Photoelectrochemical Characteristics of Pulse Electrodeposited AgIn0.5Ga0.5Se2 Thin FILMS

AgGa_0.5In_0.5Se₂ films were deposited by the pulse electrodepsoition technique at room temperature from a bath containing Analar grade 2 mM silver nitrate, 8 mM gallium nitrate, 5 mM indium nitrate and 2 mM SeO₂. The deposition potential was maintained as — 0.68 V (SCE). Tin oxide coated glass substrates (5.0 ohms/sq) were used as the substrate. The duty cycle was varied in the range of 6 - 50 %. Thickness of the films measured by surface profilometer increased from 600 to 800 nm as the duty cycle increased. Structural, optical, electrical and photoelectrochemical properties were studied. Structural characteristic was studied by using Philips x-ray diffraction unit. using CuKα radiation. Optical characteristics were studied by using U 3400 UV-VIS-NIR spectrophotometer. Photoelectrochemical cells were fabricated with the films deposited at different duty cycles. 250 W tungsten halogen lamp was used as the light source. 1 M polysulphide ( ! M each of NaOH, Na₂S, S) was used as the redox electrolyte.

The x-ray diffraction pattern of AgGa_0.5In_0.5Se₂  films deposited at different duty cycle in the range of 6 - 50 %. All the peaks observed in the films were corresponding to the diffraction lines from AgGa_XIn_1−XSe₂ and no peaks were found from GaSe – Ga₂Se₃ or InSe–In₂Se₃phases. All of the thin films were strongly oriented to the (112) plane. Peaks corresponding to the (112),(204),(312) planes of the chalcopyrite phase was observed in all cases.  The lattice parameters.’a’ and ‘c’ were calculated from the diffraction spectra using the built in software. The crystallite size was calculated using the Scherrer’s equation  increases from 20 nm – 40 nm with decrease of duty cycle.

Composition of the films was determined by the Energy dispersive x-ray analysis (EDAX) attachment of the Scanning Electron Microscope (SEM). Decreasing the duty cycle from 50% to 15% reduces the relative content of Ga and Se, but has no obvious effect on Indium relative content in the film, this can be attributed to the fact that Ga was first dissolved back into the solution due to its lowest electronegativity corresponding to the positive current during the non pulse duration t_off, leading to the loss of Ga in the deposited film, and therefore reduction of Ga–Se compound(s)

The transmission spectra of the AgGa_0.5In_0.5Se₂  films deposited at different duty cycles. The spectra exhibit interference fringes and the value of the refractive index was estimated by the envelope method. The value of the refractive index at 350 nm, calculated from the above equations was in the range of 2.90 - 2.70 for the samples deposited at different duty cycle. The films exhibited a high absorption co-efficient of the order of 10⁴ cm^-1. A plot of (αhν)²against hν exhibits linear behavior near the band edge, the band gap of the deposited films was determined to be in the range of 1.36 – 1.46 eV.

The PEC cells using the as deposited films exhibited very low photocurrent and photovoltage. In order to increase the photo output, the films deposited at 50 % duty cycle were post heated in argon atmosphere at different temperatures in the range of 450 - 525°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 and post heat treated at 500°C , an open circuit voltage of 0.52 V and a short circuit current density of 11.5 mA cm^-2 were observed at 80 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 1.35 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 2.25.     

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. The power output characteristics after 80s photoetching indicates a V_oc of 0.63V, J_sc of 15.50 mA cm^-2, ff of 0.58 and h of 7.08 %, for 80 mW cm^-2 illumination.