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Silicon surface texturing by reactive ion etching

Dekkers H. F. W., Duerinckx F., Szlufcik J., Nijs J.

Opto-Electronics Review

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2000

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Vol. 8, No. 4

311-316

EN

The high reflectivity of bare silicon substrates is reduced by roughening the surface with reactive ion etching (RIE). Silicon samples are immersed in a plasma chamber and ion assisted etching produces a high density of pits across the surface of the silicon. The obtained reflectivity is only slightly dependent on grain orientations and so RIE offers a way to effectively texture multi - crystalline substrates that cannot be easily textured using the methods commonly employed for single crystalline silicon. Typical reactive ion etching techniques produce a silicon surface with a very low reflectivity (black silicon). However, such surfaces increase carrier recombination and so the efficiency of the resulting solar cells is low. By adjusting the process conditions, the height and width of the surface features can be controlled to produce a surface that is sufficiently rough to couple light into the cell. Under specific conditions, etching pits are formed by intersections of {111} - crystallographic planes. The RIE selectivity for those planes causes a texture with {111} - surfaces and slows down the RIE process when this texture is formed. This effect causes a self-adjusting uniformity of texture over the whole substrate what might be of a use in bath reactors. Following the RIE process, short wet chemical etching removes the ion damaged silicon layer that is responsible for defect recombination of carries. The etched surfaces are described using scanning electron microscopy and finished solar cells are characterised with spectral response.

2

Rear passivation of thin multicrystalline silicon solar cells

Bowden S., Duerinckx F., Szlufcik J., Nijs J.

Opto-Electronics Review

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2000

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Vol. 8, No. 4

307-310

EN

Thinner wafers, combined with improvements in multicrystalline substrate quality have led to minority carrier diffusion lengths that equal or even exceed the device thickness. In such devices, the passivation of the rear surface plays an increasingly important role in determining device performance. We have been using an aluminium paste to passivate the rear surface of the solar cells through the creation of a back surface field. This has led to devices with efficiencies that exceed 16% efficiency using an industrial process on large 10 x 10 cm² wafers of 300 µm thick. However, the aluminium paste is expensive and a thick layer needs to be applied which leads to bending of thinner wafers below 250 µm. An alternative method uses a boron diffusion to form of a back surface firld. The boron diffusion have been adapted from high efficiency cells (where high temperatures and long diffusion times are used) to multicrystalline cells that can only tolerate lower temperatures, and for industrial applications that require shorter diffusion times and simple industrially compatible application techniques. The application of boron diffusion sources via simple screen-printed pastes and spray-on diffusion sources is reported. The boron diffusion have been characterised through lifetime measurements on test samples and spectral response measurements on finished solar cells.

3

Advanced concepts of industrial technologies of crystalline silicon solar cells

Szlufcik J., Duerinckx F., Horzel J., Van Kerschaver E., Einhaus R., De Clercq K., Dekkers H., Nijs J.

Opto-Electronics Review

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2000

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Vol. 8, No. 4

299-306

EN

This paper reports on the results of several years of investigations aiming at simplification and efficiency improvement of a low cost screen printing solar cell process. All solar cell process operations, materials and equipment have been, with this respect, critically examined, re - optimised and, if necessary, removed or replaced. A simple industrial type process for high efficiency multicrystalline and monocrystalline solar cells has been developed. The process sequence is based on screen printed contacts fired through a PECVD SiNx antireflection coating layer. The total number of processing steps has been reduced to six. All processing steps can be easily transferred into big volume roduction lines. Solar cells with average cell efficiency above 15% and 17% were respectively obtained on large area multicrystalline and monocrystalline substrates. Including in the processing sequence the advanced processes of isotropic texturisation and selective emitter increases the cell efficiency to 16.9% and 17.9% respectively for multi- and and monocrystalline silicon. A new cell concept, metallisation wrap through cell (MWT), has been recently introduced. The cell efficiency of 13.1% has been obtained in a simple cell processing sequence based on screen printed contacts.