GB2128017A

GB2128017A - Solar cell unit

Info

Publication number: GB2128017A
Application number: GB08226653A
Authority: GB
Inventors: Masaharu Nishiura; Hiroshi Sakai; Masahide Miyagi; Yoshiyuki Uchida; Hiromu Haruki
Original assignee: Fuji Electric Co Ltd; Fuji Electric Corporate Research and Development Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1982-09-18
Filing date: 1982-09-18
Publication date: 1984-04-18
Anticipated expiration: 2003-05-08
Also published as: GB2128017B

Abstract

Solar cell units each comprise a group of series-connected solar cells 18, connected in parallel (82) and/or series (81) with reverse-polarity protective diodes, the diodes are obtained by using solar cell structures of the same general type as the solar cells 18 formed on a substrate 11. Each solar cell structure includes a first conductive layer 13 or 31, an intermediate silicon layer 14 or 41 and a second conductive layer 16 or 61. Each silicon layer 14,41 includes a rectifying junction arrangement for providing photoelectric conversion. The protective diodes 81,82 make use of the rectifying property of the junction to act either as by-pass element (82) or as blocking elements (81). While normally used in combination, only the series connected (81), or parallel-connected (82), diodes may be provided in a solar cell unit, where the solar cell units, which are usually connected in a matrix arrangement, are connected only in cascade or only in parallel respectively. <IMAGE>

Description

SPECIFICATION Solar cell unit This invention relates to a solar cell unit which includes a plurality of thin film semiconductorsolarcellsformed on a substrate and connected in series with one another.

Solar cells arranged on a substrate have attracted attention as photoelectric conversion devices for converting sunlight or the artificial (e.g. lamp) light into electrical energy. Each of the cells includes a semiconductor thin film, for example, silicon having a pn junction or a Schottky barrier acting as a photo-electric conversion region, and respective electrodes formed on opposite major surfaces of the semiconductor thin film.

However, since the solar cell has a low electromotive force, a plurality of solar cells must be connected in series to form units and the units must be connected in parallel with one another in order to be useful as a power supply.

When either one or a plurality of series-connected solar cells of a unit is shaded and the desired electromotive force is not generated, the cells may be subject to reverse bias by the electromotive force generated by the other cells, thereby often destroying such shaded cells. In order to prevent this reverse bias, reverse-polarity diodes 2 have been connected as by-pass elements, in parallel with respective single solar cells 1 as shown in Figure 1, or in parallel with several series-connected solar cells 1 as shown in Figure 2 to protect them from damage or destruction due to reverse bias even if one of the solar cells 1 is shaded. If electrical noise is applied between the terminals of the solar cells connected in series, the probability that an intolerably high reverse bias is applied to the solar cells is reduced and reliability is improved by means of the diode 2.However, hitherto the diode 2 has been separately manufactured, prepared and connected to the solar cells during manufacture of the solar cell unit.

Also in the conventional configuration, if an imbalance is generated between solar cells connected in parallel, a circulating reverse current may flow through some of the solar cells to lower their output. If some of the solar cells begin to be particularly deteriorated, the deterioration is promoted by this circulating current, so worsening the output reduction. Therefore, it has been proposed to connect a diode having a polarity opposite to the polarity of the pn junction or Schottky barrier of the solar cells in series with respective parallel solar cell units to prevent the circulating reverse current from flowing. However, again, the diodes have been separately prepared for this purpose and then connected to the units, thereby increasing cost and complicating manufacture.In addition, the solar cell unit becomes inconvenient to handle because its thickness and/or area is increased.

According to this invention there is provided a solar cell unit comprising a substrate on which is formed an array of solar cell structures each comprising a first conductive layer in contact with the substrate, an insulating layer on the first conductive layer and incorporating a rectifying junction for providing photo-electric conversion, and a second conductive layer on the insulating layer, each said layer being formed as one of an array of mutually spaced islands respectively associated with the solar cells, said array of solar cell structures comprising a plurality of series-connected solar cells and a reverse-polarity protective diode arrangement in series with and/or in parallel with the series-connected solar cells of the unit, said series-connection of the solar cells being provided by interconnecting the first conductive layer of one cell with the second conductive layer of a neighbouring cell forming the next cell in the series, said protective diode arrangement comprising one or more solarcell structures of the unit array.

In one embodiment said array comprises a line of solar cell structures at one end of which at least the outermost one of the solar cell structures comprises said diode arrangement, the innermost one of said diode solar cell structures having its second conductive layer connected to a first terminal and to the first conductive layer of the outermost solar cell at the other end of said line, the second conductive layer of the solar cell adjacent said innermost diode solar cell structure being connected to a second terminal providing an opposite pole to the first terminal, whereby the diode arrangement is connected in parallel with the series-connected solar cells.

In another embodiment said array comprises a line of solar cell structures at one end of which at least the outermost one of the solar cell structures comprises said diode arrangement, the innermost one of said diode solar cell structures having a common first conductive layer with the nearest solar cell of the unit, the second conductive layer of said innermost diode solar cell structure being connected to a first terminal and the second conductive layer of the outermost of the solar cells at the other end of said line being connected to a second terminal providing an opposite pole to the first terminal, whereby the diode arrangement is connected in series with the series-connected solar cells.

Where a matrix arrangement of the solar cell units is provided by connecting groups of solar cell units in series and connecting the series-connected groups in parallel it is desirable to provide reverse-polarity diode arrangements both in parallel and in series with the seriesconnected solar cells of each unit. Alternatively the series-connected diode arrangements may be provided in only one or two (or more - but preferably such as to constitute a (small) minority) of the units of each seriesconnected group.

Embodiments of this invention will now be described, by way of example, with reference to the accompanying drawings in which: Figures 1 and 2 are circuit diagrams of prior art arrangements in which a solar cell or cells is or are connected to a diode so as to prevent reverse biassing of the solar cells; Figure 3 is a cross-sectional view of a solar cell unit forming a first embodiment of the present invention; Figures 4 and 5 are equivalent circuit diagrams of the unit shown in Figure 3; Figure 6 is a cross-sectional view of a solar cell unit forming a second embodiment of the invention; Figure 7 is an equivalent circuit diagram of the unit shown in Figure 6; and Figure 8 is a circuit diagram of a solar cell device comprising units as shown in Figure 7.

In Figure 3, there is shown formed on a glass plate 3 serving as substrate, a transparent conductive film 5 made of, for example, ITO (Indium Tin Oxide). The film 5 is formed as a linear array of islands separated by small spaces 4 by means of sputtering techniques. An amorphous silicon film 6, which fills the spaces 4 and covers the transparent conductive film 5, is provided by means of a plasma CVD technique. For example, the film 6 is formed by decomposing monosilane in a glow discharge and depositing the amorphous silicon. The film 6 is in the form of a linear array of islands, substantially corresponding to the islands of the film 5, separated by small spaces 7. A metal film 8 such as a deposited film of aluminium, which fills the spaces 7 and covers the silicon film 6 is then provided again as a linear array of islands.

The amorphous silicon film 6 comprises three layers (not shown) i.e. a p-type layer, an intrinsic layer, and an n-type layer which are successively deposited in the stated order. The production technique for forming the film 6 using the decomposition of monosilane (SiH4) in a glow discharge is well known. Diborane (B2H6) or phosphine (PH3) is added to the monosilane so as to obtain the p-type or n-type layers.

When light is applied to the amorphous silicon film through the substrate 3 and the transparent conductive film 5, photoelectric conversion is carried out by the p-n junction to act as a solar cell. Respective solar cells 9 are connected in series by the arrangement shown in Figure 3.

In this case, thetransparent conductive film island 51 at one end of a cell 91 and the metal film 82 at the other end of a cell 92 are electrically connected to a common terminal A, and the transparent conductive film island 52 of the cell 92 (that is. the metal film 83 (which is in electrical contact with the film island 52) of the cell 93 adjacent the cell 92) is connected to another terminal B.

This connection is represented by the equivalent circuit shown in Figure 4. That is, one reverse-polarity solar cell 92 is connected in parallel with the series-connected solar cells 91 to 93.

Thus since the junction acting as the photoelectric conversion region in the solar cells at the same time has an electric current rectifying function, the solar cell 92 serves as a diode, and the same electrical arrangement as is shown in Figure 2 is obtained. Accordingly, if the number of individual cells 9 (the number is 8 in the case of Figures 3 and 4) to be connected in series as a unit is determined in such a manner that each cell 9 can endure the reverse bias generated by the electromotive force of the other solar cells 9 of the unit. Thus when one of the cells 9 is shaded, the shaded cell will not be destroyed since the reverse bias due to other series-connected units is bypassed by the diode 92 in a solar cell device comprising a series-connected plurality of the units as shown in Figure 5.Corresponding to Figure 3, Figure 6 shows the structural arrangement of such a device.

If the amorphous silicon film 6 in Figure 3 comprises an n-type layer, an intrinsic layer. and a p-type layer deposited successively in that order, a unit arrangement represented by the equivalent circuit shown in Figure 7 can be obtained.

Since the diode 92 tends to function as a solar cell, electromotive force is generated if light is applied to the diode 92 and the polarity of the resulting e.m.f. is reversed with respect to that of the series-connected cells.

Therefore, it is desirable to shield the diode 92 from light.

However, only a slight difference in the open circuit voltage arises from this in practice, and the optimum operating voltage is almost unaffected. Consequently the light-shield is not always provided.

Instead of providing only a single diode 92 in each unit it may be desirable or necessary to provide an arrangement of two or more parallel-connected diodes 92 in parallel with the series-connected group of solar cells 91 to 93.

This will enhance the current-carrying capacity of the by-pass arrangement enabling more units to be connected in series in an arrangement of the kind shown in Figure 5 (where the units each have only one diode 92).

Another embodiment of the present invention will now be described with respect to Figure 8, wherein a transparent conductive film 13 made of ITO is formed, using sputtering techniques, as a linear array of islands separated by small spaces 12 on a glass plate 11 serving as substrate. In this case, a space 12 is not provided between two adjacenttransparentconductive film islands at one end of the linear array, and thus a transparent conductive film island 31 whose size is twice the normal size is formed. An amorphous silicon film 14, which fills the spaces 12 and covers the transparent conductive film 13, is provided as a linear array of islands separated by small spaces 15 by means of a plasma CVD technique as described above for the film 6 (Figure 3).Only the outer silicon film island 41, which is spaced from the inner amorphous silicon film islands and deposited on the transparent conductive film island 31, extends to the outside of the transparent conductive film island 31 so as to cover the edge of the latter.

A metal film 16 such as a deposited film of aluminium, which fills the spaces 15 and covers the silicon film 14, is provided as a linear array of islands separated by spaces 17. In this case, the spaces 15 in the transparent conductive film island 41 are not filled by the metal film 16, and one end of the metal film island 61 deposited on the silicon film 41 covers the outside edge of the silicon film 41 and extends to the substrate. The amorphous silicon film islands are composed of three layers as described above for the film 6.

When light is applied to the silicon film through the substrate 11 and the transparent conductive film, photoelectric conversion is effected by the p-intrinsic-n (p i n) junction. The respective solar cells are connected in series in an arrangement as shown in Figure 8.

In this case, the cell 81 is connected in series with, but with reverse-polarity relative to, the other cells 18.

Therefore, when one end of the metal film 61 is connected to a terminal A and the other end of the metal film 62 is connected to a terminal B, an arrangement represented by the equivalent circuit shown in Figure 9 is obtained.

The junction acting as the photo-electric conversion region in each solar cell has, at the same time. a rectifying function. Therefore, the solar cell 81 serves as a diode to prevent a reverse current from flowing through the seriesconnected solar cells 18.

Since the cell 81 can function as a solar cell, when it is exposed to light, an electromotive force of reverse polarity is caused with respect to the other cells 18 and the output voltage of the solar cell unit is reduced by this counter electromotive force. Consequently, it is desirable to shield the cell 81 (diode) from incident light.

When the solar cell units associated with the respective diodes 81 are connected in parallel as shown in Figure 10, circulating current flow is prevented even if an imbalance exists among the solar cell units or if there is a deterioration of a cell or cells. Consequently, reduction in the solar cell device output can be kept minimal.

Instead of a single diode 81 . a plurality of such diodes connected in series may be used. In this case, the terminal A position is shifted by an appropriate number of cells. Consequently, the reverse bias voltage blocking level provided by the reverse-polarity parallel connected diode arrangement will be greater than with a single diode and the serial number of the solar cells 18 for photoelectric conversion in each of the parallel arms (Figure 10) can be increased accordingly.

It is possible for the embodiments shown in Figures 3 and 8 (or Figures 5 and 10) to be effectively combined since in a practical solar cell device, solar cell units are usually series connected as in Figure 5 and the seriesconnected arrangements are then connected in parallel in a manner similar to that indicated in Figure 10 to form a matrix arrangement. Two such combined (matrix) embodiments of solar cell device are shown in Figures 11, 12 and Figures 13, 14 respectively. As can be seen from the equivalent circuit diagrams (Figures 11 and 13) both by-pass parallel-connected diodes 82 and blocking series-connected diodes 81 are incorporated in each modular unit whose individual structures are shown in Figures 12 and 14 for the respective embodiments.The difference between the two embodiments resides in the electrical connection of the blocking series-connected diode 81 in each respective unit.

Clearly, if it is not necessary or desirable to use identical modular units the equivalent circuits could be varied by positioning all the blocking series-connected diodes in one or two (or more - but preferably such as to constitute a (small) minority) of the units (e.g. at each end - i.e. units A11 B11 and AIN B1N) of each (e.g. A11 Bii to A1N B1N) series-connected arrangement of units.

In the devices shown in Figures 3, 6 and 8 light is transmitted through the glass plate substrate and the transparent conductive film and is applied to the silicon thin film. However, by using a metal plate such as a stainless steel plate as the substrate and as as electrode and utilising a transparent conductive film as the opposite electrode, the light may be applied to the silicon thin film through the transparent conductive film from the opposite side of the substrate. The conductive film 8, 16 in the embodiments shown in Figures 3, 6 and 8 may in any case be light transmissive.

Solar cells using amorphous silicon have been described in the above embodiments. However, solar cells using a polycrystalline silicon thin film deposited on the substrate can be provided instead of the amorphous silicon cells. In addition, a Schottky barrier may be formed on the silicon thin film instead of the pin or pn junction, and its rectifying property employed.

The solar cell devices described above utilise one or more solar cell structures directly as reverse-polarity diode arrangements connected in parallel and/or series with the other solar cells for preventing reverse bias or reverse currentthrough those other cells. Since the diode arrangements can be produced on the same substrate with the solar cells while at the same time, excessive cost and processing is avoided, the solar cell device can be economically manufactured. The overall size of the device is not increased. The effect of an imbalance among solar cell units connected in parallel is control lable, and the solar cell device can be effectively used with a high degree of reliability.

Claims

1. A solar cell unit comprising a substrate on which is formed an array of solar cell structures each comprising a first conductive layer in contact with the substrate, an insulating layer on the first conductive layer and incorporating a rectifying junction for providing photoelectric conversion, and a second conductive layer on the insulating layer, each said layer being formed as one of an array of mutually spaced islands respectively associated with the solar cells, said array of solar cell structures comprising a plurality of series-connected solar cells and a reverse-polarity protective diode arrangement in series with and/or in parallel with the series-connected solar cells of the unit, said series-connection of the solar cells being provided by interconnecting the first conductive layer of one cell with the second conductive layer of a neighbouring cell forming the next cell in the series, said protective diode arrangement comprising one or more solar cell structures of the unit array.

2. A solar cell unit according to claim 1 wherein said substrate and said first conductive layer are made of a light transmissive material.

3. A solar cell unit according to claim 1 or claim 2 wherein said second conductive layer is made of a light transmissive material.

4. A solar cell unit according to any one of the preceding claims wherein the solar cell structures providing said diode arrangement are shielded from incident light to prevent photoelectric conversion within said diode solar cell structures.

5. A solar cell unit according to any one of the preceding claims wherein said array comprises a line of solar cell structures at one end of which at least the outermost one of the solar cell structures comprises said diode arrangement, the innermost one of said diode solar cell structures having its second conductive layer connected to a first terminal and to the first conductive layer of the outermost solar cell at the other end of said line, the second conductive layer of the solar cell adjacent said innermost diode solar cell structure being connected to a second terminal providing an opposite pole to the first terminal, whereby the diode arrangement is connected in parallel with the series-connected solar cells.

6. A solar cell unit according to claim 5 wherein said diode arrangement comprises at least two parallelconnected solar cell structures.

7. A solar cell unit according to any one of claims 1 to 4 wherein said array comprises a line of solar cell structures at one end of which at least the outermost one of the solar cell structures comprises said diode arrangement, the innermost one of said diode solar cell structures having a common first conductive layer with the nearest solar cell of the unit, the second conductive layer of said innermost diode solar cell structure being connected to a first terminal and the second conductive layer of the outermost of the solar cells at the other end of said line being connected to a second terminal providing an opposite pole to the first terminal, whereby the diode arrangement is connected in series with the seriesconnected solar cells.

8. A solar cell unit according to claim 7 wherein said diode arrangement comprises at least two seriesconnected solar cell structures.

9. A solar cell unit substantially as described herein with reference to Figures 3 and 4 or 7 orto Figures 8 and 9 or to Figure 12 or Figure 14 of the accompanying drawings.

10. A solar cell device comprising a matrix arrangement of solar cell units according to any one of the preceding claims, groups of said units each comprising a plurality of the units connected electrically in series the series-connected groups being connected electrically in parallel, said reverse-polarity diode arrangements being provided both in parallel and in series with the seriesconnected solar cells of each unit.

11. A solar cell device according to claim 10 wherein instead of the series-connected diode arrangement being provided in every said unit, such diode arrangements are provided in a minority of the units of each seriesconnected group, remaining units of each said group having only said parallel-connected reverse-polarity diode arrangements.

12. A solar cell device substantially as described herein with reference to Figures 3 to 6 or as modified by Figure 7 orto Figures 8 to 10 orto Figures 11 and 12 orto Figures 13 and 14 of the accompanying drawings.

13. A solar cell device, comprising: a plurality of solar cells each being composed of a first conductive film, a semiconductor film including a junction, and a second conductive film, said solar cells being arranged on a substrate at desired intervals, said first conductive film contacting said substrate, said solar cells all being connected in series by connecting said first conductive film of one cell to said second conductive film of an adjacent cell in turn, said first conductive film of a first of a series of cells and said second conductive film of a last of a series of cells being connected to one terminal, and said second conductive film of a selected cell being connected to another terminal.

14. A solar cell device, comprising; a plurality of solar cells each being composed of a first conductive film, a semiconductor film including a junction, and a second conductive film, said solar cells being arranged on a substrate at desired intervals, said first conductive film contacting said substrate. two adjacent cells sharing a common first conductive film, a first terminal being connected to one of said two cells at a second conductive film thereof, and a second terminal connected to a second conductive film of a further cell.