WO2014180471A1 - Solar cell and method for producing same - Google Patents

Abstract

Solar cell having a solar cell substrate, preferably a silicon solar cell substrate, having a rough rear side and an aluminum-containing rear side contact, which is preferably printed on, and method for producing a solar cell having method steps comprising texturing a solar cell substrate at least on a rear side of the solar cell substrate, diffusing boron into the rear side of the solar cell substrate, and printing an aluminum-containing paste onto the rear side of the solar cell substrate for the purpose of forming a rear side contact.

Description

 Solar cell and process for its production

Crystalline silicon solar cells with dielectric back coating are now state of the art. Likewise, backside passivated solar cells exist.

Also, there are numerous publications on bifacial solar cells which are diffusion-coated on the backside, but which, unlike this invention, have a silver grid on the backside

There is a serious problem with solar cell types with dielectric backside passivation and full-surface backside metallization with local contacts: If the backside passivation is not tight enough, parasitic contacts occur in the area of the entire backside metallization due to locally trapped aluminum. These unwanted, parasitic contacts are electrically connected in parallel with the specially prepared desired contact openings. The parasitic contacts reduce the efficiency of the solar cells. Particularly in the case of rough, textured solar cell backs, the effect of the parasitic contacts is so strong that no gain in efficiency over conventional solar cells with full-surface aluminum back surface field (Al-BSF) is possible.

The behavior of the parasitic contacts is still poorly studied. In contrast to the desired contact openings, the parasitic contacts seem to be characterized by locally high electrical resistances and high recombination losses. Presumably, no local Al-BSF forms here in contrast to the deliberately prepared contact openings. The occurrence of these parasitic contacts is proven: If, for example, a "Centaurus" solar cell dispenses with the laser opening of the backside dielectric, the solar cell should have an efficiency of 0.0% for a 100% dense dielectric, in fact such solar cells have up to 5% efficiency Electroluminescence images (see below) show a "starry sky", with parasitic contacts distributed over the entire cell surface and on the silver pad edges.

So far, no inexpensive and sufficiently dense backside passivation was found. Since the problem of parasitic contacts occurs especially on rough, textured backsheets, they have been chemically polished so far, which is very expensive especially with multicrystalline wafers and HF / HN03 polish.

Another known alternative is a back-printed, self-firing grid, which leads to solar cells for bi-facial applications. Logically, no spitting can take place at the non-pressed areas since no metal is present here. In order to obtain a sufficiently high conductivity in a backside grid, expensive silver pastes are usually used here.

Another problem that occurs with backside dielectrically passivated multicrystalline cells is the fact that in this cell type a fill factor loss is frequently observed which can not be explained by either the recombination described above or the series resistance of the base material (transverse conductivity) alone. One hypothesis for this additional fill factor loss is that the band bending at grain boundaries and the band bending due to the positive charge of the silicon oxide or the silicon nitride lead to an interaction on the cell back, which significantly inhibits the near-surface transverse conductivity in the grain boundaries. This thesis has not yet been proven, but appears as a plausible explanatory model. Direct solutions are not known yet, but the effect seems to be less with negative surface charge (eg A1203) or not present.

The invention enables the fabrication of backside dielectrically passivated solar cells using a low cost, rough, textured backside surface provided with a boron-backfilled surface area (boron-BSF), followed by a non-perfectly dense backside dielectric and a full-surface or almost full-surface metallized back. The boron-BSF now leads to a passivation of the resulting parasitic contacts.

As an alternative to boron diffusion, a boron-doped dielectric applied over the entire area can be applied to the rear side. The P0C13 diffusion used in the further process sequence drives the boron into the Si wafer by means of co-diffusion or by a separate heating step. The reverse side is completed by a full-surface or almost full-surface metallization. Just like the boron-boride boron-boride (BSF), the boron-BSF from the boron-doped dielectric leads to a passivation of the resulting parasitic contacts, which results in an improvement in cell parameters. The process flow of the new Centaurus-Bor solar cell for multicrystalline wafers is as follows:

Process sequence 1: 1. Texturing wafers on both sides (acidic, eg NH ₃ / HF) and cleaning (chemical)

 2. Boron diffusion for passivation of parasitic contacts and / or optionally optimization of the transverse conductivity at grain boundaries or "passivation" of the grain boundaries

3. (possibly boron glass sets)

 4. Backside passivation (PECVD layers)

 5. Front-side removal of any surface-diffused layers (chemical)

 6. Cleaning

 7. Phosphorus diffusion

 8. Optionally laser diffusion for the production of selective emitters

9. Local laser opening of the dielectric of the cell backside

10. Phosphorglasät ze and cleaning

 11. Front passivation (PECVD layers)

 12. Metallization

For monocrystalline wafers, the following process sequence has instead proven itself:

Process sequence lb:

 1. Texturing monocrystalline wafers on both sides, acidic, z. B.

NH ₃ / HF, non-alkaline, and cleaning (chemical)

 2. Boron diffusion for passivation of parasitic contacts and / or optionally optimization of the transverse conductivity at grain boundaries or "passivation" of the grain boundaries

3. (possibly boron glass sets)

 4. Backside passivation (PECVD layers)

 5. Front-side removal of any surface-diffused layers (chemical). First, remove the boron glass (if any) on the front of the cell with an etching mixture containing HF, followed by an alkaline cell surface texture.

The back of the cell remains due to the backside passivation protected. Possible alternative: Use of a one-side etch unit for the one-sided removal of boron glass only on the front of the cell. The remaining boron glass on the back of the cell then protects against the alkaline texture.

6. Cleaning

 7. Phosphorus diffusion

 8. Optionally laser diffusion for the production of selective emitters

9. Local laser opening of the dielectric of the cell backside

10. Phosphorglasät ze and cleaning

 11. Front passivation (PECVD layers)

 12. Metallization

The result is a solar cell with an alkaline texture on the front. The cell edges (i.e., the approximately 150-200pm "thick" faces) and the cell backside are acid textured, and this edge treatment results in extremely little breakage.

The process flow of the new Centaurus-Boron solar cell for multicrystalline wafers with boron-doped backside dielectric looks like this:

Process sequence 2:

13. Texturing wafers on both sides (acidic, eg NH ₃ / HF) and cleaning (chemical)

 14. Boron-doped dielectric on the back of the cell. The boron is subsequently driven into the Si wafer by co-diffusion. The boron passivates the parasitic contacts

 15. Cleaning, possibly gloss edge on front side when wringing the dielectric by means of e.g. Remove HF, the chemistry used depends on the dielectric used

16. Phosphorus diffusion and tempering to diffuse boron from dielectric into silicon wafers 17. Optional laser diffusion for the production of a selective phosphorus emitter

 18. Local laser opening of the dielectric of the cell backside

19. Phosphorglasät ze and cleaning

20. Front passivation (PECVD layers)

 21. Metallization

The boron doped backside dielectric process is also feasible and useful on monocrystalline material. In this case, however, it would be preferred not to make an acidic texture in step 1.), but an alkaline (e.g., KOH / IPA). The Centaurus solar cell concept is supplemented by process points. The additional process steps are shown in italics.

Process sequence 3:

 22. Wafer sawing etching on both sides

23. Boron-doped dielectric on the back of the cell. The do ^¬ oriented dielectric acts as a back-surface field; The boron is subsequently converted into the Si by co-diffusion.

Wafer driven. The boron passivated parasitic con ^¬ tacts

 24. Cleaning and texture

 25. POC13 diffusion

26. Laser diffusion on the front to form a selective phosphorus emitter

27. Depending on the nature of the selected Rückseitendielektri ^¬ kums possibly no laser opening on the back needed.

 In this case, the AI back side must fire at random through the dielectric and make good contact with the Si wafer. The boron in the

Dielectric passivates the resulting contacts.

 28. Phosphorglasät ze and cleaning

29. Front passivation (PECVD layers) metallization; The Al paste used may be more aggressive than standard pastes and will etch locally at random through the PECVD back side.

 31. Wafer texturing on both sides

32. Boron-doped dielectric on the back of the cell. The do ^¬ oriented dielectric acts as a back-surface field; The boron is subsequently driven into the Si wafer by co-diffusion. The boron passivated parasitic con ^¬ tacts

 33. Cleaning, texture can be omitted

34. POC13 diffusion, evtl- with annealing to boron from back ^¬ sided dielectricum in the Si wafer 35. diffuse laser diffusion on the front side for forming a selective emitter phosphorus

 36. no laser opening on the back, due to boron saturation, the parasitic contacts are passivated, and due to the texture on the cell back, the AI back preferably contacts the texture tips

 37. Phosphor glassware and cleaning

 38. Front passivation (PECVD layers)

39. metallization; The Al paste used is more aggressive than standard pastes and locally randomly etches through the PECVD backside. Especially with a texture on the back, the texture peaks are not / not so thickly covered with the PECVD layer, so that the AI ^better water into the Si et passivate and saturate the parasitic contacts.

The advantages of the invention are:

Passivation of parasitic contacts on backside dielectrically passivated solar cells 2. Passivation of parasitic contacts by boron-doped dielectrics on the back of the cell

 3. Prevention of the rear laser opening step in centaurus technology through the use of a boron doped dielectric on the back of the cell, or an aggressive Al paste

 4. Possibility of dispensing with polishing processes of the cell back in the centaurus process

 5. Higher efficiency than conventional Centaurus solar cells through

 a. good passivation of the inevitable parasitic contacts

 b. good passivation of the silver solder pads

 c. Low-impedance connection of the cell edge.

 d. Good transverse conductivity between the back contact areas

 e. Possibly good passivation of the back grain boundaries by "back surface field" (see, e.g., N.P. Härder et al., 31st IEEE-PVSC 2005, p.491-494).

 f. Possibly improving the transverse conductivity of silicon near the cell backside across grain boundaries

 6. Use of inexpensive aluminum pastes instead of silver pastes. The requirements for the Al pastes depend on the concept used in FIG. 5. It is conceivable that in the process sequences 22.) -30.) And 31.) -40.) More aggressive Al pastes must be used in order to ensure contact with the dielectric used.

Possible modifications:

 • Conductors made of printed aluminum in interdigitated

back contact (IBC) solar cells Instead of boron diffusion, an a-Si: B, a boron-containing dielectric or the like can be deposited. Driving either in a separate annealing step or in 7.)

• If necessary, process steps 2-4 can be replaced by a process step (deposition of a B-containing dielectric). Annealing can be done either in a separate process step or together with 7.)

• omission of process step 9.) (local opening of the

 Back), instead using a more aggressive one

 (firing) metal-containing paste for Rückseitenkon- taktierung

To supplement this, FIG. 1 shows electroluminescent images of dielectrically passivated cells without rear-side contact opening. Both with and without boron diffusion the parasitic contacts are visible as white dots. The boron diffusion does not prevent the contact formation, but causes their passivation. In the present case, a rough back side of a solar cell substrate is understood to mean a non-polished and non-polish- or smooth-etched back side, which has a corresponding roughness. The rear side of the solar cell substrate or solar cell is that side which, during operation of the solar cell, is arranged facing away from the incident light. Preferably, at least the back has a texture.

Under a rear side contact containing aluminum is presently to be understood a backside contact whose electrical conductivity is largely determined by AI contained in the back contact. Accordingly, an aluminum-containing paste is to be understood as a paste in which, after application and contact sintering / firing, a back contact formed by this paste has an electrical conductivity. which is essentially determined by AI included in the back page contact.

The invention is not limited to the embodiments described above or illustrated in the figures - not even in terms of functional features. The previous description as well as the figures contain numerous features, which are given in the dependent subclaims in part to several summarized. However, those features as well as all other features disclosed above or in the figures will also be considered individually by those skilled in the art and combined to make meaningful further combinations. In particular, all features can be combined individually and in any suitable combination with the method and / or the solar cell of the independent claims.

Although the invention has been described by means of exemplary embodiments, the invention is not restricted by the disclosed examples, and other variants can be derived by the person skilled in the art without departing from the scope of the invention.

Claims

Having solar cell  A solar cell substrate, preferably a silicon solar cell substrate, with a rough back side;  - An aluminum containing backside contact, which is preferably printed. Solar cell according to claim 1, characterized , that the backside contact is substantially free of silver. Solar cell according to one of the preceding claims, d a d u c h e c e n e c e n e, that is textured on the back of the solar cell substrate. Solar cell according to one of the preceding claims, d a d u c h e c e n e c e n e, a dielectric passivation layer is arranged on the rear side of the solar cell, which is preferably free of aluminum oxide and is particularly preferably formed from silicon nitride and / or silicon oxide. Solar cell according to one of the preceding claims, d a d u c h e c e n e c e n e, in that the rear-side contact is designed as a planar back contact which covers the back side of the solar cell substrate by more than 50%, preferably substantially completely. 6. Solar cell according to one of the preceding claims, d a d u c h e c e n e c e n e,  the solar cell substrate is a multicrystalline solar cell substrate. 7. A process for producing a solar cell having Verfahrenssehritte  - Texturing a solar cell substrate at least on a back side of the solar cell substrate;  - the indiffusion of boron in the back of the solar cell substrate;  - Printing an aluminum-containing paste on the back of the solar cell substrate for the purpose of forming a backside contact. 8. The method according to claim 7,  characterized ,  in that a paste which is substantially free of silver is used as the aluminum-containing paste. 9. The method according to any one of claims 7 to 8,  characterized ,  a boron-containing dielectric is applied to the rear side of the solar cell substrate as the boron source for the diffusion of boron into the back side of the solar cell substrate. 10. The method according to any one of claims 7 to 8,  characterized , the boron is diffused into the backside of the solar cell substrate by BBr ₃ tube diffusion.