Alliages silicium-germanium polymorphes en couches minces pour applications photovoltaïques PDF Download
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Author: Marie-Estelle Gueunier
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Languages : en
Pages : 171
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Fabrication of high-efficiency stable multijunction solar cells requires low optical band gap materials. Thus, hydrogenated amorphous silicon-germanium alloys (a-SiGe:H) are of great interest for photovoltaic applications since the band gap can be decreased by increasing the germanium content. However, on one band, the addition of germanium in the silicon network was shown in the past to lead to a drastic deterioration of the electronic properties. On the other hand, improved electronic properties and stability had been observed on a new material, called hydrogenated polymorphous silicon (pm-Si:H). This material was deposited by PECVD in high pressure and high hydrogen dilution conditions. The aim of this thesis was to explore similar conditions of high pressure and high hydrogen dilution for the fabrication of SiGe alloys, with the hope that the improved properties observed on pm-Si:H could be extended to these alloys. The optical, structural, defect-related and transport properties of different series of alloys have been studied by a set of complementary techniques. The modulated photocurrent technique was particularly studied and some new developments were brought to this technique. We find that these new SiGe alloys exhibit some specific properties attributed to the peculiar hydrogen microstructure, which justifies that these alloys have been designed as hydrogenated polymorphous silicon-germanium alloys (pm-SiGe:H). First results of p-i-n diodes for which pm-SiGe:H alloys were incorporated in the intrinsic layer are also presented here. The efficiency of such devices is comparable to that of amorphous alloys (around 7%) and does not reveal the improved properties of the polymorphous material. We have identified that this is due to problems at the p/i interface. Solving these problems should greatly increase the performance of the devices.
Author: Marie-Estelle Gueunier
Publisher:
ISBN:
Category :
Languages : en
Pages : 171
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Book Description
Fabrication of high-efficiency stable multijunction solar cells requires low optical band gap materials. Thus, hydrogenated amorphous silicon-germanium alloys (a-SiGe:H) are of great interest for photovoltaic applications since the band gap can be decreased by increasing the germanium content. However, on one band, the addition of germanium in the silicon network was shown in the past to lead to a drastic deterioration of the electronic properties. On the other hand, improved electronic properties and stability had been observed on a new material, called hydrogenated polymorphous silicon (pm-Si:H). This material was deposited by PECVD in high pressure and high hydrogen dilution conditions. The aim of this thesis was to explore similar conditions of high pressure and high hydrogen dilution for the fabrication of SiGe alloys, with the hope that the improved properties observed on pm-Si:H could be extended to these alloys. The optical, structural, defect-related and transport properties of different series of alloys have been studied by a set of complementary techniques. The modulated photocurrent technique was particularly studied and some new developments were brought to this technique. We find that these new SiGe alloys exhibit some specific properties attributed to the peculiar hydrogen microstructure, which justifies that these alloys have been designed as hydrogenated polymorphous silicon-germanium alloys (pm-SiGe:H). First results of p-i-n diodes for which pm-SiGe:H alloys were incorporated in the intrinsic layer are also presented here. The efficiency of such devices is comparable to that of amorphous alloys (around 7%) and does not reveal the improved properties of the polymorphous material. We have identified that this is due to problems at the p/i interface. Solving these problems should greatly increase the performance of the devices.
Author: Sofia Gaiaschi
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Despite continuous effort, thin-film silicon multi-junction solar cells are still limited by the light-induced degradation of amorphous materials that they employ - hydrogenated amorphous silicon layers (a-Si:H) or amorphous silicon-germanium (a-SiGe:H) layers. To survive, this technology must fully benefit from the ease with which it allows multi-band gap photovoltaic (PV) devices to be assembled. To this end, materials that are stable under light soaking and have an electronic band gap between that of hydrogenated microcrystalline silicon (μc-Si:H, 1.1 eV) and that of a-Si:H (1.7 eV) are needed. The goal of this PhD thesis was to develop a new class of materials satisfying all these requirements by alloying carbon and silicon. Indeed, hydrogenated microcrystalline silicon-carbon alloys (μc-Si1-xCx:H) are a promising candidate for expanding the toolbox of useful materials for thin-film photovoltaics. The interest in these alloys lies in the possibility of easily varying their effective band gap by changing the amount of carbon in their composition. In this thesis, the usefulness of such materials in thin-film PV devices was probed using a broad range of deposition and characterization techniques. Using thin-film growth techniques at low temperatures (175-300° C), the range in which such electronically useful materials can be grown has been explored. It was confirmed that even in the condition of small crystallites, no stable sub-stoichiometric Si-C crystalline phase exists (i.e. no parallel for silicon-rich c-SiGe has been observed). Under all deposition techniques utilized, these materials were composed of submicron-size silicon crystallites embedded in an amorphous silicon-carbon (a-Si1-xCx:H) matrix. However, while the presence of the crystallites assures a higher conductivity compared to a-Si1-xCx:H, the carbon incorporation leads to an effective energy gap larger than that of microcrystalline silicon, supporting our investigation of these materials as promising optoelectronic layers. In the first part of this work, different Plasma Enhanced Chemical Vapor Deposition strategies have been investigated to achieve the widest range of processing conditions and to learn the most about the growth conditions required to produce a high quality μc-Si1-xCx:H material. Material properties were extensively characterized both on the structural side and also from an electrical point of view, in order to establish a correlation between the deposition parameters and the microstructural, transport and defect-related properties. The extensive set of results has allowed the proposal of a coherent growth model for such μc-Si1-xCx:H thin films. Exploiting these results, PV devices using these alloys as active layers were made. Although the absolute levels of efficiency (around 3.5 %) are not as high as state-of-the-art microcrystalline silicon, this work showed that it is possible to obtain variations in the open circuit voltage by varying the amount of carbon incorporated in such μc-Si1-xCx:H alloys. This important result shows that a process parameter other than silane dilution can be used to control this aspect of device performance. PV performances are modest so far, which is expected as these are the first ever results concerning the application of such a new class of materials as the active layer in thin-film solar cells. However, with further advancements in such materials, their replacement of the less stable a-SiGe:H is not unforeseeable.
Author: Anis Daami
Publisher:
ISBN:
Category :
Languages : fr
Pages : 190
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Cette étude porte sur la faisabilité d'application de l'alliage SiGe et du silicium poreux (SP) et de leur intégration dans les photopiles silicium à couches minces. L'intérêt de cette étude est d'augmenter le rendement des cellules solaires silicium. Le SiGe permet d'accroître le photo-courant grâce à une plus grande absorption dans l'IR. Le SP sert de couche antireflet et de passivation de l'émetteur de la cellule. Nous avons montré par spectroscopie de photoluminescence (PL), que la passivation par nitruration de la surface du silicium poreux, améliore son intensité de luminescence et augmente sa stabilité dans le temps. Ce résultat a été exploité pour améliorer la qualité de passivation au niveau de la photopile. Ensuite, nous avons caractérisé par PL l'alliage SiGe afin de contribuer à une amélioration de sa croissance et diminuer la densité de défauts dans le matériau. Cet alliage est par la suite intégré dans la base active de la photopile. Nous mettons en évidence une très bonne qualité des couches réalisées par CVD et LPE. En effet, nous observons une très faible activité des dislocations que nous corrélons avec des longueurs de diffusion, de porteurs minoritaires, supérieures aux épaisseurs des couches étudiées. Enfin, par des mesures de DL TS sur des photopiles de petites surfaces, nous montrons que les défauts présents dans l'alliage SiGe ne sont pas en concentration suffisante, pour diminuer le rendement d'une cellule solaire. Par cette étude nous avons activement participé à élaborer et conceptualiser la structure d'une photopile SiGe à couches minces. Cette dernière, sans optimisation, présente un rendement comparable à une cellule solaire tout silicium (12.3%). Ceci nous permet d'espérer encore un gain plus important après perfectionnement de la photopile (2% en absolu par rapport au Si).
Author: Sai͏̈d Sefiani
Publisher:
ISBN:
Category :
Languages : fr
Pages : 246
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DES COUCHES MINCES DE SILICIUM-CARBONE-GERMANIUM ET DE SILICIUM-GERMANIUM SONT OBTENUES PAR DECOMPOSITION THERMIQUE DE MOLECULES ORGANOGERMANOSILICIEES DANS UN REACTEUR DE DEPOT CHIMIQUE EN PHASE VAPEUR. L'ETUDE DE LA STABILITE THERMIQUE DES PRECURSEURS ORGANOMETALLIQUES EN FONCTION DE LEUR STRUCTURE CHIMIQUE, EST MENEE PAR FRAGMENTATION EN SPECTROMETRIE DE MASSE ET PAR ANALYSE DES GAZ DE PYROLYSE. LES CARACTERISATIONS CHIMIQUES ET STRUCTURALES DE LA PHASE SOLIDE MONTRENT QUE SUIVANT LA STRUCTURE CHIMIQUE DES PRECURSEURS ET LA TEMPERATURE DE DEPOT, DES ALLIAGES AMORPHES OU POLYCRISTALLINS DE SILICIUM-CARBONE-GERMANIUM ET SILICIUM-GERMANIUM SONT OBTENUS
Author: Ouafa Saadane
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ISBN:
Category :
Languages : en
Pages : 166
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The work presented in this thesis is devoted to the study of the metastability in the hydrogenated polymorphous silicon (pm-Si:H). The pm-Si:H films deposited in a plasma regime close to the powder formation, which promoted the incorporation of nanocrystallites and/or clusters in amorphous matrix of the layers. Previous results have shown the promising potentiality of the pm-Si:H for photovoltaic conversion. The aim of this study is to bring a better comprehension between the structural and optoelectronic properties of pm-Si:H films prepared under different deposition conditions. We used various methods of characterizations, and in particular some electrical techniques of characterizations: measurements of conductivity and photoconductivity (constant photocourant method, modulated photocurrent, steady state photocarrier gratings)), which we coupled with methods of structural characterizations (Fourier transform infrared spectroscopy and hydrogen effusion experiments). We particularly studied the metastability of the pm-Si:H. After a systematic study of a large number of pm-Si:H films, we highlighted the deposition conditions which lead to films with good properties transport (high deposition rate, high and excellent stability of the ambipolar diffusion length of the minority carriers, better efficiency of the solar cells). A link between hydrogen bonding and transport properties is made that shows that to exhibit good transport properties the material has to present a peculiar microstructure revealed by the hydrogen bonding. However, the hydrogen bonding and/or content in this structure has to be adjusted via the deposition conditions to reach an optimum hydrogen incorporation leading to the best layers and the best solar cell devices.
Author: Jamil Kündig
Publisher:
ISBN:
Category :
Languages : fr
Pages : 121
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