Major leap towards graphene for solar cells
Graphene was deposited onto a glass substrate. The ultrathin layer is but one atomic layer thick (0.3 Angström, or 0.03 nanometers), although charge carriers are able to move about freely within this layer. This property is retained even if the graphene layer is covered with amorphous or polycrystalline silicon. © Marc A. Gluba/HZB
Surprising result: Graphen retains its properties even when coated with silicon
Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light - at least in theory. Whether or not this holds true in a real worldsetting is questionable as there is no such thing as "ideal" graphene - a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: interactions with adjacent layers can change graphene's properties dramatically. Now, Dr. Marc Gluba and Prof. Dr. Norbert Nickel of the HZB Institute for Silicon Photovoltaics have shown that graphene retains its impressive set of properties when it is coated with a thin siliconfilm. These findings have paved the way for entirely new possibilities to use in thin-film photovoltaics.
"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," Marc Gluba explains.
To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glas; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.
Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees C, the graphene is still detectable.
"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel. Their measurements of carrier mobility using the Hall-effect showed that the mobility of charge carriers within the embedded graphene layer is roughly 30 times greater than that of conventional zinc oxide based contact layers. Says Gluba: "Admittedly, it's been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We're still having to work on that." Adds Nickel: "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it."
The researchers obtained their measurements on one square centimeter samples, although in practice it is feasible to coat much larger areas than that with graphene.
This work was recently published in Applied Physics Letters Vol. 103, 073102 (2013).
Authors: M. A. Gluba, D. Amkreutz, G. V. Troppenz, J. Rappich, and N. H. Nickel
doi: 10.1063/1.4818461
- New Helmholtz Young Investigator Group at HZB on perovskite solar cellsSilvia Mariotti starts building up the new Helmholtz Young Investigator Group ‘Perovskite-based multi-junction solar cells’. The perovskite expert, who was previously based at Okinawa University in Japan, aims to advance the development of multi-junction solar cells made from different perovskite layers.
- Hydrogen storage in MXene: It all depends on diffusion processesTwo-dimensional (2D) materials such as MXene are of great interest for hydrogen storage. An expert from HZB has investigated the diffusion of hydrogen in MXene using density functional theory. This modelling provides valuable insights into the key diffusion mechanisms and hydrogen's interaction with Ti₃C₂ MXene, offering a solid foundation for further experimental research.
- HZB and National University Kyiv-Mohyla Academy start cooperation in Energy and ClimateHelmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB) and the National University of "Kyiv-Mohyla Academy" (NaUKMA) have signed a Memorandum of Understanding (MoU). The MoU serves as the starting point for collaborative research, academic exchange, and capacity-building between the two institutions. Actions will be taken to establish the Joint Research and Policy Laboratory at NaUKMA in Kyiv. The aim of the future laboratory is to jointly develop research and policy analysis, focusing on the energy and climate dimensions of Ukraine’s EU integration.