Photoelectrochemical Cells Studies with Brush Electrodeposited AgGaSe2 Films

AgGaSe₂ films were deposited on tin oxide coated conducting glass substrates at different substrate temperature in the range of 30–80°C at a deposition current density of 5.0 mA cm^-2. The total deposition time was 15 min. The precursors used were Analar grade 0.15 M AgCl, 0.20 M InCl₃ and 0.15 M sodium thiosulphate. Thickness of the films estimated by Mitutoyo surface profilometer varied in the range of 700 – 1200 nm with increase of substrate temperature. 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. Photoelectrochemical cell studies were made on the deposited films. A 250 W tungsten halogen lamp was used for illumination.

The X-ray diffraction diagram of AgGaSe₂ films formed at different substrate temperature is shown in Fig.1. The films are polycrystalline with peaks corresponding to the (112), (204), (312), (332) and (424) reflections of the chalcopyrite phase (JCPDS 311240). The peaks are observed to increase in intensity with substrate temperature. The lattice constants estimated form the XRD data are a = 5.996 A° and c = 10.887 A° for the films deposited at 80°C substrate temperature. The crystallite size was determined from Scherrer’s equation. The crystallite size  varied in the range of 20 to 58 nm with increase of substrate temperature.

Composition of the films was estimated by recording the EDS spectrum of the films deposited at different substrate temperature. It is observed that films deposited at lower substrate temperature were silver rich, due to the more noble nature of silver compared to gallium, the Ag/Ga ratio was 1.21, as the substrate temperature increased, the films became stochiometric and at 80°C substrate temperature, the Ag/Ga ratio was unity.  

From a plot of photon energy vs. (αhν)² plot of AgGaSe₂ thin films deposited at different substrate temperature,a bandgap energy in the range of 1.94 eV –1.70 eV with increase of substrate temperature was obtained using αhν = 0. This value is comparable with the previous report on vacuum evaporated films. The lower value of band gap obtained at 80°C substrate temperature is due to the slight excess of gallium in these films. The gallium concentration increases gradually with increase of substrate temperature. This is supported by EDAX data.

Photoelectrochemical (PEC) cells were prepared using the films post heat treated at different temperatures. The films were lacquered with polystyrene in order to prevent the metal substrate portions from being exposed to the redox electrolyte. These films were used as the working electrode. Photoelectrochemical cell studies were made using 0.5 M KI and 0.05 M I₂ as the redox electrolyte. Graphite was used as the counter electrode. The light source used for illumination was an ORIEL 250 W Tungsten halogen lamp.

The PEC cells using the as deposited films exhibited low photocurrent and photovoltage. The intensity of the light falling on the films deposited at different substrate temperatures was kept constant at 60 mW cm^-2. 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. Fig.3 shows the load characteristics of the post heat treated films. 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 80°C substrate temperature, an open circuit voltage of 0.475 V and a short circuit current density of 4.3 mA cm^-2 at 60 mW cm^-2 illumination. 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^-8 A cm^-2, the ideality factor (n) was calculated from the slope of the straight line and it was found to be 1.7. The effect of photoetching on the PEC performance was studied by shorting the photoelectrode and the graphite counter electrode under an illumination of 100 mW cm^-2 in 1:100 HCl for different durations in the range  0 – 100s. Both photocurrent and photovoltage are found to increase up to 80s photetch, beyond which they begin to decrease. The power output characteristics after 80s photoetching indicates a V_oc of 0.60V, J_sc of 7.50 mA cm^-2, ff of 0.53 and h of 3.98 %, for 60 mW cm^-2 illumination.