Manufacturing technique pushes black silicon closer to commercialisation

2 min read

Scientists in the US have simplified the manufacture of solar cells using the top electrode as the catalyst that turns plain silicon into valuable black silicon.

According to the team, which has been working to fine tune the creation of black silicon, an advance in the manufacturing technique should push it closer to commercialisation.

The work led by postdoctoral researcher Yen-Tien Lu had two major attractions, according to chemist Andrew Barron of Rice University, Texas.

Gold electrodes also serve as catalysts in a process developed at Rice University to create black silicon for solar cells. Black silicon reflects little light and allows more to reach the active elements of solar cells to be turned into electricity
Gold electrodes also serve as catalysts in a process developed at Rice University to create black silicon for solar cells. Black silicon reflects little light and allows more to reach the active elements of solar cells to be turned into electricity

“One, removing steps from the process is always good,” he said in a statement. “Two, this is the first time in which metallisation is a catalyst for a reaction that occurs several millimeters away.”

The metal layer used as a top electrode is usually applied last in solar cell manufacturing, Barron said. The new method, known as contact-assisted chemical etching, applies the set of thin gold lines that serve as the electrode earlier in the process. This also eliminates the need to remove used catalyst particles.

The researchers discovered that etching in a chemical bath takes place a set distance from the lines. That distance appears to be connected to the silicon’s semiconducting properties, Barron said.

“Yen-Tien was doing the reaction with gold top contacts, adding silver or gold catalyst and getting these beautiful pictures,” he added. “I said, ‘OK, fine. Now let’s do it without the catalysts.’ Suddenly, we got black silicon, but it was etching only a certain distance away from the contact. And no matter what we did, there was always that distance.

“It told us the electrochemical reaction is occurring at the metal contact and at the silicon that’s a certain distance away.

“The distance is dependent on the charge-carrying capacity, the conductivity, of the silicon. At some point, the conductivity isn’t sufficient for the charge to carry any further.”

An electron microscope image from earlier research shows the nanoscale spikes that make up the surface of black silicon used in solar cells
An electron microscope image from earlier research shows the nanoscale spikes that make up the surface of black silicon used in solar cells

An extremely thin layer of gold atop titanium, which bonds well with both gold and silicon, should be an effective electrode that also serves for catalysis, Barron said.

“The trick is to etch the valleys deep enough to eliminate the reflection of sunlight while not going so deep that you cause a short circuit in the cell,” he added.

The electrode’s ability to act as a catalyst suggests other electronic manufacturing processes may benefit from a bit of shuffling, Barron said.

“Metal contacts are normally put down last. It begs the question for a lot of processes of whether to put the contact down earlier and use it to do the chemistry for the rest of the process,” he added.

Black silicon has a highly textured surface of nanoscale spikes or pores that are smaller than the wavelength of light. The texture allows the efficient collection of light from any angle at any time of day.

The research was published in ACS Applied Materials and Interfaces.