Brush Plated Copper Indium Selenide Films and Their Characteristics

In recent years, there has been a growing attention to ternary chalcopyrite compound CuInSe₂ (CIS) and its alloys with gallium CuIn_xGa_1−xSe₂ (CIGS) owing to their promising performance for thin film solar cell applications. The conversion efficiencies have already exceeded 19% using these materials at the laboratory scale. Over the past decades, CIS thin films have been successfully electrodeposited using direct current (d.c.) under the potentialstatic or galvanostatic condition, and periodic pulse electrodeposition technique. In this work, the brush plating technique has been employed for the first time to deposit CIS films.

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. Dislocation density and strain were calculated.

Composition of the films determined from EDAX measurement indicated that as the electrolyte temperature increased the films became more stoichiometric. At lower electrolyte temperature slight excess of copper is deposited compared to indium since, the deposition potential for copper is more positive compared to indium and a lower concentration of indium ions are available for deposition. At higher electrolyte temperatures, the concentration of the indium ion increases, hence 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.

Solar cell structure of the configuration, Mo/CuInSe₂/CuInS₂/Ag was studied.  For solar cell studies, CuInSe₂ films were brush electrodeposited at different bath temperatures on Molybdenum substrate, over this layer, n-CuInS₂ films ( 400 nm) were brush electrodeposited at 80°C. The resistivity of the CuInS₂ layer was 2.0 ohm cm. On top of the CuInS₂, a silver grid was vacuum evaporated. The solar cell parameters, open circuit voltage (Voc), short circuit current density (Jsc), series resistance (Rs),shunt resistance (Rsh), and fill factor (FF) were studied under 100 mW cm^-2. The cell area was 0.65 cm². The results show that the cells exhibit higher efficiency compared to an earlier report on pulse reverse deposited CuInSe₂ films.

The results of this work clearly point to the possibility of successfully employing the brush electrodeposition technique for the growth of CIS films with grain size in the range of 15 – 40 nm and for the preparation of films with resistivity in the range of 0.1 – 20.0 ohm cm.