The smarter E 2018

••• 13••• Innovationen ThesmarterEEurope Better opto-electronic properties than tin Germanium improves the suitability of kesterite-type semiconductors’ as absorber layers in solar cells K esterites are semiconductor compounds made of the ele- ments copper, tin, zinc, and sele- nium. These semiconductors can be used as an optical absorber material in solar cells, but so far their maximum efficiency is only 12.6 per cent, while solar cells made of copper-indium-gallium- selenide (CIGS) already attain ef- ficiencies of over 20 per cent. But kesterites are considered in- teresting alternatives to CIGS so- lar cells because they consist of common elements. Researchers at the Helmholtz-Zentrum Berlin (HZB) have investigated kesterite samples and shed light on the re- lationship between composition and the opto-electronic proper- ties. During the synthesis of the samples, the tin atoms were re- placed with germanium. The re- searchers then investigated these samples using neutron diffraction at BER II. Copper, zinc, and germa- nium can be distinguished from each other particularly well with this method and their positions be located in the crystal lattice. The result: kesterites with a slightly copper-poor and zinc-rich composition found in solar cells with the highest efficiencies also have the lowest concentration of point defects as well as the low- est disorder of copper-zinc. The more the composition was en- riched with copper, the higher the concentration was of other point defects considered to be detrimental to the performance of solar cells. Further investiga- tions showed how the energy band gap depends on the com- position of the kesterite pow- der samples. “This band gap is a characteristic of semiconductors and determines which frequen- cies of light release charge carri- ers within the material”, explains René Gunder. “We now know that germanium increases the optical band gap, allowing the material to convert a greater pro- portion of sunlight into electrical energy.” The picture shows the typical arrangement of cations in a kesterite type structure. In the back- ground the crystal structure is shown, a unit cell is highlighted. Photo: HZB Welser Profile GmbH Prochenberg 24 3341 Ybbsitz at@welser.com www.welser.com Neue Perspektiven gewinnen Besuchen Sie uns auf der Intersolar! Halle A3, Stand 105