Perovskite solar cells have seen rapid efficiency gains in recent years and are viewed as a promising technology for delivering low cost renewable electricity. Many of the highest performing devices rely on solution based ink processing, while much of the thin film electronics and optics industry already uses vacuum deposition as a clean, solvent free method for coating large areas uniformly. When perovskites are grown entirely by vacuum deposition, however, the resulting films often suffer from sub optimal crystal growth, higher defect densities and reduced stability compared with their solution processed counterparts.
The HKUST led team addressed this bottleneck by introducing a lead chloride co source into a thermal co evaporation process for forming wide bandgap perovskite layers. By carefully engineering the evaporation conditions, they directed the perovskite crystals to adopt a predominantly (100) face up orientation across the film. This orientation is associated with improved crystallinity, better resistance to light and heat induced degradation and enhanced optoelectronic properties, leading to reduced nonradiative losses and stronger operational stability.
Using this deposition recipe, the researchers fabricated an all vacuum deposited single junction perovskite solar cell with a certified maximum power point tracked efficiency of 18.35 percent on a 0.25 square centimeter device. In laboratory measurements, the same architecture reached 19.3 percent efficiency, and 18.5 percent on a larger 1 square centimeter active area, demonstrating that the process can be extended to more practically relevant device sizes while maintaining competitive performance.
The team subjected their devices to durability testing under the International Summit on Organic Photovoltaic Stability protocol to assess long term behavior. In an ISOS L 2 accelerated aging test with full spectrum, one sun equivalent illumination without ultraviolet filtering, at 75 plus or minus 5 degrees Celsius in air and held at open circuit, encapsulated cells retained 80 percent of their peak performance after 1,080 hours of continuous operation. These results place the vacuum grown devices on par with state of the art solution processed perovskites in terms of combined thermal and photostability.
"Our work addresses the core materials science problem that has held back vacuum deposited perovskites," said first author Dr. Shen Xinyi, a postdoctoral researcher in the Department of Electronic and Computer Engineering at HKUST. "By engineering the evaporation process to control crystal orientation, we have achieved extended thermal and photostability on par with state of the art solution processed counterparts, but with all the inherent advantages of a dry, industry compatible vacuum technique."
To probe how the devices behave under operating conditions, the researchers employed operando hyperspectral imaging, an advanced spectral imaging method that maps optical signals across a functioning solar cell with pixel level resolution. This capability, developed at HKUST with support from the Vice President for Research and Development Office, allowed the team to visualize the spatial and temporal evolution of key processes inside the cells, including halide segregation and trap mediated recombination pathways that influence lifetime and performance.
"Leveraging operando hyperspectral imaging, we obtained unprecedented spatiotemporal insights into device physics and revealed the factors governing extended device lifetime," explained Prof. Lin Yen Hung, who leads the HKUST team in the Department of Electronic and Computer Engineering and the State Key Laboratory of Displays and Opto Electronics. "We visualized and distinguished the processes of halide segregation and trap mediated recombination at the microscopic scale, directly linking these features to macroscopic device performance." The analysis also separated beneficial radiative recombination from detrimental loss channels, giving researchers a diagnostic tool for further optimization.
High quality vacuum deposited perovskite layers are particularly attractive for tandem solar cells, where a wide bandgap perovskite top cell is stacked on a silicon bottom cell to harvest a broader range of the solar spectrum. Using their improved films, the team achieved conformal coating on industrial standard silicon heterojunction cells with micron scale surface texture, producing 1 square centimeter perovskite on silicon tandem devices with 27.2 percent power conversion efficiency. In an eight month outdoor trial in Italy, these all vacuum deposited tandem cells maintained around 80 percent of their initial performance, underscoring progress toward stable, field ready perovskite silicon tandems.
According to the authors, the co evaporation approach is directly compatible with existing industrial infrastructure used for thin film deposition in sectors such as display manufacturing and optical coatings. By overcoming previous limitations in crystal growth and stability, the method turns vacuum deposition from a compromise solution into a leading candidate for commercial scale production of high efficiency perovskite and perovskite on silicon tandem solar cells. The research appears in Nature Materials under the title Crystal facet directed all vacuum deposited perovskite solar cells and involved contributions from the National Thin Film Facility for Advanced Functional Materials at Oxford, Eurac Research, and Universite Grenoble Alpes in partnership with the French Alternative Energies and Atomic Energy Commission.
Research Report:Crystal-facet-directed all-vacuum-deposited perovskite solar cells
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