Properties of Pulse Electrodeposited AgGaS2 Films

AgGaS₂ films were deposited on tin oxide coated conducting glass substrates at different duty cycles, in the range of 6 – 50 % at a deposition potential of  - 1.14 V (SCE). The deposition potential was fixed based on an earlier report [14]. The total deposition time was 15 min. The precursors used were Analar grade 0.10 M AgCl, 0.20 M InCl₃ and 0.15 M sodium thio sulphate. Thickness of the films estimated by Mitutoyo surface profilometer varied in the range of 400 – 900 nm with increase of duty cycle. The films were characterized by Xpertpanalytical x-ray diffraction unit with Cukα radiation. Optical measurements were recorded using an Hitachi UV–Vis-IR spectro-photometer. Composition of the films was estimated by EDS attachment to JOEL SEM. Hall Van der Pauw method was employed to estimate the electrical parameters.

The X-ray diffraction pattern of AgGaS₂ films formed at different duty cycles is shown in Fig.1. The films were polycrystalline exhibiting the peaks corresponding to the single phase AgGaS₂. Peaks corresponding to (112), (220), (204), (312) and (116) orientations of the chalcopyrite structure were observed (JCPDS No.75-0118). The evaluated lattice parameters were a = 5.76 Å and c = 10.27 Å with a c/a ratio of 1.78. The crystallite size was calculated from the Full width half maximum of the diffraction profiles using Scherrer’s equation. The crystallite size increased from 12 nm – 30 nm with duty cycle. At lower duty cycle, the Ag/Ga ratio is greater than unity. With increase of duty cycle, due to the increase in concentration of Ga, Ag/Ga ratio approaches unity.  The transmission spectra exhibit interference fringes and the value of the refractive index was estimated by the envelope method. The value of the refractive index at 550 nm, calculated from the above equations was in the range of 2.92 - 2.54 for the samples deposited at different duty cycle. This value is higher than the values  obtained on thermally evaporated AgGaS₂ films. The films exhibited a high absorption co-efficient of the order of 10⁴ cm^-1. A plot of (αhν)² against hν, exhibits linear behavior near the band edge, the band gap of the deposited films was determined to be in the range of 2.35 – 2.57 eV.

The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contact. The influence of duty cycle on the resistivity of the films is shown in Table-5.4. The films exhibit p-type conductivity. The EDAX results support the p-type conductivity, since, the non-stochiometry parameter is greater than zero. The magnitude of the resistivity increased from 400 ohm cm to 900 ohm cm as the duty cycle is increased. The resistivity values are lower than earlier report.

The photocurrent increases with an increase in voltage. The photocurrent spectra show a peak near the absorption edge. Photosensitivity is the ratio of the increase in conductivity of the material in the presence of light to the conductivity in darkness. . Thinner films exhibit moderate photosensitivity, whereas thicker films are found to exhibit higher photosensitivity. Crystallographical imperfections acting as trapping centers will enhance the photosensitivity, whereas the recombination centers decrease the photosensitivity.