Power/Performance Bits: Sept. 11

Non-toxic photoluminescent nanoparticles
Researchers from Osaka University developed a way to improve display technologies using non-toxic light-emitting nanoparticles.

In trying to replace cadmium and other toxic materials used in quantum dots, scientists have turned to non-toxic nanoparticles that emit light in an efficient manner by creating I–III–VI semiconductors, such as silver indium disulfide (AgInS2). However, the color purity of previous attempts has been poor.

To address this, the researchers tried coating the silver indium disulfide nanoparticles with a semiconducting shell made of gallium and sulfur.

“We synthesized non-toxic nanoparticles in the normal way: mix all ingredients together and heat them up. The results were not fantastic, but by tweaking the synthesis conditions and modifying the nanoparticle cores and the shells we enclosed them in, we were able to achieve fantastic efficiencies and very pure colors,” said Susumu Kuwabata, a professor at Osaka University.

Structures of silver indium sulfide/gallium sulfide core/shell quantum dots and pictures of the core/shell quantum dots under room light. (Source: Osaka University)

Rather than having a rigid arrangement of atoms, the shells are made of a chaotic material without a rigid structure.

“The silver indium disulfide particles emitted purer colors after the coating with gallium sulfide. On top of that, the shell parts in microscopic images were totally amorphous. We think the less rigid nature of the shell material played an important part in that–it was more adaptable and therefore able to take on more energetically favorable conformations,” said Taro Uematsu, a researcher at Osaka.

The coating allowed the nanoparticles to perform at a level nearly comparable to cadmium chalcogenide quantum dots. The team plans to continue work on the nanoparticles.

Double layer solar cell record
Scientists at UCLA developed a highly efficient thin-film solar cell using a double layer design to generate more energy. The base layer is made of a compound of copper, indium, gallium and selenide, or CIGS, while the top is a thin layer of perovskite.

The team’s new cell converts 22.4% of the incoming energy from the sun, a record in power conversion efficiency for a perovskite-CIGS tandem solar cell. The performance was confirmed in independent tests at the U.S. Department of Energy’s National Renewable Energy Laboratory. (The previous record, set in 2015 by a group at IBM’s Thomas J. Watson Research Center, was 10.9%.) The new device’s efficiency rate is similar to that of the poly-silicon solar cells that currently dominate the photovoltaics market.

“With our tandem solar cell design, we’re drawing energy from two distinct parts of the solar spectrum over the same device area,” said Yang Yang, professor of materials science at UCLA. “This increases the amount of energy generated from sunlight compared to the CIGS layer alone.”

The cell’s CIGS base layer, which is about 2 microns thick, absorbs sunlight and generates energy at a rate of 18.7% efficiency on its own, but adding the 1 micron-thick perovskite layer improves its efficiency. The two layers are joined by a nanoscale interface that the researchers designed; the interface helps give the device higher voltage, which increases the amount of power it can export. The entire assembly sits on a glass substrate about 2 millimeters thick.

Yang noted that devices using the two-layer design could eventually approach 30% power conversion efficiency, a goal the group will be working toward.