A. Sayari

Characterisation of the GaAs-based intermediate band solar cell with multi-stacked InAs/InGaAs quantum dots

We report on both the photovoltaic and the optical properties of GaAs p-i-n solar cell (SC) and GaAs p-i-n SC with multiple InAs quantum dot (QD) layers in the i-region. Current-voltage and impedance spectroscopy measurements have been used to characterise the two SC devices. Refractive index, extinction and absorption coefficients were deduced from spectroscopic ellipsometry. I-V measurements in dark and illumination conditions have been carried out to investigate the photovoltaic effect in these devices. Our results suggested that the presence of trap states could cause the degraded photovoltaic performance of the QD SC device. The C-V characteristics of the QD SC device show forward bias region with negative values of capacitance, which is caused by the filling process of the dots near the junction. The equivalent circuit model consisting of two-series connected RC networks with a series resistance Rs was found to give a good fit to the experimental data.

Influence of aluminium concentration in Zn0:9V0:1O nanoparticles on structural and optical properties

Abstract The (V,Al) co-doped ZnO nano-structured powders (Zn0.9-xV0.1AlxO, where x = 0.02, 0.03 and 0.04) were synthesized via the sol-gel technique and their structural and optical properties were investigated. The effect of Al concentration on the structural and optical properties of the Zn0.9-xV0.1AlxO nanopowders was studied using various techniques. The XRD patterns indicate that the samples have a polycrystalline wurtzite structure. The crystallite size increases with increasing the Al content and lies in the range of 23 to 30 nm. The lattice strain, estimated by the Stokes-Wilson equation, decreases when Al content increases. SEM and TEM micrographs show that Zn0.9-xV0.1AlxO powders are the agglomeration of nanoparticles having spherical and hexagonal shapes with dimensions ranging from 20 to 30 nm. FT-IR spectra show a distinct absorption peak at about 500 cm-1 for ZnO stretching modes and other peaks related to OH and H2O bands. Raman spectra confirm the wurtzite structure of the Zn0.9-xV0.1AlxO nanoparticles. The direct band gaps of the synthesized Zn0.9-xV0.1AlxO nanopowders, estimated from the Brus equation and the crystallite sizes deduced from XRD, are around 3.308 eV. The decomposition process of the dried gel system was investigated by thermal gravimetric analysis (TGA).

Annealing and Ni content effects on EPR and structural properties of Zn1–xNixO aerogel nanoparticles

Abstract Zn1-xNixO aerogel nanopowders with nickel concentration in the range of 0.05 ≤ x ≤ 0.25, were synthesized by the sol-gel processing technique and post-annealed in air at 500 °C. Structural, vibrational, thermal and magnetic properties of the as-prepared and annealed Zn1-xNixO powdered samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering, thermal gravimetric analysis (TGA) and electron paramagnetic resonance (EPR) spectroscopy. In addition to the ZnNiO phase, XRD analysis revealed the formation of a secondary NiO phase when the Ni content was greater than or equal to 10 %. The TEM images confirm that the particle size is in the range of 20 nm to 40 nm, in accordance with XRD results, and the particles are well dispersed. Raman scattering measurements confirm the wurtzite structure of the synthesized Zn1-xNixO nanopowders and show that intrinsic host-lattice defects are activated when Ni2+ ions are substituted to the Zn sites. Room temperature ferromagnetic order was observed in all of the samples and was strongly dependent on the Ni content and thermal annealing. These results indicate that the observed room temperature ferromagnetism in ZnNiO may be attributed to the substitutional incorporation of Ni at Zn sites.