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Hydrogenated amorphous silicon carbon alloys for solar cells

Hydrogenated amorphous silicon carbon films were grown by PECVD from silane/methane gas mixtures by fixing the methane ratio in the gas phase and by changing the rf power and pressure. The effects of the discharge parameters on the optical, electrical and structural properties were investigated. These effects were attributed to the variation of carbon content in the film. The analyses enabled us to determine the optimal growth conditions to produce a-SiC:H materials, suitable in solar cell applications, with a good photosensitivity and low defect density of states.

Monitoring of concentrated radiation beam for photovoltaic and thermal solar energy conversion applications

Methods for evaluating the light intensity distribution on receivers of concentrated solar radiation systems are described. They are based on the use of Lambertian diffusers in place of the illuminated receiver and on the acquisition of the scattered light, in reflection or transmission mode, by a CCD camera. The spatial distribution of intensity radiation is then numerically derived from the recorded images via a proprietary code. The details of the method are presented and a short survey of the main applications of the method in the photovoltaic and thermal solar energy conversion field is proposed. Methods for investigating the Lambertian character of commercial diffusers are also discussed.

Changes of hydrogen evolution thermodynamics induced by He and H2 dilution in PECVD a-Si:H films: influence on thermal crystallization

Abstract Intrinsic and n-type amorphous silicon films were deposited by plasma-enhanced chemical vapour deposition on Qz (fused silica), crystalline silicon and aluminium substrates. Different substrate temperatures (200°C and 300°C) and 50% diluted SiH 4 in He or H 2 process gas were used to change hydrogen content and microstructure of deposited amorphous material. The hydrogen evolution thermodynamics of a-Si:H films was investigated using differential scanning calorimetry to obtain the entropy change and the activation energy of hydrogen evolution process. Crystallization of a-Si:H was obtained by an isothermal annealing performed at a pressure of 30 mTorr, at 650°C, using different annealing times (30–480 s). Different entropy variations are observed in the a-Si:H films. The hydrogen evolution affects the crystallization kinetics; in fact, crystallization was delayed in the samples with a greater disorder after hydrogen evolution.

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.

Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells

Abstract The growth of polycrystalline silicon on glass by laser-assisted chemical vapor deposition has been studied with the aim of identifying a light absorber layer for solar cells, with superior material quality compared to other technologies available for low-temperature substrates. One-dimensional calculations of the thermal wave produced by laser irradiation have been used to elucidate the complex interaction of the molten silicon surface layer with the substrate during the growth. The experiments show the relevant role played by the seed layer used as the growth initiator. The morphology of the laser-crystallized films has been analysed by scanning electron microscopy and X-ray diffraction. Polysilicon films, 2 μm thick, with a compact structure consisting of 1–2-μm grains that are almost monocrystalline, have been obtained.

Application of Nd:YLF laser to amorphous silicon crystallization process

Abstract Polysilicon thin films have been obtained by Laser Induced Crystallization utilizing a Q-switched diode pumped, frequency-doubled Nd:YLF laser at 523-nm wavelength. Intrinsic and n-doped amorphous materials, of different thickness, have been deposited on Corning 1737 by LPCVD technique. The irradiation conditions have been varied in order to study their influence on crystallized material properties. Electrical and optical properties of as-deposited and crystallized films have been determined. Structural characterization has been performed to evaluate average grain size and distribution. Larger grain size has been observed in intrinsic materials compared to n-doped materials and the largest grain size (≈1 μm) has been obtained on materials having thickness of 50 nm. Critical role of doping in the crystallization process has been pointed out.

Indoor-outdoor measurement inter-comparison of c-Si concentrating photovoltaic cells at 1X to 300X

In this work the authors show results of a measurement inter-comparison campaign that saw the cooperation of the University of Applied Sciences and Arts of Southern Switzerland (SUPSI-ISAAC, indoor characterization), the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA, outdoor characterization) and the University of Cyprus (indoor and outdoor characterization). The measurement campaign is extremely useful to improve the know-how of the centres involved. Aside the well-known spectral mismatch effects, the importance of the “capacitive effect” is highlighted in this work, which arises when the tested high-efficiency CPV cell are characterized indoor on a pulsed solar simulator, depending on the duration of the pulse.

Large area a-Si/a-Si tandem module with 9.1% conversion efficiency

This work illustrates the research activity carried out at ENEA for the achievement of high efficiency large area a-Si modules by a low cost process involving a simple single chamber PECVD reactor. Device optimization leading to small area (1 cm/sup 2/) single and tandem a-Si junctions exceeding 10% efficiency is described. Details of fabrication process, such as back-reflector, microcrystalline layers, interface cleaning process, adjustment of i-layer absorption and device thermal annealing are given. In addition, the results of laser scribing interconnection technique for module realization are also described. Back scribing and modified beam power distribution were effective for the improvement of module efficiency. A pin/pin tandem module with initial efficiency of 9.1%, on active area of 750 cm/sup 2/, has been realized. This is the best efficiency ever reported by a European laboratory for a-Si large area tandem modules.