T 0074/14 21-03-2019

1. The appeal is admissible.

2. As announced in advance, the duly summoned opponent-appellant did not attend the oral proceedings. According to Rule 71(2) EPC 1973, if a party duly summoned to oral proceedings does not appear as summoned, the proceedings may nevertheless continue, the party then being treated as relying only on its written case. As the present case was ready for decision at the conclusion of the oral proceedings (Article 15(5) and (6) RPBA), the voluntary absence of a party was not a reason for delaying the decision (Article 15(3) RPBA).

3. Main Request: Article 100(c) EPC 1973

3.1 Claim 1 of the main request is largely based on claims 1, 2 and 5 of the English translation of the PCT application as originally filed. The features which are not found in the originally filed claims are essentially as follows (in italics):

(a) a method of forming a silicon based thin film solar cell (claim 1 as originally filed concerned a silicon based thin film solar cell);

(b) depositing a photoelectric conversion layer on a translucent board;

(c) an n-type silicon based low refractive index layer; and

(d) an n-type silicon based interface layer.

3.2 In the statement of grounds of appeal the opponent does not raise any objection against the change of category per se, and the Board also sees no reason to do so. In the arrangement of Fig. 2, the layers are formed in the order claimed, on an underlying translucent board 1, and so feature (b) is adequately based in the original application, even if various other deposition possibilities are mentioned in the description (see page 13, lines 6-13). Claim 1 as originally filed defined a "conducted type" silicon based low refractive index layer, which can only be taken to refer to silicon having a conductivity type (p or n), and the choice to limit to n-type, as disclosed in the arrangement of Fig. 2, does not introduce undisclosed subject-matter. The argument that only arrangements having precisely two photoelectric units are disclosed (an amorphous unit 3 and a crystalline unit 4) is contradicted by the description (page 13, lines 2-6).

3.3 The remaining objection concerns forming an "n-type silicon based interface layer". In claim 1 as originally filed this layer was simply a "silicon based interface layer". It is not disputed that the embodiment of Fig. 2 provides a basis for an "n-type silicon based interference layer" per se (page 12, line 9), but this is disclosed within the context of an arrangement in which other non-claimed features are also disclosed, in particular the interface layer being disposed between the low refractive index layer and the back electrode (page 9, lines 10-14; page 12, last paragraph; Fig. 2).

3.4 The proprietor argues that claim 1 should be seen as defining a method of manufacturing an intermediate product, which does not necessarily comprise all the features of a solar cell, in particular a back electrode, and there is therefore no requirement to include in the claim a back electrode or the relationship of the interface layer to a back electrode.

The Board does not agree that claim 1 defines a method of forming an "intermediate product", it quite explicitly defines a method of "forming a silicon based thin film solar cell". However, in relation to the present discussion, what is at issue is not the nature of the final product, but whether it is permissible to import into claim 1 a first feature from the description (forming an n-type silicon based interface layer), while not importing a second feature (the location of the interface layer between the low refractive index layer and the back electrode), given that both features are disclosed in combination.

3.5 According to consistent case law of the Boards, such an amendment introduces subject-matter which extends beyond the content of the application as filed only if there is a clearly recognizable functional or structural relationship between the features, i.e. if they are inextricably linked (see Case law of the Boards of Appeal of the European Patent Office, 8th edition 2016, II.E.1.7).

3.6 In the present case, the function of the intermediate layer is described in the application as filed as follows:

"The n type silicon based interface layer 4n is used in order to improve a contact resistance between the n type silicon based low refractive index layer 4on and a back electrode 5" (page 12, lines 11-14).

It would be apparent to the skilled person that the resistance is reduced by the combination of the material being n-type (a p-type or i-type interface layer or a layer of e.g. silicon oxide would not perform the intended function) and its position (an interface layer which was not located between the low refractive index layer and the back electrode would also not perform the intended function). These two features are therefore functionally related and inextricably linked.

3.7 As a result, the amendment in question constitutes an extension of the subject-matter of the European patent beyond the content of the application as filed, and hence the patent cannot be maintained according to the main request (Article 100(c) EPC 1973).

4. Auxiliary requests: Admission into the proceedings

4.1 Auxiliary requests 1-6 were filed approximately one month before the oral proceedings and may only be admitted and considered at the Board's discretion (Articles 13(1) and 13(3) RPBA).

4.2 In the case of auxiliary requests 1-3, the additional features of claim 1 are entirely unrelated to, and would not overcome, the Board's objection to the main request on the grounds of Article 100(c) EPC 1973. These requests therefore serve no useful purpose and are not admitted into the proceedings.

4.3 In the case of auxiliary requests 4 and 5, the additional features of claim 1 of auxiliary requests 4 and 5 are focused on the relevant objection, but neither request corresponds precisely to the embodiment of Fig. 2, according to which the back electrode 5 consists of a transparent reflecting layer 5t, and a back reflecting layer 5m. The Board makes no comment on whether this difference results in an extension of the subject-matter beyond the content of the application as filed, but merely notes that if these requests were admitted into the proceedings, this matter would have to be considered. The Board is not minded to admit requests filed at a late stage in the proceedings which raise such new issues, especially as the letter accompanying these requests contained no reasoning whatsoever indicating why these requests should be considered allowable. Hence, auxiliary requests 4 and 5 are not admitted into the proceedings.

4.4 The additional features of claim 1 of auxiliary request 6 have a basis in the application as filed (e.g. page 12, final paragraph), and this request is considered to overcome the Board's objections to the main request in relation to the ground of Article 100(c) EPC 1973, without raising any new issues. Auxiliary request 6 is therefore admitted into the proceedings.

5. Auxiliary request 6: Article 123(2) EPC

As noted in the previous paragraph, the subject-matter of claim 1 of auxiliary request 6 is found not to extend beyond the content of the application as filed. Dependent claims 2-6 are based on claims 3, 4 and 6-8 as originally filed. Auxiliary request 6 therefore meets the requirements of Article 123(2) EPC.

6. Auxiliary request 6: Article 100(b) EPC 1973

6.1 Although the opponent's objection under Article 100(b) EPC 1973 was raised in relation to the main request, it would apply equally to auxiliary request 6. The objection is that the claimed method includes open ended ranges (the low refractive index layer has no lower limit for the refractive index and no upper limit for the thickness), and hence "the thickness may be infinite and the refractive index may be negative". However, "both an infinite thickness and a negative refractive index are not practically realizable so that the invention cannot be performed over the entire range claimed."

6.2 Conventionally, the absolute lower limit for the refractive index of any material is 1 (the refractive index in a vacuum), and for a low refractive index layer formed of an alloy of n-type silicon and silicon oxides (as disclosed in the description), the lower limit would be about 1.46, i.e. the refractive index of SiO2 at 600 nm.

It is true that there has been interest in recent years in exotic materials (e.g. engineered arrays of micro-resonators or certain photonic crystals) displaying a negative refractive index. In the opinion of the Board, the fact that it may be theoretically (and even, to some extent, practically) possible to construct such materials does not mean that the term "refractive index" in a patent document must automatically be considered to extend to negative refractive indices. Where, as in the present case, it is abundantly clear that the invention concerns normal optical materials, the term "refractive index" is to be understood in its conventional sense, being restricted to positive values greater than 1. Hence, the fact that the patent does not disclose how to form an n-type silicon based low refractive index layer with a negative refractive index does not amount to a lack of disclosure within the meaning of Article 100(b) EPC 1973. The invention is simply not concerned with such materials.

6.3 Regarding the thickness of the low refractive index layer, a skilled person would not interpret claim 1 as being intended to extend, in practice, to "an infinite thickness", as suggested by the opponent.

This layer is disclosed as serving several functions (see passage bridging pages 7 and 8): it acts to generate a diffusion potential in the photoelectric conversion layer, it reflects light from its surface (hence the requirement for a low refractive index) and it should absorb as little light as possible (so that the transmitted light may be recycled by the reflective back electrode 5).

Thickness values for the low refractive index layer cited in the application are:

- in claim 1, "not less than 30 nm" (lambda/20 at the claimed nominal wavelength of lambda = 600 nm);

- in example 1 (page 15, lines 16-17), "600 Angstrom" (60 nm) i.e. lambda/10;

- on page 11, line 17, the preferable range is 500-900 Angstrom (50-90 nm).

As silicon is well known to display significant absorption in the visible range, the skilled person would, in practice, understand that the thickness of the layer should be limited to sub-wavelength values of this order.

6.4 In any event, Article 100(b) EPC 1973 requires that the patent should disclose the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art, and the Board sees no reason to doubt that the skilled person would be capable of manufacturing the claimed low refractive index layer having essentially any thickness (i.e. any thickness which could reasonably be required within the context of a method of forming a solar cell). Whether a particular thickness chosen would be optimal, sub-optimal or even poor has nothing to do with the ability of the skilled person to carry out the invention. The Board is therefore not persuaded by the opponent's arguments based on the ground of Article 100(b) EPC 1973.

7. Auxiliary request 6: Inventive step starting from D1

7.1 Document D1 discloses a photovoltaic device in which the layer 3n11 broadly corresponds to the n-type silicon based low refractive index layer of claim 1. Although no refractive index is disclosed for this layer, the stated provisional view of the Board was that, at least for some of the disclosed compositions (for example, SiO:H containing up to 25 at.%O - see Fig. 3B; claim 7), the refractive index would be less than 2.5 (in this regard, see claims 2 and 3 of the granted patent). This statement has not been challenged, and the Board sees no reason to deviate from this view.

7.2 The sole feature distinguishing claim 1 from D1 is therefore that the silicon based low refractive index layer has a thickness of not less than 30 nm.

As well as providing a light trapping function, the silicon based low refractive index layer "has a function to generate a diffusion potential in the photoelectric conversion layer" (paragraph [0016]). In a section describing such layers in general (paragraph [0005]) it is noted that they should be as thin as possible to reduce absorption, but must have sufficient thickness to generate an appropriate diffusion potential ("the conductivity type layers of the p type and the n type are preferably maintained for a smallest possible thickness within a range for generation of a sufficient diffusion potential"). Fig. 6 shows that below about 300 angstroms the conversion efficiency drops, presumably due to an insufficient diffusion potential.

7.3 It therefore appears reasonable to proceed on the basis that the distinguishing feature of claim 1 solves the problem suggested by the proprietor, i.e. it serves "to provide a high-efficient silicon based thin film solar cell".

7.4 In D1, the thickness of the corresponding layer 3n11 is given in table 1 as 50-200 Å (5-20 nm), and specific values within that range (10 nm, 15 nm) are also disclosed.

7.5 The function of the n-type layer 3n11 in D1 is to block diffusion of undesired elements from the back electrode 4 into the i-layer 3i, thereby minimizing the thermal degradation of the conversion efficiency when the device is held at a high temperature for a long time. The opponent argues that the skilled person would consider increasing the thickness of this layer to increase the blocking effect to "further decrease the thermal degradation of the solar cell".

7.6 The argument of the opponent that the analysis of inventive step should be based on the problem of reducing thermal degradation is based on the contention that the patent itself fails to disclose any technical effect associated with the distinguishing feature. However, as explained above, the Board accepts that the patent makes it plausible that ensuring that the silicon based low refractive index layer has a thickness of not less than 30 nm contributes to improving the efficiency, and no good reason can be seen why the problem should be shifted from one which is disclosed in the description to one which is nowhere mentioned or hinted at in the patent.

There is nothing in D1 suggesting that increasing the thickness of layer 3n11 would increase the efficiency, or provide a greater diffusion potential (or indeed that it would lead to any other benefit). For this reason alone, the opponent's argument from D1 fails.

7.7 For completeness, the Board does not find the arguments of the opponent persuasive even on the basis of the suggested problem of reducing thermal degradation. The disclosed layer 3n11 of SiN:H containing 25 at.%N and with a thickness of about 15 nm (column 3, lines 43-45) appears to substantially eliminate the problem of thermal degradation, as shown in Fig. 2. It is nowhere suggested in D1 that increasing the thickness of the layer would lead to a further improvement in this regard, and the opponent's contention that it would is essentially no more than speculation. In addition, to arrive at the claimed feature, the thickness of the layer would have to be doubled from 15 nm to 30 nm, which, as the skilled person would understand, would have a significant and undesirable effect on the absorption of the layer.

8. Auxiliary request 6: Inventive step starting from D2

8.1 The argument starting from D2 is based on the described embodiments corresponding to Fig. 10, and identifies layer 61 with the claimed photoelectric conversion layer, layer 63 with the claimed n-type silicon based low refractive index layer and layer 64 with the claimed n-type silicon based interface layer (similar identifications could be made in the case of Fig. 12).

8.2 Claim 1 differs from D2 in that the silicon based low refractive index layer has:

- a thickness of not less than 30 nm; and

- a refractive index not more than 2.5 at a wavelength of 600 nm.

In D2, the layer 63 is disclosed as having a thickness of 50 Å (5 nm) and no refractive indices for the layers are quoted.

8.3 As explained above (point 6.3), the Board finds it reasonable to proceed on the basis of the objective technical problem suggested by the proprietor ("to provide a high-efficient silicon based thin film solar cell").

8.4 To arrive at the claimed thickness (not less than 30 nm) the thickness of the layer 63 in D2 (5 nm) would have to be increased by a factor of at least six. There is nothing in D2 which would motivate such a measure in order to increase the efficiency, or to provide a greater diffusion potential or for any other reason.

8.5 In the statement of grounds of appeal it is argued that D3 discloses:

"a silicon based thin film solar cell having a low refractive index layer of a greater thickness, from around 10 nm to 20 nm in, for example, paragraph 27."

The opponent goes on to argue that, from this, D3 would provide the necessary motivation to arrive at the claimed subject-matter:

"Therefore, the skilled person learns from D3 that the thickness of a low refractive index layer may be greater than the 5 nm. Therefore, the skilled person would not have been dissuaded from investigating the effect of the thickness of the layer 63 in D2."

8.6 This argument fails for two reasons. Firstly paragraph [0027] of D3 is concerned with the properties of a layer of amorphous SixO1-x, whereas in D2, the only concrete example of the material of layer 63 is amorphous Si0.9N0.1 (tables 6 and 7), and so it cannot be seen why any conclusions drawn in D3 should be considered pertinent. Secondly, as stated by the opponent, the thickness of the layer referred to in D3 ("around 10 nm to 20 nm") is also below the claimed lower limit of 30 nm, and hence the Board does not see how this document (even if it were to be considered as prior art according to Article 54(2) EPC) would incite the skilled person to increase the thickness of the layer in question to 30 nm or greater.

8.7 The opponent has failed to demonstrate that it would be obvious, starting from D2, to arrive at the claimed feature that the silicon based low refractive index layer has a thickness of not less than 30 nm, and it is therefore not necessary to investigate whether the second difference (a refractive index not more than 2.5 at a wavelength of 600 nm) is obvious.

9. Auxiliary request 6: Inventive step starting from D8

9.1 In the argument starting from D8, layer 105 is identified with the claimed photoelectric conversion layer, layer 116 is identified with the claimed n-type silicon based low refractive index layer and layer 104 is identified with the claimed n-type silicon based interface layer. During manufacturing, the surface of n-type silicon layer 104 is oxidized (by exposure to oxygen or using plasma CVD) to form the silicon based oxide thin film 116 (paragraphs [0013]-[0017]), and the photoelectric conversion layer 105 is then formed on oxide layer 116 (paragraph [0018]).

9.2 The term "n-type silicon based low refractive index layer" in claim 1 implies that the layer comprises a certain amount of n-type silicon, i.e. elemental silicon doped with donor impurities. According to the contested patent, this layer is formed from an alloy layer comprising (n-type) silicon and other elements, such as oxygen, typically as silicon oxides (paragraph [0017]). The skilled person would understand that it is the n-type silicon that provides the layer's contribution to the generation of the diffusion potential in the photoelectric conversion layer, while the other components (e.g. silicon oxides) serve to reduce the refractive index to facilitate light trapping (paragraph [0016]).

9.3 In D8, however, layer 116 is formed by oxidization of the surface of the silicon layer 104 (which may be n-type), and hence layer 116 would be a thin layer entirely composed of silicon oxide (apart from small numbers of dopant phosphorus atoms, but without any n-type elemental silicon). The complete oxidation of layer 116 is confirmed in paragraph [0022], where it is described as "a typical insulator film". Layer 116 in D8 is not, therefore, an "n-type silicon based low refractive index layer" as defined in claim 1 of the patent, but an insulating layer.

If the skilled person decided to reverse the order in which the layers were deposited, as suggested by the opponent, presumably the oxide layer 116 would be formed by fully oxidizing the surface of the underlying photoelectric conversion layer 105, and the resulting oxide layer would still not be an "n-type silicon based low refractive index layer".

9.4 Hence, contrary to the submissions of the opponent, layer 116 cannot be identified with the claimed n-type silicon based low refractive index layer. Consequently, the argument of the opponent fails, and no obvious route is apparent by which the skilled person would be led to the claimed subject-matter starting from D8.

10. For the reasons given above, the Board judges that the subject-matter of claim 1 of auxiliary request 6 involves an inventive step within the meaning of Article 52(1) EPC and Article 56 EPC 1973.

11. Documents D10-D16: Admission into the proceedings

11.1 Documents D10-D15 were not admitted into the proceedings before the department of first instance, and D16 was filed for the first time with the statement of grounds of appeal. The admission of these documents into the appeal proceedings is therefore at the discretion of the Board (Article 12(4) RPBA).

11.2 The Opposition Division found (Reasons, point 5.2) that documents D10-D15 were not prima facie relevant, in the sense of being likely to prejudice the maintenance of the patent (citing T 1002/92). In the statement of grounds of appeal, the opponent challenged this decision, arguing that these documents were relevant to support inventive step arguments starting from documents cited in the notice of opposition (D1, D2 and D8), and also in new combinations which were not presented in the first instance proceedings. No reasons were given why such combinations could not have been included in the notice of opposition.

11.3 Where a Board of Appeal is requested to review a discretionary decision of a department of first instance not to admit late-filed submissions, it is generally accepted that:

"it is not the function of a Board of Appeal to review all the facts and circumstances of the case as if it were in the place of the first instance department, in order to decide whether or not it would have exercised such discretion in the same way as the first instance department ... a Board of Appeal should only overrule the way in which a first instance department has exercised its discretion if it comes to the conclusion either that the first instance department in its decision has not exercised its discretion in accordance with the right principles ... or that it has exercised its discretion in an unreasonable way, and has thus exceeded the proper limits of its discretion." (G 7/93, Reasons, point 2.6.)

11.4 To successfully challenge the decision of an Opposition Division not to admit documents into the proceedings on the grounds of lack of relevance prima facie, it is not enough merely to argue that they should in fact be considered relevant or that they could be used in an inventive step attack. Rather, it would be necessary to demonstrate that these documents are so manifestly relevant, and that the failure to recognise this was such an egregious misjudgement on the part of the Opposition Division, that the decision not to admit them constituted an unreasonable exercise of the Opposition Division's discretion.

11.5 As the opponent has not demonstrated this - or even attempted to demonstrate it - the Board sees no reason to deviate from the preliminary opinion expressed in the communication pursuant to Article 15(1) RPBA that the Opposition Division dealt with this matter in accordance with the right principles and in a reasonable way. The Board therefore confirms the decision of the Opposition Division that documents D10-D15 are not admitted into the proceedings.

11.6 No reasons were given by the opponent why D16, filed with the statement of grounds of appeal, could not have been filed earlier. Moreover, admitting this document (filed to establish common general knowledge concerning silicon oxide) would not change any of the conclusions drawn above. D16 is therefore not relevant and is not admitted into the proceedings.