DE102016225120A1 - Method for reducing the optical reflection on solar cells - Google Patents

Abstract

The present invention relates to a method for reducing the optical reflection of solar cells, which comprise at least one active layer of a semiconductor material, to which a passivation and / or antireflection layer is applied. In the method, not the layer of the semiconductor material but the passivation and / or antireflection layer is textured with a plasma etching process to form nanostructures in order to reduce the optical reflection at the solar cell. The method avoids the problems of damage and contamination of the layer of the semiconductor material and thus achieves a reduction of the optical reflection without a negative impact on the efficiency of the solar cell.

Description

Technical application

The present invention relates to a method for reducing the optical reflection of solar cells, comprising at least one active layer of a semiconductor material, on which a passivation and / or anti-reflection layer is applied, wherein the optical reflection on the solar cell by texturing to form nanostructures is reduced.

The reduction of surface reflection on solar cells is very important to increase their efficiency. The industrial standard for reducing the reflection of crystalline silicon solar cells is the wet-chemical texturing by HF / HNO ₃ or by KOH and IPA (IPA: isopropyl alcohol). These wet-chemical textures produce surface structures on a micrometer scale and reduce them by multiple reflections on the surface Solar cell whose total reflection. This technology is limited by the geometric optics (structure width> light wavelength) and therefore achieves only a moderate reduction of the reflection. A promising alternative to this is the so-called maskless plasma texturing of silicon by fluorine- or oxygen-containing plasmas, which is also known by the term black-silicone method. This technique allows near-perfect reflection of the silicon surface by plasma textures that make the silicon wafer appear black. The physical basis of this antireflection coating are nanostructures (structure width <light wavelength) on the silicon surface, which are smaller than the resolution of the incident light. As a result, the incident light experiences no refractive index jump at the heterojunction of air / silicon, which leads to a reduction of the reflection.

Up to now, capacitively coupled plasmas have been used for maskless plasma texturing of silicon. Capacitive coupling of the plasma using asymmetric electrode surfaces results in the formation of a negative self-bias between the plasma and the silicon surface. This negative self-bias of up to several 100 V causes an acceleration of the positive ions in the plasma in the direction of the silicon surface, whereby it is damaged. The surface damage induces carrier recombination centers, which significantly reduces the efficiency of solar cells.

An important process step during the solar cell processing is the passivation of the silicon surface in order to saturate open bonds, which act as recombination centers for the light-generated charge carriers and thus can significantly reduce the efficiency of the solar cell. An industrially deposited SiN passivation layer has a typical thickness of approx. 70-80 nm. The nanostructures generated with the black-silicon method have a structure width of approx. 100 nm. Therefore, a homogeneous homogeneous coverage of the plasma texture by a SiN layer, as it is produced in the solar cell processing for passivation, not possible without further process steps. In addition, plasma-induced defects and impurities at the interface of silicon and SiN reduce the efficiency of solar cells.

This is one of the main reasons why the black silicone method has not been integrated into existing manufacturing processes until today. In addition, the plasma texture increases the effective surface area of the silicon, thereby increasing the contribution of surface recombination to the overall charge carrier combination. In addition, an efficient passivation of the plasma-induced effects generated by the ion bombardment described above by means of SiN is not yet possible.

Another problem is that the plasma-exposed metallic surfaces within the process chamber result in surface contamination of the silicon surface, which also reduces carrier lifetime. Depending on the procurement of the process chamber, the silicon surface after texturing is contaminated with iron, chromium, zinc and aluminum. Although these impurities can be removed after the plasma texture step by a wet chemical HF / HCl or HF / HNO ₃ Dip or RCA cleaning. However, this additional wet-chemical process step does not belong to the state of the art in solar cell production and causes additional costs in solar cell production. Because of the aforementioned problem, therefore, the maskless anisotropic plasma texturing is not used to reduce the optical reflections in silicon solar cells on an industrial scale so far.

State of the art

Some of the above problems have heretofore been reduced or eliminated only on a laboratory scale with considerable time and expense in manufacturing.

With a capacitive coupling of the plasma, the self-bias can not be technically avoided with asymmetrical electrode surfaces. However, there are approaches through the lowest possible capacitively coupled power and through Combination with inductive plasma sources to reduce the self-bias and thus also the surface damage.

For this purpose, a combination of a capacitive and an inductive plasma source is usually used. As a result, the self-bias of several hundred volts can be reduced to less than 100 volts, as for example in Gaudig et al., Investigation of the Optoelectronic Properties of Crystalline Silicon Masonry by Plasma Marking at Different Ignition Modes, EU-PVSEC, 2014, pp. 885-888 is described.

The inadequate PECVD SiN surface passivation of plasma textures is currently being circumvented by time-consuming and cost-intensive laboratory-scale technologies. As an alternative, an Al ₂ O ₃ atomic layer deposition (ALD) is used, which is significantly more expensive and time-consuming than a PECVD SiN deposition. This technique is for example in M. Otto et al., "Conformal Al2O3 coatings on black silicon by thermal ALD for surface passivation", Energy Procedia 27, pages 361 to 364, 2012 described. In addition, Al ₂ O ₃ atomic layer deposition requires a wet-chemical pretreatment of the solar wafer (RCA cleaning), which is associated with additional costs. In addition, with heavy contamination and damage to the silicon surface can not be achieved with this method so far satisfactory result.

The object of the present invention is to provide a method for reducing the optical reflection on solar cells, which avoids the problems described above and can be realized more cost-effectively than the techniques previously used on a laboratory scale.

Presentation of the invention

The object is achieved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description and the embodiments. The proposed method can be carried out with solar cells which have at least one active layer of a preferably crystalline semiconductor material, to which a passivation and / or antireflection layer is applied. This layer is preferably a SiN layer. However, it is also possible to use other layers as the passivation and / or antireflection layer, for example the Al ₂ O ₃ layer mentioned above or a layer of silicon dioxide. In the proposed method, the optical reflection is reduced in a known manner by texturing this layer to form nanostructures. This texturing is carried out with a plasma etching process, preferably with a maskless anisotropic plasma etching process. In contrast to the known methods of the prior art, however, in the present method not the layer of the semiconductor material but the overlying passivation and / or antireflection layer is textured. Preferably, the method is applied to finished processed solar cells.

Thus, the interface between the layer of the semiconductor material and the overlying passivation and / or antireflection layer is not increased by the texturing step and is also not exposed to ion bombardment. Nor can plasma-induced impurities reach the layer containing the semiconductor material since it is covered with the passivation and / or antireflection layer during the plasma texturing process. Thus, the proposed method solves all the problems described above. However, the reflection at the passivation and / or antireflection layer is significantly reduced.

The anisotropic plasma etching process for plasma texturing preferably takes place with a fluorine-, hydrogen-, carbon- and / or oxygen-containing gas mixture, for example with an SF ₆ / O ₂ etching gas. Other fluorine and / or oxygen-containing gases can be used. Plasma texturing is also not limited to fluorine- and oxygen-containing gases. The process can be carried out in the same way as in the known black-silicon process, wherein in the proposed method, however, not the semiconductor layer but the applied on the semiconductor layer passivation and / or anti-reflection layer is textured.

The texturing of the passivation and / or antireflection coating can take place after the end of the conventional process chain in solar cell production. The texturing of the passivation and / or antireflection layer then takes place with an already completely processed solar cell, which thus already has the front and rear side contacts.

Furthermore, the texturing of the passivation and / or antireflection coating can also take place within the process chain for the solar cell production after the application of the passivation and / or antireflection coating and before the application of the front and rear contacts.

In the known black-silicon method for reducing the optical reflection of silicon solar cells according to the prior art, the crystalline silicon is directly textured by a capacitively coupled plasma. The silicon surface is exposed to accelerated ions, which damage the silicon surface. This then leads to a lower effective life of the charge carriers generated during operation of the solar cell, whereby the efficiency of the solar cell is reduced. In the method according to the present invention, however, in silicon solar cells, first a passivation and / or antireflection layer, for example a SiN layer, is deposited on the silicon surface. This well-defined and well-known standard methods are used, which ensure high carrier lifetimes. For example, the deposition can be carried out with a PECVD process without ion bombardment (no self-bias). Subsequently, only the deposited passivation and / or antireflection layer is textured by a preferably maskless plasma etching step, so that there is no damage to the underlying silicon surface. Neither the passivation properties of the passivation layer nor the electrical properties of the solar cell are impaired by this step. Therefore, by the method proposed here, higher effective lifetimes can be achieved compared to methods in which the silicon surface is textured.

In the known black-silicon method according to the prior art, the silicon surface is textured by the maskless plasma etching step in the sub-micron range. As a result, however, the effective surface area of the silicon increases by a multiple, which in turn leads to an increased contribution of the surface recombination to the total recombination rate. This results in a lower efficiency of the solar cell. In the method according to the invention, however, in the case of silicon solar cells, the passivating and / or antireflection layer previously deposited on the planar silicon surface is textured by the plasma step. As a result, there is no surface enlargement of the silicon, which leads to a lower contribution of the surface recombination rate. This in turn results in a better solar cell efficiency.

In the method according to the invention, lower plasma-induced contamination of the silicon or semiconductor material is also to be expected since the silicon or semiconductor surface itself is protected by the passivation and / or antireflection layer itself during the preferred plasma texture step. The interface between the semiconductor material and the passivation and / or antireflection layer remains unaffected by the plasma process. Therefore, the silicon or semiconductor surface comes with no contaminants from the plasma or the process chamber in contact. This in turn reduces the overall recombination rate of the generated charge carriers.

An essential advantage of the proposed method is the decoupling of the plasma texture and the surface passivation of the semiconductor layer. In contrast to the classic black-silicon method, not the silicon surface but the passivating and / or antireflection layer deposited thereon, in particular the silicon nitride layer, is plasma-textured. As a result, there is no surface enlargement, contamination or ionic bombardment of the silicon. The electrical properties of the solar cell remain unaffected. The optical properties are improved due to the reduction of the optical reflection of the interface between air and the passivation and / or antireflection layer.

The proposed method is mainly used in the manufacture of solar cells whose surface reflectivity is to be reduced by texturing. The method can be used both in the industrial mass production of solar cells as well as in the laboratory in research institutions, since the method is freely scalable.

list of figures

• 1 eine schematische Darstellung einer gemäß dem Stand der Technik entspiegelten (links) im Vergleich zu einer gemäß dem vorgeschlagenen Verfahren (rechts) entspiegelten Solarzelle;
• 2 ein erstes Beispiel für die Durchführung der vorgeschlagenen Plasmatexturierung bei der Herstellung einer Solarzelle;
• 3 ein zweites Beispiel für die Durchführung der vorgeschlagenen Plasmatexturierung bei der Herstellung einer Solarzelle; und
• 4 eine Darstellung eines schematischen Aufbaus einer Plasmaanlage, die für die Durchführung des vorgeschlagenen Verfahrens geeignet ist.

The proposed method will be explained in more detail below with reference to exemplary embodiments in conjunction with the description. Hereby show:

• 1 a schematic representation of a coated according to the prior art (left) compared to a according to the proposed method (right) coated solar cell;
• 2 a first example of the implementation of the proposed plasma texturing in the manufacture of a solar cell;
• 3 a second example of the implementation of the proposed plasma texturing in the manufacture of a solar cell; and
• 4 a representation of a schematic structure of a plasma system, which is suitable for carrying out the proposed method.

Ways to carry out the invention

The proposed method will be explained below with reference to the reduction of the optical reflection on silicon solar cells, in which on the crystalline silicon layer, an SiN passivation layer is applied. In contrast to the technique used to date for the antireflective coating of such a solar cell with the aid of the already described black-silicone method, the proposed method does not use the Surface of the crystalline silicon layer plasma-textured, but the deposited on SiN passivation layer.

1 shows in a highly schematic representation of a comparison of a silicon solar cell, which was textured with the previous method (left), and a silicon solar cell, which was textured by the method according to the invention (right). Both representations each show the silicon base 1 with the overlying silicon emitter 2 as part of the crystalline silicon layer and deposited on the silicon layer SiN passivation layer 3. In the previously known method according to the black-silicon method was first the surface of the silicon layer, so the silicon emitter 2 , plasma-textured. Subsequently, the SiN passivation layer was then applied, which then at least partially followed the textured surface contour due to its small thickness. This is in the left part of the 1 indicated schematically. However, this approach causes the problems already mentioned in the introductory part of the present specification.

In contrast, in the method according to the invention, the silicon semiconductor layer is not plasma-textured. Rather, the surface of this silicon layer, that is, the non-textured silicon emitter layer 2 , First applied the SiN passivation layer. This can be done with a deposition process by which the surface of the silicon emitter layer 2 is not exposed to ion bombardment. Only then is the surface of the SiN layer plasma-textured, as shown in the right partial image of the 1 is indicated. As a result, the silicon surface is not enlarged by this plasma texturing, the silicon surface is not exposed to ion bombardment and no plasma-induced contaminants can reach the silicon surface. The optical effect of reducing the optical reflection on the solar cell is nevertheless achieved by the texturing. At the same time, the problems associated with the previous methods are avoided, so that the efficiency of the solar cell is not reduced by this process.

In principle, the plasma texturing of the passivation layer can be carried out as a downstream process step after the solar cell production or else within the process chain of the solar cell production. 2 shows schematically the implementation of the plasma texturing after the solar cell production. The solar cell production 4 corresponds here to the industry-standard solar cell production, through which a fully processed solar cell is obtained, which already has the SiN layer, front contacts, back contacts, etc. The SiN plasma texturing step 5 is then performed at the end of this process chain to achieve the improvement of the optical properties, that is, the reduction of the optical reflection on the surface of the solar cell. The texturing of the front contact fingers by the plasma can be neglected.

In principle, the SiN layer or any other passivation layer used in the method can be optimized for electrical properties, for example via the hydrogen content for a better passivation and regeneration behavior or for reducing the specific resistance to reduce the potential-induced degradation susceptibility (PID). In addition, the optical properties of the passivation layer can also be optimized. In this case, the refractive index of the passivation layer is adapted to the refractive index of the underlying semiconductor layer in order to reduce the internal reflection at the interface of the passivation layer to the semiconductor layer.

3 shows an example of a performance of the plasma texturing step within the solar cell manufacturing process chain. In this example, a suitably adjusted four-stage solar cell production is shown, in which the plasma texturing is integrated as an additional process stage. Here, in a known manner in the first process stage a Sägeschadensätze 6 for removing sawing damage on the surface of the silicon wafer. This process step can be carried out, for example, as a plasma etching process with a fluorine-containing etching gas, for example with an SF ₆ etching gas. Also a wet-chemical damage rates 6 is of course possible. In the second process stage, a phosphorus diffusion for the POCl ₃ doping to produce the pn junction in the crystalline silicon and a subsequent Phosphorglasätze (PSG) 7 , In the third process stage, the deposition takes place 8th a SiN passivation layer on the crystalline silicon layer. This SiN layer is finally plasma-textured in the fourth process stage according to the method according to the invention in order to reduce the surface reflection. Plasma texturing 5 takes place preferably with a fluorine- or oxygen-containing gas mixture. In this case, for example, capacitively coupled, inductively coupled or capacitively and inductively coupled plasmas can be used. Other plasma sources may also be used, for example a microwave source. After texturing the surface of the SiN layer, the remaining production steps required to complete the solar cell then become 9 performed in the last process stage, in particular the application of the contact fingers on the textured SiN layer and the firing of these contact fingers through the SiN layer therethrough.

• - Prozesszeit: < 10 Minuten
• - ICP Leistung: > 200 Watt
• - CCP Leistung: < 100 Watt
• - Substrattemperatur: 20 °C
• - Prozessdruck: < 15 Pa
• - CHF₃/O₂ Fluss: < 100 sccm

In the example below, the surface of a SiN layer previously deposited on a silicon wafer is masonry textured by an anisotropic plasma etch to reduce reflection of the surface. This SiN plasma texturing is carried out in a one-step process by a fluorine-, hydrogen-, carbon- and / or oxygen-containing gas mixture. For this purpose, a combination of an inductive (ICP) and capacitive plasma source (CCP) is used. Possible process gases for SiN plasma texturing are SF ₆ , H ₂ , CHF ₃ and O ₂ . The maskless anisotropic SiN etch etches nanostructures into the SiN surface, thereby reducing the reflection of the surface. The process time of the texturing is less than 10 minutes. The process pressure is less than 15 Pa (150 μbar) and the process temperature is less than 50 ° C. The following are exemplary process parameters for the implementation of the SiN plasma texturing specified, which, however, may also turn out differently depending on the boundary conditions:

• - Process time: <10 minutes
• - ICP power:> 200 watts
• - CCP power: <100 watts
• - Substrate temperature: 20 ° C
• - Process pressure: <15 Pa
• - CHF ₃ / O ₂ flow: <100 sccm

For plasma texturing, it is possible, for example, to use a plasma system with a high-vacuum-compatible recipient, which makes it possible to produce an inductively and / or capacitively coupled plasma. In 4 an example of such a plasma system is shown. The figure shows the schematic structure of the process chamber 11 , The chamber has a wafer support in the lower part 12 on. In the wafer pad 12 is a tempering device 13 integrated, over which the launched solar cell wafer 19 can be kept at a constant temperature. The tempering device 13 allows temperature control of the wafer in the range of -50 to + 100 ° C. Furthermore, the lower part also contains the capacitive plasma source 23 for generating the plasma 10 indicated, while in the upper part of the process chamber, the coil assembly 14 the inductive plasma source is to recognize that with a high frequency generator 15 connected is. The high frequency generator operates in this example at the frequency of 13.56 MHz. In the region of the coil arrangement 14 is furthermore a gas inlet 16 for the process gases to be detected by a corresponding gas supply device 17 be supplied. The process chamber 11 is vacuum-compatible, so that a pressure of <1 × 10 ^-8 hPa ( 1 × 10 ^-8 mbar) can be generated in the chamber. About a temperature control is the solar cell wafer 19 or the electrode or wafer support 12 kept at a temperature of for example about 20 ° C. The gas supply device 17 allows the supply of fluorine, hydrogen, oxygen and / or carbon containing gases through the gas inlet 16 and a regulation of the gas supply of the individual gases. A pressure control allows adjustment of the gas pressure in the process chamber 11 , The setting of the appropriate pressure is carried out via a high vacuum pump 18 and a pressure measuring device 20 For example, from a capacitive pressure gauge and a Penning tube. The figure also shows a throttle valve 21 and retaining clips 22 for the solar cell wafer 19 , Furthermore, the system has an auto-match unit and transfer locks over which the solar cell wafer to be processed 19 in the process chamber 11 can be introduced or removed from the process chamber 11.

LIST OF REFERENCE NUMBERS

1

Silicon-based

2

Silicon emitter

3

SiN passivation

4

solar cell production

5

SiN Plasmatexturierung

6

Sägeschadensätze

7

Phosphorus diffusion and phosphorous glass etchings

8th

SiN deposition

9

remaining production steps

10

plasma

11

process chamber

12

wafer support

13

tempering

14

coil assembly

15

High-frequency generator

16

gas inlet

17

Gas supply means

18

High vacuum pump device

19

wafer

20

Pressure measuring device

21

throttle valve

22

retaining clips

23

capacitive plasma source

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Gaudig et al., "Investigation of the optoelectronic properties of crystalline silicon by maskless plasma etching at different ignition modes", EU-PVSEC, 2014, pp. 885-888 [0009]
• M. Otto et al., "Conformal Al 2 O 3 coatings on black silicon by thermal ALD for surface passivation", Energy Procedia 27, pages 361 to 364, 2012 [0010]

Claims (8)

Method for reducing the optical reflection of solar cells, in particular silicon solar cells, which have at least one active layer of a semiconductor material (1, 2) on which a passivation and / or antireflection layer (3) is applied, in which the optical reflection on the solar cell by texturing with a plasma etching process to form nanostructures, characterized in that the passivation and / or anti-reflection layer (3) is textured to form the nanostructures. Method according to Claim 1 , characterized in that the passivation and / or antireflection layer (3) is textured with a maskless anisotropic plasma etching process (5). Method according to Claim 2 , characterized in that the maskless anisotropic plasma etching (5) is carried out with a fluorine, hydrogen, carbon and / or oxygen-containing etching gas. Method according to Claim 2 or 3 , characterized in that the maskless anisotropic plasma etching process (5) is carried out with an inductively and / or capacitively coupled plasma. Method according to one of Claims 1 to 4 , characterized in that an SiN layer is textured as passivation layer (3). Method according to one of Claims 1 to 5 , characterized in that the texturing of the passivation and / or antireflection layer (3) takes place in an already fully processed solar cell having front and back contacts. Method according to one of Claims 1 to 5 , characterized in that the texturing of the passivation and / or antireflection coating (3) takes place before application of front and back contacts to the passivation and / or antireflection coating (3). Method according to one of Claims 1 to 7 , characterized in that the texturing of the passivation and / or antireflection layer (3) takes place such that the active layer of the crystalline semiconductor material (1, 2) and its interface is not attacked by the texturing process.

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