Method Of Manufacturing An Electrode System

Abstract

The invention relates to a method of manufacturing an electrode system having a monograin layer. On free grain surfaces of the monograin layer a first electrode layer is deposited. After a material-removing treatment, a deposition step is applied in which parts of enveloping layers of the grains and edges of cores of the grains adjoining pn-junctions are selectively covered with an insulating material and a second eletrode layer is deposited on parts of the cores of the grains not covered.

Description

The invention relates to a method of manufacturing an electrode system having a monograin layer, in which grains of a semiconductor material with enveloping layers and cores of opposite conductivity types are embedded in a layer of binder in a substantially monograin layer over a part of the layer thickness, a first electrode layer for contacting the enveloping layers of the grains is deposited on one side of the monograin layer with free grain surfaces, a material-removing treatment is used in which parts of the cores of the grains and parts of the enveloping layers of the grains are exposed, after which in a succeeding process step insulating material is provided on the exposed part and a second electrode layer for contacting the cores of the grains is deposited.

The invention furthermore relates to an electrode system manufactured by means of the method.

The said electrode systems may comprise monograin layers with grains operating as diodes, in which the first and the second electrode layer are deposited on oppositely located sides of the monograin layer.

One side of other electrode systems manufactured by means of the above-mentioned method is often free from electrode layers to facilitate the entrance and/or emanation of light. These electrode systems ar used, for example, in solar cells and in injection luminescent light sources.

In manufacturing the said electrode systems, the enveloping layers of the grains must be connected together and the cores of the grains must be connected together by electrode layers which may not contact each other.

It is often difficult to fulfil this requirement since in the material-removing treatment both the cores of the grains and the enveloping layers of the grains are exposed; this holds good in particular if one of the sides of the electrode system must be free from contact layers. For example, in a method of the type mentioned in the preamble (see U.S. Pat. No. 3,040,416) an insulating layer is provided on the first electrode layer and the material-removing treatment is applied on the side of said layers and the material of the grains and the material of the first insulating layer must be matched to each other in such manner and the material-removing treatment should be such that of the grains a part of the first electrode layer and a part of the enveloping layer of the grains is removed and between the grains the first insulating layer is removed over a greater part of the layer thickness without, however, entirely removing the first electrode layer from and between the grains.

After providing insulating material in the form of a second insulating layer on the exposed parts, a second material-removing treatment is carried out in which the material of the grain and the material of the second insulating layer are to be removed to an equal extent so as to be able to deposit the second electrode layer which contacts the cores of the grains and is separated from the first electrode layer.

The materials of the two insulating layers and of the grains must be accurately matched to each other and in addition very high requirements are imposed upon the material-removing treatment so that the above-described method is difficult and cumbersome and often does not produce the desired result but results, for example, in shortcircuit.

One of the objects of the invention is to provide a novel method of contacting the cores of the grains and the enveloping layers of the grains in an insulated manner and furthermore to provide an improvement of the method described in the U.S. patent specification, as a result of which the manufacture becomes easier and can be carried out in fewer steps. The invention is inter alia based on the recognition of the fact that the second material-removing treatment may be omitted when no more insulating material is used than is necessary for the insulation of the second electrode layer relative to the enveloping layers of grains.

Therefore, according to the invention, the method is characterized in that in said process step the insulating material is selectively provided on the exposed parts of the enveloping layers of the grains and on edges of the exposed parts of the cores of the grains adjoining the junctions and parts of the cores of the grains not covered during the provision of the insulating material are then contacted by depositing the second electrode layer.

One of the advantages obtained is that a second material-removing treatment may be omitted and it is achieved that the material-removing treatment to be used can be particularly simple since no relief need be provided in the monograin layer. A grinding or polishing process is preferably chosen as the material-removing treatment.

By means of the method according to the invention, good results can be obtained in providing mutually insulated electrode layers on the cores of the grains and on the enveloping layers of the grains, for example, in manufacturing the already stated electrode system the electrode layers of which are deposited on either side of the monograin layer.

An insulating layer is preferably provided on the first electrode layer and the material-removing treatment is applied to both last-mentioned layers, parts of the first electrode layer being exposed, said last-mentioned parts being covered during the selective provision of the insulating material.

In a preferred embodiment of the method according to the invention, the insulating material is provided by electrophoresis in that the exposed parts are contacted with a bath containing an insulating material suitable for electrophoretic coating and electrophoresis is carried out by giving the first electrode layer a voltage relative to the bath at which the junctions between the cores of the grains and the enveloping layers of the grains are biased in the reverse direction.

In another preferred embodiment of the method according to the invention the insulating material is provided by electrochemical polymerization in that the exposed parts are contacted with a bath containing a material which is converted into the insulating material by electrochemical polymerization and polymerization is carried by giving the first electrode layer a voltage relative to the bath at which the junctions between the cores of the grains and the enveloping layers of the grains are biased in the reverse direction.

In the above preferred embodiments, the provision of the insulating material proceeds more slowly according as more material is provided and is finally discontinued automatically.

The insulating material is preferably provided on the monograin layer prior to the material-removing treatment and after the treatment the insulating material is caused to swell so that the exposed parts of the enveloping layers of the grains and the edges of the exposed parts of the cores of the grains adjoining the junctions are covered.

Swelling may be carried out, for example, by a treatment with vapour or a solution with a swelling agent suitable for the insulating material.

The swelling agent may be removed, for example, by evaporation or extraction, in which insulating material remains in the place covered due to the swelling.

When a grinding process is used as a material-removing treatment, it is advantageous when the grains project approximately equally far from the layer of binder with their free grain surfaces. Therefore, the monograin layer is preferably obtained prior to depositing the first electrode layer in that the grains are provided as a substantially monograin layer in an adhesive layer on a substantially plane substrate and embedded in the layer of binder, after which the monograin layer is separated from the substrate to obtain the side with the free grain surfaces.

Preferably the side with the free grain surfaces is then treated with a specific solvent for the binder so as to enlarge the free grain surface.

It is achieved that the grains project approximately equally far from the layer of binder also in the case in which all the grains are not accurately equally large.

The invention furthermore relates to an electrode system manufactured by means of the method according to the invention.

The invention will now be described in greater detail with reference to a drawing and a few examples.

In the drawing:

FIGS. 1 to 5 are diagrammatic sectional views of parts of an electrode system in successive stages of manufacture by means of preferred embodiments of the method according to the invention and

FIG. 6 is a diagrammatic sectional view of a part of an electrode system in a stage of manufacture by means of a variation of the method according to the invention.

The manufacture will now be described of an electrode system having both layers on one side of the monograin layer, which systems are used in solar cells an injection luminescent light sources. An electrode system 51 (see FIG. 5) with a monograin layer (11, 21) is manufactured, in which grains 11 having a size of approximately 50 .mu.m and consisting of semiconductor material, for example gallium phosphide, having approximately 5 .mu.m thick enveloping layers 12 and cores 13 of opposite conductivity types are embedded in a layer 21 of binder (see FIG. 3) in a substantially monograin layer over part of the layer thickness.

After implantation in the usual manner in the gallium phosphide grain of ions of an element which can cause the same conductivity type in the gallium phosphide as the enveloping layers of the grains have, a first electrode layer 41 (see FIG. 4) for contacting the enveloping layers of the grains is deposited on one side 31 of the monograin layer (11, 21) with free grain surfaces, for example, by means of vapour deposition of gold. An insulating layer 42, for example of polyurethane, is provided on the first electrode layer 41. By using a material-removing treatment, for which a grinding process is preferably chosen, parts 43 of the cores 13 of the grains, cores 44 of the enveloping layers 12 of the grains and parts 45 of the first electrode layer 41 are exposed.

Insulating material 52 of materials to be described hereinafter is selectively provided in the form of a pattern of grains on the exposed parts 44 of the enveloping layers 12 of the grains, on the exposed parts 45 of the first electrode layer 41 and on edges 54 of the exposed parts 43 of the cores 13 of the grains adjoining the junctions 14.

Parts of the cores 13 of the grains not covered during the provision of the insulating material 52 are then contacted by depositing the second electrode layer 53. For that purpose, ions are implanted in a usual manner in the gallium phosphide grains of an element which can cause the same conductivity type in the gallium phosphide as the cores of the grain have, and a second electrode layer 53 is deposited, for example, by vapour-depositing gold.

The first electrode layer 41 and the second electrode layer 53 can be provided with current conductors in a usual manner.

When a grinding process is used as a material-removing treatment, it is advantageous when the grains project approximately equally far from the layer of binder with the free grain surfaces.

Therefore, the monograin layer (11, 21) is obtained prior to depositing the first electrode layer 41 by providing the grains 11 as a layer which has a thickness substantially of one grain in an adhesive layer 15 on a substantially plane substrate 6 (see FIG. 1).

The adhesive layer 15 consists, for example, of a usual rubber glue and the substrate 16 of glass. The grains 11 are embedded in the layer 21 of binder (see FIG. 2) which consists of an electrically insulating material, for example, polyurethane.

The monograin layer (11, 21) is then separated from the substrate 16 and the side 31 with the free grain surfaces is obtained (see FIG. 3).

The free grain surface may still be enlarged by a treatment with a specific solvent for a binder. In the case of polyurethane this can be carried out in a usual manner by a treatment with an alcoholic potassium hydroxide solution.

EXAMPLE 1

In the first example the insulating material 52 is provided by electrophoresis in that the exposed parts 43, 44, 45 are contacted with a bath containing an insulating material suitable for electrophoretic coating.

Electrophoresis is carried out by giving the first electrode layer 41 a voltage relative to the bath at which the junctions 14 between the cores 13 of the grains and the enveloping layers 12 of the grains are biased in the reverse direction.

The grains consist, for example, of the semiconductor material gallium phosphide. By doping with zinc the cores 13 have been made p-type conductive and by doping with sulfur the enveloping layers 12 have been made n-type conductive.

In a manner conventionally used in electrophoretic coating, a bath is used of a solution of a synthetic resin containing carboxyl groups and/or hydroxyl groups and of an organic amine or ammonia in water. The weight concentration of solid of the solution is 10 to 15 percent. Conventional materials such as epoxy-, acrylate- or alkyl resins may be used as a synthetic resin. A positive voltage of, for example, approximately +100 volt relative to the bath is given to the first electrode layer 41. During the passage of current through the bath, the current rapidly decreases to a fraction, for example, 1 to 10 percent of the original value, and the exposed parts 44, 45 and at most edges 54 of the exposed parts 43 of the cores 13 of the grains are covered. The insulating material reaches, for example, a layer thickness of 5 .mu.m. The insulating material is finally subjected to a baking treatment for 5 to 20 minutes 120.degree. to 180.degree.C. During providing the insulating material there may also be proceeded so that a certain current density is adjusted (in the order of magnitude of 0.1 mA/cm.sup. 2) and the process is discontinued when the voltage has reached a certain high value.

EXAMPLE 2

The second example differs from the first example in that in this case the insulating material 53 is provided by electrochemical polymerisation in that the exposed parts 43, 44, 45 are contacted with a bath containing a material which is converted into the insulating material by electrochemical polymerisation. The polymerisation is carried out by giving the first electrode layer 41 a voltage relative to the bath at which the junctions 14 between the cores 13 of the grains and the enveloping layers 12 of the grains are biased in the reverse direction.

In a manner conventionally used in electrochemical polymerisation a bath is used having a mixture of orthoisopropylphenol and triethylamine in a molecular ratio of 10 : 1. At a positive voltage of +200 V of the first electrode layer 41 relative to the bath, insulating material 52 is provided in a thickness of 0.6 .mu.m. The current density decreases during the process from approximately 10.sup.-.sup.3 A/cm.sup.2 to approximately 10.sup.-.sup.6 A/cm.sup.2.

EXAMPLE 3

The third example differs from the preceding examples in that the insulating material is provided on the monograin layer (11, 21) prior to the material-removing treatment and the insulating material after the treatment is allowed to swell so that the exposed parts 45 of the first electrode layer 41, the exposed parts 44 of the enveloping layers 12 of the grains and the edges 54 of the exposed parts 43 of the cores 13 of the grains adjoining the junctions 14 are covered (see FIG. 6). The insulating material is provided, for example, as an insulating layer 42. As a material for the insulating layer 42 is used, for example, polyurethane which swells by a treatment with vapour of ethyl acetate.

After removing the ethyl acetate by evaporation at 80.degree.C, insulating material 52 remains in the places covered by the swelling.

The invention is not restricted to the examples described.

For example, instead of gallium phosphide another suitable semiconductor material, for example zinc selenide, mixed crystals of gallium arsenide and gallium phosphide or mixed crystals of gallium arsenide and aluminium arsenide may be used. The cores of the grains may also show n-type conductivity and the enveloping layers of the grains may show p-type conductivity. In the latter case, of course, the insulating material used in the electrophoretic coating and the material used in the electrochemical polymerisation must be adapted to the polarity of the first electrode layer varying relative to the bath. In the case of electrophoretic coating, for example, synthetic resins on the basis of melamine are to be considered. Upon providing insulating material by means of swelling it makes no difference whether the enveloping layers of the grains have p-type or n-type conductivity.

The method according to the invention may also be used in manufacturing electrode systems in which the electrode layers are present on oppositely located sides of the monograin layer.

Claims

What is claimed is:

1. A method of manufacturing an electrode system having a monograin layer, in which grains of a semiconductor material with enveloping layers and cores of opposite conductivity types are embedded in a layer of binder in a substantially monograin layer over a part of the layer thickness, a first electrode layer for contacting the enveloping layers of the grains is deposited on one side of the monograin layer with free grain surfaces, a material-removing treatment is used in which parts of the cores of the grains and parts of the enveloping layers of the grains are exposed, after which in a succeeding process step insulating material is provided on the exposed parts and a second electrode layer for contacting the cores of the grains is deposited, characterized in that in said process step the insulating material is selectively provided on the exposed parts of the enveloping layers of the grains and on edges of the exposed parts of the cores of the grains adjoining the junctions and parts of the cores of the grains not covered during the provision of the insulating material are then contacted by depositing the second electrode layer.

2. A method as claimed in claim 1, characterized in that an insulating layer is provided on the first electrode layer, the material-removing treatment is applied to both last-mentioned layers, parts of the first electrode layer being exposed, said latter parts being covered during the selective provision of insulating material.

3. A method as claimed in claim 1, characterized in that the insulating material is provided by electrophoresis in that the exposed parts are contacted with a bath containing an insulating material suitable for electrophoretic coating and electrophoresis is carried out by giving the first electrode layer a voltage relative to the bath at which the junctions between the cores of the grains and the enveloping layers of the grains are biased in the reverse direction.

4. A method as claimed in claim 1, characterized in that the insulating material is provided by electrochemical polymerisation in that the exposed parts are contacted with a bath containing a material which is converted into the insulating material by electrochemical polymerisation and polymerisation is carried out by giving the first electrode layer a voltage relative to the bath at which the junctions between the cores of the grains and the enveloping layers of the grains are biased in the reverse direction.

5. A method as claimed in claim 1, characterized in that the insulating material is provided on the monograin layer prior to the material-removing treatment and after the treatment the insulating material is caused to swell so that the exposed parts of the enveloping layers of the grains and the edges of the exposed parts of the cores of the grains adjoining the junctions are covered.

6. A method as claimed in claim 1, characterized in that a grinding process or a polishing process is chosen as a material-removing treatment.

7. A method as claimed in claim 1, characterized in that prior to depositing the first electrode layer the monograin layer is obtained in that the grains are provided as a substantially monograin layer in an adhesive layer on a substantially plane substrate and embedded in the layer of binder, after which the monograin layer is separated from the substrate to obtain the side with the free grain surfaces.

8. A method as claimed in claim 7, characterized in that the side with the free grain surfaces is then treated with a specific solvent for the binder so as to enlarge the free grain surface.