Kerf losses due to ingot and wafer sawing can be avoided by solidifying the silicon wafers directly from the melt by the Ribbon Growth on Substrate (RGS) process, thus significantly reducing the wafer cost. However, up to now solar cells made from standard RGS material suffered from shunting problems due to current collecting structures. This resulted in lower fill factor values and hence in lower efficiencies compared to solar cells made from block-cast multicrystalline silicon (mc-Si) materials. In this contribution two novel RGS materials are presented and investigated. Solar cells processed from these new materials have fill factor values above 78%, comparable to those of mc-Si. The increased fill factor values can be explained by the absence of current collecting structures as concluded from a comparative analysis of spatially resolved Light Beam Induced Current (LBIC) measurements and Electroluminescence (EL) images, and from infrared transmission microscopy investigations. Additionally the improved material quality resulted in open-circuit voltage VOC values up to 608mV. This enhanced material quality, in combination with increased fill factor values, resulted in record efficiencies above 16% (certified by Fraunhofer ISE CalLab). This represents a significant improvement compared to the former efficiency record of 14.4% for standard RGS material.

Thermoelectric (TE) power generation presents a promising and attractive way to convert high temperature waste heat into electricity. However in many situations, such as in industrial waste heat recovery, the market is looking for affordable solutions, while the search for efficient and stable materials is still ongoing. The relatively high TE system costs, are partially due to material costs such as for Te in BiTe, or PbTe, but also due to the relatively small scale leg-and module manufacturing process. Especially in the leg manufacturing, there are often a number of steps involved such as ingot crystallization, ball milling, hot pressing or plasma sintering and leg sawing. With the availability of the ribbon-growth-on-substrate (RGS) technology, it is possible to cast semiconductor material in a net-shape form directly from the melt, thus reducing the number of manufacturing steps, material losses and production costs. In combination with a strip leg based module concept, the potential for a major cost reduction step is within reach. Besides affordability, other challenges are to demonstrate a material that is stable at elevated temperatures, and comparable or better than material produced by other semiconductor manufacturing processes.In this paper, the progress in manufacturing higher manganese silicon material by the RGS process is shown, demonstrating ZT values above 0.5. It could be shown that the phase composition can be controlled by the RGS process and that phase geometry is tunable by changing crystallization velocity. This phase structuring ability is seen as an opportunity to enhance phonon scattering by phase boundaries, and hence further improve ZT values. Controlling the Si phase in HMS was found to be key in the RGS material development process. Especially the addition of chromium doping not only changed carrier concentration and conductivity, but also resulted in a different phase composition of the cast material.In addition to the material development the strip material based module concept is introduced and first demonstrators devices are shown. (C) 2015 Published by Elsevier Ltd.

Thermoelectric power generation presents a promising and attractive way to utilize waste heat and to generate electricity in remote locations. Thanks to continuing technology development an outlook of converting a significant amount of waste energy comes within reach. The class of structured metal silicide materials (MgSi2, Mn11Si19, CrSi2) is considered a class of sustainable and cost-effective candidates for thermoelectric applications. An important way to improve the effectiveness of these materials is by nanostructuring. Nanostructured metal silicides can be obtained by using a rapid solidification technology, ribbon growth on substrate (RGS), where a mixture of liquid metal and silicon melt is solidified under controlled conditions by cooling on a solid, reusable substrate. This technology allows for low-cost and high-throughput synthesis of silicide materials with controllable structures in the nano- and micrometer range. Crystallizing materials from the melt allows for an easy way to add dopants or alloy with different metals. Controlling the structure and composition offers the opportunity of optimizing the thermoelectric properties of silicides. This paper describes the current status of development and validation process for higher manganese silicides and chromium silicides as demonstrators, as well as future perspectives for TE applications.

Ribbon Growth on Substrate (RGS) silicon wafers are casted directly from the silicon melt onto reusable substrates. Material losses by wafer sawing are omitted and high production speeds can be achieved. However, multicrystalline RGS silicon as it is produced today incorporates high densities of crystal defects and impurities limiting the efficiency of the corresponding solar cells. The local impact of crystal defects on material quality is estimated via models developed by Donolato and Micard et al.. By theoretically negating the impact of grain boundaries and dislocations, charge carrier diffusion lengths are still limited to values <100 µm. In addition to crystal defects which are common in other multicrystalline silicon materials, we found current collecting structures within grain boundaries. These structures can be associated with carbon and oxygen precipitation and are the cause for shunting phenomena. We conclude that high impurity concentrations are the dominant factor for limiting the performance of RGS silicon solar cells.

The ribbon-growth-on-substrate (RGS) silicon wafer technology is a unique casting technology for the next generation of silicon wafer manufacturing for photovoltaic application. Compared to today's cut wafer technology, the silicon yield is increased from about 40% to more than 90% due to the direct casting. This in combination with the high production volume per machine and year makes RGS wafers a promising technology for the next step towards cost effective solar electricity. However, for a successful implementation of this technology in the market, not only the wafer manufacturing process but also the compatibility of the RGS wafers with today's solar cell lines has to be demonstrated. This paper presents an overview on the lessons learned in the last 7 years of solar cell processing of RGS wafers manufactured with the laboratory equipment at ECN.

Ribbon Growth on Substrate (RGS) is a cost-effective method to produce silicon wafers directly from molten silicon without material losses due to wire sawing. This technique decouples crystallization and pulling direction of the silicon wafers which results in good wafer quality at a very high production speed of about one wafer per second. The multicrystalline wafers have typical grain sizes between 0.1 mm and 0.5 mm so that spatially resolved investigations of the effect of the crystal structure on cell performance are expected to improve wafer production and cell processing. One processed RGS solar cell was measured with Light Beam Induced Current (LBIC) and Lock-In Thermography (LIT) and the data was correlated with dislocation densities and electron microscopy investigations. In regions where the as grown wafer is thinner compared to the average wafer thickness, the internal quantum efficiency shows a tendency towards lower values. This can be traced back to high defect densities introduced by mechanical stresses during casting. An Energy Dispersive X-ray (EDX) analysis revealed carbon and oxygen impurities and possible precipitates. Also metal impurities in some of the defect-rich regions were found which may affect their electronic properties.

This paper focuses on two very promising silicon ribbon materials currently produced for research: ribbon growth on substrate (RGS) by ECN solar energy and molded wafer (MW) by GE Energy. Both materials are investigated in terms of solar cell processing and characterisation. First cell results of large area 10times10 cm2 RGS cells are presented as well as results from 5times5 cm2 cells processed from 8times12 cm2 RGS and 12.5times12.5 cm2 MW wafers

The current density–voltage (J–V) curve that characterises the performance of a solar cell is often extra rounded, resulting in reduced efficiency. When fitting to the standard one‐dimensional models, it is often found that the rounding cannot be fitted by the series resistance only. In these cases, the diode factor m or the depletion region saturation current density J0DR (depending on the model used) is increased. This behaviour could not be explained so far; this paper discusses if a nonuniform contact resistance of the front side metallisation leads to an increase of m or J0DR.

Sufficient passivation of recombination active defects in the bulk of crystalline silicon solar cells using atomic hydrogen is a key feature for reaching high conversion efficiencies. This is of special interest for promising low-cost multi-crystalline (mc) materials, as a substantial cost reduction concerning Watt-peak(Wp)-costs seems to be possible. The effectiveness of this hydrogenation is strongly influenced by the diffusion kinetics of atomic hydrogen in silicon. Oxygen impurities seem to play a major role, as they have the ability to trap hydrogen, slowing down the diffusion of hydrogen atoms. For two crystalline silicon materials the influence of different oxygen concentrations on hydrogen kinetics is discussed. We demonstrate that not only the overall oxygen concentration, but as well the thermal history of the samples has to be taken into account. Precipitation of oxygen alters the diffusion kinetics and has an influence on vacancy concentration. Faster passivation of crystal defects can be reached in low-oxygen samples.

Ribbon growth on substrate (RGS) solar cells have been processed at the University of Konstanz using an adapted industrial-type fire-through SiN process. An efficiency of 12.3% has been reached on a 5/spl times/5 cm/sup 2/ cell. This is the highest efficiency obtained on this very promising and cost-effective material using an industrial-type cell process. An important factor for the increase in efficiency was the reduced oxygen concentration of almost an order of magnitude in the current RGS wafer material compared to former RGS material. Enhanced J/sub sc/, V/sub oc/ and L/sub eff/ values in the range of 100 /spl mu/m as well as lifetimes above 4 /spl mu/s demonstrate the potential of the new low oxygen RGS material. Efficiencies well above 13% should be possible, provided a surface texture is applied and shunting mechanism can be avoided.

The objective of this chapter is to review, for photovoltaic application, the current status of crystalline silicon ribbon technologies as an alternative to wafers originating from ingots. Increased wafer demand, the current silicon feedstock shortage and the need of a substantial module cost reduction are the main issues that must be faced in the booming photovoltaic market. Ribbon technologies make excellent use of the silicon, as wafers are crystallised directly from the melt in the desired thickness and no kerf losses occur. Therefore, they offer a high potential to significantly reduce photovoltaic electricity costs when compared to wafers cut from ingots. Nevertheless, the defect structure present in the ribbon silicon wafers can limit material quality and cell efficiency.

Multi-crystalline silicon solar wafer are the working horses of the rapidly developing photovoltaic market. The availability and cost efficiency but, even more important, the improved wafer quality and the understanding how to process multi-crystalline silicon wafers into highly efficient solar cells are important factors in this development. In this paper multi-crystalline silicon wafer manufacturing technologies both in industrial production as well as under development are outlined. Important wafer characteristics such as oxygen-, carbon and metallic impurities are described and experimental links to solar cell efficiency are exemplary demonstrated.

The silicon surface passivation of monolayers of organic compounds that are bound to Si surfaces by a covalent Si−C bond has been investigated. The effective lifetime τeff of minority charge carriers in the surface-modified semiconductor has been determined by modulated free carrier absorption (MFCA) measurements. The results show that on 1−2 Ω·cm p-type Si(100) surfaces modified with a monolayer obtained from CH2CH(CH2)8C(O)OCH3 maximum effective lifetimes τeff ≥ 130 μs can be obtained. This value corresponds to a maximum surface recombination velocity Seff of 120 cm/s, a value that is similar to those obtained using other passivation techniques, which demonstrates that these monolayers provide an interesting alternative for silicon surface passivation. During these MFCA measurements an unusual time dependence of the effective lifetime is observed:  τeff rises continuously during illumination of the substrate. Kelvin probe measurements show that there is a slow shift of the Fermi level of the semiconduct...

The Ribbon-Growth-on-Substrate (RGS) silicon wafer manufacturing technology is the most promising highspeed wafer production technique under development at the moment. It has the promise to lead to a manufacturing technology, which allows silicon wafer manufacturing at the 25 MWp/a to 50 MWp/a level.A future development of this technology in the areas, RGS machine prototyping, wafer quality improvement and solar cell process optimization should lead to a commercialization of this technology in 2005.In the following a status of the RGS technology today and the most probable road ahead is outlined.

Thin crystalline silicon solar cells cannot be interconnected with standard soldering techniques because of warping and breakage. Interconnection with conductive adhesives showed excellent behavior: warping can be avoided and 80μm thin cells do not break during and after interconnection. Contact resistances measured on glued interconnections are similar to soldered contacts. Damp/heat tests show no degradation after 2500 hours at 85°C/85% humidity. Temperature cycling –40/+80°C has shown no effects after 200 cycles. I-V measurements of interconnected thin back-contact cells in mini modules show no loss in fill factor. Test mini modules made with soldered back contacts and glued front contacts show excellent performance even after over 900 cycles. Interconnection with conductive adhesives is a promising technique combining excellent mechanical properties, good conductivity, durability, and low process temperatures thus reducing stress.

In solar cell manufacturing it is difficult to optimize screen printed front side metallization, in, particular to obtain good contact over the entire cell. The Corescan enables mapping of contact resistance of full cell surfaces. Using these mappings, it is possible. to diagnose and exclude reasons for poor contact formation. In this study, several process conditions have been studied. Cross-belt temperature non-uniformity in the firing furnace turns out to be a reason for large contact resistance differences. This shows that the acceptable firing temperature range can be considerably increased if a constant temperature profile across the belt width is realized. Further, it is shown that plasma etching for isolation gives cause for an increase of the contact resistance of the fingers at the cell edges. Finally, contact resistance differences related to emitter non-uniformity are found. To summarize, the Corescan is a powerful tool that enables-easy optimization of front side metallization.

Thin crystalline silicon solar cells cannot be interconnected with standard soldering techniques because of warping and breakage. Interconnection with conductive adhesives showed excellent behavior warping can be avoided and 80mum thin cells do not break during and after interconnection. Contact resistances measured on glued interconnections are similar to soldered contacts. Damp/heat tests show no degradation after 2500 hours at 85degreesC/85% humidity. Temperature cycling -40/+80degreesC has shown no effects after 200 cycles. I-V measurements of interconnected thin back-contact cells in mini modules show no loss in fill factor. Test mini modules made with soldered back contacts and glued front contacts show excellent performance even after over 900 cycles. Interconnection with conductive adhesives is a promising technique combining excellent mechanical properties, good conductivity, durability, and low process temperatures thus reducing stress.

The Ribbon-Growth-on-Substrate (RGS) silicon wafer manufacturing technology is a very promising high-speed wafer production technique under development at the moment. It has the promise to lead to a manufacturing technology, which allows silicon wafer manufacturing at the 25 MWp/a to 50 MWp/a level. A future development of this technology in the areas, RGS machine prototyping, wafer quality improvement and solar cell process optimization should lead to a commercialization of this technology in 2005.

Presents an outline of the work done in the EC co-funded project ACE Designs. The objective of this project was to develop rear contact solar cell designs and to demonstrate their applicability as an alternative crystalline silicon technology for industrial module production. An overview of the results is given with links to the most relevant, publications for further details. The most important result of this project was that rear contact solar cells are a feasible, attractive and cost effective alternative to the well-known front contacted solar cell.