EP1228538A1

EP1228538A1 - Method and device for producing solar cells

Info

Publication number: EP1228538A1
Application number: EP00965843A
Authority: EP
Inventors: Peter Fath; Markus Spiegel; Thomas Pernau; Gernot Wandel; Rainer MÖLLER; Johann-Georg Reichart
Original assignee: Centrotherm Elektrische Anlagen GmbH and Co; Universitaet Konstanz
Current assignee: Universitaet Konstanz; Centrotherm Thermal Solutions GmbH and Co KG
Priority date: 1999-10-13
Filing date: 2000-09-12
Publication date: 2002-08-07
Also published as: TW492076B; CA2387510A1; US6734037B1; AU7644600A; WO2001028005A1

Abstract

The present invention relates to a method for producing a solar cell. Material is deposited on a multicrystalline silicon substrate. Passivation is carried out with hydrogen plasma. According to the invention, the material is deposited by means of low pressure CVD and the hydrogen passivation is carried out by supplying hydrogen plasma that is induced away from the partially processed solar cells. The invention also relates to a device for carrying out said method.

Description

Title: Method and device for producing solar cells

Bescnreibung

The present invention relates to the preambles of the independent claims. The present invention is concerned with the manufacture of photovoltaic solar cells.

In the case of photovoltaic solar cells, it is usually a question of making a certain electrical output available at the lowest possible prices. On the one hand, this requires high efficiency, on the other hand, the lowest possible production costs. The source material can make a significant contribution to costs. It is therefore desirable, if possible, to use ultra-crystalline silicon instead of high-purity single-crystal Czochralski silicon. A disadvantage of using multicrystalline silicon, however, is that it has a large number of defects which reduce the efficiency. This includes both geometric errors in the structure of the crystal lattice, such as dislocations and grain boundaries, and embedded foreign atoms, which are preferentially deposited at the grain boundaries.

In order to reduce the negative influence of such foreign atoms, etc., it has been proposed to neutralize the disadvantageous influence of at least the electronically charged defects by introducing atomic hydrogen. It is important that the atomic hydrogen must not remain on the surface of the solar cell to be manufactured, but rather that The interior of the volume must penetrate so that it can get close to the defects to be passivated.

A number of different techniques are known in the prior art by means of which the hydrogen passivation is to be achieved. It has been proposed to implant high-energy hydrogen ions in the surface area of a silicon wafer and then to drive them thermally into the interior of the volume. It has also been proposed to expose the silicon wafers to a hydrogen atmosphere at temperatures which are e.g. 700 ° C is chosen so high that the molecular hydrogen dissociates and can then diffuse into the wafer. It has also been proposed to expose the samples to a hydrogen plasma, which is generated capacitively or mechanically directly and directly on the silicon wafers. It has also been proposed to carry out the passivation in conjunction with an anti-reflective coating on the solar cell. Here, a hydrogen-containing silicon nitride layer can be deposited by PE-CVD; The hydrogen atoms contained in the superficial, hydrogen-containing silicon nitride layer then enter the solar cell volume in a subsequent processing step.

Attention is also drawn to the article "Detailed Study on Microwave Induced Remote Hydrogen Plasma Passivation of Multicrystalline Silicon" by M. Spiegel, P. Fath, K. Peter, G. Willeke and E. Bucher m 13 ^{^ h} EC-PVSEC, Nice, 1995, pages 421 ff. There it is proposed to use microwave radiation to generate a hydrogen plasma distant from the location of the solar cells to be processed and then to bring them to the solar cells. However, the known techniques are generally slow, at best with a high level of technical complexity, can be carried out industrially and / or are incompatible with typical production processes today. It is therefore only possible to a limited extent with the prior art to produce a solar cell with high efficiency at low cost.

The object of the present invention is to provide something new for commercial use, and in particular, but not exclusively, to improve the possibilities of inexpensive production of solar cells with high efficiency and to extend the methods obtained there to other areas of semiconductor production.

The solution to this task is claimed independently. Preferred embodiments are found in the subclaims.

The invention thus proposes a method for the treatment of semiconductors, in which material is deposited on a semiconductor and passivation is carried out with hydrogen plasma, the material being deposited by means of low-pressure CVD and the hydrogen passivation being carried out by supplying a hydrogen plasma induced remotely from the partially processed semiconductor.

In view of the differences in the process conditions of PE-CVD and low-pressure CVD, it was surprising to the person skilled in the art, in spite of the known passivation of solar cells produced in the PE-CVD process, that passivation with hydrogen which is microwave-induced away from the location of the partially processed solar cells also Low pressure process to a si- significant improvement in efficiency, but without making the process significantly more expensive.

Preferred semiconductors are silicon semiconductors or substantial portions of semiconductors containing silicon. The method according to the invention is particularly preferred in the production of solar cells from inexpensive silicate substrates. In particular with multicrystalline silicon substrates, good improvements of the resulting solar cells are achieved, but also e.g. in the case of poorer qualltat and thus higher defect numbers as well as in thin-film solar cells, significant improvements are possible with emcrystalline silicon substrates.

It is possible that the partially processed solar cells are at least temporarily irritated during the hydrogen passivation; this can be done by heat radiation from an IR light source and / or resistance heating. Such active heating during the hydrogen passivation instead of a temperature which has remained unchanged since a previous step is preferred in order to enable the setting of the optimal process parameters.

The low-pressure CVD with which the hydrogen passivation according to the invention is linked is preferred

Execution for the deposition of silicon nitride. This separation reaction typically takes place at around 750 ° C. Here, the actual hydrogen passivation according to the invention can be shortened by means of remotely induced hydrogen plasma, which occurs at typically 30 mm at 350 ° C., by passivating both during the deposition and during the heating and / or cooling phase. The response time for the passive tion step can be shortened if, after the LP-CVD (low-pressure CVD), a screen printing through-firing process and / or another contact-firing process takes place, since here too the hydrogen can diffuse deeper into the cell. Passivation is thus at least partially carried out during the temperature change required to carry out another or several other process steps, and moreover at least part of the hydrogen passivation is carried out simultaneously during at least one other treatment step.

It essentially becomes possible here to carry out the volume passivation by means of hydrogen plasma which is induced remotely without lengthening the solar cell process time. The additional costs of the solar cell process according to the invention are therefore only due to the system and additional operating costs which are caused by the hydrogen gas or gas mixture and the energy expenditure required for plasma reduction.

The hydrogen plasma is preferably generated by means of microwave radiation, because this enables particularly good control and / or regulation of the radiation intensity. The plasma can be generated in a manner known per se in the presence of a non-reactive, in particular noble gas, in particular helium. This in particular reduces self-reactions in the plasma from the location of the plasma induction to the location of the partially processed solar cells, so that the efficiency of the plasma treatment increases.

It is also preferred if the hydrogen plasma comes into contact with the partially processed solar cells at least during part of the passivation phase in the presence of other gases brought. The plasma chemically activates the gases used for the deposition, such as NH3, S1H4, SiH2Cl2, which increases the proportion of atomic hydrogen at the sample location and at the same time accelerates the deposition process for SiN, for example.

The solar cells can be continuous in small units, e.g. horizontally lying or vertically arranged in a hoarding order and processed in the process, but it is preferred if a batch-like process is carried out on a large number of simultaneously processed wafers, since the process conditions are particularly easy to control.

The invention further relates to a device for producing solar cells from a multicrystalline silicon substrate with a reaction space for processing the solar cells, a source for passivating hydrogen, in which the source for passivating hydrogen comprises a microwave generator for the microwave induction of plasma, which removes is arranged from the reaction space, and the reaction space is designed for the execution of a low-pressure CVD. In particular, it can be provided that the system for the simultaneous treatment of a large number of wafers is elongated and the hydrogen injection takes place transversely to the longitudinal axis, a plurality of hydrogen plasma injection openings being provided transversely to the longitudinal axis, to which a plurality of hydrogen plasma-reducing microwave arrangements are preferably provided assigned. The implementation of the method according to the invention can be at least substantially automated in such a system by means of a suitable process control. The invention is described below only by way of example with reference to the drawing. In this shows:

1 shows a first exemplary embodiment of a solar cell production system according to the invention in

Page section; 2 shows a further exemplary embodiment of a solar cell manufacturing plant according to the invention in the

Page section; 3a shows a third exemplary embodiment of a solar cell production plant according to the invention in

Side cut; 3b shows the third exemplary embodiment in cross section; 4a shows a fourth exemplary embodiment of a solar cell manufacturing plant according to the invention in FIG

Side cut for the quasi continuous

processing; Fig. 4b, the fourth exemplary embodiment in cross section.

According to FIG. 1, a system 1, generally designated 1, for the batchwise production of solar cells comprises an elongated process tube 2 made of quartz glass. At one end of the elongated process tube 2 there is a closable opening 3 which is large enough to insert a silicon wafer carrier 4a with a plurality of standing wafers 4b made of multicrystalline silicon. At the opposite end of the process tube 2, a suction opening 5 is provided, which leads to a vacuum pump (not shown). A resistance heater 6 is arranged along the circumference of the process tube 2, consisting of a heating wire winding βa and an insulation 6b against the outside. The resistance heater 6 is designed so that a temperature inside the process tube temperature of at least 770 ° C can be achieved under all process conditions. With thermocouples 7, the temperature inside the process tube 2 can be checked.

At the opening 3, a gas inlet 8 is provided for process gases which are intended to effect low-pressure material deposition (LP-CVD) on the wafers made of multicrystalline silicon. For this purpose, the gas inlet 8 is connected to suitable sources for process gases such as S1CI2H2 and NH3.

Diametrically opposite the gas inlet 8 and perpendicular to the axis of the process tube 2, a further inlet opening 9 is provided, which leads to a microwave cavity 10 and through which a hydrogen / helium mixture n is fed to the process tube 2 from a suitable source. Microwave energy of a frequency and intensity which is sufficient to ignite a plasma is fed into the microwave cavity 10.

Another, the first identical hydrogen plasma unit is provided near the suction opening 5. This injection unit also injects the hydrogen plasma generally perpendicular to the longitudinal axis of the process tube 2.

A controller (not shown) is provided in order to run a predetermined temperature characteristic with the resistance heater, to control the suction and the process gas supply in accordance with a predetermined, desired process course and to influence the passivation gas supply and excitation.

Plant 1 of the present invention can be used to manufacture solar cells, for example, as follows: The system is first ventilated and loaded with wafers made of multicrystalline silicon prepared in a conventional manner. It is then pumped off for 10 mm to a pressure below 30 mTorr at 500 ° C. in order to remove pest gases. Subsequently, a gas mixture of 90% He and 10% H2 is introduced from the loading side through the microwave resonator 10 until a pressure of 500 mTorr is reached. Then 200 W of microwave energy at 2.4 GHz m are fed into the cavity. The passivation takes place for 40 mm at 500 ° C; then the resistance heater 6 heats up to 750 ° C. at 10 ° C./min while plasma continues to be generated. After reaching 750 ° C, the temperature is increased at a reduced rate of rise from 2 ° C / mm to 770 ° C. At this temperature the plasma is switched off and the tube is briefly evacuated. Then for 1 mm NH3 ~ gas up to a pressure of 230 mTorr is introduced from the loading side. Then, depending on whether the process tube is partially or fully loaded, a mixture of 37.5 sccm dichlorosilane and 150 Sccm NH3 is fed in on the loading side for 22 to 26 mm and a pressure of 250 mTorr is fed in. Thereafter, NH3 ~ gas is spooled on the loading side for 1 mm and a pressure of

230 mTorr set. A gas mixture of 90% He and 10% H2 is then again fed into the process tube and that

Plasma ignited by feeding 200 W microwave energy at 2.4 GHz into the cavity. The hydrogen passivation is continued during a cooling phase at 7 ° C / mm from 770 ° C to 500 ° C. Then the plasma is switched off and, after a brief evacuation, the tube is "vented to discharge".

The arrangements in Fig. 2 and Fig. 3 are the example of

Fig. 1 essentially identical, but undercut m in the number of hydrogen injection units and their arrival Order. 2 shows an arrangement in which the hydrogen plasma is first conducted in a pipe 9b inside the process pipe, which increases the uniformity of the plasma deposition. Due to the multiple microwave cavities 10a, 10b, 10c and the respective feed lines 9a, 9b, 9c, particularly high plasma performances can be achieved. The throughput can also be increased in the arrangement of FIG. 3 with a number of microwave cavities 10. It is clear that the entire system can also be constructed in a modular manner in order to enable a correspondingly high throughput in production lines for very high numbers of pieces.

The F g. 4 show an exemplary embodiment of a system with which a quasi-continuous process of batchwise treatment of wafers of the present invention can be carried out. The actual reaction space, which in turn can be heated via a heater 6, is tubular and is connected to a suitable vacuum source. The semiconductor wafers to be processed are fed in via a vacuum lock 3a, so that when a boat is fed with a number of wafers to be processed, there is no impairment of the pressure conditions and the gases present in the reaction space. Furthermore, a vacuum removal lock 3b is also provided on the opposite exit side, which is sufficient to hold e boat together with the wafers contained therein. With a suitable design of the vacuum lock and the pump to be connected to it, a quasi-continuous supply of boats can take place, for example, every minute.

The boats loaded with wafers to be processed are activated by a so-called "Walkmg beam" through the reaction required by space. By arranging a plurality of microwaves cavities 10 at different plant positions, a high overall throughput is achieved with a homogeneous distribution of the atomic hydrogen over the entire plant length.

The solar cells obtained in this way are compared with solar cells obtained in the same system without hydrogen passivation. For this purpose, only a rinse with N2 was carried out in the comparison process during the temperature change phases. In comparison, the deposition phases in particular are chosen to be as long as those in the case of hydrogen passivation and the temperatures used were identical.

Photocurrent disconnection curves were determined on the inventive and comparative samples. It was found that the process according to the invention, which combines a low-pressure deposition process (LP-CVD) with passivation by hydrogen plasma generated at a distance from the location of the solar cell processing, leads to substantial increases in the lifetime of the charge carrier. The average lifespan of the method according to the invention could be increased from 1.74 μs to 6.94 μs. At the same time, the efficiency was increased by an average of 4% (relative).

It should be noted that the exchange of the gas flow direction offers advantages when the H plasma process is switched to the LP-CVD process. 2, hydrogen can also be induced via the inlet openings 9b, 9c arranged at a distance from the inlet and pumped out via the line 9a (then possibly free of a microwave generator). The process gases containing Si and N, on the other hand, were still able to semlaß 8 m let the process pipe and sucked off at the opening 5. This arrangement has the advantage that the microwave generators 10 can then only be arranged at the more easily accessible location.

It should be mentioned that the system structure and the wafer holder are simple and that the passivation before and after the cell metallization is possible, which enables particularly high flexibility. It should also be mentioned that the process parameters can easily be adapted for efficient bulk passivation, in order to adapt, for example, to foil silicon, crystalline thin layers and / or ingot-molded silicon. It is particularly advantageous that foil silicon can be used regardless of any ripple that may be present, e.g. problems with the PE-CVD process due to the flat support on an electrode. In addition, a surface passivating layer can be applied on both sides with the LP-CVD process.

In addition, due to the avoidance of layer deposition on electrodes, as occurs in the PE-CVD process, the maintenance intervals in the LP-CVD process are long and, due to the overall process, the requirements for the vacuum system are relatively low, since the lowest required pressures around 250 mTorr. The overall process is also not very sensitive to temperature homogeneity and gas distribution.

It should be mentioned that the arrangement of the thermocouples in the interior of the process tube 2 offers advantages, in particular with regard to the response times, but it may also be possible to arrange them outside the process tube 2, for example on its outer wall.

Claims

Patent claims

1. Process for the treatment of semiconductors, in which material is deposited on a semiconductor and passivation is carried out with hydrogen plasma, characterized in that the material is deposited using low-pressure CVD and the hydrogen passivation is carried out by supplying a solar cell that has been partially processed remotely induced hydrogen plasma takes place.

2. The method according to the preceding claim, characterized in that a silicon semiconductor or a substantial proportion of silicon-containing semiconductors is treated as the semiconductor.

3. Method according to one of the preceding claims, characterized in that a silicon substrate, in particular a silicon wafer, is treated as a semiconductor.

4. The method according to any one of the preceding claims, characterized in that a multicrystalline silicon substrate, in particular a multicrystalline silicon wafer, is treated as a semiconductor.

5. Method according to one of the preceding claims, characterized in that it is used in the production of a solar cell.

6. Method according to the preceding claim, characterized in that the partially processed solar cells are be heated at least temporarily during hydrogen passivation.

7. Method according to one of the preceding claims, characterized in that the low-pressure CVD is carried out to deposit silicon nitride.

8. The method according to the preceding claim, characterized in that heating is carried out by thermal radiation from an IR light source and / or a resistance heater.

9. The method according to any one of the preceding claims, characterized in that the partially processed solar cells have a temperature of between 250 and 850 ° C, in particular of app. Reach 350°C.

10. The method according to one of the preceding claims, characterized in that the passivation is carried out at least partially during the temperature change required to carry out another process step.

11. The method according to any one of the preceding claims, characterized in that during at least part of the hydrogen passivation, at least one other treatment step is carried out simultaneously.

12. The method according to the preceding claim, characterized in that the other step is a deposition process and / or screen printing firing process step.

13. The method according to any one of the preceding claims, characterized in that the hydrogen plasma is generated by means of microwave radiation.

1 . Method according to the preceding claim, characterized in that the hydrogen plasma is carried out in the presence of a non-reactive, in particular noble gas, in particular helium.

15. The method according to any one of the preceding claims, characterized in that the hydrogen plasma is brought into contact with the partially processed solar cells in the presence of other gases.

16. The method according to the preceding claim, characterized in that the gas is at least one of NH3,

S1H4, NO2 and/or S1H2CI2 is selected.

17. The method according to any one of claims 15 or 16, characterized in that the other gases are excited remotely from the semiconductors to be processed, in particular by microwaves, laser and / or UV light, preferably until plasma formation.

18. The method according to any one of claims 15 to 17, characterized in that the passivating hydrogen of the induced hydrogen plasma is generated by excitation with microwave energy, laser light and/or UV light.

19. The method according to one of the preceding claims, characterized in that the total volume of the semiconductor ters, especially the solar cell, is passivated, especially on both or all sides.

20. Method according to one of the preceding claims, characterized in that the semiconductors, in particular

Solar cells are processed in batches.

21. Device for producing semiconductor products, in particular solar cells from a multicrystal silicon substrate, with a reaction space for processing the semiconductor products, in particular solar cells, a source for passivating hydrogen, characterized in that the source for passivating hydrogen is a plasma generator, in particular a micro - Wave generator for microwave induction of plasma, which is arranged remotely from the reaction space, and the reaction space is designed to carry out a low-pressure CVD.

22. Device according to the preceding claim, characterized in that the system for the simultaneous treatment of a large number of wafers is elongated and the hydrogen injection takes place transversely to the longitudinal axis.

23. Device according to the preceding claim, characterized in that a plurality of hydrogen plasma ection openings are provided transversely to the longitudinal axis, preferably a plurality of hydrogen plasma-inducing microwave arrangements being provided

24. Device according to the preceding claim, characterized by a control which is used to execute a Method according to one of the preceding method claims is suitable.

25. Device according to one of the preceding device claims, with a motor-operated loading system

Loading the reaction space with partially processed semiconductor products, especially wafers.

26. Device according to one of the preceding device claims, wherein the reaction space has at least two openings through which semiconductor products can be supplied and removed.