Alkaline-earth aluminoborosilicate glass and the uses thereof

Details Alkaline-earth aluminoborosilicate glass and the uses thereof Alkaline-earth aluminoborosilicate glass and the uses thereof

2002-06-14

2004-01-20

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S064000, C501S067000, C501S069000, C501S070000, C313S636000

Reexamination Certificate

active

06680266

ABSTRACT:

The invention relates to aluminoborosilicate glasses which contain alkaline earth metals. The invention also relates to uses of these glasses.
When energy is being obtained by means of photovoltaics, the property of certain semiconducting materials of absorbing light from the visible spectral region as well as the near UV or IR to form free charge carriers (electron/hole pairs) is utilized. If there is an internal electric field in the solar cell, produced by a pn junction in the photoactive semiconductor material, these pairs can be spatially separated using the diode principle, leading to a potential difference and, given suitable contacts, to the flow of current. Solar cell systems which are currently commercially available contain, as photoactive material, almost exclusively crystalline silicon. This is produced as what is known as “solar grade Si”, inter alia, as a waste material during the production of high-purity silicon single crystals for complex integrated components (chips).
The possible applications for photovoltaic installations can be roughly divided into two groups. These are, firstly, applications which are not connected to the mains, which are used in remote areas on account of the lack of energy sources which are relatively easy to install. By contrast, solutions which are connected to the mains and in which solar energy is fed into an existing fixed mains remain uneconomical, on account of the high cost of solar current, and are therefore relatively rare.
Therefore, the future market development of photovoltaics, in particular for solutions which are connected to the mains, is highly dependent on the potential for reducing costs in the production of solar cells. The implementation of thin-film concepts is considered to offer great potential. In this case, photoactive semiconductor materials, in particular highly absorbent compound semiconductors, are deposited in layers which are a few &mgr;m thick on substrates which are as inexpensive as possible and are able to withstand high temperatures, for example glass. The possibilities for reducing costs lie primarily in the low consumption of semiconductor material and the excellent possibilities for automation during the production compared to wafer Si solar cell production.
Solar cells based on the II-VI compound semiconductor CdTe are a promising thin-film concept. This material satisfies essential conditions, such as a band gap which is well matched to the solar spectrum, high absorption of the incident light and very good chemical stability of the compound.
The same is true of the compound semiconductor Cu(In,Ga) (S,Se)
2
, (“CIS”). Compared to the first example, this is also more environmentally friendly, since it does not contain any Cd.
Thin polycrystalline films of CdTe can be produced by a range of methods (vapor deposition, screen printing, sublimation, spray pyrolysis, electrodeposition), but only in p-conducting form. To obtain a pn junction, what is known as a heterojunction has to be produced using a different n-conducting material, e.g. CdS.
In addition to the substrate technologies which are in widespread use in thin-film photovoltaics (semiconductor resting on bases made from materials such as glass, metal, plastic, ceramic), having said layers and a covering glass, with the light acting through the covering glass, a superstrate arrangement has also established itself in particular in CdTe photovoltaics. In this arrangement, the light from impingement on the semiconductor layer initially passes through the support material. This eliminates the need for the covering glass, which has advantages in terms of costs. To achieve high efficiencies, it is necessary for substrates of this type to have a high transparency in the VIS/UV region of the electromagnetic spectrum, which makes the use of glass a suitable solution. For example, even semitransparent glass ceramics are unsuitable, partially for cost reasons caused by the ceramicizing process.
Further demands on the substrate/superstrate material result from the structure of the solar cell and the temperature conditions during the process used for deposition of the CdTe film. With a view to achieving rapid deposition rates for good-quality CdTe, high temperatures, generally of over 650° C., are required. Accordingly, the substrate glasses should have a sufficiently high ability to withstand thermal loads, i.e. the transformation point T
g
of the glasses should be over 660° C. To prevent flaking of the semiconductor layer during the cooling which follows the coating process, the glasses must also be matched to the thermal expansion of CdTe (&agr;
20/300
≈5-6*10
−6
/K). In the case of the CIS technology, in addition to the high T
g
(>650° C.), a coefficient of thermal expansion &agr;
20/300
which is matched to the Mo layer functioning as electrode, of 4.5-5.0*10
−6
/K, is required. The soda-lime glass which has previously been used in no way satisfies these requirements, having an &agr;
20/300
≈9*10
−6
/K and a T
g
of approx. 520° C.
Furthermore, the glasses are to be sufficiently mechanically stable and chemically resistant to water and also to any reagents used in the production process, in particular in the; case of the superstrate concept, in which there is no covering glass protecting the solar module from environmental influences. For example, soda-lime glasses only have a hydrolytic resistance belonging to Hydrolytic Class 3. Furthermore, it should be possible to economically produce the glasses in sufficient quality in terms of having no or few bubbles and crystalline inclusions.
Similar demands are also imposed on glasses for lamp bulbs:
The glasses have to be able to withstand high thermal loads, since high bulb temperatures generally occur in operation. The glasses must be sufficiently resistant to devitrification to be suitable for tube drawing. For use as lamp bulb glass for lamp bulbs which include molybdenum components as electrode or supply conductor material, the thermal expansion of the glasses has to be matched to that of molybdenum (&agr;
20/300
=5.0*10
−6
/K), so that a sealed, stress-free fusion between the metal and the glass is achieved. For this application too, the glasses must be as free from bubbles as possible. Moreover, glasses for halogen lamps must be substantially free of alkali metals, since alkali metal ions disrupt the regenerative halogen cycle of the lamp.
This profile of requirements is best satisfied by aluminoborosilicate glasses which contain alkaline earth metals but little if any alkali metal. However, the known glasses for display or solar cell substrates which are described in the following documents still have drawbacks in terms of their chemical and physical properties and/or their formation options and fail to satisfy the full range of demands.
Numerous documents describe glasses with relatively high B
2
O
3
contents, for example DE 196 01 922 A, JP 58-120 535 A, JP 60-141 642 A, JP 8-295 530 A, JP 9-169 538 A, JP 10-59 741 A, JP 10-722 37 A, EP 714 862 A1, EP 341 313 B1, U.S. Pat. No. 5,374,595, DE 197 39 912 C1. These glasses do not have the required high transformation temperatures and/or have coefficients of expansion which are too low for the applications which are preferred in this document.
By contrast, B
2
O
3
-free glasses are described in U.S. Pat. No. 4,607,016, JP 61-236 631 A and JP 61-261 232 A. The absence of B
2
O
3
means that the glasses are difficult to melt and tend towards devitrification. The glasses mentioned in WO 97/30001 also do not contain any B
2
O
3
.
DE 44 30 710 C1 describes borosilicate glasses with a low boric acid content and high SiO
2
contents (>75% by weight) which means that they are highly viscous even at high temperatures and can only be melted and refined at considerable cost. Moreover, these glasses, with transformation temperatures T
g
of between 500 and 600° C., have only a relatively low thermal stability.
DE 196 17 344 C1 and DE 196 03 698 C1, in the name of the applicant, have disclosed alkali-free, tin-cont

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