How Have Transparent Photovoltaic Cells Been Developed?
In a recent study published in Journal of Power Sources, a team from the Department of Electrical Engineering at Incheon National University, South Korea, has detailed the creation of the first fully transparent solar cell. While the idea of such see-through cells is already fairly common knowledge, this is the first time that engineers have been able to translate the idea into practice.
The team, led by Professor Joondong Kim, focused on two alternative materials: titanium dioxide and nickel oxide. (This was in the knowledge that, again, traditional solar cells have to be opaque due to the components in their semiconductor layers.) And these two materials had been previously identified by other researchers for their desirable properties: titanium dioxide for being environmentally friendly, non-toxic, and ability to absorb ultraviolet light while also letting through most of the visible light range; and nickel oxide, for its high optical transparency, abundance, and ability to be manufactured at low industrial temperatures.
The crucial new finding was a product of both of the above materials combined, as the solar cell comprised a glass substrate and a metal-oxide electrode. On top, a thin layer of titanium dioxide, as the n-type ultraviolet light absorber, and nickel oxide, as the p-type visible light transmitter, were deposited. Finally, a coating of silver nanowires was added to act as the other electrode in the cell. The team then ran several tests to determine the device's absorbance and transmittance of light and, ultimately, its effectiveness as a solar cell.
A close-up of a photovoltaic cell within a see-through panel, which suggests that there is potential for future solar cells to be integrated into buildings and windows
Image credit: Biosystems Engineering via Science Direct
How Do They Compare to Traditional Solar Panels?
To convert the sun's energy into electricity, photovoltaic (PV) cells are sandwiched between layers of semiconducting materials. Each layer has unique electronic properties that energise when hit by photons from sunlight, thereby creating an electric field. The electricity is then passed through an inverter to convert it into an alternating current that can be funnelled into a grid or used directly by a home or business.
The next three subsections cover the three dominant types of solar cells currently being used.
Crystalline Silicon Photovoltaics
As one of the most abundant materials on earth, silicon is the most commonly used semiconductor material. The cells are formed by connecting silicon atoms together to form a crystal lattice. The organised structure gives the cells an upper limit of 29% efficiency, which, along with its low cost and long lifetime, make it hard to beat.
Thin-film cells are made by depositing layers of semiconductor materials on a supporting material such as glass, plastic, or metal. The most common semiconductor material used for thin-film PVs is cadmium telluride. The material has a low-cost manufacturing process, although it can only deliver up to 22% efficiency in laboratory conditions.
Perovskite cells are built with layers of materials that are printed, coated, or vacuum-deposited onto an underlying substrate. The cells are easy to assemble and can reach similar efficiencies to crystalline silicon. However, the cells don’t stand up well to moisture or extended periods of light or heat. More work is done to increase stability and solve the issue of large-scale uniform production.
When it comes to the use cases of traditional solar panels, they are typically found in solar farms and on roofs. However, not every roof is able to offer the correct orientation to take full advantage of the sun’s energy. The new transparent solar cells differ in their ability to pass the visible wavelength of light while absorbing the invisible light in the ultraviolet or infrared range to convert to electricity. This further suggests that the technology may be opened up to more applications and locations.
A building with three of its windows each hit by solar glare, which represents the solar energy harnessing potential that windows may have
Image credit: Bigstock
Could Transparent Photovoltaic Cells be Commercialised?
There has been a lot of global research into how to integrate solar energy into buildings, and the discovery by Professor Kim and his team forms an interesting part of this. Another recent development is the production of double-sided solar panels, which have demonstrated a 35% increase in efficiency rates.
The new findings are quite encouraging: in the final part of their research, the researchers demonstrated that the device could be used to power a small motor. The efficiency rate of the transparent PV cell is only at 2.1% at the moment, but that is quite good considering that it targets only a small part of the light spectrum. What’s more, the cell was highly responsive and able to work in low light conditions.
Ultimately, researchers would need to push the technology to be able to deliver a useful amount of electricity. The research is very much in its infancy, but demonstrating the practicality of a transparent solar cell is an important first step. Moreover, the team at Incheon University is confident that further improvements are possible by optimising the cell’s electrical properties.
Could We Really Generate Electricity Through Our Windows?
Five years on from the Paris Agreement, nations everywhere are still battling towards a carbon-free future. Solar energy understandably plays an essential role in the energy transition as it is the most reliable and abundant energy source available to us. However, while solar cells have become cheaper, more efficient, and environmentally friendly, their integration into everyday materials has been limited by their opaque nature.
Solar panels have become an increasingly popular source of renewable energy in Europe, especially in Italy, France, and Germany. If energy efficiency can be improved, then there is a real chance that we could see PV cells being used in our windows. In the meantime, low energy efficiency could lead to its use in smaller consumer products such as mobile phones. Either way, the new technology might just open a new window into the future of green energy.