Researchers at the University of Wisconsin–Madison, the National Laboratory of the Rockies, the University of Toledo, and the University of Colorado Boulder, have designed a tin-based perovskite with built-in protection against air and moisture, addressing a major obstacle to achieving practical lead-free perovskite photovoltaics. Devices built with the new material reached 16.2% power conversion efficiency, among the best reported for tin perovskite solar cells, while showing markedly improved durability.
Tin-based perovskites are considered the leading lead-free alternative thanks to favorable light-absorbing and electronic properties, but they are especially vulnerable to oxygen and moisture, which rapidly degrade the material and its performance, limiting practical deployment. To address this, the team examined how the organic components of these hybrid materials assemble and designed new tin perovskites intended to impede damage from oxygen and water. They synthesized and compared a family of closely related organic cations substituted with different halogen atoms (fluorine, chlorine, and bromine), and found that the chlorinated version, 4-chloro-phenethylammonium (4ClPEA), packed most tightly into the crystal structure.
The resulting layered material, (4ClPEA)2SnI4, showed stronger π-stacking interactions and tighter interlayer packing than its fluorinated or brominated counterparts, substantially impeding oxygen and water diffusion into the perovskite. Conventional tin perovskites typically begin degrading within hours or days in ambient air; the new material maintained its structure and bright photoluminescence in ambient air for several months and remained undissolved even after days immersed in water. Theoretical calculations confirmed that the tight packing makes it substantially harder for oxygen and water to penetrate and damage the tin-containing perovskite.
The team incorporated the material into working 2D/3D tin perovskite solar cells, where the 4ClPEA addition also improved film crystallinity and orientation. Beyond the 16.2% power conversion efficiency, the best devices retained more than 95% of their initial efficiency after 1,600 hours of storage in dry air, and 80% of their initial efficiency after 1,000 hours of continuous operation under simulated sunlight at 55°C, among the best stability and performance figures reported to date for tin-based perovskite solar cells.
"Tin perovskites are very promising materials for solar energy, but they are extremely sensitive to air and moisture," said University of Wisconsin–Madison's Song Jin. "We wanted to find a way to protect these materials while preserving the properties that make them attractive for solar cells." Christopher T. Triggs, the study's first author, who recently earned his doctorate in materials chemistry at UW–Madison, added: "What is exciting is that a relatively small change in the material's design produces such a large improvement in stability. It shows how designing the organic components and controlling the way perovskite structures pack together can provide powerful protection of the resulting perovskite materials from the surrounding environment." National Laboratory of the Rockies' Kai Zhu noted that the work "gives us a new design strategy for making tin perovskite solar cells both efficient and much more robust... to enable future applications."
The National Laboratory of the Rockies and the Wisconsin Alumni Research Foundation have jointly filed a patent related to the new material, and the researchers say the approach opens new strategies for designing more stable tin-based perovskites as the field works toward practical, lead-free perovskite photovoltaics.
