We describe the development and the first characterization of a compact trace-gas sensor based on cantilever photoacoustic spectroscopy (CEPAS). The sensor was characterized in order to find the optimal operating parameters (pressure, molecule absorption line and laser modulated frequency). N2O was selected as test molecule. A quality factor of 200 at 10 mbar of cell pressure were determined. Furthermore, the first test measurements showed a minimum detection level of hundreds of ppb with integration time of 30 ms.

La scienza può aiutare l'economia circolare? Relatori: Alberto Zanelli, ricercatore Istituto per la sintesi organica e la fotoreattività (ISOF-CNR); Marica Canino, ricercatrice Istituto per la microelettronica e microsistemi (IMM-CNR) Ordine e grado: Secondaria di II grado Area disciplinare: Scientifica

Monolithic tandem cells involving a top cell with Si nanocrystals embedded in SiC (Si NC/SiC) and a c-Si bottom cell have been prepared. Scanning electron microscopy shows that the intended cell architecture is achieved and that it survives the 1100 °C anneal required to form Si NCs. The cells exhibit mean open-circuit voltages Voc of 900-950 mV, demonstrating tandem cell functionality, with <=580mV arising from the c-Si bottom cell and >=320mV arising from the Si NC/SiC top cell. The cells are successfully connected using a SiC/Si tunnelling recombination junction that results in very little voltage loss. The short-circuit current densities jsc are, at 0.8-0.9 mAcm2, rather low and found to be limited by current collection in the top cell. However, equivalent circuit simulations demonstrate that in current-mismatched tandem cells such as the ones studied here, higher jsc, when accompanied by decreased Voc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that Voc is a better optimisation parameter than jsc for tandem cells where the limiting cell exhibits poor junction characteristics. The high-temperature-stable cell architecture developed in thiswork, coupledwith simulations highlighting potential pitfalls in tandemcell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright © 2016 John Wiley & Sons, Ltd.

Thermally treated silicon rich oxides (SRO) used as starting material for the fabrication of silicon nanodots represent the basis of tunable bandgap nanostructured materials for optoelectronic and photonic applications. The optical modelization of such materials is of great interest, as it allows the simulation of reflectance and transmittance (R&T) spectra, which is a powerful non destructive tool in the determination of phase modifications (clustering, precipitation of new phases, crystallization) upon thermal treatments. In this paper, we study the optical properties of a variety of as-deposited and furnace annealed SRO materials. The different phases are treated by means of the effective medium approximation. Upon annealing at low temperature, R&T spectra show the precipitation of amorphous silicon nanoparticles, while the crystallization occurring at temperatures higher than 1000 °C is also clearly identified, in agreement with structural results. The existing literature on the optical properties of the silicon nanocrystals is reviewed, with attention on the specificity of the compositional and structural characteristics of the involved material.

In this work we present the results we obtained by processing 1 ©cm p and n-type, multi-crystalline silicon wafers when a double hydrogenated amorphous /crystalline silicon heterojunction is applied to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. We have investigated the role of the multi-crystalline silicon surface conditioning, on the base of illuminated and dark current-voltage characteristic, quantum efficiency measurements, and overall photovoltaic solar cell performance. In particular, a comparison between an acidic isotexturing and a chemical polishing treatment is presented and discussed in detail. We have found that the morphology of the multicrystalline silicon surface plays a tough role in the a-Si:H properties when used as surface passivating layer. The commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film, needed in heterojunction technology. The deposition of a uniform thickness over a whole textured silicon surface of a thin amorphous layers still requires further investigation and improvement. Nevertheless the high Voc obtained on p-type doped mc- Si (625 mV) remarks the effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process.

The optical properties of silicon nanodots embedded in a dielectric matrix are studied. A critical review of the existing literature is presented, with focus on the specificity of the compositional and structural characteristics of the involved material. The results are applied to the simulation of spectrophotometric measurements taken on thermally treated silicon rich oxide (SRO) grown by Plasma Enhanced Chemical Vapor Deposition. Our results include the determination of the optical properties of the SRO host matrix. Simulation of the R&T data indicate the precipitation of amorphous silicon clusters for annealing temperatures below 1000°C, and subsequent crystallization for higher temperatures. The dielectric function of silicon nanoparticles that form after annealing is observed to closely approach the literature data obtained for samples fabricated by thermal precipitation from a silicon implanted SiO2 matrix.

We studied the passivation effect of plasma deposited intrinsic and doped hydrogenated amorphous silicon layers on industrial grade, n-type, 1 ?cm multicrystalline silicon, aimed at the fabrication of Heterojunction (HJ) solar cells based on such material. The investigated variables include the process temperature, hydrogen dilution of silane during the intrinsic layer deposition, and passivating layer thickness. The structure of the passivating layers was analysed by optical measurements. Surface photovoltage and quasi-steady state photoconductance were used to characterize untreated and treated wafers. Thin a-Si:H layers exhibited the best passivating performance.

The surface morphology and the electrical activation of P+ implanted 4H-SiC were investigated with respect to annealing treatments that differ only for the heating rate. P+ implantation was carried out in lightly doped n-type epitaxial layers. The implantation temperature was 300 degrees C. The computed P profile was 250 nm thick with a concentration of 1x10(20) cm(-3). Two samples underwent annealing at 1400 degrees C in argon with different constant ramp up rates equal to 0.05 degrees C/s and 40 degrees C/s. A third sample underwent an incoherent light Rapid Thermal Annealing (RTA) at 1100 degrees C in argon before the annealing at 1400 degrees C with the lower ramp rate. The ramp up of the RTA process is a few hundred degrees per second. Atomic Force Microscopy (AFM) micrographs pointed out that the surface roughness of the samples annealed at 1400 degrees C increases with increasing heating rate and that the critical temperature for surface roughening is above 1100 degrees C. Independently on the annealing cycle, Scanning Capacitance Microscopy (SCM) measurements showed that the P profiles are uniform over the implantation thickness and have plateau concentration around 9x10(18) cm(-3) in all the implanted samples. The fraction of P atoms activated as donors is 13% of the total implanted fluence.

An n-type 8 degrees off-axis < 0001 > 4H-SiC epitaxial wafer was processed. The n-type epilayer had doping and thickness of, respectively, similar to 3 x 10(15) cm(-3) and similar to 5 mu m. p(+)/n diodes with not terminated junctions were constructed by a selective area implantation process of 9.2 x 10(14) cm(-2) Al+ ions at 400 degrees C. The diodes had areas in the range 2x10(-4)-1 x 10(-3) cm(2). The Al depth profile was 6x10(19) cm(-3) high and 164 nm thick. The post implantation annealing process was done in a high purity Ar ambient at 1600 degrees C for 30 min. The diode current-voltage characteristics were measured in the temperature range 25-290 degrees C. Statistics of 50-100 measurements per device type were done. The fraction of diodes that could be modeled as abrupt junctions within the frame of the Shockley theory decreased with increasing area value, but was always >= 75%. The ideality factor was >= 2 only at temperatures >= 200 degrees C and bias values <= 1 V. The leakage current was extremely weak and remained of the order of 10(-9) Acm(-2) at 70 degrees C and 500 V reverse bias. 4% of the diodes reached the theoretical voltage breakdown that was 1030 V. The surface roughness of un-implanted and implanted regions after diode processing was, respectively, 2 nm and 12 nm.