Mechanical effects of chemical etchings on monocrystalline silicon for photovoltaic use

The mechanical e!ects of two etching treatments commonly applied on silicon wafers for the PV industry, are considered. The failure characteristics of this material under concentrated load are shown . In both cases, the maximum elongation and sustainable load of the etched wafers were measured to be higher than those of the original sample. The employed experimental procedure and results are presented here and a statistical data analysis substantiates the results observed. An attempt of explanation for this e!ect is o!ered based on the removal of a shallow highly defective layer induced by the etching of the material.

Electrical Properties of Silver Impurities and their Annealing Behaviour in p-Type Fz Silicon

The electrical activity of silver as well as its annealing properties in $10~\Omega$ cm p-type Fz silicon substrate are studied by means of the four-point probe and minority carrier lifetime measurements. Silver atom concentration in the range 10$^{14}$ to 10$^{15}$ cm$^{-3}$ consistently showed a donor type behaviour in the material and its presence led to a reduction of up to two orders of magnitude in the lifetime of minority carriers by the formation of deep-level traps. Isochronal annealing of silver contaminated specimens showed some gettering of the Ag impurities with resulting temperature dependent changes in the resistivity as well as the minority carrier lifetime values. Analysis of our results shows that a large fraction of the silver impurity atoms present forms the deep level defects and both the deep- and donor-levels appear to originate from the same source

Thick film microsensors based on nanosized Titania sol-gel powder

Thick films of nanostructured TiO2 and tantalum--doped TiO2 have based fabricated by screen--printing technology starting from pure titania and tantalum--doped titania powders prepared by sol--gel method. The titania powders, obtained via sol--gel, show crystalline anatase structure and the particles are homogeneous and nanosized (30 ÷ 50 nm). Two series of films each one composed by pure and Ta--doped titania samples have been obtained by firing the pastes in air atmosphere at the temperatures of 650°C and 850°C, respectively. SEM observations and electrical characterizations showed that the firing temperature strongly influences the nanostructure and the gas response of the pure titania samples. The addition of tantalum inhibits the grain sintering at the higher temperature. Moreover the electrical data show that the tantalum addition (10 at.%) does not affect the conductance of the films in air while significantly enhances the response towards CO and leaves almost unaltered or enhances its ability to sense NO2 depending on the thermal treatments

High performance photovoltaic concentrator systems

Photovoltaic concentrator systems allow for substantial reduction in system cost substituting high cost photovoltaic panels with low cost reflective surface and a small amount of high efficiency concentrator cells. Mirror based concentrator systems, complete with accurate tracking, allow for the highest concentration levels but the use of single large parabolic mirrors is expensive, adds technical complexity to the system and suffers from hot spots problems requiring the use of mixing secondary systems. The use of an array of small commercial flat mirror mounted on a metallic support structure allows for a cheaper system and greatly improves the illumination uniformity of the target. We propose a non-imaging mirror based two axis tracking concentrator capable, with no secondary concentrator, of an uniform 120 sun illuminated field

Composite mirror array photovoltaic concentrator system

The key issue in photovoltaic concentration is to substitute large surfaces of expensive silicon cells with inexpensive mirrors. The use of small metal-glass flat mirrors allows developing highly reliable modular concentrator systems, suitable for mass production

Fractal patterns for optimal charge collection on front contact concentrator cells

Concentrator solar cells must minimize surface covering due to the high value, from the energetic point of view of the active recive area. At the same time contacts must be thick and dense enough to carry the extreme current densities, often higher than 10 A/cm^2, without eccessive loss..

Composite dish systems for high efficiency concentrator photovoltaics

High level optical concentration systems can possibly offer a viable alternative to flat panel photovoltaic because large powers can be produced with relatively small amount of photovoltaic cells. At the same time the widespread application of high concentrator systems would not be hindered by the limited amount of silicon available for the flat panel market, since concentration permits the assessment of electronic grade silicon for the cells, and the system production costs would significantly gain from scale law effects. Mirror concentrators, while used nowadays mostly for thermal applications, can be usefully employed for photovoltaic once the peculiar requirements of concentrator systems have been correctly taken into account. Concentration ratios higher than 50X can be attained, for KW’s scale power plants, by the use of composite mirrors array equipped with precise two axis sun tracking systems. A set of flat mirror is mounted on a pseudo-parabolic frame allowing for an highly scalable design. While this ensures a fairly uniform illumination profile, badly needed by photovoltaic receivers, the use of a non-imaging homogenizing concentrator allows for performance and concentration boosting2. Specially designed cells, based on a design similar to the one sun cell, can be employed on concentrator systems with a significant cost reduction3 while the cooling can be based on miniaturized heat exchanger developed on silicon substrates with micromachining techniques5. Moreover the proper selection, by the use of dichroic coatings on the secondary concentrator, of the wavelength reaching the cells allows to significantly improve the cell conversion efficiency and the employable concentration ratios4. The remaining spectrum part can be sent on a separate receiver, like a different cell array of a thermal receiver. We present here an example of a small concentrator system designed and produced along these lines. The system is actually operational at the University of Ferrara and is routinely used for research purposes on the concentration subject

Synthesis of pure and loaded powders of WO3 for NO2 detection through thick film technology

Nanopowders of pure and of Mn-, Ta- and Zr-loaded (5 wt.%) WO3 were prepared and printed as thick films. Investigation of the influence of the doping on morphology, structure and gas response versus NO2 has been performed. Pure nanometric WO3 was prepared by a modified sol–gel synthesis while loading was carried out by impregnation with Mn(II), Ta(V) and Zr(IV) chlorides. Addition of Ta resulted in grain coalescence and phase transition inhibitions in the layers with respect to pure WO3 films, being the effect strongly enhanced in the filmsfired at 850 °C. The Ta-doped films turned out to be the most sensitive films with a response extending down to the sub-ppm domain

Comparison between normal and reverse thin crystalline silicon solar cells

The newly developed ingot growing techniques, as the three-grain and the columnar multigrain ingot processes, are now offering the possibility of slicing thinner wafers (=< 100 m). In this paper we present the results obtained on p type large area (=< 100 cm2) and 100 m thick wafers by using both conventional and reverse cell manufacturing technologies.The conventional cells are provided with aluminium or boron BSF plus screen-printed silver mirror or a silver-aluminium net; the reverse cells have a FSF and the deep back junction completely covered by a screen-printed or CVD silver layer.The constructing parameters have been chosen on the base of one and two dimensions modeling and both raw material and devices have been completely characterized.This work shows that very thin wafers do not introduce serious problems for the conventional manufacturing of solar cells. The efficiencies of the normal and of the reverse cells are found to be comparable and are of the same order than those of thicker cells, however at a significant lower cost. The main obtained result has to be related to the demonstration of a cell manufacturing feasibility starting from very thin wafers