Brush Plated CuInS2 Films and Their Photoelectrochemical Behaviour

Copper indium disulphide (CuInS₂), a I–III–VI₂ compound semiconductor, is a promising absorber material for thin film photovoltaics. It meets two important requisites of high solar energy conversion efficiency–it has a direct band gap of about 1.5 eV which perfectly matches the solar spectrum and has a fairly high absorption coefficient as well. The theoretical limit of the conversion efficiency of CuInS₂ based photovoltaic devices is as high as 30% while hitherto cells having 10–12% efficiencies have been made. The interest in the compound also stems from the facts that in comparison to CuInSe₂, a widely used absorber material, it is economical and non–toxic. Several methods that include wet-chemical and vacuum based approaches, have been developed to prepare CuInS₂ films.

Brush plating was carried out using Selectron Power Pack MODEL 150A-40 V. Layers were brush plated on tin oxide coated conducting substrates of about 5.0 cm² which is the negative electrode. The bath contained 5.0mM of InCl₃, 2.0mM of CuCl₂ and 1.0 mM of sodium thiosulphate at a pH of 1.5 was maintained throughout the experiment. In each case, the power unit was preset at a current density of 5.0 mA cm^-2. The electrolyte temperature was varied from 30°C - 80°C. The total deposition time was 20 min. Thickness of the CuInS₂ films measured by Mitutoyo surface profilometer varied from 1.80 μm– 2.70 μm with increase of electrolyte temperature.

XRD patterns of CuInS₂ films deposited at different electrolyte temperatures exhibited polycrystallinity with peaks corresponding to the chalcopyrite phase of CuInS₂ (JCPDS 75-0106). Peaks corresponding to CuS were not present. The crystallite size (D) of the films calculated using Scherrer’s formula increased from 300 nm -1000 nm with increase of electrolyte temperature. Dislocation density and strain were also calculated.

EDS spectra of CuInS₂ obtained from samples deposited at different electrolyte temperatures, S,Cu and In elements are detectable. It is observed that the intensity of In lines are observed to increase with increase of electrolyte temperature. For the films deposited at electrolyte temperature of 80°C, Cu/In ratio was ~1.0.

SEM micrographs for all the CuInS₂ films deposited at different electrolyte temperature exhibited smooth and homogeneous surfaces with grain size around 300 nm for electrolyte temperature of 30°C. The grain size further increased to 1000 nm for an electrolyte temperature of 80°C.

Transmittance measurements indicated interference fringes. The optical band gap E_g is in the range of 1.30 to¨1.42 eV, which is in good agreement with the value of 1.30 and 1.43 eV reported by other workers.

The magnitude of the room temperature resistivity increased from 0.56 ohm cm to 8.80 ohm cm as the electrolyte temperature is increased. Mobility is 5.76 cm²V^-1s^-1 for Cu/In ratio of unity is higher than 4.92 cm²V^-1s^-1 and carrier density value of 12.31 x 10¹⁷ cm^-3 for Cu/In ratio of unity in this study is higher than 1.2 x 10¹⁶ cm^-3 for Cu/In ratio of unity reported earlier.

            To increase the photo output, the films deposited at 80°C were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. For a film deposited at 80°C, an open circuit voltage of 0.55 V and a short circuit current density of 12.0 mA cm^-2 at 60 mW cm^-2 illumination. The photo output is higher than earlier report. The power output characteristics after 80s photoetching indicates a V_oc of 0.625V, J_sc of 16.0 mA cm^-2, ff of 0.71 and h of 11.83 %, for 60 mW cm^-2 illumination.

            The results indicate that these films can be used in photovoltaic devices.