Engineers at the University of California, San Diego have developed a way to fabricate perovskites as single-crystal thin films. The method, which uses standard semiconductor fabrication processes including lithography, produces flexible, single-crystal perovskite films with controlled area, thickness, and composition. According to the researchers, these single-crystal films show fewer defects and are more efficient and stable than polycrystalline perovskites. The new fabrication method could make perovskites more efficient for use in solar cells and optical devices. The engineers used lithography to etch a mask pattern on a substrate of hybrid perovskite bulk crystal. To keep the growth of the perovskite at the single-crystal level, they designed the mask pattern to allow control in both lateral and vertical dimensions. A single-crystalline perovskite thin film during the transfer process. Courtesy of Yusheng Lei. The researchers peeled the single-crystal layer off the bulk crystal substrate and transferred it to an arbitrary substrate, while maintaining its form and its adhesion to the substrate. They applied a lead-tin mixture with a gradually changing composition to the growth solution, creating a continuously graded electronic bandgap of the single-crystal thin film that increased carrier mobility. The researchers said that the transferred single-crystal hybrid perovskites were of comparable quality to those directly grown on epitaxial substrates, and were mechanically flexible depending on the thickness. Using their method, the researchers fabricated squares of single-crystal thin films as large as 5.5 cm2. The thickness of the single-crystal perovskite ranged from 600 nm to 100 μm. The researchers were able to control the perovskite’s thickness as well as the composition gradient in the thickness direction. Graded single-crystal perovskites. Courtesy of Yusheng Lei. “Modern electronics such as your cellphone, computers, and satellites are based on single-crystal thin films of materials such as silicon, gallium nitride, and gallium arsenide,” professor Sheng Xu said. “Single crystals have less defects, and therefore better electronic transport performance, than polycrystals. These materials have to be in thin films for integration with other components of the device, and that integration process should be scalable, low-cost, and ideally compatible with the existing industrial standards. That had been a challenge with perovskites.” The single-crystal perovskite sits on a neutral mechanical plane sandwiched between two layers of materials that allow the thin film to bend so that the single-crystal film can be incorporated into flexible thin-film solar cells and wearable devices. “Further simplifying the fabrication process and improving the transfer yield are urgent issues we’re working on,” Xu said. “Alternatively, if we can replace the pattern mask with functional carrier transport layers to avoid the transfer step, the whole fabrication yield can be largely improved.” Single-crystal perovskite films could enable more efficient flexible solar cells such as the one pictured here. Courtesy of Yusheng Lei. The researchers believe their method is the first to control the growth and fabrication of single-crystal devices precisely and efficiently. “The method doesn’t require fancy equipment or techniques — the whole process is based on traditional semiconductor fabrication, further indicating its compatibility with existing industrial procedures,” researcher Yusheng Lei said. The research was published in Nature (www.doi.org/10.1038/s41586-020-2526-z).
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