Axel Schönecker

New crystalline silicon ribbon materials for photovoltaics

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.

Review on Ribbon Silicon Techniques for Cost Reduction in PV

The shortage of Si feedstock and the goal of reducing Wp costs in photovoltaics (PV) is the driving force to look for alternatives to ingot grown multicrystalline (me) Si wafers which have the highest share in the PV market. Ribbon Si seems to be a very promising candidate as no kerf losses occur, resulting in reduced Si costs per Wp. In addition, there is no need for the energy consuming crystallization of the ingot and therefore energy payback times can be significantly reduced. The higher defect density in ribbon Si materials has to be taken into account during cell processing, but ribbon materials already commercially available show excellent efficiencies, while for the most promising techniques efficiencies are significantly lower, but very promising. In this presentation an overview of ribbon Si technologies currently under research will be given, based on available data on crystal growth as well as solar cell processing and cell parameters

Ribbon Growth on Substrate and Molded Wafer-Two Low Cost Silicon Ribbon Materials for PV

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

Screen-printed ribbon growth on substrate solar cells approaching 12% efficiency

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.

Impact of oxygen on carbon precipitation in polycrystalline ribbon silicon

This article reports experimental evidence for the effect of oxygen on carbon precipitation in polycrystalline ribbon silicon. Four sets of wafers subject to various heat treatments have been examined by infrared spectroscopy. It is found that carbon precipitation in an oxygen-containing wafer consists of two distinct steps, namely, an initial rapid oxygen–carbon coprecipitation in the very first hour annealing, followed by slow precipitation during subsequent prolonged annealing. A high oxygen content enhances carbon precipitation throughout the two steps. It is shown that the formation of interstitial carbon in the presence of excess silicon self-interstitials generated during oxygen precipitation plays an important role in increasing the carbon precipitation rate in the first hour annealing. Because of the absence of interstitial injection during the following slow precipitation process, the enhancement effect of oxygen can only arise from an increase in precipitation sites. It is proposed that the oxy...

Kinetics of hydrogenation and interaction with oxygen in crystalline silicon

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.

Compatibility of the RGS silicon wafer with industrial type solar cell processing

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 todays 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 todays 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.