Rapid, low-temperature process improves kesterite solar cell prospects

Jan. 27, 2014

Berlin, Germany and Limerick, Ireland--HZB and University of Limerick researchers have discovered a novel solid-state reaction that lets kesterite grains grow within a few seconds and at relatively low temperatures.

Gail Overton

The transformation from a layer of closely packed nanorods (top left) to a polycrystalline semiconductor thin film (top right) can be observed in by in-situ X-ray diffraction in real time. The intensities of the diffraction signals are color coded in the image at the bottom. A detailed analysis of the signals reveals that the transformation of the nanorods into kesterite crystals takes only 9 to 18 seconds.

Berlin, Germany and Limerick, Ireland--Helmholtz Zentrum Berlin (HZB) and University of Limerick researchers have discovered a novel solid-state reaction that lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin-film solar cells.

With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick fabricated films of highly ordered wurtzite nanorods that have exactly the same composition as kesterite--Cu₂ZnSnS₄. With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold observed how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. "It is interesting to see that the complete formation of the kesterite film is so fast," says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form--instead of a few larger grains. In addition, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.

Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model that can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.

The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu₂ZnSnS₄-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se₂ chalcopyrite solar cells that already achieve efficiencies above 20%. Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements that are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. "But we continue to focus on kesterites, because this is a really exciting topic at the moment."

The research is published here in Nature Communications.

SOURCE: Helmholtz Zentrum Berlin; http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13909&sprache=en&typoid=49880

About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.