With a view to one day developing transparent solar cells that can double as windows in homes and other buildings, an international team of scientists has demonstrated a new type of transparent electrode that can function as a key building block. The breakthrough overcomes some performance issues with previous efforts in this area, and lays the groundwork for advanced tandem solar cells that combine the strengths of two separate but complimentary technologies.
Silicon-based solar cells have been a mainstay of solar technology for decades, but lately we're seeing perovskite-based cells make their mark. Ever-improving efficiencies have placed these solar cells front and center in conversations about renewable energy, and one exciting possibility includes combining them with traditional silicon-based cells to lower the cost and offer greater conversion efficiencies.
Tandem solar cells are the focus of this latest research, which actually builds on earlier work around electrode materials for perovskite solar cells. Scientists had shown how ultrathin films of gold could be used as transparent electrodes for these cells, but had struggled to create a uniform layer, which led to poor conductivity. The authors of the new study have now found that using chromium as a seed layer for the gold film to form on sidesteps these issues.
“Normally, if you grow a thin layer of something like gold, the nanoparticles will couple together and gather like small islands,” says Dong Yang, assistant research professor of materials science and engineering at Penn State. “Chromium has a large surface energy that provides a good place for the gold to grow on top of, and it actually allows the gold to form a continuous thin film.”
These ultrathin electrodes proved stable as part of a functioning perovskite cell, and demonstrated high efficiency in the team's testing. This perovskite cell on its own exhibited a 19.8 percent efficiency, which the team says is a record for a semi-transparent cell. Combining it with a silicon cell to form a tandem version resulted in an efficiency of 28.3 percent, compared to the 23.3 percent offered by the silicon cell alone.
“A five percent improvement in efficiency is giant,” says study author Shashank Priya. “This basically means you are converting about 50 watts more sunlight for every square meter of solar cell material. Solar farms can consist of thousands of modules, so that adds up to a lot of electricity, and that’s a big breakthrough.”
There is still work to do to turn these findings into a commercially available solar cell, but the scientists say that they've tackled two important issues in the development of of tandem solar cells, namely, the transparency and conductivity of the top electrode.
“Transparent solar cells could someday find a place on windows in homes and office buildings, generating electricity from sunlight that would otherwise be wasted,” says Kai Wang, assistant research professor of materials science and engineering at Penn State and co-author on the study. “This is a big step – we finally succeeded in making efficient, semitransparent solar cells.”
The research was published in the journal Nano Energy.
Source: Penn State University