Ironing out the wrinkles

Solar-cell researchers have spent much of the past decade pushing perovskite photovoltaics from laboratory curiosities toward commercial viability, and the latest milestone comes from an unexpected corner of the field: the deliberately wrinkled surfaces of ultrawide-bandgap perovskites. A new analysis published in Nature Energy examines a strategy of surface reconstruction and halide homogenization that smooths out these microscopic defects, and the payoff is striking — a certified power conversion efficiency of 29.3 percent in all-perovskite triple-junction solar cells. The result, highlighted by Tomoya Nakamura, Minh Anh Truong and Atsushi Wakamiya of Kyoto University’s Institute for Chemical Research, addresses one of the most stubborn bottlenecks standing between tandem perovskite architectures and the kind of performance that could reshape the photovoltaics industry.

To understand why surface wrinkles matter so much, it helps to revisit the basic physics of multi-junction solar cells. In a single-junction device, photons above the absorber’s bandgap deposit their excess energy as heat, while photons below the gap pass through unabsorbed — a fundamental trade-off that caps efficiency around the Shockley–Queisser limit. Stacking multiple absorbers with different bandgaps circumvents this constraint: a wide-bandgap top cell harvests high-energy blue photons, while narrower-gap layers beneath capture the red and infrared remainder. All-perovskite triple-junction designs are particularly attractive because perovskites offer tunable bandgaps, solution processability and strong defect tolerance. But the topmost layer in these stacks must be an ultrawide-bandgap perovskite — approaching roughly 2 electron volts — and that is precisely where the chemistry turns hostile.

The hostility stems from bromide. To push a perovskite’s bandgap into the ultrawide regime, formulators must replace much of the iodide in the crystal lattice with the smaller, more electronegative bromide anion. High bromide content, however, destabilizes the film during deposition and annealing. As the solvent evaporates and the crystallites grow, differential stresses and uneven halide distribution produce a surface texture the researchers describe as wrinkling — nanoscale and microscale corrugations that ripple across the top interface of the film. These wrinkles are not merely cosmetic. They increase surface area where recombination can occur, disrupt the uniformity of subsequent charge-transport layers, and scatter light in uncontrolled ways. Worse still, the same high-bromide chemistry promotes halide disorder: iodide and bromide ions segregate into bromide-rich and iodide-rich domains, creating local bandgap fluctuations that act as recombination hotspots and undermine the very wide bandgap the design depends on.

The new work, discussed in the News & Views analysis, tackles both problems simultaneously through a combined surface reconstruction and halide homogenization strategy. Surface reconstruction, in essence, reprocesses the topmost region of the perovskite film after it forms, relaxing the strained, defect-rich outer layer and allowing it to recrystallize into a flatter, more orderly termination. Halide homogenization addresses the compositional side of the problem, encouraging iodide and bromide to distribute evenly through the lattice rather than pooling into segregated phases. Together, the two interventions produce an ultrawide-bandgap perovskite surface that is morphologically smooth, compositionally uniform and electronically cleaner — three qualities that wide-bandgap perovskites have historically failed to deliver at once.

The consequences for device performance are dramatic. In the reported all-perovskite triple-junction devices, the smoothed 2-electron-volt top absorber transmits its full complement of sub-bandgap photons to the middle and bottom cells while extracting high-energy charge carriers with far fewer losses. The certified efficiency of 29.3 percent represents a benchmark for all-perovskite triple-junction technology, a figure that places these fully thin-film stacks in the same conversation as the best silicon-based tandems while retaining the advantages of low-temperature, scalable fabrication. Certification matters here: independent verification distinguishes genuine reproducible performance from laboratory optimism, and a certified number signals to the field that the surface-engineering approach survives the scrutiny of recognized testing protocols.

The path to this point has been built on a sequence of steady advances that the Nature Energy analysis situates in context. Earlier work on wide-bandgap perovskites documented the efficiency ceilings imposed by halide segregation and interfacial recombination, establishing the diagnosis that the new treatment directly addresses. Subsequent studies demonstrated high-performance perovskite-based multi-junction architectures and clarified how the constituent subcells could be optically and electrically matched. The underlying research article by Zhang and colleagues, which the analysis accompanies, now closes the loop by showing that the materials problem — the wrinkled, halide-disordered surface of ultrawide-bandgap absorbers — could be engineered away rather than merely tolerated. In the ecosystem of photovoltaics research, this is how fields advance: not by a single breakthrough, but by each study removing the specific obstacle the previous one identified.

From a manufacturing standpoint, the significance of the result extends beyond the headline efficiency. Solution-processed perovskites can in principle be deposited by printing, blade coating or roll-to-roll methods at temperatures far below those required for high-quality silicon, promising low embodied energy and low cost. But every additional processing step must be compatible with the layers beneath it, and triple-junction stacks compound this challenge: the top cell’s surface must accept a charge-transport layer, then a recombination layer, then the next absorber, each without degrading what came before. A wrinkled, compositionally inhomogeneous surface jeopardizes every one of those interfaces. By flattening and homogenizing the top absorber before the stack is completed, the reconstruction strategy improves not just a single interface but the integrity of the entire device architecture — a systems-level benefit that efficiency figures alone understate.

There are also deeper scientific questions that the work reframes. Halide segregation in mixed-halide perovskites has been studied extensively under illumination, where it manifests as photoinduced phase separation; the wrinkling phenomenon tied to high bromide content during film formation is a related but distinct failure mode rooted in crystallization mechanics. Understanding how strain, surface energy and halide mobility interact during deposition — and how a post-deposition reconstruction can erase the damage — offers a template that may generalize beyond ultrawide-bandgap compositions. If the same principles can be applied to tune the surfaces of other wide-bandgap perovskites, or to heal defects in the buried interfaces of the lower subcells, the entire tandem design space opens up. The Kyoto authors, who have long studied the chemistry of perovskite film formation from both academic and industrial perspectives — Wakamiya is co-founder and chief science and technology advisor of the perovskite startup EneCoat Technologies — are well placed to translate that understanding into manufacturable processes.

Challenges remain on the road to commercialization, and the analysis is careful to keep them in view. Triple-junction perovskite devices must prove their operational stability over thousands of hours of illumination, thermal cycling and moisture exposure — tests that mixed-halide compositions, with their susceptibility to ion migration, have historically found difficult. Scaling the surface reconstruction from small-area champion cells to large-area modules without sacrificing uniformity is another hurdle, as is demonstrating that the certified 29.3 percent can be reproduced with the reproducibility that industrial partners require. And the perovskite field as a whole still faces questions of lead content and end-of-life recycling that no efficiency record can resolve. Yet the trajectory is unambiguous: a technology that could not break 20 percent a few years ago is now approaching the practical ceiling of single-junction silicon, with theoretical headroom that silicon simply does not have.

For now, the image that captures the advance is the one its title invokes: wrinkles, ironed out. Nanoscale corrugations and halide disorder that once seemed like unavoidable consequences of wide-bandgap chemistry have been shown to be engineering problems with engineering solutions. With a certified 29.3 percent efficiency in an all-perovskite triple-junction cell, the field has demonstrated that the highest-performance perovskite architectures need not be compromised by their most challenging component. As the analysis by Nakamura, Truong and Wakamiya makes clear, the recipe for next-generation photovoltaics may hinge not on exotic new materials, but on learning to make the surfaces of the ones we already have perfectly flat, perfectly mixed and perfectly quiet — at least at the level of the electron.

Subject of Research: Ironing out the wrinkles

Article Title: Ironing out the wrinkles

Article References: Nakamura, T., Truong, M. A., & Wakamiya, A. (2026). Ironing out the wrinkles. Nature Energy. https://doi.org/10.1038/s41560-026-02127-1

Image Credits: AI Generated

DOI: 10.1038/s41560-026-02127-1

Keywords: Ironing, wrinkles, scientific research, peer-reviewed research, research findings, science news