Photoelectrochemical Cells Studies with Pulse Electrodeposited CuInS2 Films

Copper Indium Sulphide(CISu) thin films were deposited on conducting glass substrates at different duty cycles in the range of 6 % – 50 %and at a current density of 10 mA cm^-2.   The precursors were solutions of 0.1 M CuCl_2, 0.1 M InCl₃ and 0.2 M of sodium thiosulphate. The deposition current density was fixed at 10 mA cm^-1. The deposition time was fixed as 60 min in all cases. Thickness of the films measured by gravimetry using a semimicro Mettler balance were observed to be in the range of 500 – 700 nm with an increase of duty cycle from 6% to 50% (sensitivity of the semimicro balance is 10 μg).

XRD patterns of CISu films deposited at different duty cycles were polycrystalline with peaks corresponding to the chalcopyrite phase of CISu. Peaks corresponding to CuS were not present. XRD peaks of the chalcopyrite phase, namely, (112), (204 ), (3 1 2). The height of the peak increased with duty cycle and the width of the peaks decreased with increase of duty cycle. The crystallite size determined using Scherrer’s equation, increased from 10 nm to 25 nm with increase of duty cycle.

Composition of the films was estimated by recording the EDS spectrum of the films deposited at different duty cycles. It is observed that films deposited at lower duty cycles were copper rich. As the duty cycle increased, the films became stochiometric. For the films deposited at 50 % duty cycle, Cu/In ratio was 1.0.

Bandgap values of 1.39 eV, 1.41 ev and 1.51 eV were obtained from Tauc’s plot. The band gaps of most of the as-prepared films correspond to theoretical value of CuInS₂ at 1.53 eV. Band gap decreases with increase of duty cycle. These changes are directly related to the phase composition of the CISu films.

            The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contact. The films exhibit p-type conductivity, which is supported by the EDAX results, since, the non-stochiometry parameter is greater than zero. The magnitude of the resistivity increased from 0.10 ohm cm to 3.67 ohm cm as the duty cycle is increased. The resistivity values of this work are lower than the earlier report of 80.5 ohm cm for the films with Cu/In ratio of unity. The values of mobility and carrier density decrease with increase of duty cycle. Mobility is 2.43 cm²V^-1s^-1 for Cu/In ratio of unity is lower than 4.92 cm²V^-1s^-1 and carrier density value of 3.998 x 10¹⁸ 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.

The PEC cells using these films exhibited low photocurrent and photovoltage. In order to increase the photo outpout, the films deposited at 50 % duty cycle were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. Photoelectrodes heat-treated at temperatures greater than this value exhibited lower open circuit voltage and short circuit current due to the reduction in thickness of the films as well as the slight change in stoichiometry. For a film deposited at 50 % duty cycle, 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 photooutput is higher than earlier report. A plot of lnJ_sc vs V_oc yielded a straight line. Extrapolation of the line to the y-axis yields a J₀ value of 5.1 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 1.85. Photoetching was done by shorting the photoelectrodes and the graphite counter electrode under an illumination of 100 mW cm^-2 in 1 : 100 HNO₃  for different durations in the range 0 – 100s. Both photocurrent and photovoltage are found to increase up to 80s photoetch, beyond which they begin to decrease. This is illustrated in Fig.4.13 for the CIS photoelectrode deposited at 50 % duty cycle. The decrease of the photocurrent  and photovoltage after 80s photoetch is attributable to separation of grain boundaries due to prolonged photoetching. 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.