T 0380/96 (Photovoltaic device/SOLAREX CORPORATION) 06-06-2001

1. The appeal is admissible.

2. The main request

2.1. Inventive step

Document D1 was identified by the Examining Division to represent the closest prior art, and was recognized as such by the appellant in his submissions during the appeal proceedings. The Board sees no reason to depart from this assessment.

Document D1 relates to a photovoltaic device in which individual photovoltaic cells are connected in series and groups of series connected cells are connected in parallel by bus bars formed by electrical contacts (24, 26) mounted across (page 2, lines 72 to 74) or bonded to (page 3, lines 104 to 106) the respective electrode segments. The device disclosed in document D1 thus has all the features referred to in the preamble of claim 1. The only difference between the invention as claimed and the prior art document D1 resides in the manner in which the bus bar is formed which provides the parallel connection. It follows that the objective problem solved by the invention claimed in claim 1 of the main request is that of providing alternative means of connecting in parallel groups of photovoltaic cells which are interconnected in series.

The appellant argued that the problem addressed by the invention is that of providing photovoltaic cells customised in respect of both size and electric output (see section V above). By providing for series connection of individual cells into groups and parallel connection of these groups, the device structure known from document D1 provides the main features which are necessary to taylor the size of individual cells as well as the size of arrays of these cells. Thus the problem as stated by the appellant is solved by the prior art, and hence it cannot be considered to be the objective problem underlying the invention.

Document D2 relates to thin-film solar cells, that is, to photovoltaic devices of the kind to which the application in suit relates. The device structure comprises a glass substrate 10, a patterned transparent front conductor 18 deposited on the glass substrate, a thin semiconductor film 20 formed on top of the front conductor and a metallic back contact 22 formed on the semiconductor material. In order to improve the electrical properties of the front contact layer, and in particular to reduce its contact resistance, metallic strip conductors 24 are deposited directly on the transparent conductive layer (column 2, lines 50 and 51). The metallic strips are preferably applied over the conductive layer either by screen printing or by evaporation through a mask (column 3, lines 4 to 6). In the case of screen printing a suitable commercially available screen printing paste is forced onto the surface through a pattern screen. The paste may contain for example silver powder glass frit and a suitable organic vehicle or binder. After application to the transparent layer the paste is fired to drive off the organic vehicle and leave the silver and glass frit behind. The glass frit fuses to the substrate providing a strong bond while the silver provides the desired electrical conductivity. (column 3, lines 7 to 15).

The appellant submitted that, with reference to the embodiments shown in Figures 2 and 8 of document D1, the skilled person would have had no incentive to look to document D2 for providing an alternative to the construction of the bus lines 24 and 26 of document D1, because the metallic strips in document D2 did not serve to provide a parallel connection between groups of photovoltaic cells; rather they served to decrease contact resistance of the front contact of the photovoltaic cells themselves.

The Board does not find the appellant's argument convincing. Document D1 clearly discloses that the subarrays of the photovoltaic device are connected in parallel by a common electrical contact structure which permits electrical connection of the array so formed to other electrical elements. In the embodiment shown in Figures 1 and 2, contacts 24 and 26 are "mounted across respective opposite ends of the array, in electrical low-resistance contact with respective opposite electrodes of each subarray" (page 2, lines 68 to 76). On page 3, it is described that electrical contacts 24 and 26 are bonded to electrode segments 42 of of the parent cell 32 and to electrode segments 54 of the parent cell 38 respectively (lines 104 to 106). Lastly, it is described on page 4, lines 89 and 90 that a contact in the form of a rail 26a can be provided as described for the previous embodiment.

In short, groups of series-connected photovoltaic cells are described in document D1 as being connected by a bus that is bonded to or mounted on the contact layers. To a skilled person, such a bus must undoubtedly have suitable electrical properties including an at least adequately low resistance. The skilled person considering alternatives for connecting those groups of cells in parallel will therefore immediately recognise that the commercially available conductive paste described in document D2 as being printed onto the front electrode layer for the purpose of providing a low electric resistance connection within each cell, will equally provide a low electric resistance connection when used as the bus connecting arrays of cells of document D1 in parallel.

The appellant also contended that according to document D2 silk screen printing was employed to apply the conductive paste, in contrast to the present invention which employed a computer controlled printing head to apply the conductive paste. This feature was not, however, a feature of the device as claimed, and hence could not be considered relevant to the issue at stake.

For these reasons the Board concludes that an inventive step was not required in arriving at the invention as claimed in the main request.

3. The first auxiliary request

3.1. Inventive step

Claim 1 of the first auxiliary request differs from claim 1 of the main request by the addition, at the end of the characterising clause, of the words "disposed on said front contact layer".

Document D1 discloses that the parallel connections are provided by contacts 24 and 26 being bonded to the electrode segments 42 and 44, respectively (page 3, lines 104 to 106). Elsewhere in document D1, the contacts are described as being mounted across the ends of the array (page 2, lines 72 to 76). A separate bus mounted on or bonded to the front contact layer is therefore already known from document D1. Bearing in mind the conclusions reached in respect of the main request, the Board concludes that even with the additional feature included in claim 1 of the first auxiliary request, the claim does not involve an inventive step.

4. The second auxiliary request

4.1. Admissibility of the amendments (Article 123(2) EPC)

Claim 1 of the second auxiliary request differs from claim 1 of the first auxiliary request in that the characterising portion of the claim additionally requires that the solidified conductive paste is "at least partially disposed under said semi-conductor material (134)".

In support of the admissibility of the amendments, the appellant relied on three specific passages in the description. They are at page 15, lines 2 to 19; page 18, lines 6 to 8; and page 18, lines 11 and 12.

The cited passage on page 15 relates to the conductive patterns being applied by way of a conductive fluid, and the preferred composition and properties of the conductive fluid are set out. There is no mention at all concerning the deposition of the semiconductor material.

In lines 6 to 8 on page 18 it is stated that the bus means is "disposed on the front contact layer 132 in the manner described above following which the steps shown in Figures 2(c) to 2(g) are performed". In the preceding sentence (lines 5 and 6), it is also pointed out that in Figures 2(c) to 2(g) "only the front contact layer is shown, but the bus means is not". Hence, neither the part of the description referred to by the appellant nor Figures 2(c) to 2(g) provide any information concerning the relative positioning of the bus and the semiconductor layer.

Lines 10 and 12 on page 18 contain a brief description of the deposition of the semiconductor layer in these terms: "... a substantially continuous film 134 of semiconductor material then is fabricated over front electrodes 118 and in first grooves 24, as shown in Figure 2(c)." There is no mention of the bus or its positioning relative to the semiconductor layer.

It follows that the parts of the description relied on by the appellant and the Figures of the drawings referred to there fail to establish that the claimed feature is derivable from the application as filed. Moreover, no other parts of the application as filed, including the plan views of Figures 3 to 6 provide any information relevant in this respect.

Since the application as filed does not contain any information which provides the required unambiguous support for the claimed feature, the Board concludes that its introduction into claim 1 goes beyond the subject matter of the application as filed, and that the second auxiliary request therefore contravenes Article 123(2) EPC.

5. The third auxiliary request

5.1. Clarity (Article 84 EPC)

Claim 1 of the third auxiliary request differs from claim 1 of the first auxiliary request by additionally specifying in the characterising clause that the solidified conductive paste is "deposited prior to deposition of said semi-conductor".

The sequence in which the depositions take place is a process feature. There is no device feature which is directly attributable to this sequence. The claim is therefore unclear.

The appellant had submitted that the sequence in which the depositions are performed can be ascertained from a cross section of the device. The Board cannot follow this argument for the following reasons.

The deposition sequence can be ascertained from the ensuing device structure only when the materials concerned are deposited one on top of the another but not if they are deposited side by side. It is not clear from the description and drawings whether the materials were deposited side by side or on top of one another.

The method steps included in the device claim 1 do not result in unambiguously defined device features, and the Board therefore concludes that the claim lacks clarity contrary to the requirement of the second sentence of Article 84 EPC.

5.2. The fourth auxiliary request

Claim 1 of the fourth auxiliary request relates to a thin-film voltaic device of the kind disclosed in document D1, which is characterised in that the scribe lines separating the back contact layer extend through the device to the front contact layer.

According to the argument submitted by the appellant, this complete gap is the result of scribing the scribe lines from the front of the device by ablation (see the argument in paragraph V(e) above), a method of scribing which is not found anywhere in the cited prior art. Moreover, the resulting structure in which the ablated semi-conductor material is completely removed between the front contact layer and the back contact layer clearly distinguishes the invention as claimed from the cited prior art document D1 where the corresponding gap 56 extends through the back contact layer only part way into the semiconductor layer 46, 48.

Document D1 discloses that in order to "prevent shorting between adjacent electrode segments 54 in a subaray, through semiconductor layer 48, grooves 56 are extended down to and preferably into at least the high-resistance layer 46" (page 3, lines 19 to 23).

The purpose of the gaps 129 of the application in suit is to provide insulation between adjacent photovoltaic cells and is thus the same as the purpose of the corresponding grooves 56 described in document D1. Moreover document D1 refers to the grooves 56 extending through the back contact layer at least as far as the high resistance portion of the semiconductor layer. This is a clear indication that it is contemplated in document D1 that the grooves 56 may extend beyond the high resistance layer. Whilst the Board accepts that the method of forming the gaps described in the application in suit may be new and superior to conventional methods for dividing the back electrode, the Board does not consider that these differences in method are reflected in the structural differences between the device disclosed in document D1 and the invention as claimed in claim 1 of the fourth auxiliary request. The Board therefore considers that in view of the disclosure in document D1, claim 1 of the fourth auxiliary request does not involve an inventive step as required by Article 56 EPC.

6. For the foregoing reasons in the Board's judgement the claims of neither the main request nor the first, second, third or fourth auxiliary request meet the requirements of the EPC.