Avinashi Kapoor

Doped zinc oxide window layers for dye sensitized solar cells

The present paper reports the fabrication of dye sensitized solar cell (DSSC), where boron doped ZnO (BZO) and aluminum-boron co-doped ZnO (AZB) thin films were used as front window electrodes. The highly crystalline zinc oxide (ZnO) nanoparticles (NPs) synthesized by the sol-gel route were used as host material for the dye. The efficiencies of the DSSCs formed using the BZO and AZB as window layers were obtained to be 1.56 and 1.84%, respectively. The enhanced efficiency in the case of an AZB window layer based DSSC is attributed to the increase in conductivity induced by co-doping of Al and B and an increase in the number of conducting pathways between the window layer and NPs provided by the nanorods. This facilitates a new approach in the window layer (doped ZnO) for DSSC application.

Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin films prepared by spin coating

Zinc oxide (ZnO) thin films were deposited on silicon substrates by a sol-gel method using the spin coating technique. The ZnO films were annealed at 700°C in an oxygen environment using different annealing times ranging from 1 to 4 h. It was observed that all the annealed films exhibited a hexagonal wurtzite structure. The particle size increased from 65 to 160 nm with the increase in annealing time, while the roughness of the films increased from 2.3 to 10.6 nm with the increase in the annealing time. Si diffusion from the substrate into the ZnO layer occurred during the annealing process. It is likely that the Si and O2 influenced the emission of the ZnO by reducing the amount of Zn defects and the creation of new oxygen related defects during annealing in the O2 atmosphere. The emission intensity was found to be dependent on the reflectance of the thin films.

Laser damage studies of silicon surfaces using ultra-short laser pulses

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablation of material. A magnified view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these ripples was between 0.5 and 2 μm and increased, on increasing the power density or number of pulses, and finally broke into parts, leaving well-ordered grains of approximate diameter 5 μm. Also for 100 or larger number of pulses, an amorphous ring in the periphery was formed. The diameter of this ring increased, on increasing either the laser fluence or the number of pulses. The formation of ripples has been explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285).

Effect of operating current dependent series resistance on the fill factor of a solar cell

The fill factor of a solar cell depends upon the series resistance, reverse saturation current, diode quality factor, operating current and voltage. Since the series resistance itself depends upon the operating current (or voltage), it makes the evaluation of fill factor very complicated. In this paper, we have evaluated the fill factor of a solar cell, taking into account operating current dependence of the series resistance.

ZnO based quantum dot sensitized solar cell using CdS quantum dots

This paper reports the fabrication of Zinc Oxide (ZnO) based quantum dot sensitized solar cell using Cadmium Sulphide (CdS) quantum dots (QDs) capped by poly vinyl alcohol (PVA). Chemical route was used to synthesize ZnO nanoparticles (NPs) as well as CdS QDs. The crystallite size of ZnO NPs was obtained to be 28 nm at 7 pH. The size of QDs decreased from 5.6 to 2.6 nm with increase in the PVA concentration from 2 to 10 wt. %. There is a blue shift in the band gap of QDs with increase in the concentration of PVA. Current-Voltage characteristic of the cell was obtained and various solar cell parameters were estimated. The efficiency of quantum dot sensitized solar cells was found to be 1.3% at AM 1.5.

Exact Analytical Analysis of Dye-Sensitized Solar Cell: Improved Method and Comparative Study

Dye sensitized solar cell (DSSC) being one of the main focus in area of photovoltaic after silicon solar cell, modeling and estimating their parameters becomes important for their analysis and enhancement in their performance. We have proposed an analytical approach having no approximation for extracting various parameters of DSSC using a well known Lambert W-function technique. The model used in present work is single diode model where effect of capacitance and inductance has been neglected. We have applied our method to six different DSSCs, which were previously reported in literature for both treated (Carboxylic acid treatment) as well as non treated solar cell. Plots of various parameters in- cluding I-V relations of different solar cells are presented in this article. To validate the present method proposed in ex- tracted parameters are compared with those present in literature. The results thus obtained demonstrate that this method proves to be superior to available methods and can be extensively applied for analyzing dye sensitized solar cells.

Hopping conduction in polycrystalline semiconductors

Measurements of dc conductivity (σ) on polycrystalline semiconductors, viz., InSb, Si, and CdTe, have been reported in the temperature range 77–300 K. The conduction mechanism near liquid‐nitrogen temperature has been identified as the hopping of charge carriers from the charged trap centers to empty traps near the Fermi level.

Fabrication and characterization of polyvinyl- alcohol-based thin-film optical waveguides

Polyvinyl alcohol (PVA)-based thin film optical waveguides are fabricated and characterized. Various waveguide parameters, namely, refractive index, propagation losses and depth, birefringence, and polarization conversion properties of dip-coated thin film optical waveguides are evaluated. The effective refractive index of PVA film is found to be between 1.506 and 1.531 for various concentrations. The propagation losses are 2.35 to 0.30 dB/cm for various thicknesses. The propagation loss reduces when the thickness of the film is increased. The low propagation losses, birefringence insensitivity, high transmission in the aged and annealed waveguides, control over film refractive index, the good adhesiveness of the film and good compatibility with substrate show that PVA-based waveguides are promising candidates for the optoelectronic field.

Four layer polymeric mode polarization filter for integrated optics

We report on our systematic study of fabrication and characterization of four-layer polymer waveguides. Various optical properties (such as refractive index, birefringence, and propagation loss) of polycarbonate and polystyrene waveguides are presented. The thin film structure consisting of glass/polycarbonate/polystyrene/air has been used for demonstrating polarization filter action because the two polymers are quite different with respect to their optical anisotropy. Modal electric field plots for both TE and TM are generated to support the observed behavior. It is also observed that the four-layer lightguide exhibits relatively low loss values compared to the monolayer configuration.

Determination of optimum load for a solar cell

An expression for load for maximum power transfer from a real solar cell, having resistive and current leakage losses, has been obtained using Lagrange’s method of undetermined multipliers for solving the transcendental current‐voltage relationship. The theoretical results are compared with the experimental measurements of the optimum load for p‐n junction solar cells for various illumination levels.