Ionic compounds lamination reaction and characteristics of photosensitive copper indium sulfide on titania nanotube arrays

This study investigates using an inorganic photosensitive CuInS2 (CIS) coating instead of an organic dye on TiO2 nanotube arrays (TNAs). The stoichiometric characteristics by use of various deposition parameters such as precursor concentrations (0.1 M, 0.05 M, and 0.01 M) and deposition cycles (1–60 cycles) are then analyzed in relation to the crystallinity and photosensitivity. TNAs are synthesized by anodic oxidation of Ti metal, modified by the TiO2 film, and are subsequently annealed at 450 °C for 30 min, producing what are named T-TNAs. They show high photocatalytic efficiency and photosensitivity under UV-illumination. The photosensitive CIS coatings on the T-TNAs are processed by an ionic compounds lamination reaction (ICLR) method. The more immersion cycles and the higher the precursor concentration of copper sulfide, the more CIS peeled off as precipitates formed, which result in less indium sulfide deposition being required for reacting with the copper sulfide to reach stoichiometry. Near stoichiometric CIS can be obtained by controlling the precursor concentration and deposition cycles of the ICLR process. Good crystallinity and n-type characteristics are achieved by controlling the precursor concentrations and deposition cycles suitably to obtain a high current density. When the Cu/In ratio is adjusted for n-type characteristics, the current density reaches at least 300 μA/cm2 under visible light illumination intensity of 100 mW/cm2.

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► Only good photocurrent density under UV irradiation for TiO2 (or TiCl4) modified titania nanotubes (T-TNAs) demonstrates the response to visible light actually due to the CuInS2 photosensitivity on T-TNAs.
► The suitable CuInS2 (CIS) film thickness can be completed to achieve the well crystallinity, coverage and high photosensitivity of CIS by combining the precursor concentration with deposition cycles.
► An n-type CIS with a lower band gap (<1.5 eV) extending the absorption of the large solar spectrum into the infra-red range demonstrates the inorganic CIS potentially instead of organic dye to use in photosensitive solar cells by simple ionic compounds lamination reaction (ICLR) process.