Photoluminescence studies of chalcopyrite and orthorhombic AgInS₂ thin films deposited by spray pyrolysis technique

Chalcopyrite (ch) and orthorhombic (o) AgInS₂ thin films were prepared by spray pyrolysis using a ratio of [Ag] / [In] = 1.5 and 0.83 respectively. AgInS₂ polycrystalline material was annealing in a sulphur atmosphere at 400 °C for 2 h. The estimated optical gap energies were 1.87 and 2.01 eV for ch-AgInS₂ and 1.98 eV for o-AgInS₂. All the deposited films exhibited n-type conductivity. Photoluminescence (PL) studies reveal in both phases several PL bands. In ch-AgInS₂ the PL bands were observed to be centered at 1.45, 1.7 and 1.88 eV at 10 K and an excitation intensity of 10 W cm^{- 2}. The 1.45 eV emission is related with indium vacancies whereas the other emissions (1.70 and 1.88 eV) are related with a donor-acceptor pair recombination and free to bound transition respectively. A new PL band was observed in the annealed sample, this band was centered at 2.02 eV at 10 K and it is related to the transition between a closed level to the conduction band and the splitting valence band (0.15 eV). PL bands in o-AgInS₂ samples were observed at 1.45 and 1.524 eV at 10 K and are related with a free to bound transition. Finally o-AgInS₂ shows two emission bands located at 1.45 and 1.59 eV, the o-AgInS₂ annealed sample in a sulphur atmosphere showed a new PL band located at 2.01 eV at 10 K, this band is related with an energy transition between a level near the conduction band to the splitting valence band (0.063 eV).