GB2030766A - Laser treatment of semiconductor material - Google Patents

Abstract

A layer or layers of amorphous material deposited on a non-crystalline amorphous substrate are subjected to treatment by exposure to a laser beam. The treatment may either recrystallise or anneal the amorphous material depending on the laser power and improves its suitability for such devices as solar cells by improving the life times and diffusion lengths within the material as compared to the untreated material. The use of P type SiC or polycrystalline silicon on superposed i and n type amorphous silicon layers to improve the transmissivity of the outer layers of solar cells, and the use of varying composition SiC layers for visible light emission devices are also disclosed.

Description

SPECIFICATION Improvements in or relating to solar cells and other devices.

The present invention relates to method of treatment of amorphous material layers to make them more suitable for the production of solar cells and other similar devices and to the solar cells and or other devices produced using the treated materials.

The possible use of thin film amorphous materials for solar cells, (or other devices such as p-n junctions, Schottky diodes, zener diodes) integrated into large area systems such as displays is extremely attractive due to the relatively low cost in terms of materials and energy. A disadvantage however, is that device performance has previously been severely limited by the short life times and diffusion lengths typical of amorphous materials. For example, the present state of the art A.M.I.

efficiencies for polycrystalline silicon solar cells is about 10% which is a factor of 2 higher than the best cell produced with amorophous material.

It is an object of the present invention to produce a treated layer of amorphous material which is substantially superior to that known at present and thus to be able to combine the advantages in terms of the low cost and large area capability of amorphous thin film with the superior device performance of the polycrystalline material.

According to the present invention there is provided a method of treating one or more layers of amorphous material deposited on a non-crystalline substrate, to improve the suitability of the material for use in solar cells or similar devices as hereinbefore defined, by subjecting the layer of amorphous material to exposure by a laser.

The present invention also provides an amorphous silicon solar cell comprising an upper layer of p type SixCi -x an intermediate layer of i,-Si and a lower layer of n Si on a suitable substrate.

The invention also provides an amorphous silicon solar cell comprising an upper layer of p type polycrystalline silicon an intermediate layer of i -Si and a lower layer of n Si on a suitable substrate.

In a preferred embodiment the amorphous material is silicon.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings, in which: Figure 1 shows a conventional schottky barrier cell, Figure 2 shows a conventional P-i-n barrier cell, Figure 3 shows a first solar cell heterostructure according to the present invention and, Figure 4 shows a second solar cell structure according to the present invention.

An amorphous silicon layer or multi-layer amorphous structure of 6,000A was grown on an amorphous non-crystalline substrate e.g. glass, metallised glass or stainless steel by known methods, for example the glow discharge decomposition of SiH4 or by sputtering. The amorphous layer or layers are then subjected to exposure by a suitable laser. The effect of the exposure to the laser is either to crystallise a controlled thickness of the amorphous structure when a high power is used to anneal the amorphous layer when a lower power is used.

The recrystallised layer possesses optical and electronic properties which make it more suitable as part of a solar cell, p-n junction or other devices.

In the case of the annealed, non-recystallised layer the properties will tend to reduce the density of the defect states in the gap.

Referring now to the drawings current practise when using amorphous-silicon (a- Si) for solar cells is to use p-i-n or Schottky barrier I - i - n structures, as shown in Figs. 1 and 2. These approaches limit the attainable solar conversion efficiency, for any quality i-Si, because of the p layer or metal Schottky barrier prevents a large proportion of the incident light from reaching the depleted i region. If these surface layers could be replaced by a relatively optically transparent material, which still forms an electrically active junction, then the device efficiency would be improved.

This invention replaces the pa -Si or metal Schottky barrier with an amorphous layer of 'p' type SixCi -x in which 0 x 1 as shown in Fig. 3. Amorphous S1 xCi -x forms a continuous series of solid solutions with a bandgap that increases as x decreases, thus a larger fraction of the incident light reaches the junction depletion than with conventional structures. This application therfore provides a heterojunction a-SiC/Si structure.

Further uses of this concept includes SixC, ~ x/SiyC, ~y/SixC1 ~ x/glass/metal or plastic substrate structures for visible light emission which would have applications for flat panel displays.

Referring now to Fig. 4 a second preferred solar cell according to the present invention is shown. Current amorphous silicon solar cells are limited in efficiency because of absorption or reflection of light by the top pa-Si or metal Schottky barrier layer. The solar cell shown in Fig. 4 reduces the wastage of input photons by forming a p-n junction between p-type polycrystalline silicon and the conventional i-na-Si layers as shown in Figs. 1 and 2. The cell is therefore more efficient than previously known solar cells.

In the above shown solar cells suitable front and rear contact metallisations are applied to realise a working structure.

Claims (9)

1. A method of treating one or more layers of amorphous material deposited on a non-crystalline substrate, to improve the suitability of the material for use in solar cells or similar devices as hereinbefore defined by subjecting the layer of amorphous material to exposure by a laser.

2. A method of treating one or more layers of amorphous material as claimed in claim 1 in which the amorphous material is silicon, or hydrogenated amorphous silicon or silicon carbide or gallium arsenide or gallium phosphide.

3. A method of treating one or more layers of amorphous material as claimed in claim 2 in which the amorphous silicon or hydrogenated amorphous silicon is deposited on the amorphous substrate using the glow discharge decomposition of Six4.

4. A method of treating one or more layers of amorphous material as claimed in any one of the preceding claims in which the exposure is at a low enough level so that the material is not recrystallised but is annealed so as to reduce the number of defect states in the gap with a consequent improvement in lifetime and diffusion lengths.

5. A method of treating one or more layers of amorphous material as claimed in any one of the preceding claims in which the exposure is at a level high enough to recrystallise at least one layer of the amorphous material.

6. A method of treating one or more layers of amorphous material substantially as described.

7. An amorphous silicon solar cell comprising an upper layer of p type SixC,-x and intermediate layer of ia-Si and a lower layer of naSi mounted on a suitable substrate.

8. An amorphous silicon solar cell comprising an upper layer of p type polycrystalline silicon an intermediate layer of ia-Si and a lower layer of naSi mounted on a suitable substrate.

9. An amorphous silicon solar cell substantially as described with reference to Fig. 3 or 4 of the accompanying drawings.