Preparation and Characterization of Tin Selenide Thin Films for Application in Photoelectrochemical Cell

Tin selenide (SnSe) thin films have been prepared using various methods such as electrodeposition, chemical precipitation, thermal evaporation and chemical bath deposition. The variation in the deposition process produced thin films exhibiting various characteristics. The films have been characterised using x-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive analysis of x-ray (EDX) and x-ray photoelectron spectroscopy (XPS). The photoactivity of the films were evaluated using linear sweep voltammetry (LSV) in the presence of K4Fe(CN)6/K3Fe(CN)6 redox system. The band gap energy and transition type was determined from optical absorbance data obtained using UV-VIS spectrophotometry technique. The electrodeposition process was carried out by varying deposition potentials, solution temperature, solution concentrations and deposition time. The films prepared were found to be polycrystalline in nature. XRD studies confirmed the formation of orthorhombic SnSe phase with the preferred orientation along the (111) plane. Annealing at 150˚C in the presence of nitrogen atmosphere was found to improve the crystallinity of the film. This treatment also results in an increase in the photoresponse of the films deposited. The electrodeposited SnSe film shows photoresponse in the cathodic region conforming the p-type nature. It was an indirect transition with a bandgap energy of about 1.08 eV. The SnSe film was also prepared using the combination of chemical precipitation and brush coating method. The crystalline powder was coated onto indium doped tin oxide glass substrates using polyvinyl alcohol solution as a binder. Tin selenide powder was prepared using chemical precipitation method in alkaline aqueous medium. The XRD data obtained confirmed the SnSe powder to be free of impurities. The coatings were subjected to annealing at various times and temperatures in order to study these effects towards the structure, morphology and composition of the materials. XRD data obtained from the films indicated formation of polycrystalline materials. Annealing at 150˚C for 1 h in nitrogen atmosphere was found to be the optimum conditions to produce films of this nature. The photoresponse behaviour of the film in the cathodic region confirms the p-type conduction. The optical absorbance data exhibited indirect transition with a bandgap energy of about 1.00 eV. The SnSe powder synthesised was also used as a source material to prepare SnSe thin films using thermal evaporation method. The combination of this experimental procedure has resulted in the reduction of time to develop a particular thin film. The currently employed solid-state method, which needs 2 days to develop metal chalcogenide thin films, could be replaced with this method, which takes not more than 5 hours. The films were found to be smooth and free of pinholes. Clear transparent SnSe films with different thickness could conveniently be prepared by this method. XRD data indicated that the preferred orientation lies along the (111) plane. The films were found to be p-type semiconductors. The thicker SnSe film indicates higher photoactivity. The optical data confirmed the indirect transition with an energy gap of 1.25 eV. Tin selenide thin films could also be deposited onto indium tin oxide glass slides in alkaline medium. The method is based on simple bath deposition technique that requires fewer chemicals, less monitoring, simple and economical. Uniform and well-adhered films were obtained upon required deposition period. XRD data for the as-deposited film confirms polycrystalline material formation. The preferred orientation lies along the (201) plane. The material covered the surface of the substrate completely. The photoresponse in the cathodic region indicates formation of a p-type semiconductor. The bandgap energy obtained was 1.25 eV corresponding to direct transition. The optical absorption in the visible region makes it possible to be used in a photoelectrochemical cell or as semitransparent layer in high-speed detectors working in visible region.

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