Synthesis and characterization of CZTS (Cu2SnZnS4) thin films for photovoltaic applications

Today, thin-film-based photovoltaic (PV) structures attract attention in the search for alternatives to Si-based solar cells. Among the thin films, the Cu2ZnSnS4 (CZTS) compound is the most remarkable with its optical and electrical properties. It is an ideal alternative in terms of PV applications with its p-type semiconductor properties in the direct band (1.5 ev) range and high optical absorption coefficient (>104 cm-1) [1]. The fact that all components in the CZTS compound are relatively cheap and abundant on earth (Cu: 50–70 ppm, Zn: 75–80 ppm, Sn: 2.2 ppm, S: 260 ppm), as well as the fact that it does not contain toxic components, has greatly increased the interest in this structure [2],[3]. Despite all these advantages, the highest efficiency value achieved in CZTS-based PV structures is 12.6% [4]. The reason why this value remains below 31%, known as the Shockley-Queisser (SQ) limit, is the complex phase structure of CZTS as a result of its polyatomic structure. Binary (such as CuS, ZnS, SnS) and triple Cu2SnS3 side phases are formed as a result of not providing the right conditions during production. The MoSx semiconductor formed between the Molybdenum (Mo) back contact and the CZTS absorber layer during heat treatment is also the main reason and light circuit use with pure sulphur is the main reason for reducing the efficiency [3],[5],[6],[7]. Within the scope of this thesis, it was tried to obtain CZTS thin films in which binary and/or ternary side phases were minimized by using two-stage synthesis. In addition, experimental studies were carried out on the usability of Graphene (Gfn) interlayer to prevent the MoSx structure formed at the interface. For CZTS synthesis, the first layer is vacuum evaporation method and the other layers are sputtered from 2 different targetswith Sn/Cu/ZnS Cu/Sn/ZnS (CZT) precursor structure soda lime glass (Soda lime glass: SLG), Mo thin film coated SLG (SLG) /Mo) and Gfn coated SLG/Mo substrates (SLG/Mo/Gfn). Gfn coating process was carried out with plasma enhanced chemical vapor deposition (PECVD) technique. The structural properties of the synthesized samples were determined by XRD, Raman, SEM and FIB-SEM, and the optical properties were determined by spectroscopic ellipsometry. As a result of the optimization studies, the layer thicknesses of high purity CZTS thin films on SLG/Mo and SLG/Mo/Graphene substrates were adjusted to be Sn: 180, Cu: 135 and ZnS: 220 nm. Sulphidation conditions were determined by XRD and Raman measurements, which could be obtained under sulphidation temperature 550 oC, sulphidation time 60 minutes and 250mTorr Ar gas flow. It was determined that the Gfn coating could not improve the CZTS crystal event by about 16% but could not prevent MoSx formation. It has been understood that the reason why MoSx phase is more dominant is that the pyramidal Mo surface morphology is suitable for the formation of multilayered and defective graphite structure instead of Gfn, and the free C atoms in this structure act as reducing agents for the S atoms and support the formation of MoSx.