Low Temperature Growth of Cu2ZnSnS4 Thin Films By Solution-Based Mist Chemical Vapor Deposition Method
Authors: Ikenoue Takumi , Watanabe Yuichiro , Miyake Masao , Hirato Tetsuji 
Published: February 2020

Introduction
The Cu-III-VI2 chalcopyrite semiconductors such as Cu(In,Ga)(S,Se)2 (CIGSSe) have been proved to be successful candidate for photovoltaic applications. CIGSSe thin film solar cells show high power conversion efficiencies over 20 % and these results promoted to explore other related semiconductors. Cu2ZnSnS4 (CZTS) is one of the promising semiconductor for absorber layer in thin film solar cells. The elements zinc (Zn) and tin (Sn) in CZTS are nontoxic and relatively abundant compared to indium (In) and gallium (Ga) in CIGSSe. CZTS thin films have a direct band gap of about 1.5 eV, which is ideal for solar cells, p-type conductivity, and high absorption coefficient (> 104 cm-1).
Various methods have been applied for CZTS thin film fabrication, including sputtering, evaporation, sulfurization of precursors, spray pyrolysis and chemical techniques. However these techniques have several problems such as high energy consumption, not suitable for large-area fabrication, and require high temperature anneal and/or sulfurization processes. To overcome the above problems, we applied a mist chemical vapor deposition (CVD) technique to fabricate CZTS thin films. This technique allows high quality film formation in atmospheric pressure without vacuum equipment and is able to be easily expanded for large area substrates. In addition, the mist CVD have already realized the thin film formations of transparent conductive oxides (TCOs) which constitute CZTS-based solar cells. A wide variety of precursor materials have been considered to be grown at lower growth temperatures using mist CVD method. In this presentation, we will show growth of CZTS thin films with this technique.
Experimental
We used fine channel type mist CVD for the present purpose. The reaction region is composed of a fine channel structure above the substrate, in which the precursor materials are effectively reacted. A solution containing precursor materials was ultrasonically atomized and the mist particles formed were supplied onto a substrate, using nitrogen carrier gas. Thin films are obtained by the thermal decomposition of precursor materials on the substrates. CZTS thin films were fabricated onto glass substrates. The precursor materials used for CZTS thin film fabrication were metal chloride, metal acetylacetonate, and/or metal acetate as Cu, Zn and Sn sources and thiourea as S source. These precursor materials were diluted in methanol with the stoichiometric concentration ratio. The growth temperatures were set below 450 oC.
The structure of the CZTS thin films has been analyzed by X-ray diffraction (XRD). The concentration and optical properties of the samples were characterized by energy dispersive X-ray analysis (EDX) and UV-visible spectrometry, respectively. Van-der-Pauw-Hall measurements were carried out at room temperature in order to characterize the electrical properties.
Results and discussion
CZTS thin films grown from a source solution which contains metal chloride (CuCl2, ZnCl2 and/or SnCl2) have various compositions far from stoichiometric ratio and observed not only diffraction peaks of CZTS but also different phase peaks indexed to Cu2-xS or ZnS. The thin films whose sources were chloride-related ones consisted CZTS + Cu2-xS or CZTS + ZnS, which depend on the growth temperatures. In contrast, CZTS thin films grown from a source solution which contains only acetylacetonate-related sources, no diffraction peaks indexed to different phase were observed and film compositions were almost stoichiometric ratio. These results indicate that chloride sources realize lower activation energy of Cu2-xS or ZnS, Therefore, Cu2-xS or ZnS were grown even lower growth temperatures at the same time as growing CZTS. On the other hand, acetylacetonate-related sources have higher activation energy of different phase and inhibit the growth of Cu2-xS and ZnS.
Conclusion
CZTS thin films were fabricated by a non-vacuum mist CVD technique. CZTS was grown in the low temperature region below 350 oC. Because of the low-cost source materials and simple production process used, the fabricated films may load to photovoltaic applications.