Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-06-09
2001-09-25
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S261000, C257S461000, C257S463000, C257S066000, C257S075000, C117S020000, C117S023000, C117S073000, C117S074000, C438S097000, C438S089000
Reexamination Certificate
active
06294726
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to silicon with a high oxygen content and a high dislocation density, to its production and to its use for the production of solar cells.
Crystalline silicon is the material from which the vast majority of all solar cells for photovoltaic conversion of sunlight into electrical energy are currently manufactured. Monocrystalline and polycrystalline silicon form the two principle variants of the silicon material used for solar cells. While monocrystalline silicon is usually pulled as a single crystal from a silicon melt using the Czochralski process, there are a number of production processes for polycrystalline silicon. The most usual processes are various block-crystallization processes in which the silicon wafers for producing the solar cells are obtained by sawing a solid polycrystalline silicon block, and various film-drawing processes or film-casting processes, in which the wafers are drawn or cast in their final thickness as a silicon film from a molten material. Examples of the film-drawing process are the EFG process (Edge-defined Film-fed Growth) (EP 0,369,574 A2) and the RGS (
R
ibbon
G
rowth on
S
ubstrate) process (EP 0,165,449 A1, DE 4,102,484 A1, DE 4,105,910 A1).
Solar cells are large-area pn diodes, in the volume of which the sunlight generates minority charge carriers which have to diffuse towards the emitter at the surface of the cell, so that they can there be separated at the pn-junction by the electric field and contribute to the external current flow. The greater the service life and therefore also the diffusion length of the minority charge carriers in the base, the more effective this process. Consequently, particular demands are imposed on the quality of the silicon material for producing solar cells: this material must be as far as possible free of impurities and crystal defects, in order to enable a maximum diffusion length of the minority charge carriers to be achieved.
Oxygen is usually the dominant impurity in silicon, since the silicon melt is usually melted in a quartz crucible (SiO
2
). Although oxygen is electrically inactive as an interstitially dissolved impurity, it is known that if there is an oxygen content of above approximately 8×10
17
atoms/cm
3
in the silicon material caused by high-temperature steps, the minority carrier service life is also reduced (J. Vanhellemont et al., J. Appl. Phys. 77 (11), 5669 (1995)), and therefore so is the efficiency of solar cells. Therefore, the rule has hitherto been that silicon material with an oxygen content of over 8×10
17
atoms/cm
3
is unsuitable for the production of solar cells.
Especially for highly productive film-drawing or film-casting processes, it is particularly difficult and expensive to produce a silicon material with a sufficiently low oxygen content. Particularly in the case of the RGS material, it is necessary to apply a stream of oxidizing gas to the solidifying surface in order to establish a sufficiently smooth surface (DE 4,105,910 A1). Consequently, a relative high oxygen concentration is established in the volume of the RGS material. In the layer of silicon which is close to the surface, the oxygen content is even greater, and in fact the surface of untreated specimens is even to a large extent covered with a layer of silicon dioxide. This is necessary and desirable in order to immobilize impurities which have segregated out. In the case of thin silicon films such as an RGS film having a thickness of around 300 &mgr;m, without the getting action of the oxygen close to the surface, the metallic impurities which have just been concentrated in this thin zone would immediately diffuse back into the interior of the film, which is still at a temperature of over 1000° C.
In view of the foregoing, the invention is based on the object of preparing silicon which, despite having a high oxygen concentration, is suitable for use in photovoltaics. Advantageously, this allows solar cells which contain this silicon to reach levels of efficiency which enable them to be used economically.
Surprisingly, it has now been discovered that silicon with a high oxygen concentration can be used in photovoltaics if it additionally has a high density of crystal lattice dislocations (referred to below as dislocations). This is even more surprising since a high dislocation density generally leads to materials which are relatively unsuitable for photovoltaics.
DESCRIPTION OF THE INVENTION
Consequently, the invention relates to silicon which has a total oxygen content of from 8×10
17
atoms/cm
3
to 1×10
19
atoms/cm
3
and a dislocation density of 1×10
5
cm
−2
to 5×10
7
cm
−2
. The dislocation density is preferably from 5×10
5
cm
−2
to 5×10
6
cm
−2
. The invention furthermore relates to a process for producing silicon with a total oxygen content of 8×10
17
atoms/cm
3
to 1×10
19
atoms/cm
3
and a dislocation density of 1×10
5
cm
−2
to 5×10
7
cm
−2
and to solar cells which contain this silicon. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
The silicon according to the invention can be produced at a low cost using highly productive processes such as film-casting or film-drawing processes, and can be used to good effect in photovoltaics. Solar cells which contain the silicon according to the invention in some instances have a higher short-circuit current than an identically processed solar cell which is based on monocrystalline Czochralski silicon, which is a particularly high-quality material for photovoltaics.
Obtaining a high oxygen content in the silicon generally involves the step of substantially saturating with oxygen a molten silicon that is in contact with silicon monoxide and/or silicon dioxide, and/or treating the molten silicon with an oxygen-containing gas before or during the crystallization. The high oxygen content in the silicon according to the invention may, for example, be achieved by applying an oxygen-containing gas or gas mixture to liquid silicon during the crystallization process. Advantageously, the gas mixture used consists of an inert gas component and a reactive gas component. An inert gas which may be used is argon, while a reactive gas component which may be used is oxygen. The high oxygen content may also be established, for example, by the controlled use of quartz (SiO
2
) crucibles and internals with a high ratio of the surface area of these quartz components which is wetted by liquid silicon during the production of the silicon material to the volume of the liquid silicon. A combination of a plurality of methods is advantageous. A high oxygen content can be achieved in a particularly advantageous manner if the silicon is produced using the RGS process as described, for example, in DE 4105910 A1, and in the process is exposed to a gas mixture which contains at least 50% by volume oxygen.
The dislocation density is set by means of the crystallization rate during the production of the silicon. High crystallization rates usually lead to high crystal defect and therefore dislocation densities. For its part, the crystallization rate is set by means of the temperature profile during the crystallization process of liquid silicon. Rapid cooling of liquid silicon causes a high crystallization rate and therefore a high dislocation density. Rapid cooling of the silicon which has just been crystallized is generally associated with steep temperature gradients; these lead to mechanical stresses which in turn bring about the formation of further dislocations.
According to the invention, by setting a suitable temperature profile it is also possible to influence the form in which the oxygen is present in the silicon. For using the silicon in photovoltaics, it is advantageous if at least 25%, preferably at least 50%, particularly preferably at least 75% of the total oxygen present is in the form
Breitenstein Otwin
Hassler Christian
Hofs Hans-Ulrich
Koch Wolfgang
Thurm Siegfried
Bayer Aktiengesellschaft
Connolly Bove & Lodge & Hutz LLP
Diamond Alan
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