Synthesis of Cu 2 (ZnCd)SnS 4 Absorber Material for Monograin Membrane Applications
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Synthesis of Cu2(ZnCd)SnS4 Absorber Material for Monograin Membrane Applications Godswill Nkwusi1, Inga Leinemann1, Jaan Raudoja1, Valdek Mikli1, Marit Kauk-Kuusik1 Mare Altosaar1, Enn Mellikov1 Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia ABSTRACT CZTS monograin powder samples were synthesized in CdI2 as flux material. The obtained materials were analysed by EDX, SEM, and Raman methods. It was found that Cd from flux was incorporated into the formed compound leading to the formation of solid solution Cu2Zn1-xCdxSnS4. The content of Cd in the compound was studied in the dependence of synthesis temperature and time. It was found that Cd content in the formed Cu2Zn1-xCdxSnS4 did not depend on synthesis duration at constant temperature and increased with temperature. The activation energy of the Cd incorporation process was estimated as 17.5 ± 2 kJ/mol. Keyword: CZTS, monograin powder, CdI2 flux, Cd incorporation INTRODUCTION Kesterite structured Cu2ZnSnS4 (CZTS) as an absorber material for solar cell is a promising semiconductor for low-cost and sustainable energy production [1-5]. Thin film technology of quaternary CZTS solar cell has been widely studied over the last decade with the contribution of different authors aimed at understanding the best preparation conditions and to improve the solar cell efficiency, that is currently 11.1 % for Cu2ZnSn(S, Se)4 synthesized in hydrazine [6]. Monograin technology has been developed as a possible alternative to thin film technology with current CZTS monograin layer (MGL) solar cell efficiency of over 8%. In the monograin technology, synthesis and isothermal recrystallization of polycrystalline powders in the presence of the liquid phase of a suitable solvent (flux) in sufficient amount, aids the formation of powders with single crystalline structure of powder grains [2]. CZTS in monograin form has been grown in different flux materials such as KI, NaI and CdI2 [7-10] . Cadmium Iodide (CdI2) had been used as a low temperature flux in our previous report [11]. It was confirmed that CZTS monograin powder can be grown in molten CdI2 at lower temperatures relative to the melting temperature of KI [12]. According to Klavina I, et al, it is known that during the synthesis of CZTS in CdI2 as flux , Cd incorporates into the crystals of CZTS forming a solid solution of Cu2(CdZn)SnS4. However, how to reach single phase absorber material in monograin powder form and how to avoid the formation of by-products was not discussed. Also, the incorporation of Cd from CdI2 into CZTS crystals affects the properties of solar cell absorber materials. Therefore, the regularities and extent of Cd incorporation into CZTS have been studied in this report. The influence of synthesis temperature and time on the Cd incorporation into CZTS is reported. Finally, based on the obtained experimental data the activation energy of the Cd incorporation was determined. EXPERIMENTAL DETAILS Copper Zinc Tin Sulfide (CZTS) monograin powder was synthesised
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