(CN) — Imagine a world where skyscrapers, symbols of the urban future, serve as solar cells to produce energy instead of consuming it.
That future might be closer than you think.
A University of Michigan research team has developed a transparent, color-neutral solar cell with greater efficiency than ever before. The material, which is still in the testing stage, could one day be used to produce high-rise windows that harvest energy from the sun.
“Windows, which are on the face of every building, are an ideal location for organic solar cells because they offer something silicon can't, which is a combination of very high efficiency and very high visible transparency,” stated lead research Stephen Forrest, professor of engineering at the University of Michigan.
Transparent solar cells are “attractive energy-conversion devices for integration onto windowpanes, skylights and building facades, providing an opportunity for increasing solar energy harvesting on building surfaces,” according to the study.
Using a carbon-based design instead of conventional silicon, Forrest’s team achieved a record 8.1% efficiency and 43.3% transparency.
“While the cells have a slight green tint, they are much more like the gray of sunglasses and automobile windows,” the researchers said in a statement.
Forrest’s version of the device was made with an indium tin oxide electrode. While a silver electrode increased the efficiency to 10.8%, with 45.8% transparency, that version's tint would limit its use in some window applications.
Overall, the test cell transparency from the study is considered to be similar to the transparency of some existing windows, advancing the technology closer to meeting consumer demand for solar panels that look and function like traditional window glass without dramatically altering the tint of the sunlight as it filters through the panes, the research team reports Monday in the journal Proceedings of the National Academy of Sciences.
“These results surpass other semitransparent solar cell technologies based on organic and other thin-film materials systems, showing a promising future for ST-OPVs as power-generating windows and other solar energy harvesting applications,” the study states.
The material created for the solar cells is “a combination of organic molecules engineered to be transparent in the visible and absorbing in the near infrared, an invisible part of the spectrum that accounts for much of the energy in sunlight.”
“The new material we developed, and the structure of the device we built, had to balance multiple trade-offs to provide good sunlight absorption, high voltage, high current, low resistance and color-neutral transparency all at the same time,” said Yongxi Li, an assistant research scientist in electrical engineering and computer science.
Study participants also developed optical coatings to simultaneously boost power and transparency — qualities that typically compete in similar designs, according to the study, which was funded by the U.S. Department of Energy Solar Energy Technologies Office, the Office of Naval Research and the Universal Display Corporation.
The practical application of these advances has significant potential.
Typically, buildings with glass facades are coated with a substance that reflects and absorbs light to reduce brightness and heating inside the structure. Forrest’s technology would allow the same buildings to capture the energy from that sunlight instead of deflecting it, allowing them to generate energy.
Currently, the United States has more than 650 skyscrapers — high-rise buildings with more than 40 floors — while China has more than 1,600. These enormous structures consume considerable energy to cool and provide electricity for the people who live in them.
Forrest and his team say that the solar cells they developed can be scaled up for mass production using materials less toxic than similar transparent solar cells. Their cells can also be placed in double-glazed windows and customized for local latitudes.
Improvements are in the works, with a goal of achieving a utilization efficiency of 7% and extending the cell life to 10 years. Next up for the team? Economic feasibility studies on retrofitting existing buildings with their advanced windows.
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