WO2010072462A1

WO2010072462A1 - Photovoltaic element

Info

Publication number: WO2010072462A1
Application number: PCT/EP2009/064884
Authority: WO
Inventors: Markus Hlusiak
Original assignee: Q-Cells Se
Priority date: 2008-12-16
Filing date: 2009-11-10
Publication date: 2010-07-01
Also published as: EP2377168A1; DE102008054756A1

Abstract

The invention relates to a photovoltaic element (1) having a photovoltaically active region, a transparent region (6) arranged on the photovoltaically active region and made of a transparent electrically conductive contact region base material for withdrawing a current photovoltaically generated in the photovoltaic active region, and an intermediate region (4) arranged between the photovoltaically active region and the transparent region (6) and made of a transparent electrically conductive intermediate region base material, wherein the intermediate region base material has a higher doping density in the intermediate region (4) than the transparent region base material in the transparent region (6).

Description

Title:

photovoltaic element

Description:

The invention relates to a photovoltaic element with a transparency region.

Such transparency regions are suitable for being arranged on a rear side of the photovoltaic element facing away from the light incidence side between a photovoltaically active region and a reflective layer. The transparency region serves to improve the reflection properties of the reflective layer. The photovoltaic element may in particular be a solar cell.

The transparency region can also be designed to remove currents generated in an active region of the photovoltaic element by charge carrier separation. The reflective layer is then formed as a contact layer. As a rule, so-called transparent conductive oxides (TCO), for example zinc oxide (ZnO), are used as the base material for such transparency regions The contact layer is preferably in the form of a full-area metallization in order to collect and forward the current removed by means of the transparency region This metallization regularly includes aluminum.

In order to improve the contact between the active region and the transparency region, the transparency region is doped to have a high conductivity. Usually, for this purpose, the entire range of transparency arranged between the active region and the metallization is highly doped, in the case of ZnO, for example, by means of aluminum doping. However, the doping of the entire transparency range results in that incident light is absorbed in the start transparency region. This decreases the transparency of the transparency region and the efficiency of the photovoltaic element is reduced.

It is therefore an object of the invention to provide a photovoltaic element with improved efficiency.

The object is achieved according to the invention by a photovoltaic element having the features of claim 1. Advantageous developments of the invention will become apparent from the dependent claims.

The invention is based on the recognition that the specific conductivity need not be increased in the entire transparency range, since the current flows substantially perpendicular to a transition surface between the active region and the transparency region. Therefore, it is sufficient to establish an optimum electrical connection between these two areas when the doping is concentrated on the intermediate area. Usually, a high lateral conductivity or transverse conductivity and good contact properties of the transparent layer are achieved in combination by high doping and thus sacrifice good transparency. By means of the invention it is possible to maintain the good contact properties. However, this sacrifices the transverse conductivity of the transparency layer in favor of good transparency.

Thus, while the intermediate region has an increased doping and thereby an electrical contact between the active region and the

Can form the intermediate region, the transparency region is doped lower to reduce absorption of the incident light due to the doping. With the incident light, not only the visible spectrum is meant here. Rather, in particular, infrared light, for example in the wavelength range of 900 to 1200 nm, be of great importance. In particular, in this spectral range, for example, silicon has a low absorption, so that the incident radiation penetrates to the back of the photovoltaic element and must be reflected there. Especially at Infrared light, the absorption is greatly affected by the doping. Accordingly, the property "transparent" refers to materials that transmit at least a substantial portion of the light from a wavelength range suitable for the photovoltaic element.

In particular, it is preferred that the intermediate region and / or the transparency region have a transparency for optical and / or infrared light of at least about 70%, 80%, 85%, 90% or 95%. This means the transparency transverse to a plane along which the intermediate region or the transparency region propagates. Advantageously, these transparency values are measured at wavelengths of the incident light arranged in the spectral range usable for silicon. For example, the above transparency values are for incident light of about 500 nm or 600 nm wavelength.

The dopants with which the intermediate region and the transparency region are doped may differ here. Furthermore, multiple dopants may be used in one of the regions or in both regions. In this context, the term "dopant" refers to that material in the transparency region or in the intermediate region which does not form the essential part of the respective region. However, the dopant influences the conductivity of the region by its presence.

Between the photovoltaic active area and the transparency area, further areas can be added to improve the

Contacting properties of the photovoltaic element serve.

Advantageously, this is a heterojunction. This may be formed, for example, as a pip ⁺ -transition, with a p-doped crystalline region, an intrinsic amorphous surface for surface passivation and a p ⁺ -doped amorphous region. This is therefore a heterojunction with intrinsic thin film or HIT

("Heterojunction with intrinsic thin layer"). - A -

The photovoltaic element is produced by forming the active region, the intermediate region, the transparency region and optionally the heterojunction on a substrate. One possible method of manufacturing the photovoltaic element comprises sputtering a plurality of layers of different targets onto a substrate on which an active region is formed. Alternatively or cumulatively, other deposition methods can be used, for example a chemical and / or physical deposition from the gas phase (CVD - "chemical vapor deposition", PVD - "physical vapor deposition").

In a preferred embodiment it is provided that the intermediate region is formed from a same base material as the transparency region. In other words, the intermediate region base material and the transparency region base material are substantially the same. This is thus a photovoltaic element in which the intermediate region and the transparency region differ only by the doping and / or the doping density.

Such a photovoltaic element can be produced by sputtering in that the intermediate region base material is present in one target as a base material with a doping and / or a doping density and the transparency region base material in another target as a base material with a further doping and / or doping density the sputtering from the two targets takes place one after the other. Alternatively, sputtering from a base material present in a target can be accomplished using a dopant gas that is differently concentrated, activated, and / or present in different deposition zones. The addition of the doping gas can also be varied over time.

Further processes for the production of the intermediate region and the transparency region with the same base material and different doping include batch processes with different temporal doping gas quantities and / or inline processes with different levels of spatial doping Separation zones. Thus, in a batch process, the deposition on substrates arranged in packages takes place with time-varying doping gas quantities and / or densities, the substrates undergo different deposition zones in the deposition plant during an inline process.

In an alternative method for producing the photovoltaic element, a thin layer of a dopant, for example an aluminum layer, is first applied to the active region. Subsequently, the transparent conductive base material is undoped or applied with only slight doping to this layer. The dopant will then diffuse into the base material. This process can be assisted by means of energy supply, for example in a warm-up step.

In an advantageous development, it is provided that the intermediate region has a doping density 10 times higher than the transparency region, preferably a doping density 40 times higher, preferably a doping density 100 times higher. For example, a doping density higher by a factor of 40 exists when the intermediate region has a doping density of about 2% and the transparency region has a doping density of about 0.05%.

According to an expedient embodiment, it is provided that the transparency region base material is substantially undoped in the transparency region. As a result, the light absorption in the transparency region is minimized. Here, however, process-related

Doping levels are present. For example, during sputtering a low doping may be necessary for the target to be sufficiently conductive. Process doping densities may range up to about 0.05%.

Preferably, it is provided that the base material in the intermediate region is doped in such a way that a tunnel junction forms between the intermediate region and the photovoltaically active region. For this For example, the intermediate region may be p- or n-doped. The region of the photovoltaically active region adjacent to the intermediate region has an opposite doping. In order to form a tunnel junction, it is possible to set both the doping density in the intermediate region and the spatial dimensions of the intermediate region, for example a layer thickness in the case of layered intermediate regions.

In an expedient embodiment, it is provided that the intermediate region has a doping density of about 0.5%, preferably about 1%, preferably about 2%. Higher doping densities may also be provided.

In an advantageous embodiment, it is provided that metallization is provided on a side of the transparency region facing away from the intermediate region. The metallization serves to dissipate the current removed by means of the transparency region from the active region. It is preferably formed over the entire surface in order to provide lateral conductivity and to reflect the electromagnetic radiation impinging thereon back into the photovoltaically active region.

According to a preferred embodiment, it is provided that a highly doped metallization junction is arranged between the transparency region and the metallization. By suitable selection of electrical and / or geometrical parameters of the metallization transition, the electrical connection properties between the transparency region and the metallization can be adjusted.

Advantageously, it is provided that the metallization is formed of aluminum. In this case, a metallization junction may be formed by driving aluminum atoms from the metallization into the transparency region base material. According to a preferred embodiment, it is provided that the photovoltaically active region is formed from silicon. The silicon may be at least partially amorphous or polycrystalline. The active region may comprise, for example, a layer sequence with a pn junction, a pin junction, and / or another transition suitable for converting electromagnetic radiation into electrical current. For this purpose, for example, a Schottky junction is suitable.

Preferably, it is provided that the intermediate region and / or the transparency region are made of indium tin oxide (ITO), of tin oxide (SnO), of zinc oxide (ZnO) and / or of another transparent conductive metal oxide (TCO). ) are formed.

In an expedient refinement, it is provided that a region of the photovoltaically active region adjacent to the intermediate region or adjacent to the intermediate region is p-doped and the intermediate region and / or the transparency region are n-doped. For example, the elements B, Al and / or Ga can be used as dopants for the p-doping of silicon. The converse case in which the region adjacent to the intermediate region is n-doped and the intermediate region and / or the transparency region are p-doped is also suitable, but the production of p-doped TCOs may be more difficult.

Appropriately, it is provided that the intermediate region and / or the transparency region are doped with aluminum, boron and / or another donor material. Depending on the base material used, it is also possible to use other elements as donor material for doping the base material in the intermediate area and / or the transparency area.

In an advantageous embodiment, it is provided that the intermediate region is formed as an intermediate layer and the transparency region as a transparency layer. In this case, the photovoltaic element at least partially layered or plate-shaped. Alternatively, the individual regions can form three-dimensional structures.

According to a preferred embodiment, it is provided that the intermediate layer has a substantially smaller thickness than the

Transparency layer. The thickness of the intermediate layer may be, for example, in a range between 5 and 30 nm.

In an expedient embodiment, the photovoltaic element is designed as a solar cell. The solar cell may be wafer-based or applied as a thin film on a substrate.

The invention will be explained below with reference to embodiments with reference to the figures. Hereby show:

Fig. 1 is a schematic not to scale sectional view of a

Photovoltaic elements with a doped transparency region;

Fig. 2 is a schematic not to scale sectional view of a

Photovoltaic elements with a doped intermediate region arranged between the transparency region and a heterojunction; and

FIG. 3 shows an embodiment of the photovoltaic element from FIG. 2, in which a metallization transition is inserted between the transparency region and metallization.

FIGS. 1 to 3 show photovoltaic elements 1 of different design. The zigzag line at the upper edge of the structures shown here indicates that the photovoltaic element 1 continues upward. Not shown in FIGS. 1 to 3 are the photovoltaically active regions of the photovoltaic elements 1 which adjoin the top and which may comprise, for example, a pn junction and / or a heterostructure. The light incident sides of the solar cells are thus not shown in FIGS. 1 to 3. 1 shows schematically in a sectional view the layer structure of a contact region of a photovoltaic element 1 with a heterojunction 2 and a known contacting. Here, the layered heterojunction 2 is covered by means of a transparency region 6, on which in turn a metallization 8 is applied. The photovoltaic element 1 may comprise a crystalline wafer. Alternatively, it may also be applied in the form of thin layers on a suitable substrate, for example glass.

The heterojunction 2 comprises a three-layer structure of a p-doped crystalline silicon layer 21, an intrinsic amorphous silicon layer 22 and a p ⁺ -doped amorphous silicon layer 23. The zig-zag line at the upper edge of the crystalline silicon layer 21 may indicate, for example to the crystalline silicon layer 21, a further semiconductor layer connects, which with the crystalline

Silicon layer 21 forms a photovoltaically active transition, for example, it may be a silicon wafer, which also serves as a substrate for the photovoltaic element 1.

The active region, not shown here, as well as the heterojunction 2 can be produced by conventional semiconductor methods. The photovoltaic element structure is not limited to the embodiment described here and also used by way of example below with a heterojunction in the contact region, but may also have other structures which are suitable for light conversion and contacting. Such structures can be constructed as (pure) semiconductor structures or as metal-semiconductor structures, for example with a Schottky junction. This applies to all photovoltaic elements listed here.

The transparency region 6 of the photovoltaic element 1 comprises as

Base material is the transparent conductive oxide (TCO) zinc oxide (ZnO) and is doped with aluminum. The usually heavy doping of the TCO serves to increase the conductivity of the transparency region 6. Such However, heavy doping has the disadvantage that, when passing through the photovoltaically active region, unabsorbed electromagnetic radiation, which is to be reflected back into the photovoltaically active region by means of a metallization 8, experiences a strong absorption.

The photovoltaic element 1 shown in FIG. 2 comprises a heterojunction 2 which follows an active region (not shown, but only indicated by the zigzag line). Between the heterojunction 2 and the transparency region 6, an intermediate region 4 is provided. In the present case, the intermediate region 4 as the intermediate region base material has the same base material as the transparency region 6, namely ZnO. In contrast to the transparency region 6 in the embodiment of FIG. 1, the transparency region 6 in the embodiment illustrated in FIG. 2 is not doped or has only a very low doping. In contrast, the intermediate region 4 has a relatively high aluminum doping. In other words, the doping of the TCO is concentrated to a substantial part or substantially completely on the intermediate region 4.

A method for producing the intermediate region 4 and the transparency region 6 comprises the steps of applying an optionally very thin or ultrathin aluminum layer to the heterojunction 2 and then covering this aluminum layer with the ZnO. In a subsequent diffusion step, for example with assisting heat treatment, aluminum atoms diffuse from the aluminum layer into the ZnO and form a doped intermediate region 4, while the remaining part of the ZnO layer forms the transparency region 6. In this method, it is possible to produce the intermediate region 4 and the transparency region 6 as a coherent layer comprising a higher doped intermediate region 4 and a lower doped or substantially undoped transparency region 6.

An estimate for an advantageous layer thickness of the thin aluminum layer gives the following calculation: In order to a doping density from 0.5% -2% in a 5-30 nm thick intermediate layer would theoretically suffice a layer of 0.025-0.6 nm. This corresponds to fractions of one to about two whole monolayers of Al atoms.

Due to the doping, the ZnO of the intermediate layer 4 is n-doped, so that a tunnel junction is formed between the intermediate layer 4 and the p ⁺ -doped amorphous silicon layer 23 of the heterojunction 2. Instead of the aluminum and the base material ZnO, other suitable combinations of substances can also be used.

On the transparency region 6, a metallization 8 is applied, which in the present case is formed from an aluminum layer. Again, the aluminum from the aluminum layer in a diffusion step in the ZnO layer of the transparency region 6 penetrate. As shown in FIG. 3, a metallization transition 9 can thereby be formed. Such a metallization transition 9 improves the electrical connection between the transparency region 6 and the metallization 8.

As explained above, the layer structure of intermediate region 4, transparency region 6 and metallization transition 9 can thus have emerged from a layer of a base material which is doped at its two opposite surfaces, either with the same dopant or with different dopants. Alternatively, the transparency region 6 can be formed from a transparency region base material, while the intermediate region 4 and / or the metallization transition 9 are formed from deviating materials and contain the same or different dopants. A possible alternative dopant is, for example, boron.

In the previously described embodiments, the intermediate region 4 is n-doped and the region adjacent to the intermediate region 4 of the heterojunction 2, in this case of amorphous silicon, is p-doped. alternative If the doping type can also be reversed in such a way that the intermediate region 4 is p-doped, however, this can be technologically more difficult to implement.

The embodiments described here can be used both on both sides as well as on all back contacted photovoltaic elements. These include, for example, emitter wrap-through structures (EWT structure), H IT heterojunction with intrinsic thin layers (HIT) and thin-film back-contacts in connection with amorphous or micromorphous semiconductors.

LIST OF REFERENCE NUMBERS

1 photovoltaic element

2 hetero transition

21 p-doped crystalline silicon layer

22 intrinsic amorphous silicon layer

23 p ⁺ -doped amorphous silicon layer

4 intermediate area

6 Transparency area

8 metallization

9 metallization transition

Claims

claims:

A photovoltaic element (1) having a photovoltaically active region, a transparency region (6) arranged on the photovoltaically active region and comprising a transparent electrically conductive transparency region base material for removing a photovoltaically generated current in the photovoltaically active region and a photovoltaically active region between the photovoltaically active region intermediate region (4) arranged in the transparent region (6), which is formed from a transparent electrically conductive intermediate region base material,

characterized,

the intermediate region base material in the intermediate region (4) has a higher doping density than the transparency region base material in the transparency region (6).

2. Photovoltaic element (1) according to claim 1, characterized in that the intermediate region (4) and / or the transparency region (6) has a transparency for optical and / or infrared light of at least approximately 70%, 80%, 85%, 90%. or 95%.

3. photovoltaic element (1) according to claim 1 or 2, characterized in that the intermediate region (4) is formed of a same base material, such as the transparency region (6).

4. photovoltaic element (1) according to any one of the preceding claims, characterized in that the intermediate region (4) has a 10-fold higher, preferably a 40-fold higher, preferably a 100-fold higher doping density than the transparency region (6) ,

5. photovoltaic element (1) according to one of claims 1 to 3, characterized in that the transparency region base material in the transparency region (6) is substantially undoped.

6. photovoltaic element (1) according to one of claims 1 to 3, characterized in that the base material in the intermediate region (4) is doped such that forms a tunnel junction between the intermediate region (4) and the photovoltaic active region.

7. Photovoltaic element (1) according to one of the preceding claims, characterized in that the intermediate region (4) has a doping density of about 0.5%, preferably of about 1%, preferably of about 2%.

8. photovoltaic element (1) according to any one of the preceding claims, characterized in that on a said intermediate region (4) facing away from the transparency region (6) a metallization (8) is provided.

9. photovoltaic element (1) according to claim 8, characterized in that between the transparency region (6) and the metallization (8) a highly doped metallization junction (9) is arranged.

10. photovoltaic element (1) according to claim 8 or 9, characterized in that the metallization (8) is formed of aluminum.

11. Photovoltaic element (1) according to one of the preceding claims, characterized in that the photovoltaically active region is formed from silicon.

12. Photovoltaic element (1) according to one of the preceding claims, characterized in that the intermediate region (4) and / or the transparency region (6) of indium-tin-oxide (ITO), of SnO, of ZnO and / or formed from a further transparent conductive metal oxide.

13. Photovoltaic element (1) according to one of the preceding claims, characterized in that an intermediate region (4) adjacent or at the intermediate region (4) adjacent region of the photovoltaically active region is p-doped and the intermediate region (4) and / or the transparency region (6) are n-doped.

14. Photovoltaic element (1) according to claim 13, characterized in that the intermediate region (4) and / or the transparency region (6) are doped with aluminum, boron and / or another donor material.

15. Photovoltaic element (1) according to one of the preceding claims, characterized in that the intermediate region (4) as an intermediate layer (4) and the transparency region (6) as a transparency layer (6) are formed.

16. Photovoltaic element (1) according to claim 15, characterized in that the intermediate layer (4) has a substantially smaller thickness than the transparency layer (6).

17. Photovoltaic element (1) according to one of the preceding claims, characterized in that the photovoltaic element (1) is designed as a solar cell.

18. Photovoltaic element (1) according to one of the preceding claims, characterized in that a heterojunction (2) is formed between the intermediate layer (4) and the photovoltaically active region.