SIFT-MS is a relatively new trace gas analysis technique that has wide application. One particular attribute of the instrument is the ability to detect and quantify volatile organic compounds to the parts per trillion in real-time without the need for sample preparation. However the issue of maintaining accuracy at these low concentrations required attention as it was evident large or polar analytes were being lost by adsorption to the SIFT instrument’s inlet system. The purpose of this research was to evaluate the performance of a passivated inlet in lowering any adsorption in the inlet system compared to the current unpassivated inlet of the SIFT instrument. Volatile concentrations of vanillin (C₈H₈O₃ 152.15 g/mol), ammonia (NH₃ 17.03 g/mol), and hydrogen sulfide (H₂S 34.08 g/mol) were measured. The results determined the passivated inlet provided a significantly better inlet response to these compounds. Consequently improved passivated inlets were installed on current models of SIFT-MS VOICE200®, and also the research laboratory VOICE100™ instrument. Having established a more reliable sampling system for very low concentrations of analyte, attention was paid to SIFT-MS flavour analysis of two foods, cheese and chocolate. The volatile matrix of these foods is highly complex and the compounds of interest are typically difficult to measure. The key aroma compounds for analysis were based on reported literature and earlier SIFT-MS studies which provided a useful framework for the current food flavour research. A significant finding from the SIFT-MS examination of Parmesan cheese is that differences in the relative concentration of some characteristic aroma compounds were a consequence of the milk type used in manufacture. Endogenous enzymes responsible for a multitude of reactions are mostly if not completely inactivated by the pasteurization temperature. A similar analysis approach was attempted for chocolate analysis. Here flavour differences were not as clearly recognised as for the cheese samples. In chocolate there are a greater number of parameters that are involved in its manufacture. Nevertheless, some recognisable differences in chocolate could be attributed to cocoa bean type and flavour additions by the manufacturer.

No description available.

No description available.

Dans le contexte des cellules photovoltaïques (PV) à base de silicium cristallin (c-Si), le développement de structures de contacts dits « passivants », qui permettent de limiter les pertes par recombinaisons des porteurs de charge à l’interface entre le métal et le c-Si, est un des principaux leviers vers l’obtention de plus hauts rendements. Une approche de contacts passivés consiste à intégrer entre le métal et le c-Si une jonction composée d’une couche de silicium poly-cristallin (poly-Si) fortement dopée sur une mince couche d’oxyde de silicium (SiOx < 2 nm).Les objectifs de ce travail sont d’une part de développer une jonction poly-Si/SiOx compatible avec la fabrication industrielle des cellules PV, et d’autre part d’améliorer la compréhension des mécanismes de passivation et de transport des charges au niveau de la fine couche de SiOx située à l’interface entre le poly-Si et le c-Si.Dans ce travail, une jonction de poly-Si/SiOx dopée au bore a été développée, le dopage de la couche étant dans un premier temps réalisé in-situ pendant l’étape de dépôt chimique en phase vapeur assisté par plasma (PECVD) de la couche poly-Si. La méthode de dépôt PECVD est répandue dans l’industrie PV et permet la fabrication de la couche poly-Si d’un seul côté du substrat c-Si. Cependant, elle induit une forte concentration d’hydrogène dans la couche déposée, ce qui entraine la formation de cloques à l’interface avec le c-Si et tend à dégrader les propriétés de passivation de surface de la jonction après recuit de cristallisation. L’optimisation des conditions de dépôt (température de dépôt et ratio de gaz H2/SiH4) a permis d’obtenir des couches de poly-Si dopées in-situ intègres. Par la suite, une méthode de dopage alternative, par le biais du dépôt d’une couche diélectrique riche en bore sur le poly-Si, a été appliquée afin de réduire l’apport en hydrogène pendant le dépôt et d’obtenir des couches de poly-Si intègres plus épaisses. L’ajout d’une étape d’hydrogénation a permis d’obtenir des propriétés de passivation de surface au niveau de l’état de l’art pour les deux types de jonctions poly-Si/SiOx développées.A la suite du développement de la jonction poly-Si/SiOx, la caractérisation physico-chimique de la couche SiOx a été réalisée et a démontré une possible amélioration de la stœchiométrie de la couche vers SiO2 ainsi qu’une dégradation de son homogénéité en épaisseur sous l’effet du recuit de cristallisation à haute température. Ces phénomènes pourraient s’expliquer par une diffusion des atomes d’oxygène à l’interface. D’autre part, l’étude du transport des charges à travers le SiOx par C-AFM a mis en évidence les limites de cette technique quant à la détermination de nano-ouvertures au sein de la couche SiOx (qui favoriseraient le transport des charges). Enfin, une méthode de caractérisation des défauts recombinants à l’interface entre une jonction de poly-Si intrinsèque et le c-Si a été mise en œuvre. Cette méthode a permis de modéliser les recombinaisons à l’interface poly-Si/c-Si via deux défauts discrets apparents dont les niveaux d’énergie dans la bande interdite et les ratios de sections efficaces de capture des électrons et des trous ont été déterminés. / In the context of high efficiency solar cells (SCs) based on crystalline silicon (c-Si), the development of "passivating" contact structures to limit the recombination of charge carriers at the interface between the metal electrode and the c-Si has been identified as the next step to further improve the photovoltaic (PV) conversion efficiency. Passivating contacts consisting of a highly doped poly-crystalline silicon layer (poly-Si) on top of a thin layer of silicon oxide (SiOx ≤ 2 nm) are particularly sparking interest as they already demonstrated promising conversion efficiency when integrated in SCs.The objectives of this work are to develop a poly-Si/SiOx passivating contact compatible with the industrial production of c-Si SCs, and to investigate the passivation and charge transport mechanisms in the region of the thin SiOx layer located at the interface between the poly-Si and the c-Si.In this work, a boron-doped poly-Si/SiOx contact was fabricated. The doping of the layer was first performed in-situ during the deposition of a hydrogen-rich amorphous silicon (a-Si:H) layer by plasma-enhanced chemical vapor deposition (PECVD). The PECVD step was followed by an annealing step for crystallization of the poly-Si layer. The PECVD presents the advantages of being widespread in the PV industry and enabling the fabrication of the poly-Si contact on a single side of the c-Si substrate. However, it induces a high concentration of hydrogen in the deposited layer, which causes the formation of blisters at the interface with the c-Si and tends to degrade the surface passivation properties of the contact after annealing for crystallization. The optimization of the deposition conditions (temperature and H2/SiH4 gas ratio) enabled to obtain blister-free in-situ doped poly Si layers. An alternative doping method consisting of the deposition of a boron-rich dielectric layer on top of the poly-Si layer was applied to reduce the hydrogen content of the deposited layer. This approach enabled to obtain thicker blister-free poly-Si layers. The diffusion of hydrogen in the contact after annealing is known to provide a further chemical passivation of the poly-Si/c-Si interface. In this work, the addition of a hydrogenation step enabled to obtain state-of-the-art surface passivation properties for the two types of poly Si/SiOx contact fabricated.After developing the poly-Si/SiOx contact, a study of the effect of the annealing step on the chemical and structural properties of the SiOx layer was performed. Results indicated a possible improvement of the stoichiometry of the layer towards SiO2 as well as a degradation of its homogeneity at the poly-Si/c-Si interface after annealing at high temperature. These phenomena could be explained by a diffusion of the oxygen atoms content in the interfacial SiOx layer. The transport mechanism of charge carriers through the SiOx layer was conducted by C-AFM. This study revealed the limits of this technique to determine the presence of pinholes within the SiOx layer (that would help the transport of charge carriers). Finally, a method for characterizing recombinant defects at the interface between an intrinsic poly-Si junction and the c-Si has been developed. This method enabled to model the recombination phenomena at the poly-Si/c-Si interface via two apparent discrete defects. Their associated energy levels in the bandgap and ratios of electron and hole capture cross sections were estimated.

Le coût de fabrication des modules photovoltaïques est un point critique pour implanter l’énergie solaire dans le mix énergétique. L’un des moyens d’abaisser ce coût est la réduction de l’épaisseur de silicium utilisé pour la fabrication des cellules photovoltaïques. Il est techniquement possible de produire des cellules photovoltaïques en silicium cristallin d’une épaisseur de quelques dizaines de micromètres d’épaisseur seulement, bien que cela représente un défi à la fois pour le procédé de fabrication de telles cellules et pour leur optimisation. Celle-ci est différente des cellules d’épaisseur conventionnelle notamment par le besoin d’un piégeage optique et d’une passivation de surface de haut niveau. Cet aspect sera étudié au travers de deux structures : un réflecteur en face arrière de la cellule, et un procédé de texturisation innovant pour limiter la gravure du silicium de la cellule, déjà mince. Enfin, l’implantation du réflecteur dans des cellules photovoltaïques sera traitée. L’optimisation du réflecteur considéré pour des cellules minces en silicium cristallin a montré de très bonnes propriétés réfléchissantes et de passivation de surface, ainsi qu’une compatibilité avec l’ensemble des étapes du procédé de fabrication. Ensuite, la texturisation avancée développée dans ce travail a montré un gain potentiel important en photogénération pour des cellules de faible épaisseur. La caractérisation de ces structures a montré des performances optiques et électriques comparables à l’état-de-l’art. Enfin, la fabrication de cellules photovoltaïques d’épaisseur standard utilisant le procédé développé pour les cellules minces a montré le gain du réflecteur développé pour la face arrière par rapport à une structure classique de cellule. De plus, la réalisation de ces cellules avec le procédé destiné aux cellules minces a permis d’établir que les étapes non-standard du procédé sont compatibles avec l’obtention de cellules photovoltaïques performantes. / The cost of fabrication of photovoltaic modules is a critical figure for settling solar power into the energy mix. One way to lower this cost is to decrease silicon use in photovoltaic cells. It is technically possible to produce crystalline silicon solar cells only a few dozens of microns thick, although this represents a challenge both for their fabrication process and their optimization. This last one is different from cells of standard thickness, especially by the need of high level light trapping and surface passivation. Two structures will be studied in order to fulfill these aspects : a reflector on the rear side of the cell, and an innovative texturing process used to limit the etching of the already thin silicon absorber. Eventually, the implementation of the rear side reflector into photovoltaic cells will be discussed. The rear side reflector optimized for thin-film crystalline silicon solar cells has shown very good passivating and reflecting properties, as well as compatibility with the overall fabrication process. Moreover, the advanced texturation process developped in this work has shown a large potential gain in photogeneration for thin solar cells. These structures have been characterized and have shown a reflectivity and a passivation level coherent with the state-of-the-art. Finally, solar cells of standard thickness have been fabricated with the thin solar cells process, and have shown an improvement from the rear side reflector in comparison with a standard cell structure. Moreover, making these cells with the thin cells process has shown that the non-standard steps of this process are compatible with high-performance solar cells fabrication.

Neste trabalho estudamos o processo de deposição do filme de silício policristalino dopado com oxigênio (SIPOS) depositado por LPCVD, a partir da mistura entre a silana (SiH4) e o óxido nitroso (N2O); para a sua aplicação como camada de passivação superficial em dispositivos de potência. As características físicas e elétricas do filme SIPOS foram analisadas em função dos seguintes parâmetros de deposição: pressão, razão gasosa entre (N2O/SiH4), espaçamento entre as lâminas de processo, tempo para a formação de uma camada de pré-oxidação entre SIPOS-Si e tempo de processo. Observamos que o espaçamento entre as lâminas de processo é um importante parâmetro de processo, pois este influi diretamente na uniformidade em espessura e na concentração de oxigênio presente nos filmes depositados. A caracterização elétrica dos filmes SIPOS foi realizada através de capacitores MSS. Verificamos a validade do modelo sobre o comportamento da condutividade elétrica em função da proporção gasosa (N2O/SiH4) proposto por Ni e Arnold. Uma vez determinado as melhores condições de processo, os filmes SIPOS foram depositados sobre diodos de potência pré processados fornecidos pela AEGIS Semicondutores Ltda. Estes diodos foram então caracterizados quanto a sua tensão de ruptura reversa e a sua corrente de fuga reversa. Os histogramas dos dados experimentais mostraram que diminuindo-se o tempo para a formação de uma camada de pré-oxidação entre a interface SIPOS-Si, temos uma diminuição da corrente reversa que flui pelo filme SIPOS. Os diodos de potência fornecidos pela Aegis Semicondutores Ltda foram projetados para suportarem uma tensão de ruptura reversa de 650 V. Os diodos passivados com SIPOS suportaram tensões de ruptura de até 1.200 V. / In this work, the SIPOS (Semi-Insulating Polycrystalline Silicon) LPCVD deposition process was studied to be applied as passivation layer in power devices. It was used a mixture of silane and nitrous oxide to promote the deposition process. The physical and electrical characteristics were analyzed in function of the follow process parameters: total pressure, gas ratio (N2O/SiH4), distance between samples in the LPCVD wafer holder; pre oxidation time and total process time. It was observed that the distance between samples in the LPCVD wafer holder is direct related to the thickness uniformity and in the oxygen concentration present in the SIPOS thin films. MSS capacitors were fabricated to perform the electrical characterization of the deposited SIPOS films. The validity of the model proposed by Ni and Arnold, to the behavior of the electrical conductivity in function of gas ratio (N2O/SiH4), was confirmed. The SIPOS thin film was deposited over pre processed diodes samples, supplied by AEGIS Semicondutores Ltda, in the best process conditions obtained in the previous experiments. The behavior of the leakage current and the breakdown voltage were analyzed. The histograms of the breakdown voltage data showed that decreasing the pre oxidation time of the SIPOS-Si interface, the leakage current through the SIPOS films decreases. The power diodes supplied by Aegis Semicondutores Ltda was designed to support a breakdown voltage of 650 V. The power diodes passivated with SIPOS films supported a breakdown voltage up to 1200 V.

[Some symbols cannot be rendered in the following metadata – please see the PDF file for an accurate version of the Abstract] ¶ Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. ¶ Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. ¶ The rate of intrinsic recombination in silicon is of fundamental importance. It has been investigated as a function of injection level for both n-type and p-type silicon, for dopant densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity silicon, have been measured. Importantly, the wafers where commercially sourced and had undergone significant high temperature processing. A new, general parameterisation has been proposed for the rate of band-to-band Auger recombination in crystalline silicon, which accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this new parameterisation, with the effects of photon recycling included. ¶ Surface recombination processes in silicon solar cells are becoming progressively more important as industry drives towards thinner substrates and higher cell efficiencies. The surface recombination properties of well-passivating SiN films on p-type and n-type silicon have been comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined. The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric in composition, rather than being silicon rich, a property which is attributed to the use of dilute silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface being determined by a high fixed charge density within the SiN film (even under illumination), has been proposed to explain the injection-level dependent Seff for a variety of differently doped wafers. The passivation obtained with the optimised SiN films has been compared to that obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown that the optimised SiN films show little absorption of UV photons from the solar spectrum and can be easily patterned by photolithography and wet chemical etching. ¶ The recombination properties of n+ and p+ emitters passivated with optimised SiN films and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances. Both planar and random pyramid textured surfaces were studied for n+ emitters, where the optimised SiN films were again found to be stoichiometric in composition. The optimised SiN films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a belt-line furnace. It was found that the surface recombination properties of SiN passivated p+ emitters was poor and was worst for sheet resistances of ~150./ . The hypothesis that recombination at the Si/SiN interface is determined by a high fixed charge density within the SiN films was extended to explain this dependence on sheet resistance. The efficiency potential of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100./ and a base resistivity of 1-2.cm found to be optimal. Open-circuit voltages of 670-680mV and efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells respectively. The recombination properties measured for emitters passivated with SiO2, both n+ and p+, were consistent with other studies and found to be superior to those obtained with SiN passivation. ¶ Stoichiometric SiN films were used to passivate the front and rear surfaces of various solar cell structures. Simplified PERC cells fabricated on 0.3.cm p-type silicon, with either a planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20.cm) demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2.cm multicrystalline silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to 17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency measurements confirmed the excellent passivation achieved with the optimised stoichiometric SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from the rear contact dots. An alternative is to have locally diffused p+ regions under the rear contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN passivation elsewhere, without using photolithography, has been demonstrated.

Neste trabalho estudamos o processo de deposição do filme de silício policristalino dopado com oxigênio (SIPOS) depositado por LPCVD, a partir da mistura entre a silana (SiH4) e o óxido nitroso (N2O); para a sua aplicação como camada de passivação superficial em dispositivos de potência. As características físicas e elétricas do filme SIPOS foram analisadas em função dos seguintes parâmetros de deposição: pressão, razão gasosa entre (N2O/SiH4), espaçamento entre as lâminas de processo, tempo para a formação de uma camada de pré-oxidação entre SIPOS-Si e tempo de processo. Observamos que o espaçamento entre as lâminas de processo é um importante parâmetro de processo, pois este influi diretamente na uniformidade em espessura e na concentração de oxigênio presente nos filmes depositados. A caracterização elétrica dos filmes SIPOS foi realizada através de capacitores MSS. Verificamos a validade do modelo sobre o comportamento da condutividade elétrica em função da proporção gasosa (N2O/SiH4) proposto por Ni e Arnold. Uma vez determinado as melhores condições de processo, os filmes SIPOS foram depositados sobre diodos de potência pré processados fornecidos pela AEGIS Semicondutores Ltda. Estes diodos foram então caracterizados quanto a sua tensão de ruptura reversa e a sua corrente de fuga reversa. Os histogramas dos dados experimentais mostraram que diminuindo-se o tempo para a formação de uma camada de pré-oxidação entre a interface SIPOS-Si, temos uma diminuição da corrente reversa que flui pelo filme SIPOS. Os diodos de potência fornecidos pela Aegis Semicondutores Ltda foram projetados para suportarem uma tensão de ruptura reversa de 650 V. Os diodos passivados com SIPOS suportaram tensões de ruptura de até 1.200 V. / In this work, the SIPOS (Semi-Insulating Polycrystalline Silicon) LPCVD deposition process was studied to be applied as passivation layer in power devices. It was used a mixture of silane and nitrous oxide to promote the deposition process. The physical and electrical characteristics were analyzed in function of the follow process parameters: total pressure, gas ratio (N2O/SiH4), distance between samples in the LPCVD wafer holder; pre oxidation time and total process time. It was observed that the distance between samples in the LPCVD wafer holder is direct related to the thickness uniformity and in the oxygen concentration present in the SIPOS thin films. MSS capacitors were fabricated to perform the electrical characterization of the deposited SIPOS films. The validity of the model proposed by Ni and Arnold, to the behavior of the electrical conductivity in function of gas ratio (N2O/SiH4), was confirmed. The SIPOS thin film was deposited over pre processed diodes samples, supplied by AEGIS Semicondutores Ltda, in the best process conditions obtained in the previous experiments. The behavior of the leakage current and the breakdown voltage were analyzed. The histograms of the breakdown voltage data showed that decreasing the pre oxidation time of the SIPOS-Si interface, the leakage current through the SIPOS films decreases. The power diodes supplied by Aegis Semicondutores Ltda was designed to support a breakdown voltage of 650 V. The power diodes passivated with SIPOS films supported a breakdown voltage up to 1200 V.

The objective of the research in this thesis is to develop manufacturable high-efficiency silicon solar cells at low-cost through advanced cell design and technological innovations using industrially feasible processes and equipment on commercial grade Czochralski (Cz) large-area (239 cm2) silicon wafers. This is accomplished by reducing both the electrical and optical losses in solar cells through fundamental understanding, applied research and demonstrating the success by fabricating large-area commercial ready cells with much higher efficiency than the traditional Si cells. By developing and integrating multiple efficiency enhancement features, namely low-cost high sheet resistance homogeneous emitter, optimized surface passivation, optimized rear reflector, back line contacts, and improved screen-printing with narrow grid lines, 20.8% efficient screen-printed PERC (passivated emitter and rear cell) solar cells were achieved on commercial grade 239 cm2 p-type Cz silicon wafers.

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination.