PHOTON SCIENCE

Perovskites are among the greatest hopes in the solar industry. These materials could replace silicon as the basic semiconducting component of solar cells, as they are just as readily available and much easier to process. Alternatively, the two could be used in a paired set as a so-called tandem cell: The perovskite solar cell at the top converts visible light into electricity, while the silicon cell at the bottom converts infrared light. This promises the highest efficiency. However, it is difficult to produce perovskites on an industrial scale. A new measuring method developed at DESY could help to remedy this shortcoming, and it goes by the name of Axolotl.

Researchers led by DESY physicist Michael Stuckelberger have developed this method: Axolotl has nothing to do with the amusing looking tailed amphibian of the same name from Mexico. It is an acronym and stands for "Analyzer of X-ray excited Optical Luminescence Offering Temporal and spectraL resolution." This is a box around one metre wide and weighing 50 kilograms, in which optical lenses, mirrors, beam splitters, sensors, cameras and a spectrograph are installed. It is optimised for the P06 beamline of the PETRA III X-ray light source at DESY but can be used anywhere in the world. In principle, Axolotl is a special form of the already established XEOL technology (X-ray Excited Optical Luminescence) which uses X-rays to illuminate materials and in turn stimulates them to glow in the optical spectrum. The appearance of this light provides information about the properties and quality of the material being examined.

In this way, perovskites can also be analysed to find out why they age more quickly than silicon, how their efficiency can be improved and how more homogeneous layers can be produced. Perovskites are minerals with a specific crystal structure. The halogens iodine and bromine are used for solar cells, but they also contain other elements in a complex combination. This combination makes perovskites an excellent absorber of solar radiation: The photons excite electrons in the atoms and thus ensure a flow of electricity. The advantage of perovskites over silicon lies in their more efficient absorption of sunlight; because they absorb sunlight around 100 times more strongly, the cells can be made correspondingly thinner.

The problem with X-ray studies of perovskites, however, is that they are much less tolerant of X-ray light than optical light. "You can virtually watch how the material ages under the ionising X-rays and changes so quickly that we can't examine it fast enough with conventional XEOL technologies," says Stuckelberger. And this is precisely what Axolotl aims to improve by optimising its setup for maximum detection efficiency and speed. This means that the setup requires much weaker X-rays and allows measurements to be taken more quickly than the material ages.

This was made possible through extreme precision: Components such as mirrors and lenses are positioned with an accuracy of less than one micrometer. "However, the whole thing shifts again just when we lift the black, fairly light lid of the light-tight box to look inside," reports Jackson Barp, who helped develop Axolotl as a doctoral student at DESY. "That's why we fitted all the critical components with motorised adjusting screws so that they can be correctly adjusted from the outside." The resulting accuracy is impressive: "The setup enables XEOL spectroscopy with a spatial resolution of less than 100 nanometres, a temporal resolution of less than 100 picoseconds and a spectral resolution of less than one nanometre in the wavelength of the luminescent light," reports Alf Mews from the Institute of Physical Chemistry at Universität Hamburg, whose team was involved in the development of Axolotl.

The effort is well worth it: Compared to optical measurements with laser light, known as "photoluminescence," X-ray light is able to penetrate straight into the material and provides correspondingly higher-resolution information about the structure, even below the surface. "More importantly, however, we can now measure several aspects simultaneously for the first time," says Stuckelberger. Laser-excited photoluminescence also provides the light spectrum and the life time of charge carriers in a solar cell. The life time is a direct measurement of the time span between the arrival of an X-ray or light pulse and the emission of the photon excited by it. This takes place within nanoseconds, and the longer the material remains in an excited state, the higher its quality.

Ideally, Axolotl should be combined with detectors to measure X-ray fluorescence, which also provide data on the distribution of chemical elements in the cells. It has long been known that the surface of perovskites exhibits a degree of folding, which forms small “mountains” and “valleys” and results in an inhomogeneous power density and is detrimental to overall efficiency. It was also shown that durability increases when cesium is added to the element mixture in addition to the anionic (i.e. negatively charged) halogens and organic, positively-charged cations. Stuckelberger and his team investigated perovskites with methylammonium, formamidinium and cesium as cations, increasing the cesium content in three stages.

They found that the cesium improves the stability of the perovskites, but at the same time reduces their efficiency. The latter is most likely related to an observed increase in folding, as bromine and iodine are more unevenly distributed with an increase in cesium content. "Such correlations can only be established with Axolotl," says Stuckelberger.

In any case, it is clear that the manufacturers of perovskite solar cells can gain a quite literally deeper understanding of their materials with Axolotl. " For perovskite-based solar cells, we are entering a period where a much deeper understanding of the material properties is essential to address the remaining issues," says Erkan Aydin, a researcher at the University of Munich who was involved in the study. “The new instrument broadens our knowledge of perovskite-based single junction and tandem solar cells, with applications for both Earth and space.”
Another partner in the pilot study with Axolotl was the Saudi Arabian King Abdullah University of Science and Technology (KAUST), which had already previously worked with Stuckelberger's team to investigate and optimise perovskites at DESY. KAUST is a world leader in the development of perovskite solar cells and last year set a world record for the efficiency of a tandem solar cell of 33.7%. "However, the reproducibility of the processing and the degradation of the devices are still a challenge," says Stefaan De Wolf, Head of the KAUST Solar Center. "Correlative, multimodal characterisation with Axolotl represents an extremely valuable asset to accelerate the maturity of perovskite solar technologies." And not only that: "In addition to solar cells, Axolotl can be used to characterise the entire range of semiconductors," adds Tobias Kipp from Universität Hamburg. "To be precise, we use it to investigate the optical properties of colloidal nanostructures with unprecedented spatial resolution."

Axolotl at the P06 beamline of PETRA III is just the beginning, Michael Stuckelberger stresses. With the planned expansion of the X-ray light source to PETRA IV, Axolotl will be able to carry out even more sensitive and precise measurements. "This will make it much easier for us to systematically improve perovskite solar cells in close collaboration with leading manufacturers."

(from DESY News)


Reference:
Novel Detection Scheme for Temporal and Spectral X-Ray Optical Analysis: Study of Triple-Cation Perovskites. Christina Ossig, Christian Strelow, Jan Flügge, Svenja Patjens, Jan Garrevoet, Kathryn Spiers, Jackson L. Barp, Jr., Johannes Hagemann, Frank Seiboth, Michele De Bastiani, Erkan Aydin, Furkan H. Isikgor, Stefaan De Wolf, Gerald Falkenberg, Alf Mews, Christian G. Schroer, Tobias Kipp, and Michael E. Stuckelberger. PRX Energy 3, 023011 (2024), DOI: 10.1103/PRXEnergy.3.023011