Physico-Chemical nanomaterials science
The phonon confinement effect in Cu2ZnSnS4 nanocrystals and the Raman characterization of possible secondary phases
Ye.O. Havryliuk1, V.M. Dzhagan1, V.O. Yukhymchuk1, M.Ya. Valakh1
1 V.E. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, 41 pr. Nauki, 03028, Kyiv, Ukraine. 
E-mail: evgenko93@isp.kiev.ua

The Cu2ZnSn(S,Se)4-type (CZTSSe) crystals are promising alternative to Si in the photovoltaic devices. They benefit due to nearly ideal for photovoltaic direct band gap energy of 1.0–1.5 eV, the high absorption (~104 cm-1), and all of the constituents elements are earth abundant, affordable and low toxic. For the last few years the power conversion efficiency (PCE) of 13.8 % has already been reached.
Advances in the synthesis of high quality semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have opened new possibilities for novel devices, including third-generation solar cells. CZTS NCs intensively investigated also for catalytic and other applications beyond photovoltaics.
Raman spectroscopy proved to be a very efficient structure diagnostic tool for CZTS and relative compounds. The aim of this our work is the investigation of properties of CZTS NCs prepared by colloidal synthesis. Along with the commonly studied size dependence of the NC properties, for CZTS compound NCs the possible formation of secondary phases of binary and ternary chalcogenides and intrinsic defect structures is considered. In this Raman study the NC samples with different sizes were obtained by multiple fractionizing the parental NC solution. It is shown that the main phonon peak of full symmetric type shifts to higher frequencies and gets narrower towards fraction with smaller NC size. Furthermore, the critical role of depositing conditions was established. The longer drying time leads to formation of Cu2-xS as a secondary phase and transformation of the remaining CZTS phase into the disordered kesterite one. Aditional, all possible binary and ternary compounds of the CZTS constituent elements were specially synthesized in the form of QD under the same conditions. During analyzing Raman spectra of synthesized nanoparticles, it was assigned all peaks for all materials. We note that in all cases there were some differences from the known spectra of bulk samples. The reasons for this can be both the known phonon confinement effect and the ability of some compounds to exhibit enhancing RS properties when we going to nano-sizes. The possibility of the presence of several modifications in the Sn-S and Cu-Sn-S systems is confirmed. It has also been shown that ternary compounds of the Cu-Zn-S and Zn-Sn-S type do not have their own peaks, but are an alloy of two binary chalcogenides.