JPH0732259B2 - Photovoltaic device manufacturing method - Google Patents

Description

The present invention relates generally to solar cell technology, and more particularly to monolithic thin-film solar cells in which a plurality of photovoltaic regions are connected in series via the thin film itself. The present invention relates to a method for manufacturing a photovoltaic device having at least

[Conventional technology]

Conventionally, an array of a plurality of cells of a thin film solar cell has been manufactured by laminating each layer including a photoactive layer of the cell in a preselected pattern. In other cases, each layer was patterned by removing the portion of the layer between each cell after being laminated. The finally obtained cells are externally interconnected to form a solar cell array. Examples of arrays formed by this method are Hanaku "Amorphous Silicon Monolithic Solar Panels", Solar Energy 23,145-147 (1979), Bowden U.S. Pat. No. 4,278,473, and Hanak U.S. Pat. No. 4,29.
No. 2,092. Such arrays are generally rather expensive and not easy to manufacture, primarily due to the steps required when patterning the photoactive layer into individual devices. The patterning process also significantly reduces the active area of the cell.

Another form of thin film photovoltaic device is described in Kwano et al., U.S. Pat.
4,281,208. In one of the embodiments disclosed therein, an unpatterned active layer having a large number of photovoltaic regions is provided. Electrical interconnections are formed beyond the edges of the active layer. The configuration is easier to manufacture than that described above, but is dimensioned by the sheet resistance of its electrodes, in which the current has to be conducted to the end of the cell where the interconnect is formed. The sheet resistance significantly attenuates any portion of the output signal generated inside the panel, making the disclosed configuration impractical for most applications. Another configuration is disclosed in the Kwano et al. Patent, in which the active layer is patterned to allow connections within the panel.

[Problems to be solved by the invention]

Therefore, in many applications, it is desirable to provide a method of manufacturing a thin film photovoltaic device in which the active layer is continuous and unpatterned, yet the photovoltaic regions of the thin film can be easily connected in series. Be done.

That is, it is an object of the present invention to provide a method for manufacturing a photovoltaic device in which the active layers in a thin film photovoltaic element are not continuous and patterned, and the photovoltaic regions of the thin film can be easily connected in series. It is in.

[Means and Actions for Solving Problems]

The method for manufacturing a photovoltaic device of the present invention, in manufacturing a photovoltaic device having a photovoltaic continuous thin film of a predetermined overall thickness that defines a plurality of photovoltaic regions, on the main surface of the transparent substrate, each Forming a plurality of spaced transparent front electrode means having at least one contact portion that is substantially thicker than the total thickness of the photovoltaic continuous thin film;
A step of forming a photovoltaic continuous thin film above the main surface of the substrate while covering the front electrode means and sandwiching the front electrode means, and separately arranging a gap on the photovoltaic continuous thin film with a space between them. Forming a plurality of back electrode means disposed substantially over a front electrode means defining a photovoltaic region therebetween and overlapping at a contact portion with the electrode means of an adjacent photovoltaic region. The photovoltaic continuous thin film is provided between each back electrode means and the contact portion overlapping with the back electrode means so that electrical connection is completed. In a preferred embodiment, each front electrode portion completes an electrical connection to the back electrode through the continuous thin film, so that the back electrode of an adjacent photovoltaic region overlaps in a predetermined region with an interconnect portion. And each interconnect comprises at least one contact or "stitch bar" having a textured or thickened portion adjacent the front surface of the continuous film. A desirable degree of unevenness is roughness on a microscopic scale.

The device manufactured by the manufacturing method of the present invention is a monolithic solar cell panel having a plurality of cells defined by spaced photovoltaic regions of a thin film active layer. Since the cells are connected across the active layer itself, the connection is completed at any desired location without patterning the layer. Thus, the limitations on manufacturing feasibility, size and shape of monolithic solar panels are eliminated. Furthermore, the back electrode is provided directly on the mounted semiconductor surface for patterning, without removing it from the processing equipment. This tolerance is advantageous with respect to the contamination of the semiconductor surface before the back electrode is provided.

The concept of the present invention is also useful in devices having polycrystalline or single crystal thin films, although microcrystal thin films (microcrystal thin films)
line thin film to MTF) It is most useful in the case of thin film devices such as photovoltaic devices. In this context, "microcrystalline"
The term is intended to cover various thin films which are generally loosely defined in the literature as "amorphous". Most such films, whether deliberately produced or not, contain microcrystalline moieties. And, more precisely, it is believed to mean any thin film lacking visible crystallinity, as is generally the case for "microcrystalline".

The overlapping electrodes of the present invention electrically connect each cell in series, thus allowing a connection to be made between the back surface of one cell and the front surface of an adjacent cell. The stitch bar of the front electrode has either an uneven surface in the vicinity of the thin film active layer or is thicker than the layer,
The active layer is applied thereon. Therefore, the electrical distance of the active layer is very small near the stitch bar, resulting in a local conductive path or "short circuit" extending through the layer. Conductivity, whether uniform or selective, can be further enhanced by heating the back electrode to diffuse the electrode or other conductivity inducing material into the active layer. With each of these configurations, the current through the active layer is limited to flow from one electrode section to the other substantially across the thickness of the layer. Since the active layer has a very high sheet resistance, there is no apparent current in it. Each cell then operates as an individual solar cell connected in series to achieve the appropriate high output voltage level.

Local heating of the back electrode in the electrode overlap portion may be added. In that case, the electrode material diffuses into the thin film to create localized highly conductive regions. This allows the bulb to flow across the active layer rather than in the plane of the active layer, as described above. Local heating may be achieved by the use of lasers, electron beam guns or other suitable means. Lasers are useful in creating highly localized conductive regions that do not unduly limit the active area of the cell. In some applications it may also be desirable to provide a thin strip of a conductivity inducing agent such as phosphorus, boron or arsenic in the interconnect region. The conductivity inducer is
Diffuses into the active layer by itself or when heated,
Conductive interconnects may be created. The conductivity inducer is, for example,
It is provided by a screen printing process or by heating a local region with a laser beam in a gas atmosphere containing an appropriate impurity gas. Either of these configurations minimizes the overlap and separation areas of the electrode parts, resulting in a highly efficient solar panel.

〔Example〕

FIG. 1 shows a monolithic thin film solar cell panel 10 constructed according to one embodiment of the present invention. The panel 10
Is defined by a plurality of solar cells 12 electrically connected in series between a pair of external lead wires 14. Each cell 12 is connected along each opposite longitudinal edge to minimize series resistance loss and is formed as a narrow strip. The connection between the cells is achieved through the unpatterned active thin film of the solar panel without interruption of the thin film. The current produced by a cell in response to incident light (hν) travels a very short distance within each cell electrode before reaching the counter electrode of an adjacent cell.

As shown in FIG. 2, the solar cell panel 10 comprises a transparent substrate 16, a transparent front electrode 18, a continuous thin film 20 of photovoltaic material and a patterned back electrode 22. The electrode 22
Comprises a plurality of back electrode portions 24 separated by voids 26 and overlying a substantially thin film elongated photovoltaic region 28. The front electrode 18 comprises a transparent conductive layer 29 and a series of contact portions or "stitch bars" 30. Layer 29
Are patterned to form a plurality of transparent electrode portions 32 that are separated by voids 34 and substantially underlie the photovoltaic regions. In this way, the photovoltaic region is
An effective sandwich is formed between 24 and the front electrode portion 32 so as to collect the current generated in the area. Further, each front electrode portion partially overlaps the back electrode portion of the adjacent photovoltaic region in the predetermined region 36.

A key feature of the present invention is the provision of a conductive path substantially across the active thin film 20 between each front electrode portion and the back electrode portion of the adjacent photovoltaic region. Interconnects for electrically connecting the photovoltaic regions in series are achieved in the overlapping portions of the electrodes without patterning of the thin film or other interruptions.

In the solar cell thin film 10 of the example, the stitch bar 30
Are sufficiently tall and narrow and / or sufficiently rough as compared to the thin film 20 to electrically short through the thin film. Although the stitch bars are shown as being provided on the transparent layer 29, they should be provided directly on the surface of the substrate 16 as long as the next applied transparent layer is electrically connected to them. Is also possible. The final configuration of the solar cell panel 10 is best shown in FIG. There, the stitch bar deforms the subsequent thin film 20 to create a relatively thin region 38 that cannot withstand the cell voltage. Electrical conduction occurs through this region 38. It is advantageous for the stitch bars to be as rough as possible on their upper surface, in order to concentrate the applied electric field and minimize the resistance of the regions 38.

Stitch bar 30 preferably has a thickness of approximately 25μ, and includes thin film 20 and patterned transparent conductive layer 29 described above.
Are approximately 6,000 and 2,000Å thick, respectively.
The thickness of the back electrode is about 25μ for screen printing and about 2μ for vapor deposition. At the location of the stitch bar 30 in one example, the membrane 20 is sandwiched between a pair of conductive elements having a thickness of at least 40 times the thickest portion of the membrane. The thickness ratio is larger in the relatively thin region 38, that is, in a portion where the thickness of the thin film is 5Å or less. This creates an effective short circuit through the membrane at each stitch bar, but the membrane 20 has a very high sheet resistance so it does not completely short the cell 12. . The sheet resistance eliminates current in the plane of the film, leaving substantially only the transverse current that extends into the photovoltaic region and between the electrodes at the overlap.

Referring to FIGS. 2 and 3 in more detail, the panel 10
The respective layers and the electrode part are substantially provided on the main surface 40 of the substrate 16. The substrate is composed of glass or other suitable transparent material that is compatible with the materials of stitch bar 30 and transparent conductive layer 29.

As the transparent conductive layer 29, indium tin oxide (TCO-indiu) is used.
It is desirable to use a transparent conductive oxide (TCO) such as tin oxide), tin oxide (TO-tin oxide) or indium oxide (IO-indium oxide). Combinations of these material layers can also be used to obtain the effect of chemical and / or electrical compatibility and to optimize the performance of the device. In order to minimize contamination, it is desirable to provide the transparent conductive layer prior to installing the stitch bar. However, the order of installation can be reversed if appropriate precautions are taken.

The transparent conductive layer 29 is initially provided as a continuous layer. For example, ITO is provided by vacuum deposition of indium and tin with the aid of a glow discharge in an oxygen atmosphere at 300 ° C. Glow discharge activates oxygen to produce high quality films. TO can be provided using a chemical vapor deposition process and IO is provided by reactive vapor deposition of indium in an active oxygen atmosphere.

The thickness of the transparent conductive layer 29 is chosen to minimize the reflection and absorption of light thereby. According to established optical principles, internal reflection losses due to transparent coatings are minimized when the thickness of the coating is an odd multiple of 1/4 wavelength of the incident light divided by the refractive index of the coating. .
For this purpose, the appropriate wavelength corresponds to the peak of the spectral response of the photovoltaic material forming the thin film. In the case of MTF silicon, it is almost 5,400 Å, and when corrected with a refractive index of 1.8, it becomes 3,000 Å, 1/4 wavelength is 750 Å,
And 3/4 wavelength becomes 2,250Å. Many conventional transparent conductive oxides are required to reduce sheet resistance to acceptable levels, but in practice a thickness of 750Å is desired to minimize light absorption.

The stitch bar is preferably provided on the transparent conductive layer by screen printing or vapor deposition through a mask. In the case of screen printing, a suitable commercially available screen printing paste is applied via a patterned screen. The paste is, for example, silver (A
g) Will include powder, glass frit and a suitable organic medium or binder. After providing the transparent layer, the paste is baked to remove the organic medium, leaving the silver and glass frit. Glass frits tend to fuse to the substrate and provide a strong bond, and metallic silver renders the stitch bar conductive. As an alternative, a commercially available screen printing paste without glass frit can be used. If stitch bars are deposited, they are made of silver, aluminum or other material that can provide high quality contacts.

Stitch bar 30 is shown as a grid or a segment of grid passing through a predetermined area 36 of overlapping electrodes, but they need not be continuous or linear. If the stitch bars are screen printed, they need to be approximately 25μ high for full operation. If vapor deposition, they require a height of about 2μ, and preferably 10μ. Aspect ratio (height / width) of the stitch bar in each case
The roughness, that is, the unevenness of the surface is a parameter that influences the local short circuit that allows the operation of the panel 10. If the width of the bars substantially corresponds to their height, or if the bars are smooth rather than rough,
Its height needs to be increased beyond the values mentioned above.
Roughness is usually the dominant factor in producing highly conductive passages. This allows the stitch bar to operate as a series of point potential sources, significantly increasing the conductivity through the active layer. In some cases,
The resulting surface irregularities or "microroughness" due to the stitch bar placement process can achieve this goal.

After providing the stitch bar, the transparent conductive layer 29 is patterned by conventional techniques such as laser scribing. In the case of panel 10, the patterning operation requires removal of the transparent conductive layer along a series of parallel lines near stitch bar 30, thereby creating front electrode portions 32 separated by voids 34. Thus, the front electrode portion is formed as a group of parallel strips generally according to the portion of the cell 12 in FIG. However, the front electrode portions 32 and cells 12 are not necessarily formed in strips, as long as each front electrode portion comprises an interconnection portion that partially overlaps the back electrode portion of the adjacent photovoltaic region. You don't have to. layer
The 29 may be provided either before or after the stitch bar 30, but is preferably not patterned until after the stitch bar is provided. The stitch bar 30 then acts as a guide for patterning the layer 29.

The thin film 20 may include any photovoltaic material that features optical coupling for converting light into electrical energy. In this embodiment, the thin film 20 is, as shown in FIG.
MTF with N ⁺ , I and P ⁺ layers 44, 46 and 48 respectively
It is silicon. Alternatively, it may be a polycrystalline or single crystalline material. Thin film 2 for MTF silicon
0 is provided by conventional glow discharge technology without patterning or masking. The thin film 20 extends continuously and completely across the stitch bar 30, the transparent conductive layer 29 and the void 34. The thickness of the thin film 20 complete with three layers having different conductivity types is about 6000Å. This dimension depends on the material of the thin film 20, but the stitch bar 30
Should in all cases be high and rough relative to the thickness of the thin film. Otherwise, there is no desired conductive path through the film.

The back electrode 22 acts as a back contact for the cell 12. It can be formed by any suitable technique such as screen printing. In the case of screen printing, conventional epoxy-based silver / graphite materials can be used to selectively provide conductive silver in the shape of back electrode portion 24. At that time, the panel cures the epoxy, so
Heated to ℃. Instead of silver in the back contact, other materials can be used, such as nickel, palladium or gold. In a commercial sense, precious metals are expected to be excluded to minimize cost. A further possibility for the back electrode is a sandwich layer in which the three materials molybdenum / aluminum / molybdenum are applied sequentially by conventional techniques. The aluminum layer provides good reflectivity to redirect the light into the thin film, while the outer molybdenum layer promises good electrical contact and good laser coupling with the thin film. Laser coupling is important in the examples described below.

Although a short circuit through the membrane 20 is achieved in many situations by the arrangements described above, it is sometimes desirable to increase the local conductivity through the membrane by heating the solar panel 10. When the stitch bar 30 is relatively tall, narrow and rough, the heating can cause the back electrode material, or the material of the stitch bar 30, to diffuse into the semiconductor film. The resulting diffused region has better conductivity than the real part of the thin film and enhances cell interconnection. The conductive metal material diffuses in the thin film as individual atoms or exists as particles that occupy lattice gaps in the film.
In the latter, the conductive regions of the thin film are composed of a mixture of metal and semiconductor particles.

When heating the panel 10 to improve conductivity, it is usually desirable to heat only the localized areas within a given overlap. In this way, the substantial part of the thin film 20 is not changed by heat and the metal electrode material is not diffused, so that a desired diffusion level can be obtained in the overlapping part. This approach is shown on the right side of FIG. 3, where a laser beam 50 is directed to the back electrode section, producing conductive areas 51 in relatively thin areas 38. A laser beam 50, the output of the laser scribe, is moved across the solar panel to heat the film along the pattern of the stitch bar 30. The heating operation causes diffusion of the metal electrode material into the thin film near region 38. The beam 50 melts the back contact and thin film material, forming a eutectic composition of both in region 51, or simply heats the material sufficiently to promote diffusion. When the back contact is aluminum and the film is MTF silicon, the two materials combine to form a conductive silicide that bridges the thickness of the film.

Local heating has been described above with respect to laser beams, but it will be readily appreciated that various other heating techniques may be used. Such techniques also include the use of electron beam sources or radiant heating wires to cause the diffusion.

The sheet resistance of the thin film determines the prevention of shorting the cell from one location of the interconnect to another. Thin film 20 of the present invention
Has a resistivity of more than 10 ⁶ Ω per unit area under the condition of sunlight incidence. If the film has a lower resistivity, some of the semiconductor material is etched away adjacent to the interconnect lines to prevent shorts. In addition, the range of operating parameters is
It is considered useful in the practice of the present invention.

Width of stitch bar 30 10 to 2500 μ Height of stitch bar 30 10 to 5000 μ Thickness of photoactive thin film 20 0.05 to 100 μ Thickness of back electrode 22 100 Å to 5000 μ Gap 26 and 24 1 to 2500 μ In practical use, solar cell panel 10 and 10 'is arranged to receive incident light (hν) through the transparent substrate 16 and the front electrode 18. The light is absorbed by the photovoltaic material in region 28 and converted into current by the front and back electrode portions of the cell for collection. The voltage at the front electrode portion of a cell is applied to the back electrode portion of an adjacent cell by the short circuit area of the thin film 20 to connect the cells in series. The voltage across the two leads 14 is the sum of the voltages of each solar cell 12 and produces a relatively high output voltage level for the panel 10 as a whole.

Good results are obtained with the MTF solar cell module made according to the present invention. The module is made as an array of 18 parallel strip cells on a 70 cm square 7059 glass substrate. The glass substrate is cleaned using a detergent first, then NH ₄ OH / H ₂ O ₂ etching, and then a water wash. The glass is then covered with tin oxide (TO) provided by chemical vapor deposition with a mixed gas containing tetramethyltin, CF ₃ Br and O ₂ . The resulting conductive oxide layer (29) has a sheet resistance of 5Ω per unit area and a light transmission of about 80% in the solar spectrum. The conductive layer is "Conductrox" by Thick Film Systems.
18 using the silver paste sold under the name "3347"
Printed with an array of parallel stitch bars (30), the sample was cauterized at 550 ° C, leaving a stitch bar approximately 0.008 inches wide. A laser is then used to scribe the TO along the lateral lines of each stitch bar to form the groove-like voids in the TO layer. The void has a width of about 0.001 inch to insulate each cell from each other. Additional laser scribing at the periphery of the substrate insulates the cell from the TO extending over the edge of the glass.

The substrate was then rinsed in methanol and MT
When placed in an F deposition reactor, a pin layer with the following characteristics is created.

The final layer of the module, the back contact (24), is deposited to a thickness of approximately 2μ by vapor deposition through a shadow mask. The mask creates a void of approximately 0.004 inches between the back contact layer portions, resulting in a total interconnect width of approximately 0.015 inches. And the module is CF ₄ + 4% O ₂
It is etched for 1.5 minutes in a 100 W glow discharge, and the short circuit between cells due to the conductive n-type back layer at the position of the void 26 is removed.

The module thus manufactured has the following characteristics.

Voc: 14.3V Isc: 70.7mA (under AM-1 illumination) Fill factor (FF): 69.3% Efficiency: 7.0% According to the above, multiple cells of a monolithic thin film solar panel are connected through a continuous photoactive thin film. It can be seen that there are provided photovoltaic devices connected in series and associated forming methods.

In the above, a certain specific embodiment of the present invention has been described as a representative, but of course, the present invention is not limited to these specific forms, and is applied to all modified examples included in the scope of the gist. It is possible. For example, the cells 12 of the solar panel 10 can have a wide variety of shapes other than the simple strip shape disclosed herein, if desired. Also, in that case, the electrodes on each side of the cells are similarly formed and interconnected in the manner described herein to electrically connect at least two cells in series.

〔The invention's effect〕

According to the present invention, it is possible to provide a method for manufacturing a photovoltaic device in which an active layer in a thin film photovoltaic element is continuous and not patterned, and the photovoltaic regions of the thin film can be easily connected in series. .

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of a monolithic thin-film solar cell panel constructed according to an embodiment of the present invention, and FIG. 2 is a detailed view of the essential parts of the same embodiment taken along line 2-2 of FIG. The figure which expands and shows a cross section, FIG. 3 is sectional drawing which shows the completed state corresponding to a part of FIG. 10 …… Solar cell panel, 12 …… Solar cell, 14 …… Lead wire, 16 …… Transparent substrate, 18 …… Front electrode, 20 …… Continuous thin film, 22 …… Back electrode, 24 …… Back electrode section , 28 …… Photovoltaic area, 30 …… Stitch bar, 32 …… Front electrode part.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Robert Earl Gay, California 91344, Granada Hills, Carrie Aveniryu 11547 (72) Inventor Gary P. Turner, California, California 91307 Canoga Park, Scarborough Peak Drive 7145 (56) References JP-A-58-155770 (JP, A)

Claims (6)

[Claims] 1. A method of manufacturing a photovoltaic device having a photovoltaic continuous thin film of a predetermined overall thickness defining a plurality of photovoltaic regions, wherein each of the photovoltaic devices is substantially light-exposed on a major surface of a transparent substrate. Forming a plurality of transparent front electrode means separated and arranged with a gap having at least one contact portion thicker than the total thickness of the electromotive continuous thin film; and covering and sandwiching the front electrode means. A step of forming a photovoltaic continuous thin film above the main surface of the substrate, and the photovoltaic continuous thin film is separated and arranged with a gap, and each determines a photovoltaic region between them. Forming a plurality of back electrode means disposed substantially over one of the front electrode means and overlapping at a contact portion with the electrode means of an adjacent photovoltaic region. The above contact that overlaps with it A method for manufacturing a photovoltaic cell device, wherein an electrical connection is completed between the solar cell and the photovoltaic section by passing the photovoltaic continuous thin film. 2. The method for manufacturing a photovoltaic cell device according to claim 1, wherein the upper surface of the contact portion is substantially rough. 3. The electrical connection between each back electrode means and the overlapping contact portion is facilitated by local heating of the back electrode means and the photovoltaic continuous thin film at the contact portion. A method of manufacturing a photovoltaic device according to claim 1. 4. The back electrode means and the photovoltaic continuous thin film,
The method of manufacturing a photovoltaic device according to claim 3, wherein heating is performed by a laser beam at a predetermined position. 5. The back electrode means and the photovoltaic continuous thin film,
The method for manufacturing a photovoltaic cell device according to claim 3, wherein heating is performed in the impurities by a laser beam at a predetermined position. 6. The method for manufacturing a photovoltaic device according to claim 1, wherein the contact portion is formed by applying a paste having a powder component by screen printing.