The element that appeared out of nowhere. Caesium unlocked lead-free perovskite

The element that was supposed to be the answer

For a decade, laboratories around the world repeated the same formula: “Just replace lead with tin.” Tin sits in the same group of the periodic table, forms identical crystal structures, and its toxicity is negligible compared to lead. Perovskite — the best photovoltaic material ever discovered — was finally supposed to shed the one flaw blocking its path to market: poisonous lead.

Everyone believed it.

They were wrong.

When Lianzhou Wang’s team at the University of Queensland finally peered inside a thin perovskite film — a layer just a few hundred nanometres thick — the picture was unambiguous. Two kinds of crystals that were supposed to be evenly mixed were separating: some sank to the bottom, others grew on top, riddled with holes. The problem was not that tin oxidizes. It lay in the pace at which crystals are born. And the solution came from an element nobody had considered: caesium.

The ideal material you are not allowed to use

Perovskites are a family of compounds with a crystal structure that absorbs light with near-total efficiency. In 2009 the first perovskite solar cell reached 3.8 percent efficiency. Fifteen years later the best lead perovskites exceed 27 percent — as much as silicon cells took half a century to achieve, but at a fraction of the cost. Perovskite can be printed like a newspaper, deposited on flexible foils, layered onto silicon in tandem cells that squeeze more current out of the same light than silicon alone can.

Except that inside every such cell sits lead. An element whose content in electronics the European Union caps at 0.1 percent of mass under the RoHS directive. A perovskite panel cannot clear that bar — and that is the wall against which all commercialization crashes. The British firm Oxford PV is already assembling perovskite–silicon tandem cells at a factory near Berlin, but it bypasses the consumer market until the lead question is solved.

“Replace lead with tin” sounds like a no-brainer. Tin is lead’s closest relative: the same valence electron configuration, the same crystal structure, a bandgap of 1.3–1.4 electronvolts — the energy range perfectly matched to the solar spectrum. On top of that, tin is not a heavy metal in the regulatory sense. Team after team tried. And team after team got a cell that died.

The false trail everyone followed

The first tin cells from the middle of the previous decade broke down within minutes. Tin in the Sn²⁺ form reacts with oxygen so eagerly that a cell could degrade before the researcher managed to measure its efficiency. Over time, remedies for the disease appeared: tin(II) fluoride, antioxidants, a nitrogen atmosphere in the glovebox. Cells began to last days, then weeks, finally months. Efficiency climbed: 6, then 10, 14 percent. In 2024 the best tin cells passed 15 percent.

The trend looked promising. Each successive year seemed to confirm: tin is catching up with lead.

But every additive acted as a bandage, not a cure. Tin fluoride patched up the effects of oxidation, antioxidants slowed degradation, but none of them raised the efficiency itself. The cells were becoming more durable, but not better. And here lay the false trail: for years the whole field treated the symptom — oxidation — believing that once it was mastered, tin would finally catch lead. Meanwhile the real disease sat deeper, in the very act of a crystal’s birth.

And the gap was not closing. Lead: 27 percent and rising. Tin: 15 and stuck. Something structural was off, but nobody could point to what.

The wall nobody knew about

The key lay in two electrons. Lead in perovskite appears as a Pb²⁺ ion with a chemically inert 6s electron pair — it sits quietly and causes no trouble. Tin as Sn²⁺ has two 5s electrons that are active, restless, and eager to escape. That explained the oxidation. But it did not explain why, even in an oxygen-free atmosphere, in a hermetically sealed box, the tin cell was still worse than it should have been.

Here the second, subtler obstacle appears. To improve the quality of the tin film, teams reached for a trick borrowed from lead perovskites: they added a large organic cation, phenethylammonium (PEA⁺), which forces the formation of two-dimensional “scaffolds” — matrices on which the proper three-dimensional FASnI₃ crystal grows. With lead, this trick worked superbly: it eliminated defects and smoothed the film.

With tin — it did not.

The reason was physical, concrete, and measurable. FA⁺ cations (small, about 253 picometres) form dense clusters in solution — seeds of 3D crystals. PEA⁺ cations (large, about 552 picometres) form loose clusters that must first stick together before reaching critical size. The effect: 2D layers crystallize more slowly than 3D layers. As the film dries from the top, the heavier, slower 2D phases sink to the bottom, while a defect-ridden 3D crystal grows on top. A sandwich of two phases that should have been mixed.

Wang’s team saw this directly, tracking the distribution of elements through the film’s depth: 2D phases gather at the bottom electrode, while charge traps appear in the top layer. Efficiency stalls, stability crumbles — no matter how carefully the tin is shielded from oxygen.

Caesium — the element that appeared out of nowhere

The solution sounds absurdly simple: add one more element to equalize the crystallization rate. That element turned out to be caesium — an alkali metal almost nobody thought about in the context of solar cells.

Not just any element. Every colloidal aggregate in solution is surrounded by a cloud of ions — like a ship with an escort. Caesium as a Cs⁺ cation is tiny: 188 picometres. When Wang and his PhD student, Dongxu He, introduced it into the solution, caesium slipped into that ionic escort around the PEA⁺ clusters and partially replaced the large organic cations. The 2D clusters shrank, their aggregation barrier dropped — and suddenly they began to crystallize at the same rate as the 3D phases.

Five mole percent of caesium was enough for the PEA₂SnI₄ clusters to grow by 75 percent and the FASnI₃ clusters by 40 percent — equalizing both processes. Instead of a sandwich with phase segregation, a homogeneous, evenly mixed 2D/3D structure emerged. The number of charge traps fell, and the carrier lifetime jumped from 6.15 to 15.13 nanoseconds — more than doubled.

The effect is measurable. A tin cell with caesium reached 17.13 percent efficiency, and the certified, independently confirmed value is 16.65 percent — a record among lead-free perovskites, measured at the Shanghai Institute of Microsystem and Information Technology. More importantly, the cell retained 93.3 percent of its initial efficiency after 6,000 hours in the dark and 92.0 percent after 1,500 hours of continuous illumination — without hermetic encapsulation.

The race you cannot see

The Brisbane work, published in Nature Nanotechnology, is not the end of the road. Seventeen percent — still a chasm separating this technology from lead’s 27 percent. But it is the first time a tin perovskite has stopped being “the inferior substitute” and become a technology with its own predictable development path.

The stakes are high. Photovoltaics today rests almost entirely on silicon, but lead-free perovskites target areas silicon cannot reach: lightweight rooftop modules that cannot carry heavy panels, flexible foils on facades, tandem layers boosting the efficiency of existing cells. Without lead — with full regulatory compliance. The perovskite market is estimated at tens of billions of dollars over the coming decade, and the first tandem-cell production lines are already starting up.

Other teams are in the lead-free perovskite race too: groups in Japan and China explore bismuth and antimony variants, laboratories in the US and Switzerland polish tin perovskites themselves. The Queensland team — working with the Australian National University and the University of New South Wales — holds the edge today: it was the first to solve a nucleation problem whose existence long went unsuspected. And the first to show that a tin cell can work for thousands of hours without encapsulation.

Where is this heading? Two paths are clearly visible. The first is pushing pure tin toward 20 percent and beyond — the threshold above which lead-free cells become a genuine commercial alternative. The second is mixing tin with lead in proportions that minimize toxicity while maximizing efficiency — an intermediate solution that part of the industry treats as the fastest route to cheaper panels. Both paths rest on the same Brisbane discovery: that a cell’s quality is decided not only by what is in it, but also by the order and pace in which that something crystallizes.

Poland watches from the sidelines. It doesn’t have to

Poland is today one of the fastest-growing photovoltaic markets in Europe — over 20 gigawatts of installed capacity, panels on every other roof. In the history of perovskites we have our own episode: Saule Technologies of Olga Malinkiewicz, the Wrocław company that was among the first in the world to try to commercialize flexible perovskite cells. Except that Saule relies on lead perovskites — and, together with the whole industry, faces the same RoHS wall.

The lead-free tin path is an opportunity for the Polish sector that need not be built from scratch. The competencies are there: perovskite teams at the Warsaw University of Technology, AGH in Kraków and Wrocław University of Science and Technology, a base of module manufacturers, and companies such as ML System from Rzeszów, which produces building-integrated photovoltaics and is listed on the Warsaw Stock Exchange. What is missing is the cell linking efficiency with the absence of lead.

If a Polish research group took up lead-free crystallization — for instance in partnership with UQ, under National Science Centre grants or Horizon Europe programs — it would land at the point where, in three to five years, the decision will be made about who supplies the panels of the future. The infrastructure already exists: Poland’s PV industry is among the fastest-growing in the Union, we have our own module makers, and the engineering workforce from technical universities will move from silicon to perovskite without trouble. What is missing is only the decision to bet on the lead-free path before others do.

Because today one thing is already clear: lead will disappear from perovskites. The question is not “whether”, but “who will be first to profit from its successor”.

Sources

He D., Chen P., Steele J. A., Wang Z., Xu H., Zhang M., Ding S., Zhang C., Lin T., Kremer F., Xu H., Hao M., Wang L., Homogeneous 2D/3D heterostructured tin halide perovskite photovoltaics, Nature Nanotechnology 20, 779–786 (2025).DOI: 10.1038/s41565-025-01905-4