Properties of Pulse Electrodeposited CuInGaSe2 Films

CuIn_1-xGa_xSe₂ films of different composition (0< x <1), were pulse electrodeposited on indium tin oxide coated glass and molybdenum substrates at 50 % duty cycle and at room temperature. The precursors used were 20 mM SeO₂, 30 mM CuCl₂ and the concentration of the Indium chloride and gallium chloride precursors were varied to obtain films of different composition.  The deposition potential was maintained at – 0.8V (SCE). The pH was maintained at 1.5 by HCl. A microprocessor controlled pulse plating unit was used. Thickness of the films measured by Mitutoyo surface profilometer was in the range of 0.5 – 1.4 micrometer with increase of Indium concentration.

The x-ray diffraction patterns of CuIn_1-xGa_xSe₂ films of different composition. All the figures indicate the prominent peaks corresponding to (112), (220)/(204), (312)/(116). These care characteristic of the chalcopyrite phase. No other phases were observed in the x-ray diffractograms indicating the formation of single phase material. The peaks shifted from CuInSe₂ side to CuGaSe₂ side as the concentration of Ga increased in the films.

Composition of the films was estimated by recording the EDS spectrum of the films deposited of different composition. It is observed that films with lower concentrations of Gallium were copper rich. As the gallium concentration increased, the films became nearly stochiometric.

The tranmssion spectra exhibits interference fringes and the value of the refractive index was estimated by the envelope method. The refractive index decreases from 2.80 to 2.35 with wavelength. The band gap of the films increased from 1.11 eV to 1.62 eV  as the gallium concentration increased ( from(αhν)² vs hν plot ). The increase in band gap at lower duty cycles is due to the small crystallites. The values of the band gap agree well with the earlier report.

The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contact. The magnitude of the resistivity increased from 0.8 ohm cm to 23.52 ohm cm as the gallium concentration is increased. The resistivity values are comparable with an earlier report.

The PEC cells using these films exhibited low photocurrent and photovoltage. The intensity of the light falling on the films deposited at different duty cycles was kept constant at 60 mW cm^-2. Films of composition CuIn_0.9Ga_0.1Se₂ exhibited maximum photo output. The photooutput was low, hence, in order to increase the photo outpout, the films of different composition were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. Films of composition CuIn_0.7Ga_0.3Se₂ exhibit maximum V_oc. The power output characteristics after 80s photoetching indicates a V_oc of 0.70V, J_sc of 20.0 mA cm^-2, ff of 0.71 and h of 14.33 %, for 60 mW cm^-2 illumination.