Development of manufacturable homojunction silicon solar cells with passivated contacts

Afin de favoriser le déploiement des énergies renouvelables, le développement de cellules solaires moins chères mais aussi plus performantes reste un enjeu pour rendre l’électricité photovoltaïque encore plus attractive. Si les technologies des cellules solaires à base de silicium à homojonction dominent le marché mondial, les performances de ces structures peuvent encore être améliorées. En effet, le contact direct entre la grille métallique et les zones fortement surdopées est source de pertes par recombinaisons des porteurs de charges. L’émergence de de nouvelles structures de cellules émergent à contacts passivés permet des solutions alternatives face à cette limitation. Ces structures visent à délocaliser la prise de contact grâce à l’introduction de couches passivantes entre le substrat de silicium cristallin et la grille de métallisation, diminuant ainsi drastiquement les phénomènes de recombinaisons au sein des dispositifs. La technologie de contacts passivés la plus connue reste celle des cellules à hétérojonction de silicium a-Si:H/c-Si. Cette technologie mature reste pour l’instant limitée car elle représente un nouveau standard industriel mais aussi car elle n’est pas compatible avec les procédés utilisant des températures excédant 250°C. De plus, l’utilisation d’indium, matériau cher et dont la ressource est limitée, dans les couches d’Oxyde Transparent Conducteur (OTC) peut représenter un frein à l’industrialisation de masse du procédé. Il est alors nécessaire de développer de nouvelles technologies de contacts passivés, compatibles avec des procédés à haute température (supérieures à 800°C), et donc intégrables dans une ligne de production existante. Des approches utilisant des OTC en combinaison avec des couches ultraminces d’oxydes, des empilements diélectriques, et des jonctions poly-silicium sur oxyde ont été investiguées afin d’améliorer les performances des cellules à homojonction. Les couches intermédiaires d’OTC développées permettent potentiellement de diminuer les pertes résistives et et celles par recombinaison au niveau des contacts. Ces travaux de thèse se sont ainsi focalisés sur le développement de couches d’oxyde de zinc dopé à l’aluminium (AZO) par pulvérisation cathodique (PC) et Atomic Layer Deposition (ALD) pour les cellules solaires à contact passivés. Ces couches, utilisées seules ou en combinaison avec des matériaux diélectriques, ont été intégrées et testées sur des dispositifs photovoltaïques fonctionnels.For the deployment of renewable energies, the development of cheaper and more efficient solar cells remains an issue to make photovoltaic electricity even more attractive. While homojunction-based silicon solar cell technologies dominate the global market, the performances of these structures can be further improved. Indeed, the direct contact between the metal grid and the highly doped junction is a source of recombination losses. To overcome these limitations, new structures are emerging such as silicon-based passivated contacts solar cells. These structures aim at integrating of passivating layers between the crystalline silicon substrate and the metal grid, thus drastically reducing the recombination phenomena within the devices. Silicon heterojunction (a-Si:H/c-Si) cells remain the most well-known passivated contact technology. Nevertheless, this mature technology is still limited by its fabrication process which is far from the industrial standard, and is hardly compatible with temperatures exceeding 250 ° C. In addition, the use of expensive and potentially toxic indium in the Transparent Conductive Oxide (TCO) layers has restrained up to now the expansion towards mass industrialization of the process. Thus, it is necessary to develop new passivated contacts technologies compatible with high temperature (above 800°C), implementable in a standard production line. This study explores new paths for passivating contact technologies thanks to ultrathin layers of oxides or dielectrics/TCO stacks deposited on silicon homojunctions as well as poly-silicon on thin oxide junctions. In order to limit the resistive losses and potentially limit recombination losses in the contacted areas, intermediate TCO layers have been developed. In this perspective, this works aims at investigating the development of Aluminum Zinc Oxide (AZO) layers by both Magnetron Sputtering (MS) and Atomic Layer Deposition (ALD) for passivated contact solar cells. These layers, also used in combination with dielectric materials have been integrated and then tested in photovoltaic devices

Développement de cellules photovoltaïques silicium à homojonction industrialisables à contacts passivés

For the deployment of renewable energies, the development of cheaper and more efficient solar cells remains an issue to make photovoltaic electricity even more attractive. While homojunction-based silicon solar cell technologies dominate the global market, the performances of these structures can be further improved. Indeed, the direct contact between the metal grid and the highly doped junction is a source of recombination losses. To overcome these limitations, new structures are emerging such as silicon-based passivated contacts solar cells. These structures aim at integrating of passivating layers between the crystalline silicon substrate and the metal grid, thus drastically reducing the recombination phenomena within the devices. Silicon heterojunction (a-Si:H/c-Si) cells remain the most well-known passivated contact technology. Nevertheless, this mature technology is still limited by its fabrication process which is far from the industrial standard, and is hardly compatible with temperatures exceeding 250 ° C. In addition, the use of expensive and potentially toxic indium in the Transparent Conductive Oxide (TCO) layers has restrained up to now the expansion towards mass industrialization of the process. Thus, it is necessary to develop new passivated contacts technologies compatible with high temperature (above 800°C), implementable in a standard production line. This study explores new paths for passivating contact technologies thanks to ultrathin layers of oxides or dielectrics/TCO stacks deposited on silicon homojunctions as well as poly-silicon on thin oxide junctions. In order to limit the resistive losses and potentially limit recombination losses in the contacted areas, intermediate TCO layers have been developed. In this perspective, this works aims at investigating the development of Aluminum Zinc Oxide (AZO) layers by both Magnetron Sputtering (MS) and Atomic Layer Deposition (ALD) for passivated contact solar cells. These layers, also used in combination with dielectric materials have been integrated and then tested in photovoltaic devices.Afin de favoriser le déploiement des énergies renouvelables, le développement de cellules solaires moins chères mais aussi plus performantes reste un enjeu pour rendre l’électricité photovoltaïque encore plus attractive. Si les technologies des cellules solaires à base de silicium à homojonction dominent le marché mondial, les performances de ces structures peuvent encore être améliorées. En effet, le contact direct entre la grille métallique et les zones fortement surdopées est source de pertes par recombinaisons des porteurs de charges. L’émergence de de nouvelles structures de cellules émergent à contacts passivés permet des solutions alternatives face à cette limitation. Ces structures visent à délocaliser la prise de contact grâce à l’introduction de couches passivantes entre le substrat de silicium cristallin et la grille de métallisation, diminuant ainsi drastiquement les phénomènes de recombinaisons au sein des dispositifs. La technologie de contacts passivés la plus connue reste celle des cellules à hétérojonction de silicium a-Si:H/c-Si. Cette technologie mature reste pour l’instant limitée car elle représente un nouveau standard industriel mais aussi car elle n’est pas compatible avec les procédés utilisant des températures excédant 250°C. De plus, l’utilisation d’indium, matériau cher et dont la ressource est limitée, dans les couches d’Oxyde Transparent Conducteur (OTC) peut représenter un frein à l’industrialisation de masse du procédé. Il est alors nécessaire de développer de nouvelles technologies de contacts passivés, compatibles avec des procédés à haute température (supérieures à 800°C), et donc intégrables dans une ligne de production existante. Des approches utilisant des OTC en combinaison avec des couches ultraminces d’oxydes, des empilements diélectriques, et des jonctions poly-silicium sur oxyde ont été investiguées afin d’améliorer les performances des cellules à homojonction. Les couches intermédiaires d’OTC développées permettent potentiellement de diminuer les pertes résistives et et celles par recombinaison au niveau des contacts. Ces travaux de thèse se sont ainsi focalisés sur le développement de couches d’oxyde de zinc dopé à l’aluminium (AZO) par pulvérisation cathodique (PC) et Atomic Layer Deposition (ALD) pour les cellules solaires à contact passivés. Ces couches, utilisées seules ou en combinaison avec des matériaux diélectriques, ont été intégrées et testées sur des dispositifs photovoltaïques fonctionnels

Développement de TCO sans Indium pour les cellules solaires silicium à contacts passivés

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Bifacial crystalline silicon homojunction cells contacted with highly resistive TCO layers

International audienceThis study explores the needed properties of Transparent Conductive Oxides (TCOs) for bifacial homojunctionsolar cells with potentially passivated contacts. TCO layers with different electrical and optical properties have been testedon both sides of n-type homojunction cells. The high lateral conductivity provided by the diffused emitter and back surfacefield (BSF) greatly reduces the constraints on TCO electrical properties in such structures. An understanding of the requiredproperties of TCO for advanced homojunction applications is given. Hence different O2-rich Indium Tin oxide (ITO) layersare analyzed optically and electrically before being implemented in homojunction solar cells to evaluate their influence onthe device performances. All in all moderately conductive TCOs are shown to be suitable for such applications, allowingbetter optical properties without inducing resistive losses in the devices

Optimizing TCO layers for Novel Bifacial Crystalline Silicon Homojunction Solar Cells integrating Passivated Contacts

International audienceThis study aims at understanding and optimizing the properties of transparent conductive oxides when applied on a homojunction doped silicon solar cells. Thus it deals with contact formation for passivated contact homojunction solar cells, hence the choice of the topic 2.2. Silicon cells: Homojunction solar cells

Contacting n+^+ poly-Si junctions with fired AZO layers: a promising approach for high temperature passivated contact solar cells

International audiencePolysilicon (poly-Si) based passivating contacts are promising to improve silicon solar cells conversion efficiency. However, the use of Transparent Conductive Oxides (TCO) has to be considered to improve lateral conductivity while maintaining good optical and surface passivation properties especially for bifacial devices. In this work different Aluminum-doped Zinc Oxide (AZO) based layers have been investigated after high temperature firing steps to contact Phosphorus-doped poly-Si layers. Contact resistivity below 100 mΩ.cm$^2$ at the AZO/n$^+$ poly-Si interface and stable implied open circuit voltage values (> 715 mV) have been obtained for firing temperatures from 550°C to 900°C. Moreover, the use of capping layers allows to maintain highly conductive AZO layers upon annealing. This novel high temperature contacting method via indium-free TCOs, is particularly interesting for the industrial integration of poly-Si based passivated contacts and provides new perspectives for advanced homojunction solar cells

ALD-grown SnO2_2 as an electron selective layer for perovskite/silicon tandem cells

International audienceThis work presents a comparative study between tin(IV) oxide (SnO2) thin films deposited either by solution process or by Atomic Layer Deposition (ALD) for an application as an electron selective layer in perovskite/silicon tandem solar cells. This study is motivated by the usually lower performances of electron selective layers made of ALD-grown SnO2 compared to ones made via solution processes. Chemical, electrical, optical and topographical properties of each type of film were investigated. In an attempt to link thin film properties to device characteristics single-junction perovskite solar cells and perovskite/silicon tandem solar cells were fabricated. Despite the high-quality electronic and optical properties of ALD-grown SnO2, perovskite-based solar cells employing such film showed limited performances. Characterization of perovskite films properties grown on both type of SnO2 did not rise significant differences and tend to indicate some hindering factors at the ALD-grown SnO2 interface with perovskite. Specifically, Kelvin force probe microscopy characterisation unveiled a larger workfunction for ALD-grown SnO2, which may create a potential barrier for electron extraction from the perovskite

TCO contacts on poly-Si layers: High and low temperature approaches to maintain passivation and contact properties

International audiencePolysilicon (poly-Si) based passivating contacts are promising to improve silicon solar cells conversion efficiency. Thin poly-Si layers usually feature high sheet resistances compared to conventional diffused junctions. Therefore the use of Transparent Conductive Oxides (TCO) has to be considered to improve lateral conductivity while maintaining good optical and surface passivation properties. Standard sputtered Indium Tin Oxide (ITO) can alter the poly-Si passivation properties. In this work different TCO-based contacts have been investigated (various materials and deposition techniques) in order to contact phosphorus-doped poly-Si layers (n+ poly-Si). The TCO-deposited samples experienced both low and high temperature annealing steps. Implied open circuit voltage above 730 mV and contact resistivity below 50 mΩ.cm2 at the TCO/n+ poly-Si interface have been obtained for both high and low temperature approaches. Thus such novel structures are very promising to contact poly-Si layers

ALD-grown SnO2_2 as an electron selective layer for perovskite/silicon tandem cells

International audienceThis work presents a comparative study between tin(IV) oxide (SnO2) thin films deposited either by solution process or by Atomic Layer Deposition (ALD) for an application as an electron selective layer in perovskite/silicon tandem solar cells. This study is motivated by the usually lower performances of electron selective layers made of ALD-grown SnO2 compared to ones made via solution processes. Chemical, electrical, optical and topographical properties of each type of film were investigated. In an attempt to link thin film properties to device characteristics single-junction perovskite solar cells and perovskite/silicon tandem solar cells were fabricated. Despite the high-quality electronic and optical properties of ALD-grown SnO2, perovskite-based solar cells employing such film showed limited performances. Characterization of perovskite films properties grown on both type of SnO2 did not rise significant differences and tend to indicate some hindering factors at the ALD-grown SnO2 interface with perovskite. Specifically, Kelvin force probe microscopy characterisation unveiled a larger workfunction for ALD-grown SnO2, which may create a potential barrier for electron extraction from the perovskite

TCO contacts for high efficiency c-Si solar cells: Influence of different annealing steps on the Si substrates and TCO layers properties

International audienceDistrict heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. The main scope of this paper is to assess the feasibility of using the heat demand-outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. Abstract Different Transparent Conductive Oxide (TCO) layers properties are evaluated after annealing steps at temperatures above 200°C, in order to study their potential use in crystalline silicon (c-Si) solar cells fabrication processes. While the conductivity of Indium Tin Oxide (ITO) layers obtained by magnetron sputtering (MS) is almost stable after annealing in air, Aluminum doped Zinc Oxide (AZO) layers deposited by Atomic Layer Deposition (ALD) need a controlled atmosphere to maintain high carrier densities and mobilities. During the annealing processes, contaminating atoms (such as Zn) diffuse into the c-Si bulk and may potentially decrease its quality. Thus, both the contamination of the c-Si bulk and the properties of the AZO layer have been analyzed. Abstract Different Transparent Conductive Oxide (TCO) layers properties are evaluated after annealing steps at temperatures above 200°C, in order to study their potential use in crystalline silicon (c-Si) solar cells fabrication processes. While the conductivity of Indium Tin Oxide (ITO) layers obtained by magnetron sputtering (MS) is almost stable after annealing in air, Aluminum doped Zinc Oxide (AZO) layers deposited by Atomic Layer Deposition (ALD) need a controlled atmosphere to maintain high carrier densities and mobilities. During the annealing processes, contaminating atoms (such as Zn) diffuse into the c-Si bulk and may potentially decrease its quality. Thus, both the contamination of the c-Si bulk and the properties of the AZO layer have been analyzed