Influence of a back side dielectric mirror on thin film silicon solar cells performance

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Réalisation de nouvelles structures de cellules solaires photovoltaïques à partir de couches minces de silicium cristallin sur substrat de silicium préparé par frittage de poudres

Les cellules photovoltaïques en couches minces de silicium cristallin sont des candidates prometteuses pour réduire le prix du watt-crête de l'énergie photovoltaïque, grâce à une très faible utilisation de silicium de haute pureté. Dans notre cas, les couches actives de silicium sont supportées par des substrats, de bas coût et compatibles avec les conditions de haute température nécessaires à une croissance cristalline rapide et de bonne qualité des couches. La société S TILE développe ces substrats, par frittage à partir de poudres de silicium, et en recristallisant les plaquettes ainsi obtenues. Le but de cette thèse est de valoriser ce substrat pour l industrie photovoltaïque et de démontrer qu il est adapté à la fabrication de cellules solaires à bas coût et rendement élevé. Ces travaux utilisent le procédé d épitaxie de silicium, qui est central pour fabriquer des cellules minces. Ils s articulent autour de deux axes principaux. Le premier est la fabrication de cellules solaires et leur optimisation sur des substrats de référence monocristallins. Dans ce cadre, de nombreuses voies ont été explorées : l utilisation de réflecteurs de Bragg en silicium poreux, l optimisation du dopage de l émetteur, la formation de gradients de dopage dans la base et l utilisation de structures à émetteur en face arrière. Ces études ont permis d évaluer le potentiel de ces différentes voies ; des résultats prometteurs pour l amélioration du rendement de conversion des cellules sur couches minces ont été obtenus. Le second axe de la thèse est la fabrication de cellules sur les substrats frittés préparés par S TILE et l application des moyens développés dans le cadre du premier axe pour améliorer ces cellules. Les rendements encoura-geants obtenus ont ainsi démontré la faisabilité de cellules solaires sur les substrats réalisés par le procédé de frittage à bas coût développé par la société S TILE.Crystalline silicon thin-film solar cells are promising candidates to reduce the watt-peak prices of photovoltaic energy, thanks to a much smaller use of high purity silicon. In our case, the active layers of silicon are supported by substrates. These substrates have low production costs and are compatible with the high temperature process steps, which are necessary to a rapid and high-quality crystalline growth. The company S TILE develops these substrates, by sintering silicon powders and recrystallizing the obtained wafers. The objective of this PhD thesis is to pinpoint the relevance of this substrate for the photovoltaics industry and demonstrate that it is adapted to the fabrication of solar cells with low cost and high efficiency. This work uses the epitaxy process, which is central to fabricate these thin-film cells. It is organized in two main axes. The first one is the fabrication of solar cells and their optimization on monocrystalline reference substrates. Several optimization pathways have been tested: the use of porous silicon Bragg reflectors, the optimization of emitter doping, the base variable doping and the use of rear emitter structures. The studies permitted to unveil the potential of each pathway; promising results were obtained for the improvement of thin-film solar cell conversion efficiency. The second axis of the thesis is the cell fabrication on the substrates prepared by S TILE and the application of the means developed in the first axis to improve these cells. Encouraging efficiencies have demonstrated the feasibility of solar cells on the substrates made by the low-cost process developed by S TILE.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Passivating Properties of Hydrogenated Amorphous Silicon Carbide Deposited by PECVD Technique for Photovoltaic Applications

AbstractAmorphous hydrogenated silicon carbide (a-SiCx:H) could be used as a passivating layer in solar cell configuration. We have deposited a-SiCx:H by plasma enhanced CVD on polished silicon wafers. Si-rich a-SiCx:H allows to reach a surface recombination velocity of 7.5cm.s-1. The hydrogenation of silicon surface dangling bonds and the electricalfield-effect near the interface are analyzed by minority carrier lifetime and C(V) measurements and additional FTIR and XPS spectroscopy. The fixed charges within the layers are found to be amphoteric. The interface trap density increases with carbon content in a-SiCx:H because of a lower hydrogen content at the a-SiCx:H/Si interface. The polarity of the fixed charge is depending on the presence of a metallic contact. As a-SiCx:H may be considered as a semiconductor, the a-SiCx:H/c-Si interface is in inversion regime at equilibrium inducing a band bending and accu- mulation when adding a metallic contact

Injection-dependent Minority Carrier Lifetime in Epitaxial Silicon Layers by Time-resolved Photoluminescence

AbstractTime-resolved photoluminescence (TRPL) is used to evaluate the injection-dependent effective minority carrier lifetime of high resistivity epitaxial silicon layers grown on highly doped CZ-Si substrates. Effective lifetimes ranging from 10μs to 200μs are estimated for excess carrier densities between 1x1017 cm-3 and 2x1016 cm-3. Standard models are used to separate the contribution from the different recombination mechanisms. The influence of the epitaxial layer and substrate parameters on the minority carrier effective lifetime measurement is discussed

Fabrication of Si tunnel diodes for c-Si based tandem solar cells using proximity rapid thermal diffusion

Increasing competitiveness of photovoltaic (PV) devices is currently an important objective in technological research, especially with the development of tandem solar cells based on c-Si as the bottom cell. For a monolithical structure, a tunnel diode in between the top and bottom cells is necessary. In this work we report on the development of the fabrication of Si tunnel junction using a combination of spin-on doping and proximity rapid thermal diffusion. A desirable attribute of this process is simplicity. Two different structures p++/n++ or n++/p++ were fabricated on (100) Si substrates. Carrier density profiles were measured by ECV to characterize the shallow doping profiles. Vertical tunnel diodes were fabricated and I(V) characteristics are presented. It is shown that device peak current densities up to 270 A/cm² are achieved using this technique, which is the best value reported with such simple technique

Spectrally-resolved measurement of concentrated light distributions for Fresnel lens concentrators

A test method that measures spectrally resolved irradiance distribution for a concentrator photovoltaic (CPV) optical system is presented. In conjunction with electrical I-V curves, it is a means to visualize and characterize the effects of chromatic aberration and nonuniform flux profiles under controllable testing conditions. The indoor characterization test bench, METHOD (Measurement of Electrical, Thermal and Optical Devices), decouples the temperatures of the primary optical element (POE) and the cell allowing their respective effects on optical and electrical performance to be analysed. In varying the temperature of the POE, the effects on electrical efficiency, focal distance, spectral sensitivity, acceptance angle and multi-junction current matching profiles can be quantified. This work presents the calibration procedures to accurately image the spectral irradiance distribution of a CPV system and a study of system behavior over lens temperature

Design and fabrication of photonic crystal thin film photovoltaic cells

International audienceWe present the integration of an absorbing planar photonic crystal within a thin film photovoltaic cell. The devices are based on a stack including a hydrogenated amorphous silicon P-i-N junction surrounded by TCO layers, with a back metallic contact. Optical simulations exhibit a significant increase of the integrated absorption in the 300-720nm wavelength range. The global electro-optical characteristics of such a new solar cell, and the impact of surface passivation, are also discussed. Carrier generation rate maps calculated by optical simulations are introduced as input data in a commercial electrical simulation software. The fabrication of such a device is finally addressed, with a specific focus on the use of low cost nanopatterning processes compatible with large areas

Light harvesting by planar photonic crystals in solar cells: the case of amorphous silicon

International audienceIn this paper, we discuss on light management in silicon thin film solar cells, using photonic crystals (PhC) structures. We particularly focus on photovoltaic devices including amorphous silicon absorbers patterned as 2D PhCs. Physical principles and design rules leading to the optimized configuration of the patterned cell are discussed by means of optical simulations performed on realistic thin film solar cell stacks. Theoretically, a maximum 40%rel increase of integrated absorption in the a-Si:H layer of the patterned cell is expected compared to the unpatterned case. Moreover, both simulation and optical characterization of the fabricated cells demonstrate the robustness of their optical properties with regards to the angle of incidence of the light and to the fabrication induced defects in the PhCs. Finally, the impact of the surface recombination due to the generation of new free surfaces with higher defect densities is addressed. We demonstrate that patterning still induces a substantial increase of the conversion efficiency, with a reasonable surface recombination velocity