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Developments in transparent solar cells help achieve record efficiency, generates power from everyday surfaces

Developments in transparent solar cells help achieve record efficiency, generates power from everyday surfaces

28-Mar-14

A new record in the efficiency of transparent solar cells has been set by a development that allows transparency levels up to 40% with an efficiency of over 7%, by Heliatek GmbH. The company already holds the world record of 12% cell efficiency for opaque (non-transparent) organic solar cells. This lab development underscores the company's strategy to supply its transparent HeliaFilm to glass manufacturers for both building integration (BIPV) and car roofs. The production of HeliaFilm is based on small molecules (oligomers). Oligomers are deposited at low temperatures in a roll-to-roll vacuum process. Their selective absorption of the solar spectrum targets different colors and transparency to convert sunlight into electricity. The production of the transparent film is possible with the introduction of transparent conductive layers at the front- and back-side of the solar cells. The transparency of the products is at the core of the film customized to meet partners' specific needs. This is a film which can combine transparency and energy generation. Both these application areas use tinted glass to reduce glare which can be done by applying a film, so changing the film to one that generates electricity as well is a drop-in for manufacturers. Using the film in glass car roofs will not only generate energy, but will also allow the car manufacturers to claim Eco-innovation credits towards CO2 emission goals.  Architectural glass panels and windows incorporating the film enable electricity to be discretely generated on the outside envelope of a building. 40% light transparency with an efficiency record of 7.2% - this measurement follows standard testing conditions using a white background.  Heliatek thus succeeded in generating an optimal energy conversion for transparent HeliaFilm. 7.2% cells mirror the same efficiency, since the partial transparency allows the usage of only 60% of the light for the energy harvesting.  Depending on the application, the balance between light let through and electricity generated can be adjusted. Importantly for these applications, the technology continues to be effective at generating electricity at low light levels, any orientation and at high temperatures, which are conditions where conventional solar lose efficiency.

Inventing a new solar technology that can compete commercially with today’s solar cells is difficult, given existing deployment methods. But a transparent photovoltaic (PV) cell would change the rules of the game. MIT researchers are making transparent solar cells that could turn everyday products such as windows and electronic devices into power generators - without altering how they look or function today. Their new solar cells absorb only infrared and ultraviolet light. Visible light passes through the cells unimpeded, so they are invisible to the eyes. Using simple room-temperature methods, the researchers have deposited coatings of their solar cells on various materials and have used them to run electronic displays using ambient light. They estimate that using coated windows in a skyscraper could provide more than a quarter of the building’s energy needs without changing its look. They are now beginning to integrate their solar cells into consumer products, including mobile device displays.
3 years ago, a team in MIT’s Organic and Nanostructured Electronics Laboratory began to tackle the problem using a different approach. Richard Lunt, then an MIT postdoc and now an assistant professor at Michigan State University, proposed making a solar cell that would absorb all the energy from the sun except the part that allows us to see. Inspired by Lunt’s idea, the team developed a transparent PV cell. The thickest layer is the glass, plastic, or other transparent substrate being coated; there are multiple layers of the PV coating. At the core of the coating are the two active layers—the absorptive semiconductor materials that get excited by sunlight and interact, creating an electric field that causes current to flow. Sandwiching those layers are electrodes that connect to the external circuit that carries the current out of the device. Since both electrodes must be transparent—not the usual reflective metal—a layer on the back of the cell can be added to reflect sunlight of selected wavelengths, sending it back for a second pass through the active layers. Finally, anti-reflective coatings can be used on both outside surfaces to reduce reflections because any light that reflects - potentially as much as 10% of the total - doesn’t go through the device.
In a detailed theoretical analysis, Lunt, Bulović, and others showed that their design should realistically be able to reach over 12% efficiency, a rating comparable to that of existing commercial solar panels. The cost of implementing the technology will vary with the application, solar cell efficiency, and other factors. But Barr cites several sources of potential cost savings over traditional solar systems. For instance, the processes used in fabricating the new transparent PVs are environmentally friendly and not energy intensive. The coatings are deposited at nearly room temperature, so the transparent PV can be laid down on essentially any type of surface. There is no need to use glass, which is a costly component in the fabrication of conventional systems. Barr, Lunt, Bulović, and Bart Howe co-founded a company called Ubiquitous Energy. They are continuing development work to optimize their transparent PVs, using different semiconductor materials and device configurations that will lead to higher efficiencies and better transparencies.

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