Is it finally time to take solar windows seriously?

For as long as the solar PV panel has been in existence, scientists and entrepreneurs have been trying to make it just transparent enough to be used in windows and other visual building-integrated PV (BIPV) applications. From super-thin silicon to perovskites, a number of bases have been tested for transparent solar with little commercial success. The solution for this see-through problem is outside of what we think of as the typical solar cell. Instead, researchers are turning to organic semiconductors.

Organic semiconductors are carbon and hydrogen-based atoms that are natural electrical insulators without needing silicon, cadmium or lead. Organic semiconductors produce energy or are “charged” when they are doped with appropriate electrodes or exposed to sunlight. Organic light-emitting diode (OLED) is the most well-known organic semiconductor and is found in the latest television models. Organic semiconductors can be dissolved to print like ink and spread thinly on a glass substrate to appear much more transparent than a solid silicon cell, making organic PV a suitable product for solar windows.

These are not huge energy generators — Ubiquitous Energy estimates that a 3-ft by 5-ft window equipped with its transparent PV coating could produce up to 200 Wh of electricity per day — but the electrification of an underutilized building product is the real win. Considering high-rise city buildings have little space for traditional rooftop solar but thousands of windows, some energy offset is better than none.

Solar Power World talked with two transparent solar companies using organic semiconductors in a number of successful demonstration projects, suggesting that the next few years will be the kick-off point for solar windows.

NEXT Energy Technologies — piloting installations in 2023

Early in 2023, outdoor apparel company Patagonia announced it had installed solar windows at its corporate headquarters in Ventura, California, furthering the company’s commitment to sustainability. The windows were fabricated and installed by Walters & Wolf, a large commercial window company operating in the Western U.S., that used a transparent PV coating from NEXT Energy Technologies. NEXT’s coating was integrated into 22 windows on the south-facing façade of Patagonia’s campus, and the power is being used to charge phones and other devices in the employee community spaces.

“The great thing about PV on commercial buildings is you’re providing power when it’s needed, where it’s needed. You’re lowering the load on the grid rather than increasing it,” said Corey Hoven, chief technology officer for NEXT.

This Patagonia installation is the first major demonstration project for NEXT, which spun out of organic semiconductor research from the University of California, Santa Barbara. Hoven said the company sees most of its potential in the commercial building market. Working with window fabricators and installers like Walters & Wolf will be the key to bringing transparent PV technology into more projects.

“We’re trying to disrupt a market without disrupting the supply chain,” he said. “There are flat glass manufacturers that make glass and sell it to window fabricators. Window fabricators, where we partner, cut pieces of glass and sell that to window glaziers who then put the frame on it and install it. Our business model is to allow existing manufacturers of windows to manufacture this themselves. Then people can buy the windows from trusted window manufacturers, and you don’t have to reinvent a supply chain from scratch.”

NEXT’s technology consists of a printed transparent photovoltaic coating on glass, which is sealed by a second piece of glass and integrated into a traditional window glazing system. By using organic semiconductors, NEXT is able to absorb more infrared energy than traditional solar, which means less semiconductor is needed to produce energy, allowing the coating to be transparent.

“You don’t realize the cells are there, it’s pretty much invisible,” Hoven said. “At the very edges of the glass are some busbars. Windows in commercial buildings have a frame system already that’s particularly good for carrying wires. You can use rapid shutoffs with power optimizers at the windows. You can connect them with string inverters, microinverters. It’s like a normal solar installation, but a lot of the [difference] is in the framing that carries those wires.”

Newer commercial windows have electrochromic properties, so window installers are already used to connecting wires. Commercial building architects and window glaziers are the most ideal customer of NEXT’s solar window product, but Hoven said there may be future opportunities for traditional solar installers.

“There’s a ton of opportunity in this market. Right now we’re doing it the most normal way for windows, but there are more opportunities, like the whole façade of the building,” he said.

NEXT Energy Technologies will continue to look for window company partners and work on additional pilot installations in 2023. The transparent solar technology is not in full-scale production yet, but Hoven said the political climate around energy efficiency right now is encouraging.

“The IRA has a huge impact for this,” he said. “Now, your façade of your building can be counted as a solar cell. That’s enormous. People have moved beyond just thinking about fields of solar panels or solar panels on a rooftop. Let’s go to the next stage and make beautiful solar.”

Ubiquitous Energy — starting significant manufacturing by 2025

Another university spinout, Ubiquitous Energy got its start at the Massachusetts Institute of Technology 11 years ago when researchers were exploring ways to make solar more lightweight and flexible. By discovering spectrum engineering and organic photovoltaics, Ubiquitous developed a thin coating that is fully transparent with no color tinting. While acknowledging the potential for solar windows in the commercial building market, Ubiquitous recently signed a manufacturing agreement with leading residential window maker Andersen for its patented UE Power technology.

“When you look at just the windows market, it’s roughly one-half residential, one-half commercial. We really wanted this technology to be ubiquitous, to be everywhere,” said Veeral Hardev, VP of strategy for Ubiquitous Energy.

Ubiquitous is currently making solar windows at its R&D pilot line in California, where it also has functioning solar windows installed on an exterior façade that provide power for overhead LED lighting.  UE Power solar windows have also been installed at Michigan State University, a commercial building in Boulder, Colorado, the NSG Pilkington glass development facility in Ohio and throughout Asia. Already a proven technology, Hardev said the relationship with Andersen will allow Ubiquitous to reach residential customers through a recognized window brand.

“We believe this puts a strong stamp of credibility and validity to what we’re doing, working with someone with that brand reputation and place in the market,” he said. “The next phase would be licensing the technology to manufacturers all around the world. We believe that’s going to be the fastest way to get this technology globally deployed.”

Hardev said there have been many semi-transparent PV products released over the years, but Ubiquitous Energy’s product is “vision glazing” that people actually want to look through.

“I want to look out and see no color or obstructions. We want the consumer to operate it just like a traditional window. You don’t even know that our technology is integrated in there, but it’s giving you all this benefit,” he said.

Residentially, power could be used locally for active window features or connected back into a smart home’s energy settings. Commercial buildings have more window area, making it more logical to feed that energy back into the grid for net metering. Obviously, UE Power windows won’t produce as much as traditional solar, but with more localities mandating renewable energy installed on new commercial buildings, solar windows could provide a significant alternative to small rooftop-solar footprints.

“We’ve certified up to 10% power conversion efficiency. That’s apples-to-apples comparison of efficiency as traditional solar,” Hardev said. “The nice thing about our technology, very similar to how thin-film solar technologies behave, they perform very well in off-angle or diffused light. You don’t really need access to direct sunlight to operate.”

Ubiquitous Energy is continuing to raise money to start a dedicated manufacturing line. Hardev said the company hopes to break ground on the line by the end of this year, with product available for both the residential and commercial spaces in the latter half of 2025.

“We didn’t really invent or develop any new method or process that requires new equipment to be developed. It’s using what the window coating industry already does as part of their day-to-day operations,” he said. “Our goal has been to not disrupt the whole flow, to not create unnecessary challenges. We want it to be something that people can easily use and get installed and integrated.”