Photoelectric Properties of Pulse Electrodeposited CdIn2Se4 Films

Among II–III–VI group ternary compounds, Cadmium indium selenide (CdIn₂Se₄) is one of the interesting semiconductors due to its optical absorption property, band gap and low electrical resistance. CdIn₂Se₄ is a direct band gap semiconductor with an energy gap of 1.73 eV which make them interesting for solar cells through photoelectrochemical route. The material in thin film form can be obtained by electrodeposition, vacuum evaporation and spray pyrolysis. In this work, the pulse electrodeposition technique was used for the deposition of CdIn₂Se₄ films for the first time.

CdIn₂Se₄ films were pulse electrodeposited at different duty cycles in the range of 6 – 50 %. The deposition potential was maintained at – 0.95 V(SCE). Titanium and conducting glass substrates were used as substrates. The films were deposited at 80°C. The precursors used were 0.1 M CdSO₄, 0.25 M In₂(SO₄)₂ and 5 mM SeO₂ in diethylene glycol. Thickness of the films measured by Mitutoyo surface profilometer increased from 600 – 950 nm with increase of duty cycle.

X-ray diffraction pattern recorded for CdIn₂Se₄ thin films prepared on titanium substrates revealed that the deposited films are polycrystalline with tetragonal structure with lattice constants (a = 5.77 Å; c = 11.59 Å). The diffraction peaks of tetragonal CdIn₂Se₄ with the lattice planes (101), (111),  (112),  (300), (311) and (104). All the peaks identified are from CdIn₂Se₄ and no additional lines corresponding to Cd, In and Se are present. The height of (111) peak increases with duty cycle, which indicates that the deposited films exhibits preferential orientation along (111) planes. It is also observed that the height of preferential peak is increased with duty cycle. The crystallite size of the deposited films are calculated using FWHM data and Debye Scherrer formula variedc from 15 – 40 nm.

Composition of the films was studied by EDS analysis of Cd_1.01In_1.96Se_4.03 for the films deposited at 50 % duty cycle which indicates the nearly stoichiometric composition.

The band gap energy of CdIn₂Se₄ varied in the range of 1.97 eV – 2.16 eV as the duty cycle decreased. This variation is due to quantum size effects. The grain size of the films deposited at lower duty cycle is lower compared to the films deposited at higher duty cycles.

The variation of photocurrent with light intensity of CdIn₂Se₄ films deposited at different duty cycles exhibited linear behaviour. The increase in photocurrent is attributed to an increase in the majority carrier concentration and/or an increase in impurity centers acting as traps for minority carriers. The variation of photocurrent with applied voltage in CdIn₂Se₄ films is also linear. Photosensitivity is the ratio of the increase in conductivity of the material in the presence of light to the conductivity in darkness It seems that some transitions that create additional free carriers effectively increase the free life time increasing the photosensitivity of the material. A plot of photosensitivity versus light intensity of CdIn₂Se₄ thin films is also linear. Thinner films exhibit moderate photosensitivity, whereas thicker films are found to exhibit higher photosensitivity. Crystallographical imperfections acting as trapping centers will enhance the photosensitivity, whereas the recombination centers decrease the photosensitivity.

Single phase CdIn₂Se₄ films could be prepared by the pulse electrodeposition technique. Films with grain size in the range of 15 nm – 40 nm cane be prepared. Films with band gap in the range of 1.97 eV – 2.16 eV can be obtained. Photosensor studies indicated linear photocurrent – voltage and linear photocurrent – illumination characteristics showing that these films can be used in optoelectronic applications.