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Content archived on 2024-04-16

Concepts for high efficiency multi-crystalline silicon solar cells multi-chess


The overall aim of the project is to bring together on European scale the expertise in multicrystalline and polycristalline silicon solar cells of different strong groups in order to be competitive with groups outside of Europe and to obtain the efficiency goals of this project : 16% on 4cm2 cells and 15-15.5% on 100cm2 cells, both for laboratory cells and 14.5% (best) and 13.5% (average) for cost-effective production (100cm2). The coast-goal is 1.5 ECU/Wp.
Gettered and textured Polyx wafers have been processed to fabricate 2 x 2 cm{2} cells. The 240 minutes 900C gettered wafers had a maximum efficiency of 15.6%. Cells which had received a hydrogen plasma treatment showed the best efficiency.

Further optimisation of the phosphorus gettering was done. The worst results are due to some impurities in the conveyor finance which are counterbalancing the effect of gettering.

It is possible to improve drastically the electrical properties of P type multicrystalline silicon samples by phosphorus diffusion. Hydrogenation of the material could be beneficial as this passivating agent is able to neutralise recombination centres introduced by impurities which cannot be gettered like oxygen. Gettered impurities are iron, copper and nickel.

The polycrystalline material studies shows such features is as grown conditions, its behaviour being comparable to single crystal silicon. Material performances at high boron concentrations are controlled by a deep recombination centre, whose concentration is determined by dopants and oxygen concentration. Even at high R values, donor compensation with phosphorus does not affect the electronic properties of the material. The formation of boron-phosphor complexes can compete with boron-oxygen recombination centres. Free oxygen concentration must be kept as low as possible.

2 deposited or grown glasses improve the bulk carrier transport properties in classical thermal processing. All doping processes give almost the same gettering efficiency, as the classical annealing is not sensitive to external contamination. As RTA is an efficient method for the detection of contamination it has been confirmed that the chemical vapour deposition deposited glass is superior to the spin on one if a rapid thermal diffusion to form the pp{+} contact.

Annealing in argon of the aluminium-silicon structure dramatically increases the effective minority carrier diffusion lengths Ln.

A fundamental study of the silicon/silica interface was carried out. The increase in electrical response that could be gained by oxidation could be counterbalanced by a decrease caused by precipitation of oxygen and metals on specific defects.

Photochemical etching is a new process to prepare the surface of silicon. Promising results were obtained using this process for microscopic texturing and passivation of the emitter of photovoltaic cells.
In the first phase of the programme the different technological strengths the groups will be further elaborated and compared. The comparison of the different techniques will be based on technological and economical grounds. Those techniques that are found successful will be taken over by the indus tries in this project and will lead to more cost-effective cells with higher efficiency, preferably on substrates made by partners within the consortium. Of course all the evaluation techniques present at the different partners will be optimally used to give feedback to the technology. In order to meet the efficiency and cost goals, mentioned above, different work topics are identified and investigated in order to increase the efficiency of the polycrystalline silicon solar cells :

Material (substrate, wafer) improvement : the material quality has to be improved at the ingot and at the wafer level. Gathering techniques are of prime importance to be studied. On the wafer level the interest especially goes to thinner wafers (100-150 microns) to reduce the overall recombination volume and to reduce the material cost.

Back surface field : because of the trend for thinner wafers the diffusion length will comparable or larger to the wafer thickness and there fore it will become sensitive to surface recombination at the back side. Different techniques will be tried to realize the BSF.

Optical confinement : because of the thinner cell interest, light will be not fully absorbed on the first passage through the cell. Optical confinement will become important and will be investigated to increase the optical pathlength.

Frontside passivation and antireflection coating : in order to increase the blue response of the cells, the emitter junction has to be doped less and/or has to be thinner. But then one has to avoid an increase in dark saturation current and hence a decrease in open circuit voltage. Therefore it is advisable to use a very effective surface passivation technique, combined with the antireflection coating.

Selective emitter: in the same context as point 4 the blue response can be enhanced by using a thin emitter. But under the metal fingers one can not passivate the emitter surface.Therefore the emitter junction has to stay deep under the metal fingers. This results in a selective emitter concept.

Contacts: contacts are and stay of prime importance in the solar cells. The contact resistance and sheet resistance have to be low and on the other hand the coverage factor has to be as small possible. New concepts are envisaged such as laser grooving, combined with wet chemical metallization and the improvement of the screenprinting process (smaller line widths). The outcome is the choice of some techniques as production-ready(performance and cost). Therefore the proposal is also well-balanced between industrial interest and research laboratory interest.


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