Science on the Hill: Turning windows into solar panels
Quantum dots are nanoscale particles that can be custom-designed to absorb sunlight in glass windows without altering the view except for a slight color-neutral tinting effect, turning windows into efficient solar collectors. Courtesy image

Gov. Susana Martinez recently announced an “all of the above” energy plan promoting the development of all sources of energy, especially renewables like solar and wind power. Given New Mexico’s border-to-border sunshine and vast expertise in energy-related research, there’s no question our state plays a significant role in the nation’s turn to a more diverse energy portfolio.

Indeed, sunlight seems like the most obvious source of clean, renewable energy — after all, the sun has powered all life on Earth for almost 4 billion years. Sunlight is abundant, free and for all practical purposes, eternal. Harvesting that light with photovoltaic cells, which convert sunlight into electricity, at a cost-competitive price, however, has proved elusive. While the price of solar cells recently has plummeted and their efficiency has gone up, challenges remain around siting vast arrays of solar-electric panels and finding ways to integrate them into buildings and other applications.

These challenges prompted a joint research team from the Center for Advanced Solar Photophysics at Los Alamos National Laboratory and the University of Milano-Bicocca in Italy to try a fresh approach to solar power. Working with intriguing materials called quantum dots, the team achieved a breakthrough in solar-concentrating technology that can turn windows into electric generators and revolutionize the way we think about where and how we generate energy.

Think about it: If windows can generate electricity for use on site, the consumer gains the advantages of free, nonpolluting power and at least partial grid independence. The grid reduces its reliance on fossil fuels, gains resilience and gets relief from peak-use demands, which slashes greenhouse gas emissions. Siting problems go away, neighbors don’t have to look at awkward photovoltaic arrays and power generation stays close to its end use.

Quantum dots make it work. As nanocrystals just 10 to 50 atoms wide and made of a light-emitting semiconductor material, quantum dots obey quantum mechanics laws, which give them properties that can be manipulated in the laboratory. They can absorb light at one wavelength, and then efficiently convert it and re-emit it at another. A bigger quantum dot emits on the red end of the spectrum, a smaller dot on the green-blue end. By adjusting the dots’ size, we can precisely tune those wavelengths to suit our purposes. A luminescent solar concentrator consists of quantum dots dispersed in a clear plastic material, which forms a waveguide. The quantum dots absorb sunlight and convert it to a wavelength suitable for the photovoltaic cells. Then the waveguide directs light toward the photovoltaic cells, which make electricity.

That sounds fine in theory, but the team had to fix a few deal-breaking problems that plagued earlier proof-of-concept demonstrations of related solar-concentrating technologies. First, the concentrators reabsorbed too much of the re-emitted light, which limited the concentrators to just a few square inches in size — hardly practical. Second, the quantum dots used in earlier concentrators were based on the toxic heavy-metal cadmium, which caused environmental problems. Third, the concentrators only absorbed a small portion of the solar spectrum, tinting the concentrators yellow and red, which was not going to work for windows.

The team solved these problems by creating quantum dots of copper, indium, selenium and sulfur — a composition without any toxic metals. A new industrial method developed by the University of Milan embeds the dots in a transparent material, where the dots absorb uniformly across the solar spectrum. The team’s quantum dots re-emit infrared light, which is invisible to the human eye and ideal for the photovoltaic cells, yet occupants in the room will notice only a slight, neutral-colored tinting effect, much like the filtering of conventional “low-E” windows that save on cooling bills.

Turning windows into solar collectors is the kind of disruptive technology that can revolutionize energy generation and consumption and contribute to the low-carbon energy portfolio mentioned in the governor’s plan. Someday, buildings ranging from your own house to urban skyscrapers might generate much of their own electricity through innocuous, luminescent, solar-concentrating windows. If this technology replaced all the glazing on the One World Trade Center building in New York City, the windows could power more than 350 apartments. How’s that for megascale impact from nanotechnology?

Victor Klimov is director of the Center for Advanced Solar Photophysics at Los Alamos National Laboratory and a member of the team developing quantum-dot solar concentrators.