Structural, Optical and Photoelectrical Properties of AgInSe2 Films

The I–III–VI₂ compounds are the ternary analogues of II–VI compounds. AgInSe₂ is a ternary analogue of CdSe, which has been used for a number of electronic devices. AgInSe₂ is a semiconductor with energy gap of 1.20 eV. They crystallize in the chalcopyrite structure, which is closely related to zinc blend structure. In the present work we employ the pulse electrodeposition technique for the first time to deposit AgInSe₂ films and study their properties.

AgInSe₂ films were deposited by the pulse electrodepsoition technique at room temperature from a bath containing Analar grade 10 mM silversulphate, 50 mM Indium sulphate and 5 mM SeO₂  at deposition potential of – 0.98V (SCE). Tin oxide coated glass substrates (5.0 ohms/ sq) were used. The duty cycle was varied in the range of 6 – 50 %. Thickness of the films measured by surface profilometer increased from 500 nm to 1000 nm as the duty cycle increased from 6 % to 50 %. Structural, optical and photoconductivity studies were made on the films.

The films deposited at different duty cycles were characterized by the X-ray diffraction (XRD) pattern. All the reflections could be indexed as the tetragonal phase of AgInSe₂ with the lattice parameters a = 6.09Å , c = 11:67Å , which were very close to the reported data (JCPDS Cards, 35– 1099). As the duty cycle increased, the intensity of the peaks increased. The height of the peak increased with duty cycle and the width of the peaks decreased with increase of duty cycle. No characteristic peaks of other impurities, such as Ag₂Se or In₂Se₃, were detected in the pattern. The crystallite size was determined from Scherrer’s equation varied from 10 to 40 nm with increase of duty cycle.

Energy dispersive X-ray (EDAX) spectrum gives the elemental composition of the as prepared samples. It contains 29.29% of Ag, 20.76% of In and 49.95% Se. This indicates that the as prepared films were indium deficient. The indium sites can be occupied by Ag atoms, acting as donors.

The transmittance spectra at RT of AgInSe₂ thin films deposited at different duty cycles exhibited interference fringes. Refractive index was calculated by envelope method and it increased from 1.25 to 1.80 with increase of wavelength from 500 nm to 1200 nm. The extinction coefficient 0.8 to 0.4 with increase of wavelength in the above range. This result is similar to that observed earlier. Photon energy vs. (αhν)² plot of AgInSe₂ thin films deposited at different duty cycles, indicated a band gap of 1.172 eV

Photoelectrochemical cell studies were made using 1M polysulphide (1M S, 1 M Na₂S, 1 M NaOH) as the redox electrolyte. Graphite was used as the counter electrode. The as deposited films exhibited very poor photo output after post annealing in argon atmosphere photo output was observed. For a film deposited at 50 % duty cycle, an open circuit voltage of 0.45 V and a short circuit current density of 4.0 mA cm^-2 at  60 mW cm^-2 illumination was observed. Both photocurrent and photovoltage are found to increase up to  80 s photoetch, beyond which they begin to decrease.  Photoetching leads to selective attack of surface states not accessible to chemical etchants.  The power output characteristics after 80s photoetching indicates a V_oc of 0.60V, J_sc of 7.50 mA cm^-2, ff of 0.53 and h of 3.0% for 80 mWcm^-2 illumination.

The results of this investigation clearly points to the possibility of depositing single phase AgInSe₂ films with grain size in the range of 10 – 40 nm which exhibit reasonable output when used as photoelectrodes.