DE102012102745A1 - Process for producing a solar cell and solar cell - Google Patents

Abstract

The invention relates to a method for producing a solar cell and to such itself. The solar cell comprises a silicon substrate with a front side facing the radiation and a rear side, a first dielectric layer running along the rear side, a second dielectric layer running along the substrate side facing away from the first dielectric layer of a material from the group consisting of silicon nitride, silicon oxide, silicon oxynitride and a metal layer extending along the substrate side of the second dielectric layer. In order to produce the solar cell reproducibly with particularly high efficiency with fewer process steps, it is proposed that the back side of the substrate has a gloss value at 60 ° irradiation angle in the range between 20% and 80% and that the first dielectric layer contains free negative charge carriers.

Description

The invention relates to a method for producing a solar cell and a solar cell

In recent years, a number of attempts have been made and methods developed to improve the efficiency of solar cells, in particular of silicon-based solar cells.

From the EP 1489667 A2 and the EP 1763086 A1 One acquires knowledge about the possibility of increasing the time until the recombination of the free charge carriers and thus the recombination rate and thus in turn the efficiency, in which on the side facing away from the light, a dielectric layer between the silicon substrate and the metallization is applied.

The EP 1763086 A1 describes the use of a dielectric layer system consisting of SiO ₂ and a SiN layer deposited thereon for electrical passivation of the solar cell rear side.

An embodiment of the in the EP 1489667 A2 In accordance with the disclosed teaching, for the dielectric passivation of the solar cell back side, a compound comprising Al ₂ O ₃ and SiO ₂ is used.

The DE 3815 512 A1 discloses a solar cell made of a n + p junction-containing doped semiconductor body, which is covered over its entire surface with a contact layer on the back. On the back of the semiconductor body, an oxide layer is additionally applied over the entire surface.

The literature on highly efficient solar cell structures based on crystalline silicon has, as an essential feature for achieving high efficiencies, a dielectric passivation of the surfaces, wherein only at defined locations a connection between a metal contact and silicon is produced (cf. MA Green, Silicon Solar Cells, 1995 ).

While metallization is necessarily always patterned on the front, in industrial solar cells the backs are often fabricated with a full area interconnection of silicon and metal to form a back surface field (BSF) to reduce recombination. Major obstacles in the industrial implementation of processes to increase the efficiency by dielectric passivation of the cell backside are the high cost of the additional manufacturing steps required for this, as well as the difficulty in industrial processes to achieve a sufficient electronic quality of the resulting layers and contacts.

Suppression of surface recombination in silicon solar cells occurs through two mechanisms. By chemical saturation of electronic bonds of the silicon they are passivated, d. H. they do not cause electronic recombination. For this purpose, layers that have an electronic band gap, ie semiconductors or dielectrics, are suitable.

By applying electrically charged layers, the concentration of a charge carrier type on the surface can also be greatly reduced due to their field effect, which also suppresses the recombination (field effect passivation). As known in the literature, various dielectrics in contact with silicon form a surface charge, particularly silicon oxide (weak positive), silicon nitride (positive), and aluminum oxide (negative). Depending on the chosen deposition process, this charge may be formed after an annealing step.

In order to be able to passivate the surface, it is first necessary to remove the surface damage resulting from the production process of the wafer. This is done by etching the wafer surface in an acidic or alkaline solution.

Depending on the method, the etching solution can be adjusted to be polishing or roughening. A particularly smoothly etched surface offers the advantage of a lower recombination, rough surfaces, however, prove to be advantageous for the solar cell front side, since the reflection is reduced.

For monocrystalline silicon, it is a good idea to etch the surface of <100> oriented wafers alkaline in such a way that <111> -oriented facets are formed, which leads to a structure of the surface in the form of pyramids. For multicrystalline silicon, this is also possible, but the grains are then textured unevenly due to their different crystal orientation. Therefore, an acidic so-called isotexture is generally used, which leads to a more uniform roughening.

The texturing of the front side in combination with the smooth etching of the back side represents the optimum with regard to the solar cell efficiency. However, this combination requires that at least one of the two processes is applied on one side, which represents a considerable investment outlay. In general, only one of the two processes will be applied one-sidedly; This is also because a merely one-sided removal of the by the Surface damage induced stresses leads to a strong bending of the wafer.

In order to produce a back-passivated solar cell, it is necessary that an emitter be formed only on one of the two sides, generally the front side. In the application of dopants from the gas phase, however, this happens initially on both sides. If solid or liquid doping sources are used, this is unilaterally, but it is also to be expected that the doping on the wafer backside will be somewhat bypassed.

To avoid this, the wafer backside can be provided with a protective layer before doping. In industrial processes, however, after doping, a layer is removed from the back of the wafer whose thickness is slightly greater than the penetration depth of the dopant, ie typically between 0.5 and a few μm.

For electronically passivating the back of the solar cell, a layer is found that offers a favorable combination between field effect passivation and bond saturation. As known from the literature, very good passivation can be produced with silicon oxide, which can be produced by thermal oxidation of the silicon or deposited by CVD processes.

However, SiO _x offers little resistance to sintering of the back metal layer in the annealing step required for contact formation. Silicon nitride is much more stable in connection with metals, but due to its high positive charge density it produces an n-type influential n-type inversion layer. In this layer, minority carriers of the base are collected and directed to the metal contacts, where they are lost by recombination (Lit .: Dauwe). Since it contains negative charges, AlO _{x is} a good alternative, but it can not be economically deposited in layer thicknesses that would ensure a sufficient temperature stability. Because of the difficulties described, combinations of these materials appear to be very advantageous, frequently using SiO _x or AlO _x as the first layer and SiO _x or SiN as the second layer.

The WO 2006/097303 A1 describes that sufficient surface passivation quality is achieved only when using very thick layers; Further, the formation of contacts by locally introducing holes into the layers and applying a conductive material to the back surface will be described. Only the front side of the wafer is textured.

The DE 100 46 170 A describes a method for contact formation between the back metal layer and silicon, after the application of the metal layer and optionally sintering with metal paste, wherein the designated contact area locally z. B. is heated by laser radiation, so that the metal penetrates the dielectric layer and connects to the silicon.

• 1. Löcher (”Pinholes”) in den abgeschiedenen Schichten durch während der Beschichtung anhaftende oder aufliegende Partikel,
• 2. Schwankungen in der Schichtdicke bis hin zu Löchern durch die unebene Struktur der Oberfläche selbst in Verbindung mit dem gewählten Abscheideverfahren, insbesondere Stellen mit sehr geringer Dicke bei gerichteter Abscheidung,
• 3. Punktweise oder flächige Ablösung der Schichten während der Abscheidung bzw. bei der nachfolgenden Prozessierung, insbesondere in thermischen Prozessen wie der Kontaktsinterung (”Blistering”).

Unintentional "parasitic" contacts between Si and the backside metal layer often form in industrial practice, which increase the carrier losses on the back of the cell and thus reduce the efficiency. This happens mainly through the following mechanisms:

• 1. Holes ("pinholes") in the deposited layers by particles adhering or resting during the coating,
• 2. variations in the layer thickness up to holes due to the uneven structure of the surface itself in connection with the chosen deposition method, in particular points with very small thickness in directional deposition,
• 3. Point-wise or planar detachment of the layers during the deposition or in the subsequent processing, in particular in thermal processes such as contact sintering ("blistering").

• – Polieren oder Glattätzen der Waferrückseite, indem nach einem Texturschritt der Wafer einer einseitigen Ätze unterzogen wird, oder indem nach einer beidseitigen Glattätzung die Rückseite maskiert und nur die Vorderseite texturiert wird,
• – Aufbringen sehr dicker Schichten von mehr als z. B. 100 nm oder Stapeln von mehreren Schichten

At the corresponding points, the parasitic contacts are formed during the sintering of a metal layer applied to the passivation layer on the back of the cell. The avoidance of these loss mechanisms takes place according to the classical teaching by the following measures:

• Polishing or smooth etching of the wafer back side, by subjecting the wafer to a one-sided etching after a texture step, or by masking the back side after a two-sided smooth etching and texturing only the front side,
• - Apply very thick layers of more than z. B. 100 nm or stacking of multiple layers

• – Ablation per Laser,
• – Maskieren der Oberfläche und lokales Ätzen,
• – Lokales Aufbringen eines Ätzmediums und anschließende thermische Aktivierung.

The opening of the layer for forming local contacts can be done by the following techniques:

• Ablation by laser,
• Masking the surface and local etching,
• - Local application of an etching medium and subsequent thermal activation.

The methods known from the prior art have a large number of necessary process steps. This results in high process costs and fluctuation margins (error propagation) of the efficiency of the solar cells produced.

On this basis, the present invention is based on the object, a method for producing a solar cell and a solar cell in such a way to further develop that the solar cells can be produced consistently with very high efficiency reproducible with fewer process steps.

To solve the problem, it is proposed, but not by way of limitation, in substance:
Passivation of the solar cell back using an at least double-layered dielectric layer such that losses due to parasitic contact are substantially avoided.

This is achieved by the combination of bilateral (substantially) symmetrical texture (characteristic: roughness on the back remains) with deposition of the following layer sequence: {therm. SiO ₂ (optional)}, Al ₂ O ₃ , SiN _x or SiO _x .

Examples:

• A. loss of insufficient etch in combination with SiO / SiN layers,
• B. AlO does not avoid parasitic contact (unopened cells),
• C. AlO improves rough surface efficiency compared to SiO _x . AlO _x is to be designed in such a way that no blistering occurs in combination with the subsequent processing.

The process is carried out in such a way that substantially no increased recombination occurs despite the increased surface area. Roughness-reducing etching of the entire back surface takes place exclusively after diffusion, regardless of whether the diffusion is performed on one or both sides. The removal is as high as necessary to remove the diffused layer, so typically greater than 0.5 microns, but less than z. B. 5 microns. Due to the contained negative charges, the losses are minimized by parasitic contacts. The layer structure also helps ensure that even on a rough surface as possible no parasitic contacts are formed.

Preferred process sequences are the following, where appropriate, process steps can be omitted, replaced or changed in order.

Method I:

• A. Provision of silicon wafers with p-doping, multi- or monocrystalline.
• B. etching of both surfaces for Sägeschadenentfernung and / or texture without separate etching of the back, d. h., the refining is omitted.
• C. Diffusion of an n-type dopant at least into the anterior surface.
• D. preferably removal of the phosphorus glass before or after step E.
• E. Removal of the diffusion layer on the back of the wafer, thereby removing between preferably 0.5 microns and 5 microns. The etching is carried out such that a gloss value at 60 ° incidence angle is in the range between 20% and 80%. Gloss value is the relationship between direct and diffuse diffusion.
• F. deposition of a negative charge containing dielectric layer in a layer thickness preferably between 5 nm and 100 nm
• G. deposition of preferably silicon nitride in a layer thickness of preferably 40 to 400 nm
• H. Local opening of the layers
• I. Applying an aluminum layer to the back by vapor deposition or screen printing
• J. Preparation of the metal-semiconductor contact by annealing at temperatures of preferably more than 550 ° C.

Method II:

Process steps A.-F. according to method I:

• G. deposition of silicon nitride or silicon oxide in a layer thickness of preferably 40 to 400 nm
• H. Applying an aluminum layer on the reverse side by vapor deposition or screen printing
• I. Making local metal-semiconductor contacts by locally heating the backside metal by means of laser radiation such that the metal penetrates the dielectric layers and bonds to the silicon

In particular, the invention relates to a solar cell comprising a silicon substrate having a facing front surface which is textured and has an n-doped region, a backside having a p-doped region, a first dielectric layer extending along the backside, a substrate facing away from the substrate Side of the first dielectric layer extending second dielectric layer consisting of or containing a material selected from the group consisting of silicon nitride, silicon oxide, silicon oxynitride and a side facing away from the substrate side of the second dielectric layer extending metal layer.

A relevant solar cell is characterized in that the back side of the substrate has a gloss value at 60 ° irradiation angle in the range between 20% and 80% and that the first dielectric layer contains free negative charge carriers.

In a further development, it is provided that the first dielectric layer consists of a material or contains one of the group of aluminum oxide, doped silicon oxide.

Preferably, the invention provides that the first dielectric layer has a layer thickness D1 with 5 nm <D1 <100 nm.

Furthermore, the second dielectric layer should preferably have a layer thickness D2 with 40 nm <D2 <400 nm.

In a further development, the invention proposes that a silicon oxide-containing or silicon oxide-containing layer having a thickness D3 and preferably 1 nm ≦ D3 ≦ 10 nm extends between the first dielectric layer and the substrate.

• • Ätzen beider Oberflächen des Substrats
• • Diffusion eines n-dotierten Materials zumindest in strahlenzugewandter Vorderseite des Siliciumsubstrats
• • Ätzen der Rückseite des Substrats unter Vermeidung eines Glanzätzens
• • Substratrückseitiges Abscheiden einer ersten dielektrischen Schicht aus einem Material, das nach dem Abscheiden freie negative Ladungsträger enthält
• • Abscheiden einer zweiten dielektrischen Schicht aus einem Material oder ein Material enthaltend aus der Gruppe Siliciumnitrid, Siliciumoxid, Siliciumoxinitrid entlang der ersten dielektrischen Schicht
• • Abscheiden einer Metallschicht entlang der zweiten dielektrischen Schicht.

The invention is in particular characterized by a method for producing a solar cell, comprising the method steps

• • etching both surfaces of the substrate
• Diffusion of an n-doped material, at least in the radiation-facing front of the silicon substrate
• • Etch the back of the substrate while avoiding a bright etch
• Substrate-back-depositing a first dielectric layer of a material containing free negative charge carriers after deposition
• Depositing a second dielectric layer of a material or a material comprising the group of silicon nitride, silicon oxide, silicon oxynitride along the first dielectric layer
• Depositing a metal layer along the second dielectric layer.

In this case, provision is made in particular for the metal layer to be contacted in regions with the substrate.

Advantageous embodiments:

• - The negatively charged layer is AlO _x
• Deposition of AlO _x with ALD (Atomic Laser Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition)
• Thermal oxidation of the wafer surface before deposition of the layers d. H. between step E and F.
• - Execution of contacts as lines
• - Substrate temperature at SiN deposition preferably> 320 ° C.
• - Boron doping of the contact areas before the layer deposition (step G)
• - Version with selective emitter

Further details, advantages and features of the invention will become apparent not only from the claims, the features to be taken from them - alone and / or in combination - but also from the following description of a preferred embodiment.

The single FIGURE shows an embodiment of a solar cell, wherein a solar cell backside RS by means of an at least double-layered dielectric layer 23 . 24 is passivated in such a way that losses due to parasitic contact are substantially avoided. This is achieved by the combination of bilateral (substantially) symmetrical texture T, wherein the roughness is retained on the back RS and with deposition of the following layer sequence: {therm. SiO ₂ (optional)}, Al ₂ O ₃ , SiN _x or SiO _x , viewed from the back RS.

In other words, a first dielectric layer is initially formed on the rear side RS after optionally depositing a silicon oxide layer having a thickness of preferably between 1 nm and 10 nm 23 applied, which contains free negative charges after deposition. On this preferably consisting of alumina layer 23 or another layer containing free negative charge carriers after application, such as e.g. For example, a fluorine doped silicon oxide layer becomes a second dielectric layer 25 applied, which may consist of silicon nitride, silicon oxide or silicon oxynitride. On the second dielectric layer 24 then becomes a backside metal layer 25 , in particular an aluminum layer and with the silicon substrate 21 contacted.

Furthermore, it can be seen from the schematic representation that the rear side RS is not smoothly etched, but rather can be termed textured, since the rear side RS is also etched during the etching of the upper side OS.

In other words, the prior art smooth etching is not performed. This results in an enlarged rear side surface in comparison to known solar cells.

The AlO _x layer 23 is designed so that no "blistering" occurs in combination with the subsequent processing.

The process control is carried out in such a way that substantially no increased recombination occurs despite the increased surface area of the rear side RS. Roughness-reducing etching of the entire back surface takes place exclusively after diffusion, regardless of whether the diffusion is performed on one or both sides. The removal is as high as necessary to remove the diffused layer, so typically greater than 0.5 microns, but less than z. B. 5 microns. Due to the contained negative charges in the first dielectric layer 23 the losses are due to parasitic contacts minimized. The layer structure also helps ensure that even on a rough surface as possible no parasitic contacts are formed.

In more detail, the production of a solar cell will be described below:
The sole FIGURE shows a solar cell produced according to the method previously described as method II., Having a non-brightly etched reverse side RS. The solar cell comprises a silicon wafer 21 with p-doping, in which a multi- or monocrystalline formation can be present. First, the surfaces, ie the front side OS and back RS are etched in order to remove sawing damage or to form a texture T. An otherwise performed according to the prior art separate Glanzätzen the back RS omitted. Then at least in the surface of the front 18 an n-doped substance such as phosphorus diffused. Then formed phosphorus glass is removed. Finally, the back is etched to remove diffused dopant. The removal amounts to between preferably 0.5 .mu.m and 5 .mu.m. According to the embodiment is then on the back RS dielectric layer 23 a layer thickness is preferably deposited between 5 nm and 100 nm, which contains negative charges after deposition. Optionally, a silicon oxide layer may be previously applied directly to the back RS, with a thickness between 1 nm and 10 nm being preferred. On the first dielectric layer 23 then becomes a second dielectric layer 24 deposited from a material such as silicon nitride, silicon oxide or silicon oxynitride, wherein the thickness is preferably between 40 nm and 400 nm. Finally, on the free outside of the second dielectric layer 24 a metal layer, in particular an aluminum layer 25 applied. This can be done by vapor deposition or screen printing. Then, a contact between the metal layer 25 and the substrate 21 produced. In this case, by means of laser radiation local metal-semiconductor contacts 26 by locally heating the backside metal such that the metal forms the dielectric layers 23 . 24 penetrates and connects to the silicon.

The inventive method allows the production of an improved Si solar cell with as few manufacturing steps, in particular in addition to the standard process (1 diffusion step, screen printing contacts).

In this case, known technologies, such as modified emitter profiles, selective emitters and passivated backside are combined according to the invention. Surface roughness on the back surface leads to parasitic bonding and, in combination with conventional coating techniques and bonding techniques, electrical losses. These losses are avoided according to the invention, without having to etch the back especially smooth.

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 1489667 A2 [0003, 0005]
• EP 1763086 A1 [0003, 0004]
• DE 3815512 A1 [0006]
• WO 2006/097303 A1 [0019]
• DE 10046170 A [0020]

Cited non-patent literature

• MA Green, Silicon Solar Cells, 1995 [0007]

Claims (8)

A solar cell comprising a silicon substrate having a facing surface facing the front, which is textured and has an n-doped region, a backside having a p-doped region, a first dielectric layer running along the backside, a second one extending along a substrate side of the first dielectric layer dielectric layer consisting of or comprising a material from the group consisting of silicon nitride, silicon oxide, silicon oxynitride and a metal layer extending along the substrate side of the second dielectric layer, characterized in that the back side of the substrate has a gloss value at 60 ° irradiation angle in the range between 20% and 80% and that the first dielectric layer contains free negative charge carriers. Solar cell according to claim 1, characterized in that the first dielectric layer consists of a material or contains one of the group of alumina, doped silica. Solar cell according to claim 1 or 2, characterized in that the first dielectric layer has a layer thickness D1 with 5 nm <D1 <100 nm. Solar cell according to at least one of the preceding claims, characterized in that the second dielectric layer has a layer thickness D2 with 40 nm <D2 <400 nm. Solar cell according to at least one of the preceding claims, characterized in that extends between the first dielectric layer and the substrate consisting of a silicon oxide or silicon oxide-containing layer having a thickness D3 with preferably 1 nm ≤ D3 ≤ 10 nm. Process for producing a solar cell, in particular according to claim 1, comprising the process steps: • etching both surfaces of the substrate Diffusion of an n-doped material, at least in the radiation-facing front of the silicon substrate • Etch the back of the substrate while avoiding a bright etch Substrate-back-depositing a first dielectric layer of a material containing free negative charge carriers after deposition Depositing a second dielectric layer of a material or a material comprising the group of silicon nitride, silicon oxide, silicon oxynitride along the first dielectric layer Depositing a metal layer along the second dielectric layer. A method according to claim 6, characterized in that between the substrate and the first dielectric layer, a silicon oxide layer or a silicon oxide-containing layer is deposited. A method according to claim 6 or 7, characterized in that the metal layer is partially contacted with the substrate.

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