DE19525720C2 - Manufacturing process for a solar cell without front-side metallization - Google Patents

Description

The invention relates to a manufacturing method for a Solar cell without front-side metallization, in which both the n and p contacts on the back are located. The front will not be touched shadowed and can be used without restriction Light exposure can be used.

A solar cell without front-side metallization is 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, pp  157 to 165 known. For their production, in several mask steps differently doped areas generated side by side and by applying one multilayered metal structure metallized or contacted. The metal structures are applied by Thin film process.

The disadvantage here is that the method several Mask steps required and is therefore complex.  

From JP 5-75149 A a solar cell is without Front metallization known, which is also only possible through is a complex process to produce. So be with With the help of several masking and etching steps lowered areas on the back of the solar cell. An oxide layer is applied thereon, on which in turn Screen printing process the two rear contacts in shape an aluminum-containing silver paste applied and be branded. The aluminum penetrates when baked through the oxide layer into the substrate.

Against this background, the invention exists problem in specifying a method by which a Solar cell without front-side metallization, which adheres well Contacts and high efficiency, simple and is safe to manufacture.

This problem is solved by that in claim 1 specified procedures. Advantageous further developments of the Procedures result from the subclaims.  

The invention uses an advantageous combination of thick film and thin film processes for the production of the p- and the n-contacts. A similar method for a conventional solar cell with front and rear contacts is known for example from WO 93/19492 A1. As is known, the semiconductor body of the solar cell produced according to the invention consists of p-doped silicon, the front side of which is textured to improve the light incidence geometry. On the back there is a passivation layer, preferably an oxide layer. Finger-shaped openings in which the n-contacts are in the form of metallic thin-film contacts are recessed in this passivation layer. The p-contacts (thick film contacts) are also finger-shaped, intermesh without contact with the finger-shaped thin-film contacts and consist of a printed silver-containing paste. Underneath the finger-shaped contacts are highly doped n ⁺ or p ⁺ regions, which do not overlap with each other and between which a pn junction is formed. The combination of thick film and thin film technology allows alternating contacts to be created in a line grid, which can achieve a grid spacing of at least approx. 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.

In the manufacturing method according to the invention, the contacts and are applied 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 regions in the region of the openings, since only there can the dopant penetrate into the semiconductor body. By appropriately selecting the diffusion conditions, it is possible to precisely limit the highly doped areas or to 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 suitable boron or aluminum connections or metallic aluminum.

In the subsequent sintering process, the thick film contacts are hardened and sintered onto the semiconductor body. At the same time, the dopant contained is driven into the semiconductor body and a good conductive connection between the p ⁺ regions and the printed thick film contacts is made through the passivation layer.

The thick film contacts are printed so that the p ⁺ - doped regions do not overlap with the previously produced in the region of outlets, to n ⁺ -doped areas. Nevertheless, the geometry of the thin-film and thick-film contacts is optimized to 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 deposition of both the nickel layer and the layer of the further metal can be light-induced or can be supported by light irradiation. Charge carrier pairs are generated in the semiconductor body and separated in the internal field of the semiconductor body or the solar cell. As a result, electrons are available at the p ⁺ -doped areas for reducing the metal ions. This method has the advantage that the solution containing the metal ions does not have to have any reducing agents. Stable metallization baths, such as those used in electroplating, are therefore sufficient for light-induced chemical deposition. It should be noted that in addition to the light-induced metal deposition in the n ⁺ -doped areas, the contacts applied over the p ⁺ -doped areas also dissolve. It was found that the printed thick film structures are well suited as a so-called "sacrificial anode". In addition to accelerating the deposition process, the light-induced metal deposition has the further advantage that it can be carried out at lower temperatures than a corresponding “dark deposition” or a corresponding galvanic deposition, for example already at room temperature.

That with the light-induced metal deposition from the Dick 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 step of tempering, both liability and 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.  

Figs. 1 to 6 show cross-sections with reference to schematic views of a solar cell, various methods levels of Her position,

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

Embodiments

Fig. 1: The semiconductor body 1 has a p-doped CZ wafer is chosen with <100 <orientation. 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 , in the exemplary embodiment 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 openings, which correspond to the geometric structure of the desired thin-film contact, can be defined 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 opening represents that in the embodiment 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 the openings 4 in a manner known per ⁺ -doped regions 5 n generated in the semiconductor body 1 by means of a phosphorus diffusion.

Fig. 4: In the next process step, the thick film contacts 6 are produced by printing a conductive paste containing silver. 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 freige in the trenches or openings 4 laid n ⁺ doped surface 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.

In the exemplary embodiment, a metallization bath is usually used 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 brief immersion in or short treatment 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 back 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 about 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 currentlessly from any copper-containing bath, for example from a copper sulfate solution.

Because of the oxidation sensitivity of the nickel layer 8 , the reinforcing copper layer 9 is 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 this case, the trench floor can be provided with a further texturing, the production of which is described, for example, in EP 0 567 764 A1. The depth of the trench can be 15 µm, but can also be made shallower 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 countersunk contacts show good adhesion even with 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 compared to the rest of the upper surface of the oxide layer, as shown 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.

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. 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 (8)

1. A method for producing a solar cell comprising the steps:

• 1. - Texturing of the front side of a semiconductor body ( 1 ) by crystal-oriented etching of a surface;
• 2. - Generation of a passivation layer ( 3 ) at least on the back of the semiconductor body opposite the texturing;
• 3. - structuring of the passivation layer ( 3 ) on the back to create finger-shaped openings ( 4 ) therein;
• 4. - Carrying out a phosphorus diffusion, n ⁺ -doped regions ( 5 ) in the semiconductor body ( 1 ) being formed exclusively in the region of the openings ( 4 );
• 5. - Printing a silver-containing paste, which also contains boron or aluminum compounds or aluminum as p-type dopant, on the back between the finger-shaped openings ( 4 );
• 6. - Generation of thick film contacts ( 6 ) by burning in the paste and simultaneously driving in the p-dopant to produce p ⁺ -doped regions ( 7 );
• 7. - Re-etch the openings ( 4 );
• 8. - Self-adjusting generation of the thin-film contacts ( 8 , 9 ) by chemical metallization of the semiconductor body exposed in the openings; and
• 9.- Annealing the semiconductor body.

2. The method according to claim 1, 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. 3. The method as claimed in claim 2, in which the openings ( 4 ) are etched free with an acidic solution containing palladium and fluoride ions, a palladium seed layer forming on the surface of the semiconductor body ( 1 ) which is exposed in the openings. 4. The method according to claim 2 or 3, in which the deposition of silver, or of nickel and Silver, or copper, or nickel and copper is light-induced. 5. The method according to any one of claims 1 to 4, one more over the textured front Anti-reflective layer is generated. 6. The method according to any one of claims 1 to 5, wherein the structuring of the passivation layer ( 3 ) is carried out with a laser. 7. The method according to any one of claims 1 to 5, in which a photoresist technique is carried out for structuring the passivation layer ( 3 ) and in which the openings are etched into the passivation layer. 8. The method according to any one of the preceding claims, characterized in that an oxide layer ( 3 ) is generated as a passivation layer by oxidation of the semiconductor body ( 1 ).