Ga doping of nanocrystalline CdS thin films by electrodeposition method for solar cell application: the influence of dopant precursor concentration

Ga doping of CdS thin films has been achieved using a simplified cathodic electrodeposition method and with glass/indium tin oxide (glass/ITO) as a substrate. CdCl2, Na2S2O3 and GaCl3 were used as precursors. The Ga-doped and un-doped CdS films obtained were characterized for their structural, optical, luminescence, compositional and morphological properties using state-of-the-art X-ray diffraction (XRD), spectrophotometry, room-temperature photoluminescence (PL), energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM), respectively. XRD results show that the presence of Ga ions in the deposition electrolyte and post-deposition annealing promote crystallinity of deposited CdS films, with estimated crystallite sizes of the films in the range (5-22)nm after annealing. Optical characterization results show that incorporation of Ga atoms into the crystal lattice of CdS results in increase in energy bandgap of the films, which makes them advantageous for application as window/buffer layers in solar cells. PL results show a single green emission peak whose intensity increases as Ga-content of the films increases. EDX results show a direct relationship between the percentage atomic Ga composition of the CdS:Ga films and the molar concentration of GaCl3 in the deposition electrolyte. SEM images reveal smooth surfaces of doped and un-doped CdS films. However, after annealing, cracks begin to develop in the films grown with electrolytic GaCl3 concentration in excess of 0.004M, thus indicating a possible threshold in GaCl3 concentration for obtaining device-grade CdS:Ga films. The entire work presents one of the strengths of electrodeposition as a reliable semiconductor growth technique for device application.