Photoelectrochemical Behavior of Indium Sulphide Films

Indium sulfide is a non-toxic material and is an n-type semiconductor with an energy band gap of 2.5 eV, and has been used efficiently as a photoconducting material. In₂S₃ is a III–VI compound and it crystallizes in tetragonal structure with the lattice parameters, a = 7.619 Å and c = 32.329 Å. In₂S₃ finds many applications in the preparation of green and red phosphors and in the manufacture of picture tubes for color television and dry cells. In this work, the pulse electrodeposition technique has been employed for the first time to deposit In₂S₃ films and the results are presented and discussed.

In₂S₃ films were deposited by the pulse electrodeposition technique at different duty cycles in the range of 6 – 50 %. The deposition potential was – 0.4 V (SCE). The deposition was done at room temperature and the precursors were 0.1 M InCl₃ and 0.06 M sodium thiosulphate in di-ethylene glycol. The total deposition time was maintained as 60 min for all the duty cycles. Thickness of the films measured by surface profilometer was in the range of 400 nm – 700 nm for the films deposited at different duty cycles.

X-ray diffraction patterns of the films deposited at different duty cycle exhibit polycrystallinity with peaks corresponding to the tetragonal structure (JCPDS 25 – 0390). The films exhibited preferred orientation in the (109) direction. The intensity of the peaks increased with duty cycle. Crystallite size measured from the XRD data using Scherrer’s equation was in the range of 50 – 90 nm with increase of duty cycle. The other parameters like strain, dislocation density were estimated.

Composition of the films was obtained from energy dispersive x-ray analysis (EDAX). In L and S K lines were observed in the spectrum. The average ratio of the percentage of In:S was 2:3 showing that the samples were in good stoichiometric ratio.

Transmission versus wavelength spectrum recorded in the wavelength range, 500–2500 nm for the films deposited at different duty cycles. It can be observed that the measured transmittance of the films at 800 nm, increased from 59.1% to 78% with the increase of duty cycle from 6 % to 50 %. Bandgap estimated from Tauc’s plot varied in the range, 2.18–2.04 eV as the duty cycle increased.  These values agree with the literature values.

The room temperature electrical resistivity of In₂S₃ films decreases from 10⁶ ohm cm to 10³ ohm cm with increase of duty cycle.

Photoelectrochemical (PEC) cells formed with the films deposited at different duty cycles did not exhibit any photoactivity. Hence, they were post heat treated in argon atmosphere at different temperatures in the range of 400 - 525°C. The power output characteristics of the PEC cells made using the photoelectrodes deposited at 50 % duty cycles. It is observed that the PEC output parameters, viz., open circuit voltage and short circuit current were found to increase for the electrodes heat treated up to a temperature of 500°C. A plot of lnJsc vs Voc yielded a straight line and the reverse saturation current density J₀ was 2.5 x 10^-7 A cm^-2. The ideality factor (n) was calculated from the slope of the straight line and it was found to be 1.80. The photo output is higher than an earlier report. It is observed that during photoetching, the open circuit voltage and short circuit current density increase from 4.5 mA cm^-2 to 5.8 mA cm^-2 and from  0.43 to 0.48 V respectively for an intensity of 60 mWcm^-2.

            Nanocrystalline films with band gap in the range of 2.04 – 2.17 eV can be deposited with prospective use in photovoltaic application.