Buildings with glass facades include a coating that reflects and absorbs some of the light, both visible and infrared, from the sun. This coating both reduces the brightness and heating within the building so that the people inside are more comfortable. Researchers, however, have been looking at ways to incorporate transparent solar panels into the design of these facades so that the energy is used to power the building instead of wasted.
The reason that researchers have been looking into incorporating solar panels into glass facades is that they provide an ideal location for organic solar cells. Since windows are on the face of buildings, they are constantly exposed to a large amount of sunlight. The trick to incorporating solar cells into these facades, though, is using cells that are both efficient and transparent so that they can generate energy while functioning as a window.
Historically, it has been difficult to create both a transparent and efficient solar panel. Thanks to Stephen Forrest, a Professor at the University of Michigan, and his team, however, transparent solar panels have hit a record of 8.1% efficiency. These solar panels are made from organic, or carbon-based, cells that are both transparent and efficient, unlike the conventional silicon cells which are only efficient. This research puts us one step closer to using skyscrapers as a power source.
The Solar Cell Material
The new solar cell materials are made from a combination of organic molecules. According to Yongxi Li, an assistant researcher, the materials had to account for multiple factors such as good absorption, high voltage, high current, low resistance, and transparency. As a result, these molecules are engineered so that they are transparent in visible light and able to absorb infrared light. More so, the researchers developed a coating that boosts both infrared-generated power and transparency.
Additionally, the team created a transparent, or color-neutral, version of this solar panel that was made from indium tin oxide electrode. The panel was 10.8% efficient with 45.8% transparency due to its silver electrode. This product, however, has a slightly greenish tint, somewhat comparable to the tint on sunglasses, which might make this material unacceptable for certain windows despite improved efficiency.
How these transparent solar cells are measured is by the light utilization efficiency. This measures how much energy is available as electricity or as transmitted light on the interior side. Previous color-neutral solar cells only have 2-3% light utilization efficiency. The organic panels, however, have increased ratings, with the indium tin oxide cell having a light utilization efficiency of 3.5% and the silver counterpart having a rating of 5%.
Both versions can be manufactured large scale, which boosts probability of use. More so, these options use fewer toxic materials than other transparent solar cells, allowing them to be more environmentally-friendly. These panels can also take advantage of the fact that they are the most efficient when sun hits them at a perpendicular angle. This will allow transparent organic solar cells to be customized based on latitude or placed between panes of double-glazed windows.
The research is published as “Color-Neutral, Semitransparent Organic Photovoltaics” in the Proceedings of the National Academy of Sciences. Forrest and Li, as well as Xia Guo, Zhengxing Peng, Boning Qu, Hongping Yan, Harald Ade, and Maojie Zhang are listed as the authors. The research team also included researchers at North Carolina University, Soochow University in China, and SLAC National Accelerator Laboratory. More so, the findings are based on work by the U.S. Department of Energy Solar Energy Technologies Office and the Office of Naval Research and Universal Display Cooperation.
Despite setting the new record, Forrest and his team are continuing to develop the technology. The next goal is to reach 7% light utilization efficiency and extend the cell lifetime to about 10 years. Additionally, the team is investigating the cost to install transparent solar cell windows into new and existing structures.