Imagine a car whose windows and sunroof can help top up its battery while parked under the sun, or a pair of smart glasses whose lenses can harvest light to power built-in electronics.

Such applications could become more feasible with a new type of ultrathin transparent solar cell developed by scientists from Nanyang Technological University, Singapore (NTU Singapore).

Led by Associate Professor Annalisa Bruno, the NTU researchers created perovskite solar cells that are about 10,000 times thinner than a strand of human hair and around 50 times thinner than conventional perovskite solar cells.

Despite their thinness, the devices achieved some of the highest power conversion efficiencies reported for ultrathin perovskite solar cells to date.

Published recently in the scientific journal ACS Energy Letters, their findings could pave the way for solar cells that can be integrated into buildings, vehicles and wearable devices without significantly changing their appearance.

Because the new solar cells are semi-transparent and colour-neutral, they could potentially be incorporated into windows and façades without significantly changing how a building looks.

“The built environment accounts for roughly 40 per cent of global energy consumption, so technologies that seamlessly convert buildings’ surfaces into power-generating assets are gaining urgency,” said Assoc Prof Bruno, who is from NTU’s School of Physical and Mathematical Sciences and School of Materials Science and Engineering.

“Our perovskite solar cells offer distinct advantages as they can be manufactured using simple processes at relatively low temperatures. They can also be tuned to absorb specific wavelengths while remaining transparent, and could potentially be scaled over large areas, reducing their carbon footprint,” added Prof Bruno, who is also Cluster Director, Renewables & Low-Carbon Solutions and Energy Storage, Energy Research Institute @ NTU (ERI@N).

Unlike conventional silicon solar cells, these perovskite-based devices are capable of generating electricity even under indirect sunlight and diffuse light conditions. This makes it particularly suited for Singapore’s urban environment, where vertical building surfaces and frequent cloud cover often limit direct solar exposure.

As an example, if the technology were scaled up while maintaining similar performance, large glass façades could be transformed into active surfaces for solar power generation.

Preliminary estimates suggest that a deployment across a major glass-fronted building, such as an office tower at Raffles Place or Marina Bay, could theoretically generate several hundred megawatt-hours of electricity annually.

Depending on the usable glass area and building orientation, this level of energy generation would be equivalent to the annual electricity consumption of about 100 four-room HDB flats.

Manufacturing near-invisible solar cells

Perovskite solar cells are made up of several layers, including a semiconductor layer that absorbs sunlight and converts it into electricity.

To make the ultrathin cells, the NTU team used an industrially compatible method known as thermal evaporation. In this process, source materials are heated in a vacuum chamber until they evaporate. The vapour then settles on a surface, where it forms a thin film.

The method allows very thin and uniform perovskite layers to be deposited over large areas. It also avoids the use of toxic solvents and helps reduce defects in the solar cells, improving their ability to convert light into electricity.

By adjusting the process, the researchers were able to control the thickness of the perovskite layer and create both opaque and semi-transparent devices.

The team believes this is the first time ultrathin perovskite solar cells have been made entirely using vacuum-based processes. This could make the technology more suitable for large-scale industrial production in the future.

Using the technique, the researchers produced ultrathin perovskite absorber layers down to 10 nanometres while retaining useful solar-cell performance.
In opaque devices, the cells achieved power conversion efficiencies of about 7 per cent, 11 per cent and 12 per cent for perovskite layers measuring 10, 30 and 60 nanometres respectively.

A semi-transparent cell with a 60-nanometre-thin perovskite layer allowed about 41 per cent of visible light to pass through, while converting sunlight into electricity at 7.6 per cent efficiency.

The researchers said this is among the best reported performances for semi-transparent perovskite solar cells made with similar materials.

This will allow daylight to pass through while still generating a useful amount of electricity, which is important for applications such as solar windows, glass façades and tinted building surfaces.

First author of the paper, Dr Luke White, a former PhD student at the Energy Research Institute @ NTU, the School of Physical and Mathematical Sciences, and the School of Materials Science and Engineering, said: “By precisely controlling thermal evaporation, we are able to adjust the transparency of the solar cells. This opens up new possibilities for sustainable architecture, such as tinted windows that generate electricity.”

Giving an independent comment, Professor Sam Stranks, Professor of Energy Materials and Optoelectronics, Department of Chemical Engineering and Biotechnology, University of Cambridge, said: “This approach offers a high level of control over film thickness and uniformity, which will be needed if semi-transparent solar cells are to move towards larger-area applications.”

“Semi-transparent perovskite solar cells are an exciting route to harvesting energy from surfaces that are difficult to use with conventional silicon panels, such as windows, façades and lightweight electronics. The results reported here show a promising balance between transparency and power generation in very thin devices, while the next critical tests will be long-term stability, durability and performance over larger areas,” he added.

Powering sustainable cities

Prof Bruno is a pioneer in the field of perovskite solar cells. Her earlier work on thermally evaporated perovskite solar cells has been scaled up, advancing the field of perovskite solar cells and paving the way for industry adoption.

Her innovations are supported by the NTU Innovation and Entrepreneurship initiative, which helps research teams accelerate and translate promising ideas from laboratories to commercialisation.

A patent for the development of the ultrathin perovskite films in a novel structure has been filed through NTUitive, the University’s innovation and enterprise company.

The researchers are now in talks with companies to validate and standardise the thermal evaporation process used in this study. They will also work to improve the long-term stability, durability and large-area performance of the perovskite solar cells before they can be commercially deployed.

As cities become denser and electricity demand grows, buildings are increasingly being seen not just as energy consumers, but as potential sources of clean energy.

Solar panels are already widely used on rooftops. But the vertical surfaces of buildings, including windows and glass façades, remain largely untapped.

Their breakthrough marks an important step towards transparent solar cells that can be integrated into everyday surfaces, from building windows to vehicles and wearable electronics, helping cities generate more clean energy without requiring additional land.

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