Kesterite solar cells: germanium promises better opto-electronic properties than tin
The picture shows the typical arrangement of cations in a kesterite type structure. In the background the crystal structure is shown, a unit cell is highlighted. © HZB
Specific changes in the composition of kesterite-type semiconductors make it possible to improve their suitability as absorber layers in solar cells. As a team at the Helmholtz-Zentrum Berlin showed, this is particularly true for kesterites in which tin was replaced by germanium. The scientists examined the samples using neutron diffraction at BER II and other methods. The work was selected for the cover of the journal CrystEngComm.
Kesterites are semiconductor compounds made of the elements copper, tin, zinc, and selenium. These semiconductors can be used as an optical absorber material in solar cells, but so far have only achieved a maximum efficiency of 12.6 per cent, while solar cells made of copper-indium-gallium-selenide (CIGS) already attain efficiencies of over 20 percent. Nevertheless, kesterites are considered interesting alternatives to CIGS solar cells because they consist of common elements, so that no supply bottlenecks are to be expected. A team led by Professor Susan Schorr at the HZB has now investigated a series of non-stoichiometric kesterite samples and shed light on the relationship between composition and the opto-electronic properties. During the synthesis of the samples at the HZB, the tin atoms were replaced with germanium.
Neutron diffraction at BER II
The researchers then investigated these samples using neutron diffraction at BER II. Copper, zinc, and germanium can be distinguished from each other particularly well with this method, and their positions can 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 lowest disorder of copper-zinc. The more the composition was enriched with copper, the higher the concentration was of other point defects considered to be detrimental to the performance of solar cells. Further investigations showed how the energy band gap, as it is known, depends on the composition of the kesterite powder samples.
The effects of Germanium
“This band gap is a characteristic of semiconductors and determines which frequencies of light release charge carriers within the material”, explains René Gunder, first author of the work. “We now know that germanium increases the optical band gap, allowing the material to convert a greater proportion of sunlight into electrical energy.”
Kesterites: Candidate for solar cells and photocatalysts
“We are convinced that these kinds of kesterites are not only suitable for solar cells, but can also be considered for other applications. Kesterites acting as photocatalysts might be able to split water into hydrogen and oxygen using sunlight, and to store solar energy in the form of chemical energy,” explains Schorr.
Published in CrystEngComm (2018): “Structural characterization of off-stoichiometric kesterite-type Cu2ZnGeSe4 compound semiconductors: From cation distribution to intrinsic point defect density”; R. Gunder, J. A. Márquez-Prieto, G. Gurieva, T. Unold and S. Schorr
DOI: 10. 1039/c7ce02090b
The last news on kesterite research was published in December 2017
- Thin nickel films boost green hydrogen productionUltra-thin nickel oxide coatings can significantly improve the performance of anion exchange membrane (AEM) water electrolysers, a promising technology for producing green hydrogen. This was demonstrated by a team led by HZB scientist Dr. Michelle Browne. The results show that a nickel oxide film only 10 nanometres thick outperformed conventional nickel oxide powder electrodes while using less than 1% of the catalyst material. The study is published in the Journal of Materials Chemistry A,
- Detailed insights at BESSY II into the oxidation processes of copperBefore pure copper oxide builds up, complex superstructures such as ‘29’CuxO, are formed. The new results on the '29' CuxO superstructure have relevance for catalyst research and corrosion protection in the development of safe copper containers for nuclear waste repositories.
- BESSY II: Evaporated perovskites in tandem solar cells improvedPerovskite-silicon tandem solar cells achieve significantly higher efficiencies than silicon solar cells on their own. One particularly attractive method is co-evaporation of the perovskite precursor molecules on top of the silicon subcell. Scientists at HZB have analysed film growth on the nanoscale at BESSY II and found a new way to improve the quality of the perovskite layer: adding a thin seed layer of caesium chloride between the two sub-cells promotes uniform perovskite growth and suppresses the formation of undesired lead iodide at the interface.