The invention relates to a photovoltaic cell having a substrate glass made of aluminosilicate glass having high thermal resistance, low processing temperature, and high crystallization resistance.

Soda-lime glass, also referred to as alkaline-alkaline-earth-silicate glass, is among the oldest known glass types and is the most frequently used type of glass with respect to quantity ("normal glass"). It is used in many fields, for example, as plate glass, such as mirror or window glass, as packaging glass, for example, for bottles, food packages, and drinking glasses, or in automobile construction, as composite safety glass. Known soda-lime glasses have the disadvantage they only have a relatively low thermal resistance, for example, having a transformation temperature ("Tg") in the range of approximately 490 to 530°C. The application of soda-lime glasses is significantly restricted in this way.

The properties of glass and the products produced therefrom must be specially adapted for many applications. Therefore, a need exists to vary and modify the known glass compositions in such a manner that the properties of the glasses are suitable for special applications. However, it is always problematic in this context that the reduction or increase of the proportion of one component can already trigger a plurality of effects which act differently on the glass properties. The procedures and effects in the event of replacement or modification of multiple components in a glass composition are still more complex, the individual glass components mutually influencing one another, so that simple relationships are typically not predictable. It is therefore relatively difficult to provide custom-tailored glass compositions for special applications.

There are numerous publications in the prior art which are concerned with glasses according to the species.

Thus, JP 07-101748 A describes low-aluminum, alkaline glasses for plasma display panels, the glass composition being synthesized from 0.3 - 2.5 wt.-% Li ₂ 0, 7.0 - 12 wt.-% Na ₂ 0, 1.5 - 4.5 wt.-% K ₂ 0, 0 - 5.0 wt.-% MgO, 6.0 - 9.0 wt.-% CaO, 0 - 5.0 wt.-% SrO, 3.5 - 15.0 wt.-% BaO, 2.0 - 4.5 wt.-% A1 ₂ 0 ₃ , 57.0 - 68.0 wt.-% Si0 ₂ , 0 - 5.0 wt.-% Zr0 ₂ , and 0 - 0.5 wt.-% Ce0 ₂ , the sum of Li ₂ 0 + Na ₂ 0 + K ₂ 0 being 9.0 - 16.0 wt.-%. US 5,858,897 describes a glass composition for a substrate, which is suitable in particular for a flat display, preferably a plasma display (PDP, plasma display panel). The glass composition essentially comprises: 59 - 72 wt.-% Si0 ₂ , 1 - 15 wt.-% A1 ₂ 0 ₃ , 0.5 - 9 wt.-% MgO, 0.5 - 1 1 wt- % CaO, 0 - 6 wt.-% SrO, 0 - 5 wt.-% BaO, 4 -19 wt.-% MgO + CaO + SrO + BaO, 0 - 9 wt.-% Na ₂ 0, 4 - 21 wt.-% ₂ 0, 10 - 22 wt.-% Na ₂ 0 + K ₂ 0, and 0.5 - 10.5 wt.-% Zr0 ₂ , the difference between the Si0 ₂ content and the Al ₂ 0 ₃ content being 50 to 71 wt.-% and the relative density being less than 2.6.

Furthermore, EP 0 769 481 Al discloses a glass composition for a substrate, in particular for plasma displays, the glass composition comprising: 52 - 62 wt.-% Si0 ₂ , 5 - 12 wt.-% A1 ₂ 0 ₃ , 0 - 4 wt.-% MgO, 3 - 5.5 wt.-% CaO, 6 - 9 wt.-% SrO, 0 - 13 wt.-% BaO, 17 - 27 wt.-% MgO + CaO + SrO + BaO, 7 - 14 wt.-% Li ₂ 0 + Na ₂ 0 + ₂ 0, 0.2 - 6 wt.-% Zr0 ₂ , and 0 - 0.6 wt.-% S0 ₃ . Such a high SrO proportion in the glass composition has great disadvantages, however. SrO is a relatively costly material, so that the production of the glass becomes significantly more expensive. The advantages asserted in EP 0 769 481 A 1 , that the transformation temperature is to significantly increase and the thermal coefficient of expansion is to rise, could not be reproduced according to the invention. Rather, an elevated SrO content does not display any positive influence on the properties and effects for the areas of application according to the invention; SrO is therefore not provided in the high quantities described here according to the invention.

Furthermore, EP 0 879 800 Al describes solarization-stable aluminosilicate glasses, which are suitable for use in display technology, in particular for plasma display panels, and have the following composition:

Si0 ₂ 45 - 68 wt.-%

A1 ₂ 0 ₃ > 5 - 18 wt.-%

Na ₂ 0 0 - 5 wt.-%

20 > 9 - 15 wt.-% with

Na ₂ 0 + K ₂ 0 > 10 wt.-%

CaO 0 - 10 wt.-%

SrO 0.5 - 18 wt.-%

BaO 0 - 10 wt.-%

CaO + SrO + BaO 8 - < 17 wt.-% Zr0 ₂ 1 - 6 wt.-%

Ti0 ₂ 0.2 - 5 wt.-%.

US 5,958,812 A relates to thermally stable glass compositions, which comprise from

Si0 ₂ 45 - 68 wt.-%

A1 ₂ 0 ₃ 0 - 20 wt.-%

Zr0 ₂ 0 - 20 wt.-%

B ₂ 0 ₃ 0 - 10 wt.-%

Na ₂ 0 2 - 12 wt.-%

20 3.5 - 9 wt.-%

CaO l - 13 wt.-%

MgO 0 - 8 wt.-%,

the sum of Si0 ₂ + Al ₂ 0 ₃ + Zr0 ₂ < 70 wt.-%, the sum of A1 ₂ 0 ₃ + Zr0 ₂ > 2 wt.-%, and the sum of Na ₂ 0 + ₂ 0 > 8 wt.-%, and if desired, the oxides BaO and/or SrO being provided in quantities such that: 1 1 wt.-% < MgO + CaO - BaO + SrO > 30 wt.-%.

The glass compositions are used for the production of fire-resistant window panes, in particular plasma shields, electroluminescence shields, and cold cathode shields (field emission displays).

Furthermore, US 2005/0003136 Al discloses a glass composition which obtains a higher strength through chemical treatment. The glass composition comprises from

Si0 ₂ 59 - 68 wt.-%

Al ₂ 0 ₃ 9.5 - 15 wt.-%

Li ₂ 0 0 - 1 wt.-%

Na ₂ 0 3 - 18 wt.-%

20 0 - 3.5 wt.-%

MgO 0 - 15 wt.-%

CaO 0 - 15 wt.-%

SrO 0 - 4.5 wt.-%

BaO 0 - 1 wt.-%

Ti0 ₂ 0 - 2 wt.-%

Zr0 ₂ 1 - 10 wt.-%. The glass composition is used, for example, as a glass substrate for a magnetic recording medium.

Furthermore, aluminosilicate glasses having high thermal resistance and low processing temperature are known from DE 10 2009 042 796.1 , filed on 25 September 2009. However, experiments have shown that the glasses do have the required physical properties, but in specific cases do not display satisfactory results with respect to the crystallization resistance.

In particular with respect to renewable or regenerating energy, i.e., energy from sources which either renew themselves in a short time or whose usage does not contribute to the exhaustion of the source, novel and improved possible uses of glasses are increasingly significant. The limited resources of fossil energy carriers, the need for environmental and climate protection, and the efforts for lower dependence on energy exporters or overall for a more sustainable energy provision require targeted refinements of the existing technologies. For these available sustainable energy resources, such as solar radiation (solar energy), very special requirements on the glasses to be used must additionally be considered.

A demand therefore exists to provide glasses which do not display the disadvantages of the prior art, are modified with respect to their properties, and may be used in particular in the field of converting solar energy into electrical current (photovoltaics).

Accordingly, the present invention is based on the object of avoiding the disadvantages of the prior art, and providing an alternative to soda-lime glasses, which have a higher thermal carrying capacity (Tg) in comparison to soda-lime glasses, but nonetheless have the lowest possible processing temperatures (VA) and do not tend toward crystallization and are used in

photovoltaics.

The object of the present invention is achieved according to the invention by a photovoltaic cell, preferably a thin-film photovoltaic cell, comprising a substrate made of aluminosilicate glass, the aluminosilicate glass comprising a glass composition which has Si0 ₂ and A1 ₂ 0 ₃ as well as the alkaline oxide Na ₂ 0 and the alkaline earth oxides CaO, MgO, and BaO, and optionally further components, the glass composition containing

10 to 16 wt.-% Na ₂ 0, preferably > 10 to 14 wt.-%, more preferably > 10 to 13 wt.-%, > 0 to < 5 wt.-% CaO, preferably 0.1 to 4.9 wt.-%, more preferably 1 to 4.7 wt.-%, still more preferably 3 to 4.7 wt.-%, particularly preferably 3 to < 4.5 wt.-%, and

> 1 to 10 wt.-% BaO, preferably 1.1 to 10 wt.-%, more preferably 1.25 to 10 wt.-%, still more preferably 1 .5 to 10 wt.-%, particularly preferably 1 .75 to 10 wt.-%, and

the ratio (wt.-%) of CaO:MgO is in the range of 0.5 to 1.7.

The object of the invention is also the use of an aluminosilicate glass as a substrate glass, superstrate glass, and/or cover glass for photovoltaic cells, preferably thin-film photovoltaic cells, in particular those based on composite semiconductor material, such as CdTe, CIS, or CIGS, the aluminosilicate glass comprising a glass composition which has Si0 ₂ and A1 ₂ 0 ₃ as well as the alkaline oxide Na ₂ 0 and the alkaline earth oxides CaO, MgO, and BaO, and optionally further components, the glass composition containing

10 to 16 wt.-% Na ₂ 0, preferably > 10 to 14 wt.-%, more preferably > 10 to 13 wt.-%,

> 0 to < 5 wt.-% CaO, preferably 0.1 to 4.9 wt.-%, more preferably 1 to 4.7 wt.-%, still more preferably 3 to 4.7 wt.-%, particularly preferably 3 to < 4.5 wt.-%, and

> 1 to 10 wt.-% BaO, preferably 1.1 to 10 wt.-%, more preferably 1.25 to 10 wt.-%, still more preferably 1 .5 to 10 wt.-%, particularly preferably 1.75 to 10 wt.-%, and

the ratio (wt.-%) of CaO:MgO is in the range of 0.5 to 1.7.

According to a preferred embodiment of the invention, an aluminosilicate glass is provided or used as a substrate, in particular as a substrate glass, superstrate glass, and/or cover glass of a photovoltaic cell, preferably a thin-film photovoltaic cell, which comprises or consists of the following glass composition (in wt.-% on oxide basis):

Si0 ₂ 49 - 69 wt.-%

B ₂ 0 ₃ 0 - 2 wt.-%

preferably B ₂ 0 ₃ 0 wt.-%

Al ₂ 0 ₃ > 4.7 - 15 wt.-%

preferably A1 ₂ 0 ₃ > 9 - 15.5 wt.-%

more preferably A1 ₂ 0 ₃ > 1 1 - 15.5 wt.-%

Li ₂ 0 0 - 4 wt.-%

preferably Li ₂ 0 0 - < 0.3 wt.-%

Na ₂ 0 10 - 16 wt.-% ₂ 0 > 0 - 8 wt.-% preferably ₂ 0 > 0 - < 5 wt.-%

in particular ₂ 0 > 2 - < 5 wt.-%, the sum of Li ₂ 0 + Na ₂ 0 + K ₂ 0 being > 12 - 19 wt.-%, and

MgO > 0 - 6 wt.-%

preferably MgO > 2 - 5.5 wt.-%

CaO > 0 - < 5 wt.-%

preferably CaO 2 - < 5 wt.-%

SrO 0 - 8 wt.-%

preferably SrO 0 - 5 wt.-%

more preferably SrO 0 - < 3 wt.-%

particularly preferably SrO 0 - < 0.5 wt.-%

BaO > 1 - 10 wt.-%

preferably BaO 1.1 - 9 wt.-%

particularly preferably BaO 2 - 8.5 wt.-%, the sum of MgO + CaO + SrO + BaO being 7 - 16 wt.-%, and

F 0 - 3 wt.-%

preferably F > 0 - 0.3 wt.-%

Ti0 ₂ 0 - 6 wt.-%

preferably Ti0 ₂ > 0.1 - 5 wt.-%

Fe ₂ 0 ₃ 0 - 0.5 wt.-%

Zr0 ₂ > 0 - 6 wt.-%

preferably Zr0 ₂ 1 - 6 wt.-%

particularly preferably Zr0 ₂ 1.5 - 5 wt.-%, the sum of BaO + Zr0 ₂ being 4 - 1 wt.-% preferably the sum of BaO + Zr0 ₂ being 5 - 15 wt.-%,

Zn0 ₂ 0 - 5 wt.-%

preferably Zn0 ₂ 0.5 - 3 wt.-%

Ce0 ₂ 0 - 2 wt.-%

W0 ₃ 0 - 3 wt.-%

Bi ₂ 0 ₃ 0 - 3 wt.-%

M0O3 0 - 3 wt.-%, the ratio (wt.-%) of CaO:MgO being in the range of 0.5 to 1.7.

Typical refining agents, such as sulfates, chlorides, Sb ₂ 0 ₃ , As ₂ 0 ₃ , Sn0 ₂ may be added to the above glass/the glass melt.

The composition of the aluminosilicate glass used according to the invention is preferably in the range:

Si0 ₂ 48 - 58 wt.-%

B ₂ 0 ₃ 0 - 1 wt.-%

preferably B ₂ 0 ₃ 0 wt.-%

A1 ₂ 0 ₃ 12 - 16 wt.-%

preferably A1 ₂ 0 ₃ > 12 - 15 wt.-%

Li ₂ 0 0 - 1 wt.-%

preferably Li ₂ 0 0 - < 0.3 wt.-%

Na ₂ 0 10 - 14 wt.-%

K ₂ 0 1 - 5 wt.-%

preferably ₂ 0 2 - 4 wt.-% the

sum of Li ₂ 0 + Na ₂ 0 + ₂ 0 being 1 1 - 17 wt.-%, and

MgO 1.5 - 6 wt.-%

CaO 3 - 4.5 wt.-%

preferably CaO 3 - < 4.5 wt.-%

SrO 0 - 3 wt.-%

preferably SrO 0 - < 0.5 wt.-%

BaO 3 - 10 wt.-%

preferably BaO 4 - 9.5 wt.-%

particularly preferably BaO 5 - 8.5 wt.-%, the

sum of MgO + CaO + SrO + BaO being 7 - 18 wt.-%, and preferably the

sum of MgO + CaO + SrO + BaO being 12 - 17 wt.-%,

F 0 - 3 wt.-%

preferably F > 0 - 1 wt.-%

Ti0 ₂ 0 - 3 wt.-%

preferably Ti0 ₂ 0 - 2 wt.-% particularly preferably Ti0 ₂ 0 - 1 wt.-%

Fe ₂ 0 ₃ 0 - 0.5 wt.-%

Zr0 ₂ > 0 - 7 wt.-%

preferably Zr0 ₂ 1 - 6 wt.-%

particularly preferably Zr0 ₂ 1.5 - 6 wt.-%, the

sum of BaO + Zr0 ₂ being 8 - 15 wt.-%

preferably the sum of BaO + Zr0 ₂ being 10 - 15 wt.-%,

more preferably the sum of BaO + Zr0 ₂ being 8 - 13 wt.-%,

Zn0 ₂ 0 - 5 wt.-%

preferably Zn0 ₂ 0.5 - 2 wt.-%

Ce0 ₂ 0 - 2 wt.-%

W0 ₃ 0 - 3 wt.-%

Bi ₂ 0 ₃ 0 - 3 wt.-%

Mo0 ₃ 0 - 3 wt.-%, the ratio (wt.-%) of CaO:MgO being in the range of 0.5 to 1.7.

Typical refining agents, such as sulfates, chlorides, Sb ₂ 0 ₃ , As ₂ 0 ₃ , Sn0 ₂ may be added to the above glass/the glass melt.

The object of this invention is accordingly an aluminosilicate glass developed for very special applications, which is suitable in particular for use in a photovoltaic cell, preferably a thin-film photovoltaic cell. The aluminosilicate glass can be used as an alternative for a soda-lime glass, a significantly higher transformation temperature Tg > 580°C, preferably > 600°C being achieved than soda-lime glasses. The transformation temperature Tg of the aluminosilicate glass according to the invention is in the range of approximately 590°C to 625°C, for example. Simultaneously, a lower processing temperature is obtained for aluminosilicate glasses ("VA") < 1200°C, preferably < 1 150°C. Thus, for example, processing temperatures according to the invention are in the range of approximately 1 100°C to approximately 1 170°C, for example.

In addition, the thermal expansion which is characteristic for soda-lime glasses, of approximately 8.5 to 10 x l O^/K. (coefficient of thermal expansion), is achieved in the temperature range of 20 to 300°C by the glasses used according to the invention. To obtain these characteristic properties, in particular a high transformation temperature Tg and a low processing temperature as well as a high expansion, the glass has high contents of Na ₂ 0 of 10 wt.-%, preferably > 1 1 wt.-%, the upper limit being < 16 wt.-%. In particular for the very special applications of the glasses according to the invention as substrate glasses, for example, as C1GS substrate glasses (copper-indium-gallium sulfide and/or selenide) and CdTe substrate glasses (cadmium tellurium) for photovoltaics, a Na ₂ 0 content of > 10 wt.-% is an essential feature. Sodium provides a significant contribution in this case to increasing the efficiency, in that sodium ions may diffuse into the CIGS or CdTe layer. The high sodium content therefore decisively contributes to the fact that the high Tg value according to the invention is achieved simultaneously with low processing temperature and high expansion of the aluminosilicate glass used according to the invention.

Furthermore, it is significant in the scope of the invention that in the glass used according to the invention, the contents of CaO are in the range of > 0 wt.-% to < 5 wt.-%. The CaO content is particularly preferably < 4.9 wt.-%, more preferably < 4.7 wt.-%, still more preferably < 4.5 wt.- %, in particular < 4.3 wt.-%, particularly preferably < 4.0 wt.-%. The lower limit on CaO is > 0 wt.-%, preferably 0.1 wt.-%, particularly preferably, the lower limit is > 0.5 or 1.0 wt.-%, very particularly preferably > 2 wt.-%, to set the described physical properties and a sufficient crystallization resistance of the glasses. CaO is therefore preferably provided in the

aluminosilicate glass used according to the invention in a quantity of 0.1 to 4.9 wt.-%, more preferably in a quantity of 1 to < 4.7 wt.-%, still more preferably in a quantity of 3 to 4.7 wt.-%, particularly preferably in a quantity of 3 to < 4.5 wt.-%, very particularly preferably in a quantity of 3 to < 4.3 wt.-%.

The desired properties of the glass may be implemented to a particularly high degree if the quotient (wt.-%) of CaO/MgO is set in the range of 0.5 to 1.7, preferably in the range of 0.8 to 1.6, and very particularly preferably in the range of 1.1 to 1.55. To determine the quotient, the quantity of CaO in wt.-% is divided by the quantity of MgO in wt.-% and a numeric value without units is obtained, which is to be in the range claimed according to the invention. By setting the quotient (wt.-%) of CaO/MgO in the described range, the crystallization resistance can be increased, the described physical properties being achieved simultaneously. It is particularly advantageous in the scope of the invention if aluminosilicate glasses are selected, the sum MgO + CaO + SrO + BaO being in the range of 7 to 18 wt.-%, particularly preferably 7 to 17 wt.-%, in particular 10 to 16 wt.-%. This can also contribute to providing the desired physical properties of the glasses to a high degree.

Therefore, aluminosilicate glasses are used according to the invention. As the main components, these comprise Si0 ₂ and A1 ₂ 0 ₃ as well as the alkaline oxide Na ₂ 0 and the alkaline earth oxides CaO and MgO and optionally further components.

The basic glass typically preferably contains at least 48 wt.-%, more preferably at least 49 wt.-%, particularly preferably at least 50 wt.-% Si0 ₂ . The highest quantity of Si0 ₂ is 69 wt.-% Si0 ₂ , preferably 64 wt.-%, particularly preferably 60 wt.-%. A preferable range of the Si0 ₂ content is 50 to 60 wt.-%, still more preferably 52 to 58 wt.-%.

The quantity of A1 ₂ 0 ₃ is preferably > 4.7 wt.-%, more preferably > 5 wt.-%, still more preferably > 7 wt.-%, particularly preferably > 9 wt.-%, very particularly preferably > 1 1 wt.-%. The A1 ₂ 0 ₃ content is particularly preferably < 16 wt.-%, preferably < 15 wt.-%, and in a particularly preferred embodiment < 14.5 wt.-%, in order to allow good fusibility. Ranges of > 5 to 16 wt.-% are very particularly preferred, in particular ranges of 1 1 to 15 wt.-%. The content can be varied as a function of the intended use. Exceeding the A1 ₂ 0 ₃ content of 16 wt.-% has the disadvantage of worsened fusibility. Falling below the A1 ₂ 0 ₃ content of 4.7 wt.-% has the disadvantage that the chemical resistance of the glass is worsened and the tendency toward crystallization increases.

Of the oxides of the alkaline metals lithium, sodium, and potassium, as already noted, sodium oxide is of substantial significance in particular. ^" Na ₂ 0 is included according to the invention in a quantity of > 10 to 16 wt.-%, in particular in a quantity of > 10 to 15 wt.-%, more preferably in a quantity of 10 to 14 wt.-%, in particular in a quantity of 10 to 13 wt.-%, very particularly preferably in a quantity of > 1 1 to 13 wt.-%. The content according to the invention of K ₂ 0 is preferably > 0 to 8 wt.-%, more preferably > 0 to < 5 wt.-%, particularly preferably > 2 to < 5 wt.-%. According to the invention, the Li ₂ 0 content is preferably 0 to 4 wt.-%, more preferably 0 to 1 .5 wt.-%, particularly preferably 0 to < 0.3 wt.-%. The addition of Li ₂ 0 can be used to set the thermal coefficient of expansion (CTE) and to reduce the processing temperature. However, the Li ₂ 0 content is particularly preferably < 0.3 wt.-% or the glass is completely free of Li ₂ 0. Up to this point, there have been no indications that Li ₂ 0 would act similarly as Na ₂ 0, since its diffusion is presumably too high. In addition, Li ₂ 0 is costly as a raw material, so that it is advantageous to use smaller quantities or to leave it out entirely (Li ₂ 0 = 0 wt.-%).

Exceeding the respective specified alkaline oxide content has the disadvantage that the corrosion of an existing glass contact material worsens. Falling below the respective alkaline oxide content has the disadvantage that the fusibility is worsened.

The sum Li ₂ 0 + Na ₂ 0 + ₂ 0 is preferably in the range > 1 1 to 19 wt.-%, more preferably in the range > 12 to 17 wt.-%.

The oxides of calcium, magnesium, optionally barium, and, to a lesser extent, also strontium are used as alkaline earth oxides.

According to the invention, CaO is provided in the glass composition according to the invention in the range of > 0 to < 5 wt.-%, more preferably in the range of 0.1 to 4.9 wt.-%, still more preferably in the range of 1 to 4.7 wt.-%, in particular 2 to 4.7 wt.-%, particularly preferably in the range of 3 to 4.7 wt.-%, more particularly preferably in the range of 3 to < 4.5 wt.-%, very particularly preferably in the range of 3 to < 4.3 wt.-%.

MgO is preferably used in the range of > 0 to 6 wt.-%, more preferably in the range of 0.1 to 6 wt.-%, still more preferably in the range of 1.5 to 6 wt.-%, particularly preferably in the range of 1 .5 to 5.5 wt.-%, very particularly preferably in the range of 2 to 5.5 wt.-%. In order to achieve the described properties of the aluminosilicate glasses of the invention, according to the invention, the quantities of CaO and MgO, each in wt.-%, are selected in the described ranges in such a manner that the quotient CaO:MgO is set in the range of 0.5 to 1 .7, preferably in the range of 0.8 to 1 .6, very particularly preferably in the range of 1 .1 to 1.55. By setting the quotient (wt.- %) of CaO:MgO in the described range, the crystallization stability is increased and the described physical properties are achieved to the desired extent.

In the scope of the invention, a quantity specification of > X wt.-% for a component in the glass composition means that it contains more than X wt.-%. For example, > 0 wt.-% of a component means that it contains a quantity of more than 0 wt.-%, for example, a quantity of 0.1 wt.-% or more.

In the scope of the invention, a quantity specification of < Y wt.-% for a component in the glass composition means that it contains less than Y wt.-%. For example, < 5 wt.-% of a component means that it contains a quantity of less than 5 wt.-%, for example, a quantity of 4.9 wt.-% or less.

BaO is used in the present invention in the range of > 1 to 10 wt.-%, preferably in the range of 1 .1 to 10 wt.-%, still more preferably in the range of 1 .25 to 10 wt.-%, particularly preferably in the range of 1 .5 to 10 wt.-%, very particularly preferably in the range of 1.75 to 10 wt.-%.

Furthermore, ranges of > 1 to 9 wt.-%, more preferably 2 to 9 wt.-%, particularly preferably 2 to 8.5 wt.-% are preferably used. The advantage of a greater BaO content is that it can be used to increase the transformation temperature Tg of the glass composition. The advantages of a small BaO content are essentially the lower density and therefore a weight reduction of the glass, as well as the savings in cost of the expensive components per se. According to the present invention, a BaO content in the range of > 1 to 10 wt.-% is used. The low density is

advantageous in particular during the transport of the glasses for further processing, in particular if the products produced from the glass are to be installed in portable devices. A very particular advantage of a low density and therefore a lower weight of a substrate glass for photovoltaic applications, for example, is that modules/panels in which the glass according to the invention is used, for example, may be constructed on foundations (e.g., industrial flat roofs) which only have a small ultimate load. This can advantageously be achieved to a still higher extent, for example, in that BaO is replaced by SrO.

The Zr0 ₂ content is particularly advantageously to be set according to the invention in the range of > 0 to 7 wt.-%, more preferably 1 to 6 wt.-%, still more preferably in the range of 1 .5 to 5 wt.- %.

The sum of BaO + Zr0 ₂ is advantageously set in the range of 4 to 1 5 wt.-%, more preferably in the range of 5 to 15 wt.-%, still more preferably in the range of 7 to 13 wt.-%, particularly preferably in the range of 8 to 12.5 wt.-%. Setting the sum in the described range is advantageous since in this way the required physical properties, in particular the glass transformation temperature (Tg) and the processing temperature (VA), may be set and the addition of these components (together with MgO) makes it possible to reduce the CaO content and thus increase the crystallization stability.

SrO is preferably provided in the glass according to the invention in the range of 0 to 8 wt.-%, more preferably 0 to < 6 wt.-%, still more preferably 0 to 5 wt.-%, very preferably 0 to < 3.5 wt.- %, in particular in the range of 0 to < 0.5 wt.-%. SrO is generally used to increase the

transformation temperature Tg of the glass. However, SrO can also be entirely absent in the glass composition according to the invention (SrO = 0 wt.-%). Particularly disadvantageous effects, as are asserted in the prior art, could not be established in this way.

According to the invention, the sum of MgO + CaO + SrO + BaO is preferably in the range of 7 to 18 wt.-%, more preferably in the range of 7 to 17 wt.-%, still more preferably in the range of 9 to 16 wt.-%, particularly preferably in the range of 12 to 16 wt.-%.

According to the invention, B ₂ 0 ₃ is preferably provided in a quantity of 0 to 2 wt.-%, more preferably 0 to 1 wt.-%, very particularly preferably 0 to 0.5 wt.-%. According to a particularly preferred embodiment, the glass is free of B ₂ 0 ₃ . This is preferable since B ₂ 0 ₃ is of toxicological concern (teratogenic), on the one hand and is an expensive component, on the other hand, which significantly increases the quantity price. In addition, higher proportions of B ₂ 0 ₃ have the disadvantage that they vaporize during the glass fusion, precipitate in an interfering manner in the exhaust gas area, and change the glass composition per se. Furthermore, the addition of B ₂ 0 ₃ is disadvantageous in special applications. Thus, it has been shown that a B ₂ 0 ₃ content greater than 1 wt.-% in a substrate glass has a disadvantageous effect on the efficiency of a thin-film solar cell, since boron atoms from the substrate glass reach the semiconductor layers via evaporation or diffusion, where they possibly cause effects, which are electrically active and could reduce the performance of the cell through increased recombination.

Furthermore, Zn0 ₂ is preferably included in a quantity of 0 to 5 wt.-%, preferably in a quantity of 0.5 to 3 wt.-%. A particularly preferred embodiment contains Zn0 ₂ in contents of 0.5 to 2.0 wt.-%, preferably 0.7 to 1.5 wt.-%. In addition, other components, such as W0 _3j M0O3, Bi ₂ 0 ₃ , Ce0 ₂ , Ti0 ₂ , Fe ₂ 0 ₃ , ZnO, and/or F or also further components may also be provided independently of one another in the

aluminosilicate glass according to the invention.

W0 ₃ , o0 ₃ , Bi ₂ 0 ₃ are each preferably provided independently of one another in the aluminosilicate glasses according to the invention in a quantity of 0 to 3 wt.-%, preferably 0 to 1 wt.-%. These components are preferably used to set the UV edge of the glass and may also be used as redox buffers during the refining.

Ti0 ₂ and also Ce0 ₂ may typically be added for UV blocking of the glass. Depending on the area of application, the glass used according to the invention may be provided in the form of a cover glass or envelope tube, for example, and have doping using, for example, Ti0 ₂ and/or Ce0 ₂ , in order to keep harmful UV radiation away from components below the glass. The Ti0 ₂ content is preferably in the range of 0 to 6 wt.-% according to the invention, more preferably in the range of > 0.1 to 5 wt.-%, still more preferably in the range of > 0.1 to 3 wt.-%. According to the invention, Ce0 ₂ is preferably provided in a range of 0 to 2 wt.-%, more preferably 0 to 1 wt.-%, still more preferably 0.1 to 0.5 wt.-%. If Ce0 ₂ is added to the glass, it is preferable if Sn0 ₂ is added to the glass in corresponding quantities, in order to prevent coloration of the glass.

Fe ₂ 0 ₃ is preferably used in a quantity of 0 to 0.5 wt.-% and is typically used to set the UV blocking, but can also be used as a redox buffer for the refining.

Furthermore, fluorine can be added to the glass of the invention in the form of fluoride salts, such as NaF, to improve the fusibility. The quantity which is used in the glass composition is preferably 0 to 3 wt.-%, more preferably 0 to 1 wt.-%, the glass still more preferably contains F in a concentration of 0.05 to 0.2 wt.-%.

The aluminosilicate glasses used according to the invention are to be free of niobium oxide except for unavoidable contaminants, in contrast to the prior art according to US 5,434, 1 1 1 .

Typical refining agents may be used, if they do not disadvantageously influence the chemical and physical properties of the glass composition used according to the invention. For example, refining using sulfates, chlorides, Sb ₂ 0 ₃ , As ₂ 0 ₃ , and/or Sn0 ₂ is possible. The refining agents are each preferably included per se in the glass in a quantity of > 0 to 1 wt.-%, the minimum content preferably being 0.05, in particular 0.1 wt.-%.

The aluminosilicate glass employed according to the invention can be used in any arbitrary shape, for example, as flat glass, tubular glass, block glass, fiberglass, bar glass, for example, round, oval, structured or non-structured; however, tubular glasses are very particularly preferred with respect to the use in the field of photovoltaics.

The aluminosilicate glasses in the scope of the present invention are suitable, for example, for producing plate glass, particularly according to the float method. In addition, the aluminosilicate glasses are suitable for producing tubular glass, the Danner method being particularly preferred. However, the production of tubular glass according to the Velio or A-traction method is also possible. Glass tubes may also be produced which preferably have a diameter of at least 5 mm to at most 15 cm. Particularly preferred tube diameters are between 1 cm and 5 cm, preferably 1 cm and 3 cm, with preferred wall thicknesses of at least 0.5 mm to at most 3 mm, preferably 0.8 mm to 2 mm.

The use according to the invention of the aluminosilicate glasses, in particular as an alternative for soda-lime glasses, is based on the higher thermal resistance than these glasses.

The areas of application of the present invention are in the field of solar technology as substrate glasses for photovoltaic applications of all types, preferably technologies which do not operate on the basis of silicon, for example, CIS, C1GS, or CdTe.

The area of use for the aluminosilicate glasses provided according to the invention are solar applications (photovoltaic applications), i.e., applications in which a glass having a high thermal expansion and relatively high temperature stability (processing stability) is required, but the hot shaping of the glass can be performed at the lowest possible temperatures (a so-called "short glass"). The processing temperature (VA) of the glass is to be as low as possible to allow cost- effective production of these glasses. The processing temperature (VA) in the present application is the temperature at which the glass has a viscosity η of η = 10 ⁴ dPa. Depending on the glass composition, the processing temperature VA, at which a viscosity η = 10 ⁴ dPa is reached, can vary. The aluminosilicate glasses used according to the invention fulfill these requirements to a high degree, since they provide processing temperatures < 1200°C and are therefore particularly well suitable for such applications.

The aluminosilicate glasses used according to the invention are particularly suitable as substrate/superstrate glasses or also as cover glasses in photovoltaic cells, in particular for thin- film photovoltaic cells, comprising layers containing cadmium and/or tellurium in metal form or containing copper, indium, gallium, sulfur, and/or selenium in metal form. Superstrates are substrate glasses, the substrate glasses functioning as a quasi-cover glass, since the coated glass is "inverted" in thin-film photovoltaics and the layer lies on the bottom side and the light is incident on the photovoltaic layer through the substrate glass.

The aluminosilicate glasses according to the invention are therefore particularly suitable for technologies based on CdTe, as well as technologies which are based on copper-indium-gallium- sulfide-selenide, so-called CIS or CIGS. C1GS stands for Cu(lni. _x ,Ga _x ) (Si _-y ,Se _y ) ₂ and is a known thin-film technology for solar cells and stands as an abbreviation for the employed elements copper, indium, gallium, sulfur, and selenium. Important examples are Cu(In,Ga)Se ₂ (copper- indium-gallium-diselenide) or CuInS ₂ (copper-indium-disulfide). These materials are

distinguished in particular in that they already effectively absorb the sunlight as direct semiconductors in a relatively thin layer of a few micrometers. The deposition of such thin photoactive layers requires high processing temperatures to achieve high efficiencies. Typical temperatures are in the range of 450 to 600°C, the maximum temperature being limited by the substrate. For large-area applications, glass is frequently used as the substrate, as is well known. In order that the coefficient of thermal expansion (CTE) is adapted to the semiconductor layers, up to this point float soda-lime glass has been used as the substrate, for example, as disclosed in DE 43 33 407 and WO 94/07269. Soda-lime glass has a transformation temperature of approximately 525°C, as already explained, however, and therefore limits all processing processes to approximately 500°C. Otherwise, so-called bulges occur and the glass begins to sag. This is all the more true the larger the substrate to be coated and the closer the processing temperature approaches the transformation temperature Tg of the glass. Bulges and sags of the substrate result in problems in particular in in-line processes or plants, whereby the throughput and the yield drop dramatically. The aluminosilicate glasses used according to the invention therefore represent an alternative to soda-lime glasses in this area of use in particular and may advantageously replace them, since higher method temperatures may be used in the vapor deposition of semiconductor layers than in the case of typical soda-lime glasses, without the substrate disadvantageously deforming. A desired higher temperature during the coating method additionally results in higher deposition rates and very good crystalline quality of the created layers.

In order that chipping of the layers does not occur during the cooling after the application of the semiconductor layers, it is necessary for the substrate glass to further be adapted to the coefficient of thermal expansion of the semiconductors applied thereon (e.g., approximately 8.5 x 10 ^"6 /K for CIGS). The thermal adaptation to the CIGS layer is decisive in this case, and not, as one could assume, to that of the back contact material, such as molybdenum, since it can absorb a greater CTE difference because of the ductility. However, the CTE should also not be too low for the molybdenum layer (not less than approximately 4 x l O^/K), since otherwise chipping of the layer occurs. These properties are also provided by the aluminosilicate glasses selected according to the invention.

The aluminosilicate glasses according to the invention are therefore used in the field of solar technology, for photovoltaic cells, in particular thin-film photovoltaic cells, in particular those based on composite semiconductor material, such as CdTe, CIS, or CIGS. The glasses of the invention are therefore preferably used as thin-film photovoltaic cell substrate or super substrates or cover glasses. Substantially less photoactive material is required in this case for an efficient conversion of sunlight into electricity than in the case of typical crystalline silicon-based photovoltaic cells. The small semiconductor consumption and the high degree of automation of the manufacturing result in significant cost reductions with this technology.

A further advantage upon use of the aluminosilicate glasses according to the invention in photovoltaic technology is their high content of sodium. It is known that sodium can be incorporated in a semiconductor and thus increase the efficiency of a solar cell through improved chalcogen incorporation in the crystal structure of the semiconductor. The substrate glass can thus also be used, in addition to the property as a carrier, for the targeted discharge of sodium ions/atoms into the semiconductor. The aluminosilicate glasses used according to the invention are very particularly well suitable for the above-mentioned technologies, since the processing capability and deposition of

semiconductor layers can be performed at higher temperatures than the traditionally used soda- lime glasses because of higher temperature stability, which results in significant advantages. A criterion for this purpose is the so-called transformation temperature Tg. On the other hand, particularly high temperatures are not required for the melting and hot shaping process of the aluminosilicate glass according to the invention, which allows cost-effective production.

According to a particularly preferred embodiment, the substrate of the photovoltaic cell is provided in the form of a tubular glass. According to a further preferred embodiment according to the invention, the substrate of the photovoltaic cell is implemented in the form of an inner tube and is enclosed by a protective tube in the form of an outer tube, the inner tube and optionally also the outer tube containing or consisting of the aluminosilicate glass, which has already been explained in detail.

The present invention is explained in greater detail hereafter on the basis of examples, which are to illustrate the teaching according to the invention, but are not to restrict it.

Examples

In the scope of the teaching according to the invention, glass compositions were selected for examples 1 to 16 and glasses were produced therefrom. For this purpose, 1 L platinum crucibles were used for the melting, into which the raw materials were brought to a melting temperature of 1580°C over 3 hours and held and stirred for 4 hours at this temperature therein. Subsequently, the melt was poured into graphite molds preheated to 500°C. The casting mold was placed in a cooling furnace preheated to 650°C after the casting, which was cooled down to room

temperature at 5°C/hour. The glass compositions of comparative examples 1 to 3 were produced similarly. The compositions and properties of the glasses to be used in the scope of the teaching according to the invention according to examples 1 to 16 and the compositions of comparative examples 1 to 3 are summarized in following tables 1 to 4: Table 1

n.d. = No devitrification measurable (according to the carrier plate method) Table 2

n.d. = No devitrification measurable (according to the carrier plate method) Table 3

No devitrification measurable (according to the carrier plate method) Table 4

[in wt.-%] Comparative Comparative Comparative example 1 example 2 example 3

Si0 ₂ 56.90 50.90 57.8

B ₂ 0 ₃ - - -

A1 ₂ 0 ₃ 9.90 13.50 9.90

Li ₂ 0 - - -

Na ₂ 0 1 1.00 10.10 1 1.00

20 4.20 2.50 4.20

MgO 0.50 2.50 2.50

CaO 10.90 1 1.00 8.00

SrO - - -

BaO - 8.40 -

Zr0 ₂ 6.50 1.00 6.50

Ti0 ₂ - - -

F 0.1 0.1 0.1

Sum 100.00 100.00 100.00

Alpha 9.23 9.52 9.01

Tg 620 601 614

VA 1 100 1039 1 123

OEG 1 160 1085 1 105

UEG 900 925 925 gmax°C 1055 995 1060 gmax 5.30 5.50 2.00 μm/min

CaO/MgO 21.8 4.4 3.2

Sum of 1 1.40 21.90 10.50

MgO+CaO+

SrO+BaO alpha coefficient of thermal expansion at 20 to 300°C [x l O^K ^"1 ]

Tg transformation temperature [°C]

VA processing temperature, at which the viscosity is 10 ⁴ dPas [°C]

OEG upper devitrification temperature [°C]

UEG lower devitrification temperature [°C]

Kgmax°C temperature having maximum crystal growth speed [°C]

gmax maximum crystal growth speed in [^m/minute]

The devitrification or crystallization behavior was ascertained according to the carrier plate method explained hereafter in the case of rising temperature control.

Carrier plate method with rising temperature control: a) Sample preparation

The sample is pulverized in a mortar and subsequently screened in a sieve to a grain fraction > 1.6 mm. b) Performance

There are two types of carrier plates, using which one or two samples may be tempered simultaneously in a gradient furnace. The plates made of device platinum have conical indentations in two rows offset to one another at a distance of 5 mm each to one another, into which sample pieces of equal size are laid. In the case of glasses of known tendency to crystallize, the furnace is set to approximately 50 above the expected OEG, in the case of unknown glasses, a range having T _mjn ~ Tg + 50°C is set.

The occupied plate is inserted centered into the gradient furnace. A ceramic pin is located as a stop in the rear furnace part in the furnace used.

For tempering in the high-temperature furnace, the plate is brought to the desired insertion depth with the aid of a corundum rod. If a different tempering time is not specified, the plate is first tempered in the furnace for 5 minutes, after half of the time the temperature is read off and written down. Since the temperatures are measured using an external thermocouple element in high-temperature furnaces, the minimum tempering time is 15 minutes, in most cases the samples are tempered for 60 minutes.

Subsequently, the temperature values are plotted on millimeter paper as a function of the furnace length and connected using a curve. After passage of the tempering time, the plate is removed from the furnace, permitted to cool down, and subsequently the sample is microscopically studied for crystallization.

The examples 1 , 3, 6, 7, 9 and 12 according to the invention, in which the crystallization stability was ascertained using the above-described carrier plate method, did not display any

devitrification. The devitrification refers in this case to the procedure in which the glass crystallizes out below the transformation range. In comparative examples 1 to 3, in contrast thereto, glasses were obtained which displayed strong tendency to crystallize, since upper and lower devitrification temperatures, a temperature having maximum crystal growth speed, and the maximum crystal growth speed could be specified. The examples and comparative examples therefore prove that the setting of the CaO/MgO ratio in the range according to the invention results in aluminosilicate glasses which are surprisingly crystallization-stable.

Therefore, aluminosilicate glasses specially adapted for the photovoltaic field are provided for the first time by the present invention, which provide an alternative to soda-lime glasses, which have a higher thermal carrying capacity at a Tg > 580°C, but display a processing temperature of VA < 1200°C, and do not tend toward devitrification, so that the fields of use may be significantly expanded in comparison to soda-lime glasses.