Ahmed S.G. Khalil

Advanced liquid cooling for concentrated photovoltaic electro-thermal co-generation

We demonstrate an advanced packaging approach with an embedded silicon micro-channel water cooler where the photovoltaic cell is electrically connected by a metallization on the silicon substrate. The backside of the silicon substrate contains the micro-machined fluidic channels thereby minimizing the thermal resistance compared to a state — of — the — art package. This leads to a reduced temperature drop between the photovoltaic cell and the coolant, allowing an increase in the temperature of recovered heat. A low-pressure drop split-flow fluid manifold is implemented to distribute the coolant from one single input to the micro-channel array and back from two outlet ports. A thermal resistance of 0.12 cm2K/W was demonstrated, which allows for the removal of 100W/cm2 heat (>1000 suns) at a ΔT of 12K. Direct chip attached silicon coolers enable higher overall concentration factor thereby reducing photovoltaic cell cost. An additional benefit of silicon is its inertness against corrosion and the matching thermal expansion coefficient which allows building of systems with a very long lifetime. The split flow configuration reduces pumping power to about 5% of the system photovoltaic output. More complex manifold micro-channel systems are proposed to minimize the pumping power to a level below 1% and to cool arrays of cells on a single large substrate.

Enhancing the absorption capabilities of thin-film solar cells using sandwiched light trapping structures

A novel structure for thin-film solar cells is simulated with the purpose of maximizing the absorption of light in the active layer and of reducing the parasitic absorption in other layers. In the proposed structure, the active layer is formed from an amorphous silicon thin film sandwiched between silicon nanowires from above and photonic crystal structures from below. The upper electrical contact consists of an indium tin oxide layer, which serves also as an antireflection coating. A metal backreflector works additionally as the other contact. The simulation was done using a new reliable, efficient and generic optoelectronic approach. The suggested multiscale simulation model integrates the finite-difference time-domain algorithm used in solving Maxwells equation in three dimensions with a commercial simulation platform based on the finite element method for carrier transport modeling. The absorption profile, the external quantum efficient, and the power conversion efficiency of the suggested solar cell are calculated. A noticeable enhancement is found in all the characteristics of the novel structure with an estimated 32% increase in the total conversion efficiency over a cell without any light trapping mechanisms.

Simulating the dispersive behavior of semiconductors using the Lorentzian-Drude model for photovoltaic devices.

Unique light-trapping structures that improve the efficiency of thin-film solar cells require advanced computational methods that can simulate the propagation of light through the thickness of each material in the solar cell. The simulations community that uses the Lorentz-Drude (LD) model cannot precisely simulate the propagation of light through the entire spectrum of the Sun, due to the difficulty in extrapolating the coefficients of each solar cell material. In this paper, a new technique for modeling dispersive and absorptive material over the Suns entire wavelength range (200-1700 nm) using the LD model is suggested. The new numerical models are used for simulating light propagation through various one-dimensional light-trapping structures, including metal backreflectors and distributed Bragg reflectors. All the numerical simulation results show agreement with previously published theoretical and experimental results. The proposed simulation technique will help the simulations community in using the LD model to simulate the propagation of light in solar cells more accurately.

Inkjet Printing of a 20 GHz Coplanar Waveguide Monopole Antenna Using Copper Oxide Nanoparticles on Flexible Substrates : Effect of Drop Spacing on Antenna Performance

Coplanar monopole antennas printed using copper oxide nanoparticles on flexible substrates are characterized in order to study the effect of the ink drop spacing on the antenna parameters. Polyethylene Terephthalate and Epson paper were the chosen flexible substrates, and the antennas were designed to operate at 20 GHz. A maximum conductivity of 2.8 × 107 Ω−1m−1 was obtained for the films printed on Polyethylene Terephthalate using a drop spacing of 20μm. The corresponding antenna achieved a gain and an efficiency of 1.82 dB and 97.6%, respectively. Experiments showed that smaller drop spacings lead to bulging of the printed lines while the antenna performance worsens for longer ones. At the same drop spacing, antennas printed on Epson paper substrate showed a −10 dB return loss bandwidth which extended from 17.9 GHz to 23.3 GHz, leading to a fractional bandwidth of 26.0%.

A comparative study on the photocatalytic degradation of organic dyes using hybridized 1T/2H, 1T/3R and 2H MoS2 nano-sheets

MoS2 is a very attractive material and has been well studied for potential applications in various areas. However, due to the wide variety of factors affecting the molecular and electronic structure of MoS2, several contradictory reports about the adsorptive and photocatalytic properties of such materials have been published. In most of these reports, the effect of the actual phase of the materials on the properties was neglected. Here, different phases of MoS2 nanosheets (1T/2H, 1T/3R and 2H) have been obtained using the hydrothermal method with different Mo : S molar ratios and different autoclave filling ratios. The obtained materials have been thoroughly characterized using Raman, UV-vis, powder XRD, SEM, TEM and XPS measurements in order to accurately identify the existing phases in each material. A comparative study of the photocatalytic organic dye degradation efficiency under white light irradiation has been conducted using methyl orange to correlate the different activity of each material to the respective phase composition. The results indicate a much higher performance of the 1T/2H phase compared to the 2H and 3R phases. Detailed computational studies of the different phases revealed the emergence of mid-gap states upon introducing 1T sites into the 2H lattice. This leads to the improvement of the photocatalytic activity of 1T/2H compared to the other prepared materials.

Effect of Inorganic Ions on Pyrite Catalyzed Fenton Reaction

Industrial wastewaters contain not only organic pollutants but also considerable amounts of inorganic ions that may affect the efficiency of wastewater remediation processes. Thus, it is important to study the influence of such ions in the respective chemical reactions. Here we performed Fenton reactions for the decolourization of Rhodamine B dye using the naturally occurring mineral pyrite (FeS2) as heterogeneous catalyst in presence of different inorganic ions that are typically present in wastewater. Additionally, the effect of milling time of the pyrite powder on the performance of the heterogeneous Fenton reaction has also been investigated.

Comprehensive study of various light trapping techniques used for sandwiched thin film solar cell structures

Thin film solar cells (TFSCs) where first introduced as a low cost alternative to conventional thick ones. TFSCs show low conversion efficiencies due to the used poor quality materials having weak absorption capabilities and to thin absorption layers. In order to increase light absorption within the active layer, specially near its absorption edge, photon management techniques were proposed. These techniques could be implemented on the top of the active layer to enhance the absorption capabilities and/or to act as anti-reflecting coating structures. When used at the back side, their purpose is to prevent the unabsorbed photons from escaping through the back of the cell. In this paper, we coupled the finite difference time-domain (FDTD) algorithm for simulating light interaction within the cell with the commercial simulator Comsol Multiphysics 4.3b for describing carrier transports. In order to model the dispersive and absorption properties of various used materials, their complex refractive indices were estimated using the Lorentzian-Drude (LD) coefficients. We have calculated the absorption profile in the different layers of the cell, the external quantum efficiency and the power conversion efficiency achieved by adding dielectric nanospheres on the top of the active layer. Besides that, the enhancement observed after the addition of dielectric nanospheres at the back side of the active layer was computed. The obtained results are finally compared with the effects of using textured surface and nanowires on the top in plus of cascaded 1D and 2D photonic crystals on the back.