Keywords: solar energy (232)

Science Highlight    07.12.2017

Solar energy: Defects in Kesterite semiconductors studied using neutrons

Backscattered electron micrograph of kesterite powder. Grey grains are attributed to CZTSe. Black background is the epoxy matrix.
Copyright: HZB

Degree of the Cu/Zn disorder of the kesterite phase according with the cation ratios Cu/(Zn+Sn) and Zn/Sn.
Copyright: HZB

A research team at the HZB has precisely characterised for the first time the various types of defects in kesterite semiconductors. They achieved this with the help of neutron scattering at the BER II research reactor and at Oak Ridge National Laboratory in the USA. The findings point to a means of guided optimisation for kesterite solar cells.

Kesterites are an economical and environmentally friendly materials which are semiconducting as well as being able to convert light into electricity. Concerning their crystal structure, kesterites resemble species of chalcopyrite semiconductors with the chemical formula Cu(In,Ga)Se2. In kesterites such as Cu2ZnSnSe4 (CZTSe) though, the rare-earth elements indium and gallium are replaced by zinc and tin, which are abundant in the earth’s crust and are far less expensive. Currently, the best kesterite-based thin film solar cells achieve an efficiency levels of up to 12.6 per cent with the kesterite layer only one micron thick. They are therefore interesting candidates for highly cost-effective as well as flexible solar modules that are non-toxic and robust. Even though single-crystal silicon solar cells or thin layers of chalcopyrite achieve efficiency levels of far more than 20 per cent, these are considerably more expensive.

Some defects may increase efficiencies

The highest efficiency levels are achieved in kesterite-thin film solar cells with CZTSe-absorber layers that contain somewhat less copper in comparison to the chemical formula and somewhat more zinc, i.e. present a copper-poor/zinc-rich composition. This kind of stoichiometric deviation leads to defect sites in the crystal structure that apparently increase the efficiency level, though. “We have now precisely characterised these defect sites and ascertained their local concentrations with the help of neutron diffraction experiments”, explains Dr. Galina Gurieva from the HZB Structure and Dynamics of Energy Materials group.  

Neutron diffraction to catalogue defect types

Neutron diffraction is the ideally suited method for this purpose. The two elements (Cu+ and Zn2+) can be extremely well differentiated from one another using neutrons, which is not the case for conventional X-ray diffraction studies. The team investigated 29 different CZTSe powder samples using the neutron sources SNS at Oak Ridge National Laboratory (USA) and BER II at the HZB. They were successful in their effort to exactly catalogue various defect types. For instance, they were able to identify sites in the crystal structure at which a copper atom was missing (copper vacancies, VCu), as well as sites that were occupied by another element instead of the anticipated element, such as a zinc atom in place of a tin atom (ZnSn anti site).

Annealing disorder

Beside the presence of point defects, which can be explained via stoichiometry deviations, the so called Copper/Zinc disorder plays an important role as well: Disorder means that copper atoms are sitting on Zinc sites in the crystal structure and vice versa. This disorder can be reduced via a heat treatment (annealing), which enhances often the efficiency of CZTSe based solar cells.

Concrete clues for further optimisation

 “The point defects we investigated experimentally really correspond quite well to the theoretical model of potential defects”, declared Gurieva. “We can deduce concrete clues from this study as to which point defects in which concentration to expect in the given composition of kesterite thin films ”, says Gurieva. “This may help to optimise kesterite based solar cells further.”

 

Publication in Journal of Applied Physics (2018): Intrinsic point defects in off-stoichiometric Cu2ZnSnSe4; Galina Gurieva, Laura Elisa Valle Rios, Alexandra Franz, Pamela Whitfield, Susan Schorr.

DOI:10.1063/1.4997402

 

arö


           



You might also be interested in
  • <p>Marcus B&auml;r, here in EMIL lab at HZB, has accepted a professorship at FAU in South-Germany.</p>NEWS      11.01.2019

    Marcus Bär accepts W2 professorship for X-ray spectroscopy in Erlangen-Nuremberg

    Prof. Marcus Bär has accepted a professorship for X-ray spectroscopy at the Friedrich-Alexander Universität Erlangen-Nürnberg (FAU). Bär heads the Department of Interface Design at Helmholtz-Zentrum Berlin (HZB). The new W2 professorship was established in cooperation with HZB and Forschungszentrum Jülich in order to strengthen the Helmholtz-Institute Erlangen-Nürnberg für Renewable Energy (HI ERN). In the future, Bär will also be working on HI ERN research topics at HZB, thereby contributing to the intensification of cooperation. [...]


  • <p>Marko Topic, Bernd Rech, Janez Krc, Benjamin Lipovsek, Steve Albrecht, Marko Jost, Borut Pahor (f.l.t.r)</p>NEWS      19.12.2018

    Slovenian president awards HZB scientist with "Apple of Inspiration"

    Marko Jošt, Steve Albrecht and Bernd Rech, researchers at the Helmholtz-Zentrum Berlin (HZB) and the Technische Universität Berlin (TUB) receive a rare award today, 19 December 2018. In the slovenian Grand Hall of Presidential Palace, the Slovenian President, Borut Pahor, presents them with the "Apple of Inspiration". It honours achievements in culture, science, sport and society. The Berlin scientists are the first foreigners who receive the prize together with colleagues from the University of Ljubljana. [...]


  • <p>Steve Albrecht is heading the junior investigator group "Perovskit Tandem Solar Cells" at the HZB.</p>NEWS      19.12.2018

    Steve Albrecht is Junior Professor at the Technical University of Berlin

    The Technische Universität Berlin (TU) and the Helmholtz-Zentrum Berlin (HZB) have appointed Prof. Dr. Steve Albrecht to the joint junior professorship "perovskite solar cells" as of December 15, 2018. Since 2016, Albrecht has headed the junior investigator group "Perovskit Tandem Solar Cells" at the HZB, which is funded by the Federal Ministry of Education and Research.

    [...]




Newsletter