L. Lancellotti

Gas detection with a porous silicon based sensor

Abstract Porous silicon (PS) layers with 60% porosity and 80 μm thick were prepared from n-type silicon wafer. We present the sensitivity of PS photoluminescence to 250 ppm of carbon monoxide. Besides the variation of conductivity of the device due to presence of organic vapors such as chloroform, methanol, ethanol and toluene have been carried out.

Cyclododecane as support material for clean and facile transfer of large-area few-layer graphene

The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge. We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements. Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the cyclododecane transfer method and outperformed reference cells made by standard methods.

Nanostructured porous silicon for gas sensor applications

Abstract The response of two different types of nanostructured gas sensor to oxygen has been investigated. The first (optical) is based on the photoluminescence quenching effect of a porous silicon sample, the second on the changes of the electrical conductance v. environment of a porous silicon free standing membrane on an insulating neutral substrate. The response of both the devices to oxygen have been measured and compared. The optical based gas sensor exhibits a quenching following the Stern-Volmer model. The corresponding reactivity rate constant is found to depend on a characteristic nanodimension of the wire. The electrically operated sensor is more sensitive to oxygen and shows an opposite behavior if exposed to a reducing environment.

A procedure for assessing the reliability of short circuited concentration photovoltaic systems in outdoor degradation conditions

Abstract A procedure for assessing the reliability functions of photovoltaic concentration (CPV) systems, subjected to outdoor degradation, will be described in this work. The evaluation of the concentrator cells failure rate, the understanding of the origin of these degradation modes and how they affect the performances of concentration cells is an essential step to improve their reliability and to accelerate their competitiveness. The reliability evaluation methodology introduced here, as the cells are deployed outdoors and then are subjected to the actual sources of stress (e.g. ambient temperature, solar concentrated irradiance, cells working temperature and the variations of these parameters over days and seasons), will furnish values of failure rates very close to the true ones.

Induced degradation on c-Si solar cells for concentration terrestrial applications

Abstract There are several factors that can affect the reliability of concentration solar cells and influence the final devices performances. Electrical characterizations like the dark I – V curve in reverse and forward bias, the I – V curve under illumination and the quantum efficiency are generally used to test the stability of the cells. For this reason also in the packaging processes, used for making external connections, great attention is devoted to guarantee maximum reliability versus thermally induced expansions, contractions and/or stresses which might cause a degradation of the device performance. After the observation of the effects due to the packaging process of ENEA c-Si concentration solar cells, we analyze the attitude of the completely “packed” device to dissipate heat as compared to a commercial c-Si back contact concentration solar cell. Then we propose accelerated degradation tests to study, through electrical characterizations, the reliability of ENEA solar cell against high light intensities and humidity. In particular, for both ENEA and commercial devices the effects of each considered degradation mechanism on final behavior will be reported.

Doping of multilayer graphene for silicon based solar cells

In the present work we have tested the effects of graphene doping by nitrate ions and chlorine anions on graphene/n-silicon Schottky barrier solar cells, by the exposure to nitric acid and thionyl chloride vapors. In both cases the graphene doping process had beneficial effects on the power conversion efficiency (PCE) and thionyl chloride doping showed a better improvement than the one obtained with nitric acid. A solar cell with an initial PCE of 1.99% could be increased to 4.02% by nitric acid doping treatment while a solar cell with an initial PCE of 1.49% could be increased to 4.32% by thionyl chloride doping treatment.

MoOx as hole-selective collector in p-type Si heterojunction solar cells

We investigated the possible application of molybdenum oxide (MoOx) on the backside of p-type SHJ solar cells as substitute for the silicon-based back surface field layer. Solar cells with 4 cm2 area were fabricated on FZ c-Si(p) wafers, passivated with ultrathin i-a-Si:H buffers. A nanocrystalline n-SiOx emitter was applied while on the backside we applied 20 nm-thick p-type a-Si:H or evaporated MoOx (10 nm). Symmetric samples were additionally prepared to compare the effects on wafer passivation of MoOx versus the more conventional p-a-Si:H layer. For flat devices we have observed a Voc increase of ∼40 mV with MoOx replacing p-a-Si:H, with fill factors ∼73% in both the cases. Globally an efficiency increase of 1% absolute has been achieved moving to the MoOx hole collector. The feasibility of the MoOx/Ag backside configuration has been demonstrated also for textured p-type SHJ solar cells, reaching so far an efficiency of 18.1%.

Relevance Of TCO workfunction in n-silicon oxide emitter - c-Si (p) heterojunction solar cell

The amorphous /crystalline silicon heterojunction solar cells have largely demonstrated their usefulness to reach high efficiency. We have adopted a different and wider bandgap emitter based on silicon oxide, n-SiOx. A central role in this type of structure is played from the TCO workfunction whose value affects strongly the heterojunctions band structure at the emitter interface. RF magnetron sputtered TCO obtained with different deposition parameters, have been made in order to optimize their use in our heterojunction solar cell. Numerical simulation on the SiOx HJ, with TCO having proper workfunction value, show potential efficiency conversion well over the 23%. New Roman Bold font. An example is shown next.