DE4217428A1 - High performance silicon crystalline solar cell structure - has more highly doped layer integrated in lightly doped layer in area below metallic contact - Google Patents

Abstract

The solar cell structure is preferably a crystalline silicon solar cell with np structure based on p-type silicon substrate and n-type emitter on the front side facing the sun. On the side of the solar cell exposed to the radiation is a layer of lightly doped n+ or p+ layer (2 or 7). In the area of the ohmic contact surface (3/6 or 8/11) of the metallic contact (6 or 11) in the lightly doped layer (2 or 7), more highly doped n++ or p++ layers (3 or 8) are integrated. USE/ADVANTAGE - Esp.for one sided or two-sided illumination. Can be manufactured more simply and achieve an optimal degree of effectiveness.

Description

The invention relates to a high-performance solar cell structure tur, in particular for a one-sided or two-sided Irradiation is provided, preferably crystalline sili Zium solar cell with NP structure based on p-type Si silicon substrates and n-type emitters attracted to the sun turned front.

In order to obtain solar cells with the highest efficiency, is it u. a. necessary to realize structures that one the highest possible collection efficiency for the La produced own manure carriers. This includes minimizing the Recombination in the areas near the surface.

For the common crystalline silicon solar cell with np structure based on p-type silicon substrates and n-type emitter on the front of the sun facing leg this holds the use of passivation layers to the Surfaces, advantageous in combination with a back egg tenfeld, which results from a higher doped p⁺ layer animals. Because in the area of the ohmic contact area a very There is a high recombination rate, their area percentage be as low as possible without resistance to contact transition loss of position occur.  

Theoretical calculations (e.g. Saitoh et al., Techn. Di gest Int. Photovoltaic Science and Engineering Conf., Tokyo, p. 83 (1987)) for determining the design of the doped zones have given the following guidelines for optimal design : For areas with a high surface recombination rate (e.g. ohmic contact areas) the surface concentration of the doping should be high (e.g. <10 ²⁰ cm ^-3 for n-type emitters) and the doped zone flat (e.g. < 0.3 µm). For areas with a low surface recombination rate (e.g. areas with surface passivation), the surface concentration of the doping should be low (for example 5-10 × 18 ¹⁸ cm ^-3 ) and the doped zone deep (for example 1-2 µm).

Previous high-performance silicon solar cells mostly resemble the structure shown in FIG. 1 (J. Zhao et al., Proceed. 22nd IEEE Photovoltaic Specialists Conference, Las Vegas, October 7-11, 1991), often in a simplified form. The more heavily doped n ⁺⁺ or p ⁺⁺ layer is initially produced through photolithographically defined openings in a masking oxide by diffusion of a dopant. After removing the masking oxide on the corresponding cell side, there is a full-surface diffusion to produce the low-doped n⁺ or p⁺ layer areas. Further process steps are the removal of the doped glass layers remaining on the surfaces after diffusion, the growth of the passivation oxide, and the photolithographic definition of the openings in the passivation oxide for the production of the ohmic contact between the more highly doped n ⁺⁺ and p ⁺⁺ layers and then contact the metal, which is usually applied using vacuum evaporation technology. Finally, a two-layer anti-reflective coating (AR layer) is usually applied.

This manufacturing process is very complex and therefore little inexpensive. Because of the tolerances in the photolithography The higher doped layer must also be phased areas are made significantly larger than those in ohms contact to the silicon metallic contact surfaces, which leads to less than optimal results.

Another structure that is easier to realize is shown in FIG. 2. Here, first (in the example shown only on the front side) the less doped layer is produced and the oxide which acts equally as a passivation, masking and antireflection layer is applied thereon. This then trenches are produced in silicon using an Nd-Yag laser beam or mechanical processes. The resulting disturbances of the crystal structure in the trench area require a subsequent wet chemical etching step to remove the disturbed zone.

The higher-doped n ⁺⁺ and p ⁺⁺ regions are then produced. After reducing the oxide thickness to the thickness of λ / 4 · n (λ = wavelength of light at the reflection minimum, n = refractive index) required for an optimal anti-reflection effect, the metal contacts are produced by electroless deposition of Ni / Cu layers in the trenches.

This manufacturing process, which at first seems simple, has however, there are also several disadvantages: the reduction of the oxide thickness on λ / 4 · n is complex and difficult to control. Except this gives the silicon oxide because of its not optimal Refractive index (1.4 instead of 2.3) is not the optimal optical Adjustment for the cell embedded in the module. Besides, he require the galvanic contact manufacturing processes complex Measures to reduce environmental pollution. The trenches also reduce the stability of the solar cell, in particular the thin ones aimed at saving material Solar cells. The various wet chemical processes  finally the trench etching step, there are also one Mass production in a continuous process ent against, whereby a reduction in manufacturing costs only limited is possible.

Ultimately, all structures proposed so far fulfill for high-performance solar cells not the perfect way theoretically just to achieve optimal efficiencies conditions and also require too expensive pro processes for their manufacture or have others like the u. a. disadvantages mentioned above.

The invention is therefore based on the object of a high power solar cell structure of the type mentioned create that in a simplified manner compared to the previous gene process can be prepared that the theoretical Meets requirements and with the optimal us degree of efficiency can be achieved.

The object is achieved in that deep, low-doped n⁺- or p⁺-layers are provided on the solar cell sides intended for irradiation, and in these layers in the area of the ohmic contact surfaces of the metallic contacts, flat the surface of higher doped n ⁺⁺ or p ⁺⁺ layers are integrated.

An advantage of the invention is that the new Structure for mutual irradiation can be provided. Is only one-sided radiation pre-applied see, a simplified variant is also conceivable. So you can the new one only on the side intended for irradiation Structure executed and on the other hand one of the be knew simpler structural features (e.g. only one homogeneous doped zone) can be used.  

A method according to the invention for producing an above high-performance solar cell structure described fol process steps on:

• a) on the end faces of a silicon substrate low doped deep n⁺ layer or a low do deep p⁺ layer (with low surface conc tration) manufactured over the entire surface,
• b) on the low doped, deep layers creates a passivation oxide layer,
• c) on the passivation oxide layers are absorption layers of silicon nitride, titanium oxide or tantalum oxide upset,
• d) in the area of the intended ohmic contact areas or of the metallic contacts are in the absorption layers and the passivation layers diffusion window opened by irradiation with laser light,
• e) in the window openings, the higher doped flat n ⁺⁺ or p ⁺⁺ layers (with high surface concentration) are made by diffusion of dopants, and
• f) the metallic cones on the absorption layers clocking the window openings into the absorption layer and fill in the passivation oxide layers brings.

A further development of the method according to the invention draws is characterized in that after the application of the absorption Layers of silicon nitride, titanium oxide or tantalum oxide a silicon oxide layer in egg on each of these layers ner thickness required for diffusion masks is applied, and that these masking oxide layers when irradiated abge with laser light in the areas of the window openings will be worn and in the manufacture of the more highly endowed Layers of the absorption layers during the diffusion protect the process.  

Further refinements of the method according to the invention are described in subclaims 4 to 12.

In the drawing, an embodiment according to the inven tion is shown, and that FIGS . 3 and 4, which follow the prior art and a Fig. 1 and 2 described above , each show a schematic representation of a high-performance solar cell cross-sectional structure.

These high-performance solar cell structures consist of a silicon substrate 1 , on the end faces of which a low-doped deep n⁺ layer ( 2 ) or a low-doped p⁺ layer ( 7 ), in each case a passivation oxide layer ( 4 ) or ( 9 ), each have an absorption layer ( 5 , 10 ), which can be designed as an anti-reflective layer, and electrical contacts ( 6 ) and ( 11 ) are produced. The structure shown in FIG. 4 additionally has the silicon layers (masking oxide layers) ( 12 ) and ( 13 ) covering the absorption layers (AR layers) ( 5 ) and ( 10 ), their meaning and manufacture is described below.

Both structures shown, which are each intended for a bilateral irradiation, have in the area of ohmic contact areas 3/6 or 8/11 on the surface of higher doped flat n ⁺⁺ or p ⁺⁺ layers, which in the low-doped n⁺- or p⁺-layers ( 2 ) or ( 7 ) are integrated and which are in electrical contact with the electrical contacts ( 6 ) or ( 11 ).

The method for realizing the structures shown in FIGS . 3 and 4 begins with the full-area production of the low-doped deep layers ( 2 ) and ( 7 ) by means of the usual methods, for example diffusion methods. The passivation oxide layers ( 4 ) and ( 9 ) are produced next (for example by thermal oxidation).

A diffusion mask is required to produce the more highly doped layers ( 3 ) and ( 8 ) by means of diffusion of doping atoms, for which a sufficiently thick (<100 mm) silicon oxide layer is usually used. The most flexible and cost-effective method for opening the diffusion window in this oxide is the use of a laser beam, which, however, must not lead to damage to the underlying silicon layers. Therefore, the use of the (long-wave) Nd-YAG laser used so far is ruled out. Also, the use of a short-wave excimer laser is initially not a solution. B. is so low for laser light of wavelength λ = 240 nm in silicon oxide, the underlying silicon layer would be damaged by penetrating laser light.

In order to avoid such damage, the absorption layers ( 5 ) and ( 10 ) are first produced on the passivation oxide layers ( 4 ) and ( 9 ). For this, e.g. B. some 100 Å thick layers of silicon nitride, titanium oxide or tantalum oxide can be used. These absorb laser light with λ = 240 nm even at such low radiation levels that are not sufficient to damage silicon. Since these layers are also common as anti-reflective coatings, the thickness of λ / 4 · n was tested. Experiments showed that the absorption layers of this thickness very well meet the requirement to carry out removal without damage to the underlying silicon layers. Although z. B. find silicon nitride as a diffusion mask use, however, the diffusion would adversely affect its optical properties so much that it is no longer sufficient for a good anti-reflective coating.

Therefore, a silicon oxide layer ( 12 ) or ( 13 ) is applied over the absorption layers ( 5 ) and ( 10 ) designed as an AR layer by means of conventional methods (e.g. CVD deposition) in the thickness required for diffusion masks, which at the same time protects the underlying absorption layer during diffusion. As a further process step, he then follows the opening of the diffusion window by irradiation with laser light which penetrates through the masking oxide layer into the absorption layer and is absorbed therein, as a result of which the layer material evaporates and the masking oxide layer is also lifted off. The window openings produced in this way immediately followed by the production of layers ( 3 ) and ( 8 ) with higher doping on the surface, preferably by means of conventional diffusion processes, since the passivation oxides with thicknesses of 100-300 Å required for good optical and passivating properties do not pose any impairment for the diffusion. The masking oxide layers ( 12 , 13 ) can either remain wholly or at least partially on the surfaces, since they are optically neutral in the embedded cell (see FIG. 4), or z. B. by conventional Ätzverfah ren in hydrofluoric acid mixtures (see Fig. 3), if after the production of the contacts ( 6 , 11 ), a second AR coating to achieve a double-layer AR layer is to be applied. Since in particular the silicon nitride layers tempered at the high diffusion temperatures are insoluble in weak hydrofluoric acid solutions, complete removal of the masking oxide is possible without any problems in this case. The subsequent establishment of contact can be carried out using all customary methods, e.g. B. the inexpensive screen printing technology can be used for this.

Modifications of the structure shown in FIGS . 3 and 4, such as with dot or line-shaped areas ( 3 ) or ( 8 ) highly doped on the surface, and the associated window openings of the same size in combination with finger-shaped or full-surface contact layers, are also possible while maintaining the basic new structural features and the process for their production.

The above described an essential feature of the Erfin Process step shown for opening windows in masking layers without damage to underlying Silicon layers could also advantageously be used in the Manufacture of other electronic components used will.

Claims (11)

1. High-performance solar cell structure, which is provided in particular for one-sided or two-sided radiation, preferably crystalline silicon solar cell with np structure based on p-type silicon substrates and n-type emitter on the front facing the sun, characterized in that on the sides of the solar cell seen for irradiation deep n an or p⁺ layers ( 2 or 7 ) which are low doped on the surfaces, and that these layers ( 2 or 7 ) in the area of the ohmic contact areas ( 3/6 or 8/11 ) of the metallic contacts ( 6 or 11 ) flat n ⁺⁺ or p ⁺⁺ layers ( 3 or 8 ) which are more highly doped on the surface are integrated. 2. A method for producing a high-performance solar cell structure according to claim 1, characterized by the following process steps:

• a) on the end faces of the silicon substrate ( 1 ), a lightly doped deep n⁺ layer ( 2 ) or a lightly doped deep p hergestellt layer ( 7 ) (with a low surface concentration) is produced over the entire surface,
• b) a passivation oxide layer ( 4 , 9 ) is produced on each of the low-doped deep layers ( 2 , 7 ),
• c) on the passivation oxide layers ( 4 , 9 ) from sorption layers ( 5 , 10 ) made of silicon nitride, titanium oxide or tantalum oxide,
• d) in the area of the intended ohmic contact surfaces ( 3/6 or 8/1 ) of the metallic contacts ( 6 or 11 ) in the absorption layers ( 5 , 10 ) and the passivation layers ( 4 , 9 ) diffusion windows are opened by irradiation with laser light ,
• e) the higher doped flat n ⁺⁺ or p ⁺⁺ layers ( 3 , 8 ) (with high surface concentration) are produced in the window openings by diffusion of dopants, and
• f) on the absorption layers ( 5 , 10 ), the metallic contacts ( 6 , 11 ), which fill the window openings in the absorption layers ( 5 , 10 ) and passivation oxide layers ( 4 , 9 ).

3. The method according to claim 2, characterized in that after the application of the absorption layers (5, 10) of silicon nitride, titanium oxide or tantalum oxide to these layers (5, 10) each serving as a Maskierungsoxidschicht silicon oxide layer (12 or 13) in one for Diffusion masks required thickness is applied, and that these masking oxide layers ( 12 , 13 ) are removed during the irradiation with laser light in the areas of the window openings and in the manufacture of the more highly doped layers ( 3 , 8 ) the absorption layers ( 5 , 10 ) protect during the diffusion process. 4. The method according to claim 3, characterized by the use of a CVD deposition method for producing the silicon oxide layers ( 12 , 13 ). 5. The method according to claim 2 or 3, characterized in that the deeply doped n⁺ or p⁺ layers ( 2 , 7 ) are produced by a diffusion process on the end faces of the silicon substrate ( 1 ). 6. The method according to claim 2, 3 or 5, characterized by growing the passivation oxide layers ( 4 , 9 ) by thermal oxidation on the deep doped layers ( 2 , 7 ). 7. The method according to claim 2 or 3, characterized in that the absorption layers ( 5 , 10 ) serve as anti-reflective coatings and are applied with a thickness of λ / 4 · n, where λ be the wavelength and n the refractive index of the material of the absorption layers to draw. 8. The method according to claim 2, 3 or 7, characterized in that an anti-reflective layer is applied to the absorption layers ( 5 , 10 ) and the contacts ( 6 , 11 ), which optionally on the absorption layers th ( 5 , 10 ) existing masking oxide layers ( 12 , 13 ) are removed beforehand. 9. The method according to claim 8, characterized by an etching process in hydrofluoric acid mixtures for removing the masking oxide layers ( 12 , 13 ). 10. The method according to claim 2, 3, 7 or 8, characterized in that the anti-reflective coatings ( 5 , 10 ) by vapor deposition, spin coating, spraying, by thermal processes or by a CVD process who who. 11. The method according to claim 2 or 8, characterized in that the production of the metallic contacts ( 6 , 11 ) is carried out by screen printing technology.