DE102009021971A1 - Method for manufacturing solar cell utilized for power generation, involves completely covering surface of substrates by intermediate layer or secondary product till completion of diffusion process - Google Patents

Abstract

The method involves providing of p-or n-doped semiconductor substrates i.e. silicon, and diffusing of n-doped and p-doped atoms for production of wafers with an emitter (12). A rear side and an upper side of the p-or n-doped semiconductor substrates are contacted. An intermediate layer i.e. oxide layer, that is semi-transparent for doped atom is applied on a surface of the substrates, before completion of the diffusion. The intermediate layer or a secondary product completely covers the surface till completion of diffusion process. A silicon oxide layer is produced in a reaction chamber. An independent claim is also included for a solar cell that comprises a base.

Description

The The invention relates to a method for producing a solar cell with the features of the preamble of claim 1 and a Solar cell with the features of the preamble of the claim 16th

solar cells are used to generate electricity using light, ie electromagnetic Radiation whose wavelength especially, but not necessarily, is in the visible spectral range. With the use of the word "light" in this Document is not restricted accompanied by the visible spectral range.

Of the physical effect on which this type of power generation is based, is the internal photo effect, more precisely the photovoltaic effect. The internal photoelectric effect that occurs in semiconductors are through Interaction with the incident light electrons from the valence band lifted into the conduction band, so electron-hole pairs generated, the the conductivity of the semiconductor.

Around Use such a charge separation efficiently for power generation to be able to it is necessary to prevent recombination and to direct the charge carriers dissipate. This can be done by providing an over the generating area of the charge carrier falling electric field can be achieved. An easy way to generate a suitable electric field in the semiconductor therein, by appropriate doping of the semiconductor, a transition to create between a p-type doping and an n-type doping.

At a p-n junction it comes to the formation of space charges, as the respective majority carriers in the each differently doped area diffuse and a corresponding elicit electrical field through the charge carriers generated in the transition region in the be sucked off corresponding direction. The area with the electric Field is called the space charge zone.

A solar cell typically has the following structure: The side which faces the light is referred to below as the "top side". There is a near-surface strong n- or p-type doping, typically with a doping level of 10 ²⁰ cm ^-3 to 10 ²¹ cm ^-3 , one weaker, typically with a doping level of 10 ¹⁵ cm ^-3 to 10 ¹⁶ cm ^-3 , p - or n-doped substrate. The heavily doped layer is commonly referred to as an emitter, the weakly doped layer as a base. In order to minimize the recombination at the back, a strong doping, the back surface field, is usually applied there - heavily p-doped for a p-base or heavily n-doped for an n-base. This reflects charge carriers in the direction of space charge zone and emitter.

incident Light, which is used to generate electricity, is in the base, the back surface field, the emitter or in the space charge zone - under Generation of an electron-hole pair - absorbed. The power needed to generate electricity Minority carriers in the space charge zone through the electric field to the right contacts pulled out. In the rest Areas of the substrate diffuse the charge carriers and thus can - if they do not recombine - the space charge zone to reach.

in the heavily doped emitters easily recombine the minority carriers, therefore would like if possible the emitter thin and low doped shape.

Around the solar cell as intended for power generation to be able to use it is still necessary to contact base and emitter. This happens at the bottom z. B. with a surface contact. At the top this is not desirable because thus the incidence of light would be shielded. Therefore, on the top the solar cell usually only a finger or grid-like Contact provided, in particular, about the use of metal pastes is reached.

One typical main component of pastes for the contacting of p-doped surfaces is aluminum, for the contacting of n-doped surfaces is silver.

Especially for finger and lattice-type contacts gives rise to the problem that a The lower the contact resistance, the easier it is to achieve the sheet resistance of the emitter to be contacted is. The sheet resistance becomes common in the unit Ω / sq indicated and typically determined by a four-point measurement. However, low-layer-resistance emitters have a high surface recombination rate, which adversely affects the efficiency of a solar cell.

To date, two approaches are known in the prior art to circumvent this problem. In industrial mass production, the sheet resistance of the emitter to be contacted is limited to a value that still allows a contact. Typical values achieved here are between 50 Ω / sq and 75 Ω / sq, depending on the metal paste used for the contacting. In this case, because of the associated with such sheet resistors significant surface recombination a clear deterioration of the Wirkungsgra of the accepted.

Also It is known to produce solar cells with selective emitter. In this case, the area under the contact fingers by corresponding Control of the diffusion process targeted as low impedance, good contactable emitter is designed while in the area between the Contact fingers a high-impedance emitter is generated. This approach leads to better efficiency, but is expensive because a larger number of process steps is necessary.

The The object of the invention is a process for the preparation a solar cell and a solar cell to provide a has high-impedance, yet good contactable emitter.

These Task is solved by a process for producing a solar cell having the features of claim 1 and a solar cell with the features of Claim 16. Advantageous embodiments and developments The invention are specified in the subclaims.

The Invention is based on the finding that the contact resistance a high - impedance emitter can be reduced when placed on the surface of the p- or n-doped semiconductor substrate before the completion of the Eindiffundierens one for Doping atoms semitransparent intermediate layer is applied, the intermediate layer or a derived product derived from it until the completion of the diffusion process, the surface completely covered.

to execution the method according to the invention for the production of a solar cell must be a p- or n-doped semiconductor substrate to be provided. To create an emitter will be an indiffusion of n-type dopant atoms using a p-type semiconductor substrate or of p-doping atoms when using an n-doped semiconductor substrate in the surface of the made p- or n-doped semiconductor substrate. Indeed is inventively before Completing the diffusion process the surface with one for the dopant atoms Semitransparent intermediate layer covered. This intermediate layer or a derived product from it covers the surface until to complete the diffusion process completely, what they in particular of intermediate layers for masking, as used in the production of Solar cells with selective emitter used differs. The diffusion process is considered to be finished when the desired sheet resistance is reached. Further high temperature steps, such. B. PECVD processes or sintering, basically can influence the achieved diffusion profile, apply not as part of the diffusion step according to this patent.

Finally back and top of the p- or n-doped Semiconductor substrate provided with contacts.

Advantageous is when, after the completion of the indiffusion, the surface of the etched emitter etched is going to be residues of the Diffusion process and the intermediate layer or its secondary product because this improved reproducibility and Long-term stability during the the Eindiffundens set sheet resistance ensured can be.

In a particularly preferred embodiment For example, the substrate provided will undergo a surface treatment to produce a textured surface subjected to the p- or n-doped semiconductor substrate. Thereby is achieved that the respective applied intermediate layer - presumably as a consequence of mechanical stresses that occur during film growth occur - in dependence have varying properties of texturing. Through her the diffusion of the doping atoms at different Placement of the texture, such as local minima and maxima of the textured surface, varies strongly inhibited, which makes a local change in a particularly simple way the degree of doping is achieved.

When Interfacial layers which have a texture-dependent thickness variation Since this is a particularly simple and direct control the local concentration and / or the achieved local diffusion profile of the Doping atoms that are set when diffusing allow. The maxima and minima of the degree of doping and / or the maximum and minimal variations of the diffusion profile are so close together, that each of the later to be applied to the top Contact fingers covered several maxima and minima.

It is particularly advantageous if, instead of applying the intermediate layer before the step of diffusion, it is possible to diffuse part of the desired doping atoms before applying the intermediate layer, since this reduces the time required for the application of the intermediate layer. The surface concentration by this diffusion should be above 10 ²⁰ cm ^-3 .

Especially Good as intermediate layers oxide layers, as these are can be produced easily and easily controllable and also already thin oxide layers can effectively hinder the diffusion of the doping atoms.

The parameters for generating the intermediate Layer and / or the parameters of the diffusion process are preferably adjusted so that the emitter generated by the diffusion after diffusion has a sheet resistance greater than 80 Ω / sq to effectively suppress recombination processes.

As a p- or n-doped semiconductor substrate is particularly suitable because of the good controllability of the silicon technologies used for solar cell production and the good availability of silicon p- or n-doped silicon. In particular, boron is suitable for p-type doping and phosphorus for n-type doping. Both multicrystalline and Czochralski-grown silicon substrates can be used. For these semiconductor substrates, the use of an intermediate layer of SiO _x or SiO ₂ with an average thickness between 10 nm and 50 nm and this superimposed thickness variation has proven particularly useful.

Such a layer can be produced particularly well by dry, thermal oxidation or by wet thermal oxidation, but it is also conceivable by spin-on applied oxide from chemical solutions or by CVD (chemical vapor deposition ), z. As PECVD or LPCVD, applied SiO _x . Suitable layers are achieved when the oxidation is carried out for between 10 and 120 minutes at temperatures greater than 850 ° C.

Especially it is advantageous if the dry, thermal oxidation or the wet, thermal oxidation in the same reaction space as the Diffusing done becomes. In particular, both can Steps one after the other in a tube furnace, without the In the meantime, the wafer to be processed must leave it. This leads to Time savings in the manufacturing process.

When the doping atoms have diffused in, POCl ₃ and, in the case of n-doped silicon substrates BBr _3, p-doped silicon substrates have proven to be particularly suitable as sources for the doping atoms.

A particularly advantageous embodiment of the method provides that after the etching of the diffusion residues from the textured surface of the emitter and before contacting the back side and / or the textured side of the p- or n-doped semiconductor substrate, an antireflection layer is applied to the textured surface of the emitter becomes. For silicon-based systems, a Si _x N _y layer has proven particularly useful here. In order to be able to produce a good contact despite this intermediate layer, this could in principle prior to contacting at the sites to be contacted z. B. be removed with a laser, but for industrial applications, it is cheaper to burn the applied as contacts metallizations or metallic pastes by another, often referred to as "firing" thermal treatment after contacting in the antireflection layer and sintered into the emitter and so to ensure electrical contact with the emitter.

The Solar cell according to the invention has a contacted base and a contacted emitter, where the emitter or base has a surface texture facing the contact having local maxima and minima, and wherein the contacting more than one of the local maxima is covered. This indicates the textured Emitter and / or the textured base between the local maxima the surface texture and adjacent local minima of the surface texture a change the concentration of the doping atoms or a change in the local distribution profile the doping atoms on. Through the localized changes in the concentration of ambient atoms is due to highly doped areas enables a good contact while low doped regions have a high sheet resistance and thus guarantee a low recombination probability.

The Invention will be further explained with reference to the following figures. It shows:

1 a schematic two-dimensional representation of a section through a solar cell according to the invention

2 a block diagram of a process for producing an embodiment of the invention

1 shows a schematic two-dimensional representation of a section through a solar cell 10 according to an embodiment of the invention. In this case, the arrow points the irradiation direction of the light on the solar cell 10 at; accordingly, the top of the solar cell 10 the closer side of the arrow and the bottom of the solar cell 10 the side farther from the arrow. The solar cell 10 has a z. B. with a doping level of 10 ¹⁶ cm ^-3 atoms p-doped base 11 and one to the base 11 towards the top of the solar cell 10 adjacent, strong - z. B. with a doping degree of 10 ²⁰ cm ^-3 or 10 ²¹ cm ^-3 atoms - n-doped emitter 12 leading to the top of the solar cell 10 out with an antireflection coating 13 is covered. A typical thickness for the base 11 is 200 μm. A typical thickness for the emitter 12 is 0.25 μm. The in 1 sharp transition between base 11 and emitter 12 is in reality a transition layer which is typically 0.3 μm thick. The base 11 is at the bottom of the solar cell with a surface contact 14 contacted, which may for example consist of aluminum or an aluminum-containing paste. The to the surface contact 14 adjacent area the base 11 is advantageously with a heavy doping of typically 10 ²⁰ cm ^-3 atoms, the back surface field 16 , provided - heavily p-doped for a p-base or heavily n-doped for an n-base. This reflects charge carriers in the direction of space charge zone and emitter.

The emitter is through the antireflection layer with a contact finger 15 contacted, which may for example consist of silver or a silver-containing paste.

The emitter has surface texturing consisting of a variety of surface textures. The shape of the individual surface textures depends on the respective semiconductor substrate and on the type of, ie the procedure for, texturing, in the case of silicon pyramids with a height of up to 10 μm. In 1 is a section through these pyramids shown. The pyramid peaks represent local maxima of the surface texturing, between each two pyramid peaks there is a local minimum of the surface texturing. The areas below the local maxima 12a of the emitter and the areas below the local minima 12b In this case, the emitter has different concentrations of doping atoms which result from the thickness variations predetermined by the shape of the surface textures within the intermediate layer present during at least part of the step of diffusion of the doping atoms of the emitter. If a particularly thin intermediate layer is formed on the local maxima of the surface texturing, the areas are 12a heavily doped and provide the good connection to the contact finger 15 ago. If the formation of a particularly thin intermediate layer occurs at the local minimums of texturing, the areas take over 12b this function.

2 Fig. 12 shows a block diagram of a process for making an embodiment of the invention.

In step 100 a boron-doped silicon wafer is provided. For this purpose, silicon blocks made of high-purity silicon are cast into polycrystalline blocks or single crystals are obtained by the Czochralski drawing process. The doping atoms are added to the melt. Subsequently, about 200 microns thick wafers are sawn from the blocks. In addition to these methods, there are other standard methods for making doped silicon wafers, e.g. B. by tape drawing of silicon films.

In step 200 the surface texturing takes place. This can, for. Example, when using a monocrystalline grown silicon substrate by alkaline etching of a [100] section with NaOH or KOH, optionally mixed with isopropanol, and leads to the formation of pyramids, since etched [100] levels faster than [111] levels of silicon become. At the same time, any damage that might have occurred during the sawing of the wafers is removed. In the case of using a multicrystalline substrate, texturing by etching with a solution of HF, HNO ₃ and water is possible.

In step 300 the intermediate layer is applied. This is done here by generating a SiO _x or SiO ₂ layer by means of dry, thermal oxidation for between 10 and 120 minutes at over 850 ° C, in particular for 105 minutes at 860 ° C. The oxidation preferably takes place in a tube furnace.

Basically but also other possibilities the oxidation conceivable, for. By wet, thermal oxidation, a, for example by spin-on, applied oxide of chemical solutions or with CVD (chemical vapor deposition), z. PECVD or LPCVD.

In step 400 the diffusion of phosphorus atoms leads to the generation of a structured emitter. This is possible, for example, in a tube furnace supplying POCl ₃ in a carrier gas, typically nitrogen. This forms on the surface, which is wetted, phosphorus-silicate glass as a diffusion source.

It is possible, too, to generate the diffusion sources by spraying or imprinting and then z. B. with a running through an IR oven conveyor belt Temperature suspend.

In step 450 the electrical insulation between emitter and base takes place by removing the phosphor layer from the rear side. Alternatively, this can also be done after step 700 by introducing a laser trench on the front, through the emitter, done.

In step 500 Etching of the phosphosilicate glass, the so-called PSG etching, the z. B. can be performed with 3% hydrofluoric acid at room temperature. Alternatively, the etching can be carried out, for example, by energy-assisted plasma etching or reactive ion beam etching. In this process, the SiO ₂ layer produced or the secondary product of this layer which has formed during the process steps that have taken place after its production is also removed at the same time.

In step 600 the emitter is provided with an antireflection coating. This can for example consist of silicon nitride and by SiN _x sputtering, LPCVD (low pressure chemical vapor depositi On) or PECVD (plasma enhanced chemical vapor deposition) are applied, the latter layer is particularly suitable.

In step 700 then the contacting of base and emitter, z. B. by screen printing a grid-like structure with silver-containing paste on the front, largely ganzflächigem screen printing with aluminum-containing paste on the back and screen printing with aluminum-silver paste of structures for soldering the solar cells on the back. Subsequently, the sintering of the pastes takes place in an infrared belt furnace. There, different temperatures are set in different heating zones, which are traversed with a suitably selected belt speed.

10

solar cell

11

Base

12

emitter

12a

Area with high / low doping

12b

Area with low / high doping

13

Anti-reflective coating

14

surface contact

15

contact fingers

16

Back Surface Field

Claims (16)

Method of producing a solar cell comprising the steps of providing a p- or n-doped semiconductor substrate - diffusing n-doping atoms when using a p-doped semiconductor substrate or p-doping atoms when using an n-doped semiconductor substrate in the surface of the p- or n-doped semiconductor substrate for producing diffused wafers with an emitter ( 12 ) - contacting the back and the top of the p- or n-doped semiconductor substrate, characterized in that is applied to the surface of the p- or n-doped semiconductor substrate prior to the completion of Eindiffundens a semitransparent for doping atoms intermediate layer, said intermediate layer or one of their resulting secondary product until the completion of the diffusion process completely covers the surface. Method according to claim 1, characterized in that that after the diffusion in an etching of the residues of the Eindiffundier of the diffused wafers for removing residues of Eindiffundier and the intermediate layer or a secondary product resulting from it he follows. Method according to claim 1 or 2, characterized in that after provision of the p- or n-doped semiconductor substrate a surface treatment for producing a textured surface of the p- or n-doped Semiconductor substrate takes place. Method according to any preceding claim, characterized characterized in that the thickness of the intermediate layer is on a length scale, which is smaller than the width of the contacts on top of the p- or n-doped ones Halbeitersubstrats is varied. Method according to any preceding claim, characterized characterized in that the diffusion of n-doping atoms when using a p-type semiconductor substrate or p-type dopant atoms when using an n-doped semiconductor substrate in the surface of the p- or n-doped semiconductor substrate partially before coating the surface of the p- or n-doped semiconductor substrate with the intermediate layer carried out becomes. Method according to any preceding claim, characterized characterized in that the intermediate layer is an oxide layer. Method according to any preceding claim, characterized in that the emitter ( 12 ) has a sheet resistance> 80 Ω / sq after diffusion. Method according to any preceding claim, characterized characterized in that the p- or n-doped Semiconductor substrate is p- or n-doped silicon. A method according to claim 8, characterized in that an SiO _x - or SiO ₂ layer is produced with an average thickness between 10 nm and 50 nm as the intermediate layer. Method according to one of claims 6 to 9, characterized that oxidation by means of dry, thermal oxidation or effected by means of moist, thermal oxidation. Method according to claim 10, characterized in that that the dry, thermal oxidation for 10 to 120 minutes at more as 850 ° C carried out becomes. Method according to one of claims 9 to 11, characterized in that the generation of the SiO _x - or SiO ₂ layer in the same reaction space as the diffusion is performed. Method according to one of claims 8 to 12, characterized in that in p-doped silicon POCl ₃ and n-doped silicon BBr ₃ as Source is used for the doping atoms. Method according to one of the preceding claims, characterized in that prior to contacting the rear side and / or the upper side of the p- or n-doped semiconductor substrate, an antireflection coating is applied to the surface of the emitter ( 12 ) is applied. Method according to claim 14, characterized in that that after contacting the back and / or the top of the p- or n-doped semiconductor substrate is subjected to a thermal treatment. Solar cell with a base ( 11 ) and a contacted emitter ( 12 ), the emitter ( 12 ) has a surface texture with local maxima and minima facing the contacting, and wherein the contacting covers more than one of the local maxima, characterized in that between the local maxima of the surface texture and adjacent local minima of the surface texture a change in concentration or distribution profile of the doping atoms ( 12a . 12b ) in the emitter having the texture ( 12 ) is present.