Rami Ghannam

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.

Ultra‐High‐Concentration Photovoltaic‐Thermal Systems Based on Microfluidic Chip‐Coolers

The electrical efficiency of a photovoltaic‐thermal system for coolant inlet temperatures ranging from 25 °C to 75 °C and concentrations from 500 to 1500 suns was investigated experimentally and theoretically. In this system absorbed radiation and thermal losses from the electric circuit are collected in a thermal circuit. This allows one to directly drive a thermal desalination process thereby contributing to an improved system efficiency. A triple‐junction solar cell was tested in two different configurations. At 1500 suns the electric efficiency of a silicon microchannel cooler package exceeded the efficiency of a reference package with a copper cooler by 2% and it remained fully functional up to concentrations of 4930 suns. We present a general model for concentrated photovoltaic‐thermal systems in which the standard efficiency modeling approaches for triple‐junction cells are extended by temperature and concentration dependencies. The currents were modeled both following the Shockley‐Queisser and a “...

Bi 2 Te 3 as an active material for MEMS based devices fabricated at room temperature

This paper reports, for the first time, on the possibility of using thin films based on bismuth telluride (Bi-Te) alloys as a MEMS surface micromachined structural layer. Furthermore, it is also demonstrated for the first time that the thermoelectric properties of the deposited films can be optimized at room temperature. Developing this material at such low temperature is very attractive for realizing low cost, high performance, miniaturized energy harvesters that can replace batteries in low power applications such as autonomous sensors network, medical implants, pico satellites, etc.

Topological alignment of NLCs using nanoscale metallic grooves

Liquid crystal on silicon (LCoS) devices have been exploiting the ever-diminishing CMOS silicon process, which is pushing towards 45nm dimensions. Consequently, such fine metallic structures are bound to influence the alignment of the liquid crystal material. To illustrate this, a number of 1D metal patterns with differing mark-to-space ratio were fabricated using an Electron-Beam exposure technique. The results confirmed Dwight Berremans topological alignment theory regarding the pitch of the surface topography and how this influences the quality of the planar alignment. Patterns with a metal to spacing ratio of 1:1 were shown to yield higher contrast ratios and hence better planar alignment. Such findings could be useful for developing non-intrusive alignment methods for nanoscale LCoS devices.

Switching properties of VAN LCoS devices with ultra-microscale electrodes

Nanolithographic fabrication techniques may soon enable electrically-driven LCoS devices to be manipulated using ultra-nanoscale CMOS transistors. However, questions as to the switching properties of such LCoS devices arise due to the diminishing dimensions of their transistors. Thus, experimental investigations into the response times and the onset-threshold voltages for LCoS devices were embarked upon. Such measurements were obtained for various electrode dimensions and cell gaps. Furthermore, an interdigitated (IDT) electrode pattern was used to drive the homeotropically-aligned NLC material in a direction parallel to the bounding planes of the cell. Experimental findings revealed that faster response times were achieved when the electrode spacings were decreased. Such results have shown that a 10μm-thick device with an electrode pitch of 2μm can achieve a switch-on time of < 5ms. In addition, decreasing the electrode spacing results in the threshold voltage to drop. The results therefore indicate that improvements in a LCoS devices switching properties can be realised by using smaller electrode dimensions.

Non-display applications and the next generation of liquid crystal over silicon technology

The next generation of applications for liquid crystal (LC) over silicon technology will be non-display oriented systems such as adaptive optical interconnects, optical switches and optical image processors. These new non-display applications have a different set of material parameters, which means that existing display-based materials are not entirely optimal. This is particularly the case when the application is driven by phase modulation at high frame rates (more than 1 kHz). An example of such a non-display application is in adaptive optical interconnects. Optical data transmission between printed circuit boards is becoming more and more important as the data rate in electronic systems increases into the gigahertz region. One way of avoiding the data bottlenecks in board to board interconnects is to use optical links to transmit the data. Recent research into free-space optical links has shown that a high level of manufacturing tolerance must be used to maintain the link. However, one way of avoiding these limitations is to use a reconfigurable LC phase hologram as a beam-steering element to compensate for movement between the boards and maintain the optical data path.

Optical investigation of porous TiO2 in mesostructured solar cells

Porous TiO2 films are a crucial part of mesostructured solar cells (MSCs), both dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). However, the literature does not provide a clear description of the optical properties especially of the refractive index and scattering for those films relevant to MSCs. In DSSCs, two different porous TiO2 layers are included, the mesoporous active layer and the blocking layer. While the first is essential for the charge separation, electron collection and ion conduction, the second is intended for suppressing the loss of generated electrons to the electrolyte. Both layers consist of the same chemical compound, TiO2, but they have different porosities. For PSCs, the perovskite is deposited on a mesoporous TiO2 structure for enhancing the I–V characteristics This paper investigates TiO2 films really used in fabricated MSCs. We utilize a technique allowing the determination of the effective refractive index and the film porosity for two different film kinds fabricated using sol-gel methods, discussed in our previous work, to determine the thickness of TiO2 films typically used in fabricating MSCs.

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.