Seeing Clearly [+]Enlarge Credit: ACS Nano

The vast real estate of windows in office buildings and skyscrapers could be a fruitful field for harvesting solar energy—if lightweight solar cells could be made with a high enough efficiency and appealing aesthetics. Now researchers at Oxford University report semitransparent solar cells that might do the trick (ACS Nano 2013, DOI: 10.1021/nn4052309).

For use in windows, solar cells need to absorb enough light to produce sufficient energy, but let enough pass through to be transparent. Organic photovoltaic materials can absorb infrared light and pass visible light, but they have very low efficiencies. Inorganic semiconductors, such as amorphous silicon, absorb strongly in the visible part of the spectrum, so the films have to be very thin to be transparent, thus decreasing the amount of photons they capture. They also tend to give windows a brownish or reddish tint, which architects dislike.

The Oxford team, led by physicist Henry J. Snaith, made their solar cells using perovskites, a class of mineral-like crystalline materials that has recently grabbed much attention among researchers in photovoltaics. Perovskites have properties similar to inorganic semiconductors and show sunlight-to-electricity conversion efficiencies of more than 15%.

To make their semitransparent cells, the researchers first deposited a film of the perovskite CH 3 NH 3 PbI 3 - x Clx onto glass coated with fluorine-doped tin oxide. They made the film by mixing methylammonium iodide and lead chloride and spin-coating the solution along with a solvent, such as dimethylsulfoxide, onto the glass. They then heated the resulting film to temperatures ranging from 90 to 130 °C. As the solution cooled, it underwent a process called dewetting; it formed droplets on the glass surface, leading to islands of crystalline material with empty spaces in between as the solvent evaporated. The islands absorb photons and convert them to electrons, while light striking the empty areas passes through. The result was a transparent solar cell with a grayish tint.

As the transparency of the film increases, efficiency decreases. The most transparent cells, which let through about 30% of incoming light, converted light to electricity with efficiencies of 3.5%. The darkest films, only 7% transparent, had efficiencies near 8%. Snaith says the ideal coating would let through about half the light and have a 5% conversion efficiency. “We think there’s a lot of scope to improve it further,” he says. He’s formed a company, Oxford Photovoltaics, with the hope of commercializing a device by 2017.

Snaith says the next step is to determine the stability of the material. A practical solar cell should function for several years. But even if it stops generating electricity, the window containing the cell should maintain its color and transparency for at least a decade. Also, the solar cells, which would most likely be inserted between two panes of glass in a standard double-glazed design, would need transparent electrodes to transmit the electricity. Snaith says those would probably consist of metal nanowires or spin-coated indium tin oxide.