WO2000061520A1

WO2000061520A1 - Silicon moulded parts with electronic surface passivation that is stable over time

Info

Publication number: WO2000061520A1
Application number: PCT/EP2000/002712
Authority: WO
Inventors: Christian HÄSSLER; Michael Mager; Harald Kraus
Original assignee: Deutsche Solar Gmbh
Priority date: 1999-04-09
Filing date: 2000-03-28
Publication date: 2000-10-19
Also published as: AU4111800A; DE10080839D2; DE19916061A1; DE10080839B4

Abstract

The invention relates to silicon moulded parts with a passivating surface coating. Said coating is a sol-gel material and is applied using wet-chemical methods.

Description

Silicon molded parts with stable electronic surface passivation over time

The present invention relates to molded silicon parts with stable electronic passivation of the silicon surfaces by applying a surface coating.

Due to the disturbed crystal perfection, the surfaces of crystalline silicon molded parts represent surfaces of high so-called surface recombination speed. Charge interfaces can effectively recombine via interface states and thus reduce the performance of electronic components, the structures of which are usually produced particularly close to the surface. To produce high-quality components, a recombination of charge carriers on the surface of the pane must be prevented.

Surface recombination is particularly important for solar cells. The number of charge carriers generated by light decreases with increasing depth of penetration of the light into the usually disk-shaped silicon semiconductor material, i.e. maximum charge carrier concentrations are reached directly below the surface of the pane. To achieve high levels of efficiency, it is therefore essential to effectively prevent recombination of the charge carriers on the surface of the pane.

Low surface recombination speeds are also useful for analytical purposes, such as process control using lifetime measurements according to the μPCD

(Microwave Photoconductive Decay) process required on monitor panes. In order to be able to make statements about the quality of the silicon volume here, the surface of the wafer has to be largely negligible as a recombination center. The reduction in the surface recombination rate is usually achieved in silicon technology by applying a silicon oxide or silicon nitride layer. Silicon oxides are generated by oxidation of the silicon surface in an oxygen stream at temperatures from 800 ° C to 1200 ° C. Typical pyrolysis times range from a few minutes to a few hours; alternatively the

Oxidation also takes place with the addition of water vapor (SM Sze in Physics of Semiconductor Devices, John Wiley & Sons, New York, 1981, pages 66-67 or A. Goetzberger, B. Voß, J. Knobloch in Solar Energy: Photovoltaik, Teubner-Verlag , Stuttgart, 1994, pages 158-160). Resistively heated tube furnaces are usually used to apply the oxides, in which the ones to be coated are used

Slices are oxidized batch-wise.

SiN layers are primarily used in solar cell technology. They are produced by deposition in a plasma CVD (Chemical Vapor Deposition) system at temperatures around 400 ° C. For the separation of high quality

Up until now, shifts have only been possible in batch mode, there are further technical problems e.g. in the temperature homogeneity within the deposition system (an overview is given in A.G. Aberle et al., Proceedings of the 14 "European Solar Energy Conference (Barcelona), 1997, pages 684-689).

The deposition of silicon oxide or silicon nitride leads, according to the known methods, to a time-stable and also a very good electrical passivation of the silicon surfaces. Surface recombination speeds v _s of less than 100 cm / s are achieved (untreated surface approx. 10 "- 10 ⁶ cm / s), which can be used, for example, for highly efficient solar cells with efficiencies of greater than 15%. However, the separation of Oxides and nitrides have the disadvantage that complex and expensive high-temperature processes have to be used which additionally only allow a limited throughput (batch processes). In addition to the generation and deposition of oxides or nitrides, other methods for passivating silicon surfaces are based on electrostatic charging of the surfaces or the use of reactive liquids, such as hydrofluoric acid, HF, or alcoholic iodine solution (L. Li et al. In J. Appl. Phys. 77 (3), (1995) 1323-1325). The application can take place at room temperature. However, such passivations have the disadvantage of only being of a temporary nature. As a result, they are in principle not suitable for such molded silicon parts that are designed for long-term use, for example in the field of solar cell technology.

Also with iodine-containing, rosin-based coatings, as in

As described in EP 0 706 207, very good electronic passivation is achieved for a short time, but this is also not permanent. The passivating effect decreases significantly after only a few hours, as described in W. Arndt et al., Electrochemical Society Proceedings Volume 95-30, pages 44-53.

The object of the present invention was therefore to provide molded silicon parts with a time-stable electronic passivation of the surface without using the high-temperature processes which have the disadvantages mentioned.

Surprisingly, it has now been found that the object can be achieved in that the surface coating is a sol-gel material.

The sol-gel material can be produced by wet-chemical application of a sol-gel solution.

Silicon moldings with a passivating surface coating were found, which are characterized in that the coating is a sol-gel material.

A time-stable electronic surface passivation in the sense of the invention is understood to mean that there is no decrease in the passivating effect over a period of one month. Lose according to the current state of technology Coatings applied wet-chemically (eg rosin containing iodine) have a passivating effect very quickly within hours, as already explained above.

Sol-gel materials in the sense of the invention are three-dimensionally cross-linked, optionally organically modified amorphous glasses, which are obtained by hydrolysis and condensation reactions of low molecular weight compounds, such as e.g. Silicon alkoxides can be obtained. They have at least one structural element of the formula (I)

[R ^, - (E), -] _J -SiR ² _k (O _1/2 ) ₄ . _k , (I), where

i is 0 or 1, j is 1, 2 or 3 and k is 0, 1 or 2 and k + j <3

E is oxygen or sulfur as well

R ^{1 is} an optionally substituted alkyl or aryl radical or a bridging alkylene or arylene radical and R ^{2 represents} an optionally substituted alkyl or aryl radical.

The sol-gel materials preferably have a structural element of the formula (II)

[R ^, ], - SiR (0 _1/2 ) ₄ . _k . _l (II). in which

j is 1, 2 or 3 and k is 0, 1 or 2 and k + j <3 R ^{1 is} a bridging C, - to C ₈ -alkylene radical and

R ^{2 represents} an optionally substituted alkylene or aromatic residue. The sol-gel solutions particularly preferably have a structural element of the formula (III)

[R'f-SiR ^ O, ^ (III), where

j is 1, k is 0, 1 or 2 and k + j <3

R ^{1 is} a bridging C, to C ₄ alkylene radical and

R ^{2 is} a methyl or ethyl radical.

The formulation "O1 / 2" in formulas (I), (II) and (III) denotes bridging difunctional oxygen, e.g. thus a structural element Si-O-Si.

If in formula (I), (II) or (III) R ^{1 is} an alkylene radical, this is preferably linked to a chain-shaped, star-shaped (branched), cage-shaped or, particularly preferably, ring-shaped siloxane radical. The ring-shaped siloxane radical can be, for example, [-O-Si (CH ₃ ) -] ₃ or [-O-Si (CH ₃ ) -] ₄ .

The sol-gel materials with chain, star, cage or ring-shaped siloxane residues are formed when appropriate polyfunctional organosilanes are used in the sol-gel solution, e.g. B. polyfunctional silanols and / or alkoxides. Their oligomers, d. H. their hydrolysis and / or

Condensation products. The following may be mentioned as preferred compounds:

a) Si [(CH ₂ ) ₂ Si (OH) (CH ₃ ) ₂ ] ₄

b) cyclo- {OSiMe [(CH ₂ ) ₂ Si (OH) (CH ₃ ) ₂ ]} ₄ c) yc / o- {OSiMe [(CH ₂ ) ₂ Si (OC ₂ H _s ) ₂ ) (CH ₃ )]} ₄

d) c ^ c / o- {OSiMe [(CH ₂ ) ₂ Si (OCH ₃ )) (CH ₃ ) ₂ ]} ₄

e) cyc / o- {OSiMe [(CH ₂ ) ₂ Si (OC ₂ H ₅ ) ₃ ]} ₄

The sol-gel materials are produced after application of the sol-gel coating solution to the silicon surface by evaporating volatile constituents and curing. The application can be carried out according to all common methods, such as dipping, spraying, flooding, spinning, knife coating or pouring, the

Silicon molding can be coated on the entire surface or only on parts thereof.

The sol-gel materials are produced, for example, by mixing suitable low-molecular compounds in a solvent, followed by

Adding water and optionally catalysts, the hydrolysis and / or condensation reaction is initiated. The execution of such sol-gel processes is basically known to the person skilled in the art. For example, the syntheses of polyfunctional organosilanes and organosiloxanes as well as processes for the production of corresponding sol-gel coating solutions, from which in turn the sol-gel

Materials of the formula (III) can be obtained, described in EP 0 743 313, EP 0 787 734 and WO 98/52992.

The hardened sol-gel material forms a highly transparent layer, which is particularly important for use on solar cell surfaces, since no appreciable light absorption may occur in the passivation layer. This would significantly reduce the efficiency of the solar cell.

A molded silicon part according to the invention, coated with sol-gel material, has surface recombination speeds in the range of approx. 1000 cm / s, while significantly higher values in the range of 10 ⁵ - 10 ⁶ cm / s are found for untreated silicon surfaces.

To reduce the reflection of visible light, one or more anti-reflective layers can additionally be applied on, under or between several layers of sol-gel material. By appropriate modification of the sol-gel material, e.g. by incorporating highly refractive substances such as titanium oxides, this can even reduce the reflection of incident light.

In a preferred embodiment of the present invention, the coated silicon moldings are silicon wafers with a wafer thickness between 100 μm and 3 mm. The silicon wafers or layers are particularly preferably solar cells. The layer thickness of deposited silicon layers is usually 100 μm or less.

In a further preferred embodiment of the invention, the solar cells are those in which the n-doped layer comprises the so-called emitter, i.e. represents the side facing the light during operation of the solar cell.

The disc-based solar cells are e.g. by producing a pn-doping transition - for example by diffusing an n-doping from the gas phase into a p-doped wafer - and then applying front and rear contact (an overview regarding cell processing is provided by A. Goetzberger, B. Voß, J. Knobloch in "Solar Energy: Photovoltaics", Teubner Verlag Stuttgart 1994). The p-doping is preferably achieved by deliberate contamination of the

Silicon with an element from the 3rd main group (e.g. boron), which generates n-doping through targeted contamination with an element from the 5th main group (e.g. phosphorus). The solar cells based on silicon layers (layer thickness about 50 μm or less) can e.g. by depositing silicon from the gas or liquid phase onto suitable substrate materials (e.g. glass, ceramics or

Silicon itself). The pn doping transition can here can be generated for example by admixing the doping elements in the process gases.

In the following, the invention will be explained using examples.

Preliminary remarks

The effect of the surface passivation was checked by lifetime measurements according to the μPCD (Microwave Photoconductive Decay) method. Charge carriers are generated by a laser pulse and the decay of this photo-generated charge carrier concentration is detected by microwave reflection. The disc was installed during the measurement so that the side of the disc coated with the sol-gel material was facing the laser and microwave measuring head. The time constant of the decay of the charge carrier _{concentration} is the so-called effective lifetime τ _efT , which is generally given by

^T ef ^{= X} bulk ^{+ τ} surf ^■ (1)

Τ _{bul is} the volume lifespan of the silicon material and τ _{surf is} the lifespan that results from the recombination effect of the surface.

Still applies

^L surf d ² / (π ² D) + d / 2S (2)

where d is the slice thickness, S the surface recombination speed and D the diffusion coefficient of the minority charge carriers (approx. 10 cm ² / s for holes in the present n-type Si).

For silicon wafers of common thicknesses (mm range) and unpassivated surfaces (high values of the surface recombination speed S in the range from 10 ⁵ -

10 ⁶ cm / s), τ _{blll is} much larger (approx. 20 μs) than τ _surf . so that the measured effective lifetime τ _{cf (} according to equation (1) is approximately equal to τ _surf . Example 1: Preparation of a sol-gel solution based on cyclo-

{OSi (CH ₃ ) [(CH ₂ ) ₂ Si (CH ₃ ) ₂ (OH)]} ₄

22.0 g ςyc / o- {OSi (CH ₃ ) [(CH ₂ ) ₂ Si (CH ₃ ) ₂ (OH)]} ₄ (33.9 mmol) and 28.3 g Si (OC ₂ H ₅ ) ₄ (135.8 mmol) are dissolved in 44.8 g of ethanol (pA) and with stirring, 4.8 g of one

0.1 n solution of p-toluenesulfonic acid in water. The mixture is stirred at room temperature for 60 minutes and can then be used as a coating solution. It can be processed for about 3 hours. The sol-gel coating thus produced can be dried at room temperature, the coating being grip-resistant after a few minutes and after a few hours

Final hardness reached, or at elevated temperature.

Example 2:

One of the surfaces of silicon wafers n-doped with phosphorus (surface

10x10 cm) with a thickness of approx. 300 - 400 μm was coated with the sol-gel solution from Example 1.

Before coating, the pane was removed by etching in an acid mixture of HF (40%), HNO ₃ (65%) and H ₃ P0 ₄ (85%) in a mixing ratio of 1: 3: 3 on the surface and about 20 μm then the natural oxide layer is removed by etching in HF (5%). The solution from Example 1 was applied to one side in a layer thickness of about 15 μm after rinsing the pane with deionized water using a doctor blade; the sol-gel coating was dried at room temperature.

Coating with a sol-gel material increases the effective service life τ _f from 8.9 μs to 15 - 16 μs (see Fig. 1). Since the volume lifespan τ _bl ., _K is not changed by the surface coating, this increase in the effective lifespan can be increased according to equation (1) to τ _sιπ , and thus according to equation (2) to a significant reduction in surface _{recombination} . speed S can be reduced. From an effective service life of 16 μs after coating, an upper limit for the surface recombination speed S of approx. 2 × 10 ³ cm / s can be estimated from the condition τ _surf > τ _eff (follows from equation 1) with the help of equation (2). By coating with a sol-gel material, S was reduced by approximately two orders of magnitude. Fig. 1 also shows that the effective lifespan is stable at around 15 μs even after 165 days.

Example 3:

In this case, multicrystalline silicon solar cells with pn doping transition were provided with a sol-gel coating. The solar cells were made from 5x5 cm ² large p-doped (boron-containing) multicrystalline silicon wafers with a thickness of approx. 350 μm. The boron concentration of the wafers was about 2χl0 ¹⁶ cm ^"3. The pn junction of the solar cells was produced by phosphorus diffusion (n-doping) from the gas phase at 820 ° C. for about 1 hour. The back contact was then used as Four-layer system (Al / Pd / Ti / Ag) evaporated and sintered for about 1 hour at 620 ° C. The front contact consists of a vapor-deposited lattice structure of the layer sequence Ti / Pd / Ag.

The efficiency of the solar cells produced in this way was determined under white light with an irradiance of 100 mW / cm ² to be 7.72% (solar cell 1) and 7.70% (solar cell 2). The solar cells were then provided with a sol-gel coating on the front side of the solar cells (n-doped emitter side). There was no further pretreatment of the solar cell surface. The coating with the sol-gel solution from Example 1 with a thickness of approximately 15 μm was applied again using a doctor blade. Drying was carried out either at room temperature

(Solar cell 1) or by tempering at 130 ° C for 1 hour in a drying cabinet. After coating, efficiencies of 9.15% (solar cell 1) and 9.31% (solar cell 2) were determined, ie the reduced surface recombination due to the coating led to a significant increase in efficiency of approximately 20%. the relative increase in efficiency for the coating dried at 130 ° C was slightly higher than for that dried at room temperature. The increase in efficiency is sustained, so after 41 days efficiencies of 9.17% (solar cell 1) or 9.24% (solar cell 2) were measured, and after 55 days 9.07% (solar cell 1) or 9.25% ( Solar cell 2). Example 4:

A solar cell was produced analogously to Example 3 and coated analogously to Example 3 with a sol-gel material. The drying took place at room temperature. In this case, the increase in efficiency was monitored over a longer period of time in order to test the stability of the passivation effect. Before coating, the efficiency was 6.84%, directly after coating the efficiency rose by 11.1% relative to 7.6%. Efficiency was 7.67% after 42 days, 7.68% after 56 days, 7.66% after 66 days, 7.63% after 79 days, 7.68% after 87 days and 7.75% after 101 days . Obviously, the coating allows for

Sol-gel material ensures stable surface passivation of solar cells even over several months.

Example 5: Preparation of a sol-gel solution based on CH ₃ Si (OC ₂ H ₅ ) ₃

10 g of CH ₃ Si (OC ₂ H ₅ ) ₃ are dissolved in 3.5 g of isopropanol (p. A.) and 1.5 g of a 0.1 N solution of p-toluenesulfonic acid in water are added with stirring. The mixture is stirred at room temperature for 60 minutes and can then be used as a coating solution.

Example 6:

Analogously to Example 2, a silicon wafer n-doped with phosphorus (after analog pretreatment) was coated with the sol-gel solution from Example 5 and dried at room temperature.

The coating with the sol-gel material increases the effective lifespan τ _l from 7.1 μs to a little more than 20 μs, whereby the increase remains in the long term (measuring time 150 days) (see Fig. 2). This value corresponds to an upper limit of the surface recombination speed S of approx. 10 × 10 cm / s.

Claims

claims

1. Molded silicon parts with passivating surface coating, characterized in that the coating is sol-gel material.

2. Molded silicon parts according to claim 1, characterized in that the sol-gel material is constructed from a network which has at least one structural element of the formula (I)

[R ¹ - (E) _i -] _r SiR ² _k (Oι _{/ 2} ) ₄ . _k , (I), where

i is 0 or 1, j is 1, 2 or 3 and k is 0, 1 or 2 and k + j <3

E is oxygen or sulfur as well

R ^{1 is} an optionally substituted alkyl or aryl radical or a bridging alkylene or arylene radical and

R is an optionally substituted alkyl or aryl radical,

having.

3. Silicon molded parts according to claim 1 and 2, characterized in that the

Sol-gel material is built up from a network that at least one structural element of the formula (II)

[R'l-SiR C / z kj (II), where j is 1, 2 or 3 and k is 0, 1 or 2 and k + j <3

R ^{1 is} a bridging Ci to C ₈ alkylene radical and

R ^{2 is} an optionally substituted alkyl or aryl radical,

having.

4. Silicon molded parts according to claim 1 to 3, characterized in that the

Sol-gel material is built up from a network that has at least one structural element of the formula (III)

[R ¹ ] _j -SiR ² _k (O _1/2 ) ₄ . _k . _j (HI), where

j is 1, k is 0, 1 or 2 and k + j <3 R ^{1 is} a bridging Ci to C ₄ alkylene radical and

R ^{2 is} a methyl or ethyl radical,

having.

5. Molded silicon parts according to one of claims 2 to 4, characterized in that the alkenyl radical Rl is linked to a chain, star, cage or ring-shaped siloxane radical.

6. molded silicon parts according to one or more of claims 1 to 5, characterized in that the sol-gel coating additionally with an anti¬

Reflective layer is provided.

7. molded silicon parts according to one or more of claims 1 to 6, characterized in that the surface thereof is only partially coated.

8. A process for the production of molded silicon parts according to one or more of claims 1 to 7, characterized in that the sol-gel coating solution is applied wet-chemically to an optionally pretreated silicon surface.

9. A method for producing molded silicon parts according to claim 8, characterized in that the drying and / or curing of the wet layer takes place at a temperature below 200 ° C.

10. A method for producing molded silicon parts according to one or more of claims 8 to 9, characterized in that the sol-gel coating solution mixed with other embedding or coating materials customary for molded silicon parts, in particular those materials which form an anti-reflex -Layer are suitable, is applied.

11. Use of the silicon molded parts according to one or more of the claims

1 to 7 for the production of solar cells, solar modules or silicon components.

12. Solar cells, solar modules and silicon components containing silicon molded parts according to claims 1 to 7.