DE3006716A1 - Electroplating of metal onto large photoelectric device - esp. onto silicon solar cell, where lamp generates voltage for electroplating one side of substrate - Google Patents

Abstract

Device has a pn-junction and one surface (a) is of one conductivity type, and at least part of a second surface has opposite conductivity (b). An electric contact is formed on surface (a), and the device is immersed in an electroplating bath contg. an anode. A voltage is applied to the anode and to surface (a) for electroplating the latter surface (a); and a lamp directs radiant energy onto surface (b). In the pref. process, the device is a silicon solar cell with a surface (a) coated with an antireflection layer (ARL) contg. holes via which a pattern of metal silicide is formed on the Si substrate, esp. the silicide of Pd or Pt; and the silicide is plated with Ag; Cu; or a mixt. of Sn and Ni. Lamp creates a voltage on surface (b) so both surfaces (a,b) can be plated with desired thicknesses.

Description

Electroplating method

The invention relates to a method for electroplating and relates in particular on such a method in which a metal layer is applied a surface of a photoelectric device is plated without a external electrical contact is connected to the surface.

Photoelectric devices and especially solar cells exist often from a semiconductor die, which forms a single large pn junction. An electromagnetic radiation like sunlight, which on this transition occurs, generates electrical carriers in the device and thus an electrical one Current. For a useful application this current must be collected and transferred to an external Circuit are fed. The collection is done by metal samples that are in ohmic Make contact with the two sides of the pn junction. As the photoelectric device generates high currents at very low voltage (which is essentially the forward voltage of the pn junction), it is necessary that the metal pattern has a path low resistance forms to the resistance losses of the generated current limit a minimum. The metallic pattern must be in its geometric dimensions be limited in particular on the front of the photoelectric device, in order to limit the amount of incident radiation energy to a minimum, which is intercepted by the metal. The intercepted radiation is actually going lost for the purpose of generating electricity. Therefore it is necessary that the Metal pattern on the front made of narrow strips of a particularly good conductive material Material. This requirement applies to so-called concentrator cells to a greater extent Degree at which the incident radiation is concentrated will and can be many times higher than the normal incident radiation. Since the The current generated is roughly proportional to the incident radiation, it can be too much lead to high current densities, eliminating the need for a very low resistance the metallic pattern is of particular importance.

When manufacturing a flat, plate-shaped silicon solar cell, which is designed for a specific operation in the sun can be a satisfactory Metal patterns can be achieved by making a thin, appropriately patterned layer are applied from an ohmic contact and a barrier material and can then be further treated by dip soldering. By immersion soldering Made a thick, low resistance conductor that is useful for many applications is sufficient.

However, the solder itself does not have a sufficiently low resistance, to produce satisfactory concentrator cells. It is therefore a material with better conductivity such as silver or copper required. So far there is but not a satisfactory one, sufficiently economical and with a sufficient one Efficiency-equipped process to make thick layers of materials with sufficient ~ to generate conductivity. Processes carried out in a vacuum, such as spraying or evaporation require much too much time and money.

Non-electrical plating is therefore unsatisfactory, because there is no satisfactory non-electric bath for plating silver or copper there.

In addition, all conventional electrolytic processes have significant Disadvantage. For example, a thick layer of silver on top of the thin ohmic Contact and applying a barrier material must be electrical contact to be made of the material, which then as one of the electrodes while plating works. When the contact with the material in a single point or is made at a finite number of discrete locations, results however, a large voltage drop from the contact or contacts to the on Spaced parts of the metal pattern. This high resistance usually leads to a high specific resistance of the thin layer of the material.

When plating begins, the resistance will keep it going distant parts of the metal pattern are at a lower potential than such Parts of the pattern which are arranged adjacent to the contacts. this leads to causes the plated layer to be highly uneven in thickness because the Plating speed is proportional to the applied voltage, which is a plating current density causes which is assigned to the voltage applied in each case. A possible solution this problem is to plate very slowly and with very low currents. When the currents are very low, the ohmic voltage drops are corresponding low, and the potential changes or fluctuations in potential across the platelet can at least be kept relatively small. However, this solution would be economically unacceptable because the plating process is too slow. The problems with electrolytic plating become even more apparent when one tries to plate both sides of the die at the same time, independently whether or not the back metallization is in a pattern.

The invention is based on the i u f g a b e, a method for To create plating of the type explained in more detail, in which with particularly Little effort and at the same time a high working speed achieved particularly uniform metal layer on both sides of a device can be.

The patent application in particular serves to solve this problem laid down characteristics.

According to the essential basic idea of the invention, the photoelectric Device placed in electrolytic plating bath, it becomes a surface of the Device electrically contacted and a voltage is applied to it while at the same time the opposite surface of the device with the help of a corresponding Radiation source is acted upon by radiation to which the photoelectric Facility appeals. The electricity generated in the facility in response to the irradiation takes place, provides a uniform flow of current, which in turn again a particularly uniform electroplating with a uniform distributed metal layer guaranteed.

This embodiment of the method according to the invention enables a simultaneous and uniform plating of both sides of a photoelectric Furnishings. The plating is controlled independently on the two surfaces, by independently adjusting the light intensity and voltage accordingly.

The invention is explained below, for example, with reference to the drawing described; these show: FIG. 1 a schematic representation of a plate with a photo element, Fig. 2 in a schematic representation an arrangement for carrying out the method according to the invention and FIG. 3 in a schematic representation an arrangement for performing an alternative embodiment of the invention Procedure.

Fig. 1 illustrates a photo element 10, which for example can be designed in the form of a silicon solar cell. The solar cell is made essentially of a silicon wafer 11 with a diameter of 75 mm, on which a single large pn junction is formed. Both sides of the transition are metallized, with a metallic pattern 12, which makes an ohmic Makes contact with the face of the wafer. The metal pattern which is representative of a large number of possible patterns is designed in such a way that that there is a path with low resistance for the collection of the current which is generated at any point on the surface of the cell. In addition, the amount of incident radiation is reduced to a minimum, which is intercepted by the metal and thus for the purpose of generating electricity get lost.

During the production of the solar cell 10 is on the silicon wafer the pn junction is formed with the help of diffusion or ion implantation, and it is the surface with an anti-reflective coating such as silicon nitride or a others Insulating material fitted. It will then be in the topping etched out an opening in the shape of the desired metal pattern. This opening offers the possibility to electrically connect the surface on one side of the pn junction to contact. The manufacturing process is then continued that in conventional Put a thin layer of palladium on the wafer by dipping it and through it the opening is created. About 50 angstroms of palladium material are used in this process upset. In such a process in which palladium on such a If the substrate is applied by plating, the palladium only reaches the exposed Silicon, but not on the silicon nitride. The platelet is then at a Temperature of about 300 ° C over a period of 15 minutes in a nitrogen atmosphere sintered to form palladium silicide. The formation of the silicide is guaranteed a good ohmic contact with the free silicon surface. After sintering an additional layer of about 300 ingætröm of palladium is applied to the platelet plated by a conventional non-electrical process, and the plate is then sintered in the same way as it was above has been described. The palladium layer forms a barrier layer that prevents that then penetrate metals into the silicon, because by such Penetration process adversely affects the life of the minority carriers of the cell or the performance of the cell may be impaired in some other way could. Other materials like platinum can be used to make the silicide to form, and other materials such as nickel or chromium can be used to serve as a barrier.

The plate equipped with the palladium layers is prepared for to be plated with a thick layer of silver to reduce the resistance along the To diminish metal patterns. Fig. 2 schematically illustrates an arrangement for plating the silver according to the invention. The plate 14, which after the The above description is immersed in a plating bath 16. That Bath is a conventional electrolytic silver plating bath, for example one aqueous solution of silver potassium cyanide. The bath 16 is in a suitable container 18 arranged. A radiant energy source 20 is arranged so that the platelet 14 is applied with a radiant energy, as indicated by the reference symbol 22 is indicated. If the chip 14 is a silicon solar cell, it can be used as a source of radiant energy 20, for example, a quartz halogen lamp can be used which emits radiant energy which is generally similar to the solar spectrum for which the silicon solar cell is photoelectrically sensitive.

The container must be made of a chemical resistant with respect to the bath 16 Material exist and must also be permeable to the radiant energy 22. According to an alternative embodiment, the plate can be placed horizontally in the container 18 arranged and irradiated from a point above the bath. Of the The container 18 then does not need to be transparent to the radiation. In order to comply with the Representation Plating silver onto a single silicon wafer is a quartz beaker a suitable container 18.

A silver plate 24 dissolves in the bath and holds the bath during of the plating process saturated with silver.

The plating process is carried out by turning on the power source 20 is so ~ that radiant energy 22 impinges on the plate. The one on the tile impinging Radiant energy creates a current in the solar cell.

The plating speed is a function of the radiation intensity, which impinges on the platelet, as the generated current of the radiation intensity is proportional. For optimal plating results, the intensity should be adjusted to deposit about 3 micrometers of silver in one minute. If the plating is done much faster, the silver will not adhere good, so that there is a tendency towards detachment.

If the plating occurs at a significantly slower rate, Although a thin layer of high quality is created the process then becomes too expensive, because of the considerable time required. A silver layer from 5 to 25 micrometers thick will generally be appropriate for a concentrator cell, being the exact thickness of the application, the cell size, the geometry of the pattern and the like depends.

Fig. 3 shows an alternative structure for performing a further embodiment of the method according to the invention. This more preferred Embodiment of the method according to the invention provides a generation of still more even layers on the front of the plate and is at the same time to it able to produce uniform layers on the back of the wafer. The layer on the back of the platelet can either be all over Surface of the rear side extend or can optionally be patterned. In both Cases, the surface can be prepared in the same way as the front, namely with layers of palladium silicide and palladium. An electrical DC voltage source 26 is arranged between the silver electrode 24 and the rear side of the plate 14. The positive terminal of the voltage source 26 is connected to the silver electrode 24, and the negative terminal is with the back or the P-side of the plate 14 connected. If now the front of the plate 14 is irradiated and a bias voltage is applied from the voltage source 26, one occurs simultaneous plating on the front and on the back of the wafer 14. The speed of plating on the front and back depends in each case independently and in a controllable manner on how the radiation intensity or the external current can be set. The radiation intensity is determined according to the above explanations brought to an optimal value.

Optimal plating on the back of the plate is achieved with an external bias voltage that has an external bias current between Generates 0.5 and 1.5 amps. These currents apply to a representative solar cell diameter of 75 mm and must be used for cells of a different size or for a plurality of cells be adjusted accordingly. When the front of the cell is irradiated and the external bias is reduced to about 50 milliamperes or less, runs the plating process continues normally on the front surface, while on the other hand No plating occurs on the back of the wafer.

The irradiation of the front side improves the uniformity of the Plating on the back and overcomes all the difficulties that come with the ohmic voltage drop through the palladium layer are connected to points, which are arranged at a distance from the electrical contact.

The method according to the invention was given above with particular reference on plating a layer of silver.

However, the process works in the same way for plating other materials such as copper or mixtures of tin and nickel. Copper can from a watery bath be titrated by copper potassium cyanide. Mixtures of tin and nickel can be plated from an aqueous bath, which Contains tin chloride, nickel chloride, ammonium hydroxide and ammonium fluoride. The tin-nickel mixtures are used as a protective cap over other metal layers to protect against Environmental influences found. With all of these applied metals it is necessary determine the optimal plating speed. This largely depends on the surface below.

While the above description applies in particular to silicon solar cells it should be noted that the invention is not limited thereto. Photo elements or photoelectric elements can be made of materials other than silicon and can, if necessary, after appropriate modifications for example, can be used in the source of the radiant energy used. However, the method does not work if there is no element which is photoelectric appeals to. If only a silicon wafer without a pn junction in the one in the figures 2 or 3 would be introduced, no plating takes place on the irradiated surface.

Blank page

Claims (6)

Patent claims 1. A method for the electrolytic application of Metal on larger surfaces of a photoelectric device with a pn junction, wherein one of these surfaces has a first conductivity type and at least part of a second surface is of the opposite type of conductivity, wherein the photoelectric device is immersed in a plating bath which Contains ions of the metal to be applied, as a result of which the surface is electrically contacted with the first conductivity type that an anode (24) is immersed in the plating bath and that a voltage (26) between the surface with the first conductivity type and the anode is applied so that this completes a circuit through the bath, and that the section the second surface with the opposite conductivity type with the aid of a corresponding one Radiation source with radiant energy QO, 22) is irradiated. 2. Method of applying metal to the two major surfaces a silicon solar cell with a pn junction, which has an anti-reflective coating having, which is arranged over a first of the larger surface, wherein the first major surface is doped to a first conductivity type, with parts the first major surface is exposed through openings in the anti-reflective coating are and there is a second major surface that is on a second Conductivity type is doped, thereby showing that a layer Made of metal silicide is applied by applying a layer of a material which metal silicide is formed from, namely on the second of the larger ones surfaces and on the parts of the first major surface that the assembly is heated so that a metal silicide is in contact with the solar cell a barrier metal is formed above the metal silicide is that an electrolytic plating bath (16) is used, which ions of the Metal to be plated contains that the solar cell (14) in the electrolytic Plating bath is immersed that the first surface of the solar cell by means of a corresponding radiation source is irradiated with a radiation energy (20, 22), to which the solar cell responds photoelectrically that the second surface of the Solar cell is contacted that an anode (24) is immersed in the bath that a voltage (26) is applied between the second surface and the anode, so that thereby an electrical circuit is closed, and that the voltage (26) and the intensity of the radiation energy (20, 22) are set in such a way that that independently controlled the plating speed on the larger surfaces will. 3. The method according to claim 2, characterized in that g e k e n n z e i c hn e t, that the material from which metal silicide is formed, from that group is selected, which consists of palladium and platinum. 4. The method according to claim 2, characterized in that g e k e n n z e i c hn e t, that a layer of palladium with the help of the application of the barrier metal a non-electrical process is applied, 5. The method according to claim 2, it is noted that the metal to be plated is made up of that Group is selected which consists of silver, copper and mixtures of tin and nickel consists. 6. Method of aleatrolytic application of metal to larger ones Surfaces of a photoelectric device with a pn junction, which is a having a first P-type surface and a second N-type surface, thereby it is noted that the device (14) and an electrode (24) in a plating bath (16) containing ions of the metal which is to be deposited that electrical contact with the first surface is dated P-type and with the electrode (24) that is made between these two parts a voltage (26) is applied that the first P-type surface is more negative is biased as the electrode and that the second N-type surface with a Radiant energy (20, 22) is irradiated from a corresponding energy source.