Semiconductor, Heal Thyself


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Prof. David Cahen, Dr. Leeor Kronik. Movement in solid material

Self-healing is normally the province of living creatures, but now a Weizmann Institute-led research team has discovered that it can occur in semiconductors as well. This finding 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 be built only from materials that are either very expensive or unstable. One type of experimental semiconductor could provide a solution. Copper indium gallium diselenide is inexpensive 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 outer space.
Now this mystery has been solved by an international team headed by 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. That study, conducted by Cahen and his colleagues at the European Synchrotron Radiation Facility in Grenoble, showed that in some cases the bonds between certain atoms of copper indium diselenide can be broken relatively easily.
Cahen's group also showed that copper atoms can move inside these semiconductor crystals. This finding was most surprising: Such movement is uncommon in solid, nonliving materials and is extremely unusual in materials used in electronic devices, where atomic mobility is viewed as ana?thema. Moreover, seeing it in a semiconductor known for its stability was particularly unexpected.
Wandering copper atoms
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 to 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 by 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 in designing other self-stabilizing materials.