Researchers from the University of Oxford, The Hong Kong University of Science and Technology (HKUST), RISE Research Institutes of Sweden, HZB and Université Grenoble Alpes (CEA) have developed a multi-source co-evaporation strategy that enhances the crystal quality of vacuum-deposited perovskite films. The team stated that this advance brings all vacuum-deposited single-junction perovskite cells as well as perovskite-on-silicon tandem solar cells closer to scalable production.
Schematic of device architecture for all-vacuum-deposited WBG PSCs. Image from: Nature Materials
Many perovskite solar cells use solution “inks" in their design, while many industrial thin-film products (from OLED displays to optical coatings) are produced by vacuum deposition - a clean, solvent-free process that can coat large areas very uniformly. However, when perovskites are fabricated entirely by vacuum deposition, the crystals can form in less-than-ideal ways, leaving the films more defect-prone and significantly unstable.
The team has found that introducing a lead chloride (PbCl2) “co-source” during thermal co-evaporation can effectively direct how the perovskite crystals grow. The approach yields a highly ordered wide-bandgap perovskite (1.67 eV) with many grains aligned in a (100) “face-up” orientation, which is a hallmark of a more crystalline film that better resists light- and heat-driven degradation, resulting in improved optoelectronic properties and stronger stability under light and heat stressors.
Using this newly developed deposition recipe, the team achieved the first certified performance for an all-vacuum-deposited wide-bandgap perovskite solar cell, reaching a maximum-power-point-tracked power conversion efficiency of 18.35% on a 0.25 cm2 device. In the lab, the cells achieved 19.3% efficiency and delivered 18.5% on the more challenging 1 cm2 cell size.
To test durability, the team followed the International Summit on Organic Photovoltaic Stability (ISOS) protocol. Under the stringent ISOS-L-2 accelerated ageing test: full-spectrum, 1-sun-equivalent illumination with no ultraviolet filter, at 75 ± 5 °C in air, operated at open circuit, the encapsulated cells retained 80% of their peak performance after 1,080 hours.
“Our work addresses the core materials-science problem that has held back vacuum-deposited perovskites,” HKUST's Dr. Shen Xinyi explained. “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 glimpse inside the devices as they operated, the team used operando hyperspectral imaging, an advanced “spectral camera” that maps optical signals across a working solar cell, pixel by pixel - a capability developed at HKUST. HKUST's Prof. Lin Yen-Hung remarked: “Leveraging operando hyperspectral imaging, we obtained unprecedented spatiotemporal insights into device physics and revealed the factors governing extended device lifetime. 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.” This analysis also differentiated beneficial radiative recombination from detrimental non-ideal pathways, providing a powerful diagnostic tool for future optimization.
High-quality vacuum-deposited perovskite layers are especially valuable for tandem solar cells, where a perovskite top cell is stacked on a silicon bottom cell to harvest more of the solar spectrum. Using their improved films, the team achieved conformal coating on industrial-standard silicon heterojunction cells with micron-scale texture, delivering 27.2%-efficient 1-cm2 perovskite-on-silicon tandem solar cells. In an outdoor trial in Italy, their all-vacuum-deposited tandem cells maintained approximately 80% of their initial performance after 8 months of real-world operation, highlighting progress toward stable perovskite-on-silicon tandems.
“This co-evaporation method is directly compatible with existing industrial infrastructure for thin-film deposition,” Prof. Lin emphasized. “It transforms vacuum deposition from a compromised alternative into a frontrunner for producing high-performance, stable perovskite solar cells and tandem cells, offering a clear pathway from the lab to the factory floor.”
