Among renewables, solar energy has been growing rapidly and driving the world towards a low-carbon future. We’ve been seeing a wave of innovations in the field, like perovskite solar cells, solar tiles and roofs, and organic panels. But what if we could harvest solar energy from the windows and skylights of our homes and skyscrapers, or even from our car windows and cellphone screens? Let’s explore transparent solar panels and how they stack up against conventional panels. Could transparent solar cells be the future of solar energy? Or is the future not so clear

In modern society we see glass everywhere, ranging in size from a small cellphone screen to huge windows and skyscraper façades. It’s estimated that there are 5-7 billion square meters of glass surfaces in the United States … so imagine that number for the whole world. And in most buildings these large areas of glass remain an unused energy potential. 1

According to the International Energy Agency (IEA), buildings and the construction sector add up to one-third of the total energy consumption in the world. That represents 40% of total direct and indirect carbon emissions into the atmosphere. And even though we’ve seen some progress there, which I’ve talked about in a previous video, there’s still a long pathway to decarbonize buildings around the world. 2

Designs that employ passive heating and natural illumination through triple-glazed windows and use rooftop solar panels look great, but what if we could go one step further? What if we could tap into all of that solar energy potential passing through the glass façades and windows, not only to provide natural illumination and heating, but to produce electricity? Enter transparent solar panels.

Photovoltaic glass, as this technology is also called, is a see-through type of solar panel, and unlike traditional mono or polycrystalline silicon panels … which are opaque … this new technology is manufactured to provide some level of transparency. 3 4

In theory these panels can be employed in anything from small mobile devices such as cellphones and laptops to huge windows of skyscrapers. But while standard solar panels absorb visible light to produce power, a truly transparent solar panel works based on a counter-intuitive idea: capturing invisible light and letting visible light pass through it.

The term “transparent solar panel” makes most of us think of something that’s as transparent as a glass window, which is a little misleading. Technically, this term can refer to both a panel that’s completely or only partially see through, so let’s take a look at the two types of transparent solar panels.

1) Partially transparent solar panels
These panels are manufactured using a thin-film technology in which ultra-thin layers of semiconductor material are deposited over a sheet of glass. Then, another sheet of glass is glued on top to form a tasty semiconductor sandwich.

These ultra-thin layers of semiconductor material (and we’re talking about the thickness of a few microns) is what makes these solar panels become transparent, and each layer you add increases the opacity just a little bit. While current silicon panels, like you have on your rooftop, achieve efficiencies above 20%, the efficiency of standard thin-film solar panels is considerably lower at around 7-10%. 3

A great application of partially transparent solar panels is in office buildings where the sunlight is intense, so the panels are installed in order to reduce the sunlight level that goes into the building. 5 In order to control the internal heating of buildings, engineers take into account the Solar Heat Gain Coefficient (SHGC) when designing windows and skylights. The SHGC basically measures the amount of available solar heat that can pass through the glass. The higher this percentage, the more heat goes into the building. 6

The SHGC of Onyx’s panels, one of the leaders in transparent PV glass for buildings, range from 10% to 40%, limiting the solar heat that goes into the building while producing electricity at the same time. 7

2) Fully transparent solar panels
The dream of having solar panels that would look like regular glass started with a group of researchers at Michigan State University (MSU), which created the first fully transparent solar cell in 2014.

The team created the transparent luminescent solar concentrator (TLSC), which uses organic salts to capture only a specific portion of the solar spectrum containing specific wavelengths of invisible light, such as ultraviolet (UV) and infrared (IR), while allowing normal visible light to pass through.

When sunlight hits the panel the UV and IR lights are turned into a different type of infrared that glows. Then this new wavelength moves to the edges of the window, where it’s converted into electricity by solar cells.

Since the materials don’t absorb or emit light in the visible spectrum, they look transparent to the human eye. The transparent solar panel created by the researchers achieved an 86% transparency level. Unfortunately, the efficiency of this prototype was very low (less than 1%), but they said that there’s a potential of reaching 10% efficiency. 4 8 9

Compared to conventional crystalline silicon-based panels, it’s obviously a big negative that the efficiency of transparent solar panels is so low. But when it comes to solar panels, efficiency isn’t everything. The low efficiency of transparent solar panels means that we’d need a larger area covered in order to produce the same amount of power as a 20%-efficient rooftop solar array. However, given how transparent solar panels can be integrated into buildings there’s more potential surface area to compensate for this lower efficiency.

An interesting advantage of partially transparent solar panels is that thin-film solar cells can achieve high efficiencies even when they’re not at the optimal tilting angle, which is great for buildings and skyscrapers where the windows are installed vertically. Thin-film cells are also more tolerant to heat, having a lower decrease in efficiency than traditional crystalline silicon solar panels at high temperatures. 10

Fully transparent solar panels have a clear advantage over partially transparent versions, since they can turn any glass sheet or window into a photovoltaic cell, they look better, and have a wider range of use cases. But their big negative isn’t just their low efficiency. Transparent solar panels utilize organic solar cells, which degrade more rapidly than inorganic solar cells. Thin-film, for example, last from 10-20 years, while panels based on crystalline silicon have an average lifespan of 25 years. 3

Although there are still many improvements to be made, the benefits and wide range of applications … mainly in the building sector … has led companies around the world to explore the technology.

Onyx Solar is one of the leaders in transparent PV glass for buildings. The company offers two technologies of solar glass, one based on amorphous silicon and another on crystalline silicon, with options ranging from 0% transparency up to 38%. Their panes use layers of heat-treated safety glass and can be customized to fit specific projects into anything like curtain walls, canopies, or even floors, but I’ll step on that in a minute. 11 7

An interesting project using their Photovoltaic Skylight is the McDonald’s Flagship Restaurant in Walt Disney, in Florida. The building became a zero-energy building and the first zero-emission restaurant in the U.S. The system is expected to produce over 2M kWh of electricity over 35 years and will have a payback time in less than 4 years. 12

Another product from Onyx is their PV Façade. Among its projects, the company created the façade for FEMSA’s headquarters, which is the largest Coca-Cola bottling plant company in the world (Monterrey, México). It consists of 370 large-sized grey amorphous silicon glass modules with a 20% transparency, and can produce about 17,200 kWh of energy. Overall, this project is expected to have a payback time in less than 1 year and reduce HVAC energy demand by 24%. 13

But you could also take a huge step into the future … and onto a photovoltaic floor … with the company’s PV pavers. They’re designed to be used for pedestrian traffic and can withstand up to 400kg (~880lbs). On top of that, they can be customized in a wide range of colors and use the same installation procedures as metal framing and wood floors.

One project that’s gaining traction is a Manhattan penthouse that used 50 glass pavers for an innovative and solar harnessing terrace. The photovoltaic system reduced 19% of the penthouse’s HVAC demands and produces green electricity at the cost of about $0.012.14 Talk about taking a walk on the bright side.

And when it comes to cost, their PV glass costs about $50 per square meter, while a typical solar panel cost usually ranges from $40-110 per square meter, and a typical double-pane window costs around $24-45 per square meter here in Massachusetts … including material and installation costs. 15 16

As cool as those are, transparency levels aren’t good enough for applications where you need clear windows that look like common glass. For that you’d want to look at the Australian-based ClearVue, which has been manufacturing solar panels that are clear … with a minor tint … and have no visual distortion, achieving a transparency of 70%. Their technology is composed of several layers, but in short, their glazing system consists of nano and microparticles scattered over an interlayer of thin film sandwiched within panes of glass.17 18 19

The price per square meter for a triple-glazed, low-emissive, insulated glass unit from ClearVue is $400. Comparatively, a common triple pane window ranges from $270-430 per square meter. 20

One of their projects was the Warwick Shopping Centre Atrium in Perth, Australia. The installation includes 18 triple-glazed, low-emissive PV glass panes that provide power to the indoor lighting, outdoor signage and the display of the shopping center. And according to an article from the company, each solar window of this building was able to produce approximately 0.1 kWh from 6-7 hours of sun. 21

Depending on the project, the company estimates the financial payback period can be as low as a year. The carbon payback is less than 9 years, compared to 100 years for typical double-glazed windows. They claim that by using inorganic materials, these PV windows can produce power for more than 20 years, with a 0.5% efficiency loss per year.

But again … when we come back to efficiency it’s not that great. Today, their industry-standard 3.5 x 1.6 meters solar window can give 30 W/m² while a typical 340W solar panel can give about 200W/m². That means to produce the same power of a rooftop PV system, we’d need 6 times more area for these transparent solar panels to equal out.22 23

But there’s where we get back to some of the really interesting research being done. Aiming to increase the efficiency, the University of Michigan, and the team led by Stephen Forrest, has been getting interesting results.

They’ve been exploring the use of organic materials in the panels, but the main challenge is how to prevent rapid degradation. To overcome that problem the team has been adding layers to the sun-facing side of the glass. They tried multiple materials in various tests, but the latest one with a fullerene material has shown some promise.

They tested this new transparent solar panel under several conditions, like simulated sunlight that could shine the equivalent of 1 sun up to 27 suns, and temperatures up to 150ºF (65ºC). The conclusion was that the solar cells could still provide 80% of their power even after 30 years. They’ve also achieved a 40% transparency and believe it’s possible to increase that number to 60%. And they’re also working hard to raise the 10% efficiency up to 15% at higher transparencies. 24

It’s not hard to see that transparent solar panels still have a challenging pathway to full transparency at higher efficiencies (or at least efficiencies closer to typical solar panels), and power densities at a reasonable costs for the market. Today, with an efficiency lower than 10% and power densities of 30W/m², this technology still requires a considerably larger area to produce the same amount of power that rooftop PV systems can give. But it’s also important to remember that the University of Michigan increased the original <1% efficiency in 2014 to about 10% in 2021. That’s significant and pretty exciting … hopefully the technology becomes more cost-competitive in time.

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