DE19525720A1 - Solar cell mfg system - has interleaved thick-film and thin-film contacts at rear surface of doped silicon substrate with surface oxidation layer - Google Patents

Abstract

The mfg. system, used to provide a solar cell without a front contact, employs a crystalline silicon substrate with a p-type conductivity. It is structured at the front surface and provided with an oxidation layer at the rear surface. Finger-shaped thick-film contacts (6) are applied to the rear surface using a metal paste containing Ag. Further finger-shaped metallic thin-film contacts are formed in openings in the oxidation layer, so that the fingers of the different contacts are interleaved. A highly doped p-type zone lies beneath the thick-film contacts. A highly doped n-type zone lies beneath the thin-film contacts, without any overlap between the doped zones.

Description

The invention relates to a solar cell without a front panel tallization in which both the n and the p contacts are on the back. The front side is not shaded by any contact and can limits to be used for light irradiation.

A solar cell without front metallization is included for example from R. A. Sinton, P. J. Verlinden, R. A. Crane, R. M. Swanson, C. Tilford, J. Perkins and K. Garrison, "Large Area 21% Efficient Si Solar Cells", Proc. of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, 1993, Pages 157 to 161 known. They are used to manufacture them areas doped differently in several mask steps created side by side and by applying one more layered metal structure metallized or contacted. The The metal structures are applied by thin layering process.

The disadvantage here is that the method involves several masking steps te required and is therefore complex.

The problem of the present invention is a method for Generation of a solar cell without front-side metallization indicate which one is easy, safe and quick to perform and which is a solar cell with high efficiency and leads to adherent contacts.

This object is achieved by a solar cell Claim 1 solved. Advantageous embodiments of the invention and a method for producing this solar cell the other claims.  

The invention uses an advantageous combination of Dick film and thin film processes for the production of the p- and the n contacts. A similar procedure for a conventional So Lar cell with front and rear contacts is included known for example from WO 93/19492. The semiconductor body of the solar cell according to the invention consists of, as is known p-doped silicon, the front of which to improve the Light incidence geometry is textured. Be on the back there is a passivation layer, preferably one Oxide layer. In this passivation layer are finger-shaped Opened openings in which the n-contacts are in shape metallic thin film contacts. Even the p contacts (Thick film contacts) are finger-shaped, grip with the finger-shaped thin-film contacts into each other without contact and consist of a printed silver-containing paste. Below the finger-shaped contacts there are highly doped n⁺ or p⁺ areas that are not overlap with each other and between each one pn junction forms. By combining thick film and thin film technology can alternate contacts in a grid of lines are generated that have a grid spacing of at least about 200 µm.

The thin-film contacts preferably consist of a Nic kelschicht in the openings of the passivation layer di is applied directly to the semiconductor surface and with another metal is reinforced. This other metal can be copper or silver. The nickel layer stands out by good adhesion to the silicon semiconductor body. The reinforcing metal ensures sufficient electrical Conductivity.

The contacts and are applied in the manufacturing process the self-adjusting generation of the highly doped areas. There ensures a high level of procedural security. Au It also becomes possible to change the distance between each  finger-shaped contacts towards a short distance optimize the up to 200 µm grid spacing or up to 100 µm clear distance between the individual contacts can.

The definition of thin-film contacts requires a structure turation step, with the openings in the passivation layer, usually in the oxide layer on the back the solar cell are generated. For structuring a Photoresist layer applied and by exposure and development be structured. The openings in the passivation layer are then produced by etching, whereby the lead practicing photoresist structure serves as an etching mask. Possibly can close the openings by etching the semiconductor body Ditches are deepened, which better adhesion of the thin Enable and depend on layer metallization in these trenches gig of semiconductor material an improvement in effectiveness degrees. It is also possible to open the openings directly through laser structuring or mechanical processing testify, here also depending on the structuring depth the public in the passivation layer to form trenches in the semiconductor body can be deepened.

The subsequent phosphorus diffusion leads to the generation of n⁺-doped areas in the area of the openings, since only there the dopant can penetrate into the semiconductor body. A suitable choice of diffusion conditions makes it possible Lich to delimit the highly-doped areas precisely, or de control their size precisely.

Thick film contacts (p contacts) are defined at Imprint. This includes screen, stencil and pad printing drive suitable. While Struk can reach up to approx. 80 µm with the their processes have even narrower structure widths of up to approx. 20 µm to be expected.  

The thick film contacts are in the form of an electrically conductive printed paste. In addition to the guidelines ability-imparting metal particles, in particular silver, still inorganic oxide and optionally binder. While the oxide surcharges ver the adhesion on the semiconductor body averaging, the binder becomes a processable loading the texture of the paste is set. As a further component the electrically conductive paste also contains a dopant Material surcharge for generating p-type doping. Corresponding the paste contains, for example, suitable boron or aluminum connections or metallic aluminum.

In the subsequent sintering process, the thick film contacts hardened and sintered onto the semiconductor body. Soon the dopant contained in the semiconductor body by driven in and a good conductive connection between the p⁺ areas and the printed thick film contacts through the passivation layer.

The thick film contacts are printed so that the p⁺-doped areas not with those previously in the area of the opening overlap gene produced n⁺-doped regions. Yet the geometry of the thin-film and thick-film contacts optimized a minimum distance from each other.

In the next step, the thin-film contacts are replaced by che Mix metallization of the semiconductor body in the openings of the passivation layer. Because the chemical separation preferably on a highly doped and electrically good conductive semiconductor surface takes place, it can be targeted done in the openings. Through an intermediary Etching step is the semiconductor surface in the openings of a thin oxide layer formed in the meantime. To an acidic solution containing fluoride ions can serve.

A nickel layer is first used for the thin-film contacts deposited. This can be opened quickly and easily  (Cleaned) semiconductor surfaces in a purely chemical way separate. The nickel layers produced show one good adhesion to the semiconductor body. Preferably the Nickel layer only thinly deposited and then with egg nem another, electrically highly conductive metal reinforced. The other metal can also be by chemical deposition or by galvanic reinforcement of the nickel layer be brought.

It is also possible to deposit nickel by previous ones Accelerate generation of a palladium seed layer. This can be treated with a short treatment of the semiconductor surface a solution containing Pd ions. Contains this also fluoride ions in an acid medium, so it can previous etching step is omitted.

In a further embodiment of the invention, the Ab separation of both the nickel layer and the layer of other metals are induced by light, or by light radiation are supported. Thereby, in the semiconductor body pairs of carriers generated and in the internal field of the Semiconductor body or the solar cell separately. Thereby ste at the p⁺-doped areas electrons to reduce the Metal ions available. This procedure has the advantage that the solution containing the metal ions does not contain any reducti must have onsmittel. For light-induced chemical ab Therefore, stable metallization baths like the one above are sufficient find use in electroplating, for example. It is note that in addition to the light-induced metal deposition in the n⁺-doped areas simultaneously via the p⁺-do contacts in the areas affected. It it was found that the printed thick film structures are well suited as so-called "sacrificial anodes". In addition to one The light-induced acceleration of the deposition process te metal deposition has the further advantage that at low higher temperatures than a corresponding "darkness"  dung "or a corresponding galvanic deposition by can be performed, for example at room temperature.

That with the light-induced metal deposition from the thick Metal contacts in solution depends on the metal Type of metal to be deposited. With light-induced Ab The thick film contact separates nickel and copper mainly dissolved the aluminum contained as a dopant. It is therefore advantageous to use aluminum in the Make thick film paste higher than usual. At the lichtin reduced deposition of silver go both aluminum and also silver from the thick film contact in solution. Also because of that must have a larger structure when applying the thick film contacts turbreite or a higher cross section are provided, the can compensate for this loss.

Another metal is preferred over the nickel layer Silver deposited, which is also light-induced without current from known chemical or galvanic silver baths can follow. However, a cyanide is preferred for this purpose Silver bath used.

In a further tempering step, liability as well also ver electrical conductivity of the thin film contacts improves.

On the front of the already completed solar cell can also passivate or anti-pass flex layer can be applied from a dielectric. Ge thin layers of silicon oxide are suitable, for example, Silicon nitride, titanium oxide or layer combinations of the materials.

In the following, the invention is illustrated by means of an embodiment game and the associated eight figures explained in more detail.  

The position of Fig. 1 to 6 with reference to schematic cross sections of a different solar cell process steps and their Her, and

FIGS. 7 and 8 show possible geometric arrangements of thick-film and thin-film contacts on the back.

Embodiments

Fig. 1: As a semiconductor body 1, a p-type CZ wafer with <100 <orientation is chosen, for example. However, it is also possible to use a corresponding polycrystalline silicon wafer.

By means of basic crystal-oriented etching, a texturing 2 for improving the light incidence geometry is first generated on the surface of the semiconductor body 1, if appropriate upstream. This creates a roughened surface with pyramid-shaped elevations, which are no longer shown in the other figures. To passivate the surface, a passivation layer 3 , for example an oxide layer, is now produced at least on the back. On the front, this passivation layer 3 can simultaneously serve as an anti-reflective layer. A silicon nitride layer or a layer combined from both materials can also serve the same purpose. It is also possible to produce a titanium oxide layer as an anti-reflective layer over the oxide layer.

Fig. 2: apertures 4 are now generated in the passivation layer 3 to define the thin-film contacts (planar) which can extend, where appropriate, in the form of trenches into the silicon of the semiconductor body 1 (grave art).

The definition of the openings, which corresponds to the geometric structure of the desired thin-film contact, can be done for example by photolithography and subsequent oxide etching. The openings can then be deepened into trenches by basic etching of the semiconductor body 1 (not shown in the figure).

A direct creation of openings 4 or of trenches extending into the semiconductor body 1 is achieved by laser writing or by mechanical removal, for example with the aid of a diamond-tipped tool.

A is preferably used for the photolithographic definition Positive photoresist is used, which after exposure and Ent winding the etching mask for the wet chemical oxide or nitride represents opening, for example, in hydrofluoric acid Solution is carried out.

Trenches can be created in silicon in a basi solution with the remaining oxide or nitride as an etch mask. For particularly thin wafers is because of the risk of breakage preferred planar technology without trenches.

In the exemplary embodiment, openings with a width of approx. 20 to 50 µm at a distance of approx. 200 µm to each other testifies.

Fig. 3: Subsequently, in a known manner n⁺-doped regions 5 in the semiconductor body 1 generates in the openings 4 by a phosphorus diffusion.

Fig. 4: In the next process step, the thick film contacts 6 are produced by printing a conductive paste containing silver for example. The paste also contains aluminum as a p-type dopant. A screen printing method is preferably used for the application. Other suitable application methods are stencil or pad printing, writing pastes, rolling or similar methods. The thick film contacts 6 are applied next to the openings only on the surface areas of the back, which are still covered with the passivation layer 3 .

Fig. 5: When baking or sintering the printed thick film contacts 6 at about 750 to 850 ° C, the additional dopant contained in the paste is partially driven into the semiconductor body 1 and generates p⁺-doped regions 7th These ensure a good ohmic contact between the semiconductor body 1 and the thick film contacts 6 .

Fig. 6: Subsequently, the thin-film contacts by metallization of the trenches or openings 4 freige n⁺-doped surface lay of the semiconductor body 1 he witnesses. For this purpose, a nickel layer 8 is first photo-induced electrolessly deposited on the exposed n⁺-doped semiconductor surfaces.

For example, a usually serves as the metallization bath nickel sulfamate bath used for electroplating. It is ever but also possible to use other nickel baths at for example, baths for chemical nickel deposition, which can be acidic to basic. It is in reducing agents present in the chemical deposition baths in principle not necessary, but the deposition can be un support.

Immediately before the nickel deposition, the wafers are freed from the thin oxide film in both the planar and trench technology by briefly immersing them in or briefly treating them with an acidic solution containing fluoridions, which is deposited during the baking process of the thick film contacts 6 on the in the openings 4 or trenches exposed semiconductor surface has formed. For nickel deposition itself, the substrates are irradiated with the rear side facing upwards in the metalization bath with a suitable radiation source, for example a 75 watt incandescent lamp or a 150 watt IR lamp at approx. 50-60 ° C. in a slightly basic chemical deposition bath. The nickel deposition on the semiconductor surface begins after only a few seconds. Already after about one to two minutes, a well-adhering, sufficiently thick nickel layer 8 is produced, via which a further metal layer 9 can be produced directly for reinforcement. After a deposition time of 1 to 3 minutes, a nickel layer with a thickness of 0.2 to 2 μm is obtained.

As a further metal layer 9 , an approximately 5 to 20 μm thick copper layer is produced, for example by chemical deposition from a corresponding commercially available copper deposition bath, or analogously to the production of the nickel layer 8 photo-induced currentless from any copper-containing bath, for example from a copper sulfate solution.

Because of the sensitivity of the nickel layer 8 to oxidation, the reinforcing copper layer 9 is preferably deposited immediately after the nickel layer has been produced. The copper layer in turn can be protected against oxidation by applying a further thin layer, for example by means of a thin coating of silver or tin.

However, the further metal layer 9 is preferably produced by light-induced deposition of an approximately 4 to 10 μm thick silver overlayer. For this purpose, a silver salt solution based on KAg (CN) ₂, which was previously only suitable for galvanic silver deposition, can be used. The photo-induced deposition is carried out at room temperature. Due to the clear silver salt solution, the silver deposition can also be carried out in tray operation, with several wa fer one behind the other at a short distance in the tallization baths and z. B. be exposed obliquely from above. When exposed to a 75 watt incandescent lamp from a distance of approx. 30 cm, the silver layer 9 is deposited in the thickness mentioned within 1 to 3 minutes at room temperature.

Fig. 6 shows sections of an opening 4 with abge different now complete thin-film contact. If the opening is recessed into a trench, this can have a V-shaped cross section. But it can also be created with a flat bottom, i.e. with a trapezoidal or rectangular cross section. In that case, the trench floor can be provided with a further texturing, the production of which is proposed, for example, in European patent application EP-A-0 567 764. The depth of the trench can e.g. B. 15 microns, but can also be made flatter or deeper. With the specified layer thicknesses, thin-film contacts can be produced in trenches, the surface of which does not protrude above the level of the oxide layer 3 . Such sunk contacts show good adhesion even under heavy mechanical stress and are therefore well protected against tearing or other damage. However, it is also possible to make the thin-film contacts thicker so that they are raised in relation to the rest of the upper surface of the oxide layer, as is shown, for example, in FIG. 6.

With the method according to the invention, the production is fine structured thin film contacts with structure widths up to significantly improved and simplified down to 20 µm.

The trench technology known per se also has the advantage that the contacts generated in the trenches have a larger con have clock area to the semiconductor as a corresponding planar applied contacts with the same footprint. In the Gra bentechnik can therefore be used with the same performance of the contacts produce narrower metallization paths that too less shading of the solar cell surface ren.

The thin film contacts created by chemical deposition are stabilized in a tempering step at 300 to 450 ° C. This also improves the efficiency of the solar cell.  

Fig. 7 shows a schematic representation of a possible arrangement of the thick film and thin film contacts on the back of the solar cell. The contacts are each in the form of a comb, the wider "comb back" each representing the bus structure for the finer "teeth" (= finger-shaped contacts). The "teeth" of the comb-shaped thick film structure 6 are in the teeth of the likewise comb-shaped thin film structure 7 in a contact-free manner such that a minimum distance between the structural edges of approximately 100 μm remains.

Fig. 8 shows a further possibility for the geometric design of the contacts on the back of the solar cell according to the invention. The finger-shaped contacts are designed here in the form of double-comb structures, wherein combs are pushed into one another without contact under different polarities.

Claims (12)

1. Solar cell

• - With a crystalline semiconductor body ( 1 ) made of p-Si
• - with a textured front ( 2 )
• - With an oxide layer ( 3 ) on the back
• - With finger-shaped thick film contacts ( 6 ) printed on the oxide layer from a silver-containing paste
• - With finger-shaped, in openings ( 4 ) in the oxide layer ( 3 ) applied metallic thin layer contacts ( 8 , 9 ),

wherein the fingers of the different finger-shaped contacts te engage alternately without contact, and wherein a highly doped p⁺ region ( 7 ) below the thick film contacts and a highly doped n⁺ region ( 5 ) below the thin layer contacts are arranged such that the highly doped regions do not overlap. 2. Solar cell according to claim 1, in which the thin-film contacts ( 8 , 9 ) consist of a nickel layer ( 8 ) reinforced with a further metal, the further metal ( 9 ) being copper or silver. 3. Solar cell according to claim 2, in which a palladium seed layer is arranged between the nickel layer ( 8 ) and the semiconductor body ( 1 ). 4. Solar cell according to one of claims 1 to 3, with the finger-shaped contacts in the form of a comb Structure are formed. 5. A method for producing a solar cell according to claim 1 with the steps

• - Texturing of the front side by crystal-oriented etching of a surface of the semiconductor body ( 1 )
• - Oxidation of the semiconductor body to produce an oxide layer ( 3 ) at least on the opposite side of the texturing
• - Structuring the oxide layer ( 3 ) on the back to create finger-shaped openings ( 4 ) therein
• - Carrying out a phosphorus diffusion to produce an n⁺-doped region ( 5 ) in the semiconductor body ( 1 ) in the region of the openings ( 4 )
• - Generation of the thick film contacts ( 6 ) by printing and baking an aluminum and silver-containing paste on the back between the finger-shaped openings ( 4 )
• - Open the openings again
• - Generation of the thin film contacts ( 8 , 9 ) by chemical metallization of the exposed in the openings semiconductor body and
• - Annealing the semiconductor body.

6. The method according to claim 5, in which a nickel layer ( 8 ) is first chemically deposited for metallization and this is then reinforced by chemical deposition of a further metal ( 9 ) selected from copper and silver. 7. The method according to claim 6, wherein the free etching of the openings ( 4 ) is carried out with an acidic solution containing palladium and fluoride ions, where a palladium seed layer is formed on the surface of the semiconductor body ( 1 ) exposed in the openings. 8. The method according to claim 6 or 7, in which the deposition of silver, or of nickel and sil ber, or of copper or of nickel and copper light indu adorned. 9. The method according to any one of claims 5 to 8,  with an antire over the textured front flex layer is generated. 10. The method according to any one of claims 5 to 9, wherein the structuring of the oxide layer ( 3 ) is carried out with a laser. 11. The method according to any one of claims 5 to 9, in which a photo lacquer technique is carried out for structuring the oxide layer ( 3 ) and in which the openings are etched into the oxide layer.