DE102011086351A1 - Process for producing a solar cell with PECVD combination layer and solar cell with PECVD combination layer - Google Patents

Abstract

Provided are a method of manufacturing a solar cell (100, 200) comprising the steps of providing a crystalline silicon substrate (101, 201); Doping at least a first region of the silicon substrate (101, 201) to produce a base (102, 202) and a second region of the silicon substrate (101, 201) to produce an emitter (103, 203); Forming a combination layer (105, 205) by performing plasma nitriding or plasma oxynitriding in a plasma of NH 3, N 2 O or a mixture of these gases to form a base film (105a, 205a) of amorphous SiN x or amorphous SiO x N y and immediately performing a plasma nitriding immediately or plasma oxynitriding subsequent PECVD process using at least one silicon-containing process gas to deposit a silicon capping layer (105b, 205b) of amorphous SiNx, amorphous SiCxNy or amorphous SiOxNy, and contacting base (102, 202) and emitter (103, 203) and a A solar cell (100, 200) comprising a crystalline silicon substrate (101, 201) having a base (102, 202), an emitter (103, 203) and contacts (106, 107, 206, 207), the solar cell (100, 200 Further, a combination layer (105, 205) with an at least partially applied directly on a surface of the crystalline silicon substrate (101, 201) en, by plasma nitriding or plasma oxynitriding in a plasma of NH3, N2O or a mixture of these gases generated base film (105a, 205a) of amorphous SiNx or amorphous SiOxNy and with a directly adjacent to the base film (105a, 205a), by subsequent PECVD deposition has silicon cover layer (105b, 205b) of amorphous SiNx, amorphous SiCxNy or amorphous SiOxNy obtainable using at least one Si-containing process gas.

Description

The invention relates to a method for producing a crystalline solar cell and a crystalline silicon solar cell having the features of the preamble of claim 3.

Crystalline solar cells made of silicon are generally known devices for generating electricity using light, in particular sunlight. Hereinafter, the "top side" and the "front side" of the solar cell are understood to mean the side facing the light when the solar cell is used as intended; accordingly, the "bottom side" and the "back side" are the side away from the light when the solar cell is used as intended.

A crystalline silicon solar cell is based on a monocrystalline or polycrystalline silicon substrate which is weak, typically p- or n-doped, with a degree of doping of 10 ¹⁵ cm ^-3 to 10 ¹⁶ cm ^-3 , and a strong, opposing near-surface doping, ie a strong n-type doping. Doping in p-doped substrate and a strong p-type doping at n-doped substrate, which is typically carried out with a doping level of 10 ²⁰ cm ^-3 to 10 ²¹ cm ^-3 .

The heavily doped layer is commonly referred to as an emitter, the weakly doped layer as a base. To minimize recombination at the backside, a heavy doping, the back-surface field, can be attached there, which is heavily p-doped for a p-base or heavily n-doped for an n-based. This reflects charge carriers in the direction of space charge zone and emitter.

Incident light, which is used to generate electricity, is absorbed in the base, the back surface field, the emitter or in the space charge zone - creating an electron-hole pair. The minority carriers required for power generation are drawn in the space charge zone through the electric field to the correct contacts. In the other areas of the substrate, the charge carriers diffuse and can - if they do not recombine - reach the space charge zone and flow away from there via electrical contacts.

A widely used in the manufacture of crystalline solar cells method for depositing layers, in particular anti-reflection layers in the application of the per se known PECVD (Plasma-Enhanced Chemical Vapor Deposition) process for depositing silicon-containing layers, in particular of SiN _x - and SiC _x N _y layers , The popularity of this method is due to the fact that the layers applied by this method can also contribute to a surface passivation of the substrate, ie reduce the recombination rate of minority carriers on the surface and thereby improve the efficiency of the solar cell.

In the PECVD process, process gases containing the elements from which the layer to be deposited is made flow in a high-frequency alternating field, which leads to the formation of a plasma, that is to say in particular an at least partial fragmentation of the process gas molecules. The resulting molecular fragments, atoms or radicals of the process gases strike the surface of the wafer arranged on a process boat and form there the desired antireflection layer. Basically, a layer formation at room temperature is possible, but this leads to poor quality antireflection coatings. For this reason, the PECVD process is usually carried out at elevated temperature, often between 400 ° C and 500 ° C, which implies the need to heat in particular the process boats with the wafers thereon to this temperature.

Known crystalline silicon solar cells with deposited using PECVD processes, in particular silicon-containing layers have a number of problems.

First, for the desired efficient use of a solar cell, the fullest possible use of light incident on the solar cell is essential. In order to achieve this, it is customary to arrange a transparent antireflection layer at least on the front side of the solar cell in order to reduce the reflection of the incident light as far as possible, as far as possible avoiding absorption losses in the antireflection layer.

However, known antireflection layers for crystalline silicon solar cells produced using PECVD methods have a number of disadvantages: In the case of solar cells coated therewith, subsequent gelling of the solar cell with metal contacts, for example in frontgrid silver plating, results in ghost plating, in which larger surface areas occur the solar cell to be metallized than actually desired, resulting in a reduction in the efficiency of the solar cell. Furthermore, it has been shown that the degree of surface passivation achieved with such antireflection layers is not yet optimal.

Secondly, the application of known crystalline silicon solar cell layers which can be produced using PECVD processes involves a relatively high expenditure of time and energy, resulting from the fact that, as mentioned above, heating of the PECVD system and in particular of the PECVD system Process boat carrying the wafers, which is usually done with a pipe heater, increases the time and energy required for the manufacturing process and that the process boat is also occupied in the implementation of the PECVD process and must be etched back after a number of processes.

Third, the conventional PECVD-accessible SiN-based passivation layers are only for the passivation of n-type silicon surfaces, but not for p-type silicon surfaces, unless an SiO ₂ layer is first applied in an additional step.

Fourthly, it has recently been found that when interconnecting many solar modules constructed of conventional crystalline silicon solar cells, in particular using conventional PECVD coating techniques, high system voltages of typically 1000 volts can be applied by means of a string leading to potential-induced degradation (Potential Induced Degradation, PID). These problems are at least partly due to the fact that SiN layers produced by means of conventional PECVD processes do not have sufficient insulation at such high system voltages that leakage currents can flow from the glass plates of the modules into the silicon.

These problems are solved by a combination layer and / or solar cells with such a combination layer having the features of patent claim 3, which are accessible by a method having the features of patent claim 1 within the scope of the production of a crystalline silicon solar cell. Advantageous embodiments of the invention are the subject of the dependent claims.

The method according to the invention for producing a crystalline silicon solar cell comprises at least the following steps: providing a crystalline silicon substrate; Doping at least a first region of the silicon substrate to create a base and a second region of the silicon substrate to produce an emitter; Forming an antireflection combination layer by performing plasma nitriding or plasma oxynitriding in a plasma of NH ₃ , N ₂ O or a mixture of these gases to produce a base film of amorphous SiN _x or amorphous SiO _x N _y and pass tion of a directly following the plasma nitriding or plasma oxynitriding PECVD process using at least one silicon-containing process gas for depositing a silicon capping layer of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _y ; and contacting the base and emitter to provide metal contacts.

Immediately thereafter, it should be understood that the wafer is processed further in the same plant without leaving the plant. Between the deposition of the layers to be combined, there may be a period during which the wafer in the plant is exposed to the process gas itself, an inert protective gas or vacuum conditions.

It is expedient to carry out the steps in the stated order, with the exception of the two doping steps for generating base and emitter, which are equally well executable in reverse order.

Essential to the invention here is the embodiment of the step of forming a combination layer. Apart from the fact that this process step achieves a layer which can be used in different ways for crystalline silicon solar cells with the advantages depending on the particular use discussed below, a number of further advantages result from this process procedure.

This reduces the PECVD process time. On the one hand, this is due to the fact that the heating of the apparatus and in particular of the process boat is already carried out by the upstream process of plasma nitriding or plasma oxinitration, and in a more homogeneous manner than with a conventional tube furnace.

On the other hand, the layer thickness to be applied subsequently by the PECVD process is reduced, so that a shorter process time for this process results. This, in turn, has the advantage that the process boat also experiences less occupancy, so it only has to be etched back after more trips, which saves costs.

Further, during plasma nitriding or plasma oxynitriding, there is a desirable partial etch back of the emitter dead layer.

Finally, for this type of process, p-type solar cells also have dielectric backside passivation with only one PECVD process and without creating a near-surface inversion layer in the silicon.

In an advantageous embodiment of the invention, hydrogen is further diffused into the base film at a time at which the combination layer is already formed. This leads to a further improvement of the surface passivation.

Particularly advantageous, this process step in the so-called "firing" or "Firing" the Metal contacts, which is anyway necessary for the production of the electrical contact through the antireflection layer, are performed. The temperature at which this partial step of contacting the solar cell is carried out, however, should be 20 to 30 ° C higher than in conventional methods. The temperature set at the oven should typically be around 885 ° C instead of around 850 ° C.

However, in this regard, it should be noted that, firstly, the actual temperatures to which the wafer is exposed at this selected temperature are typically about 50 ° C lower, and secondly, the temperatures at the wafer are very dependent on the particular furnace used, so that the temperatures of a particular Any process usually to be adapted to the furnace used. The comparison of temperature settings used in the same oven for different processes is therefore more meaningful than the indication of temperature settings of a furnace.

The solar cell according to the invention has a crystalline (ie mono- or polycrystalline) silicon substrate, which is provided by respectively opposite doping at least with a base and an emitter. It also has contacts for connecting the solar cell to a circuit, which allow a discharge of charge carriers from the space charge zone and is covered at least on portions of its upper side with a combination layer, which applied by at least partially directly on a surface of the crystalline silicon substrate, by plasma nitriding or plasma oxynitriding in a plasma of NH ₃ , N ₂ O or a mixture of these gases generated base film of amorphous SiN _x or amorphous SiO _x N _y and with a directly adjacent to the base film, then by PECVD deposition using at least one Si-containing Process gas obtainable silicon top layer of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _{y is} configured.

It is explicitly pointed out that the properties of the base film depend on the manner of its preparation, so that, for example, an amorphous SiN _x film formed by plasma nitriding in NH ₃ plasma has different properties on a crystalline silicon substrate than one produced by a PECVD process using at least one Si-containing process gas, for example by using SiH ₄ and NH ₃ generated amorphous SiN _x film. This is understandable for the reason that the silicon is provided in the first case of the crystalline silicon substrate, in the second case by the Si-containing process gas. Likewise, an amorphous SiO _x N _y film formed on a crystalline silicon substrate by plasma oxynitriding in a plasma of N ₂ O or N ₂ O and NH _{3 has} different properties than one produced by a PECVD process using at least one Si-containing one Process gas generated amorphous SiO _x N _y film. Thus, it is clear that the manufacturability of the respective film implicitly dictates properties of the film through a particular process and thus constitutes a feature by which these films are distinguishable from each other.

The films produced according to the invention have a higher density and greater etch resistance to hydrofluoric acid than films produced by conventional methods. The films according to the invention are etched at less than half the etch rate at hydrofluoric acid concentrations of 1-10% HF than conventionally prepared PECVD SiN films at the same etching conditions.

In a preferred embodiment of the solar cell, this has an antireflection layer, wherein the combination layer forms the antireflection layer.

In an alternative or additional embodiment it can be provided that the solar cell has a backside passivation layer, and that the combination layer forms the backside passivation layer.

Surprisingly, it has been shown that the combination layer - possibly due to the properties that are imprinted on the base film via plasma nitriding or plasma oxynitriding in the abovementioned, silicon-free process gases - leads to a significant reduction in ghostplating and at the same time to a significantly improved surface passivation, which is also found in p conductive silicon is effective.

The above effects become particularly strong when the base film is of amorphous SiN _x , which has a higher density than SiN _x films producible with pure PECVD deposition processes, or if the base film is of amorphous SiO _x N _y and has a higher density than pure PECVD deposition processes producible Si-O _x N _y films.

It has been found to be particularly useful to use a base film having a thickness between 5nm and 15nm. In particular, results with a base film thickness of about 10 nm are particularly good. As mentioned, in plasma nitriding or plasma oxynitriding in a process gas atmosphere containing no silicon, the silicon incorporated into the base film comes from the crystalline silicon substrate. For example, this thickness dependency of the effects obtained may possibly be related to a decreasing availability of silicon as the thickness of the base film increases, which then affects the composition of the base film, particularly with respect to its stoichiometry.

A further improvement of the surface passivation is achieved if hydrogen, in particular diffused hydrogen, is present in the base film.

A further surprising advantage results when solar cells according to the invention are used with the combination layer producible by the process according to the invention for a plurality of solar modules which are connected in series with a string, since then a noticeable reduction in the PID effect results.

The invention will be further explained with reference to figures. It shows:

1 A first embodiment of a solar cell with a combination layer according to the invention as an antireflection layer, and

2 A second exemplary embodiment of a solar cell with a combination layer according to the invention as backside passivation.

It should be noted that for reasons of clarity, the dimensions of the solar cell, in particular the relative thickness of individual layers to each other, is not drawn to scale.

1 shows a solar cell 100 with a crystalline silicon substrate 101 , In the crystalline silicon substrate 101 is by doping a p-doped base 102 and a heavily n-doped emitter 103 and a heavily p-doped back-surface field area 104 been generated. On top of the crystalline silicon substrate 101 is at least partially an antireflection coating 105 applied as a combination layer of one directly on a surface of the crystalline silicon substrate 101 applied, by plasma nitriding or plasma oxynitriding in a plasma of NH ₃ , N ₂ O or a mixture of these gases producible base film 105a of amorphous SiN _x or amorphous SiO _x N _y and with one directly to the base film 105a adjacent, obtainable by subsequent PECVD deposition using at least one Si-containing process gas silicon topcoat 105b is formed of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _y . Through a back contact 106 made of metal and a grid-like frontal contact 107 The solar cell is made of metal 100 provided with connections or contacted.

2 shows a solar cell 200 with a crystalline silicon substrate 201 in which by doping a p-doped base 202 and a heavily n-doped emitter 203 , On top of the crystalline silicon substrate 201 is at least partially an antireflection coating 204 applied, which is shown here as a conventional antireflection coating, but also, as in 1 , Can be performed as a combination layer according to the invention. On the other side of the crystalline silicon substrate 201 - The use of the word "bottom" is deliberately avoided here because it is a bifacial solar cell is a backside passivation layer 205 arranged as a combination layer of a directly on a surface of the crystalline silicon substrate 201 applied, by plasma nitriding or plasma oxynitriding in a plasma of NH ₃ , N ₂ O or a mixture of these gases producible base film 205a of amorphous SiN _x or amorphous SiO _x N _y and with one directly to the base film 205a adjacent, obtainable by subsequent PECVD deposition using at least one Si-containing process gas silicon topcoat 205b is formed of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _y . Through a grid-like back contact 206 made of metal and a grid-like frontal contact 207 The solar cell is made of metal 200 provided with connections or contacted.

LIST OF REFERENCE NUMBERS

100, 200

solar cell

101, 201

crystalline silicon substrate

102, 202

Base

103, 203

emitter

104

Back-surface field area

105, 204

Anti-reflective coating

205

Rückseitenpassivierungsschicht

105a, 205a

base layer

105b, 205b

silicon top layer

106, 107, 206, 207

Contact

Claims (9)

Process for producing a solar cell ( 100 . 200 comprising the steps of: providing a crystalline silicon substrate ( 101 . 201 ); Doping at least a first region of the silicon substrate ( 101 . 201 ) to create a base ( 102 . 202 ) and a second region of the silicon substrate ( 101 . 201 ) for generating an emitter ( 103 . 203 ); Forming a combination layer ( 105 . 205 by performing a plasma nitration or a plasma oxinitration in a plasma of NH ₃ , N ₂ O. or a mixture of these gases to produce a base film ( 105a . 205a of amorphous SiN _x or amorphous SiO _x N _y and immediate performance of a PECVD process directly following the plasma nitriding or plasma oxynitriding using at least one silicon-containing process gas for depositing a silicon covering layer ( 105b . 205b ) of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _y ,; and contacting Base ( 102 . 202 ) and emitter ( 103 . 203 ). Process for producing a solar cell ( 100 . 200 ) according to claim 1, characterized in that at a time when the combination layer ( 105 . 205 ) is already formed, hydrogen in the base film ( 105a . 205a ) is diffused. Solar cell ( 100 . 200 ) with a crystalline silicon substrate ( 101 . 201 ) with a base ( 102 . 202 ), an emitter ( 103 . 203 ) and contacts ( 106 . 107 . 206 . 207 ), characterized in that the solar cell ( 100 . 200 ) furthermore a combination layer ( 105 . 205 ) having an at least partially directly on a surface of the crystalline silicon substrate ( 101 . 201 ) applied by plasma nitriding or plasma oxynitriding in a plasma of NH ₃ , N ₂ O or a mixture of these gases base film ( 105a . 205a ) of amorphous SiN _x or amorphous SiO _x N _y and one directly to the base film ( 105a . 205a ) adjacent, by subsequent PECVD deposition using at least one Si-containing process gas obtainable silicon top layer ( 105b . 205b ) of amorphous SiN _x , amorphous SiC _x N _y or amorphous SiO _x N _y . Solar cell ( 100 ) according to claim 3, characterized in that the solar cell ( 100 ) an antireflection coating ( 105 ) and that the combination layer ( 105a . 105b ) the antireflective layer ( 105 ). Solar cell ( 200 ) according to claim 3, characterized in that the solar cell ( 200 ) a backside passivation layer ( 205 ), and that the combination layer ( 205a . 205b ) the backside passivation layer ( 205 ). Solar cell ( 100 ) according to any preceding claim, characterized in that the base film ( 105a . 205a of amorphous SiN _x , which has a higher density than SiN _x films which can be produced with pure PECVD deposition processes, or that the base film ( 105a . 205a ) of amorphous SiO _x N _y and has a higher density than SiO _x N _y films producible with pure PECVD deposition processes. Solar cell ( 100 . 200 ) according to one of claims 3 to 6, characterized in that the base film ( 105a . 205a ) has a thickness between 5nm and 15nm. Solar cell ( 100 . 200 ) according to one of claims 3 to 7, characterized in that in the base film ( 105a . 205a ) Hydrogen, in particular diffused hydrogen, is present. Use of solar cells ( 100 . 200 ) according to one of claims 3 to 8 for a plurality of solar modules, which are connected in series with a string.

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