Properties of Pulse Plated CuInS2 Films

Among ternary semiconductor compounds belonging to the I-IIIVI₂ series,CuInX₂ (X=Se, Te, S) are promising materials for photovoltaic applications because of the suitability of their electrical and optical properties [1]. CuInS2 (CISu) semiconductors crystallize on both, chalcopyrite or sphalerite structure. It is a promising material to be used as absorber layer in photovoltaic devices because of its direct gap of about 1.55 eV and its high absorption coefficient. In this work, the glavanostatic pulse electrodeposition technique has been employed for the deposition of CISu films.

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.

Surface morphology of the films deposited at different duty cycles was studied by Atomic force microscopy. It is observed that the surface roughness and grain size increases with duty cycle. The surface roughness increases from 0.85 nm to 2.50 nm as the duty cycle increases from 6% to 50 %. Fine grains are observed for the films deposited at 6 % duty cycle.

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.

  Nanocrystalluine films with higher carrier density and low resistivity can be deposited.