Photovoltaic cell having a coloured appearance, particularly for a watch dial

Abstract

There is described a colored photovoltaic cell (1) with a semiconductor, preferably silicon, which has high efficiency and a pleasing colored appearance, allowing it to be used as a dial for a watch or another electronic apparatus powered by the cell. The cell includes a reflective metal substrate (2) serving as the bottom electrode, a stack of hydrogenated amorphous silicon layers forming p-i-n junctions (8), and a transparent top electrode (9). The latter may be coated with a layer (16) of slightly diffusing lacquer, which may be colorless or colored. The respective thicknesses e1 of the top electrode and e2 of the silicon are combined as a function of the refractive indices of the materials so as to produce an interferential reflection in a predetermined reflection spectrum.

Description

[0001] The present invention concerns a photovoltaic cell, also called a solar cell, which can be used to form a dial for a watch or another electronic apparatus and to provide the watch or apparatus with electric power.

[0002] The invention concerns more particularly a coloured photovoltaic cell of the type including from the bottom to the top a substrate, a bottom reflecting electrode placed on the substrate or integrated therein, an active photodiode part formed of semiconductor layers, and a top electrode. The invention also concerns a watch dial including such a coloured photovoltaic cell.

[0003] The use of photovoltaic cells as dials for wristwatches encounters several problems which may of a technical or aesthetic nature. First, when the watch is worn, the cell or a set of cells connected in series has to generate sufficient power not only in the short term, but also in terms of the annual energy balance of the storage element (accumulator), to take account of periods of poor light, in particular the winter. As the surface area of a watch dial is limited, the cell has to have sufficient photovoltaic efficiency. Further, one wishes to have as thin a cell as possible. Photovoltaic cells made of amorphous silicon are advantageous in this respect, particularly if the substrate is metal in nature and forms one of the current collector electrodes.

[0004] On the other hand, aesthetic criteria are important in the field of horology. They result in the use of a single cell being preferred to that of several juxtaposed cells connected in series, in order to avoid the lines of separation and interconnection which remain visible between the cells. One primordial aesthetic aspect is that of colour. The amorphous silicon cells deposited on a metal substrate have in themselves a greyish appearance which is unattractive. Various publications proposing solutions to avoid this drawback will be mentioned hereinafter. Finally, in addition to obtaining attractive, varied and sufficiently light colours, one may wish the dial to give either specular reflection, or diffuse reflection.

[0005] European Patent No. 872 783 discloses a watch dial, formed of a single hydrogenated amorphous silicon photovoltaic cell (&agr;-Si:H), whose top electrode is formed of a transflective, i.e. semi-reflecting, metal layer, which preferably reflects between 60% and 85% of the incident light. This permits a simple and thin construction, giving the dial a metallic lustre, but the photovoltaic efficiency is reduced by the fact that most of the incident light is reflected towards the exterior or absorbed in the metal layer and thus does not reach the silicon.

[0006] Another category of solutions known, in particular, from European Patent Nos. 788 037 and 819 995, consists in placing on the photovoltaic cell a plate formed of various translucent coloured layers themselves providing the desired appearance of the dial. However, these plates have to be diffusing to conceal the silicon, which on the one hand reduces the quantity of light reaching the photodiode and on the other hand gives the dial a milky appearance lacking in lustre. Moreover, this plate increases the total thickness of the construction.

[0007] The object of the present invention is to create a photovoltaic cell allowing the aforementioned drawbacks to be avoided when it is used as a watch dial or an apparatus imposing the same aesthetic criteria. In particular, the invention should allow a wide choice of colours for the dial, without excessively reducing the portion of incident light which reaches the photodiode. An additional object is to offer the designer the choice between a specular reflection and a diffuse reflection of the incident light.

[0008] According to a first aspect of the invention, there is provided a photovoltaic cell of the type indicated hereinabove, characterised in that the pair of respective thicknesses of the top electrode and the active photodiode part is selected as a function of the respective refractive indices of their materials so as to generate an interferential reflection of the incident light according to a predetermined reflection spectrum.

[0009] Thus, the multi-layered structure of the photovoltaic cell constitutes a reflective interferential filter with which it is possible to obtain a wide range of colours of the reflected light, simply by an appropriate choice of the thicknesses of the layers generating the interferential reflection, by using the constituent materials of the photovoltaic cell. In practice, said thicknesses are selected from the ranges of thickness compatible with the proper operation of the photovoltaic cell, in particular with regard to the power which it provides.

[0010] The parts of the spectrum in which a substantial fraction of the incident light is reflected towards the exterior may be relatively narrow, so that most of the incident ambient light is absorbed by the photodiode to generate the electric power. All other conditions being equal, this type of cell thus allows currents to be obtained which are significantly higher than those of a cell with a semi-reflective metallic top electrode covered with coloured gloss. The photodiode can advantageously be made in a conventional manner from hydrogenated amorphous silicon. Preferably, the substrate is metal and serves both as the bottom electrode and the reflector.

[0011] Preferably, the active part of the silicon photodiode has a thickness comprised between 100 and 600 nm and the top electrode has a thickness comprised between 60 and 300 nm, the pairing of these two thicknesses resulting in a determined colour of the reflected light. In particularly preferred ranges of thickness, the active part of the silicon photodiode has a thickness comprised between 250 and 450 nm and the top electrode has a thickness comprised between 70 and 150 nm. Below these lower limits, the sheet resistance of the top electrode increases, creating undesirable ohmic losses, and the power conversion in the silicon is reduced if the thickness of the silicon is too small. Above the aforementioned upper limits, the deposition of the top electrode becomes too expensive and the mechanical hold of the silicon on the substrate becomes problematic.

[0012] In order to further enlarge the palette of available colours, a development of the invention consists in that the top electrode is covered with a clear or transparent lacquer, which may also be diffusing. This layer may further contain colorants or pigments, thus exhibiting a predetermined absorption spectrum in order to modulate the colour of the reflected light by absorbing certain wavelengths. These techniques allow dials simulating the appearance and colours of conventional watch dials to be obtained.

[0013] Other features and advantages of the present invention will appear from the following description of various embodiments, given by way of non-limiting example with reference to the annexed drawings, in which:

[0014] FIG. 1 is a schematic partial cross-section of a first embodiment of a photovoltaic cell according to the invention, forming a watch dial;

[0015] FIG. 2 is a similar view to FIG. 1, showing a second embodiment of the invention;

[0016] FIG. 3 is a similar view to FIG. 1, showing a third embodiment of the invention;

[0017] FIG. 4 shows the reflectance as a function of the wavelength, obtained with the structure illustrated by FIG. 1 for different pairs of thickness of the silicon and the transparent top electrode;

[0018] FIG. 5 shows the RGB co-ordinates of the reflected light for the structures whose reflection spectrum is shown in FIG. 4;

[0019] FIG. 6 shows the reflectance as a function of the wavelength, obtained with the structures respectively illustrated in FIGS. 1, 2 and 3, for a chosen pair of respective thickness of the silicon and the transparent top electrode; and

[0020] FIG. 7 shows the absorption spectrum of a colorant used in one of the cases shown in FIGS. 3 and 6 (curve 26).

[0021] The watch dial shown in FIG. 1 is formed by a photovoltaic cell 1 according to the present invention, including a metal substrate 2 which serves as bottom electrode 3 of the cell. Substrate 2 is preferably made of stainless steel, but other metals such as aluminium or a metal substrate coated with chromium may be used. The top surface 4 of the substrate reflects light, either specularly, or in a diffused manner, in order to improve the power efficiency. Substrate 2 supports a stack of three thin layers 5, 6 and 7 of hydrogenated amorphous silicon, respectively of types n, i and p or conversely, to form the active part of a n-i-p or p-i-n junction photodiode, indicated by reference 8. Onto the stack of silicon layers there is applied a transparent top electrode 9 formed of a thin layer of conductive oxide, for example a layer of indium oxide doped with tin (ITO), or a layer of tin oxide doped with antimony.

[0022] In the watch, the connection of the electrodes to the charge circuit of the electric accumulator is effected in a conventional manner on an edge of the dial formed by cell 1. A central hole 11 is arranged through the dial, to allow the shafts of the watch hands to pass.

[0023] Of course, bottom electrode 3 could include a reflective metal layer made of a material distinct from substrate 2, in the event that the material of the substrate was not compatible with the adjacent n or p type layer 5.

[0024] The silicon and ITO layers can be deposited by conventional methods allowing the thickness of the layers to be carefully controlled, for example RF plasma deposition for the silicon and cathodic sputtering deposition for the ITO. Examples of methods for manufacturing photovoltaic cells of this type in batches are explained in particular in U.S. Pat. Nos. 4,485,125 and 5,457,057 and European Patent No. 948 060.

[0025] The layers of silicon forming the active photodiode part 8 have a controlled total thickness e2 and a refractive index having a real part of approximately 4. The ITO forming top electrode 9 has a controlled thickness e1 and a real refractive index of approximately 2, absorption being able to be ignored in the particular case of such a layer. Consequently, the two constituent layers 8 and 9 of the photovoltaic cell, arranged between the air and the metal substrate, form an interferential optical system having a reflectance R(&lgr;), where &lgr; is the wavelength of the incident light 10, which has a spectral intensity Io(&lgr;). The light 14 reflected by cell 1, of spectral intensity Io(&lgr;)R(&lgr;), has a coloured appearance dependent on the reflectance R(&lgr;) defined by the indices and thicknesses of the elements of the interferential system.

[0026] Knowing the refractive indices of the materials used to make the photovoltaic cell, one can calculate the interferential reflection spectrum and the corresponding chromatic indices as a function of thicknesses e1 and e2 and select the combinations of thickness providing the desired colours, taking account of the constraints imposed to obtain good electrical and mechanical characteristics for the photoelectric cell. The same operations may be made with other materials having other refractive indices.

[0027] FIG. 2 shows a second embodiment wherein the structure of photovoltaic cell 1 described with reference to FIG. 1 is completed by a layer of clear or diffusing lacquer 16, applied onto top electrode 9. This layer, having a refractive index of approximately 1.5 and a thickness of the order of 1 &mgr;m to several tens of &mgr;m, modifies the reflectance R(&lgr;) of the subjacent interferential system, because its refractive index is different to that of the ITO. Moreover, a diffusing lacquer reproduces the appearance of conventional dials better, while reducing or removing the angular dependence of the interferences in the reflected light. In this example, the lacquer of layer 16 has no intrinsic absorption, i.e. no colour of its own. Since its thickness is relatively large, its only notable influence in the interferential reflection is that of its refractive index modifying the optical conditions at the interface between the lacquer and top electrode 9. Further, the lacquer constitutes mechanical and chemical protection for electrode 9.

[0028] FIG. 3 shows a third embodiment wherein the structure of photovoltaic cell 1 described with reference to FIG. 1 is completed by a layer of coloured lacquer 18, which is transparent or slightly diffusing, applied onto top electrode 9. In addition to producing the same effects as the layer of colourless lacquer 16, coloured lacquer 18 has a transmission spectrum T(&lgr;) which modifies the reflected light 14 the spectrum of which is obtained by convoluting spectra Io(&lgr;), R(&lgr;) and T(&lgr;). By this means, it is possible to modify the colour of the reflected light, for example in order to remove certain undesirable components of the interferential reflection spectrum. This allows a great number of nuances of colour to be obtained by using the principles of the present invention.

[0029] FIG. 4 is a spectral diagram of the reflectance R as a function of the wavelength &lgr; for three examples of photovoltaic cells having the structure illustrated in FIG. 1, for three different pairs of thicknesses e1 and e2. FIG. 5 shows the coordinates of colours R (red), G (green) and B (blue) for these three examples.

[0030] Spectrum 21, shown in continuous lines, corresponds to thickness values e2=280 nm of silicon and e1=80 nm of ITO. According to FIG. 5, the reflected light will have a dominant blue colour.

[0031] Spectrum 22, shown in dot-and-dash lines, corresponds to the same value e1=80 nm as in the preceding example, but with a value e2=420 nm for the silicon thickness. It can be seen that the spectrum is thus modified in the green and red region and that the reflected light will also have a dominant blue colour but slightly different.

[0032] Spectrum 23 shown in dashed lines, corresponds to the same value e2=420 nm as in the preceding example, but with a value e1=60 nm instead of 80 nm for the ITO thickness. It can be seen that the spectrum is thus greatly modified in several regions and that the reflected light will have a dominant magenta colour. It can thus be deduced that the thicknesses of the ITO electrode has a preponderant importance in the examples considered.

[0033] FIG. 6 is a spectral diagram of the reflectance R as a function of the wavelength &lgr; for three examples of photovoltaic cells having the structures illustrated respectively in FIGS. 1, 2 and 3, for a same pair of values e2=450 nm and e1=90 nm of the respective thickness of silicon and ITO. Spectrum 24, shown in a continuous line, corresponds to the lacquer free version of FIG. 1 and has a strong blue dominance. Spectrum 25, shown in dot-and-dash lines, corresponds to the version of FIG. 2, with a layer of colourless lacquer 16 of index 1.5 having a thickness of several &mgr;m. In comparison to spectrum 24, it is greatly reduced in blue and much more marked in green and red. Spectrum 26, shown in a dashed line, corresponds to the version of FIG. 3, with a layer of lacquer 18 mixed with a blue colorant whose absorption spectrum &agr;(&lgr;) is shown in FIG. 7, this layer having a thickness of several &mgr;m. It can be seen that the addition of colorant reduces the quantity of reflected light, especially in yellow and red.

[0034] The examples given hereinbefore show that the present invention provides those skilled in the art with the means to design photovoltaic cells having the most simple structure possible and sufficient efficiency, while exhibiting a predetermined colour, so that they are well suited to serve as dials for watches or other portable apparatus the aesthetic appearance of which is an important parameter.

Claims

1. A coloured photovoltaic cell including from the bottom to the top a substrate, a reflective bottom electrode placed on said substrate or integrated therein, an active photodiode part formed of semiconductor layers, and a transparent top electrode,

wherein the pair of respective thicknesses of the top electrode and the active photodiode part is selected as a function of the respective refractive indices of their materials, so as to produce an interferential reflection of incident light according to a predetermined reflection spectrum.

2. A photovoltaic cell according to

claim 1, wherein said semiconductor is hydrogenated amorphous silicon of types n, i and p.

3. A photovoltaic cell according to

claim 1, wherein said top electrode is coated with a layer of clear lacquer.

4. A photovoltaic cell according to

claim 3, wherein said layer of lacquer is diffusing.

5. A photovoltaic cell according to

claim 3, wherein said layer of lacquer contains colorants or pigments.

6. A photovoltaic cell according to

claim 1, wherein said substrate is metal and acts both as said bottom electrode and as a reflector.

7. A photovoltaic cell according to

claim 2, wherein said active photodiode part made of silicon has a thickness comprised between 100 and 600 nm and said top electrode has a thickness comprised between 60 and 300 nm, the pairing of said thicknesses leading to a determined colour of the reflected light.

8. A photovoltaic cell according to

claim 7, wherein said active photodiode part made of silicon has a thickness comprised between 250 and 450 nm and said top electrode has a thickness comprised between 70 and 150 nm.

9. A watch dial including a photovoltaic cell according to

claim 1.

10. A watch dial according to

claim 9, formed by a single photovoltaic cell, having a substrate which constitutes a substrate of the watch dial.