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The windows of a car parked in the sun or the lenses of smart glasses can be future charging points for a battery. And the technology has already reached that point thanks to scientists from the Nanyang Technological University in Singapore (NTU) who have just published in ACS Energy Letters a new type of transparent, ultra-thin solar cell based on perovskite, a semiconductor material with compositional versatility that conventional silicon cannot match. In short. The team, led by Associate Professor Annalisa Brunohas managed to manufacture cells just 10 nanometers thick. To have an even greater dimension: a human hair measures about 70,000 nanometers, that is, if that hair were the Eiffel Tower, this film would be a sheet of paper placed next to it. However, there is an even more revealing piece of information from the study, since the natural roughness of the surface on which the cell is deposited—about 2.8 nanometers according to microscopy measurements of the paper itself—represents almost a third of its total thickness. But the milestone is not in its form. The real paradigm shift proposed by this technology is the end of exclusive dependence on direct sun. Unlike conventional silicon panels, these perovskite devices generate electricity under indirect light and diffuse light conditions, making them especially useful in high-density urban environments where vertical facades and frequent cloud cover limit direct solar exposure. “Buildings consume about 40% of the world’s energy, so we urgently need technologies that turn their facades into energy generators,” explains Bruno. According to the team’s initial calculations, if we covered the glass façade of a large skyscraper (such as those in the Marina Bay financial district) with this technology, we could theoretically generate hundreds of megawatt-hours per year. We are talking about covering the annual consumption of about 100 four-bedroom apartments. These are preliminary figures, of course, but the potential is there. The secret is in evaporation. How do you keep a window looking like a window while generating energy? The answer is that these cells are semitransparent and neutral in color, with no apparent dye that reveals their presence. To manufacture them, the team used a vacuum thermal evaporation process: the base materials are heated in a vacuum chamber until they evaporate and are deposited on a surface forming an ultrathin and uniform film. Without toxic solvents, without the usual defects of solution methods. What distinguishes this work from previous attempts — and there have been many, the study compares its results to decades of studies — is that it is the first time ultrathin perovskite cells have been made using entirely vacuum processes, from start to finish. That is not a minor detail because vacuum processes are already used by the large-scale semiconductor industry, which considerably shortens the path to industrial manufacturing. The data, but with nuances. Let’s get to the numbers, which is where this technology really comes into its own. In their completely opaque versions, these sheets manage to transform 7%, 11% and 12% of the light they receive into energy, using minimum thicknesses of 10, 30 and 60 nanometers. What if we want the window to remain a window? The 60 nanometer semi-transparent model allows 41% of visible light to pass through and maintains a non-negligible efficiency of 7.6%. According to the researchers, it is the best that has been seen to date with this type of materials But here the real tension of this type of engineering appears: the more transparent, the less efficient. The study identifies the 30 nm cell as the one that best balances both variables—it has the highest potential for combined light utilization efficiency—but allows less visible light to pass through than the 60 nm cell. There is no perfect solution; There is a compromise that each application will have to negotiate according to its priorities. But what about stability? This is where any perovskite technology has to prove its maturity. The data from the study itself shows that 100 nm cells last projected for about 15,400 hours before degrading to 80% of their initial performance. The 60 nm ones, 5,800 hours. The 10 nm ones, 4,100 hours. These are figures that speak of a laboratory, not of a window exposed to rain, temperature changes and years of use. Professor Sam Stranks, from the University of Cambridge, sums it up precisely in a separate commentary on the study– The balance between transparency and generation is promising, but the next critical tests will be long-term stability, durability and performance on large surfaces. The roofs are already occupied. The next frontier of urban solar energy is the millions of square meters of glass that cover our buildings, cars and devices, surfaces that until now were passive by definition. The progress of the NTU team, already patented through NTUitive and in conversations with companies to validate the process, points in that direction. There is still a way to go, especially in real durability. But for the first time, that path has an industry-compatible manufacturing method, cells that operate with a fraction of the available light and a thickness that makes the word “invisible” not a marketing metaphor, but a technical description fairly close to reality. 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