Photovoltaic device useful as a mirror

Abstract: A photovoltaic device that comprises an insulating transparent substrate (1) coated with a light-receiving surface electrode (6) formed of a thin metal film which in turn is overlaid with an amorphous semiconductor layer (3) and a back electrode (4), said thin metal film being adapted for use as a mirror.

Description: FIELD OF THE INVENTION This invention relates to photovoltaic devices for use in solar cells, photosensors and other applications. More specifically, this invention relates to photovoltaic devices that are advantageously used in such applications as where the surface electrode is utilized as a mirror. BACKGROUND OF THE INVENTION A conventional amorphous silicon solar cell system comprises a plurality of unit solar cells that are formed by superposing transparent electrodes, amorphous silicon (a-Si) film regions and back electrodes on a single glass substrate. The individual unit solar cells are connected in series in such a manner that the back electrode of one unit cell contacts the transparent electrode of an adjacent unit cell via a conductive path. Solar cells are inherently designed to utilize the electric energy directly converted from sunlight and no considerations have been made to adapt solar cells to applications that have nothing to do with this photovoltaic conversion of sunlight to electricity. Since conventional solar cells use tin oxide or indium-tin oxide in transparent electrodes, their reflectance is too low to enable use as mirrors (for the reflectance of SnO 2 as compared to metals, see Fig. 2). Another problem with the prior art is the difficulty involved in attempts at reducing the cost of materials. Currently, thin films of tin oxide or indium-tin oxide are deposited by a thermal CVD process or sputtering but this requires the use of not only expensive oxide targets but also a complicated sputtering apparatus such as one using a radio-frequency magnetron. JP-A-60 240 163 discloses a device with an insulating transparent substrate with a light-receiving surface electrode of thin metal, overlaid with a semiconductor layer and a back electrode. EP-A1-0 450 468 relevant for determining novelty of the present invention according to Art, 54(3) EPC discloses a photovoltaic device comprising: an insulating transparent substrate ; and a plurality of solar cells having a plurality of light receiving surface electrodes formed of a thin metal film coated on the transparent substrate, an amorphous semiconductor layer over the surface electrodes, a plurality of back electrodes, and a plurality of conductive paths, each connecting a surface electrode of one of said cells and a back electrode of an adjacent cell, to connect adjacent cells in series through the amorphous semiconductor layer; wherein the metal film is formed of aluminum or nickel, has a thickness of 10 nm or less, and has a transmittance of approximately 30% to 80% over most of the visible spectrum. SUMMARY OF THE INVENTION An object, therefore, of the present invention is to provide a photovoltaic device that is useful not only as solar photovoltaic cell but also as a mirror. The present invention provides a photovoltaic device as set out in claim 1. The use of a thin metal film as the surface electrode eliminates the need to heat the substrate and permits the use of not only an inexpensive metal target but also the formation of a desired surface electrode with a simple sputtering apparatus, whereby the materials cost can be reduced. Further, the thin metal film has a sufficiently high reflectance to be used as a mirror. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the sequence of major steps for fabricating the photovoltaic device of the present invention; and Fig. 2 is a graph showing the reflectance characteristics of three types of solar cells using SnO 2 , Al and Ni films, respectively, in the surface electrode. DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention is described below with reference to Fig. 1 which shows the sequence of major steps for fabricating the photovoltaic device of the present invention. First, as shown in Fig. 1(a), a thin film of Al or Ni not thicker than 200 Å (10Å = 1nm) is deposited on a glass substrate 1 by DC sputtering to form a thin-film metal electrode 6. DC sputtering may be performed at an argon pressure of 5 Torr with a power of 0.17-0.65 kW being applied. In the next step, the thin-film metal electrode is worked with a YAG laser operating at a wavelength of 1.06 µm to form a plurality of surface electrodes 61-63. Then, as shown in Fig. 1(b), an a-Si film 3 having a pin junction is formed in a thickness of ca. 0.8 µm by a plasma-assisted CVD process. Subsequently, as shown in Fig. 1(c), a patterned metallic back electrode or conductive printed electrode 4 is formed in such a manner that it partly overlaps an adjacent thin-film metal electrode 6 via the a-Si film 3. The two overlapping electrode portions allow for interconnection between adjacent unit solar cells. One method for establishing interconnection between adjacent unit solar cells was already proposed in commonly assigned JP-A-2-268472 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to this method, a laser beam emitting at 1.06 µm is applied to the overlapping electrode portions form the substrate on the back side, whereupon the thin-film metal electrode 6 and the a-Si film 3 in each photovoltaic region and the back electrode 4 in an adjacent photovoltaic region are heated to melt, forming a conductive path (51 or 52) made of the resulting alloy layer. This conductive channel permits the back electrodes 41 and 42 to be connected respectively to thin-film metal electrodes 62 and 63, thereby producing an a-Si solar cell system in which the individual unit solar cells are connected in series, as shown in Fig. 1(d). The solar cell system constructed in the embodiment under consideration had a very high conversion efficiency as demonstrated by an output of 4.7% for incident sunlight of 200 lux and 5.2% for 1,000 lux. When the surface electrode was formed of an Al or Ni film, the solar cell system had such a higher reflectance than when the conventional tin oxide film was used (see Fig. 2) and hence exhibited satisfactory performance as a mirror. Therefore, when the solar cell system having the surface electrode formed of a thin metal film is incorporated in a hand-held electronic calculator, the latter can also be used as a mirror. As described on the foregoing pages, the surface electrode in the photovoltaic device of the present invention is formed of a thin metal film and this eliminates the need to heat the substrate during the formation of surface electrodes, which therefore can be formed by a DC magnetron sputtering process with a simple apparatus and at high production rate. Further, the surface electrodes can be formed at a lower cost using inexpensive metal targets. As another advantage, the photovoltaic device of the present invention can also be used as a mirror, which contributes to the manufacture of commercial products of much added value that incorporate said photovoltaic device. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope of the claims.

Claims: A photovoltaic device comprising a) an insulating transparent substrate (1); and b) a plurality of solar cells having c.1) a plurality of light receiving surface electrodes (61-63) formed of a thin metal film (6) coated on the transparent substrate (1), c.2) an amorphous semiconductor layer (3) over the surface electrodes (61-63), c.3) a plurality of back electrodes (41, 42), and c.4) a plurality of conductive paths (51, 52), each connecting a surface electrode (62; 63) of one of said cells and a back electrode (41; 42) of an adjacent cell, to connect adjacent cells in series through the amorphous semiconductor layer (3); d.1) wherein the metal film (6) is formed of aluminum or nickel and d.2) is thicker than 10 nm and no thicker than 20 nm (200 Å), e) and wherein the plurality of solar cells has a reflectance of around 12 % over most of the visible spectrum to perform as a mirror.

Inventors: SICHANUGRIST PORPONTH, TANAKA NORIMITSU, SICHANUGRIST, PORPONTH, TANAKA, NORIMITSU, Sichanugrist, Porponth, c/o Showa Shell Sekiyu KK, Tanaka, Norimitsu, c/o Showa Shell Sekiyu KK
Applicants: SHOWA SHELL SEKIYU, SHOWA SHELL SEKIYU KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L27
IPC codes: H01L31, H01L27