double-paned solar window amid verdant background

A new design for windows that can harvest solar electricity combines quantum dot engineering and a double-paned architecture to ratched up power conversion efficiency. [Image: Los Alamos National Laboratory]

Researchers at the Los Alamos National Laboratory (LANL), USA, led by OSA Fellow Victor Klimov, have devised a prototype large-area (230-cm2) solar window that cleverly combines a double-paned architecture and cheap-to-manufacture quantum dots (QDs) to boost solar energy collection efficiency compared with single-pane models (Nat. Photon., doi: 10.1038/s41566-017-0070-7). The key to the scheme: engineering the QDs embedded in the upper and lower panes to capture and rechannel photons from different parts of the solar spectrum—and to keep re-absorption of photons to a minimum.

Luminescent solar concentrators

What are colloquially called “solar windows” are more precisely known as luminescent solar concentrators (LSCs). In these devices, window materials are impregnated with a fluorophore or emissive material that absorbs a share of the solar photons that pass through the window and re-emits the light at a different wavelength. The windowpane acts as a 2-D waveguide, channeling the re-emitted light through total internal reflection to the edges of the window. There, solar cells embedded in the window frame harvest the captured photons and convert them to electricity.

The LSC concept has been around since the 1970s, but until recently, research has been hung up on the difficulty of finding good fluorophores to capture and re-emit solar energy in a workable way. In particular, dye molecules, the conventional choice for the job, tend to cover only a limited fraction of the solar spectrum and to reabsorb the re-emitted photons at alarming rates, cutting down the window’s overall photon harvest and power conversion efficiency.

QDs to conquer reabsorption

In the past five years, however, interest in LSCs for practical solar windows has re-ignited with advances in several photonic technologies, most notably luminescent QDs. These semiconductor nanocrystals can be tuned, both chemically and by particle size, to absorb and re-emit at substantially different wavelengths, offering a potential way out of the reabsorption paradox.

As a result, a number of experiments with QD-enabled LSCs have borne interesting fruit in recent years. In early 2017, for example, an Italian-U.S. team engineered single-pane solar windows that reportedly converted 2.85 percent of incident radiation to electrical power, while remaining transparent across the visual spectrum.

A double-paned window

Klimov’s team, which also included LANL postdocs Kaifeng Wu and Hongbo Li, wanted to see if additional engineering could enable a higher-efficiency result. To do so, the researchers focused on splitting the solar spectrum—in particular, on a stacked, double-paned solar window in which each pane would focus on sucking usable energy out of different wavelength components of sunlight. (The concept, according to the researchers, is analogous to multijunction solar cells, which combine different semiconductor materials to efficiently capture different parts of the solar spectrum.)

The team’s device consists of two thin, 15.24×15.24-cm glass panes, separated by a 2-cm air gap. On the upper glass layer, the researchers deposited a slurry containing CdZnS-based QDs tuned to absorb photons at relatively high blue- and ultraviolet-light energies (with absorption onset at wavelengths of around 440 nm). Crucially, the dots had been chemically doped with Mn2+ ions, which pushed the re-emitted photons to longer wavelengths and, thus, to energies below the dots’ absorption-onset range.

According to the scientists, that effectively makes the dots in the upper layer reabsorption-free, and thus allows a very high percentage of re-emitted photons to be channeled through the windows to the solar cells at the edges. The bottom glass layer, coated with a film containing CuInSe2-based QDs, then picks up the lower-energy infrared and visible-light bands of the solar spectrum not tapped by the top layer.

Bigger is better

In their tests of the prototype, the LANL researchers found that the double-paned design with highly tuned QDs could achieve power conversion efficiencies of 3.1 percent—50 percent higher than single-layer devices using the same materials, according to the team. And the scientists note that the tandem architecture’s efficiency gains should scale well with increasing window size, rising to 100 percent or more for window sizes of greater than a half-meter square.

The team sees potential uses for the devices not just in creating semitransparent, electricity-generating windows on urban buildings—a commonly discussed use-case for LSCs—but also in other applications. For example, LSCs could be used to create new, energy-harvesting types of building siding; as high-efficiency solar concentrators that can boost the efficiency of existing, conventional rooftop photovoltaic cells; and even in outer space, in applications such as lightweight optical antennas. Particularly given what he calls, in a press release, the “low-cost, solution-processable materials” involved, Klimov argues that the new double-paned LSCs, and potentially even more evolved variants, “offer a new way to bring down the cost of solar electricity.”