FR2481522A1 - Solar cell mfr. using single firing step - by utilising simultaneously fireable screen printing inks for application of dopants, contacts, leads and encapsulant - Google Patents

Abstract

Solar cells are mfd. from Si discs of a single conductivity type by screen printing dopant and conductive inks onto the discs, the dopant elements being provided to reverse the conductivity type of the surface locally, and the conductive, pref. metallic inks being provided to form electrical contacts. The sintering of the inks and the diffusion of the dopants are then carried out simultaneously in a single firing to form a semiconducting junction and the contact regions. The method is simple, viable, automatic and inexpensive. Stresses between components due to heating and cooling are minimised by operating with a single firing cycle, using suitably chosen materials with as nearly as possible, matching expansion coeffts.

Description

Method for manufacturing solar cell modules
The invention relates to a method for manufacturing solar cell modules.

Such a module is conventionally in the form of a panel whose front face is exposed to the sun. Starting from this face it successively comprises - a front wall transparent to solar radiation, - photovoltaic cells which can take, for example, the form of silicon discs which are arranged behind this wall to receive this radiation and in which a semiconductor junction has been created , with a conductive grid on the front and a rear contact on each cell to collect the electric current produced and connections between conductive grids and rear contacts of neighboring cells to connect the cells in series, and sometimes in parallel, and a rear protective wall .

 Other module structures are however possible, the cells being able for example to be protected individually. However, whatever its structure, such a module has the function of ensuring the assembly, protection and connection of the photovoltaic cells that it comprises.

 These cells are made from semiconductor silicon discs of a specific type of conductivity; by diffusion of a doping element of the opposite type from a face of the disc, so as to invert the conductivity type of the semiconductor over a fraction of the thickness of the disc. An internal semiconductor junction is thus created . This diffusion takes place at a temperature which is of the order of 900 to 11000C, and for a period which are carefully determined so as to obtain good efficiency of photovoltaic conversion. Any subsequent heating to a temperature above 6500C may affect this efficiency.

 The modules are produced later. It is delicate, even if good quality photovoltaic cells are available. In fact, these modules must first have good qualities of transmission of light up to the other cells, and of the electric current from these cells and between them. They must then have good mechanical strength and excellent tightness, that is to say guarantee the physical and chemical protection of the cells and connections, so as to ensure maintenance over time of the optical and electrical qualities of the assembly. .

As regards the production of electrodes for collecting the current on the cells, that is to say the conductive grid and the rear contact, it is known to use conductive pastes used by screen printing. French patent No. 2,348,897 of April 21, 1976 is entitled "Ohmic contacts on silicon from screen-printing pastes and process for implementation" and describes this use.
The screen-printing pastes used to make thick conductive layers consist mainly of - an active material, which is generally a finely divided conductive metal, or a mixture of several metals, - a passive material, such as a glass sealing, whose role consists in baking the dough to make the conductor integral with the chosen substrate, - a temporary organic binder, with thixotropic properties, suitable for screen printing and capable of being eliminated by pyrolysis without charcoal, - and a solvent which can be removed by drying.

The passive part, namely the sealing glass, has a coefficient of expansion as close as possible to that of silicon, and a low sealing temperature, necessarily below 650 C, so as not to modify the electrical properties and avoid the diffusion of impurities. This latter obligation prevents the glass expansion coefficient from being as close as possible to that of silicon, which compromises the reliability of long-term sealing.

 As a temporary binder, simple and known mixtures are used, such as for example a solution of ethyl cellulose in terpineol. Such a binder is removed when it is heated to the cooking temperatures of the dough, without leaving a residue.

 In the screen-printing paste, before baking, the temporary binder can represent 10% to 35% by weight, relative to the active and passive materials, although these proportions can be adapted at will to modify the rheological properties of the screen-printing paste.

In the paste obtained after baking, the active material can represent from 90 to 99% by weight, while the passive material can represent from 10 to 1%.

 With regard to the mechanical strength, the rigidity of the module can be ensured by the front wall, which takes the form of a flat and smooth glass plate on the rear face of which the other elements are glued or sealed. The front plate of the module is then easily cleaned of various dusts by rain, which keeps it transparent and avoids costly maintenance operations.

An important problem posed by the manufacture of the modules results from the fact that they are subjected to temperature variations which generate periodic differential expansions. This can result in the appearance of fractures which firstly constitute optical interfaces capable of reflecting an excessive fraction of the radiation received, therefore of reducing the energy yield, and which then lead to the introduction of moisture, and therefore to the degradation of cells. By limiting ourselves here to the case where the rigidity of the module is ensured by the front plate, it is first known, to solve this problem, to choose for this plate a glass whose coefficient of thermal expansion is close to that of the cells. Then various arrangements are used to ensure the connection between the front plate and the cells.

One of them consists in using a material for binding and encapsulating the cells which is sufficiently flexible so that its deformations absorb without damage the differences in expansion between the various elements. This material can again be a moldable silicone resin.

(In fact, whatever the nature of the front and rear walls of the photovoltaic modules, the cell encapsulation material currently most widely used is this type of resin). This material is expensive and its implementation is relatively delicate and difficult to make automatic.

 It has also been proposed to bond silicon cells to the glass front plate by electrostatic welding. Such a process is still experimental.

The present invention aims to obtain that the manufacturing of solar cell modules is easy, reliable, automatic and inexpensive.

 It relates to a process for manufacturing solar cells, characterized in that one starts from silicon discs of a single type of conductivity, and firstly, it is brought by screen printing and printing of inks doping and conductive on these discs, on the one hand a doping element allowing to surface invert the type of conductivity, on the other hand metallic elements intended to make electrical contacts on these discs, so as to be able, in a second time, simultaneously carry out on the one hand the diffusion of the doping element in silicon at high temperature, to form a semiconductor junction, on the other hand the sintering of the conductive inks to form the contacts of the cells.

 It relates more particularly to a method for manufacturing solar cell modules, such a module comprising - talc photovoltaic cells having the form of silicon disks each comprising an internal semiconductor junction, and surface electrical contacts for collecting current product, these contacts being a conductive grid on the front face receiving solar radiation and a rear contact on the rear face, - connections for electrically connecting these cells to each other and to the outside via these contacts, - and protective layers to protect these cells from bad weather, - the method of manufacturing modules comprising - operations for manufacturing photovoltaic cells provided with their contacts, with the use of "inksw including a solvent which can be removed by drying, an organic binder which can be removed by pyrolysis, and a permanent element, - and operations to assemble these cells, from mi the connections and the installation of the protective layers are put in place, and being characterized by the fact that these cell manufacturing operations are as follows - deposition, on a front face, of silicon disks of a determined type of conductivity , of a doping ink, the permanent element of which is a doping element corresponding to the opposite type of conductivity, and drying of this ink, - deposition and drying of a "conductive" ink, the permanent element of such an ink containing a metallic powder, this deposit being made through a screen printing screen in order to form the conductive grid on this same front face, - deposition and drying of a conductive ink on the rear face of the silicon discs in order to form the rear contact, - pyrolysis of the organic binders of these inks, - and heating of the discs at high temperature so as to simultaneously produce on the one hand the surface diffusion of said doping element in silicon to form said rod semiconductor tion and secondly the sintering of said conductive inks to form the conductive grid and the rear contact.

This process also advantageously comprises the following steps before said high temperature heating operation, - depositing a sealing ink, the permanent element of which is a glass powder, on areas of a "front" protective plate glass on which the photovoltaic cells must be sealed, - elimination of the solvent and the binder of this ink, - and application of the front faces of said discs on these areas, so that the high-temperature heating operation not only achieves diffusion of the doping element and the formation of the conductive grid and the rear contact, but also the sealing of the cells on this front plate, which achieves both the assembly of the cells and the establishment of a layer of protection of their front faces.

Although the expression "silicon disk" used above corresponds to the usual circular shape of photovoltaic cells, it should be understood that it could just as easily be different shapes, for example rectangular plates.

Using the attached schematic figures, we will describe below without limitation, how the invention can be implemented. It should be understood that the elements described and shown can, without departing from the scope of the invention, be replaced by other elements ensuring the same technical functions. When the same element is represented in several figures, it is designated by the same reference sign.

FIG. 1 represents a view of a module manufactured according to the invention, two cells of this module being represented in section at two levels along the line A-A of FIG. 2.

FIG. 2 represents a cut perspective view of a cell of the same module.

 Figures 3 to 7 show perspective views of the module of Figure 1 at different stages of its manufacture.

 FIG. 8 represents a perspective view of the finished module, seen through the transparent front plate.

 The method according to the invention is easy to implement automatically because, to form the cells with their electrical connections, and to carry out their assembly and their protection, it makes a new application of various techniques known independently, namely screen printing and printing of sealing glasses and inks containing suitable permanent elements, and diffusion of doping elements from the surface of a semiconductor wafer.

The module represented in FIGS. 1 and 2 comprises photovoltaic cells CP sealed at the rear of a front plate PA. At each cell there is, from this front plate, a layer of sealing glass AS, substantially coextensive with the cell, a conductive grid GC, the intervals of which are filled by the layer AS, then the silicon disk in which a semiconductor junction was formed to enable it to constitute a photovoltaic cell, then the rear contact CA of this cell and finally a rear protective layer PR made of glass.

 CO connections are made between the conductive grid GC of a cell and the rear contact CA of a neighboring cell to put them electrically in series. Contact between such a connection CO and the grid GC is made by means of a conductive pad PC deposited against the rear face of the front plate PA and penetrating between the conductive grid GC and this plate. An insulating glass stud is deposited between the connection CO and the front plate PA, on the edge of the cell CP, the rear contact CA of which is thus connected so as to isolate this connection CO from the edge of this cell.

The rear protective layer PR covers the assembly and is sealed to the front plate PA on the edges, not shown, of this with only two sealed passages for the electrical connection of the module to an external circuit.
Figures 3 to 8 show the main steps of a preferred embodiment of the method of manufacturing such a module according to the invention. Starting from a front plate made of PA glass, having a coefficient of thermal expansion similar from that of silicon, these main stages are 'the following
A - On the cells
- Screen printing of the rear AC contact (conductive ink)
- Drying (passage oven)
- Screen printing of a doping ink layer on the front
- Drying, pyrolysis and pre-sintering
- Screen printing of the conductive grid before GC (conductive ink)
- Drying, pyrolysis and pre-sintering (pass-through oven)
B - On the glass front plate
- Screen printing of AS sealing layers (glass-based ink)
(fig. 3)
- Drying (passage oven)
- Screen printing of PC conductive pads (conductive ink) (fiv. 4)
- Drying, pyrolysis and pre-sintering (pass-through oven)
C - Cell formation and assembly
- Installation of cells on the front plate PA (fig.5)
- High temperature heating under pressure to ensure
both the diffusion of the doping element, the sintering of
conductive inks and sealing on the front side
- Printing of insulating pads in PI glass (ink based on
glass) (fig. 6)
- Drying, pyrolysis and pre-sintering (pass-through oven)
- Printing of CO connections between PC conductor pads and
AC rear contacts of cells (conductive ink) (fig. 7)
- Drying, pyrolysis and pre-sintering (pass-through oven)
- Spraying an enamel on the rear face to form the PR layer
- Drying, pyrolysis, sintering.

The front plate PA can be constituted by a glass plate of the type known under the brand "Pyrex 732" from 3 to 6 mm thick.

CP cells can be monocrystalline silicon cells.

They have the form of discs 300 microns thick, 100 mm in diameter. They can be formed from homogeneous discs of type N or P. The "doping" ink used for the formation of the internal semiconductor junction contains as doping element either phosphorus or arsenic to carry out a surface doping of N type in a P type disc, either boron or aluminum to carry out P type doping in a N type disc.

 For example, it is possible to start from type P discs having a resistivity of 0.5 to 1 ohm centimeter, to deposit a layer of doping ink containing 2.5 to 5 × 10 8 grams per square centimeter of the surface of the disc, this element being phosphorus. The high temperature heating intended to ensure the diffusion of this element then lasts 30 to 45 minutes between 1000 and 1100 C.

 The conductive inks used are based on chemical silver powder or possibly other metals and preferably contain a small proportion of glass of the order of 3%.

The glass used to make the sealing layers AS can, in the context of the present invention, be chosen from glasses which soften at the high temperature necessary for the diffusion of the doping element. This makes it possible to choose a glass having particularly good characteristics for ensuring permanent and reliable sealing on silicon, these characteristics being above all the proximity of the coefficients of thermal expansion and elasticity.

This advantage is due to the fact that the sintering operation of the sealing glass can be carried out at the same time as that of diffusion, whereas, if this sintering operation had to be carried out after the diffusion operation, the need not to altering the photovoltaic properties of the cells would have meant choosing a glass that would soften at low temperatures.

Drying operations can be carried out in ovens for 10 minutes at 1560C, to remove organic solvents.

 Pyrolysis can last 1 minute at 2100-4500C, to remove organic binders, and pre-sintering, 1 minute at 550-5000C.

The sealing of the silicon discs and the realization of the electrical connection between the conductive grid and the conductive pads are obtained by sintering the glass plate assembly 2 silicon discs, under a pressure between 0.25 and 1 Kg / cm2 , under the temperature and duration conditions chosen to ensure the diffusion of the doping element in the silicon
Tests have shown
- that it is necessary to remove all of the organic binder before carrying out the final sintering operation,
- that it is preferable to carry out a pre-sintering before carrying out the assembly of silicon discs and glass plate,
- that the sintering pressure is a more critical parameter for the quality of electrical connections than for the quality of sealing.

Thus, too high a pressure causes densification of the conductive deposits, which can be favorable to a reduction in the electrical resistance of the layer, but can induce the introduction of sealing glass between the grid and the conductive pads or between the grid and the silicon disc.

 Measurements of contact resistances, at the interface between the conductive grid and the conductive pad, have shown that these resistances can be less than or equal to a few milliohms for suitably chosen sintering conditions. These values are within an acceptable range for interconnection between cells in a module.

In addition to the limited number of operations it involves, this method has two other advantages. On the one hand, it makes it possible to adapt the thermal expansion coefficients and the elasticity modules of the various constituents. On the other hand, it is easy to automate and the electronics industry very widely uses automatic screen printing machines allowing to make a thousand prints per hour.
It should be noted that the attachment of the cells by means of a sealing glass can be replaced by a direct attachment by using the known method of "electrostatic welding". In addition, it is possible to use for the rear protection a sheet of plastic material (example: polyethylene terephthalate, known under the brand "Mylar") formed and glued hot.

Claims (3)

1 / Method for manufacturing solar cells, characterized in that one starts with silicon discs of a single type of conductivity, and firstly, it is brought by screen printing and printing of doping and conductive inks on these discs, on the one hand a doping element making it possible to reverse the type of conductivity superficially, on the other hand metallic elements intended to make electrical contacts on these discs, so as to be able, in a second time, to realize simultaneously on the one hand the diffusion of the doping element in the silicon at high temperature, to form a semiconductor junction there, on the other hand the sintering of the conductive inks to form the contacts of the cells. 2 / Method for manufacturing solar cell modules, such a module comprising - photovoltaic cells (CP) having the form of silicon discs each comprising an internal semiconductor junction, and surface electrical contacts for collecting the current produced, these contacts being a conductive grid (GC) on the front face receiving solar radiation and a rear contact (CA) on the rear face, - connections (CO) to electrically connect these cells to each other and to the outside via of these contacts, - and protective layers (PA, PR) to protect these cells from bad weather, - the method of manufacturing modules comprising 9 - operations for manufacturing photovoltaic cells provided with their contacts, with the use of inks "including a solvent which can be removed by drying, an organic binder which can be removed by pyrolysis, and a permanent element, - and operations for assembling these cells, placing them e of the connections and installation of the protective layers, - and being characterized in that these cell manufacturing operations (CP) are as follows - deposition, on a front face of silicon disks of a conductivity type determined, of a doping ink whose permanent element is a doping element corresponding to the opposite type of conductivity, and drying of this ink, - deposition and drying of a "conductive" ink, the permanent element of such ink containing a metallic powder, this deposit being made through a screen printing screen in order to form the conductive grid (GC) on this same front face, - deposit and drying of a conductive ink on the rear face of the silicon discs. view of forming the rear contact (CA), - pyrolysis of the organic binders of these inks, - and heating of the discs at high temperature so as to simultaneously produce on the one hand the surface diffusion of said doping element in silicon to form said semiconductor junction and secondly the sintering of said conductive inks to form the conductive grid (GC) and the rear contact (CA). 3 / A method according to claim 2, characterized in that it further comprises the following operations before said heating operation at high temperature - depositing a sealing ink, the permanent element of which is glass powder, on zones (AS) of a glass "front" protection plate (PA) on which the photovoltaic cells (CP) must be sealed, - elimination of the solvent and of the binder of this ink, - and application of the front faces of said discs on these areas (AS), so that the high temperature heating operation not only diffuses the doping element and forms the conductive grid and the rear contact, but also seals the cells (CP) on this front plate (PA), which achieves both the assembly of the cells and the establishment of a protective layer (PA) of their front faces.