Quantum-Dot Coating Could Pull Solar Energy From Your Windows

Engineers inch toward the goal of achieving efficient conversion of sunlight energy

3 min read
Photograph of solar-cell coated window
Photo: Los Alamos National Laboratory

In big cities, sometimes buildings that don’t have a lot of roof space for solar cells still have large windows that could harness light for electricity. Researchers at the Los Alamos National Laboratory, in New Mexico, reported yesterday in Nature Energy that a thin film of quantum dots on everyday glass could be the key to achieving acceptable efficiency in window photovoltaic systems at low cost.

Mostly, engineers have tried using modules of connected solar cells to capture sunlight falling on windows. Some wondered if it would be possible to do it with less cells. Taking advantage of a mechanism for capturing the light falling on a window and then directing it to a single solar cell “simplifies the device; it makes it less expensive,” says Victor Klimov, a nanotechnology engineer at Los Alamos.

At first, engineers tried using organic dyes as a way to concentrate the light. The problem with that, Klimov says, is that the dyes absorb the light they produce because it appears very similar to the incoming rays from the sun. In 2013, engineers instead started investigating nanometer-scale semiconductors called quantum dots because they allow customizing properties such as what kinds of light they absorb and which ones they don’t.

In the new research, Klimov and his team found that a thin layer of quantum dots on normal glass could have a lifetime of up to 14 years and about 1.9 percent overall energy conversion efficiency. To make these devices practical they’ll have to reach 6 percent, he says, so they’re getting close.

What’s more, adding quantum dots to window glass is surprisingly easy: A machine pours a slurry of quantum dots and PVP polymer onto the glass and a blade spreads it out to form a thin sheet.

The quantum dots consist of a CdSe inner core, a Cd1−xZnxS outer shell, and are coated in silica for protection from oxidation—with the outer shell acting like an absorber. When a photon hits a dot, an electron in the shell is kicked out of its valence band into the conduction band, leaving a hole. The rogue electron and hole jump to the core, where they recombine to produce a new photon with lower energy.

By design, the shell only absorbs high energy photons, and the new photon from the core is free to propagate throughout the glass and quantum dot layers via internal reflections. Eventually, the propagating photons would arrive at the glass edges—where one or more solar cells could pick them up.

In 2015 research, members of the team had tried dispersing quantum dots directly inside a polymer. However, in polymer materials such as this, many of these photons would scatter and escape the material. The optical properties of the new thin quantum-dot layer on glass are such that there’s minimal scattering and the light tends to propagate much longer, Klimov says.

“This is important for showing that these nanocrystals may be used to make large-area and cost-effective diffuse light concentrators,” Vivian Ferry, an energy and electronics researcher at the University of Minnesota who was not involved in the study, but has worked with solar cells and quantum dots, writes in an email. 

When the researchers tested absorption and stability properties, they also found the manufactured device held its own.

“If you’re serious about applications,” Klimov says, “Stability must be comparable to the stability of the solar cell.”

He believes the application technique is inexpensive and accessible enough for the glass industry to use. A coating could even be scraped off and re-used.

Still, there’s plenty of work to do before reaching the break-even point on energy conversion, he says. To meet the efficiency goal, he’s now tinkering with the concentrations of quantum dots used and their absorption properties.

For comparable light concentrators of a similar size, color, and transparency, the Los Alamos system is “pretty good,” writes David Patrick, an energy researcher at Western Washington University who was not involved in the study but has worked on solar light conversion. 

A correction to this article was made on 11 October 2016. The inner core and outer shell materials were inadvertently reversed.

The Conversation (0)

Smokey the AI

Smart image analysis algorithms, fed by cameras carried by drones and ground vehicles, can help power companies prevent forest fires

7 min read
Smokey the AI

The 2021 Dixie Fire in northern California is suspected of being caused by Pacific Gas & Electric's equipment. The fire is the second-largest in California history.

Robyn Beck/AFP/Getty Images

The 2020 fire season in the United States was the worst in at least 70 years, with some 4 million hectares burned on the west coast alone. These West Coast fires killed at least 37 people, destroyed hundreds of structures, caused nearly US $20 billion in damage, and filled the air with smoke that threatened the health of millions of people. And this was on top of a 2018 fire season that burned more than 700,000 hectares of land in California, and a 2019-to-2020 wildfire season in Australia that torched nearly 18 million hectares.

While some of these fires started from human carelessness—or arson—far too many were sparked and spread by the electrical power infrastructure and power lines. The California Department of Forestry and Fire Protection (Cal Fire) calculates that nearly 100,000 burned hectares of those 2018 California fires were the fault of the electric power infrastructure, including the devastating Camp Fire, which wiped out most of the town of Paradise. And in July of this year, Pacific Gas & Electric indicated that blown fuses on one of its utility poles may have sparked the Dixie Fire, which burned nearly 400,000 hectares.

Until these recent disasters, most people, even those living in vulnerable areas, didn't give much thought to the fire risk from the electrical infrastructure. Power companies trim trees and inspect lines on a regular—if not particularly frequent—basis.

However, the frequency of these inspections has changed little over the years, even though climate change is causing drier and hotter weather conditions that lead up to more intense wildfires. In addition, many key electrical components are beyond their shelf lives, including insulators, transformers, arrestors, and splices that are more than 40 years old. Many transmission towers, most built for a 40-year lifespan, are entering their final decade.

Keep Reading ↓ Show less