Marco Merli

NANOSTRUCTURED PURE AND NB-DOPED TIO2 AS THICK FILM GAS SENSORS FOR ENVIRONMENTAL MONITORING

Abstract Thick films of nanostructured TiO 2 and Niobium-doped TiO 2 have been fabricated by screen-printing technology starting from pure Titania and Niobium-doped Titania powders; the powders were prepared by laser pyrolysis method which provides nanosized particles. The laser powders are crystalline with anatase structure and an average specific surface area of approximately 100 m 2 g −1 ; their grain size ranges from 10 up to 25 nm, the particles are spherical monocristalline and without internal porosity. In this work we evidentiated that TiO 2 -based thick film sensors exhibit a suitable sensitivity to atmospheric environmental monitoring provided that the microstructural properties of the materials are suitably correlated to the required electrical features. Moreover nanostructured particles were obtained at high firing temperature by adding a proper metal ions to inhibit the grain sintering. Finally, prototype sensors based on pure titania sensing film have been prepared and tested in field for environmental monitoring application.

Nanostructured thick-film gas sensors for atmospheric pollutant monitoring: quantitative analysis on field tests

Thick-film gas sensors were fabricated by screen-printing technology starting from different nanostructured semiconducting oxide powders both n-type and p-type; ultrafine and homogeneously sized ceramic powders were prepared using chemical methods such as sol–gel techniques and thermal decomposition of heteronuclear complexes. The temporal evolution of the conductivity changes in the various semiconducting oxide thick films were studied in comparison with the concentrations of CO, NO, NO2 and O3 in the atmosphere as measured by the analytical equipment. An air quality control experiment in an urban area through some arrays of nanostructured thick-film sensors without an in situ calibration is described. Through the calibration performed directly in the field, one of the monitoring units by means of the conventional equipment, it was possible to evaluate the pollutants’ concentration in various points of the city. The good results obtained with this experiment allow us to consider the feasible use of the innovative station to measure the pollution due to the automotive traffic.

Array of thick film sensors for atmospheric pollutant monitoring

Abstract Chemical routes such as sol–gel techniques and thermal decomposition of heteronuclear complexes have been used to prepare nanosized powders of n-type (SnO2, TiO2 and In2O3) and p-type (LaFeO3) semiconducting oxides. Thick film gas sensors have been fabricated using these powders, printed on laser pre-cut 96% alumina substrates, each 2×2 mm element being provided with a heater, Au interdigitated contacts and a Pt-100 resistor for controlling the operating temperature. The films have been fired at different temperatures suitable to keep the grain size at the nanometer level. Electrical responses to some major polluting gases (CO, NO, NO2 and O3) obtained in the field have been compared with international standard analytical techniques.

Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples

Abstract The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C.

Effect of thickness and surface treatment on silicon water reflectance

Abstract In this paper we present reflectance measurements on silicon wafers and the dependence of the reflectance on the thickness, on the texturization and on the back metallization. Silver and aluminum have been deposited using screen printing and also vacuum evaporation as a reference. The work was intended as a contribution to the problem of light trapping that becomes important for thin wafers for commercial, low cost, silicon cells. For the case of wafers with polished surfaces, the results are analysed to yield the reflectivity of the back surface and the total energy absorbed in the sample. Texturization, on the contrary, introduces diffraction phenomena making the evaluation of the absorbed energy more complex.

Shallow junctions for ULSI technology

Abstract. Post-implantation annealing conditions suitable to obtain very shallow p + -n and n + -p junctions with good electrical characteristics are critically examined by considering transient enhanced diffusion phenomena, dopant electrical activation, regrowth of the amorphous layer and radiation damage removal. Since activation energy of the diffusion processes is lower than that of damage recovery, high temperature rapid thermal annealings results, in principle, the best technique to fabricate shallow junction. The characteristics of diodes with junction depth of about 0.1 μm obtained by low energy implantation and annealing at 1000°C for 10 s are reported and discussed. Finally, ion implantation of B and As through a film of molybdenum or molybdenum silicide has been considered as a promising approach to shallow junction formations. Preliminary results related to devices fabricated by means of this self-aligned procedure are presented.

Minority electron diffusion coefficient from lifetime measurement combination

Minority electron diffusion coefficients as a function of dopant density are evaluated from the comparison between a steady state diffusion length (photoelectromagnetic effect) and a transient lifetime (photoconductivity decay) measurement. The reliability of the method is discussed with reference to the limits inherent to the two techniques. Within the limits of the experimental data dispersion, minority values comparable to the majority ones for the doping range 3×1014

A reverse silicon solar cell

Abstract In order to increase the efficiency of large-area solar cells made from thin (less than or equal to 200 μm) crystal wafers, without eliminating the thick film process sequence, an innovative back junction cell with a front contact grid on the lit side was developed. The major advantages of the reverse cell over the conventional cell are as follows: 1. (1) the reverse cell is suitable for the collection of long-wavelength light reflected on the back surface and at the same time of the short wavelengths owing to the opportunity of forming a shallow front surface p + layer without the risk of leakages from the contact metallization. 2. (2) the series resistance of the back and front contacts can be greatly reduced. In this work we carried out a process sequence for reverse cell production: up to now our best fill factor was 78% which is probably the highest value obtained using thick film technology. In addition, using numerical modelling, we analysed the spectral responsivity, the short-circuit current and the dependence of the efficiency on the front surface field depth. The behaviour of the short-circuit current with increasing lifetime was also investigated.

Lifetime measurements in solar cells of various thicknesses and the related silicon wafers

Abstract Large area (100 cm2 or larger) Czochralski-grown (CZ) silicon wafers combined with an appropriate low-cost manufacturing technology will probably be one of the best choices for large-scale and low-cost solar cell production for several years. The aim of this work is to test if a technology based on thick film techniques limits the solar cell efficiency by lowering the minority carriers lifetime of the CZ commercial wafers. To decouple bulk and surface effects on carrier lifetime we used samples of various thicknesses for the wafers and for the related solar cells. The experimental methods used were based on the measurements of the photoconductivity decay for the starting material, and the photocurrent decay and spectral responsivity for the devices. The results clearly show that the back surface field (BSF) practically eliminates the back surface recombination and that the technological processes do not significantly degrade the raw material lifetime.

Comparison between normal and reverse thin crystalline silicon solar cells

Abstract 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 μm). In this paper we present the results obtained on p type large area (≥ 100 cm 2 ) and 100 μm 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.