Photoelectrochemical Studies on Brush Plated CuInSe2 Films

Copper Indium selenide (CIS) films were deposited by the brush plating technique for the first time. Analar grade Copper sulphate (0.03M), Indium sulphate (0.05 M) and 0.005 M Selenium oxide was used for the deposition of films. The deposition current density was 1.0 mA cm^-2. The films were deposited at different electrolyte temperatures in the range of 30 - 80°C. Thickness of the films estimated by Mitutoyo surface profilometer increased from 400 nm to 800 nm.

The typical XRD patterns of  CIS films deposited at different electrolyte temperatures exhibit the chalcopyrite structure which is easily identified for the films (JCPDS card no. 00-040-1487). The films deposited at lower electrolyte temperatures, show a poor crystallinity with weak and broadened diffraction peaks. As the electrolyte temperature increases, the diffraction peaks become sharp and the peak intensity also  increases. Three well defined characteristic peaks at 26.6°, 44.1° and 52.4°are observed. The crystallite size of the films calculated using Scherrer’s equation varied from 15 – 40 nm with duty cycle.

Composition of the films determined from EDAX measurement indicated that as the electrolyte temperature increased the films became more stoichiometric. The Cu/In ratio approaches unity at 80°C.

The band gap of the films increased from 1.05 eV to 1.17 eV as the electrolyte temperature decreased. The increase in band gap at lower temperatures 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 0.1 ohm cm to 20.0 ohm cm as the electrolyte temperature is increased. The resistivity values are comparable to earlier report on three source evaporated films.

Photoelectrochemical (PEC) cells were prepared using the films deposited on titanium substrates 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 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 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 deposited at 80°C, exhibited maximum photo output. In order to increase the photo output, these films deposited were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. Photoelectrodes heat-treated at temperatures greater than 525°C 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. The photovoltaic parameters are shown in Table 4.4. For a film deposited at 80°C, an open circuit voltage of 0.49 V and a short circuit current density of 12.0 mA cm^-2 at 60 mW cm^-2 illumination. The photo output is higher than earlier report.

It was observed that 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 5.1 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 1.85.         

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 decrease of the photocurrent and photovoltage after 80s photoetch is attributable to separation of grain boundaries due to prolonged photoetching. The power output characteristics after 80s photoetching indicates a V_oc of 0.6 V, J_sc of 18.0 mA cm^-2, ff of 0.71 and h of 12.78 %, for 60 mW cm^-2 illumination.

The power out put and efficiency obtained are encouraging and indicates that these electrodes could be used in practical cells. Also the process can be scaled up for production of large area electrodes