Empa, the Swiss materials research institute, has described a study in which researchers observed clear light emission at room temperature from single crystals of BaZrS₃, a candidate material for next-generation solar cells. Made of barium, zirconium, and sulfur, the material is expected to offer high stability and excellent light absorption. In earlier experiments, however, its emission was weak and the measurement results varied widely. By using high-quality single crystals, the researchers were able to examine its emission properties in more detail. The photoluminescence quantum yield was low, however, at about 0.005%, and the emission decay time was short, at about 1 nanosecond. The results show the material's potential, and they also point to what needs to be improved when making thin films suitable for solar cells.
Emission properties revealed by high-quality single crystals
The researchers involved were Rasmus Svejstrup Nielsen and Mirjana Dimitrievska of Empa, and Susan Schorr of the Helmholtz-Zentrum Berlin (HZB) in Germany, among others. The crystals used in the experiments were produced at HZB, and the results were published online on July 17, 2025, in the peer-reviewed journal Advanced Optical Materials. Empa's announcement is based on this paper.
BaZrS₃ is a type of material known as a chalcogenide perovskite. "Perovskite" does not refer to a specific chemical substance but to a type of crystal structure. Its composition differs from that of the lead halide perovskites that are widely studied for solar cells, and BaZrS₃ contains no lead. It is expected to be a stable material that absorbs light efficiently, but being able to absorb light does not guarantee that the generated charges can be extracted efficiently.
The research team therefore focused on photoluminescence (PL), the emission that occurs when a material is illuminated. When a semiconductor absorbs light, it generates charge-carrying particles called carriers: electrons and holes. When electrons and holes recombine, the energy may be released as light or lost without producing light. The latter is called non-radiative recombination, and it is one factor that lowers solar cell performance. By examining the intensity and wavelength distribution of the emission, researchers can probe what is happening inside the material.
In earlier studies of BaZrS₃, emission spectra were reported with broadened peaks or overlapping components, and in some cases no emission could be detected at room temperature. In the high-quality single crystals used here, however, a clear, symmetric single peak was observed. Based on results such as measurements at varying excitation intensities, the team interprets this emission as arising mainly from band-to-band transitions in the semiconductor. The significance is that, unlike the complex emission caused by defects, the material's intrinsic emission properties can now be examined in detail.
According to the published paper, the photoluminescence quantum yield under excitation at a wavelength of 532 nm was about 0.005%. This indicator shows how many of the absorbed photons are re-emitted as light, and it was estimated by comparison with a reference sample of known quantum yield.
However, photoluminescence quantum yield is a different metric from the power conversion efficiency of a solar cell. Nor does the "clear emission" reported here mean that efficiency comparable to commercial light-emitting materials was achieved. What it means is that emission, which was weak and indistinct in conventional BaZrS₃ samples, could be clearly observed thanks to high-quality crystals. The result shows that, with appropriate crystal growth techniques, the material's intrinsic optical properties can be investigated more accurately.
Clear emission, but short carrier lifetime
The emission decay time obtained from time-resolved photoluminescence measurements was 1.0 ± 0.2 nanoseconds. One nanosecond is one billionth of a second, and this serves as a guide to how quickly light-generated carriers recombine. It has nothing to do with the product lifetime of a solar cell, meaning how many years it can be used. The measurements used pulsed light at a wavelength of 639 nm, and the ±0.2 ns value corresponds to the time resolution of the measuring instrument.
The crystals used here had not undergone passivation, a treatment that suppresses the effects of surface defects. The research team evaluated it as promising that clear emission could be observed even under these conditions, but pointed out that the short carrier lifetime remains a challenge. Obtaining clear emission and sustaining carriers for a long time are separate issues.
To investigate the cause of the short carrier lifetime, the team performed Raman spectroscopy under multiple wavelengths and polarization conditions and compared the results with first-principles calculations. Raman spectroscopy uses the energy change that occurs when light hits a material to probe the vibrations of the atoms that make up the crystal. The quantized collective vibrations of atoms in a crystal are called phonons, and interactions between electrons and phonons are also involved in processes by which carrier energy is lost in forms other than light.
Based on changes in the Raman signal under different polarization conditions and the vibrational properties obtained from calculations, the team believes that interactions between electrons and lattice vibrations are related to the short carrier lifetime. However, the emission decay measurements alone cannot identify how much loss is caused by crystal defects and how much by lattice vibrations. The short decay time actually observed must be distinguished from the team's interpretation of its cause.
The paper also notes a caution when comparing emission properties across different studies. Depending on the excitation laser's wavelength and intensity and the integration time during measurement, the effects of local heating and surface conditions can change, and the emission peak position may shift. Even for materials with the same chemical composition, it is difficult to compare emission intensity or performance unless measurement conditions are matched.
Controlling sulfur defects is key to solar cell applications
The single crystals were produced by chemical vapor transport, a method that grows crystals while transporting raw materials at high temperature. By keeping the hot side at 1150°C and the cold side at 1050°C and maintaining this temperature gradient for 240 hours, the researchers grew crystals 4 mm long. This is a sample preparation method for studying the material's properties in detail, and it does not demonstrate that large-area thin films can be mass-produced.
In Empa's official announcement, Dimitrievska said of the results:
"The single-crystal results agree very well with theory."
She also explained that when BaZrS₃ is made into thin films, defects introduced during synthesis and processing greatly suppress emission. The paper likewise suggests that the weak emission and complex spectra reported in past studies may stem from imperfections introduced during crystal fabrication rather than from unavoidable properties of the material itself. Using the high-quality single crystals obtained here as a benchmark is expected to make it easier to investigate at which stage of thin-film fabrication the properties are degraded.
Simply "reducing defects" is not enough to determine suitable manufacturing conditions, however. A first-principles calculation study published in 2024 in PRX Energy by Zhenkun Yuan and colleagues showed that sulfur vacancies, where sulfur atoms are missing from the crystal, and interstitial sulfur, where extra sulfur atoms sit in gaps in the crystal lattice, affect electronic properties differently.
Sulfur vacancies act as shallow donors that supply electrons, and are related to the material's n-type character, in which electrons are the main charge carriers, and to the difficulty of making p-type semiconductors, in which holes are the main charge carriers. Interstitial sulfur, on the other hand, can be a major center of non-radiative recombination, where carriers recombine without emitting light. Under sulfur-rich conditions, the formation of sulfur vacancies may be suppressed, but interstitial sulfur may also form more readily. In other words, supplying more sulfur does not necessarily improve performance.
These are, however, results from a separate computational study, and they do not directly identify the types of defects present in the single crystals measured here. Even so, they suggest that optimizing thin-film manufacturing conditions requires not just reducing the number of defects but determining which types of defects should be suppressed most. Nielsen and colleagues also list adjusting the material's composition and treatments to reduce the effects of defects as future research topics.
The focus going forward is whether the clear emission seen in high-quality single crystals can be reproduced in thin films, and whether charges can be extracted efficiently as a solar cell while keeping carrier losses low. If emission quantum yield and decay time can be measured under unified conditions and linked to actual device performance, the requirements for turning BaZrS₃ into a practical, highly stable optoelectronic material will become more concrete.