One of the hindrances to large-scale solar adoption, especially in cities, is where to install the chunky panels. Rooftops? Skyscraper walls? Vast open spaces that dense urban centers barely have in the first place? Researchers from Nanyang Technological University (NTU) Singapore say they may have solved part of this problem with solar cells so thin they are invisible enough to install directly onto windows.
The team claims to have developed ultrathin, semi-transparent perovskite solar cells that are roughly 10,000 times thinner than a strand of human hair and about 50 times thinner than conventional perovskite solar cells, while still retaining some of the highest efficiencies yet reported for devices in this ultrathin category.
Their work, recently published in ACS Energy Letters, could eventually pave the way for electricity-generating windows, glass facades, smart glasses, vehicle sunroofs, and other surfaces that currently sit passively in the sunlight.
The idea of transparent solar cells is not entirely new. Researchers around the world have spent years trying to create photovoltaics that can blend into glass and urban infrastructure. The problem is that solar panels are fundamentally designed to absorb sunlight. The more light a solar cell captures, the less transparent it tends to become.
Existing commercial solar panels are also physically bulky systems consisting not just of photovoltaic materials but also of thick protective glass, encapsulation layers, metallic contacts, mounting hardware, and structural framing. Typical residential solar panels weigh around 18 to 23 kg (40 to 50 lb) each and generate roughly 350 to 450 W of power under ideal conditions.
A modern office tower can easily consume several gigawatt-hours of electricity annually. Now, imagine the sheer amount and weight of solar panels powering such a building independently. Where could they possibly be mounted? In this context, the roofs appear ridiculously small. An alternative would be vast open land, but many cities simply do not have it.
What of the walls, you ask? They are everywhere and in abundance. Well, mounting heavy opaque panels across glass skyscraper facades radically changes the appearance, weight, and thermal characteristics of the building itself. But what if they didn’t have to be heavy, or bulky, or even visible at all?
These questions form the basis of NTU researchers’ technology aimed at turning glass surfaces that already dominate modern cities into active power-generating systems.
The team, led by Associate Professor Annalisa Bruno from NTU’s School of Physical and Mathematical Sciences and School of Materials Science and Engineering, developed the new devices using perovskites: a class of crystalline materials that have become one of the hottest areas in solar research over the last decade due to their potentially low manufacturing costs, high efficiencies, and ability to function under lower-light conditions.
NTU Singapore
The researchers fabricated ultrathin perovskite absorber layers measuring just 10 nanometers thick while still retaining useful photovoltaic performance. For perspective, a human hair is typically around 80,000 to 100,000 nanometers thick.
Unlike conventional silicon solar cells, which perform best under direct sunlight, perovskite-based devices can continue generating electricity even under indirect or diffuse lighting conditions. That is particularly relevant in high-rise cities, where skyscrapers create heavily shaded urban canyons and cloud cover frequently reduces direct solar exposure. Instead of relying solely on rooftops facing the sun, vertical glass surfaces across entire city blocks could theoretically generate power throughout the day.
The researchers tested multiple thicknesses. Opaque devices with 10-, 30-, and 60-nanometer perovskite layers achieved power conversion efficiencies of roughly 7%, 11%, and 12%, respectively. Meanwhile, a semi-transparent version using a 60-nanometer-thick layer achieved 7.6% efficiency while still allowing roughly 41% of visible light to pass through the device. Modern solar panels achieve above 20% efficiency. However, when you consider the relatively zero weight, low-light performance, and other beneficial characteristics of the new perovskite, the technology shines through.
That balance between transparency and efficiency is one of the central engineering challenges in transparent photovoltaics. The more transparent a device becomes, the less sunlight it absorbs and, therefore, the less electricity it generates. The NTU team says their results rank among the best reported performances for semi-transparent perovskite solar cells made using similar materials.
Importantly, the devices were also described as color-neutral, meaning they would not dramatically tint windows or radically alter the appearance of glass-covered buildings. According to the researchers, the cells’ transparency can be adjusted during manufacturing by precisely controlling the thickness of the deposited perovskite layers.
The real breakthrough of the technology may not simply be the thinness of the solar cells, but how the NTU team manufactured them. The researchers used an industrially compatible vacuum-based technique called thermal evaporation, in which materials are heated in a vacuum chamber until they vaporize and settle onto a surface as an ultrathin film. According to the team, this may be the first time ultrathin perovskite solar cells have been produced entirely using vacuum processing, an approach already widely used in semiconductor and display manufacturing.
Unlike liquid chemical processing methods commonly used for experimental perovskite cells, the vacuum-based technique allows highly uniform, large-area films with precise thickness control while avoiding toxic solvents and reducing structural defects that can hurt efficiency and scalability.
The researchers estimate that if scaled successfully, the technology could theoretically turn the glass facade of a tower like New York’s One World Trade Center into a solar-generating surface, producing several hundred megawatt-hours a year, roughly enough electricity to power about 40 average US homes annually.
“The built environment accounts for roughly 40% of global energy consumption, so technologies that seamlessly convert buildings’ surfaces into power-generating assets are gaining urgency,” said Bruno.
NTU Singapore
Reality, however, is more complicated.
Perovskite solar cells have generated enormous excitement for years, but commercialization has repeatedly run into one major obstacle: durability. Perovskites are notoriously vulnerable to moisture, oxygen, heat, and prolonged ultraviolet exposure. Laboratory prototypes can produce impressive efficiencies, but maintaining performance over years of real-world exposure remains one of the field’s biggest unsolved challenges.
Prof. Sam Stranks from the University of Cambridge, who was not involved in the research, called the work promising but noted that “the next critical tests will be long-term stability, durability and performance over larger areas.”
That last point is particularly important. Producing tiny high-performance samples in a lab is very different from manufacturing thousands of square meters of defect-free solar glass for skyscrapers.
Still, if the durability and scaling problems can eventually be solved, the implications could be significant.
Modern cities are already covered in enormous amounts of glass that currently do little more than admit light while simultaneously increasing cooling loads inside buildings. Converting even a fraction of those surfaces into electricity generators could create entirely new forms of distributed urban power generation without requiring additional land.
The potential applications also extend well beyond architecture. The NTU researchers specifically point to vehicle windows, sunroofs, wearable electronics, and smart glasses as possible future use cases. Lightweight, semi-transparent photovoltaics could someday allow devices to continuously recharge from ambient light without requiring visible solar cells. Imagine how cool it would be to power smart glasses all day just by wearing them.
The research team has already filed a patent for the ultrathin perovskite film structure through NTUitive, the university’s commercialization arm, and says it is now working with industry partners to validate and standardize the thermal evaporation manufacturing process. For now, the technology remains in the research stage.
Source: Nanyang Technological University
