Can nanostructured germanium pave the way to portable photovoltaics?

December 07, 2015 // By Paul Buckley
Researchers at the Technical University of Munich (TUM) and the Ludwig Maximillians University of Munich (LMU) have discovered a procedure using nanostructured germanium to produce thin robust and porous semiconductor layers for portable photovoltaics and battery electrodes.

The material is ideal for use in small, light-weight, flexible solar cells or electrodes that improve the performance of rechargeable batteries.

By integrating suitable organic polymers into the pores of the material, the scientists can custom tailor the electrical properties of the ensuing hybrid material. The design not only saves space, it also creates large interface surfaces that improve overall effectiveness.

“You can imagine our raw material as a porous scaffold with a structure akin to a honeycomb. The walls comprise inorganic, semiconducting germanium, which can produce and store electric charges. Since the honeycomb walls are extremely thin, charges can flow along short paths,” explained Professor Thomas Fässler, chair of Inorganic Chemistry with a Focus on Novel Materials at TU Munich.

To transform brittle, hard germanium into a flexible and porous layer the researchers had to apply a few tricks. Traditionally, etching processes are used to structure the surface of germanium. However, the top-down approach is difficult to control on an atomic level. The new procedure solves the problem.

Together with his team, Fässler established a synthesis methodology to fabricate the desired structures very precisely and reproducibly. The raw material is germanium with atoms arranged in clusters of nine. Since these clusters are electrically charged, they repel each other as long as they are dissolved. Netting only takes place when the solvent is evaporated.

Netting can be easily achieved by applying heat of 500°C or it can be chemically induced, by adding germanium chloride, for example. By using other chlorides like phosphorous chloride the germanium structures can be easily doped. This allows the researchers to directly adjust the properties of the resulting nanomaterials in a targeted manner.

To give the germanium clusters the desired porous structure, the LMU researcher Dr. Dina Fattakhova-Rohlfing has developed a methodology to enable nanostructuring: Tiny polymer beads form three-dimensional templates in an initial step.

In the next step, the germanium-cluster solution fills the gaps between the beads. As soon as stable germanium networks have formed on the surface of the tiny beads, the templates are removed by applying heat. What remains is the highly porous nanofilm.

The deployed polymer beads have a diameter of 50 to 200 nanometers and form an opal structure. The germanium scaffold that emerges on the surface acts as a negative mold – an inverse opal structure is formed which is why the nanolayers shimmer like an opal.