EP2586067A2

EP2586067A2 - Solar cell

Info

Publication number: EP2586067A2
Application number: EP11705887.5A
Authority: EP
Inventors: Hans-Joachim Krokoszinski
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2010-04-26
Filing date: 2011-03-01
Publication date: 2013-05-01
Also published as: CN102893406B; KR20130073900A; DE102010028189A1; JP2013526045A; CN102893406A; WO2011134700A2; US20130104975A1; DE102010028189B4; US9209321B2; WO2011134700A3

Abstract

The invention relates to a solar cell comprising: a passivation layer sequence provided with a plurality of island-shaped recesses in which the passivation layers are completely removed, on the second main surface; a thin first metal layer arranged on the passivation sequence layer and in the recesses on the substrate surface; a thin dielectric cover layer covering the first metal layer and comprising a first regular arrangement of narrow linear openings and a second regular arrangement of essentially wider linear or elongate island-shaped openings, the first and the second opening arrangements being aligned at an angle, in particular perpendicular to each other. Said solar cell also comprises a second metal layer which is highly conductive and can be soldered on the open surface in the openings of the first and second opening arrangements, said second metal layer making contact with the first metal layer.

Description

description

title

The invention relates to the field of the semiconductor components, in particular the solar cells, and relates to a crystalline solar cell with an n- or p-doped semiconductor substrate and a passivated back. State of the art

Silicon solar cells with a "passivated backside" have an improved optical mirroring and a much improved passivation of the rear surface compared to the previously standard aluminum back surface field (BSF), see A. Götzberger et al., "Sonnenenergie: Photovoltaics ", BG Teubner Stuttgart, 1997. The cell concept thus produced is called" passivated emitter and rear cell "(PERC), where the dielectric passivation adapted to the backside doping is locally opened at many small points, so that the metal layer deposited on the passivation layer can contact the semiconductor, but only at a small area fraction of the back surface, to minimize the strong recombination of the electron-hole pairs on metallized surfaces.

The metallization of the rear side is in most cases made of aluminum and is deposited over the entire area on the entire back, usually with vacuum vapor deposition or sputtering.

Documents which relate to such solar cells and methods for their production and are associated with rear-side structuring steps and / or the rear-side driving in of dopants are, for example, DE 19525720 C2, DE 102007059486 A1 or DE 102008 013446 AI and DE 102008033 169 AI (both latter of ErSol Solar Energy AG).

The efficient production of reliable connection structures, in particular under the influence of soldered joints, is concerned for example with JP 2005027309 A, DE 102008017312 A1 or DE 102008020796 A1.

The method of "Laser Fired Contacts" (LFC) is known, in which the metallization on the back passivation is fired through the passivation layer with laser pulses, so that a prior opening of the passivation layer becomes unnecessary, see "Laser beam method for fabrication crystalline silicon solar cells ", dissertation Eric Schneiderlöchner, Albert Ludwig University Freiburg im Breisgau (2004) or the (former) DE 199 15666 AI.

For cells on p-doped wafers, the formation of "local BSF regions" at the contact points in the backside passivation is done simply by alloying the aluminum into the p- or p ⁺ -surface. Si-eutectic temperature of 577 ° C. For this, it is necessary that the backside passivation layer and also the front emitter (after previously sintering the front silver paste) survive these temperatures without damage.

For cells on n-doped wafers, where the backside emitter (pn junction) is made with boron or aluminum doping, the formation of the Al-Si eutectic, which typically melts to a few microns, would increase a breakthrough through the thin p ⁺ emitter into the n-base and there lead to a short circuit to the base. Therefore, the metallization with low temperatures (eg 400 ° C, optionally also in forming gas) must be tempered to a sufficiently good ohmic

Contact the emitter surface without damaging the cell. This makes it impossible to fire a normal screen-print paste to subsequently deposit a solderable silver layer on the aluminum. All PERC technologies known from the prior art have the following disadvantages: 1) Common to all concepts is that the back at the end of the

Process with a large aluminum layer is present, which does not have solderable pad areas.

2) The low-resistance of the metallization necessary for the power line can only be achieved by means of a sufficiently thick aluminum layer, as a rule

4 μιτι be prepared. If the vapor deposition or sputtering rate is set high to keep the process times short, the

Heat input and thus the temperature of the wafer in the process chamber strong too. If the rate is kept low, more time or a longer run-time system with additional vapor or steam must be used

Sputter sources are provided for the aluminum coating. In any case, the compromise involves higher investment and / or process costs.

3) An alternative is a thinner aluminum layer which can be produced in a reasonably short time with a moderate deposition rate, but then a chemical or galvanic strengthening of the Al backside metallization is needed. After all, this takes place at very moderate temperatures (<90 ° C) and thus does not represent a thermal load on the almost finished solar cells.

4) If one finds a suitable method to chemically or galvanically reinforce aluminum, but ultimately the entire back would be reinforced with silver, which would be a significant cost factor.

Disclosure of the invention

Proposed is a solar cell with the features of claim 1 and a method for producing such, which has the features of claim 10. Advantageous further developments of the inventive concept are the subject of the respective dependent claims.

The proposed solar cell is characterized by a thin dielectric cover layer covering the first metal layer, which has a first regular arrangement of narrow line-like openings and a second regular arrangement of substantially wider lines or elongated island-like openings, wherein the first and second opening arrangement at an angle, in particular transversely to each other, are aligned. It is further characterized by a highly conductive and solderable on the exposed surface second metal layer in the openings of the first and second opening arrangement, which contacts there the first metal layer.

The solar cell according to the invention with a passivated and metallized large back side, which was chemically or galvanically reinforced in the open in the dielectric cover layer narrow finger areas and busbar or Lötpadbereichen current collecting, possesses the case in any case the following advantages over the prior art:

1) The solar cell has in addition to the usual large-area

Aluminum layer with local contacts to the semiconductor surface also solderable areas (busbars or soldering pads).

2) The chemically / galvanically reinforced, d. H. very conductive fingers on the back collect the power everywhere on the large wafer surface and pass it low impedance to the busbars or pads. The generated current, leaving the local contact points from the solar cell, must travel only a small distance in the metal layer sequence to the next plating finger. Therefore, this metal layer sequence of very thin metal layers, for. B. 0.5 μιτι aluminum with 0.1 μιτι nickel seed layer consist. It can therefore be produced with moderate deposition rates in such a short time that the cells are not heated too much.

3) The large-area metal layer sequence remains even after completion of the chemical or galvanic reinforcement by the dielectric cover layer against chemical attack, z. B. Corrosion in the module, over 25 years life protected.

4) Perform thin metallization and local amplification

unlike the hitherto conventional screen printing metallization of the backside, not a wafer bending. This makes it possible to increase the wafer further reduce thickness / cell thickness and thus save costs for silicon.

In a technologically and cost-effective embodiment, the second metal layer comprises at least one metal of the group comprising Pd, Ni, Ag, Cu and Sn. In particular, in this case the second metal layer has a sequence of metal layers, in particular the layer sequence Pd / Ni / Ag or Ni / Cu / Sn.

Furthermore, it is provided that the second metal layer is produced by a chemically or galvanically deposited reinforcing layer.

A further embodiment provides that the thin dielectric cover layer comprises at least one of the silicon oxide, silicon nitride,

Comprising alumina, aluminum nitride and titanium oxide.

In a first variant, the substrate is a p-doped silicon substrate with a phosphorus-doped emitter on the first main surface. In an alternative embodiment, the semiconductor substrate is an n-doped silicon substrate having a boron- or aluminum-doped p ⁺ emitter on the second major surface.

In particular, the first and second openings in the thin dielectric capping layer are formed by masked ion etching, particularly in the same plant in which the capping layer is formed. Alternatively, provision may be made for the first and second openings in the thin dielectric cap layer to be formed by laser ablation or by etching using an etch paste applied by screen printing or ink jet printing. drawings

Incidentally, advantages and expediencies of the invention emerge from the following description of an exemplary embodiment with reference to the figures. From these show:

Fig. 1 to 3 in perspective schematic representations of an initial situation and a first and second process section of the production of a

Embodiment of the solar cell according to the invention,

4A and 4B are schematic plan views of two embodiments

a contact structure of the solar cell as well

5 and 6 in schematic perspective views of a third process section.

In all figures, it remains unclear whether it is a p-doped or n-doped wafer. In the case of the n-type material, the emitter may be located either on the front side or on the back side. In the latter case, a doping for a front surface field (FSF) is usually produced on the front side. The starting point of the following description is almost complete

produced on two sides to be contacted solar cell on p- or n-doped crystalline silicon 1 (Figure 1). The front side 2a is already completely processed, d. H. doped homogeneously or selectively, with passivation / antireflection coating or layer sequence and provided with a solderable contact grid. The back side 2b may be undoped or a homogeneous doping 3

which represents an emitter or a BSF.

The surface is coated by means of chemical vapor deposition (CVD) or physical vapor deposition (PVD, ie vapor deposition or sputtering) with a passivation layer or layer sequence 4 which has been selected and optimized with regard to doping polarity and doping concentration and which is provided with a technique known per se from the prior art locally removed (opened). The resulting dot matrix, in which the local contact openings 5 are arranged, depends on the sheet resistance of the surface: in the case of undoped surfaces (PERC cell), the distance between the points D

smaller than with doped surfaces (PERT cell, ie full-area doped cell).

The first process step according to the invention begins with the full surface coating (known per se from the prior art) with a metal layer sequence 6 (FIG. 2), it being possible to use either vapor deposition technology or sputtering technology. It may be z. B. aluminum which is optionally covered in the same plant with a thin nickel-containing seed layer (not shown) for later chemical or galvanic reinforcement. Alternatively, of course, other metals or metal layer sequences can be selected, for. Titanium / palladium / silver or nickel chromium / nickel / silver.

Finally, in all cases (in the same system, without interruption of the vacuum), the entire backside is covered with a thin dielectric thin film 7. In this case, all the deposited layers lie both on the passivation layer sequence 4 and on the semiconductor surface exposed in the local openings 5 in the passivation layer. The covering layer 7 may be an oxide or a nitride of aluminum or silicon. It can be evaporated or reactively sputtered from the metal target or deposited by RF sputtering from the dielectric target. In the second step, with a suitable technique, the dielectric

Cover layer in the form of many, narrow preferably equidistant and parallel to each other arranged lines 8 open (Figure 3). In this case, for example, the following techniques known per se from the prior art can be used: laser ablation, screen-printed or ink-jet printed etching paste, inkjet-masked wet-chemical etching. The distance W of the parallel openings 8 is typically of the order of 1mm to 10mm, preferably 2mm to 5mm. The width of the openings is typically about ΙΟΟμιτι to 1mm, preferably 200pm to 500 μιτι. In 90 ° to the course of the narrow openings 8 are wider, ie opened in the same opening step, wider areas 9a and 9b in the dielectric cover, in which later solderable busbars or solder pads are to be generated. Bus bar strips 9a (FIG. 4A) can have any desired width optimized for the soldering process. It could only be 2 busbars. Pad areas 9b (FIG. 4B) can be as many as desired and of any size. They can be arranged along 2 or 3 lines, depending on the number and location of the busbars on the front. In the third step, the metal surface exposed in the openings 8, 9a, 9b in the covering layer is reinforced in a suitable chemical or galvanic deposition process with a highly conductive and readily solderable metal layer sequence (FIG. 5). Depending on the exposed metal, for example, it may be palladium, nickel and silver or nickel, Copper and tin act. If aluminum with a nickel seed layer has been chosen, nickel will likely be deposited in the first bath, which will then be covered with sufficient silver. In the same amplification process, the wider busbar or pad regions 9a or 9b are also post-amplified and solderable with the same layer sequence 11 (FIG. 6). The post-reinforced narrow fingers 10 open either directly into the reinforced busbar surfaces IIa or solder pad surfaces IIb, or into narrow connecting lines between the pads (see FIG. Incidentally, the embodiment of the invention is not limited to the above-described examples and emphasized aspects, but only by the scope of the appended claims.

Claims

claims

1. A solar cell with an n- or p-doped semiconductor substrate, in particular made of silicon, which has a serving as a light incident side first main surface and serving as a back second main surface, a passivation layer sequence with a plurality of island-like recesses in which the passivation layers completely a thin first metal layer on the passivation layer sequence and in the recesses on the substrate surface, a thin dielectric covering layer covering the first metal layer, the first regular arrangement of narrow line-like openings and a second regular arrangement substantially wider line-like or elongated island-like openings, wherein the first and second opening arrangement are oriented at an angle, in particular transversely to each other, and a highly conductive and at the exposed O. Surface solderable second metal layer in the openings of the first and second opening arrangement, which contacts there the first metal layer.

The solar cell according to claim 1, wherein the second metal layer comprises at least one metal of the group comprising Pd, Ni, Ag, Cu and Sn.

Solar cell according to claim 2, wherein the second metal layer has a sequence of metal layers, in particular the layer sequence Pd / Ni / Ag or Ni / Cu / Sn.

Solar cell according to one of the preceding claims, wherein the first metal layer comprises at least one metal of the group comprising Al, Ti, Pd, Ag, NiCr, Ni and Ag.

Solar cell according to claim 4, wherein the first metal layer has a series of metal layers, in particular the layer sequence Ti / Pd / Ag or NiCr / Ni / Ag or Al / Ni seed layer.

Solar cell according to one of the preceding claims, wherein the second metal layer has a chemically or galvanically deposited reinforcing layer.

A solar cell according to any one of the preceding claims, wherein the thin dielectric capping layer comprises at least one material of the group comprising silicon oxide, silicon nitride, aluminum oxide, aluminum nitride and titanium oxide.

A solar cell according to any one of the preceding claims, wherein the semiconductor substrate is a p-doped silicon substrate having a phosphorus doped emitter on the first major surface.

A solar cell according to any one of claims 1 to 7, wherein the semiconductor substrate is an n-doped silicon substrate having a boron or aluminum doped p ⁺ emitter on the second major surface.

A method of manufacturing a solar cell according to any one of the preceding claims, wherein the first metal layer is formed by a vacuum vapor deposition or sputtering method.

The method of claim 10, wherein the second metal layer is formed using a chemical or galvanic amplification bath.

The method of claim 10 or 11, wherein the first and second openings in the thin dielectric capping layer are masked. Ion etching are formed, in particular in the same plant in which the cover layer was produced.

13. The method of claim 10, wherein the first and second openings in the thin dielectric cap layer are formed by laser ablation or by etching using an etch paste applied by screen printing or ink jet printing.