Nanocomposites and Nanomaterials

Raman investigations of Cu2ZnSn(SxSe1-x)4 nanocrystals with different chemical composition
S. Kakherskyi, R. Pshenychnyi, О. Dobrozhan, M. Ermakov, A. Opanasyuk
Department of Electronics and Computer Technology, Sumy State University. Rimsky-Korsakov str., 2, Sumy, UA-40007, Ukraine. 
E-mail: s.kacherski@ekt.sumdu.edu.ua

Thin-film solar cells (SC) based on the absorbers of Cu (In, Ga) (S, Se)2 (CIGS) and cadmium telluride (CdTe) absorbent materials are considered as an alternative to silicon-based devices due to their lower cost and large-scale ground use. . However, they have not been widely used due to the lack of In and Ga (CIGS) in the earth's crust or use during the production of environmentally hazardous Cd, both in the absorption (CdTe) and in the window layer (CdS), which creates problems in their disposal.
Recently, there is an active study of the five-component compound Cu2ZnSn(SxSe1-x)4 (CZTSSe) for its use as absorbers of thin-film SC. This material has a direct bandgap with a width corresponding to the Shockley-Kweiser optimum, a high coefficient of optical absorption, fairly large mobility of charge carriers, and so on. The compound contains only widespread in the earth's crust and harmless elements, cheap in extraction. However, to reduce the cost of production, the corresponding SC must be manufactured using low-energy non-vacuum methods, for example, using the method of spray pyrolysis, sol-gel, solvothermal, or other chemical methods.
We are developing the technology of printing SC absorbers with printers with the replacement of ordinary inks with specially developed ones, which are created based on CZTSSe nanocrystals suspension (NC). To create such nanoinks, NC compounds of different chemical compositions were synthesized and their chemical and structural-phase composition were studied using X-ray and Raman spectroscopy methods.
NCs solid solutions were obtained by polyol synthesis in triethylene glycol (TEG) at temperature 280 0С in an inert argon atmosphere. A mixture of salts CuCl22H2O, Zn(CH3COO)22H2O, SnCl22H2O for synthesis was taken in a molar ratio of 0.6: 1: 0.45 to increase the zinc content in the final product. The elementary S and Se were introduced into the solution in molar ratios xS : (1-x)Sе (х = 0; 0,1; 0,25; 0,4; 0,5; 0,6; 0,75; 0,9; 1). 0.150 g of polyvinylpyrrolidone (PVP) was used as a stabilizer of the formed NCs final phase. A feature of the synthesis was the use of amorphous selenium and sulfur as sources of chalcogens, instead of expensive selenium urea and traditional thiourea.
After reaching a temperature of 120 ° C, the air was pumped out of the flask and inert argon gas was charged. The mixture was incubated for 30 minutes to dissolve PVP and quickly heated to the synthesis temperature. After exposure to the synthesis temperature, the mixture was cooled to room temperature and the synthesized product was separated from TEG and dissolved PVP by centrifugation. Residues of the organic component were washed with ethanol with vigorous shaking followed by centrifugation. Washed woofer was dried at 60 ° C for 12 hours.

Fig 1. Raman Spectra of CZTSSe nanocrystals with different chemical composition: х = 1- 0; 2- 0,1; 3- 0,25; 4- 0,4; 5- 0,5; 6- 0,6; 7- 0,75; 8- 0,9; 9-1 

A DRON 4-07 diffractometer was used to determine the structural characteristics of the synthesized LFs. The analysis was performed in Ni-filtered Kα- radiation of the copper anode. Measurements were performed in the mode of continuous registration (speed -16 deg/min, step 0.1 deg) in the range 2θ to 20 to 80, where 2θ - Bragg angle. The study of Raman spectra was performed using a RENISHAW inVia Reflex microspectrometer and a solid-state laser with a wavelength of 785 nm as excitation sources.
In fig. 1 shows Raman spectra from nanocrystals with different chemical compositions. The presence of the main peak of CZTS - 332 cm-1 confirms the synthesis of nanocrystals with the required chemical composition and single-phase structure to create nanoinks. At the same time, particles with a high content of selenium were still two-phase.
It was found that the NCs of all studied compositions had a kesterite structure.