This combination is also extremely light and stable against radiation so that it could be suitable for applications in satellite technology in space.
The researchers put the new solar cells through extensive tests to determine how they would hold up to high levels of irradiation, equivalent to more than 50 years in space at the International Space Station (ISS) orbit. They retained more than 85 percent of their initial performance after being exposed to these harsh conditions.
In conclusion, the team reported:
Our work identifies perovskite/CIGS tandem solar cells that can be processed on flexible foils, as a cheap, readily stowable and ultra-lightweight space PV technology with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based triple- and quadruple-junction absorbers.
Dr. Albrecht and HZB also hold the world record for perovskite/silicon (PS/Si) tandem solar cells with a 29.15% efficiency that would be suitable for large surface areas.
The National Renewable Energy Lab (NREL) in the U.S. has created a new branch on the famous NREL chart and “2-terminal” tandem cells, made up of CIGS and perovskite, now have a separate category. NREL recently broke a record for a six-junction solar cell.
This latest record will now allow researchers to further investigate these tandem solar cells for “terrestrial, high-altitude, and space applications.”
