20.08.2013

Scientists develop new production technology for zinc oxide thin-films

Tailor-made nanostructures to upgrade the efficiency of solar cells

Zinc oxide is one of the most important ingredients of today’s nano materials. At present, it is often used for many applications, e.g. for gas sensors or laser materials. Extremely thin layers of zinc oxide (ZnO) are also a component of solar cells and organic semiconductors. The conditions in the production of these layers with a thickness of only a few hundred nanometres will have a decisive impact on the exact properties of the material.

The SEM picture shows: The new processing method can form different morphologiesof zinc oxide, for example circular vesicles (top) or foam like structures (bottom).

Scientists from Technische Universität München and DESY currently developed a promising way of manufacturing ZnO nano layers. This would help to increase the efficiency and the number of applications, and result in a considerable reduction of production costs. Today, the scientists published the outcome of their experiments, which were achieved – among others – with X-ray analysis at the synchrotron light source PETRA III, in the scientific journal ChemSusChem (DOI 10.1002/cssc.201300291).

The broad range of zinc oxide applications is based on the large number of inner structures that this material can adopt – the so-called morphologies. By means of different fabrication conditions, zinc oxide can be moulded into nano rods, tetra pods or ribbon-type shapes. In the production process now published by the Munich-Hamburg team of researchers, the scientists developed and systematically investigated a method, in which the environmental conditions were specifically altered, thus creating different morphologies in zinc oxide.

“With this method, we are able to arrange the zinc oxide layer on the nano scale in a foam-like or worm-like manner, and also in circles,” says first author Kuhu Sarkar from TU München.

For their experiments, the scientists used a thin-film polymer as a kind of nano scale mould for the zinc oxide structure. Dissolved in a mix of liquids, a preliminary stage of zinc oxide (ZAD) and a polymer solution were applied to a silicon substrate with the so-called spin coating process. When the solution is applied to the substrate which rotates with several thousands of revolutions per minute, it is dispersed to a uniform nano layer with the thickness of a few hundred nanometres, which subsequently dries out. During this process, the scientists altered the relative proportion of the solution components water, polymer and preliminary stage of zinc oxide.

After production, the samples were heated. In this process, the so-called calcination, water and polymer evaporate from the thin-film sample – leaving the crystalline zinc oxide structure with its morphology, varying in relation to the components of the used solution.

In their systematic investigation, the scientists developed a phase diagram which shows the dependence of the ZnO morphology from the components of the original solution. For this purpose, they analysed the nano scale structure of the ZnO layers with the extremely intensive X-ray radiation at beamline P03 of DESY’s synchrotron light source PETRA III. In a next step, the scientists will analyse all phases of the production process with synchrotron radiation. This will give them additional approaches to influence the nano layer.

“With our production technology, we want to create as many different zinc oxide crystal structures and morphologies as possible; this will enable us to precisely adjust key functionalities as conductivity and mechanical properties of the structures,” says DESY scientist Stefan Roth.

Moreover, scientists are thinking of simplifying the production processes with the use of other coating methods. This would make possible uniform nano layers on very large surfaces. “In solar cells, there are up to seven nano layers of different materials on top of each other,” says Peter Müller-Buschbaum from TU München. “The precise understanding on how to produce these layers is the key for next generations of these alternative energy carriers which are highly efficient, long-lasting and cost-effective.”