Chlorinated Cation Unlocks Durable Tin Perovskite Solar Cells in Open Air

Tin halide perovskites have long been the tantalizing alternative to lead-based perovskites in next-generation solar cells. They offer a narrower bandgap that is ideal for harvesting more of the solar spectrum, and they replace toxic lead with an environmentally friendlier metal. Yet for all their promise, tin-based perovskites have been crippled by a fatal flaw: they degrade rapidly when exposed to air. Oxygen and moisture attack the tin(II) cation, oxidizing it to tin(IV) and destroying the crystal structure that makes the material such an effective light absorber. Now, a team of researchers led by groups at the University of Wisconsin–Madison, the National Laboratory of the Rockies, and the University of Toledo reports a deceptively simple molecular solution that could finally change the calculus for lead-free photovoltaics.

Writing in Nature Materials, the team describes how a single chemical modification—a chlorine atom placed at the para position of a phenethylammonium cation—transforms the stability of two-dimensional tin iodide perovskites. The resulting material, based on the 4-chloro-phenethylammonium (4ClPEA) cation, forms ultrastable two-dimensional and quasi-2D tin halide perovskites that retain bright photoluminescence for several months when simply left out in ambient air. When incorporated into 2D/3D perovskite solar cells, the chlorinated cation delivers devices with a power conversion efficiency of 16.2 percent, alongside operational stability exceeding 1,000 hours at 55 degrees Celsius in air—a combination of efficiency and durability that few tin-based devices have ever approached.

The key insight behind the work lies in how the organic spacer cations that separate the inorganic tin iodide sheets influence what happens between those sheets. In the layered Ruddlesden–Popper architecture of two-dimensional perovskites, bulky organic cations cap the corners of the octahedral framework and stack on top of one another between the inorganic slabs. The researchers systematically compared a family of halogen-substituted phenethylammonium cations—4XPEA, where X is hydrogen, fluorine, chlorine, or bromine—in the model compound (4XPEA)2SnI4. Single-crystal X-ray diffraction revealed that the chlorinated variant packs the organic layers more tightly than any of its siblings, driven by stronger π-stacking interactions between the aromatic rings.

That tighter packing is not merely a crystallographic curiosity. Using computational modeling and diffusion analysis, the team showed that the densely interlocked organic layers in (4ClPEA)2SnI4 substantially impede the diffusion of oxygen and water molecules into the vulnerable inorganic sheets. In effect, the chlorinated aromatic rings act as a molecular raincoat, sealing the gaps through which air would otherwise infiltrate and oxidize the tin(II) centers. The fluorinated and brominated analogues, by contrast, leave the interlayer region more permeable, and their films degrade noticeably faster under identical conditions. The result establishes a direct structure–property relationship: the tighter the interlayer packing, the greater the resistance to oxidative degradation.

Stability alone, however, would be of limited value if the material could not be turned into a high-quality solar absorber. The second half of the study addresses the notoriously uncontrolled crystallization of tin halide perovskites. Tin-based films tend to crystallize too quickly and too chaotically during deposition, producing pinholes, poor orientation, and defective grain boundaries that accelerate degradation and squander charge carriers. Here again, the 4ClPEA cation proved decisive. When added to three-dimensional tin iodide perovskite precursor solutions, it steers film growth toward markedly improved crystallinity and preferred crystallographic orientation, yielding dense, well-ordered 2D/3D heterostructured films.

Grazing-incidence wide-angle X-ray scattering measurements confirmed the enhanced orientation and phase purity of the treated films, while time-of-flight secondary ion mass spectrometry mapped the distribution of the organic cation through the film thickness. Density functional theory calculations provided a mechanistic underpinning, indicating favorable interactions between the chlorinated aromatic ring and the iodide species at the perovskite surface—an anion–π interaction that helps template orderly growth. The combination of a well-oriented 3D absorber with a protective 2D capping layer is precisely the architecture that the field has been pursuing for lead-based perovskites, and the study demonstrates that it can be realized in the tin system with the right molecular tool.

The device results are striking in context. Lead-free tin perovskite solar cells have historically lagged far behind their lead-containing counterparts, with certified efficiencies only recently crossing the 14 percent threshold and most devices failing within hours or days of operation. The 16.2 percent efficiency achieved with the 4ClPEA-based 2D/3D films places this work among the best-performing tin perovskite photovoltaics reported to date. More importantly, the devices did not merely perform well on the bench immediately after fabrication; they endured more than 1,000 hours of continuous operation at an elevated temperature of 55 degrees Celsius in ambient air, conditions that combine thermal stress, oxygen exposure, and moisture—the three horsemen of perovskite apocalypse.

The broader significance of the work extends beyond a single efficiency number. Tin halide perovskites are considered essential building blocks for the next generation of all-perovskite tandem solar cells, where a narrow-bandgap tin or tin–lead bottom cell would be paired with a wide-bandgap lead top cell to push efficiencies beyond what silicon can deliver. Lead toxicity, however, remains a persistent regulatory and public-acceptance obstacle, and studies quantifying the biological impact of lead leakage from perovskite modules have underscored the risk of assuming any safe threshold. A durable, efficient, entirely lead-free absorber would remove that obstacle while simplifying encapsulation requirements, and the chlorinated-cation strategy offers a generalizable design principle: choose spacer cations whose substituents promote tight interlayer packing and strong surface interactions.

The researchers also emphasize that the approach is rooted in fundamental chemistry rather than device engineering tricks. By comparing the full series of halogenated phenethylammonium cations, they isolated the effect of the substituent on π-stacking, interlayer spacing, and barrier properties, showing that chlorine occupies a sweet spot among hydrogen, fluorine, and bromine. The team has filed a patent application on the technology, and the design motif—electron-withdrawing substituents on aromatic spacer cations to tighten packing and passivate surfaces—could plausibly be extended to germanium-based perovskites, low-dimensional emitters, and other tin-containing optoelectronic devices where air sensitivity has been the limiting factor.

Challenges remain before tin perovskite solar cells can compete commercially. The efficiency gap with lead-based devices, which now exceed 27 percent, is still substantial, and scaling the chemistry from laboratory cells to modules will require further optimization of deposition and encapsulation. But the demonstration that a single atom of chlorine, placed with chemical intent on an organic cation, can buy months of ambient stability and a thousand hours of hot operation represents a genuine inflection point. For a field that has watched its most promising lead-free materials crumble within days, the message is clear: the path to practical tin photovoltaics may run through the careful engineering of the molecules that sit between the crystals, not just the crystals themselves.

Subject of Research: Stable 2D/3D tin halide perovskite solar cells enabled by a chlorinated organic spacer cation

Article Title: A chlorinated organic cation enables stable 2D/3D tin iodide perovskite photovoltaics

Article References: Triggs, C. T., Chen, L., Xie, J., Weadock, N. J., Zhang, Z., Ye, J. Y., Kerner, R. A., Taddei, M., Yang, F., Addison, B., Wang, X., Liu, T., Harvey, S. P., Toney, M. F., Beard, M. C., Yan, Y., Zhu, K., & Jin, S. (2026). A chlorinated organic cation enables stable 2D/3D tin iodide perovskite photovoltaics. Nature Materials. https://doi.org/10.1038/s41563-026-02726-z

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02726-z

Keywords: tin halide perovskite, perovskite solar cells, lead-free photovoltaics, 4-chloro-phenethylammonium, 2D/3D heterostructure, air stability, π-stacking, crystallization control, power conversion efficiency, Ruddlesden–Popper perovskite, tin oxidation, Nature Materials