The solar panels people are most familiar with are the dark, photovoltaic (PV) cells often seen on roofs, but strides have been made in the last few years to develop transparent versions that could eventually be fitted into windows and other glass surfaces. Known as luminescent solar concentrators (LSC), these devices so far haven't proven as efficient or scalable as regular panels, but now a team at Los Alamos National Laboratory has demonstrated a new technique that could make for larger, more practical solar energy-harvesting windows.

The key to LSCs are molecules known as flurophores embedded within the glass surface, which absorb the light that hits them and re-emit it as lower energy photons. These photons are then guided to the edges of the surface, where strips of conventional PV cells lie in wait to catch them. Over the years, the technology has advanced from visibly studded sphelar cells, to semi-transparent tinted windows, right up to fully transparent planes of energy-producing glass.

The problem is, aesthetically and practically, clear glass would be ideal. Yet those devices can lack in the efficiency department, converting just one percent of the solar energy received. Though the Los Alamos-designed LSCs are colored, the upside is they manage a much more impressive 10 percent solar conversion efficiency rate, thanks to the quantum dots embedded inside.

"We are developing solar concentrators that will harvest sunlight from building windows and turn it into electricity, using quantum-dot based luminescent solar concentrators," says lead scientist Victor Klimov. "The quantum dots used in LSC devices have been specially designed for the optimal performance as LSC fluorophores and to exhibit good compatibility with the polymer material that holds them on the surface of the window."

These quantum dots are tiny spheres made by encasing one material inside another. Adjusting their physical properties can maximize their efficiency: how much light they absorb is controlled by the size and composition of the shell, while the spectrum of light they emit can be tuned by fiddling with the core.

"This tunability is the key property of these specially designed quantum dots that allows for record-size, high-performance LSC devices," says Klimov.

To manufacture the LSCs, the team used a "doctor-blade" technique, which involves applying the quantum dots contained in a polymer material to standard plates of glass, then using a blade to wipe away the excess liquid. With this, they were able to produce colored-glass LSCs of up to 90 x 30 sq cm (14 x 4.7 sq in), with the 10 percent conversion efficiency.

The research has been published in the journal Nature Energy.