Photoelectrochemical Behaviour of Pulse Plated CuIn0.6Al0.4Se2 Thin FILMS

CIAS thin films were deposited by the pulse plating technique using non aqueous ethylene glycol solution. 0.2 M Al₂(SO₄)₃, 0.1 M In₂(SO₄)₃, 0.02 M CuSO₄, 0.05 M SeO₂. The films were deposited at 80°C and at different duty cycles at a constant current density of 5 mA cm^-2. Tin oxide coated glass (5 ohms/sq) were used as substrates. Thickness of the films measured using Mitutoyo surface profilometer was in the range of 600 nm – 1100 nm with increase of duty cycle from 6 – 50 %.  

The X-ray diffraction pattern of CIAS films indicated polycrystalline nature of thr films, the peaks corresponding to the single phase CIAS. Peaks corresponding to (112), (220), (204), (312) and (116) orientations of the chalcopyrite structure were observed, similar to earlier reports. The evaluated lattice parameters were around a = 5.62 Å and c = 10.99 Å . The crystallite size was calculated from the Full width half maximum of the diffraction profiles using Scherrer’s equation, The crystallite size increased from 15 nm – 40 nm as the duty cycle decreased from 50 % - 6 % .

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 Al and Se, but has no obvious effect on Indium relative content in the film, this can be attributed to the fact that Al 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 Al in the deposited film, and therefore reduction of Al–Se compound(s). 

Transmission spectra of the CIAS films deposited at different duty cycles.  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)² against hν, exhibits linear behavior near the band edge, the band gap of the deposited films was in the range of 1.97 – 2.01 eV. This value agrees well with earlier report on thermally evaporated  CuIn_0.6Al_0.4Se₂ films.

Photoelectrochemical (PEC) cells were prepared using the films deposited at 80°C and at different duty cycles. Photoelectrochemical cell studies were made using 1.0 M Na₂S,  1.0 M NaOH and 1.0 M S, 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 these 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. For a film deposited at 50 % duty cycle and post heat treated at 500°C , an open circuit voltage of 0.50 V and a short circuit current density of 10.0 mA cm^-2 were observed at 60 mW cm^-2 illumination. Both V_oc and J_sc increased with increase of intensity. Beyond 80 mW cm^-2 illumination, V_oc was found to saturate as is commonly observed in the case of photovoltaic cells and PEC cells, J_sc is found to linearly increase with intensity of 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 1.15 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 2.05.     

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. This is illustrated in Fig.5 for the photoelectrode deposited at 50 % duty cycle. The decrease of the photocurrent  and photovoltage after 80s photoetch may be attributed to separation of grain boundaries due to prolonged photoetching. The power output characteristics after 80s photoetching indicates a V_oc of 0.60V, J_sc of 7.53 mA cm^-2, ff of 0.53 and h of 3.98 %, for 60 mW cm^-2 illumination.