Advanced cost-effective crystalline silicon solar cell technologies

An overview is given concerning current industrial technologies, near future improvements and medium-term developments in the field of industrially viable crystalline silicon terrestrial solar cell fabrication (without concentration).     

8% Efficient thin‐film polycrystalline‐silicon solar cells based on aluminum‐ induced crystallization and thermal CVD

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline‐silicon (pc‐Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc‐Si layers obtained by aluminum‐induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc‐Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk‐Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc‐Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc‐Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin‐on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a‐Si layers that yield much higher V_oc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc‐Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc‐Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.     

Analytical study of PPV-oligomer- and C60-based devices for optimising organic solar cells

A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C₆₀, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I–V characteristics of several other devices were investigated. It is found that the presence of C₆₀ in a bulk heterojunction solar cell introduces fill factor reducing shunting paths.     

Greenhouse gas emissions in the nuclear life cycle: A balanced appraisal

In order to combat global warming, a detailed knowledge of the greenhouse gas (GHG) emissions associated with different energy conversion technologies is important. For nuclear energy, GHG emissions result from different process stages of the whole fuel cycle. A life-cycle assessment offers the possibility to properly calculate these emissions. In the past, both indirect energy use and GHG emissions were studied by many researchers. Most of the studies result in low indirect emissions comparable to wind turbines. However, some of the studies in the literature obtain high results adding up to a significant fraction of the direct emissions from a CCGT.     

Theoretical calculation and experimental evidence of the real and apparent bandgap narrowing due to heavy doping in p-type Si and strained Si1−xGex layers

Based on an analytical approach developed by Jain and Roulston, the different contributions to the bandgap narrowing at T = 0 K due to heavy doping in highly p-type doped Si and strained Si_1-xGe_x-layers are calculated for x < 0.3. The valence band in Si and in strained Si_1-xGe_x layers is not parabolic, it is highly distorted. To take the non-parabolicity into account a dopant concentration-dependent density of states effective mass is defined. within the framework of this formalism we find that the bandgap narrowing (BGN) in Si is not appreciably affected due to the band distortion. The situation for strained Si_1-xGe_x layers is quite different, in that the BGN increases significantly at doping levels exceeding 10¹⁹ cm⁻³. In the earlier published work, BGN of the Si_1−xGe_x layers was either slightly smaller or about the same as in Si. Now at high doping levels, BGN becomes considerably higher than in Si. We will show that the effect of the strain on the Fermi energy is much larger than on the BGN, which will cause a large change in the effective valence band offset. Comparison will be made then between our theoretical calculations and experimental results obtained on two different device structures. The modified effective valence band offset that we have calculated is in very good agreement with the experimental value derived from the published work on long-wavelength optical detectors. The apparent bandgap narrowing in strained p-type Si_1-xGe_x-layers is also calculated and compared with the experimental results on Heterojunction Bipolar Transistors fabricated in our laboratory.     

Porous Titanium Coatings Through Electrophoretic Deposition of TiH2 Suspensions

In the biomedical field, modification of titanium surfaces to improve the osteoinductive and antibacterial behavior is widely investigated. This functionalization can be further ameliorated by providing a porous coating with high loading capacity for bioactive materials and drug delivery carriers at the implant surface. In this work, a new powder metallurgical processing route used to deposit such porous pure titanium coatings on Ti based substrates is presented. The coatings were prepared by electrophoretic deposition (EPD) of TiH₂ powder suspensions followed by dehydrogenation and sintering in vacuum. The use of hydrides allowed to lower the sintering temperature below that of the α–β transition of the Ti6Al4V substrate. Measurement of the tensile bond strength confirmed a strong adhesion of the porous coating. Deposition of powders with different grain sizes resulted in porous titanium coatings with varying thickness, pore morphology, and surface roughness. The possibility to extend this coating technique to complex shaped implants is highlighted.     

Effect of carbon and tungsten content on the phase equilibria,microstructure, and mechanical properties of (Nb,W)C–Ni cermets

With the assistance of thermodynamic simulation, the NbC–Ni based cermets with different W and C additions were designed and sintered in liquid state at 1390°C for 90 min in vacuum. By controlling the carbon and tungsten content, (Nb,W)C–Ni based cermets were prepared with varied phase constitution, microstructure, and mechanical properties. The microstructure, composition of phases, grain size, and equilibrium phases were investigated using scanning electron microscopy, electron probe microanalysis, EBSD, and X-ray diffraction. The simulation reasonably predicted the experimentally observed phase constitutions. Depending on the additions, detailed analysis indicated that the cermets were composed of either a combination of cubic (Nb,W)C solid solution and Ni alloy binder or with an additional carbon-deficient phase. Furthermore, mechanical analysis showed a strong dependence of its mechanical properties (Vickers hardness, indentation toughness, and flexural strength) on the phases and NbC grain size.