Physicists at Lund University in Sweden have succeeded in making tiny solar energy-harvesting antennas (nanowires) using three different materials that fit the solar spectrum better than existing silicon solar cells. Because nanowires are lightweight and require very little material per unit area, they are now scheduled to be tested in solar-powered satellites, where efficiency is a key factor combined with light weight. A few days ago, new solar cells were sent into space.

A group of nanoengineering researchers working on solar cells from Lund University made a breakthrough last year when they were able to build photovoltaic monowires with three different bandwidths. In other words, this means that the same nanowire consists of three different materials that react to different parts of sunlight. Results published Today’s Materials Energyand later in more detail Nano Research.

“The biggest challenge was getting current flowing between the materials. It took more than a decade, but it finally paid off,” said Magnus Borgström, professor of solid state physics, who co-authored the papers with then-PhD student Lucas. Grahovina

There are about ten research groups around the world that are actively focusing on nanowire solar cells.

“The challenge was to bridge the various exclusion zones in solar cells, and now that door has finally opened,” says Magnus Borgström.

An alternative to silicon in the future

Different-bandwidth solar cells, known as tandem solar cells, have so far been predominantly found in satellites and are the subject of intense research. The aim of the research is to significantly increase efficiency, possibly doubling (about 20%) the efficiency of existing commercial silicon solar cells.

“Silicon solar cells soon reached their maximum efficiency limit. Therefore, attention is now focused on the development of tandem solar cells. “Satellite-mount options are too expensive to put on a roof,” says Magnus Borgström.

The most common way to build tandem solar cells is to stack different semiconductor materials that can absorb different parts of the solar spectrum. Silicon-based tandem solar cells are gaining a lot of attention and involve stacking thin translucent films of another light-trapping material on top of silicon.

Researchers from Lund use a slightly different approach. They developed a method for forming extremely thin rods of semiconductor material on a substrate. The advantage is a small amount of material per unit area, which can reduce production costs and be a more environmentally friendly alternative.

The nanometer-thick rods are made of three materials that contain varying amounts of indium, arsenic, gallium, and phosphorus. In the lab, the researchers have achieved an efficiency of 16.7% so far. A colleague, Yang Chen, demonstrated that nanowire solar cells have the potential to achieve 47% efficiency using the existing structure. To achieve even higher efficiency, a larger bandwidth is required.

In the next step, he and his colleagues will optimize the ternary diodes by improving the tunnel connections that connect the different materials in the structure and try to reduce the effect of the surface of the nanowires, which is crucial at the nanoscale.

In addition to improved light absorption, nanowire solar cells are characterized by durability, as they can withstand, for example, harmful radiation in space better than film-based tandem solar cells.

“A nanowire layer can be compared to a very rare layer of nails. If some aggressive protons appear from time to time, it probably doesn’t matter if they land between the wires and destroy some of the wires. If they hit the normal thin film, the damage could be worse.”

Testing in space in the spring

These advantages led to the installation of nanowire solar cells on a research satellite that was recently launched into space in the second week of January by collaborative researchers at the Cal Institute of Technology, USA.

“Most of our digital communications are controlled by satellites powered by solar panels. Satellites transmit GPS, television broadcasts, data traffic, cell phone calls, and weather data.

The satellite will be in orbit throughout the spring and results are expected to be announced on an ongoing basis. Magnus Borgström believes tandem solar cells will also emerge on Earth in the long run, but at least initially, silicon-free solar cells will be used in niche applications such as clothing, windows and decor.