Scientists crack a stubborn crystal flaw in kesterite solar cells, unlocking a certified efficiency record of 15.3%
Kesterite solar cells have long been one of photovoltaics’ most appealing promises — built entirely from Earth-abundant, non-toxic elements, and theoretically capable of competing with mainstream thin-film technologies. Yet despite years of intensive research, their real-world efficiency has stubbornly lagged behind that potential.
The reason may have been hiding in the manufacturing process itself. A structural flaw forming deep inside the material during production has quietly limited how well these cells can perform — and until now, no one had found a reliable way around it. A new approach may have finally changed that.
A solar cell built for sustainability — but held back by its own making
Kesterite — shorthand for Cu2ZnSn(S,Se)4 — is made entirely from copper, zinc, tin, sulfur, and selenium. All five are relatively abundant and non-toxic. That puts kesterite in sharp contrast to competing thin-film technologies like CIGS, which depends on indium and gallium, or cadmium telluride, which relies on tellurium — elements that are scarcer, more expensive, and carry real supply-chain risks at scale.
Together, these defects trap charge carriers — electrons and holes generated by sunlight get caught before reaching the electrodes, bleeding away efficiency.
The practical appeal is straightforward: a solar cell that doesn’t depend on rare materials could be manufactured cheaply and deployed widely. That’s the promise kesterite has carried for decades. But the numbers haven’t matched it. Despite sharing a crystal structure similar to CIGS, kesterite cells have consistently underperformed their thin-film rivals by a wide margin.
The hidden culprit: how crystals grow in the wrong direction
To form the final light-absorbing layer, manufacturers expose a precursor film to selenium vapor at high temperature — a step called selenization. In practice, something goes wrong almost immediately.
Selenium reacts at the back interface of the film before the rest of the material has a chance to organize properly. This early reaction creates a low-melting-point Cu(S,Se) phase, triggering what researchers describe as “reverse crystallization.” Grains begin forming from the bottom of the film upward rather than from the top down.
The result is a bilayer structure: two distinct crystalline zones separated by horizontal grain boundaries. Voids form. Unwanted secondary phases appear. Together, these defects trap charge carriers — electrons and holes generated by sunlight get caught before reaching the electrodes, bleeding away efficiency. The defect locks in during the very first stages of a rapid, high-temperature process, and by the time the film cools, there’s no undoing it.
Decoupling heat and selenium: the strategy that changes the sequence
The researchers’ solution was to change when selenium arrives — not just how much. Their thermal-decoupled selenization strategy engineers a vertical selenium concentration gradient during the initial stage of the process. The top of the film sees selenium first; the back interface sees it later.
Think of it like controlling the order in which layers of a cake solidify. If the top sets first, weak seams at the base never get a chance to form. The same logic applies here: by the time selenium reaches the back interface in meaningful quantities, the temperature has moved outside the range where Cu(S,Se) forms uncontrollably.
What emerges is a fundamentally different crystal structure. Instead of the problematic bilayer, the absorber grows as columnar grains spanning the full thickness of the film — top to bottom, no horizontal boundaries, no voids.
Record efficiency and what the numbers mean
Those structural improvements translate directly into performance. The team’s best device achieved a power conversion efficiency of 15.7% in the lab. Independently certified, it measured 15.3% — a new record for kesterite solar cells.
Certified efficiency matters because lab measurements can vary depending on equipment and conditions. Independent certification removes that uncertainty and places kesterite on the same scoreboard as every other solar technology. The 15.3% figure is the one that counts for comparison. With fewer boundaries interrupting the path from absorption to current collection, carriers move more freely and recombine less.
To put that in context: CIGS cells routinely exceed 20%, and perovskites have pushed past 26% in the lab. Kesterite has historically struggled to break 13% in certified tests — so reaching 15.3% is a meaningful step forward, even if a gap remains.
Beyond kesterite: a technique with wider reach
The researchers also applied the thermal-decoupled selenization approach to solution-processed CIGS solar cells and observed similarly positive results. That’s a significant finding — it suggests the technique isn’t a narrow fix for one material but a general framework for controlling crystallization in copper-based chalcogenide semiconductors as a class.
A clearer understanding of crystallization kinetics could accelerate progress across multiple thin-film technologies at once. For kesterite specifically, closing the efficiency gap with CIGS would validate the case for abundant-material solar at scale. The next step is to see whether the approach holds up in larger-area devices and under conditions closer to commercial production. If it does, kesterite’s long-deferred promise may finally be moving within reach.
More information is a avilable here:Wu, Z., Wei, H., Shao, Z. et al. Thermal-decoupled selenization enables kesterite solar cells with 15.3% certified efficiency. Nat Commun 17, 7453 (2026). https://doi.org/10.1038/s41467-026-74180-z
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
