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

Electrical Properties of Monocrystalline Thin Film Si for Solar Cells Fabricated By Rapid Vapor Deposition with Nano-Surface Controlling Double Layer Porous Si in H2

International audienceIntroduction To reduce the Si thickness with maintaining the high quality is a promising approach to reduce the cost of monocrystalline Si solar cell. A major method to fabricate monocrystalline thin Si is epitaxy by Chemical Vapor Deposition (CVD) and Layer Transfer Process (LTP) as shown in Fig. 1. A seed layer and a sacrificial layer such as double layer porous Si (DLPS) which consist of a Low Porous Layer (LPL) and a High Porous Layer (HPL) are fabricated on the surface of a monocrystalline Si wafer, and then Si is epitaxially deposited on the seed layer. This wafer can then be reused in LTP, thus further reducing the material cost of these Si cells. There remain two challenging issues: (ⅰ) crystal defect introduced during epitaxy caused by the roughness of the seed layer 1) and (ⅱ) low deposition rate and yield of epitaxy by CVD. To solve problem (ⅰ), we proposed a Zone Heating Recrystallization (ZHR) method 2) to smoothen the DLPS surface as shown in Fig.2. The structure of the DLPS surface can be modified by using an upper lamp heater to scan the surface in one direction and a bottom heater to pre-heat Si substrate. To solve problem (ⅱ), we proposed a Rapid Vapor Deposition (RVD) method 3) as shown in Fig.3. By depositing Si under a high vapor pressure by heating the source Si to over 2000℃, the deposition rate of over 10 μm/min with a higher yield is achieved. By applying both the ZHR and RVD methods, we successfully reduced the roughness of a DLPS surface and obtained monocrystalline Si with Si wafer level. The critical effect of lowering the roughness of a DLPS surface to R ms < 0.3 nm wa

Refining of metallurgical silicon for crystalline solar cells

International audienceA plasma-retining technique is applied to upgraded metallurgical grade silicon (UMG) to produce solar grade silicon for multi-c silicon ingots at direct costs lower than 15€/kg. Using oxygen and hydrogen as reactive gases injected in the plasma, boron is removed from the material mainly in form of BOH and BO. The boron volatili- Zation time has been reduced to 50 min compared to previous processes, by increasing the temperature of the silicon bath. At the same time, the Al, Ca, C, O concentrations are strongly reduced. From a Íirst batch of puritied UMG Silicon, multi-crystalline ingots (l2kg), wafers (125X125mm2) and solar cells have been produced for an evaluation of this intermediate material. The obtained solar cells gave efticiencies of up to ll.7%. Process development towards an up-scaled pilot equipment is on the Way to further increase the puritication efticiency

Contribution au photovoltaïque de première génération : du matériau silicium aux couches diélectriques

Le photovoltaïque est un secteur industriel en pleine expansion depuis la fin des années 90, avec une croissance de 30 à 40% de la production par an. Les activités de recherches présentées dans le cadre de cette HDR constituent un accompagnement au développement et à l'amélioration des performances des cellules photovoltaïques en silicium cristallin. La première partie concerne les propriétés du silicium " photovoltaïque ", qui présente des concentrations en impuretés plus élevées que le silicium utilisé en microélectronique. Nous abordons le principe de la purification par plasma réactif, permettant en particulier de réduire les concentrations en Bore du silicium. Nous abordons ensuite les propriétés électroniques du matériau compensé, contenant à la fois les impuretés de type P et N. Ce type de matériau, finalement peu étudié jusqu'à présent et dont l'usage augmente dans le photovoltaïque, présente des propriétés particulières en terme de mobilité. Nous nous intéressons aussi aux propriétés du silicium tri-dopé Bore-Phosphore-Gallium. Le gallium est ici volontairement introduit lors du tirage des lingots pour permettre de maîtriser le type et de mieux contrôler la résistivité du matériau final. La deuxième partie concerne la réalisation, par dépôt chimique en phase vapeur assisté par plasma (PECVD), et la caractérisation de couches minces de diélectriques (nitrure, oxydes et oxynitrures de silicium). Ces couches sont utilisées dans la passivation et pour la réalisation des couches antireflet des cellules photovoltaïques en silicium. Nous présentons les principaux résultats obtenus, en faisant la liaison entre les propriétés physico-chimiques de ces couches et leurs caractéristiques optiques et électroniques. Nous nous intéressons plus particulièrement à la problématique de la passivation en face arrière des cellules minces, dont l'objectif consiste à remplacer la traditionnelle couche en aluminium sérigraphié par un diélectrique plus fin

Ingénierie de compensation pour cellules solaires en silicium

Cette thèse s intéresse aux effets de la compensation des dopants sur les propriétés électriques du silicium cristallin. Nous montrons que le contrôle du dopage net, qui est indispensable à la réalisation de cellules solaires à haut rendement, s avère difficile dans les lingots cristallisés à partir de silicium contenant à la fois du bore et du phosphore. Cette difficulté s explique par la forte ségrégation du phosphore durant la cristallisation, qui donne lieu à d importantes variations de dopage net le long des lingots de silicium solidifés de façon directionelle. Pour résoudre ce problème, nous proposons le co-dopage au gallium pendant la cristallisation et prouvons l efficacité de cette technique pour contrôler le dopage net le long de lingots de type p ou n fabriqués à partir d une charge de silicium contenant du bore et du phosphore. Nous identifions les spécificités du matériau fortement compensé ainsi obtenu comme étant: une forte sensibilité de la densité de porteurs majoritaires à l ionisation incomplète des dopants, une réduction importante de la mobilité comparée aux modèles théoriques et une durée de vie des porteurs qui est déterminée par la densité de porteurs majoritaires et dominée après éclairement prolongé par les centres de recombinaison liés aux complexes de bore et d oxygène. Pour permettre la modélisation de cellules solaires à base de silicium purifié par voie métallurgique, nous proposons une paramétrisation des propriétés fondamentales du silicium compensé mentionnées ci dessus. Nous étudions également la dégradation de la durée de vie des porteurs sous éclairement dans des échantillons de silicium de type p et n présentant une large gamme de niveaux de dopage et de compensation. Nous montrons que le défaut bore-oxygène est issu d un complexe formé à partir de bore substitutionnel pendant la fabrication des lingots et activé sous injection de porteurs par une reconfiguration du défaut assistée par des charges positives. Finalement, nous appliquons le co-dopage au gallium pour la cristallisation de silicium UMG et démontrons que cette technique permet d augmenter sensiblement la tolérance au phosphore sans compromettre le rendement matière de l étape de cristallisation ou la performance des cellules solaires avant dégradation sous éclairement.This thesis focuses on the effects of dopant compensation on the electrical properties of crystalline silicon relevant to the operation of solar cells. We show that the control of the net dopant density, which is essential to the fabrication of high-efficiency solar cells, is very challenging in ingots crystallized with silicon feedstock containing both boron and phosphorus such as upgraded metallurgical-grade silicon. This is because of the strong segregation of phosphorus which induces large net dopant density variations along directionally solidified silicon crystals. To overcome this issue, we propose to use gallium co-doping during crystallization, and demonstrate its potential to control the net dopant density along p-type and n-type silicon ingots grown with silicon containing boron and phosphorus. The characteristics of the resulting highly-compensated material are identified to be: a strong impact of incomplete ionization of dopants on the majority carrier density, an important reduction of the mobility compared to theoretical models and a recombination lifetime which is determined by the net dopant density and dominated after long-term illumination by the boron-oxygen recombination centre. To allow accurate modelling of upgraded-metallurgical silicon solar cells, we propose a parameterization of these fundamental properties of compensated silicon. We study the light-induced lifetime degradation in p-type and n-type Si with a wide range of dopant concentrations and compensation levels and show that the boron-oxygen defect is a grown-in complex involving substitutional boron and is rendered electrically active upon injection of carriers through a charge-driven reconfiguration of the defect. Finally, we apply gallium co-doping to the crystallization of upgraded-metallurgical silicon and demonstrate that it allows to significantly increase the tolerance to phosphorus without compromising neither the ingot yield nor the solar cells performance before light-induced degradation.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Electrical properties of boron, phosphorus and gallium co-doped silicon

à paraître dans Energy ProcediaInternational audienceA number of ingots were grown from solar grade poly Silicon, to which Boron, Phosphorous and Gallium were added as dopants. The introduction of Gallium as a third dopant allowed for a better control of the resistivity and the doping type during ingot growth. Measured resistivity in this material is shown to be systematically higher than that calculated using Scheil's law for the dopants distribution and Klaassen's model for the majority carrier mobility. This resistivity underestimation is shown to be, at least partially, due to a reduction of the majority carrier mobility in highly compensated Si compared to Klaassen's model. A similar reduction is observed for the minority carrier mo- bility. We propose a correction term in the mobility calculation, to allow a greater accuracy in the prediction of the resistivity and mobility of compensated solar grade silicon

Understanding of the influence of localized surface defectivity properties on the performances of silicon heterojunction cells

International audienceThe industrial fabrication process of silicon heterojunction (SHJ) solar cells can induce locally depassivated regions (so-called defectivity) because of transportation steps (contact with belts, trays, etc.) or simply the environment (presence of particles at the wafer surfaces before thin film deposition). This surface passivation spatial heterogeneity is gaining interest as it may hinder the SHJ efficiency improvements allowed by incremental process step optimizations. In this paper, an experimentally supported simulation study is conducted to understand how the local a-Si:H/c-Si interface depassivation loss impacts the overall cell performance. The defectivity-induced cell performance drop due to depassivated regions was attributed to a bias-dependent minority carrier current flow towards the depassivated region, which is shown to affect all current-voltage (I(V)) parameters, and in particular the fill factor. Simulation was used further in order to understand how the defectivity properties (spatial distribution, localization and size) impact the induced performance losses. In the light of all results, we propose ways to mitigate the defectivity influence on the cell performances

Laser Ablation of Sub-Stoichiometric Silicon Oxide for Rear Side of PERC Thin Si Solar Cells

International audienc

Highly Microcrystalline Phosphorous-doped Si:H Very Thin Films Deposited by HF-PECVD: R.O2.6

International audienceFinely tuning crystallinity and doping of Si, Ge and SiGe thin films is a keypoint into obtaining high quality devices for above-IC near infrared SiGe-based image sensors. This study focuses on the structural and electrical properties of HF-PECVD deposited microcrystalline silicon (µc-Si:H) contact layers. µc-Si:H typically exhibits an amorphous incubation layer which can span up to 100 nm before nucleation of the crystalline phase. In previous work done on SiGe-based P-I-N sensors the n and p type layers typically have a thickness of the order of a few tens of nanometers. It is thus of great interest to ensure doped layers exhibit an incubation layer as thin as possible, independently from the doping level, as electrical and optical properties of amorphous and microcrystalline silicon differ significantly. In this work, phosphorus doped µc-Si:H was deposited by capacitive coupled plasma HF-PECVD (13,56 MHz) from a SiH4 H2 PH3 gas mixture with varying phosphine level. Two sets of deposition conditions were investigated, i.e. « low power » and « high power » conditions. For each of these conditions, thick samples (> 100 nm) were deposited for gas flow ratios ? = PH3/SiH4 between 0 and 1 % and samples of variable thickness were deposited for a ratio ? = 0.1% . The influence of phosphine concentration in the gas mixture on dopant concentration, active dopant concentration and crystallinity was studied by SIMS, Hall effect measurement and Raman spectroscopy respectively. High doping level were attained, reaching up to 1.6x10^20 cm-3. Topography and surface defects were investigated by AFM and SEM while thickness was measured by spectroscopic ellipsometry. Deposited films show « low power » samples reach a high crystallinity for films as thin as 40 nm whereas « high power » samples transition to highly crystalline films only between 40 and 80 nm despite exhibiting clear signs of early crystallite nucleation. These « low power » conditions also contribute to a better incorporation of active phosphorus in the thin film. For both set of deposition parameters the rise of blister-like surface defects due to compressive stress in P doped µc-Si:H shows there is a thickness limit for the n-type layer above which the films no longer sticks to the substrate

Ingénierie nanophotonique pour cellule solaire tandem pérovskite/silicium

National audienc