DE102011115028A1 - Photovoltaic multiple solar cell - Google Patents

Abstract

The invention relates to a photovoltaic multiple solar cell, comprising at least one upper and one lower subcell, wherein, starting from a front side of the multiple solar cell designed for incidence of light, first the upper subcell is arranged, which is formed as a pin structure, with a p-type solar cell. doped p-type layer, an n-doped n-type layer and an intrinsic i-layer arranged between p- and n-layer, wherein p-, i- and n-layer are each formed as amorphous silicon layers, optionally with further substances, and wherein further below the upper part of the cell, optionally with the interposition of one or more intermediate layers, the lower part cell is arranged, which has a p-type emitter region in an area facing the upper part cell and an n-doped base region in a region facing away from the upper part cell. The invention is characterized in that at least the base region of the lower part cell is formed in a crystalline silicon layer.

Description

The invention relates to a photovoltaic multiple solar cell according to the preamble of claim 1. Photovoltaic multiple solar cells are characterized by the fact that several pn junctions are arranged one above the other, so that compared to solar cells with only one pn junction typically the spectrum of incident electromagnetic radiation better utilized and thus a higher efficiency can be achieved. Photovoltaic multiple solar cells are constructed of semiconductor materials and are referred to as tandem solar cells with two pn junctions and triple pn junctions as triple solar cells.

Particularly in thin-film technology, the formation of semiconductor layer systems in which a plurality of pn junctions are stacked is known. In this case, the individual layers of the thin-film solar cell are typically deposited by means of chemical vapor deposition (CVD). In particular, the plasma-enhanced PECVD (plasma enhanced chemical vapor deposition) method is used.

The layers can be deposited in different degrees of crystallization, for example as amorphous or microcrystalline layers. However, this typically has the disadvantage that defects with energy levels in the bandgap of the pn junction greatly affect the diffusion length of minority carriers, especially when compared to solar cell-based solar cells. For this reason, in the case of thin-film solar cells, the pn junction is typically formed as a pin junction: a p-doped layer (p-layer) is followed by an undoped (intrinsic) layer (i-layer), which in turn has an n-doped layer (n-layer) follows. This achieves a functional separation between doping layers and the i-layer formed primarily for absorbing the incident electromagnetic radiation, so that an optimization of the individual layers for the respective purpose is possible. In the case of a pin structure, the pn junction thus extends over the intrinsic layer, and correspondingly a charge carrier separation of the charge carrier pairs generated in the intrinsic layer by absorption of electromagnetic radiation takes place through the electric field established by the p and n layers.

The term "microcrystalline" refers to material layers whose crystal sizes are substantially in the range of 1 nm to 100 nm in an amorphous matrix. By contrast, in the case of a purely "amorphous" layer, there are no crystallites.

To increase the efficiency of a thin-film solar cell, the stacking of several pin structures is known. For example, describes EP 2 017 895 A2 a silicon multiple solar cell, in which two sub-cells, which are each formed as a pin structure, are arranged one above the other.

A thin-film multiple solar cell in training as a triple solar cell is, for example, in J. Yang, A. Banerjee, S.Guha, "Triple junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies" Appl. Phys. Lett. 70 (22), 2 June 1997 described.

Basically, in the case of multi-layer photovoltaic photovoltaic cells, there is the challenge of stacking semiconductor layers with different lattice constants one over the other and, if possible, avoiding defects which lead to efficiency-reducing effects, in particular recombination losses.

For this reason, a large number of different layer structures are known from the prior art.

Proceeding from this, the present invention has the object to provide a photovoltaic multiple solar cell, which has an alternative, preferably inexpensive producible structure in particular by using inexpensive materials.

This object is achieved by a photovoltaic multiple solar cell according to claim 1. Advantageous embodiments of the inventive multiple solar cell can be found in claims 2 to 15.

The multiple solar cell according to the invention is a photovoltaic solar cell, d. H. it is used to convert incident electromagnetic radiation into electrical energy and constitutes a semiconductor structure.

The multiple solar cell according to the invention comprises at least one upper and one lower subcell, wherein, starting from a front side of the multiple solar cell designed for incidence of light, the upper subcell is initially arranged, which is designed as a pin structure. The pin structure comprises a p-doped p-layer, an n-doped n-layer and an intrinsic i-layer arranged between p and n-layer, which i-layer is formed comprising amorphous silicon.

The upper part cell is thus formed in the basic structure according to a pin subcell known per se, which in particular has the interposed i-layer as an absorber for the electromagnetic radiation.

Furthermore, in the case of the multiple solar cell according to the invention below the upper part cell, optionally with the interposition of one or more intermediate layers, the lower part cell is arranged. The lower part cell has a p-doped emitter region in an area facing the upper part cell and an n-doped base region in a region facing away from the upper part cell.

In the basic structure, the multiple solar cell according to the invention is thus formed at least as a tandem solar cell with two pn junctions, starting from the front side formed for the incidence of light, the doping sequence pn-pn takes place, each optionally with the interposition of one or more intermediate layers. Here and below, the term pn junction also includes a pin junction, unless explicitly stated otherwise.

It is essential that at least the base region of the lower part cell is formed in a crystalline silicon layer.

The multiple solar cell according to the invention thus differs from previously known multiple solar cells in that a combination of a lower part cell, in which at least the base region is formed in a crystalline silicon layer, with an upper, designed as a pin structure subcell.

The invention is based on the Applicant's finding that the arrangement of a pin structure, which is essentially formed of amorphous silicon layers, can advantageously take place with respect to the band gap adaptation on a lower solar cell whose base region is formed as a crystalline silicon layer. Thus, the formation of such a multiple solar cell is possible for the first time with improved lattice matching, and the solar cell structure according to the invention in particular opens up the possibility of using crystalline silicon materials with poorer material quality and thus shorter service life of the miniature charge carrier and yet due to the design as a multiple solar cell an increased efficiency achieve.

Typically, crystalline silicon layers are used to form solar cells with only one pn junction. In particular, the use of monocrystalline silicon, Czochralsky silicon or multicrystalline silicon is known. By contrast, in the case of less expensive materials, such as UMG silicon (upgrated metallurgical-grade silicon), by contrast, there are typically low lifetimes of the minority carriers, so that a significantly lower efficiency of the solar cell is achieved.

The multiple solar cell according to the invention thus makes it possible, in particular, to use such cost-effective crystalline silicon materials, since an increase in efficiency is achieved by the design as a multiple solar cell with an arrangement of an upper part cell, which is designed as a pin structure.

In contrast to previously known silicon wafer solar cells, the multiple solar cell according to the invention thus for the first time has the upper part cell formed of amorphous silicon layers. In contrast to previously known multiple solar cells, in the case of the solar cell according to the invention, for the first time, the typical structure is dispensed with, for example, by a light-transmissive substrate.

In the case of the solar cell according to the invention, on the contrary, the lower part cell with its crystalline silicon layer serves as the substrate on which the upper part cell is formed.

It is within the scope of the invention that the emitter region of the lower subcell is also formed in the crystalline silicon layer by introducing dopants. It is also possible that the emitter region of the lower part cell is formed as a separate layer, which on the upper Partial cell facing side of the base region is arranged, in particular, the emitter region may also be formed as an amorphous silicon layer.

The abovementioned and following formulations of the formation of a layer as an amorphous silicon layer or as a crystalline silicon layer include, where appropriate, the layers having further substances, such as, for example, dopants or alloys.

As described above, in the multiple solar cell of the present invention, the basic structure of a wafer solar cell is combined with the structure of thin film multiple solar cells, with the crystalline silicon layer serving as a supporting substrate of the lower divided cell.

Preferably, therefore, the crystalline silicon layer of the base region of the lower part of the cell is formed with a thickness in the range 50 microns to 300 microns, preferably in the range 75 microns to 250 microns.

To form the emitter region of the lower subcell in the crystalline silicon layer, it is advantageous for the lower subcell to be formed from a crystalline silicon layer which has the p-doped emitter region on the side facing the upper subcell, preferably an emitter region diffused with p-dopant, and essentially underlying the n-doped base region.

For the alternative embodiment of the solar cell according to the invention, in which the emitter region of the lower subcell is formed as a layer, it is advantageous that the base region of the lower subcell is formed in a crystalline silicon layer and that on the upper part cell facing side of the crystalline silicon layer, an amorphous silicon layer is arranged, in which amorphous silicon layer, the emitter region is formed, so that between the base and emitter layer, a hetero-pn junction is formed.

In this case, it is advantageous for the amorphous silicon layer of the lower subcell in which the emitter region is formed to have a thickness in the range from 5 nm to 20 nm. The amorphous silicon layer may also be deposited in a further preferred embodiment as a layer stack consisting of an intrinsic layer (thickness preferably in the range 2nm-10nm) a-Si and a layer a-Si (p) (thickness preferably in the range 5nm-20nm) become.

It is within the scope of the invention that further known layers and structures are formed or arranged to increase the efficiency in the solar cell according to the invention. This applies in particular to layers and structures for increasing the light coupling, such as antireflection layers or layer systems with antireflection effect and / or optical structures, such as pyramidal structures.

It is likewise advantageous that an n-doped BSF layer is arranged on the side of the base of the lower part cell facing away from the upper part cell, which BSF layer is formed as an amorphous silicon layer. The BSF layer preferably has a thickness in the range 5 nm to 50 nm. The doping of the BSF layer is preferably in the range of 10 ¹⁹ atoms / cm ³ -10 ²¹ atoms / cm ³ . Likewise, this BSF layer can also be realized by thermal diffusion of phosphorus atoms; Diffusion can significantly improve wafer quality through gettering.

It is known that by arranging a so-called back surface field, the effective surface recombination velocity at the backside for minority charge carriers can be reduced and thus the efficiency can be increased. For this purpose, the BSF has a higher doping with respect to the base for the same doping type.

Preferably, the p-layer of the upper part cell is listed with a thickness in the range 5 nm to 20 nm.

Furthermore, it is advantageous that the p-layer of the upper part cell is formed as a layer system of at least two layers, comprising a first layer comprising a first layer of microcrystalline silicon and a second layer of amorphous silicon. This leads to a further increase in the efficiency of the solar cell.

An increase in efficiency can be achieved in that, in an advantageous embodiment of the solar cell according to the invention, the i-layer of the upper part cell is formed as a germanium-alloyed amorphous silicon layer, preferably with an alloying degree in the range 10% to 40%.

Furthermore, it is advantageous that the i-layer of the upper part cell has a thickness in the range 50 nm to 400 nm.

The n-layer of the upper part cell preferably has a thickness in the range 5 nm to 30 nm.

A further increase in efficiency is achieved in a preferred embodiment of the solar cell according to the invention in that at least one of the amorphous silicon layers, preferably more of the amorphous silicon layers, more preferably, all amorphous silicon layers are formed as hydrogenated layers.

The term "hydrogenated" refers to material layers in which a high proportion of hydrogen (5-20 atomic percent) is present in the material due to the process. This is typically illustrated by ": H" in the material designation.

A further increase in efficiency is achieved by forming a third pn junction in the solar cell according to the invention. In such a so-called tripple solar cell, a further increase in the overall efficiency compared to a multiple solar cell with two sub-cells, since an additional light output is ensured.

Therefore, a third subcell is preferably arranged on the side of the lower subcell remote from the upper subcell, which third subcell is designed as a pin structure, with a p-doped p-layer, an n-doped n-layer and one between p and n-layer arranged intrinsic i-layer, so that a pin junction is formed.

p-, i- and n-layer of the third subcell are preferably formed as amorphous layers, in particular amorphous silicon, silicon-germanium and pure amorphous germanium layers.

A further increase in efficiency is achieved in a partial modification thereof by forming the i-layer of the third subcell of microcrystalline germanium.

Furthermore, an increase in efficiency is possible in that a back reflector layer, preferably with a thickness in the range from 30 nm to 200 nm, is arranged on the side of the lower part cell facing away from the upper part cell, optionally with the interposition of further intermediate layers.

Such a back reflector layer is known per se and fulfills the function that electromagnetic radiation, typically in the longer wavelength light range, which was not absorbed in the solar cell structure, reflected by the back reflector and thus passes through the solar cell structure again to increase the light output.

Further advantageous features and preferably embodiments are described below with reference to the figures and the embodiments. Showing:

1 a first embodiment of a solar cell according to the invention, wherein the emitter of the lower part cell is formed as an amorphous silicon layer;

2 a second embodiment of the solar cell according to the invention, wherein the emitter and the base of the lower part cell are each formed as doping regions in a crystalline silicon wafer;

3 the simulated external quantum efficiency of the solar cell according to 1 and

4 the simulated external quantum efficiency of a third embodiment of a solar cell according to the invention, which substantially corresponds to the first embodiment, wherein the intrinsic absorber layer of the upper part cell is formed as an amorphous silicon layer.

The representations of the solar cell structures in the figures are in each case schematic representations, which represent a partial section of the respective exemplary embodiment. Advantageously, the solar cell according to the invention is formed in a manner known per se. In particular, it is within the scope of the invention that the solar cell according to the invention known per se layers and / or structures to increase the light input to the front, the light reflection at the back and / or to reduce the Surface recombination speed of the minority carrier has additional doping regions and / or intermediate layers.

Like reference numerals in the figures indicate the same or equivalent elements.

1 shows a photovoltaic multiple solar cell comprising an upper part cell 1 and a lower part cell 2 , Starting from the - in all figures - overhead lying formed for the incidence of light front first, the upper part cell 1 is arranged. The upper part cell 1 is formed as a pin structure, with a p-doped p-layer 3 , an n-doped n-type layer 5 and an intrinsic i-layer sandwiched between p and n layers 4 ,

p-, n- and i-layer are each formed as amorphous silicon layers, wherein the i-layer additionally comprises a germanium alloy.

Furthermore, below the upper part cell 1 the lower part cell 2 arranged, which in a region facing the upper part cell has a p-doped emitter region, which in the embodiment according to 1 as emitter layer 6 in the form of an amorphous silicon layer, which is p-doped, is formed.

In one of the upper part cell 1 remote area of the lower part cell 2 the lower subcell has an n-doped base region. This is in accordance with the embodiment 1 as n-doped crystalline silicon wafer 7 educated.

On the lower back side of the silicon wafer 7 is additionally a back contact layer 8th arranged for the removal of majority charge carriers from the base of the lower part cell and thus for electrical contact. This rear-side contact layer may be formed as an n-doped amorphous silicon layer and / or as a metallic layer, in particular as a silver layer or as an aluminum layer.

On the front is above the p-layer 3 the upper part cell 1 a translucent, electrically conductive TCO layer 9 arranged. The use of such layers is known per se, on the one hand for increasing the incidence of light and on the other hand for discharging majority charge carriers, ie for electrical contacting of the solar cell.

This in 2 schematically illustrated second embodiment of a solar cell according to the invention corresponds in basic construction to the in 1 represented, in particular with regard to the denoted by identical reference numerals layers.

In contrast to the structure according to 1 has the second embodiment according to 2 a lower part cell 2 on, whose emitter and base region are both formed by means of corresponding dopants in a silicon wafer, so that the silicon wafer thus at the upper part of the cell 1 side facing a p-doped emitter region 6 ' and underlying an n-doped base region 7 ' having.

The back contact layer is in the second embodiment as formed by screen printing aluminum layer 8th' executed.

As mentioned above, the multiple solar cell according to the invention in particular has the potential, starting from silicon wafers with comparatively low lifetimes, such as UMG silicon to achieve an increase in efficiency due to the arrangement of the upper part cell.

For this purpose, simulations were carried out, which were included in the 3 and 4 are shown. A decisive factor for the efficiency of a solar cell is the conversion of incident electromagnetic radiation in charge carrier pairs which can be dissipated at the contacts. It is customary to specify the external quantum efficiency for this wavelength-resolved.

In the 3 and 4 are simulations of the external quantum efficiency for solar cells according to the invention according to the first embodiment ( 1 ) carried out. The high light output is clearly shown by the combination of an upper and lower part cell.

Compared to a silicon thin film tandem solar cell consisting of an amorphous top cell and a microcrystalline bottom cell, this new structure has the advantage that the bottom cell clearly shows has increased current, since the layer thickness is large enough for the complete absorption of sunlight and the material quality is better compared to microcrystalline silicon. Here, the efficiency can be significantly increased compared to conventional tandem technology.

Compared to the conventional wafer solar cell, the proposed structure is advantageous when using low lifetime wafers such as multicrystal-crystalline wafers and UMG silicon wafers. With these materials, low short-circuit currents are to be expected when used in conventional solar cells. The use of this type of wafers in the proposed structure has the advantage that, although the short-circuit current is lowered, but the higher voltage through the series connection of the two sub-cells, the open clamping voltage is increased, which can result in increased efficiency as a result.

In the in 4 shown simulation was different from the representation of the first embodiment according to 1 as an i-layer 4 assumed only an amorphous silicon layer of the upper part of the cell, so that the production costs are further reduced. It turns out that even in this embodiment, an overall yield is still achieved, which justifies an implementation of this solar cell concept.

In the following table, the essential data of a trained as a triple solar cell further embodiment are given. Furthermore, further preferred materials or value ranges are given in brackets. It is within the scope of the invention to design the multiple solar cell according to the invention from combinations of the stated materials / value ranges. Furthermore, it is within the scope of the invention to form a solar cell as a tandem solar cell with the data of the table with omission of the lower part cell, ie the data of the "middle part cell" of the table find in this case for the lower part cell of the tandem solar cell application. subcell layer material thickness endowment bandgap upper part cell p a-Si: H, (a-SiO: H) 10 nm (5-20 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) - i a-SiGe: H (a-Si: H) 300 nm (5-400 nm) 0 1.8 eV (1.7-1.9 eV) n a-Si: H 10 nm (5-20 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) - middle part cell p q-Si: H, (μc-SiC: H or a-SiC: H or doped emitter or a-Si: H (i) / a-Si: H (p)) 10 nm (5-20 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) - n c-Si wafers 250 μm (50-300 μm) 10 ¹⁶ cm ^-3 10 ¹⁵ -10 ¹⁷ cm ^-3 - lower part cell P μ-Si: H, a-Ge: H, μc-Ge: H 10 nm (5-20 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) - l a-Ge: H, a-SiGe: H, μc-SiGe: H, μc-Ge: H 100 nm (50-500 nm) 0 0.7-1.0eV n μ-Si: H, a-Ge: H, μc-Ge: H 10 nm (5-20 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) -

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2017895 A2 [0005]

Cited non-patent literature

• J. Yang, A. Banerjee, S.Guha, "Triple junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies" Appl. Phys. Lett. 70 (22), 2 June 1997 [0006]

Claims (15)

Photovoltaic multiple solar cell, comprising at least an upper and a lower subcell ( 2 ), starting from a trained for the incidence of light front side of the multi-solar cell, first the upper part cell ( 1 ), which is formed as a pin structure, with a p-doped p-layer ( 3 ), an n-doped n-type layer ( 5 ) and one between p- and n-layer ( 5 ) arranged intrinsic i-layer, wherein p-, i- and n-layer ( 5 ) are each formed as amorphous silicon layers, optionally with other materials, and wherein further below the upper part cell, optionally with the interposition of one or more intermediate layers, the lower part cell ( 2 ), which in a region facing the upper part cell has a p-doped emitter region and an n-doped base region in a region facing away from the upper part cell, characterized in that at least the base region of the lower part cell is formed in a crystalline silicon layer. Solar cell according to claim 1, characterized in that the crystalline silicon layer of the base region of the lower part cell has a thickness in the range 50 microns to 300 microns, preferably in the range 75 microns to 250 microns. Solar cell according to one of the preceding claims, characterized in that the lower part cell ( 2 ) is formed of a crystalline silicon layer having on the upper part of the cell side facing the p-doped emitter region, preferably an emitter region diffused with p-type dopant and substantially underlying the n-doped base region. Solar cell according to one of the preceding claims 1 to 2, characterized in that the base region of the lower part cell is formed in a crystalline silicon layer and that on the upper part cell facing side of the crystalline silicon layer, an amorphous silicon layer is disposed, formed in which amorphous silicon layer, the emitter region is so that between base and emitter layer ( 6 ) a hetero-pn junction is formed. Solar cell according to claim 4; characterized in that the amorphous silicon layer of the lower subcell has a thickness in the range 5 nm to 20 nm. Solar cell according to one of the preceding claims, characterized in that on the side facing away from the upper part cell base of the lower part cell, an n-doped BSF layer is arranged, which BSF layer is formed as an amorphous silicon layer and / or by diffusion of a dopant, and preferably has a thickness in the range 5 nm to 50 nm. Solar cell according to one of the preceding claims, characterized in that the p-layer ( 3 ) of the upper part cell has a thickness in the range 5 nm to 20 nm. Solar cell according to one of the preceding claims, characterized in that the p-layer ( 3 ) of the upper part cell is formed as a layer system of at least two layers, with a first microcrystalline silicon layer and a second amorphous silicon layer. Solar cell according to one of the preceding claims, characterized in that the i-layer ( 4 ) of the upper part cell is formed as a germanium-alloyed amorphous silicon layer, preferably with an alloying degree in the range of 10% to 40%. Solar cell according to one of the preceding claims, characterized in that the i-layer ( 4 ) of the upper part cell has a thickness in the range 50 nm to 400 nm. Solar cell according to one of the preceding claims, characterized in that the n-layer ( 5 ) of the upper part cell has a thickness in the range 5 nm to 30 nm. Solar cell according to one of the preceding claims, characterized in that at least one of the amorphous silicon layers, preferably more of the amorphous silicon layers, more preferably all amorphous silicon layers are formed as hydrogenated layers. Solar cell according to one of the preceding claims, characterized in that on the side facing away from the upper part cell of the lower part cell, a third subcell is arranged, which third subcell is formed as a pin structure, with a p-doped p-layer ( 3 ), an n-doped n-type layer ( 5 ) and one between p- and n-layer ( 5 ) arranged intrinsic i-layer ( 4 ), so that a pin junction is formed. Solar cell according to claim 13, characterized in that the i-layer ( 4 ) of the third subcell comprising microcrystalline germanium, or amorphous germanium or amorphous germanium silicon. Solar cell according to one of the preceding claims, characterized in that on the side facing away from the upper part cell side of the lower part cell, optionally with interposition of further intermediate layers, a back reflector layer is arranged, preferably with a thickness in the range 30 nm to 200 nm.

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