Divergent Solid-state Conversion Pathways in Evaporated All-perovskite Tandem Solar Cells
By revealing divergent solid-state conversion mechanisms in sequentially evaporated wide-bandgap and narrow-bandgap perovskite stacks, this study establishes bandgap-specific control as a design principle to enable the first evaporated all-perovskite tandem solar cell with a 19.2% efficiency and enhanced thermal stability.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Solar energy has long been limited by a fundamental ceiling: a single layer of material can only capture a specific slice of the sun's light, letting the rest pass through or turning it into wasted heat. To break this barrier, scientists have turned to stacking two different layers on top of each other, creating a tandem cell where the top layer catches high-energy light and the bottom layer catches the rest. The most promising material for this job is a class of crystals called perovskites, which can be tuned to absorb different colors of light. While these cells have reached impressive efficiency levels in the lab, they are currently made using liquid chemicals. This method involves toxic solvents and messy steps that are difficult to scale up for large, flexible, or industrial-sized panels. A cleaner, more precise alternative is to build these layers from scratch in a vacuum, evaporating solid materials until they form a film, much like frost forming on a window. However, while this vacuum method works well for simple layers, scientists have struggled to understand how to make the complex, mixed-material layers needed for tandem cells without the help of liquids.
A team of researchers has now solved this puzzle by watching, in real time, how these vacuum-deposited layers transform into working solar cells. They discovered that the process is not a single, uniform event but rather two entirely different pathways depending on the type of material being made. When creating the top layer, which is designed to absorb blue and green light, the materials mix together quickly and evenly before the final crystal structure even forms. The researchers found that the color of light this layer absorbs is determined simply by how much of a specific bromine-containing ingredient is added, regardless of the ratio of other ingredients. The system acts like a well-mixed soup where the final flavor depends on the total amount of a key spice, not the proportion of other ingredients.
In stark contrast, the bottom layer, which is designed to absorb red and infrared light using a mix of tin and lead, follows a much more chaotic path. As soon as the organic component is added, a reaction begins at the interface between the layers, forming a solid crust that traps the remaining materials underneath. This early formation creates a barrier that prevents the ingredients from mixing thoroughly. Instead of a uniform blend, the final crystal ends up with a vertical gradient, where the top is rich in lead and the bottom is rich in tin. The researchers realized that this uneven mixing happens because the crystal lattice forms and hardens faster than the metal atoms can swap places to find their perfect spots. This insight explains why previous attempts to make these cells were inconsistent; the process was trying to force a uniform mix in a system that naturally resists it.
Armed with this understanding of how the materials behave, the team optimized the heating and timing for each layer separately. They successfully created a complete tandem solar cell where both the top and bottom layers were formed entirely through this dry, vacuum-based method, without a single drop of liquid solvent. The resulting device converted sunlight into electricity with an efficiency of 19.2 percent, a significant milestone for this specific manufacturing technique. Furthermore, the cells proved to be remarkably durable; when stored in a warm environment for 1,200 hours, they retained 80 percent of their initial performance. This work demonstrates that by tailoring the manufacturing process to the specific physical behavior of each layer, it is possible to build high-quality, solvent-free solar cells that are ready for the next generation of flexible and large-scale solar power.
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