U.S. patent application number 09/738761 was filed with the patent office on 2001-06-28 for photovoltaic cell having a coloured appearance, particularly for a watch dial.
Invention is credited to Fischer, Diego, Saurer, Eric, Viennet, Rene, Ziegler, Yvan.
Application Number | 20010004900 09/738761 |
Document ID | / |
Family ID | 4232453 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010004900 |
Kind Code |
A1 |
Ziegler, Yvan ; et
al. |
June 28, 2001 |
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 e.sub.1 of the top electrode and e.sub.2 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.
Inventors: |
Ziegler, Yvan; (Villiers,
CH) ; Fischer, Diego; (Neuchatel, CH) ;
Saurer, Eric; (Bevaix, CH) ; Viennet, Rene;
(Neuchatel, CH) |
Correspondence
Address: |
GRIFFIN & SZIPL, PC
SUITE PH-1
2300 NINTH STREET, SOUTH
ARLINGTON
VA
22204
US
|
Family ID: |
4232453 |
Appl. No.: |
09/738761 |
Filed: |
December 18, 2000 |
Current U.S.
Class: |
136/256 ;
136/258; 136/291 |
Current CPC
Class: |
H01L 31/056 20141201;
Y02E 10/52 20130101 |
Class at
Publication: |
136/256 ;
136/258; 136/291 |
International
Class: |
H01L 031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 1999 |
CH |
2381/99 |
Claims
What is claimed is:
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.
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
(.alpha.-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 e.sub.2 and a refractive index
having a real part of approximately 4. The ITO forming top
electrode 9 has a controlled thickness e.sub.1 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(.lambda.), where .lambda. is
the wavelength of the incident light 10, which has a spectral
intensity I.sub.o(.lambda.). The light 14 reflected by cell 1, of
spectral intensity I.sub.o(.lambda.)R(.lambda.), has a coloured
appearance dependent on the reflectance R(.lambda.) 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 e.sub.1 and e.sub.2 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 .mu.m to several tens of
.mu.m, modifies the reflectance R(.lambda.) 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(.lambda.) which modifies
the reflected light 14 the spectrum of which is obtained by
convoluting spectra I.sub.o(.lambda.), R(.lambda.) and T(.lambda.).
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 .lambda. for three examples of
photovoltaic cells having the structure illustrated in FIG. 1, for
three different pairs of thicknesses e.sub.1 and e.sub.2. 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 e.sub.2=280 nm of silicon and e.sub.1=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 e.sub.1=80 nm as in the preceding example, but with a
value e.sub.2=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 e.sub.2=420 nm as in the preceding example, but with a value
e.sub.1=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 .lambda. for three examples of
photovoltaic cells having the structures illustrated respectively
in FIGS. 1, 2 and 3, for a same pair of values e.sub.2=450 nm and
e.sub.1=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 .mu.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 .alpha.(.lambda.) is shown in FIG. 7, this
layer having a thickness of several .mu.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.
* * * * *