Characteristics of CuIn1−xGaxS2 thin films synthesized by chemical spray pyrolysis

CuIn1−xGaxS2 multi-component semiconductors thin films were prepared by chemical spray pyrolysis on glass substrates using different concentrations of gallium in the spray solutions (y=([Ga3+]/[In3+]) varying from 0 to 20 at% by a step of 5 at%). Samples were characterized using X-ray diffraction, Raman spectroscopy, Atomic Force Microscopy, photoluminescence spectroscopy, spectrophotometric and Hall effect measurements. The X-ray spectra reveal that the CuIn1−xGaxS2 thin films are of chalcopyrite crystalline phase with a highly (1 1 2) preferential orientation. The best crystallinity is obtained for 10 at% Ga incorporation since the maximum (1 1 2) peak intensity and grain size are obtained at this Ga incorporation rate. The level of the residual microstrain and dislocation network seems to be reduced respectively to the values 0.09% and 4×108 lines mm−2 for an optimum y=10 at% for which the crystallinity of CuIn1−xGaxS2 thin layers is the best one. Raman spectra indicate that the sprayed thin films are grown only with CH-ordering. Optical analysis by means of transmission T(λ) and reflection R(λ) measurements allow us to determine the direct band gap energy value which increases by increasing the Ga content and it is in the range 1.39-1.53 eV, indicating that CuIn1−xGaxS2 compound has an absorbing property favorable for applications in solar cell devices. Photoluminescence measurements are performed on CuIn1−xGaxS2 crystals and the analysis reveals that the emission is mainly due to donor-acceptor pair transitions. The film resistivity (ρ) and Hall mobility (μ) are strongly affected by Ga incorporation rate. The lowest resistivity (ρ=0.1 Ω cm) and maximum value of Hall mobility (μ=0.5 cm2 V−1 s−1) are also obtained for the thin layers prepared with y=10 at%. Finally, we reported two new structures for CuInS2/β-In2−xAlxS2/ZnO:Al and CuIn1−xGaxS2 (y=10 at%)/β-In2-xAlxS2/ZnO:Al solar cells to investigate the effect of gallium incorporation on the photovoltaic parameters. We found that the Ga-containing cell shows conversion efficiency (η=1.6%) higher than the Ga-free reference cell due to higher open-circuit voltage (Voc=540 mV) and short-circuit current density (Jsc=10 mA cm−2).