Solar absorbers using CZTS nanocrystals: new insights in their properties from Raman spectroscopy Yevhenii Havryliuk 1 , V. Dzhagan 3 , O. Selyshchev 1,2 , D.R.T. Zahn 1,2 1 Chemnitz University of Technology, Germany, 2 Center for Materials, Architectures, and Integration of Nanomembranes (MAIN), Germany, 3 V.E. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Ukraine Quaternary semiconductor Cu 2 ZnSnS 4 (CZTS) is a promising material for photovoltaic applications. The conversion efficiency already reached about 13%, but further improvement is precluded by coexistence of multiple structural modifications, impurity phases, antisite and other point defects. Most of these impurities can be probed by Raman spectroscopy, some even more reliably than by such conventional structural tool as X-ray diffraction (XRD). Unlike strongly overlapping diffraction patterns of CZTS and secondary phases, their vibrational patterns are very different. Further advantage of Raman spectroscopy is the ability to selectively detect tiny inclusions of other phases by resonantly exciting them with a suitable laser excitation. Furthermore, fast photoinduced effects can be detected. Here we investigate the CZTS NCs obtained by a low-temperature "green" aqueous colloidal synthesis in the form of liquid "inks". Raman spectra of the as-synthesized solutions showed the NCs possess kesterite structure. In order to clarify the possibility of the formation of secondary phases, NC samples of potential compounds were prepared under the same conditions. Then size-selected CZTS NC samples produced by fractionationing the parental colloids allowed the effect of phonon confinement to be separated from other factors influencing the Raman spectra. We found that for CZTS NCs the phonon-confinement effect of the strongest Raman-active mode is opposite to that of most semiconductors due to an anomalous phonon dispersion of that mode in CZTS. Most device applications require thin films with well defined parameters. We investigated CZTS NC films formed by drop-casting, spin- and spray-coating, as simple, fast, and scalable fabrication methods. We found that the conditions of film formation critically influence its crystallinity and other properties. In particular, a long drying time and the presence of humidity lead to the formation of the secondary phase Cu 2-x S and a transition from the ordered to the disordered kesterite phase. Improving the film crystallinity by flash-lamp annealing was observed in a certain range light powers. Summarizing, Raman spectroscopy has proven to be a very efficient tool for rapid and non-destructive structural characterization of CZTS NC films and excellently complements the commonly used techniques based on diffraction. References 1. O. Selyshchev, Ye. Havryliuk, M.Y. Valakh, V. O. Yukhymchuk, O. Raievska, O. L. Stroyuk, V. Dzhagan and D.R.T. Zahn, ACS Appl. Nano Mater. , 2020, 3 , 5706–5717. 2. Ye. Havryliuk, O. Selyshchev, M. Valakh, A. Raevskaya, O. Stroyuk, C. Schmidt, V. Dzhagan and D.R.T. Zahn, Beilstein J. Nanotechnol. , 2019, 10 , 222–227. 3. Ye. Havryliuk, M.Y. Valakh, V. Dzhagan, O. Greshchuk, V. Yukhymchuk, A. Raevskaya, O. Stroyuk, O. Selyshchev, N. Gaponik and D.R.T. Zahn, RSC Adv. , 2018, 8 , 30736–30746.
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