While conventional rooftop solar panels are now widely used to help cut electricity bills for many types of buildings, the cost savings from adopting photovoltaics could be much greater if solar panels can double as windows for homes and offices. Recently, a team of researchers at South Korea’s Ulsan National Institute of Science and Technology (UNIST) has unveiled see-through solar cells that are capable of achieving a conversion efficiency rate of up to 12%. Commercialization of this technology could transform many large commercial and residential buildings into their own solar power stations in the near future.
Permit application, municipal regulations, aesthetic issues, and space availability are some well-known constraints that hinder the deployment of rooftop solar panels. Therefore, companies and research institutions involved in the solar sector have been working on “solar glass” – a transparent photovoltaic material. Many kinds of solar glass materials have been developed over the recent years, along with solutions that integrate window designs with existing photovoltaic technologies. If some of them do eventually arrive onto the market one day, then “solar windows” could become entry-level green energy products that can complement many architectural designs.
There are still several technical hurdles to overcome in the development of solar glass. In the aspect of performance, most prototypes of see-through solar cells have yet to reach a conversion efficiency rate of 20%, which is the level already attained by most conventional solar cells. Furthermore, see-through solar cells are not colorless because they are made of the same materials (e.g., silicon, polymers, dyes, and organic materials) as other solar cells. These materials, which come in various colors (mostly blue and brown), allow for the capture of as much of the solar spectrum as possible. However, they also produce a tinted hue when light passes through them. Hence, it will take some time before fully transparent solar windows hit the market.
The solution that the UNIST team has devised does not require new materials nor a radically different manufacturing method. It entails a simple modification of crystalline silicon wafers. The result of their R&D efforts could generate huge opportunities in the solar energy market.
In their lab, UNIST researchers took crystalline silicon cells that are 1 centimeter square in area and drill tiny holes in them to let the light shine through. The holes are 100 micrometers in diameter, or around the same thickness as a human hair. Through this process, cells become see-through or semi-transparent. Each of them looks like a mesh, in which the solid part is still blue colored. The UNIST team stated that the conversion efficiency rate of their prototypes can reach 12%, which is lower than the 20% attained by normal opaque cells but three to four percentage points higher compared with transparent cells developed elsewhere. According to UNIST researchers, the conversion efficiency rate has to be raised to at least 15% before their see-through cells are viable for commercialization.
The UNIST team also pointed out that its texturing process for wafers could give birth to a new kind of solar cells that may be suitable to become part of a window panel. Normally, the installation of building-integrated photovoltaic systems requires mounting solar panels directly on the outer walls of buildings. Therefore, solar panels must be tilted and fixed at the optimum angle for receiving sunlight. A conventional solar cell could lose as much as 30% of its efficiency if it is oriented at a 90-degree angle. In contrast, a see-through solar cell like the ones developed by UNIST researchers loses just around 4% of its efficiency when positioned vertically.
Kwangyong Seo, a member of the UNIST team and co-author of the paper on this subject, said that he and his colleagues want to raise the conversion efficiency rate of their see-through solar cells to the commercialization threshold as quickly as possible. They also plan to begin the development of a transparent electrode.
(Image credit: Ulsan National Institute of Science and Technology.)