July 1, 1999 Self-healing is normally the province of living creatures, but now a Weizmann Institute-led research team has discovered that it can occur in a semiconductor. This finding, presented in June at the European Materials Research Conference in Strasbourg and soon to be published in the scientific journal Advanced Materials, may help create better solar cells and other electronic devices.
Solar cells, which convert sunlight into electricity, could offer a perfect way of using solar energy. But, unfortunately, such devices can only be built from materials that are either very expensive or unstable with respect to radiation or other environmental factors.
One type of experimental semiconductor could provide an answer. Copper indium gallium diselenide doesn't cost much because only very small amounts of it are needed. It is also extremely stable, a characteristic that has long baffled the scientific community because it appears to defy common sense: Copper indium gallium diselenide is so complex that one would expect it to be easily disrupted, yet it manages to survive intact for long periods of time under harsh conditions, including those present in space.
Now this mystery has been solved by an international team consisting of Prof. David Cahen of the Weizmann Institute's Materials and Interfaces Department, working with consultant Dr. Leeor Kronik of Tel Aviv University and colleagues from France's CNRS and Germany's Stuttgart University.
Their discovery is based, among other things, on a study in which crystals of a related material, copper indium diselenide, were examined using high-energy X-rays. In that study, conducted by Cahen and other colleagues at the European Synchrotron Research Facility in Grenoble, it was shown that in some cases the bonds between certain atoms of copper indium diselenide can be broken relatively easily.
Cahen's group had also shown that copper atoms can move inside these semiconductor crystals. This finding was most surprising: Such movement is uncommon in solid, nonliving materials, and extremely unusual in materials used in electronic devices, where atomic mobility is viewed as anathema. Moreover, seeing it in a semiconductor known for its stability was particularly unexpected.
Another even more surprising finding provided the explanation for the material's mysterious stability. Once some atomic bonds have been broken, the copper atoms, which are capable of moving throughout the crystal, wander around until they reach the damaged spot and undo the effects of the damage. This "self-repair" mechanism stems from the material's tendency to try and stay close to equilibrium.
"Now we understand how solar cells made of copper indium gallium diselenide manage to survive and function effectively in hostile environments such as those encountered on satellites: Once damaged, for example by radiation, this 'smart' material simply 'heals' itself and restores its previous function," Cahen says.
This research may lead to more extensive use of copper indium gallium diselenide and help design other self-stabilizing materials.
Funding for this research was provided by the German Federal Ministry of Education, Science, Research and Technology and the Israel Ministry of Science, as well as by the German-Israeli Foundation for Scientific Research and Development, the Israel Science Foundation and the Minerva Foundation.
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