S. De Wolf

Solar cells from upgraded metallurgical grade (UMG) and plasma-purified UMG multi-crystalline silicon substrates

High impurity concentrations do not allow the direct use of upgraded metallurgical grade (UMG) Si for PV production. A newly developed prototype inductive plasma-purification system andprocess allowedthe significant reduction of the elements B, C, O, P, Al, Ca, Fe and Ti, depending on the duration of the treatment. Based on this type of purification, it is shown that subsequent appropriate low-cost cell-processing yields homogeneously distributed energyconversion efficiencies throughout the cast ingots. Stabilisedcell efficiencies of up to 14.7% were already experimentally shown to be attainable on highly B-doped (ro0:1O cm) 102 cm 2 multi-crystalline Si substrates of high purity. On plasma-purifiedUMG p-type 0.1–0.2 O cm ingots, efficiencies of up to 12.38% are reached, to be compared with about 10.12% on the same material without prior plasma treatment. Some light-induced degradation is present on processedsamples, which is most likely linkedto the presence of metastable boron–oxygen complexes in the material, andresults in stabilisedefficiencies of, respectively, 12.19% and 10.00%. r 2002 Elsevier Science B.V. All rights reserved.

Towards back contact silicon solar cells with screen printed metallisation

In this paper, a generic cost effective cell process based on screen printing for a variety of rear contacted solar cells is presented. After discussing the choice of the method for contact isolation, being one of the major problems in the field of back contact solar cells, results of solar cell structures based on this cell process are discussed. The discussed cells have become known as the metallisation wrap through (MWT), metallisation wrap around (MWA) and emitter wrap through (EWT) structures. The results for the screen printed versions of the MWT and MWA solar cells categorise amongst the best reported efficiencies for large area low cost back contact solar cells. The preliminary results on EWT cells indicate the high potential of the cell design.

Light-induced degradation of very low resistivity multi-crystalline silicon solar cells

An J/sub sc/ degradation under illumination has been measured for finished solar cells processed from multicrystalline B-doped Si-substrates with resistivities below 0.1 /spl Omega/ cm. This phenomenon has been studied as function of the different applied processing steps and as function of the boron- and oxygen-concentration of the substrate. The observed effect is likely related to a reversible formation of boron-oxygen complexes, introducing traps in the bandgap. This behaviour is similar to what has been reported in literature for carrier lifetime instabilities of 1 /spl Omega/ cm Cz-Si. The degradation was found to be fully reversible by a low-temperature anneal at about 200/spl deg/C, provided that the degradation causing defects have not been passivated by hydrogenation.

Advanced dry processes for crystalline silicon solar cells

Significant cost reduction of bulk crystalline silicon solar cells requires the removal of the technological barriers that impede the development of a high throughput, low cost, and reliable industrial process on thin substrates. Present industrial surface conditioning and rear surface passivation processes do not meet the requirements for yield and performance on thin substrates. In addition, large-scale production brings about the issue of the environmental impact of PV processing and its related externalities, which may contribute a significant part of the final costs and are so far, underestimated or belittled. In this paper we present an advanced, plasma-based processing technology, suitable for industrial production of bulk silicon solar cells on thin substrates and capable of meeting the PV market growth challenge with a low environmental impact and a competitive cost. The modified processing steps include plasma etching and texturing, dielectric passivation and rear side local contact schemes. Each step is compatible with the standard process sequence, can be integrated separately in the production line and leads to improved performance and/or cost reduction.

Electronically-coupled up-conversion: an alternative approach to impurity photovoltaics in crystalline silicon

The traditional approach to harnessing the impurity-photovoltaic effect to improve solar cell performance is plagued by additional recombination caused by the impurity centres. This extra recombination channel is usually deemed to outweigh the benefits of additional generation of electron–hole pairs via sub-band-gap absorption through the impurity levels. Here we consider an alternative approach that restricts the impurity levels to a film with a wider band gap at the rear of a solar cell, isolating the impurities from the minority carriers generated in the base, but, in principle, still allowing impurity-generated carriers to contribute to the cell current. Initial proof-of-concept experiments show that implantation of silicon ions into amorphous silicon films on the rear of crystalline silicon wafers results in the desired increase in sub-band-gap absorptance of infrared photons, without degrading the surface passivation properties of the amorphous layer. However, these defect states are not thermally stable, and in any case do not result in additional carriers being injected into the silicon wafer itself, either because the infrared-generated carriers relax back to the valence band before the second photon can be absorbed, or because the free carriers recombine before reaching the wafer interface. Subsequent attempts involving implantation of iron and erbium impurities to generate stable absorption centres in the amorphous silicon films also failed to inject additional carriers into the crystalline wafer. Possible modifications that may alleviate these problems are discussed.

Selective junction separation techniques for multi-crystalline silicon solar cells

This paper addresses the need for new junction separation techniques that are low-cost, selective, have a high throughput and high-yield. Pre- as well as post-diffusion selective approaches as integrated into an existing low cost screenprinted metallisation processing scheme are discussed and compared to state-of-the-art technology.