Effect of sulfur substitutions on optical, electrical and structural properties of Ge(SxSe1−x)2 system

Glasses in the system Ge-Se-S were prepared with different Se/S ratios in order to investigate the compositional dependence of selected physical properties. We report the results of a systematic study examining the UV-vis transmission, dc electrical conductivity and X-ray diffraction of the system Ge(S_xSe_1−x)₂ with x=0, 0.1, 0.4 and 1.0 where replacement of S by Se was made. The changes in the optical energy gap, E_g, (from 1.95 to 2.43 eV) and band tail width, E_e, (from 103 to 243 meV) behave contrarily to the change in refractive index, n, (from 2.3 to 2) with the progressive replacement of S by Se. This behavior was discussed and interpreted with the changes in cohesive energy. The analysis of defects in the prepared films was carried out by the examination of activation energies obtained from dc electrical conductivity. The analysis of the X-ray diffraction pattern revealed a remarkable reduction in the intensity of the first and second diffraction peaks with the progressive replacement of S content, which confirms a change in the intermediate range order structure: reorganization of the structural properties.     

Nano ZnO thin films synthesis by sol–gel spin coating method as a transparent layer for solar cell applications

Zinc oxide nano-structured thin films have been synthesized by low-temperature and cost-effective sol–gel spin coating method. Zinc oxide films with good adherence have been deposited on soda lime glass substrates with two thicknesses 250.15 and 311.32?nm. High transmission (>95%) zinc oxide films with proper interference fringes in the visible and near infrared region have been obtained. Film thickness, optical constants and dispersion parameters have been calculated accurately by using Swanepoel method, which basely depends on the interference fringes of the transmission spectra. Zinc oxide films have direct optical band gap, its values slightly change with the annealing temperatures and film thickness. The X-ray diffraction studies indicated the hexagonal wurtzite structure for zinc oxide films with preferred orientation along (002) plane. Raman spectroscopy confirmed the hexagonal structure for the films. The average particle size is in the nano-scale and the crystallinity level increases with the annealing temperatures and film thickness.

     

Effect of Co60 γ-irradiation on the optical properties of thin films from the system GeSe3–Sb2Se3–ZnSe

Optical transmittance in the range from 200 nm to 1100 nm is measured for fresh and γ-irradiated thermally evaporated chalcogenide films of GeSe₃, Sb₂Se₃, ZnSe, (GeSe₃)₈₀(Sb₂Se₃)₂₀ and (GeSe₃)₇₀(Sb₂Se₃)₁₀(ZnSe)₂₀. The effect of ZnSe incorporation with both GeSe₃, Sb₂Se₃ results in amorphous γ-radiation sensitive (GeSe₃)₇₀(Sb₂Se₃)₁₀(ZnSe)₂₀ composition as obtained from the estimated optical parameters. Optical energy gap, E_g, for (GeSe₃)₇₀(Sb₂Se₃)₁₀(ZnSe)₂₀ film shows a noticeable decrease from 1.81 eV at 0 kGy to 1.52 eV at 690 kGy and conversely the corresponding band tail width, E_e, increases from 0.123 eV at 0 kGy to 0.138 eV at 690 kGy. By contrast, the estimated values of E_g and E_e for (GeSe₃)₈₀(Sb₂Se₃)₂₀ compositions, show no change with different γ-irradiation doses in the same range. The obtained results could be explained in terms of the band edge shift into the energy gap due to either the formation of localized states at the edges or weakening in the composition cohesive energy as reformation of new weaker bonds appear.     

Correlation of structure parameters of absorber layer with efficiency of Cu(In,Ga)Se<Subscript>2</Subscript> solar cell

Polycrystalline Cu(In, Ga)Se₂ with Ga-content x=Ga/(In+Ga) ranging from 0.0 (CuInSe₂) to 1.0 (CuGaSe₂) in heterojunction thin film solar cells were grown by multi-source evaporation. Solar cells with a highest efficiency of η=15.3% need a composition of x≈0.2. At this composition, the c/a ratio of the lattice constants for the tetragonal lattice equals c/a=2, indicating ideal tetragonality. These results suggest that low electronic defect densities occur at x≈0.2, due to the smallest possible crystallographic distortion of the tetragonal lattice at this composition. Cells with high efficiencies require grain sizes above 145 nm and a high preferred orientation (P₂₀₄/P₂₂₀ pole density ratio) for the grains.