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|Growth and Characterization of Cu2ZnSn(SSe)4 Thin Films for Solar Cell Applications
|Mohanty, Bhaskar Chandra
|Cu2ZnSnS4 (CZTS) has emerged as a leading absorber candidate for thin film solar cells over the last few years due to its highly favorable optoelectronic properties. In addition, it has an earth abundant, non-toxic, and inexpensive constituent that promises development of cost-competitive solar cells. However, the formation of the single phase, which is crucial to the performance to the photovoltaic devices, has been found to be a big challenge in CZTS since a single phase exists within a much smaller chemical potential window. The tendency to form binary phases, selective re-evaporation of elements from the film, reaction at contacts, etc. further make it difficult to control the intended composition and the formation of single phase. Consequently, there is extensive ongoing efforts to design synthesis protocols for the fabrication of single phase CZTS with desired properties. Various physical vapor deposition techniques, especially the sputter deposition one, which is being widely used in the optoelectronics industry, have performed poorly for the CZTS based devices. In this work, efforts have been made to fabricate CZTS thin films by RF magnetron sputtering of a single elementary target. The resulting films were post-selenized at different conditions to obtain Cu2ZnSn(SSe)4 (CZTSSe) thin films. The proposed route of film synthesis greatly simplifies the process steps in fabrication of these films. Based on the results, reaction mechanism of evolution of kesterite phase has been proposed. The films grown from a target containing powders of Cu, Zn, Sn and S taken in stoichiometric proportion were always Cu -poor in spite of a large variation in the sputtering and post-deposition sulfurization parameters. This stoichiometric deviation of the precursor film was primarily due to the poor sputter yield of Cu. The Cu-deficiency led to the formation of spurious secondary phases. Cu deficiency in the precursor film was compensated by growing a Cu interlayer between successively sputtered Cu-Zn-Sn-S precursor layers. Inserting Cuinterlayer improved compositional stability through thermally activated diffusion of Cu from this intermediate layer towards both the sides and resulted in the formation of kesterite CZTS even at a modest sulfurization temperature of 500 °C. The obtained phase-pure films have a bandgap of ~1.58 eV and shows excellent photoresponse behavior characterized by increase in current by three orders of magnitude at a bias of 3 V upon white light illumination, typically that required for a potential absorber layer in thin film solar cells. In an alternate approach, the Cu deficiency was mitigated by preparing a target from elemental powders with 12.5% excess of Cu. By intuitively manipulating the post-sulfurization process carried out in a quasi-open environment using sulfur flakes, single phase CZTS films could be obtained. The influencing mechanism of post-sulfurization process was elucidated from systematic variation in the dwell time, temperature and the sulfur amount. A high temperature or a shorter dwell time yielded a small-grained microstructure associated with the presence of secondary phases. By using the target with excess copper target, phase pure CZTS films with better microstructural features were obtained for sulfurization at 500 C for 60 min with 1.0 g of sulfur flakes. This film exhibited an optical bandgap of ~1.58 eV indicating its photovoltaic potential. A device in the Mo/CZTS/Ag configuration showed typical features of a Schottky junction. The obtained current-voltage characteristic was analyzed to estimate saturation current, ideality factor and series resistance in correlation with the properties of the CZTS film. Based on the results of efforts on preparing phase pure CZTS films by inserting a Cu interlayer for films grown from a target of stoichiometric composition or by carefully manipulating the sulfurization process for films prepared from a target containing excess Cu, a single step synthesis route was designed that did not require any post-deposition heat treatment. An elevated substrate temperature of 450 °C during sputter deposition using a single elementary target with excess Cu provided enough energy for the reaction and formation of kesterite phase. These films have an optical bandgap of ~1.6 eV and white light sensitivity >200% at a bias potential of 5 V, highly suitable for photovoltaic and photocatalytic activities. Detailed electro-impedance analyses have been carried out to confirm the p-type conductivity, carrier concentration and a carrier life time. Finally, the CZTS thin films which were synthesized in a single process step were selenized and the influencing mechanisms of selenization parameters have been established. The process parameters such as selenization temperature, dwell time and selenium amount were found to significantly affect the composition, bandgap, microstructure and the reaction pathway leading to the formation of single phase CZTSSe thin films.
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