WO2014016954A1

WO2014016954A1 - Solar cell

Info

Publication number: WO2014016954A1
Application number: PCT/JP2012/069141
Authority: WO
Inventors: 翔士佐藤; 悟司東方田
Original assignee: 三洋電機株式会社
Priority date: 2012-07-27
Filing date: 2012-07-27
Publication date: 2014-01-30
Also published as: JPWO2014016954A1

Abstract

A solar cell, comprising: a photoelectric conversion unit that receives incident light and generates power; and a collecting electrode electrically connected to the photoelectric conversion unit and including silver. The collecting electrode includes a network structure having silver particles reciprocally deposited therein and flakes, being particles of a conductive material, having a ratio between the major axis and the thickness thereof of major axis/thickness ≥ 10, and having an average particle diameter of at least 2-5 µm. The flake ratio is no more than 60 wt%.

Description

Solar cell

The present invention relates to a solar cell.

As shown in FIG. 10, the solar cell is provided with comb-like fingers 12 and bus bars 14 on the front and back surfaces of the photoelectric conversion unit 10. The fingers 12 and the bus bar 14 are used as collecting electrodes that extract the electric power generated by the photoelectric conversion unit 10 to the outside of the solar cell.

Finger 12 and bus bar 14 are formed by applying a conductive paste on photoelectric conversion unit 10 using screen printing or the like. As the conductive paste, for example, a resin-type conductive paste in which a resin material such as an epoxy resin is used as a binder and conductive particles such as silver particles are mixed as a filler can be used. Furthermore, the shape of the conductive particles may be a mixture of flakes and spheres, a mixture of particles having different sizes, or an uneven shape on the surface.

JP 2011-181966 A

Incidentally, further improvement in output is desired in a solar cell including fingers and bus bars in which conductive particles and flaky particles are mixed.

One aspect of a solar cell according to the present invention includes a power generation unit that generates power upon receiving light, and a collector electrode including silver that is electrically connected to the power generation unit. The collector electrode includes silver particles. It includes a network structure welded to each other and flakes that are particles of a conductive material, and the ratio of the flakes is 60% by weight or less.

According to the solar cell of the present invention, the output can be improved in the solar cell having a network structure on the collector electrode.

It is the top view seen from the light-receiving surface side of the solar cell in embodiment of this invention. It is the top view which looked at the solar cell in embodiment of this invention from the back surface side. It is sectional drawing of the solar cell in embodiment of this invention. It is a cross-sectional schematic diagram of the collector electrode (finger and bus bar) in the embodiment of the present invention. It is a cross-sectional schematic diagram of the collector electrode (finger and bus bar) in the embodiment of the present invention. It is a figure which shows the characteristic of the solar cell in embodiment of this invention. It is the top view and cross-sectional schematic diagram of the collector electrode (finger and bus bar) in embodiment of this invention. It is a cross-sectional schematic diagram of the collector electrode (finger and bus bar) in the embodiment of the present invention. It is a cross-sectional schematic diagram of the collector electrode (finger and bus bar) in the embodiment of the present invention. It is a figure which shows the structure of the conventional solar cell.

The solar cell 100 according to the present embodiment includes a photoelectric conversion unit 20, fingers 22, and a bus bar 24 as shown in FIGS. FIG. 1 is a plan view of the solar cell 100 on the light receiving surface side. FIG. 2 is a plan view of the back side of the solar cell 100. FIG. 3 shows a cross-sectional view along line AA in FIGS.

Note that the drawings are schematic, and the ratios of dimensions and the like are different from actual ones for easy understanding of the explanation. Moreover, the part from which the relationship and ratio of a mutual dimension differ also in between drawings is contained.

Here, “light-receiving surface” means a main surface on which sunlight mainly enters from the outside of the solar cell, and “back surface” means a main surface opposite to the light-receiving surface. For example, more than 50% to 100% of sunlight incident on the solar cell 100 enters from the light receiving surface side.

The photoelectric conversion unit 20 has a semiconductor junction such as a pn or pin junction, a crystalline semiconductor material such as single crystal Si or polycrystalline Si, a thin film semiconductor material such as an amorphous Si alloy or CuInSe, or GaAs. It is made of a semiconductor material such as a compound semiconductor material such as InP. Further, an organic material such as a dye-sensitized type may be used.

For example, the photoelectric conversion unit 20 includes a substrate 20a, an i-type amorphous silicon layer 20b, a p-type amorphous silicon layer 20c, a transparent conductive layer 20d, an i-type amorphous silicon layer 20e, and an n-type amorphous silicon layer. 20f and the transparent conductive layer 20g can be comprised. An i-type amorphous silicon layer 20b and a p-type amorphous silicon layer 20c are provided on the upper surface side of the substrate 20a. Further, a transparent conductive layer 20d is provided on the p-type amorphous silicon layer 20c. On the other hand, an i-type amorphous silicon layer 20e and an n-type amorphous silicon layer 20f are provided on the lower surface side of the substrate 20a. Further, a transparent conductive layer 20g is provided on the n-type amorphous silicon layer 20f. Such a solar cell 100 having the photoelectric conversion unit 20 has an intrinsic (i-type) amorphous semiconductor film interposed between a crystalline semiconductor and a pn junction formed of a p-type amorphous semiconductor film. By inserting, the conversion efficiency is dramatically improved.

The i-type amorphous silicon layer 20b, the p-type amorphous silicon layer 20c, the i-type amorphous silicon layer 20e, and the p-type amorphous silicon layer 20f are formed by PECVD (Plasma Enhanced Chemical Vapor Deposition), Cat-CVD ( (Catalytic Chemical Vapor Deposition), sputtering, or the like. For PECVD, any method such as RF plasma CVD, high-frequency VHF plasma CVD, or microwave plasma CVD may be used. For example, an i-type amorphous silicon layer 20b having a thickness of about 5 nm is formed on the light-receiving surface side of the substrate 20a, and a p-type amorphous silicon layer 20c having a thickness of about 5 nm is further formed. Further, an i-type amorphous silicon layer 20e having a thickness of about 5 nm is formed on the back side of the substrate 20a, and an n-type amorphous silicon layer 20f having a thickness of about 20 nm is further formed.

The transparent

conductive layers

20d and 20g include, for example, at least one metal oxide such as indium oxide, zinc oxide, tin oxide, or titanium oxide, and these metal oxides include tin, zinc, tungsten, A dopant such as antimony, titanium, cerium, or gallium may be doped. The transparent

conductive layers

20d and 20g can be formed by a film forming method such as an evaporation method, a CVD method, or a sputtering method.

However, the material constituting the photoelectric conversion unit 20 is not particularly limited, and various materials can be used.

The finger 22 and the bus bar 24 are provided on the transparent

conductive layers

20d and 20g. The finger 22 is an electrode for collecting carriers generated by the photoelectric conversion unit 20. In order to collect carriers from the photoelectric conversion unit 20 as evenly as possible, the fingers 22 have a linear shape having a width of about 100 μm, for example, and are arranged every 2 mm. The bus bar 24 is a current collecting electrode for carriers collected by the plurality of fingers 22. The bus bar 24 has a linear shape having a width of 1 mm, for example, and is arranged so as to intersect the fingers 22. That is, a plurality of narrow fingers 22 and a wide bus bar 24 are combined to form a comb-shaped collector electrode. The numbers and areas of the fingers 22 and the bus bars 24 are appropriately set in consideration of the size and resistance of the solar cell 100. The bus bar 24 may not be provided.

In addition, it is preferable that the installation area of the finger 22 and the bus bar 24 provided on the light receiving surface side of the solar cell 100 is smaller than the back surface side. That is, on the light receiving surface side of the solar cell 100, the light shielding loss can be reduced by making the area that blocks the incident light as small as possible. On the other hand, it is not necessary to consider incident light on the back surface side, and a collecting electrode may be provided so as to cover the entire back surface of the solar cell 100 instead of the fingers 22.

In the above configuration, the finger 22 and the bus bar 24 are formed of a conductive paste. As the conductive paste, for example, a resin-type conductive paste using a resin material such as an epoxy resin as a binder and conductive particles such as silver particles as a filler can be used. The binder is mixed mainly for adhesion. In order to maintain reliability, the binder is required to be excellent in moisture resistance and heat resistance. As the binder, for example, at least one selected from an epoxy resin, an acrylic resin, a polyimide resin, a phenol resin, a urethane resin, a silicone resin, or a mixture or copolymerization of these resins may be applied. The filler is mixed for the purpose of obtaining electrical conductivity of the finger 22 and the bus bar 24. Silver particles having different sizes may be mixed, or those having an uneven shape on the surface may be mixed. Moreover, you may add a solvent to an electrically conductive paste as needed. The solvent may be butyl carbitol acetate (BCA) or the like.

Further, in the present embodiment, conductive particle flakes are also mixed with the conductive paste. The flake means that the ratio of the long diameter to the thickness of the conductive material powder particles is long diameter / thickness ≧ 10 and the average particle diameter is 2 to 5 μm or more. The flake-shaped conductive particles can be obtained by, for example, processing granular conductive particles using a pulverizer using balls as pulverizing media such as a rolling mill, a planetary mill, a tower mill, and a medium agitating mill. Obtainable. For example, the flakes may be made of a material containing silver.

The finger 22 and the bus bar 24 can be formed by applying such a conductive paste in a desired pattern on the transparent

conductive layers

20d and 20g by a method such as screen printing and offset printing, and curing by heating. . At this time, by adjusting the characteristics and heating temperature of the silver particles, a network structure 30 in which a large number of silver particles are welded to each other can be provided as shown in the schematic cross-sectional view of FIG. When the silver particles of the finger 22 and the bus bar 24 have the network structure 30, it is possible to confirm a structure in which more than half of the silver particles of the finger 22 and the bus bar 24 are welded and connected to each other in the observation range of the microscopic observation. However, when a solar cell created by a low-temperature process such as a HIT solar cell is applied as the solar cell 100, it is cured in a temperature range (200 ° C. or less) in which the thermal damage to each amorphous semiconductor layer is small, and the network structure It is preferable to use a conductive paste forming 30.

In a solar cell module in which the solar cells 100 are modularized, a plurality of solar cells 100 are connected to each other by conductive connection members called tabs. The connecting member may be connected along the bus bar 24. Furthermore, a laminated film having a structure in which a sealing material on the light-receiving surface side having transparency, such as glass and translucent plastic, and a resin film such as PET (Polyethylene Etherephthalate) or an aluminum (Al) foil are sandwiched between resin films, etc. You may seal with the sealing material of the back surface side which consists of. For sealing, a light-transmitting filler such as EVA may be used.

By the way, as in the present embodiment, by providing the finger 22 and the bus bar 24 with the network structure 30 of silver particles, the conductivity of the finger 22 and the bus bar 24 is improved, and the resistance loss of the solar cell 100 is reduced. Can do.

Further, when the silver particle network structure 30 is formed, the shrinkage stress in the finger 22 and the bus bar 24 increases due to the welding of the silver particles. Thereby, the stress difference between the finger 22 and the bus bar 24 and the transparent

conductive layers

20d and 20g also increases, and the finger 22 and the bus bar 24 easily peel off at these interfaces. When the finger 22 or the bus bar 24 is peeled off from the transparent

conductive layers

20d and 20g, the contact resistance is increased, and the characteristics of the solar cell 100 are deteriorated.

On the other hand, by mixing the conductive particle flakes 32 in the fingers 22 and the bus bar 24, the network structure 30 of the silver particles is divided by the flakes 32, and the contraction stress in the fingers 22 and the bus bar 24 can be reduced. The flakes 32 may be randomly distributed within the network structure 30. Accordingly, as shown in the schematic cross-sectional view of FIG. 5, the ratio of the flakes 32 in the finger 22 and the bus bar 24 as shown in FIG. 5A is low to the flake 32 as shown in FIG. 5B. By setting it as a state, the cutting | disconnection location of the network structure 30 of a silver particle can be increased, and the increase in contraction stress can be suppressed. However, an increase in the flake 32 ratio results in an increase in the bulk resistance of the fingers 22 and bus bar 24.

FIG. 6 shows the relationship between the ratio of the flakes 32 included in the finger 22 and the bus bar 24, the short circuit current density Isc, the fill factor FF, and the maximum power generation amount Pmax. The horizontal axis of FIG. 6 shows the ratio of translucency by the flakes 32 included in the fingers 22 and the bus bar 24 in weight%, and the vertical axis shows the case where the ratio of the flakes 32 is 24% by weight. The relative values of the short circuit current density Isc, the fill factor FF, and the maximum power generation amount Pmax are shown.

When the ratio of the flakes 32 in the finger 22 and the bus bar 24 exceeds 60% by weight, the silver particle network structure 30 is not formed even if the conductive paste is cured. The desired electrical and mechanical properties were not obtained. Therefore, the ratio of the flakes 32 in the fingers 22 and the bus bar 24 is preferably 60% by weight or less.

Further, from FIG. 6, as the ratio of the flakes 32 included in the finger 22 and the bus bar 24 increases, the short-circuit current density Isc also increases. This is because, as the flake 32 ratio is higher, the thixotropy ratio of the conductive paste is increased, the bleeding at the time of printing the finger 22 and the bus bar 24 is reduced, and as a result, the light shielding loss due to the finger 22 and the bus bar 24 is reduced. Conceivable. Further, the fill factor FF increases as the ratio of the flakes 32 contained in the finger 22 and the bus bar 24 increases from 0 to 24% by weight, shows a maximum value at 24% by weight, and gradually decreases when the ratio exceeds 24% by weight. did. When the ratio of the flakes 32 is reduced when the ratio of the flakes 32 is less than 24% by weight, the number of portions where the network structure 30 of the silver particles is divided by the flakes 32 is reduced. Thereby, the shrinkage stress in the finger 22 and the bus bar 24 is increased, the finger 22 and the bus bar 24 are easily peeled off, and it is assumed that the fill factor FF is decreased because the contact resistance is increased. On the other hand, when the ratio of the flakes 32 is increased in the range where the ratio of the flakes 32 exceeds 24% by weight, the number of places where the silver particle network structure 30 is divided by the flakes 32 increases. Thereby, it is guessed that the bulk resistance of the finger 22 and the bus bar 24 increased, and the fill factor FF decreased.

Due to the influence of the short-circuit current density Isc and the fill factor FF, the maximum power generation amount Pmax of the solar cell 100 becomes a high value when the ratio of the flakes 32 is 10 wt% or more and 30 wt% or less, and reaches a maximum value at 24 wt%. Indicated. For this reason, it is preferable that the ratio of the flakes 32 in the finger 22 and the bus bar 24 is 10 wt% or more and 30 wt% or less.

Further, as shown in FIG. 7, the fingers 22 and the bus bars 24 have a thickness distribution in the width direction (W direction) perpendicular to the longitudinal direction (L direction), and the film thickness becomes thinner toward the end. The thicker the fingers 22 and the bus bars 24, the greater the stress between silver particles due to the network structure 30. Therefore, it is preferable to increase the ratio of the flakes 32 in the region where the fingers 22 and the bus bars 24 are thick, and to increase the number of cut points of the network structure 30 of silver particles. For example, as shown in FIG. 7, in the width direction perpendicular to the longitudinal direction of the fingers 22 and the bus bar 24, it is preferable that the density of the flakes 32 at the center is higher than the density of the flakes 32 at the ends.

Also, as shown in FIG. 8, a texture structure 34 may be provided on the transparent

conductive layer

20d or 20g. In this case, when the network structure 30 of silver particles is formed in the concave portion of the texture structure 34, the stress in the network structure 30 is the direction in which the fingers 22 and the bus bars 24 are peeled from the side walls of the texture structure 34 (arrow direction in the figure). It becomes easy to act on. Such local concentration of stress facilitates the finger 22 and the bus bar 24 to be peeled off from the transparent

conductive layer

20d or 20g. Therefore, when the finger 22 and the bus bar 24 are formed on the texture structure 34, it is preferable to have the flakes 32 in a region in the recess of the texture structure 34. Furthermore, as shown in FIG. 8, it is preferable that the ratio of the flakes 32 in the region in the concave portion of the texture structure 34 is higher than that in the region above the texture structure 34.

In order to make the ratio of the flakes 32 in the area in the concave portion of the texture structure 34 higher than that in the upper layer area, conductive pastes having different flake 32 ratios may be applied in multiple layers. For example, as the first layer, a conductive paste having a higher flake 32 ratio is applied so as to fill the concave portion of the texture structure 34, and as the second layer, a conductive paste having a lower flake 32 ratio exceeds the texture structure 34. What is necessary is just to apply | coat to an upper layer area | region.

On the other hand, considering the entire finger 22 and bus bar 24, it can be said that the network structure 30 of silver particles is cut by the texture structure 34 in the lower layer, and the stress is relaxed. Therefore, when considering the entire finger 22 and the bus bar 24, it is preferable to have the flakes 32 in a region above the texture structure 34 that is not divided by the texture structure 34. Moreover, as shown in FIG. 9, it is preferable to make the ratio of the flakes 32 in the upper layer higher than the lower layer having the texture structure 34.

In such a configuration, conductive pastes having different flake 32 ratios may be applied in multiple layers. For example, as a first layer, a conductive paste having a lower ratio of flakes 32 is applied so as to fill the concave portion of the texture structure 34, and as a second layer, a conductive paste having a higher ratio of flakes 32 exceeds the texture structure 34. What is necessary is just to apply | coat to an upper layer area | region.

The present invention has been described above based on the embodiments. However, the present invention is not limited to the above embodiments, and modifications may be made without departing from the spirit of the present invention. Example) may be combined.

DESCRIPTION OF SYMBOLS 10 Photoelectric conversion part, 12 Finger, 14 Bus bar, 20 Photoelectric conversion part, 20a Substrate, 20b i-type amorphous silicon layer, 20cp p-type amorphous silicon layer, 20d Transparent conductive layer, 20e i-type amorphous silicon layer 20 f n-type amorphous silicon layer, 20 g transparent conductive layer, 22 fingers, 24 bus bar, 30 network structure, 32 flakes, 34 texture structure, 100 solar cell.

Claims

A photoelectric conversion unit;
A collector electrode containing silver electrically connected to the photoelectric converter;
With
The collector electrode is
A network structure in which silver particles are welded to each other;
Flakes that are particles of conductive material,
The solar cell whose ratio of the said flakes is 60 weight% or less.
The solar cell according to claim 1,
The solar cell whose ratio of the said flakes is 10 to 30 weight%.
The solar cell according to claim 1 or 2,
The collector electrode is a solar cell in which the density of the flakes in the center portion is higher than the density of the flakes in the end portion in the width direction perpendicular to the longitudinal direction of the collector electrode.
The solar cell according to any one of claims 1 to 3,
The said collector electrode is a solar cell which is formed on the texture structure and has the said flakes in the area | region in the recessed part of the said texture structure.
The solar cell according to any one of claims 1 to 3,
The said collector electrode is a solar cell which is formed on the texture structure and has the said flakes in the area | region of the upper layer from the said texture structure.
A solar cell according to any one of claims 1 to 5,
The flakes are solar cells in which the ratio of major axis to thickness is major axis / thickness ≧ 10 and the average particle diameter is 2 to 5 μm or more.