DE102011010306A1 - Process for producing a crystalline silicon solar cell while avoiding unwanted metal deposits - Google Patents

Abstract

Method for producing a crystalline silicon solar cell, in which, for the purpose of forming an emitter (52) on a first side of a silicon substrate (50), dopant is diffused into the silicon substrate (50), followed by a silicon nitride layer (56) on the first side of the silicon substrate (50) is deposited (16), subsequently openings (58) are made locally in the silicon nitride layer (56) on the first side of the silicon substrate (50), subsequently in the openings (58) by means of plating (20) metal contacts (60) ) are formed (20), the emitter (52) on the first side of the silicon substrate (50) being partially etched back (12) before the deposition (16) of the silicon nitride layer (56) and subsequently before the deposition (16) of the silicon nitride layer ( 56) a silicon oxide layer (54) with a thickness in the range from 1.0 nm to 10 nm is formed (14) on the first side of the silicon substrate (50).

Description

The invention relates to a method for producing a crystalline silicon solar cell according to the preamble of claim 1 and to a solar cell produced by this method.

From the international patent application WO 2008/039067 A2 It is known to wet-chemically oxidize the surface of a silicon substrate prior to the deposition of a silicon nitride layer as a dielectric layer. This serves to improve the surface passivation in solar cells with screen printed contacts.

Another possibility for forming metal contacts in solar cells is to locally form openings in dielectric layers, which are arranged on a surface of a silicon substrate used. In these openings, metal is subsequently deposited by means of plating. It has been found that in plating, be it electroplating or electroless plating, metal is also partly deposited on unopened areas of the dielectric layer, for example on unopened areas of a silicon nitride layer. Such unwanted metal deposits are often referred to as "ghost deposits" or "ghostplating. They are undesirable because they lead to additional Abschattungsverlusten and affect the efficiency of the solar cell.

Against this background, the present invention seeks to provide a method of the generic type with which the formation of ghost deposits can be avoided without reducing the efficiency of the finished solar cell.

This object is achieved by a method having the features of claim 1.

Furthermore, the invention is based on the object of providing a solar cell with contacts formed by plating without ghost deposits.

This object is achieved by a solar cell having the features of claim 12.

Advantageous developments are each the subject of dependent claims.

The method according to the invention provides, for the purpose of forming an emitter on a first side of a silicon substrate, to diffuse dopant into the silicon substrate, subsequently depositing a silicon nitride layer at least on the first side of the silicon substrate, subsequently introducing openings locally into the silicon nitride layer on the first side of the silicon substrate and subsequently forming metal contacts in the openings by means of plating. Prior to depositing the silicon nitride layer, the emitter on the first side of the silicon substrate is partially etched back. Thereafter, however, prior to deposition of the silicon nitride layer, a silicon oxide layer having a thickness in the range of 1.0 nm to 10 nm is formed on the first side of the silicon substrate.

It has been shown that in this way ghost deposits can be avoided and thereby solar cells with reduced shading losses can be produced.

The described design of the emitter is not necessarily limited to the first side of the silicon substrate, but may also be on other sides of the silicon substrate. The deposition of the silicon nitride layer, the formation of the silicon oxide layer as well as the etching back of the emitter are also not necessarily limited to the first side of the silicon substrate.

Plating in the present sense is to be understood as meaning electroplating, which is sometimes referred to as galvanic metal deposition or electroplating, or electroless plating, which is sometimes referred to as chemical metal plating or electroless plating. A metal contact in the present sense is a contact formed from one or more metals or one or more metal alloys.

The openings are preferably introduced into the silicon nitride layer by means of laser ablation. Advantageously, a hydrogen-containing silicon nitride layer is deposited on the first side of the silicon substrate. In this way, in the course of solar cell production, hydrogen can be diffused from the silicon nitride layer to passivate defects into the silicon substrate. This is particularly advantageous when using multicrystalline silicon substrates. Due to the small thickness of the silicon oxide layer formed before the deposition of the silicon nitride layer, the diffusion of hydrogen from the silicon nitride layer into the silicon substrate through the silicon oxide layer is not impeded to any significant extent.

Preferably, the metal contacts are formed in the openings by electroplating. In this embodiment variant, the invention has a particularly advantageous effect, since in electroplating ghost deposits occur to a greater extent than in the case of electroless plating.

It has proved to be advantageous to etch back the emitter wet-chemically. Preferably he etched back in an etching solution containing hydrofluoric acid and nitric acid or hydrofluoric acid and ozone.

In an advantageous embodiment variant of the method, the etch resistance of the emitter increases its layer resistance value by 10 Ω / sq to 50 Ω / sq. When using a multicrystalline silicon substrate, in practice, after the emitter has been etched back, a maximum limit has been established for its sheet resistance value of 90 Ω / sq. If monocrystalline silicon substrates are used, this upper limit may be higher.

Preferably, the silicon oxide layer is formed by wet-chemical oxidation. This can be done, for example, in a solution comprising ozone, hydrogen peroxide or nitric acid. Particularly preferably, the silicon oxide layer is formed in an oxidation solution of deionized water and ozone dissolved therein.

In an alternative embodiment variant, the silicon oxide layer can be formed by means of a gas-phase oxidation. This is preferably done in an ozone-containing gas atmosphere. Advantageously, this is energetically excited, for example by irradiation of electromagnetic radiation, preferably of ultraviolet light.

In practice, it has been found useful to form the silicon oxide layer in a thickness in the range of 2 nm to 3 nm.

Advantageously, the silicon oxide layer is locally removed in the openings introduced into the silicon nitride layer. This can again be done by means of laser ablation, and particularly preferably in the same laser ablation step, in which the openings are also introduced into the silicon nitride layer.

According to a development of the method, the openings are introduced into the silicon nitride layer by means of laser ablation and, by means of laser diffusion, emitter regions which are more heavily doped are formed in regions of the openings. Laser diffusion means that the silicon substrate is locally heated by means of laser radiation, which is already irradiated for the laser ablation, in such a way that a rearrangement of the dopant occurs in the regions of the openings, ie an emitter profile is locally changed. Under such a rearrangement of dopant also falls the activation of electrically inactive dopant. By means of such a laser diffusion locally stronger doped emitter regions and thus a so-called selective emitter can be formed in regions of the openings.

In the practical implementation of laser ablation, both in conjunction with homogeneous and selective emitters, laser-induced chemical processes, sometimes referred to as laser chemical processing, have been proven; For example, a laser-induced chemical etching, which is sometimes referred to as laser chemical etching. Alternatively, the laser ablation can also be realized by means of laser beam evaporation, which, like the laser-induced chemical methods, enables laser diffusion for the purpose of forming a selective emitter.

If a selective emitter is formed in the manner described, a stronger diffusion can be used to form the emitter on the first side of the silicon substrate. On the one hand, this facilitates the process control on the other hand, impurities present in the silicon substrate can be better gettered with a stronger diffusion of dopant, which has an advantageous effect on the efficiency of the finished solar cell.

For the sake of completeness, it should be mentioned that in the case of a selective emitter, the abovementioned, advantageous upper limits for emitter layer resistance values can be exceeded apart from more heavily doped emitter regions. Higher sheet resistance values in these ranges can have an advantageous effect on the efficiency of manufactured solar cells.

Furthermore, the invention will be explained in more detail with reference to figures. Show it:

1 schematic representation of a first embodiment of the method according to the invention

2 schematic representation of a second embodiment of the method according to the invention

1 shows an embodiment of the method according to the invention. In this first dopant is in a silicon substrate 50 diffuses 10 and in this way an emitter 52 educated. In the present embodiment, the dopant is only in the top of the solar cell substrate 50 diffused. This can be done in a manner known per se, for example by diffusion of dopant from one to the top of the silicon substrate 50 applied, dopant-containing solution. The dopant used is based on the volume doping of the silicon substrate 50 vote. In principle, both an n-doped and a p-doped emitter can be formed. Furthermore, there is basically the possibility of doping not only in the top surface of the silicon substrate but diffuse into the entire surface. In this case, appropriate measures should be taken to avoid a short circuit between the emitter 52 and a later to be applied back contact 62 to prevent.

In the further course of the process, the emitter 52 partially etched back 12 , In the present embodiment, this is done wet-chemically in a solution containing hydrofluoric acid and nitric acid.

In the following, the silicon substrate 50 for the purpose of training 14 a silicon oxide layer 54 dipped in deionized water in which ozone is dissolved. In this way, wet-chemical oxidation of the entire surface of the silicon substrate takes place 50 ,

Furthermore, a hydrogen-containing silicon nitride layer is deposited 16 , This is usually done by chemical vapor deposition (CVD). The in the presentation of the 1 downwardly facing backside of the silicon substrate 50 is from the silicon nitride layer 56 kept free. This can be done, for example, by placing the silicon substrates in pairs back to back in a coating system used or by the back is brought to a boat for investment, which is retracted into the coating system used.

In the further course of the process, on the first side of the silicon substrate 50 , So in the present embodiment, on the top of the silicon substrate 50 , local openings 58 into the silicon nitride layer 56 brought in 18 , This takes place in the illustrated embodiment by means of laser ablation. In this embodiment, in addition, in the openings 58 the silicon oxide layer 54 locally removed.

Finally, in the openings 58 by electroplating 20 Front-side contacts 60 educated. These are metal contacts, which have, for example, nickel and silver or nickel, copper and silver. In the context of electroplating 20 will also be the back contact 62 on the back of the silicon substrate 50 educated. This consists therefore of the same material as the front side contacts 60 ,

The presentation of process steps known per se for the formation of back surface fields, so-called back surface fields, has been described in US Pat 1 omitted for the sake of clarity. However, corresponding process steps known per se can readily be integrated.

The lowest part in 1 illustrated next to the last process step of electroplating 20 also an embodiment of the solar cell according to the invention.

The embodiment of 2 is different from the one of 1 in that at the local introduction 28 of openings 58 into the silicon nitride layer 56 also a laser diffusion takes place 28 , Here, in areas of the openings 58 more heavily doped emitter regions 64 formed, which together with the remaining areas of the emitter 52 form a selective emitter. As in the case of the embodiment of 1 For example, laser ablation, and in conjunction therewith laser diffusion, can be realized by laser-induced chemical etching. Instead of a laser-induced chemical etching, a laser beam evaporation method can be used.

The lowest part in 2 illustrated schematically next to the last process step of electroplating 20 In addition, a further embodiment of the solar cell according to the invention.

LIST OF REFERENCE NUMBERS

10

Indiffusion dopant

12

Etching emitter

14

Form silicon oxide layer

16

silicon nitride deposition

18

Inserting openings

20

electroplating

28

Introduce openings and laser diffusion

50

silicon substrate

52

emitter

54

silicon oxide

56

silicon nitride

58

opening

60

Front contact

62

Back contact

64

more heavily doped emitter region

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

• WO 2008/039067 A2 [0002]

Claims (12)

Process for producing a crystalline silicon solar cell, in which - for the purpose of forming an emitter ( 52 ) on a first side of a silicon substrate ( 50 ) Dopant in, the silicon substrate ( 50 ) is diffused ( 10 ); - Subsequently, a silicon nitride layer ( 56 ) on the first side of the silicon substrate ( 50 ) is deposited ( 16 ); Following on the first side of the silicon substrate ( 50 ) local openings ( 58 ) into the silicon nitride layer ( 56 ) are introduced; - subsequently in the openings ( 58 ) by means of plating ( 20 ) Metal contacts ( 60 ) be formed ( 20 ); characterized in that - before deposition ( 16 ) of the silicon nitride layer ( 56 ) the emitter ( 52 ) on the first side of the silicon substrate ( 50 ) is partially etched back ( 12 ); - subsequently before the deposition ( 16 ) of the silicon nitride layer ( 56 ) on the first side of the silicon substrate ( 50 ) a silicon oxide layer ( 54 ) is formed with a thickness in the range of 1.0 nm to 10 nm ( 14 ). Method according to claim 1, characterized in that the metal contacts ( 60 ) in the openings ( 58 ) by electroplating ( 20 ) be formed ( 20 ). Method according to one of the preceding claims, characterized in that by the indiffusion ( 10 ) of dopant into the silicon substrate ( 50 ) an emitter ( 52 ) is formed with a sheet resistance value in the range of 50 Ω / sq to 70 Ω / sq. Method according to one of the preceding claims, characterized in that the emitter ( 52 ) is etched back wet-chemically ( 12 ). Method according to one of the preceding claims, characterized in that by etching back ( 12 ) of the emitter ( 52 ) whose sheet resistance value is increased by 10 Ω / sq to 50 Ω / sq. Method according to one of the preceding claims, characterized in that the silicon oxide layer ( 54 ) is formed by wet-chemical oxidation ( 14 ). Method according to one of the preceding claims, characterized in that the silicon oxide layer is formed by means of a gas phase oxidation. Method according to one of the preceding claims, characterized in that the silicon oxide layer ( 54 ) is formed in a thickness in the range of 2 nm to 3 nm ( 14 ). Method according to one of the preceding claims, characterized in that in the openings ( 58 ) the silicon oxide layer ( 54 ) is removed locally, preferably by means of laser ablation. Method according to one of the preceding claims, characterized in that a hydrogen-containing silicon nitride layer ( 56 ) is deposited ( 16 ). Method according to one of the preceding claims, characterized in that the openings ( 58 ) by laser ablation in the Sillziumnitridschicht ( 54 ) ( 28 ) and thereby by means of laser diffusion in areas of the openings ( 58 ) more heavily doped emitter regions ( 62 ) be formed ( 28 ). Solar cell produced by the method according to one of claims 1 to 11.

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