Transparent conductive film system.

The invention relates to a transparent, conductive layer system, in particular for solar cells or heatable window elements, with at least one metal layer arranged between semiconductor layers.

Transparent, conductive layers are used, for example, in the manufacture of resistors, heatable windows, antistatic windows, transparent electrodes, infrared reflectors, optical filters and selective sensors (KL Chopra, S. Major, DK Pandya, Thin Solid Films<u style="single">102</u> (1983) 1).

The materials used are either metals such as gold, silver, platinum or transparent, conductive oxides such as cadmium stannate, tin oxide or indium oxide, which are also known as TCO (= Transparent Conductive Oxides). The group of TCO materials includes, for example: SnO₂ (= TO = tin oxide); SnO₂: Sb (= ATO = antimony doped tin oxide); SnO₂: F (= FTO = fluorine doped tin oxide); In₂O₃: Sn, among others (= ITO = indium tin oxide).

In addition to tin, titanium, antimony, fluorine, fluorine and tin are also known as dopants for indium oxide.

The combination of metal layers with poorly conducting oxides or sulfides is also known. The embedding of a thin silver layer between two tin oxide layers in a heatable windshield is also known. To produce antistatic windows it is sufficient to use a TCO layer with a relatively low conductivity, so that the transparency is only slightly affected. In the case of a heatable windscreen or an a-Si solar cell, in addition to high transparency, there must also be low electrical resistance. However, when using TCO layers, high conductivity can only be achieved with a high layer thickness.

A higher conductivity can be achieved with thin metal layers, which already have a sheet resistance of 10 Ω / □ with layer thicknesses of 10 nm. However, the transmission of a metal layer deposited on glass with a layer thickness d &lt;30 nm is already below 50%.

The object of the present invention is to design a transparent, conductive layer system of the type described at the outset in such a way that both good electrical conductivity and high transparency are provided.

The object is achieved in that the semiconductor layers contain a conductive oxidic heavy metal compound and have a surface resistance of less than 40 Ω / □.

The layer system according to the invention has the advantage that the electrical conductivity as with a thicker metal layer and the optical transmission as with a thinner metal layer are achieved at the same time.

The transmission of the layer system is particularly high if the semiconductor layers adjacent to the metal layer have a high refractive index. This is advantageously achieved in that at least one of the semiconductor layers contains an oxidic tin compound.

According to a particularly noteworthy proposal of the invention, there is good electrical conductivity and at the same time high optical transmission when the semiconductor layers have indium oxide mixed with tin dioxide. Tin dioxide doped with fluorine or antimony can also be used as the material for the semiconductor layers.

In the case of a layer system applied to glass or acrylic glass as a substrate, for example, such semiconductor layers also act as a reflection-reducing layer and can be optimized for a specific transmission maximum by adapting their layer thickness. The optical layer thickness n · d must be equal to or equal to a multiple of half the wavelength λ of the transmitted light. For a wavelength of λ = 550 nm, a layer thickness of around 140 nm results with a refractive index of n≈2. The electrical resistance of such a layer is around 10 Ω / □. If this layer is divided into two halves, each 70 nm thick, and a metal layer with a thickness of 5 to 30 nm and a surface resistance of 20 Ω / □ to 3 Ω / □ is arranged between them, a surface resistance in the range of 7 Ω / □ can be obtained up to 2 Ω / □, whereby the transparency of this layer system is significantly higher than that of an equally thick metal layer without adjacent semiconductor layers.

In order to avoid reflection losses, in particular at the outer interface bordering on air, when the semiconductor layer with a refractive index n = 2 borders on the medium air with n = 1, it is proposed in an embodiment of the invention that one or more transition layers can be arranged above the semiconductor layer whose refractive indices have decreasing values. Likewise, reflection losses between the second semiconductor layer and the substrate can be reduced if a transition layer is also arranged between them, the refractive index of which lies between that of the semiconductor layer and that of the medium.

In particular, there is an improved transmission with the following arrangement of the layer system:<tables id="tabl0001" num="0001"><img file="EP0252489A2_D0001.tif" /></tables> It is particularly expedient if the transition layers do not have a constant refractive index, but instead have a refractive index gradient, the refractive index decreasing from the semiconductor layer to the medium.

A reflection-optimized layer system with the layer arrangement ITO / Ag / ITO has, for example, a sheet resistance of 7 Ω / □ and a transmission of over 70% with a thickness of the silver layer of 10 nm. The layer thickness of the semiconductor layers is advantageously between 10 and 1000 nm, preferably between 50 and 500 nm. Indium oxide mixed with tin oxide is advantageously used as the material. The layer thickness of the metal layer can vary between 1 and 100 nm depending on the required conductivity and transmission. However, a layer thickness between 5 and 30 nm is preferred. For the metal layer, a material from the group of copper, silver, gold, platinum, aluminum, chromium, iron or nickel is recommended. These can therefore be used as pure metals or as alloys or mixtures.

Materials known from optical application, such as silicon dioxide, aluminum oxide or magnesium fluoride, can be used for the reflection-reducing layers.

When using the layer system according to the invention, for example for a heatable window pane, two opposite strip-shaped sections on the edge of the pane are provided with metallic contacts on the uppermost semiconductor layer, which can be connected to the electrical connections.

In the case of a solar cell, the layer build-up begins on a substrate that will later serve as a cover plate, with a semiconductor layer that preferably contains ITO and has a thickness of 10 to 1000 nm. This is followed by a 1 to 100 nm, preferably 5 to 30 nm thick metal layer from the group of the metals mentioned above. Photosensitive material such as silicon is applied to the metal layer, which is p-doped towards the metal layer and n-doped towards the back. On the back of the silicon layer, a metal layer can be arranged as a back contact, the layer thickness of which should be less than 5 to 30 nm if the solar cell is to convert incident light from both the front and the back into electrical energy. If the solar cell is only intended to detect radiation from the front, the back contact can be of any thickness and at the same time serve as a substrate.

Further details, advantages and features of the invention result not only from the claims, the features to be extracted from them - individually and / or in combination - but also from the following description of preferred exemplary embodiments shown in the drawing.

Show it:<ul id="ul0001" list-style="none"><li>Fig. 1 shows a cross section of a layer system according to the invention intended for heating window elements in a schematic representation and</li><li>Fig. 2 shows a cross section through an inventive layer system in a solar cell in a schematic representation.</li></ul>

1 shows a schematic representation of a layer system (10) which is intended as a heater for a window element (11). The window element (11), which can also be referred to as a substrate and which can be a glass pane or a film, has on its upper side a semiconductor layer (12) which can consist of indium oxide mixed with tin oxide. The layer thickness of the semiconductor layer (12) is approximately 70 nm. A metal layer (14), which consists of silver and can have a thickness of 15 nm, is applied to the semiconductor layer (12). The metal layer (14) is then covered by a further (second) semiconductor layer (16) which, like the semiconductor layer (12), has a thickness of 70 nm and can consist of indium oxide mixed with tin oxide.

In order to reduce any reflection losses, a dielectric layer (18) made of silicon oxide, for example, can be applied to the semiconductor layer (16), the refractive index of which is smaller than the refractive index of the semiconductor layer (16). The thickness of the dielectric layer (18) can be, for example, 180 nm. A further reduction in reflection can now be achieved by additionally applying a further layer (20) or (22), whose refractive index is lower than that, to both the dielectric layer (18) and the underside of the substrate (11) of the covered layers is. Magnesium fluoride, the layer thickness of which can be 210 nm, should be mentioned as a suitable material for the layers (20) and (22).

Furthermore, a contact strip (24) or (26) is applied to the side edges of the layer system (10) composed of layers (12), (14) and (16), each with a feed line (28) or (30 ) connected is. Electrical current is conducted via the feed line (28) and (30) and the contact strips (24) and (26) to the layer system (10), preferably to the metal layer (14), in order to cause the substrate (11) to heat up.

The layer system according to the invention enables the electrical power required for heating to be fed in even at low voltages (eg 12 V). The structure of the layer system (10) according to the invention further ensures that the transparency is not noticeably impaired.

The materials of the outer layers used in the production of known layer structures consist of dielectric-metal-dielectric are insulators. As a result, contacting the layer structure is problematic in the case of small contact areas. Small contact areas are a prerequisite for high transparency of the entire arrangement. By means of the layer system according to the invention, however, the required current densities can be impressed even with small contact areas, so that there is no noticeable influence on the transparency.

In a layer system according to the invention, two solderable electrical contact tracks can be produced, for example, on the back of an ITO layer using, for example, screen printing, PVD / CVD coating, galvanic processes or plasma spraying (flame spraying), to which leads can be soldered.

In the structure shown in FIG. 1, the refractive indices of the individual layers have the following values:

The substrate (11), if it is glass, has the refractive index n = 1.5, the semiconductor layers (12) and (16) the refractive index n = 2, the dielectric layer (18) in the form of SiO₂ has the refractive index n = 1.5 and the antireflection layer (20) or (22) in the form of magnesium fluoride has the refractive index n = 1.3.

2 shows a layer system (32) according to the invention which is used in a solar cell (34). In the solar cell (34), the reference symbol (36) denotes a cover plate, which also serves as a support for the layer system (32). The cover plate is preferably made of glass and has a refractive index n = 1.5.

On the outside, the cover (36) is covered with a reflection-reducing layer (38), which preferably consists of magnesium fluoride, has a refractive index n = 1.3 and has a layer thickness of 10 to 280 nm, preferably 20 to 100 nm. The inside of the cover (36) is provided with a semiconductor layer (40) which consists of indium oxide mixed with tin oxide and preferably has a thickness of 70 nm and which has a refractive index n = 2. On the side of the semiconductor layer (40) facing away from the cover (36) there is a metal layer (42), preferably in the form of silver with a thickness of 20 nm. Above the layer (42) is a layer (46) made of photosensitive material such as Solar silicon arranged, which is p-doped in the area facing the metal layer (42), for example, and n-doped in the opposite, that is to say the rear area. Finally, the silicon layer (46) is covered on the outside by an electrically conductive back contact (48) in the form of a metal layer made of, for example, aluminum with a thickness of 0.5 to 2 μm.

Instead of the solar cell (34) described in the exemplary embodiment with a cover (36) serving as a carrier substrate and a relatively thin silicon layer (46), the layer system according to the invention can of course also be used in solar cells whose silicon layer is thicker, for example in the form of a silicon wafer , so that this serves as a carrier for the layer system (32). In this case, the substrate (36) is no longer required, but can be replaced by a layer of SiO₂ with n = 1.5.