

“For a photovoltaic material, obviously absorbing light is the first part and converting that solar energy into electrical energy is the second part,” says Ames Laboratory scientist Emily Smith. “So you want a material that does both efficiently. Germanium has some desirable characteristics for photovoltaic materials, but unfortunately it doesn’t absorb light well.”


“We aren’t actually measuring absorption,” Smith explains. “We measure the luminescence — the amount light given off after a photon is absorbed.”

“The fact that germanium doesn’t absorb light well is a simple way of saying it’s an indirect bandgap material,” Smith adds, “and we are trying to make a more direct bandgap material, one that absorbs light better.”



The specific method used is called Successive Ion Layer Adsorption and Reaction, or “SILAR,” which was first adapted to group IV colloids several years ago.


"We have been developing the expertise required to grow intricate core/shell and other well defined nanoparticles for many years," Vela says. “Through our collaboration with Emily Smith’s group, we hope to continue making inroads in our ability to manipulate and direct energy flows at the nanoscale."


“The atoms are in a very specific location within the nanocrystal core and when you apply the shell around the nanocrystal, the atoms of the shell may not match perfectly with the atoms of the core,” Smith says. “With the germanium only material used previously, the core and shell didn’t match perfectly.”

“When we studied the germanium-tin particles, we proposed that they worked better because the spacing of the atoms better matches the spacing of the atoms we used in the coat layer,” she says. “By doing that, you’re getting a more perfect shell that’s less likely to cause chemical changes to the surface of the nanoparticle core.”



Source: Ames Laboratory