FR2675633A1 - Photovoltaic device with reinforced insulation and its method of production - Google Patents

Abstract

The subject of the invention is the production of breaks in the material forming cuttings (19, 23) particularly through a semiconducting layer interposed between two electrode layers (5, 9) of a thin-film photovoltaic device. To do that, one of the electrode layers (9) in contact with the semiconducting layer is produced in such a way as to have cuttings in the material (23) and, in the extension of these cuttings, the semiconducting layer is removed selectively by chemical etching to form an electrical insulation rim (21) between the said electrode layers. Application especially to amorphous silicon solar cells.

Description

 The invention relates first of all to a thin-film photovoltaic device for which it has been sought to solve the problems of electrical insulation between the layers, in order to avoid the risks of short-circuits.

Currently, many photovoltaic or solar thin-film devices are made so as to understand
- a substrate,
a first layer forming an electrode formed on said substrate,
a photovoltaic conversion semiconductor layer formed on said first electrode layer,
- And a second layer also forming an electrode formed on said photovoltaic conversion layer.

 On a certain number of these devices, it is known to provide cuts through the second electrode layer and, substantially in the extension, through the photovoltaic conversion layer, these cuts thus interrupting the continuity of the layers concerned.

 Such cuts are encountered in particular on "semi-transparent" photovoltaic cells or devices, that is to say partially transparent combining the traditional functions of a solar structure generating an electric current with that of a panel allowing passage light by filtering it.

 An example of such a structure can be found in US-A-4,795,500.

 This patent describes a solar module comprising a transparent insulating substrate, a transparent front electrode layer deposited on the substrate, a photovoltaic conversion semiconductor layer deposited on said front electrode, and a metal rear electrode layer deposited on the semi-layer. conductive, at least this rear electrode having a plurality of holes formed through it to therefore ensure a passage for the light.

 However, the production of such holes, orifices or passages poses a certain number of problems, particularly when the cuts in question have to be formed through the photovoltaic conversion layer in order to provide the person looking through the module with certain comfort. visual assured by a neutrality of the colors (which is not the case when the photovoltaic conversion layer, for example produced on the basis of amorphous silicon, is preserved, the light which crosses the module then presenting an orange-red color whose vision can be difficult to bear).

 Another exemplary embodiment of such a partially transparent cell can be found in French patent application FR-A-91 02075 filed by the applicant and in which to make a visually neutral semi-transparency homogeneous, continuous trenches are produced. preferably through the rear electrode and the photovoltaic conversion layer.

 However, each time that the cuts, holes or trenches are formed through this photovoltaic conversion layer, it is clear that risks of short circuits can appear between the electrodes on the occasion of an edge defect, this risk being of course proportional to the dimension or the length of the cuts thus produced.

 Independently of these problems specific to partially transparent modules, such risks of short-circuits can generally exist at the places where electrical insulation of the active part of the module must be carried out with respect to its outer edge. peripheral.

 The object of the invention is therefore to propose a satisfactory solution both technically and industrially to all of these problems.

 In this case, this solution consists, according to an important characteristic of the invention, in producing the photovoltaic conversion layer so that it has, at the location of said cuts, an electrical insulation flange between said first and second layers. electrodes.

 According to an additional characteristic of the invention, this rim, which therefore forms an advance of material from the photovoltaic conversion layer towards the interior of the cut considered, will preferably extend continuously along the entire edge of said cut.

 Thus, with the presence of such a material barrier, will it be possible to systematically avoid the formation of short circuits between the electrodes at the edge of the cuts, that these are formed over most of the surface of the device to form passages through which an incident light can pass through the photovoltaic device over its entire thickness, or else these same cuts are in the form of trenches provided at least for some of them near the periphery of the device to form then a peripheral trench continues through not only the second electrode layer and the semiconductor layer but also, substantially in the extension, through the first electrode layer, the cut made through this first electrode layer then preferably presenting itself a rim also forming a projection towards the inside of the cut.

 In addition to the device which has just been presented, the invention also relates to a method for producing such interruptions of material through a photovoltaic conversion layer.

 Indeed, the realization of such cuts poses a real problem insofar as it is well known to those skilled in the art that chemical etching through a mask regularly causes an underlying attack leading to the formation of an overhanging mask border (phenomenon often referred to as "undercutting").

 However, the existence of such an overhanging border necessarily increases the risks of short circuits between the electrodes, if the mask is formed by the rear electrode (or second electrode layer).

 And in general, to the knowledge of the applicant, a method has never been proposed to make short-circuit barriers at the location of such cuts, with all the necessary reliability and for industrially bearable costs.

 The purpose of the process of the invention is of course to propose a solution to these problems.

More specifically, in the context of this process, it is proposed
a) producing the second electrode layer, so that it locally has, in contact with the photovoltaic conversion layer, interruptions of material forming the abovementioned cuts,
b) then, substantially in the extension of said cuts, selectively removing said photovoltaic conversion layer by chemical etching so that in these places this layer itself has cuts limited laterally by an electrically insulating rim of said electrode layers.

 It will be noted that, in particular if the second electrode layer of step a) above is chosen so that it comprises a stainless metal resistant to soda (that is to say not dissolved by it) and the photovoltaic conversion layer so that it is based on amorphous silicon, it is then possible to chemically erode this last layer by spraying with sodium hydroxide solution.

Other characteristics and advantages of the invention will appear from the description which follows, given with reference to the appended drawings given solely by way of nonlimiting examples and in which
FIG. 1 illustrates schematically and in partial view a cross section of a photovoltaic device according to the invention according to line II of FIGS. 2 or 3,
FIGS. 2 and 3 each schematically show, according to a local plan view, an embodiment of a zone belonging to the photoelectrically active surface of a photovoltaic device with partial transparency,
FIGS. 4A, 4B, 4C and 4D schematically illustrate, in local cross-section, the main steps which can be followed in order to produce the structure of FIG. 1,
FIG. 5 illustrates, in a schematic plan view, a photovoltaic device having over its entire periphery an isolation trench according to the invention,
FIG. 6 shows in schematic transverse section the isolation trench of FIG. 5, at the location of the detail marked VI in this same FIG. 5,
FIGS. 7A, 7B, 7C, 7D and 7E also show in a transverse schematic section the main steps of an embodiment process which can be followed to produce the perimeter insulation slice of FIGS. 5 and 6,
- And Figure 8 shows, in cross section, a photovoltaic conversion layer being chemically etched.

 In FIG. 1, we therefore see first of all illustrated a detail of a photovoltaic device 1, produced according to the technique known as "thin layers".

 Conventionally, the device in question comprises, stacked on a support substrate 3, a series of thin layers 5, 7, 9.

 The substrate 3 can be produced for example from glass or from polymer which can be transparent (in particular in the case of the production of a semitransparent module).

 The thin layers which surmount it successively comprise a layer 5 serving as a front electrode (for example made of transparent conductive oxide, TCO) a few hundred nm thick disposed on the substrate 3, a semiconductor layer 7 ensuring the conversion photovoltaic and, surmounting the latter, a layer 9 serving as a rear electrode.

The "rear" electrode 9 may in particular be made of metal and will then be opaque. However, it can also be envisaged to produce it with electrically conductive and transparent materials, of the same type as those used for the production of the front electrode 5, namely in TCO (such as for example SnO2, In202,
ZnO ...) or with stacks of very thin metal layers and TCO metal oxide layers. It will be noted that such stacks are described for example in the publication EP-A-0 204 562.

 As regards the photovoltaic conversion layer 7, it may, according to a conventional technique, be for example produced by successive stratification of three pine sublayers, each of these sublayers being made from an appropriate semiconductor material such for example as hydrogenated amorphous silicon.

 As mentioned above, difficulties arise with such devices when it is desired to make cuts, holes, trenches, passages. ... forming interruptions of material through at least some of the thin layers, the risk being that during the production of these "cuts" (we will often use this generic term in the following), short-circuits may occur between the electrode layers 5 and 9.

 Such cuts are encountered in particular when it is desired to produce a photovoltaic device with partial transparency.

 Examples of embodiments can be found in patent US-A-4,795,500 or in French patent application FR-A-91,02075 where it is indicated that "semi-transparency" can be obtained in particular by holes 11 (figure 2) or trenches 13 (FIG. 3) formed through the rear electrode 9 and preferably, in the extension, through the photovoltaic conversion layer 7 (see FIG. 1), or even further through the front electrode 5 (see FIG. 6), these cuts then being, whatever their shape, formed on the photoelectrically active surface of the cell, surface marked 15 in FIG. 5. Thus, the light L can pass through, at the place of these cuts, the entire photovoltaic device as indicated by the two-way arrows in FIG. 1.

 It will be noted that the technique of cutting the electrode layer 5 could possibly be advantageous in particular in the context of the production of a solar module with an inverted structure and with partial transparency, as described for example in the French patent application FR-A -91 02 074.

 Besides in the context of the production of photovoltaic devices with partial transparency, it will be easily understood that these short-circuit problems can also arise at the places where electrical insulation of the active surface (such as 15) of the cells must be ensured. , that is to say in practice near their perimeter border, as illustrated in phantom 17 in FIG. 5.

 In accordance with the invention, in order to avoid all these risks of short circuits, whether on conventional photovoltaic devices or with partial transparency, the solution proposed in this case consists in providing, at the place of the cut 19 made at through the photovoltaic conversion layer 7, an advance of material 21 projecting towards the interior of the cut considered and thus forming an electrical insulation rim between the first and second electrode layers 5 and 9.

 Thus, the cut 19 formed in the photovoltaic layer 7 will have a transverse dimension d1 less than the transverse dimension d2 of the underlying cut 23 previously formed through the upper electrode layer 9.

 To increase the reliability of the results, it is advisable to produce the rim 21 so that it extends continuously along the entire edge of the cuts 19 considered.

 From the tests which have been carried out, it has been observed that favorable results are obtained when the flange (s) 21 has (have) a width 1 of between approximately 5 and 50 μm (microns).

 Such characteristics will of course be found in the context of the construction of the continuous perimeter trench illustrated in FIGS. 5 and 6 and which is therefore intended to provide electrical insulation of the active part of the cell with respect to its outer edge.

 Note, however, that in this case, the stripe or cut 25 which has been formed through the first electrode layer 5 will preferably have a transverse dimension d3 less than the transverse dimension d1 of the cut 19 made through the photovoltaic conversion layer 7 which overcomes it. Thus, we will then obtain a triple isolation trench with edges in staircase steps.

 The structure of the devices to be obtained in accordance with the invention having been described, we will now focus more particularly on the presentation of their production process.

 In general, this process which therefore aims to produce interruptions of material forming cuts through a photovoltaic conversion layer interposed between a first and a second electrode layer is characterized in that the second layer will be produced electrode (such as layer 9 in the examples illustrated) so that it locally has material interruptions (such as 23) thus forming cuts through the thickness of this second layer. Then, substantially in the extension of these cuts, we will selectively remove, in 19, the photovoltaic conversion layer 7 by chemical etching so that at this location, this layer has the insulation flange 21.

 If a continuous trench of peripheral insulation (such as 17) is to be produced in FIG. 5, care will also have been taken to have carried out through the electrode before 5 the cut in continuous trench 25.

 In FIGS. 7A to 7E, the main steps of a technique which can be used to obtain the expected results have been shown.

 First of all, in FIG. 8A, it can be seen that the first electrode layer 5 can be deposited first (for example by sputtering under partial vacuum) on the surface of the substrate 3.

 It is then possible to carry out the continuous trench through the electrode 5 (FIG. 7B).

 For this, one can in particular use ablation by laser, such as a laser of the "YAG" type with a beam wavelength of the order of 1.06 Clm for a layer of SnO2 of approximately 600 nm. thick, the trench 25 being able to have a width d3 of a few tens to a few hundred microns.

 After that, we will be able to deposit on this peripheral trench layer the photovoltaic conversion layer 7, such as a layer of hydrogenated amorphous silicon with p-i-n structure with a thickness of the order of 200 to 400 nm (FIG. 7C).

 After which, the metal layer forming the second electrode 9 can be deposited over the layer 7.

 To obtain the desired cuts through this electrode 9, and this of course in the vertical extension of the trench 25, it is advisable first to continuously deposit the layer (or sub-layers) of material, then to selectively etch the electrode with respect to said trench 25, for example by a laser technique (conceivable in particular with a laser having a pulse width less than or equal to the picosecond).

 But one could also use a technique called revelation (or "patterning by lift off") mentioned in particular in the publications US-A-4,443,651 or JP-A-611,098,686.

 Provision could even be made not to deposit the electrode layer 9 at the locations situated in the vertical extension of said trench 25, in particular by then using a metallization technique by evaporation or by sputtering through a suitable metal mask.

 Be that as it may, care will have been taken to make through this upper layer 9 a trench of width d 2 greater than the width d3 of the trench of layer 5 (FIG. 7D).

 Having reached this stage, the conversion layer 5 will have to be removed selectively between the two trenches 23 and 25, this so as to create the communication trench (s) 19 there with isolation flange (s) 21.

 For this, a chemical etching technique will advantageously be used so as to selectively erode layer 7.

 More specifically, and in particular in the case of such a layer based on amorphous silicon, in the presence of a rear electrode 9 based on a stainless metal resistant to soda, it will be advantageous to use a detergent of sodium hydroxide which can in particular be sprayed or sprayed in the liquid phase on the semiconductor layer, at the cut-off locations, this for example from spray nozzles such as 27 (FIG. 8).

 In particular, it is possible to use a concentrated sodium hydroxide solution between about 5 and 40% (and preferably of the order of 10%) brought to a temperature of 45 to 65 ° C. and provide an attack time of a few minutes.

 It should be noted that this operation does not require any particular masking of the rear face of the module (that is to say on the side of the electrode 9), an operation which would be very delicate taking into account the dimensions of etching to be carried out. On the contrary, the production of this protective edge 21 is "naturally" obtained thanks to the masking function that the only metal 9 exerts with regard to the etching of the layer 7, which makes the process particularly easy to implement industrially.

 The structure of FIG. 7E is thus obtained (identical to that of FIG. 6).

 Apart from the initial production of the cut 25 through the first electrode layer 5, the technique which has just been presented is entirely applicable to the production of the structure of FIG. 1, that is to say to the production of a partially transparent photovoltaic device having passages for light.

 For greater clarity, we have also shown in FIGS. 4A to 4D the main steps in producing such a structure.

 In these figures, it will only be noted, with regard to FIGS. 4A and 4B, that the case (among others) has been illustrated where the local ablation of the metal layer 9 is carried out by "lift off" ( revelation), that is to say in the present case by depositing an adherent but peelable paste 29, this on the photovoltaic conversion layer 7 previously deposited and only at the places where it is desired to make the passages 11 or 13 useful for semi-transparency, these patterns of revelation paste then being, as illustrated in FIG. 4B, covered, like the whole of the layer 7, by the layer forming the rear electrode 9, the abovementioned technique of " lift off "allowing by revelation the withdrawal of the dough 29 and of the metal which surmounts it where the dough has been deposited, thus making it possible to obtain the cutting patterns 23 illustrated in FIG. 4C. It then only remains to conduct the chemical etching with soda lye of the underlying photovoltaic conversion layer 7 in order to obtain the desired semi-transparency patterns with their border 21 of electrical insulation.

Even if the nature of the semiconductor layer 7 is relatively important, it should be clear that it could, as far as it is concerned, be produced with a nip structure, even with a p / n junction or the like and from other semiconductors such as CuInSe2, CdS,
CdTe, or GaAs, the nature of the solution used for its chemical etching then being adapted accordingly.

 It could also not be uninteresting to produce in accordance with the invention a structure of the "tandem" type, that is to say a structure today conventional comprising a double stack of thin layers, separated or not by a layer electrically insulating transparent connection.

Claims (13)

 1. - Thin-film photovoltaic device comprising  - a substrate (3),  - a first layer forming an electrode (5) formed on said substrate,  a photovoltaic conversion semiconductor layer (7) formed on said first layer,  - a second electrode layer (9) formed on said photovoltaic conversion layer, cuts (19, 23) being formed through this second electrode layer (9) and, substantially in extension, through the photovoltaic conversion layer , these cuts interrupting the continuity of the layers concerned, characterized in that at the place of said cuts (19, 23), the photovoltaic conversion layer (7) has a flange (21) of electrical insulation between said first and second layers (5, 9) forming an electrode.  2. - Device according to claim 1 characterized in that said flange (21) which forms an inward projection of the cut considered extends continuously along the entire edge of this cut  3. - Device according to claim 1 or claim 2 characterized in that the flange (21) has a width (l) between 5 and 50 microns approximately.  4. - Device according to any one of claims 1 to 3 characterized in that said layer (7) of photovoltaic conversion is made based on amorphous silicon (aSi: 3fi and said second layer (9) forming an electrode comprises a stainless metal resistant to soda.  5. - Device according to any one of the preceding claims, characterized in that the substrate (3) and said first electrode layer (5) are transparent to natural light and said cuts (19, 23, 25) are provided on the 'essential of the surface (15) of the device to form passages (11, 13) through which an incident light (L) can pass through the device over its entire thickness.  6. - Device according to any one of the preceding claims, characterized in that the cuts (19, 23, 25) are in the form of trenches provided at least for some of them near the periphery of the device where they extend to form a continuous peripheral trench (17) formed through not only the second electrode layer (9) and the photovoltaic conversion layer (7) but also, substantially in the extension, through the first electrode layer (5 ), the cut-off (25) formed through this first electrode layer itself having a rim forming an advance towards the inside of the cut-off relative to the rim (21) of the photovoltaic conversion layer.  7. - Method for making interruptions of material forming cuts (19, 23, 25) through a semiconductor layer of photovoltaic conversion (7) interposed between a first layer and a second layer forming electrodes (5, 9) d '' a thin film photovoltaic device, characterized in that  a) said second electrode layer (9) is produced in contact with the photovoltaic conversion layer (7) so that it locally has interruptions (23) of material forming cuts through the thickness of said second layer,  b) and, substantially in the extension of these cuts (23), the photovoltaic conversion layer (7) is selectively removed by chemical etching so that at this location this layer itself has said cuts (19) which include a rim (21) of electrical insulation between said first and second layers forming an electrode.  8. - Method according to claim 7 characterized in that  - Before step a) of claim 7 and over the entire periphery of the device near its outer edge (18), said first layer (5) forming an electrode is produced with a continuous trench (25) formed over the thickness of this layer, and  - after having deposited on this said first layer (5) the photovoltaic conversion layer (7), itself surmounted by said second layer forming an electrode (9), steps a) and b) of claim 7 are carried out while providing the cuts (23) of the second layer (9) during step a) so that they have at least part of the form of trench extending above, substantially in the extension, of the trench (25) of the first layer (5) with a greater trench width.  9. - Method according to claim 7 or claim 8 characterized in that during step b) of claim 7 is chemically eroded the photovoltaic conversion layer (7) with a sodium hydroxide solution.  10. - Method according to any one of claims 7 to 9 characterized in that said second layer (9) forming electrode of step a) of claim 7 comprising a stainless metal resistant to soda and said photovoltaic conversion layer (7) being based on amorphous silicon, this photovoltaic conversion layer (7) is chemically eroded with a sodium hydroxide solution.  11. - Method according to claim 9 or claim 10 characterized in that said sodium hydroxide solution is concentrated between about 5 and 40% and the chemical erosion of the photovoltaic conversion layer (7) is carried out by spraying at a temperature between about 45 and 65 C.  12. - Method according to any one of claims 7 to 11 characterized in that to carry out step a) of claim 7, firstly depositing said second electrode layer (9) substantially uniformly on the photoelectric conversion layer (7) then selectively removing areas of material to form through this second layer (9) said cuts (23).  13. - Method according to any one of claims 8 to 12 characterized in that to produce said first layer (5) forming an electrode with its trench (25), this first layer is first deposited in a substantially uniform manner on a substrate (3) then selectively removing this first layer (5) to form through it said continuous trench (25).