In broad terms, this thesis is concerned with the measurement and interpretation of carrier lifetimes in multicrystalline silicon. An understanding of these lifetimes in turn leads to a clearer picture of the limiting mechanisms in solar cells made with this promising material, and points to possible paths for improvement. The work falls into three broad categories: gettering, trapping and recombination. A further section discusses a powerful new technique for characterising impurities in semiconductors in general, and provides an example of its application. Gettering of recombination centres in multicrystalline silicon wafers improves the bulk lifetime, often considerably. Although not employed deliberately in most commercial cell processes, gettering nevertheless occurs to some extent during emitter formation, and so may have an important impact on cell performance. However, the response of different wafers to gettering is quite variable, and in some cases is non-existent. Work in this thesis shows that the response to gettering is best when the dislocation density is low and the density of mobile impurities is high. For Eurosolare material these conditions prevail at the bottom and to a lesser extent in the middle of an ingot. However, these conclusions can not always be applied to multicrystalline silicon produced by other manufacturers. Low resistivity multicrystalline silicon suffers from a concurrent thermally induced degradation of the lifetime. This had previously been attributed to the dissolution of precipitated metals, although we note that the crystallographic quality also appears to deteriorate. The thermal degradation effect results in an optimum gettering time for low resistivity material. Edge-defined Film-fed Growth (EFG) ribbon silicon was also found to respond to gettering, and even more so to bulk hydrogenation. Evidence for Cu contamination in the as-grown EFG wafers is presented. Multicrystalline silicon is often plagued by trapping effects, which can make lifetime measurement in the injection-level range of interest very difficult, and sometimes impossible. An old model based on centres that trap and release minority carriers, but do not interact with majority carriers, was found to provide a good explanation for these effects. These trapping states were linked with the presence of dislocations and also with boron-impurity complexes. Their annealing behaviour and lack of impact on device parameters can be explained in terms of the models developed. The trapping model allowed a recently proposed method for correcting trap-affected data to be tested using typical values of the trapping parameters. The correction method was found to extend the range of useable data to approximately an order of magnitude lower in terms of carrier density than would be available otherwise, an improvement that could prove useful in many practical cases. High efficiency PERL and PERC cells made on gettered multicrystalline silicon resulted in devices with open circuit voltages in the 640mV range that were almost entirely limited by bulk recombination. Furthermore, the injection-level dependence of the bulk lifetime resulted in decreased fill factors. Modelling showed that these effects are even more pronounced for cells dominated by interstitial iron recombination centres. Analysis of a commercial multicrystalline cell fabrication process revealed that recombination in the emitter created the most stringent limit on the open circuit voltage, followed by the bulk and the rear surface. The fill factors of these commercial cells were mostly affected by series resistance, although some reduction due to injection-level dependent lifetimes seems likely also. Secondary Ion Mass Spectroscopy on gettered layers of multicrystalline silicon revealed the presence of Cr and Fe in considerable quantities. Further evidence strongly implied that they resided almost exclusively as precipitates. More generally, injection-level dependent lifetime measurements offer the prospect of powerful contamination-monitoring tools, provided that the impurities are well characterised in terms of their energy levels and capture cross-sections. Conversely, lifetime measurements can assist with this process of characterising impurities, since they are extremely sensitive to their presence. This possibility is explored in this thesis, and a new technique, dubbed Injection-level Dependent Lifetime Spectroscopy (IDLS) is developed. To test its potential, the method was applied to the well-known case of FeB pairs in boron-doped silicon. The results indicate that the technique can offer much greater accuracy than more conventional DLTS methods, and may find applications in microelectronics as well as photovoltaics.

<p>Patterning of multi crystalline silicon Solar cell is prepared with photolithography etching. Electroless plating is used to get metallization of Nickel contacts. SEM analysis of Nickel deposition and measurement of contact resistance for series and shunt resistance is done. To increase the fill factor, the screen printed electrodes are subjected to different firing temperatures there by increasing the efficiency of solar cell. Nickel-silicide formation at the interface between the Silicon and Nickel enhances stability and reduces the contact resistance, resulting in higher energy conversion efficiency.</p><p> </p>

Patterning of multi crystalline silicon Solar cell is prepared with photolithography etching. Electroless plating is used to get metallization of Nickel contacts. SEM analysis of Nickel deposition and measurement of contact resistance for series and shunt resistance is done. To increase the fill factor, the screen printed electrodes are subjected to different firing temperatures there by increasing the efficiency of solar cell. Nickel-silicide formation at the interface between the Silicon and Nickel enhances stability and reduces the contact resistance, resulting in higher energy conversion efficiency.

No description available.

Ring-opening polymerization (ROP) is a promising approach to accessing well-defined polyesters with superior (bio)degradability and recyclability. However, the living/controlled polymerization of glycolide (GL), a well-known sustainable monomer derived from carbon mono/di-oxide, has never been reported due to the extremely low solubility of its polymer in common solvents. Herein, we report the first living/controlled anionic ROP of GL in strong protic fluoroalcohols (FAs), which are conventionally considered incompatible with anionic polymerization. Well-defined polyglycolide (PGA, Ð < 1.15, Mn up to 55.4 kg mol-1) and various PGA-based macromolecules are obtained at room temperature for the first time. NMR titration and computational studies revealed that FAs simultaneously activate the chain-end and monomer without being involved in initiation. Low boiling point FAs and PGA can be recycled through simple distillation and sublimation at 220 oC in vacuo, respectively, providing a promising sustainable alternative for tackling plastic pollution problems. Well-defined multicrystalline multiblock polymers are essential model polymers for advancing crystallization physics, phase separation, self-assembly, and improving the mechanical properties of materials. However, due to the different chain properties and incompatible synthetic methodology, multicrystalline multiblock polymers with more than two crystallites are rarely reported. Herein, by combining polyhomologation, ring-opening polymerization, and “catalyst switch” strategy, we synthesized the first pentacrystalline pentablock quintopolymer, polyethylene-b-poly(ethylene oxide)-b-poly(e-caprolactone)-b-poly(L-lactide)-b-polyglycolide (PE-b-PEO-b-PCL-b-PLLA-b-PGA). The novel “fluoroalcohol-assisted catalyst switch” enables the first successful incorporation of a high melting point polyglycolide into the complex multiblock polymer. Solid-state NMR spectroscopy, X-ray diffraction, and differential scanning calorimetry revealed the existence of five different crystalline phases.

The overall goal of this research is to improve fundamental understanding of the hydrogen passivation of defects in low-cost silicon and the fabrication of high-efficiency solar cells on thin crystalline silicon through low-cost technology development. A novel method was developed to estimate the flux of hydrogen, released from amorphous silicon nitride film, into the silicon. Rapid-firing-induced higher flux of hydrogen was found to be important for higher defect passivation. This was followed by the fabrication of solar cell efficiencies of ~ 17% on low-cost, planar cast multicrystalline silicon. Solar cell efficiencies and lifetime enhancement in the top, middle, and bottom regions of cast multicrystalline silicon ingots were explained on the basis of impurities and defects generally found in those regions. In an attempt to further reduce the cost, high-efficiency solar cells were fabricated on thin crystalline silicon wafers with full area aluminum-back surface field. In spite of loss in efficiency, wafer thinning reduced the module cost. Device modeling was performed to establish a roadmap towards high-efficiency thin cells and back surface recombination velocity and back surface reflectance were identified as critical parameters for high-efficiency thin cells. Screen-printed solar cells on float zone material, with efficiencies > 19% on 300 μm and > 18% on 140 μm were fabricated using a novel low-cost fabrication sequence that involved dielectric rear passivation along with local contacts and back surface field.

Innerhalb dieser Arbeit werden die Wechselwirkungen von interstitiellem Eisen mit Defekten, wie Versetzungen und Ausscheidungen, untersucht. Dazu wurde zunächst das Löslichkeitsverhalten von Eisen im p-dotierten mc-Silizium im Temperaturbereich von 550°C bis 800°C gemessen. An Versetzungsstrukturen konnte in einigen Bereichen eine deutlich erhöhte Konzentration an gelöstem Eisen im as cut Zustand und nach dem Tempern der Probe festgestellt werden. Diese wurde mit der Struktur der Versetzungsanordnung erklärt. An vorhandenen Ausscheidungen im Silizium konnte eine Abnahme der gelösten Eisenkonzentration beobachtet werden. Die Dichte und Morphologie der Ausscheidungen haben dabei einen großen Einfluss. Der Eintrag des Eisens in das Silizium aus Tiegeln mit unterschiedlicher Reinheit während der Kristallisation wurde untersucht. Dabei zeigte sich ein deutlicher Einfluss der Tiegelreinheit auf die Konzentration am Rand des Blockes, jedoch nur einen geringen Einfluss auf die Eisenkonzentration im Blockinneren.

Cette thèse présente le développement d'un équipement permettant le dopage Bore des cellules photovoltaïques à base de silicium de type n. Un four de diffusion, appartenant à la société Semco Engineering a été développé pour tirer profit du procédé LYDOP (Leaktight Yield Doping en anglais), breveté par la société. Ce dernier a permis la mise au point d'un procédé de diffusion du Bore, régulé sous basse pression, intégrant une source dopante gazeuse à base de BCl3 afin d'effectuer le dopage de plusieurs plaques de silicium simultanément. Les principaux paramètres influençant le procédé de dopage ont été étudiés pour obtenir un dopage très uniforme sur plaque et sur nacelle. Une large gamme de résistances carrées d'émetteurs (de 40 à 100 ohm/sq) a été obtenue avec une uniformité inférieure à 5% sur plaque et sur nacelle. Le développement du procédé de dopage a conduit à étudier deux méthodes de fabrication d'une cellule photovoltaïque sur silicium multicristallin de type n. Plusieurs méthodes pour la formation de l'émetteur Bore sur une seule face ont été présentées : masquage (SiNx, SiO2), dopage back-to-back ou gravure chimique. De cette étude, deux procédés de fabrication (flowcharts) ont été développés pour la fabrication de cellules photovoltaïques : la première par gravure à l'hydroxyde de potassium (KOH) de l'émetteur, la seconde en effectuant le dopage bore des cellules en position back-to-back (dos à dos). Un rendement sur cellule de 13,2% et 14,4% a été obtenu respectivement pour chacune des flowcharts. Ces résultats, limités principalement par les étapes de passivation et de métallisation permettent de démontrer l'utilisation du procédé Bore comme solution à la formation des émetteurs p+. / This thesis presents the development of an equipment for boron doping of n-type multicrystalline silicon solar cells. A diffusion furnace was developed by Semco Engineering Company. It was built using LYDOP (LeakTight Yields DOPing) technology, patented by Semco. This one permits a simultaneous doping of a big amount of silicon wafers using regulated low pressure processes. Boron diffusion process development was carried out using LYDOP's specifications with BCl3 as gaseous doping source. Main parameters have been studied to control diffusion process. Several sheet resistance values of emitters were achieved (from 40 to 100 ohm/sq) with uniformity under 5% within wafer and within boat by tuning process parameters. Doping process development leads us to investigate how to create a single side emitter with n-type multicrystalline solar cells. Two fabrications flowcharts were presented: one using KOH emitter etches on backside and the other using back-to-back positioning during boron diffusion. Comparison between both flowcharts carried out to 13,2% and 14,4% efficiencies solar cells, respectively on each flowchart. Results are limited by passivation and metallization of emitters. However boron diffusion process demonstrate that LYDOP technology is well adapted to develop n-type solar cells.

Die vorliegende Arbeit befasst sich mit den Untersuchungen zum Einfluss der Züchtungsbedingungen auf die Eigenschaften von multikristallinen (mc) Silizium-Kristallen. Im Mittelpunkt stehen Züchtungsexperimente mit einer gezielten Variation der Züchtungsaufbauten und Züchtungsgeschwindigkeiten. Die gezüchteten Kristalle wurden umfassend charakterisiert im Hinblick auf die Kohlenstoffkonzentration, die Kornstruktur, die Vesetzungsdichte, Verteilung der Ausscheidungen und Ladungsträgerlebensdauer. Zusätzlich wurde die Versetzungsanordnung in Abhängigkeit von der Wachstumsrate bzw. Abkühlrate systematisch untersucht. Als Ergebnis wurde gezeigt, dass die Züchtungsbedingungen die Kohlenstoffkonzentration, die Versetzungsdichte, die Bildung von den Ausscheidungen sowie die Ladungsträgerlebensdauer beeinflussen können, jedoch nicht die Korngröße. Es wurde ein direkter Zusammenhang zwischen Ausscheidungsgebieten und erhöhte Versetzungsdichte beobachtet. Im Rahmen der Arbeit wurde festgestellt, dass die endgültige Versetzungsstruktur sich als Resultat von Gleit- und Erholungsprozessen darstellt.

The usage of more inexpensive silicon feedstock for the crystallization of multicrystalline silicon blocks promises cost reduction for the photovoltaic industry. Less expensive substrates made out of metallurgical silicon (MG-Si) are used as a mechanical support for the epitaxial solar cell. Moreover, conventional inert solar cells can be produced from up-graded metallurgical silicon (UMG-Si). This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Solar cell processing steps can also exert a change in the values of mechanical strength of processed multicrystalline silicon wafers until the fabrication of a solar cell. Bending tests, fracture toughness and dynamic elastic modulus measurements are performed in this work in order to research the mechanical behavior of multicrystalline silicon crystallized with different qualities of silicon feedstock. Bending tests and residual stress measurements allows the quantification of the mechanical strength of the wafers after every solar cell processing step. The experimental results are compared with theoretical models found in the classical literature about the mechanical properties of ceramics. The influence of second phase particles and thermal processes on the mechanical strength of silicon wafers can be predicted and analyzed with the theoretical models. Metals like Al and Cu can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, amorphous silicon oxide particles (SiOx) lower the mechanical strength of multicrystalline silicon due to thermal residual stresses and elastic mismatch with silicon. Silicon nitride particles (Si3N4) reduce fracture toughness and cause failure by radial cracking in its surroundings due to its thermal mismatch with silicon. Finally, silicon carbide (SiC) and crystalline silicon oxide (SiOx) introduce thermal residual stresses but can have a toughening effect on the silicon matrix and hence, increase the mechanical strength of silicon wafers if the particles are smaller than a certain size. The surface of as-cut wafers after multi-wire sawing presents sharp micro-cracks that control their mechanical behavior. Subsequent removal of these micro-cracks by texture or damage etching approximately doubles the mechanical strength of silicon wafers. The mechanical behavior of the wafers is then governed by defects like cracks and particles formed during the crystallization of multicrystalline silicon blocks. Further thermal processing steps have a minor impact on the mechanical strength of the wafers compared to as-cut wafers. Finally, the mechanical strength of final solar cells is comparable to the mechanical strength of as-cut wafers due to the high residual thermal stress introduced after the formation of the metallic contacts which makes silicon prone to crack.