JP2005353836A - Solar cell element and solar cell module using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell element in which optical loss by surface electrode is reduced, short circuit current density and a curve factor can be improved by properly balancing them, and sufficient conversion efficiency can be obtained. <P>SOLUTION: The solar cell element has the surface electrode on the light receiving face-side of a semiconductor substrate of polycrystalline silicon. If an area when the surface electrode is viewed in terms of plane from the vertical direction of the light receiving side is set to be Sa (cm<SP>2</SP>), the surface area of a region where the surface electrode is arranged to be Sb (cm<SP>2</SP>) in the light receiving face of the solar cell element, a relation of 1.10≤Sb/Sa≤2.10 is satisfied. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solar cell element having a front electrode on the light receiving surface side and a solar cell module using the solar cell element.

  A solar cell converts incident light energy into electrical energy. Major solar cells are classified into crystalline, amorphous, and compound types depending on the type of materials used. Of these, most of the crystalline silicon solar cells currently on the market are in the market. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single-crystal silicon solar cells have the advantage that the substrate quality is good and the efficiency can be easily increased, but the substrate is expensive to manufacture. On the other hand, the polycrystalline silicon solar cell has the advantage that it can be manufactured at a low cost although it has the disadvantage that it is difficult to increase the efficiency because the quality of the substrate is inferior. In recent years, conversion efficiency of about 18% has been achieved at the research level due to the improvement of the quality of the polycrystalline silicon substrate and the advancement of cell technology.

  On the other hand, since polycrystalline silicon solar cells of mass production level are low cost, they have been distributed on the market and are now mainstream products of solar cells.

A general configuration of a bulk silicon solar cell 110 described in Patent Document 1 is shown in FIG. FIG. 4A shows a cross-sectional structure. As shown in the figure, P (phosphorus) atoms and the like are diffused at a high concentration on the light incident surface side of a p-type bulk region 105 made of a p-type silicon semiconductor substrate, thereby forming a pn junction with the p-type bulk region. The reverse conductivity type region 104 is formed, and an antireflection film 106 made of a silicon nitride film, a silicon oxide film or the like is further provided. Further, on the opposite side of the light incident surface, a p ⁺ type region 107 which is a p ⁺ region containing a large amount of p type semiconductor impurities such as aluminum is provided.

When light is incident from the light incident surface, photogenerated carriers are generated in the semiconductor region 103 composed of the reverse conductivity type region 104, the p-type bulk region 105, and the p ⁺ -type region 107. Therefore, on the light incident surface side, front side electrodes (a bus bar electrode 101 and finger electrodes 102 (described later)) whose main component is a metal material such as silver are provided, and on the opposite side, a back surface collecting electrode 108 made of aluminum. A back surface output electrode 109 mainly composed of silver is provided.

  FIG. 4B is a view of the front electrode viewed from the light incident surface (front surface) side. In the figure, reference numeral 101 denotes a bus bar electrode, and 102 denotes a finger electrode. As described above, the front electrode generally includes the finger electrode 102 (branch electrode) having a narrow line width and the bus bar electrode 101 (trunk electrode) having a large line width to which at least one end of the finger electrode 102 is connected. Yes.

  In order to reduce the power loss at the front electrode as much as possible, a metal material is usually used for the front electrode, and it is common that silver having a low resistivity is the main component, for example, a screen printing method. It is formed by applying a silver paste or the like and baking it.

In recent years, environmental problems have been addressed, and higher conversion efficiency is required for solar cells. Therefore, various ideas have been made for the front side electrodes (bus bar electrode 101, finger electrode 102) arranged on the light incident surface. For example, it is possible to reduce the optical loss (reflection loss) by thinning the wires, or to make the electrodes orthogonal to each other in order to carry the electrons collected on the finger electrodes 102 to the bus bar electrode 101 with as little loss as possible. It has been done in general.
JP-A-8-274356 Japanese Patent Application Laid-Open No. 11-312665

  The conversion efficiency of the solar cell element has the following relationship.

Conversion efficiency η 短 絡 Short circuit current density Jsc × open circuit voltage Voc × fill factor FF
Therefore, in order to increase the conversion efficiency η and increase the efficiency, it is only necessary to increase any one of the above-described factors among the open circuit voltage Voc, the short circuit current density Jsc, and the curve factor FF.

  According to the above-described method for reducing the optical loss by thinning the front electrode, it is possible to substantially increase the light energy incident on the solar cell element and improve the short-circuit current density Jsc. However, on the other hand, there is a problem that the electrode resistance increases as the line width decreases. This electrode resistance becomes a series resistance component of the solar cell element, particularly worsens the fill factor FF, and the conversion efficiency cannot be improved. As described above, it is difficult to uniformly increase all of these factors related to the conversion efficiency.

  The present invention has been made in view of such a problem, and while reducing optical loss due to the front-side electrode, improving the short-circuit current density and the fill factor while maintaining an appropriate balance, and achieving good conversion efficiency. It aims at providing the solar cell element which can be obtained.

  In order to achieve the above object, the inventors have studied from various angles about reducing the electrical loss due to the series resistance component of the front electrode. As the series resistance component of the front electrode, the resistance of the front electrode itself and the contact resistance between the front electrode and the semiconductor region are involved, and in particular, the contact between the latter front electrode and the semiconductor region. We found that resistance is a problem. And as a result of intensive studies in view of this knowledge, the inventors have reached the configuration of the present invention that can improve the contact resistance between the front electrode and the semiconductor region.

That is, the solar cell element according to claim 1 of the present invention is a solar cell element having a front side electrode on the light receiving surface side of a polycrystalline silicon semiconductor substrate, and the front side electrode is viewed in a plan view from the vertical direction on the light receiving surface side. the area when Sa (cm ^2), of the light receiving surface of the solar cell element, the surface area of the front electrode is provided area when the Sb (cm ^2),
1.10 ≦ Sb / Sa ≦ 2.10 (1)
Satisfy the relationship.

  Formula (1) is a range in which the ratio of the surface area of the light receiving surface of the solar cell element in the region where the front electrode is provided to the area of the front electrode is greater than 1.10 and less than 2.10. As a result, the substantial contact area between the front electrode and the solar cell element is increased, and the adverse effect due to the electrical loss due to the series resistance component can be reduced. Nothing.

  As a result, the solar cell element of the present invention reduces the optical loss due to the front electrode and improves the short-circuit current density and the fill factor while maintaining an appropriate balance, so that a good conversion efficiency can be obtained. it can.

  As for the area Sa of the front side electrode, after photographing the solar cell element from the vertical direction on the light receiving surface side and digitizing the surface image, for example, by using a known image processing method, By binarizing with a threshold value such that the front electrode is separated, it is possible to separate the location of the front electrode and the other location, and to obtain the area.

  Furthermore, the surface area Sb of the light receiving surface of the solar cell element in the region where the front electrode is provided is a predetermined acid selected according to the material type of the front electrode (for example, if the electrode is mainly composed of silver, The surface electrode may be removed by removing the front electrode with water, and the surface area of the portion where the electrode was provided may be measured. The surface area can be measured by either contact or non-contact methods, but it is desirable to use an AFM (Atomic Force Microscope) from the viewpoint of accuracy. In addition, when using AFM, since the size of the observable region is limited, by measuring a plurality of locations from a predetermined location where the front electrode is provided and statistically processing, It is possible to determine whether it is within range.

  The front electrode includes a substantially straight bus bar electrode and a plurality of finger electrodes having at least one end connected to the bus bar electrode.

  The solar cell module of the present invention is a solar cell module having a plurality of plate-like solar cell elements arranged at predetermined intervals and electrically connected to each other, wherein the plurality of solar cell elements are: The solar cell element of the present invention was included.

As described above, the solar cell element of the present invention is a solar cell element having a front-side electrode on the light-receiving surface side of a polycrystalline silicon semiconductor substrate, and when the front-side electrode is viewed in plan from the vertical direction on the light-receiving surface side the area Sa (cm ^2), of the light receiving surface of the solar cell element, the surface area of the front electrode is provided area when the Sb (cm ^2),
1.10 ≦ Sb / Sa ≦ 2.10 (1)
The solar cell element of the present invention has a good conversion efficiency because it reduces the optical loss due to the front electrode and improves the short-circuit current density and the fill factor while maintaining an appropriate balance. Can be obtained.

  Hereinafter, embodiments of a solar cell element according to the present invention will be described in detail with reference to the drawings.

  First, the structure of the solar cell element according to the present invention will be briefly described. FIG. 2A is a diagram showing a cross-sectional structure of the solar cell element according to the present invention, FIG. FIG. 2B is a diagram showing an example of the electrode shape on the non-light-receiving surface side (back surface side).

As shown in FIG. 2A, P (phosphorus) atoms and the like are diffused at a high concentration on the light incident surface side of the p-type bulk region 5 made of a p-type silicon polycrystalline semiconductor substrate, and the p-type bulk region is separated from the p-type bulk region. A reverse conductivity type region 4 having a pn junction formed therebetween is formed, and an antireflection film 6 made of a silicon nitride film, a silicon oxide film or the like is further provided. Further, on the opposite side of the light incident surface, a p ⁺ type region 7 which is a p ⁺ region containing a large amount of p type semiconductor impurities such as aluminum is provided, and the solar cell element 10 according to the present invention is configured. Yes. And as shown in FIG.2 (b), FIG.2 (c), the front side electrode (bus-bar electrode 1, finger electrode 2) which has metal materials, such as silver, as a main component is on the light-incidence surface side of the solar cell element 10. FIG. On the opposite side, a back surface collecting electrode 8 made of aluminum and a back surface output electrode 9 mainly composed of silver are provided.

When light is incident from the side of the antireflection film 6 that is the light incident surface side of the solar cell element 10, it is absorbed by the semiconductor region 3 including the reverse conductivity type region 4, the p-type bulk region 5, and the p ⁺ -type region 7. -Photoelectric conversion generates electron-hole pairs (electron carriers and hole carriers). Photoelectron-generated electron carriers and hole carriers (photogenerated carriers) cause photovoltaic power to be generated between the substantially linear front-side electrode provided on the front side of the solar cell element 10 and the back-side electrode provided on the back side. The generated photogenerated carriers are collected by these electrodes and guided to the output terminal.

  FIG. 2B is a diagram illustrating an example of the front electrode viewed from the light incident surface (front surface) side. In the figure, 1 is a bus bar electrode, 2 is a finger electrode, and 6 is an antireflection film. The front side electrode is generally composed of a finger electrode 2 (branch electrode) having a narrow line width and a bus line electrode 1 (stem electrode) having a large line width to which at least one end of the finger electrode 2 is connected and being substantially linear. Yes. In order to reduce the power loss at the front electrode as much as possible, a metal material is usually used for the front electrode, and it is common that silver having a low resistivity is the main component, for example, a screen printing method. It is formed by applying a silver paste or the like and baking it.

As shown in FIG. 2B, the back side electrode includes a back surface collecting electrode 8 and a back surface output electrode 9. The back collector electrode 8 is preferably formed using aluminum which acts as a p-type semiconductor impurity element with respect to silicon as a semiconductor substrate. The reason is that a p ⁺ type region 7 is formed on the surface layer on the back side of the silicon substrate. This p ⁺ type region 7 is also called a BSF (Back Surface Field) region, and photogenerated electron carriers reach the back collector 8. This serves to reduce the rate of recombination loss and improves the short circuit current value Jsc. And also since the p ⁺ In -type region 7 minority carriers (electrons) density is reduced, which serves to reduce the diode current amount in the area in contact with the p ⁺ -type region 7 and the rear surface collector electrode 8 (dark current quantity) In addition, the open circuit voltage Voc is improved. As the back output electrode 9, a metal material mainly composed of silver having the same low resistivity as that of the front electrode is often used. For example, a silver paste is applied by screen printing or the like, and then fired. It is formed.

  Next, the structure of the front side electrode which concerns on the solar cell element of this invention is demonstrated using FIG.

  Fig.1 (a) is the elements on larger scale of A part of the front side electrode shown in FIG.2 (b), FIG.1 (b) shows the front side electrode shown in FIG.1 (a) in the cross section of a BB direction. FIG.

  As shown in FIG. 1A, Sa represents an area when the bus bar electrode 1 and the finger electrode 2 which are front side electrodes are viewed in a plan view from the vertical direction on the light receiving surface side. 1A is a partially enlarged view of a portion A in FIG. 2B, and the same applies to other portions of FIG. 2B that do not appear in FIG. 1A. Moreover, in FIG. 1A, the surface area of the light receiving surface of the solar cell element which is covered with the front electrode and cannot be seen, that is, the portion immediately below the region where the front electrode (bus bar electrode 1 and finger electrode 2) is provided. Is Sb. At this time, the configuration represented by the following equation (1) is established.

1.10 ≦ Sb / Sa ≦ 2.10 (1)
The expression (1) will be described. As described above, the denominator Sa corresponds to the area of the front electrode, and the numerator Sb is the surface area of the light receiving surface of the solar cell element in the region where the front electrode is provided. / Sa is a measure of the ratio at which the front electrode contacts the light receiving surface of the solar cell element. In order to help understanding, FIG. 1B shows one dimension dropped by looking from the cross-sectional direction. Since Sa is the area of the front electrode in plan view, in FIG. 1B, it is a one-dimensional straight line as represented by a double-headed arrow C1, and Sb is a two-dimensional uneven shape surrounded by the region C2. Represented as: Actually, since this also extends in the depth direction of FIG. 1B, Sa is a two-dimensional (virtual) plane, and Sb is a surface having three-dimensional unevenness.

  Here, the surface area of the light-receiving surface of the solar cell element is determined so that the ratio of the formula is larger than 1.10 and smaller than 2.10, thereby substantially reducing the front electrode and the solar cell element. The contact area can be increased. When Sb / Sa is 1.10 or less, if the contact area is increased to improve the FF, the light receiving area decreases and the short circuit current is reduced, and the electrode area is decreased to improve the short circuit current. If this is done, there is a problem that hinders the conversion efficiency improvement of the solar cell element that FF is lowered, and when it is 2.10 or more, it becomes difficult to fill the silicon substrate surface with the electrode material by the screen printing method. There is.

  Thus, according to the configuration of the front side electrode according to the solar cell element of the present invention, the substantial contact area between the front side electrode and the solar cell element increases and becomes an appropriate value. Can reduce the adverse effects of As a result, the fill factor FF is not deteriorated.

  As a result, the solar cell element of the present invention reduces the optical loss due to the front electrode and improves the short-circuit current density and the fill factor while maintaining an appropriate balance, so that a good conversion efficiency can be obtained. .

  In order to reduce the value of Sb / Sa in the equation (1), the surface of the semiconductor substrate under the electrode may be made flat or an insulating film may be interposed between the electrode and the semiconductor substrate. In order to increase the surface, the surface of the semiconductor substrate under the electrode may be roughened or a depression may be formed. In particular, in order to obtain an appropriate range of the present invention, it is desirable to make the surface of the semiconductor substrate under the electrodes rough. In addition, when the electrodes are formed by screen printing, it is desirable to uniformly form fine irregularities having an irregularity height of 2 μm or less. By doing so, it is possible to fill the electrode material along the shape of the semiconductor substrate even in screen printing, and it is possible to more effectively reduce the series resistance.

  Further, in the solar cell element of the present invention, the bus bar electrode 1 included in the front electrode is described as an example in which two bus bar electrodes 1 are provided in one solar cell element, but may be three or more. In particular, in order to prevent energy loss of light on the light receiving surface, when the line width of the finger electrode 2 is reduced, when the bus bar electrode 1 is two, the fill factor FF is deteriorated by the series resistance component in the finger electrode 2. Although there is a tendency, if the number of the bus bar electrodes 1 is three, the length of the finger electrode 2 can be shortened, and the deterioration of the fill factor FF due to the series resistance component of the finger electrode 2 can be suppressed. As the area Sa when the front electrode according to the present invention is viewed in plan, when the area Sa is in the range of 4% to 7% with respect to the area of the light receiving surface of the solar cell element, the light energy loss is optimally reduced. This is preferable because it is less likely to receive the resistance component of the electrode.

In the solar cell element of the present invention, the short-circuit current density Jsc obtained by dividing the short-circuit current Isc defined by JIS C 8913 (1998) by the substrate area is 35.5 mA / cm ² or more, and JIS C 8913 (1998). It is desirable that the FF specified by) is 0.75 or more. In such a solar cell element having a high short-circuit current density and FF, further severe control is required in designing the electrode. Therefore, the effect of the solar cell element according to the present invention can be sufficiently exhibited.

The short-circuit current Isc is preferably 8000 mA or more. The value of the short-circuit current Isc can be adjusted according to the size of the solar cell element. When the short-circuit current density Jsc is 35.5 mA / cm ² in the solar cell element of the present invention, the light receiving surface area is 15 cm × It is necessary to have a size of 15 cm or more. By designing the size of the solar cell element so that the short-circuit current Isc is within this range, there is an excellent effect that the manufacturing cost can be reduced.

  Next, FIG. 3A shows an example of the solar cell module of the present invention configured to include the solar cell element of the present invention. As shown in the figure, the plurality of plate-like solar cell elements 10 arranged at a predetermined interval with the light incident surface facing the light receiving surface side of the solar cell module 17 are made of a conductive material typified by metal. The tabs 11 are electrically connected to each other to constitute a solar cell element group.

  In addition, in these solar cell element groups, it is effective if at least one solar cell element 10 of the present invention is included, but in order to achieve the effects of the invention, the solar cell element group is configured. It is more desirable that all the solar cell elements that perform the solar cell element 10 of the present invention.

  The solar cell element group described above is hermetically sealed with a filler 13 mainly composed of ethylene vinyl acetate copolymer (EVA) or the like between the translucent panel 12 and the back surface protective material 14, and the solar cell module 17. Is configured. The output of the solar cell module 17 is connected to the terminal box 16 via the output wiring 15. The terminal box 16 is further connected to an external load (not shown).

  FIG. 3B shows a partially enlarged view of the internal structure of the solar cell module shown in FIG. As shown in the figure, the bus bar electrode 1 that is an output extraction electrode of the solar cell element 10 and the back surface output electrode 9 on the back side of the adjacent solar cell element 10 are electrically connected by a tab 11.

  The tab 11 connects the entire length of the back surface output electrode 9 and the bus bar electrode 1 or a plurality of locations by thermal welding such as hot air to connect the solar cell elements 10 to each other. As the material constituting the tab 11, for example, a material obtained by processing a copper foil having a thickness of about 100 to 500 μm and covering the entire surface with a solder of about 20 to 70 μm to have a predetermined width and length is preferably used. it can. In addition, the solder coating on the surface of the copper foil is not essential, and when provided, it may be on both sides or one side of the copper foil.

  Next, a process for forming the solar cell element according to the present invention shown in FIG.

First, a p-type silicon substrate is prepared. In FIG. 2A, at least the p-type bulk region 5 is included in the substrate. At this time, it is desirable to use B (boron) as the p-type semiconductor impurity element, the concentration is about 1 × 10 ¹⁶ to 1 × 10 ¹⁷ / cm ^3, and the specific resistance value of the substrate is 0.2 to 2Ω at this time.・ It becomes about cm.

  The substrate thickness is 500 μm or less, more preferably 350 μm or less. As the substrate, a polycrystalline silicon substrate or a single crystal silicon substrate obtained by slicing a polycrystalline silicon ingot cast by a casting method is used. In addition, doping may include an appropriate amount of a doping element alone at the time of manufacturing a silicon ingot, or may include an appropriate amount of a B-containing silicon block whose doping concentration is already known. Thereafter, in order to remove the mechanical damage layer on the surface layer portion of the substrate accompanying the slicing of the substrate, the surface layer portions on the front surface side and the back surface side of this substrate are each about 10 to 20 μm with NaOH, KOH, hydrofluoric acid, hydrofluoric acid, or the like. Etching and then cleaning with pure water or the like.

  Next, a concavo-convex structure having a light reflectivity reduction function is formed on the surface side of the substrate serving as a light incident surface (not shown). In forming the concavo-convex structure, an anisotropic wet etching method using an alkaline solution such as NaOH used for removing the substrate surface layer portion described above can be applied, but the silicon substrate is a polycrystalline silicon substrate formed by a cast method or the like. In this case, since the crystal plane orientation in the substrate plane varies randomly for each crystal grain, it is very difficult to uniformly form a good concavo-convex structure that effectively reduces the light reflectivity over the entire substrate. It is. Therefore, when the silicon substrate is a polycrystalline silicon substrate, it is desirable that a good concavo-convex structure can be easily formed over the entire substrate by performing gas etching by RIE (Reactive Ion Etching) method or the like (for example, Patent Document 2). See).

  This uneven structure is closely related to the above-described equation (1). In order to reduce the value of Sb / Sa, it is only necessary to reduce the depth of the unevenness and reduce the direction of increasing the lateral size of the unevenness, that is, the aspect ratio (vertical / horizontal) of the unevenness. Further, in order to increase the value of Sb / Sa, it is only necessary to increase the depth of the unevenness and increase the direction of decreasing the lateral size of the unevenness, that is, increase the aspect ratio (vertical / horizontal) of the unevenness. In order to increase the aspect ratio of the unevenness, the reaction pressure during etching may be decreased, and in order to decrease it, the reaction pressure may be increased.

  In the case of the wet etching method, as described above, the crystal plane orientation in the substrate surface varies randomly for each crystal grain, so that it is difficult to form a concavo-convex structure uniformly, and the gas etching method such as the RIE method is used. In comparison, it is difficult to freely adjust Sb / Sa with good reproducibility.

Next, an n-type reverse conductivity type region 4 is formed. As the n-type doping element, P (phosphorus) is preferably used, the doping concentration is about 1 × 10 ^{18 to} 5 × 10 ²¹ / cm ³ , and the sheet resistance is an n ⁺ type having a sheet resistance of about 30 to 300 Ω / □. . As a result, a pn junction is formed between the p-type bulk region 5 described above. In addition, the preferable area | region of sheet resistance is 60-300 ohms / square, and when it is smaller than this range, there exists a problem that a short circuit current cannot be improved, and since a diffusion layer will become too shallow when it exceeds this range, There is a problem that the diffusion layer is destroyed when the light-receiving surface electrode is formed, or that sufficient adhesion strength cannot be obtained.

As a manufacturing method, it is formed by diffusing a doping element in a surface layer portion of a p-type silicon substrate at a temperature of about 700 to 1000 ° C. using a thermal diffusion method using POCl ₃ (phosphorus oxychloride) as a diffusion source. At this time, the thickness of the diffusion layer is about 0.2 to 0.5 μm, and this can be set to a desired thickness by adjusting the diffusion temperature and the diffusion time.

In a normal diffusion method, a diffusion region is also formed on the surface opposite to the target surface, but this portion may be removed later by etching. At this time, the reverse conductivity type region 4 other than the surface side of the substrate is removed by applying a resist film on the surface side of the silicon substrate, removing the resist film by etching using a mixed solution of hydrofluoric acid and nitric acid. To do. Further, as will be described later, when the p ⁺ type region 7 (BSF region) on the back surface is formed with an aluminum paste, aluminum as a p-type dopant can be diffused to a sufficient depth at a sufficient concentration. The influence of the n-type diffusion layer in the shallow region that has already been diffused can be ignored, and it is not necessary to remove the n-type diffusion layer formed on the back side.

  Note that the method of forming the reverse conductivity type region 4 is not limited to the thermal diffusion method. For example, using a thin film technology and conditions, a crystalline silicon film including a hydrogenated amorphous silicon film or a microcrystalline silicon layer is used as a substrate temperature. You may form at about 400 degrees C or less. However, when forming using thin film technology, it is necessary to consider the temperature of each process described below and to determine the order of formation so that the process temperature is as low as the subsequent process.

  Here, when the reverse conductivity type region 4 is formed using a hydrogenated amorphous silicon film, the thickness is 50 nm or less, preferably 20 nm or less, and when it is formed using a crystalline silicon film, the thickness is 500 nm or less. The thickness is preferably 200 nm or less.

  When the reverse conductivity type region 4 is formed by the above thin film technology, if an i type silicon region (not shown) is formed with a thickness of 20 nm or less between the p type bulk region 5 and the reverse conductivity type region 4, the characteristics are improved. It is effective for.

Next, an antireflection film 6 is formed. As the material of the antireflection film 6, a Si ₃ N ₄ film, a TiO ₂ film, a SiO ₂ film, a MgO film, an ITO film, a SnO ₂ film, a ZnO film, or the like can be used. The thickness is appropriately selected depending on the material, and realizes the non-reflection condition for incident light (if the refractive index of the material is n and the wavelength of the spectral region to be non-reflection is λ, (λ / n) / 4 = d is (This is the optimum film thickness for the antireflection film). For example, in the case of a commonly used Si ₃ N ₄ film (n = about 2), if the non-reflection target wavelength is 600 nm, the film thickness may be about 75 nm.

As a manufacturing method, a PECVD method, a vapor deposition method, a sputtering method, or the like is used, and the film is formed at a temperature of about 400 to 500 ° C. The antireflection film 6 is patterned in a predetermined pattern in order to form front side electrodes 1 and 2 to be described later. As a patterning method, an etching method (wet or dry) used for a mask such as a resist, or a method in which a mask is formed in advance when the antireflection film 6 is formed and the antireflection film 6 is formed and then removed can be used. As another method, a so-called fire-through method in which the front-side electrodes 1 and 2 and the reverse conductivity type region 4 are brought into contact with each other by applying and baking an electrode material directly on the antireflection film 6 is also common. There is no need for patterning. This Si ₃ N ₄ film has a surface passivation effect during formation and a bulk passivation effect during subsequent heat treatment, and has the effect of improving the electrical characteristics of the solar cell element together with the antireflection function. is there.

  Next, a front side electrode and a back side electrode are formed by applying a silver paste on the surface of the substrate and applying an aluminum paste and a silver paste on the back side and baking (hereinafter referred to as a printing baking method).

First, as the back collector 8, an aluminum paste in which aluminum powder, an organic vehicle, and a glass frit are added in a paste form by adding 10 to 30 parts by weight and 0.1 to 5 parts by weight with respect to 100 parts by weight of aluminum, For example, it is baked by printing by a screen printing method and baking at 600-850 degreeC for about 1 to 30 minutes simultaneously after drying. At this time, p ⁺ -type region 7 (BSF region) is formed at the same time, which prevents aluminum from diffusing into the silicon substrate and recombining carriers generated on the back surface. At this time, the aluminum dope concentration in the p ⁺ type region is set to about 1 × 10 ^{18 to} 5 × 10 ²¹ / cm ³ . Note that B (boron) can also be used as the p-type semiconductor impurity element.

When the p ⁺ type region 7 is formed by using the printing and baking method, as described above, the n type region formed on the back side of the substrate simultaneously with the formation of the reverse conductivity type region 4 on the front side of the substrate. Can be eliminated.

Moreover, among the metal components in the paste not used in formation of the p ⁺ -type region 7 that remains on the p ⁺ -type region 7 can also be directly used as a part of the rear-side electrode, in this case the residual component Need not be removed with hydrochloric acid. In this specification, it is assumed that the back collector electrode 8 mainly composed of aluminum remaining on the p ⁺ type region 7 is present. However, when the back collector electrode 8 is removed, an alternative electrode material may be formed. As this alternative electrode material, it is desirable to use a silver paste that will be the back collector electrode 8 described later in order to increase the reflectance of long-wavelength light reaching the back surface.

The p ⁺ type region 7 (back surface side) can be formed by a gas diffusion method instead of the printing and baking method. In this case, it is formed at a temperature of about 800 to 1100 ° C. using BBr ₃ as a diffusion source. At this time, a diffusion barrier such as an oxide film is formed in advance in the reverse conductivity type region 4 (surface side) already formed. Further, when the antireflection film 6 is damaged by this process, this process can be performed before the antireflection film forming process. The doping element concentration is about 1 × 10 ^{18 to} 5 × 10 ²¹ / cm ³ . As a result, a Low-High junction can be formed between the p-type bulk region 5 and the p ⁺ -type region 7.

Further, the method for forming the p ⁺ -type region 7 is not limited to the printing and baking method or the gas diffusion method. For example, a hydrogenated amorphous silicon film or a crystalline silicon film containing a microcrystalline silicon phase is used by using a thin film technique. Or the like may be formed at a substrate temperature of about 400 ° C. or lower. At this time, the film thickness is about 10 to 200 nm. At this time, if an i-type silicon region (not shown) having a thickness of 20 nm or less is formed between the p ⁺ -type region 7 and the p-type bulk region 5, it is effective for improving the characteristics. However, when forming using thin film technology, it is necessary to consider the temperature of each process described below and to determine the order of formation so that the process temperature is as low as the subsequent process.

  Next, the front side electrodes 1 and 2 and the back surface output electrode 9 of the present invention are formed. These are silver pastes prepared by adding 10-30 parts by weight and 0.1-5 parts by weight of silver powder, organic vehicle, and glass frit to 100 parts by weight of silver. After printing and drying, the printed surface is baked at 600 to 800 ° C. for about 1 to 30 minutes.

As these electrode materials, it is desirable to use a material containing at least one low-resistance metal such as silver, Cu, or aluminum, but silver is most preferable in terms of resistivity. As a manufacturing method, vacuum film forming methods such as a sputtering method and a vapor deposition method can be used in addition to a printing and baking method using a paste containing these metals. In particular, in the printing and baking method using a paste, the metal-containing paste to be the front-side electrodes 1 and 2 is directly printed on the antireflection film 6 by the so-called fire-through method without patterning the antireflection film 6, and baking treatment is performed. By doing so, electrical contact can be made between the front-side electrodes 1 and 2 and the reverse conductivity type region 4, which is very effective in reducing the manufacturing cost. The formation of the front side electrodes 1 and 2 may be performed prior to the formation of the p ⁺ type region 7 on the back side.

Furthermore, in order to particularly increase the bonding strength between the electrode and the semiconductor region, the paste and the printing and baking method include a slight amount of an oxide component such as TiO ₂ in the paste, and in the vacuum film forming method, the electrode and the semiconductor region are separated from each other. A metal layer mainly composed of Ti may be inserted at the interface. In the case of the back side electrode, it is desirable that the Ti main component metal layer has a thickness of 5 nm or less and suppresses a reduction in reflectance due to the insertion of the metal layer. The back surface collecting electrode 8 is preferably formed on the entire back surface of the substrate in order to increase the reflectance of long-wavelength light reaching the back surface.

  In addition, since the back surface collecting electrode 8 and the back surface output electrode 9 are overlapped and thickened, the back surface collecting electrode 8 does not cover the back surface output electrode 9 as much as possible after forming the back surface output electrode 9 for output extraction. Thus, it is desirable to form in such a state that conduction can be obtained. Note that the order of forming the back output electrode 9 and the back collector 8 may be reversed. Further, the back side electrode may not have the above-described structure, and may have a structure composed of a bus bar portion and a finger portion mainly composed of silver similar to the front side electrode.

  Finally, a solder region is formed on the front side electrode and the back side electrode by a solder dipping process as necessary (not shown). In addition, when it is set as the solderless electrode which does not use a solder material, a solder dipping process is abbreviate | omitted.

  The solar cell element of this invention is implement | achieved by the above.

  Next, the process for forming the solar cell module according to the present invention will be described with reference to FIG.

  As the translucent panel 12, glass, polycarbonate resin, or the like is used. As the glass, white plate glass, tempered glass, double tempered glass, heat ray reflective glass and the like are used, but generally white plate tempered glass having a thickness of about 3 mm to 5 mm is often used. In the case of a polycarbonate resin, a resin having a thickness of about 5 mm is often used.

  As the filler 13, a material having translucency, heat resistance, and electrical insulation is preferably used. In addition to an ethylene vinyl acetate copolymer (EVA) having a vinyl acetate content of 20 to 40%, polyvinyl butyral (PVB ) Etc. as a main component, and a sheet-like form having a thickness of about 0.4 to 1 mm is used. In the production of the solar cell module 17, the filler 13 is often arranged on both the front side and the back side of the solar cell element, and these are thermally cross-linked and fused to other members in the laminating step under reduced pressure. And integrate.

  The back surface protective material 14 is made of a weather-resistant fluorine-based resin sheet sandwiching an aluminum foil so as not to transmit moisture, a polyethylene terephthalate (PET) sheet deposited with alumina or silica, or the like.

  As already described, the tab 11 is made of, for example, a conductive material whose main component is copper foil and whose surface is coated with solder. This is cut into a predetermined length and used by soldering to the bus bar electrode 1 which is an output extraction electrode of the solar cell element 10 and the back output electrode 9 on the back side.

  To actually wire the tab 11, first, one end of the tab 11 is bonded to the bus bar electrode 1 of the solar cell element 10 by soldering with hot air or a hot plate. Subsequently, when the other end of the tab 11 is made into a module, it is soldered and bonded in the same manner to the back surface output electrode 9 on the back side of the adjacent solar cell element 10. In the case of parallel connection, the bus bar electrodes 1 of adjacent solar cell elements 10 may be bonded together. This is repeated to produce a solar cell element group in which a plurality of solar cell elements 10 are connected.

  In addition, although it will be effective if at least one solar cell element 10 of the present invention is included in the solar cell element group, in order to achieve the effect of the invention satisfactorily, the sun constituting the solar cell element group It is more desirable that all the battery elements are the solar battery element 10 of the present invention.

  The output wiring 15 transmits electrical output from the group of solar cell elements 10 connected by the tab 11 to the terminal of the terminal box 16, and is usually a copper foil having a thickness of about 0.1 mm to 0.5 mm and a width of about 6 mm. The whole surface of which is coated with a solder of about 20 to 70 μm is cut into a predetermined length and soldered to the electrodes of the solar cell element 10.

  Here, the translucent panel 12, the front-side filler 13, the solar cell element group in which the tab 11 and the output wiring 15 are connected to the plurality of solar cell elements 10, the back-side filler 13, and the back surface protective material. 14 laminates are bonded and integrated. That is, after setting the laminated body of each member in a device called a laminator that pressurizes while heating in a reduced pressure state, the pressure is reduced to about 50 to 150 Pa in order to remove the air inside the solar cell module 17, and 100 to 200 ° C. The pressure is applied while heating at a temperature of 15 minutes to 1 hour. As a result, the fillers 13 respectively arranged on the front side and the back side are softened and crosslinked and fused, so that the members can be bonded and integrated to produce the panel portion of the solar cell module 17.

  Furthermore, the terminal box 16 is attached to the back surface of the panel portion of the solar cell module 17 manufactured by the above-described method using an adhesive. The terminal box 16 is used to connect the output wiring 15 from the solar cell element 10 and a cable (not shown) for connecting to an external circuit, and is usually modified with a modified PPE resin in consideration of light resistance against ultraviolet rays and the like. Made in black. Further, the general size of the terminal box 16 is generally about 100 × 60 × 20 mm in a general solar cell module having an output of about 160 W.

  Usually, a module frame (not shown) is often provided for each side of the panel portion of the solar cell module 17. The module frame is often made by extrusion molding of aluminum, and the surface thereof is subjected to anodizing. And this module frame is inserted in each outer periphery side of the panel part of a solar cell, and each corner part is fixed with a bis | screw etc. By providing such a module frame, mechanical strength and weather resistance can be imparted, and the module can be easily handled when a solar cell module is installed.

  The solar cell module of this invention is implement | achieved by the above.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  In the above description, the solar cell element provided with two of the front side electrode and the back side electrode has been described. However, the present invention is not limited to this, and a solar cell of a type in which all electrodes are provided on the light receiving surface side (front side). It may be an element.

  Moreover, although demonstrated as the front side electrode by the example provided with the substantially linear bus-bar electrode and the several finger electrode by which one end was connected to this, it is not restricted to this.

  Further, in the above description, the solar cell element using the p-type polycrystalline silicon substrate has been described. However, even when the n-type polycrystalline silicon substrate is used, the same applies if the polarity in the description is reversed. The effect of the present invention can be obtained by this process.

  In the above description, the case of single junction has been described. However, the present invention can be applied even to a multi-junction type in which a thin film junction layer made of a semiconductor multilayer film is stacked on a bulk substrate-use junction element. it can.

  Furthermore, in the above description, the polycrystalline silicon substrate using the casting method is taken as an example, but the substrate need not be limited to the one using the casting method.

  Further, in order to satisfy the formula of 1.10 ≦ Sb / Sa ≦ 2.10, the method of making the surface of the polycrystalline silicon substrate rough by reactive ion etching has been described as an example, but the present invention is limited to this. However, it is also possible to form a groove in advance in an electrode formation scheduled region by, for example, laser or dicing, and embed an electrode material in the groove. Further, the shape and number of the grooves are not limited, and a structure such as a line shape, a dot shape, or a combination thereof can be used.

  Further, by forming a convex portion in advance at the electrode formation position, the formula of 1.10 ≦ Sb / Sa ≦ 2.10, which is the configuration according to the present invention, may be satisfied. This convex part can be formed by leaving only the electrode formation planned position and etching other regions. Also in this case, the shape and number of the convex portions are not limited, and a structure such as a linear shape, a dot shape, or a combination thereof can be used. Further, this method can be applied to a so-called selective emitter in which the surface concentration of the diffusion layer under the electrode is made high and deep, and the surface concentration of the diffusion layer in other regions is lowered and made shallow.

  Furthermore, it goes without saying that the expression of 1.10 ≦ Sb / Sa ≦ 2.10 can also be satisfied by performing reactive ion etching after forming grooves and protrusions under the electrodes as described above. .

  Hereinafter, the experimental result of the solar cell element produced along the above-mentioned embodiment is demonstrated. However, the present invention is not limited to these examples.

  As a substrate, a flat p-type polycrystalline silicon substrate having a size of 150 mm × 150 mm manufactured by a casting method was used, and a solar cell element was formed with the configuration shown in FIG.

  The front electrode according to the solar cell element of the present invention was printed and fired using a paste mainly composed of silver. The overall pattern of the front electrode is such that the length of the bus bar electrode 1 arranged in line symmetry with respect to the vertical center line of the substrate is 148.8 mm, the width of the bus bar electrode 1 is 2 mm, and between the center lines of the two bus bar electrodes 1 The distance from one end of the substrate to the other end of the finger electrode 2 arranged at a distance of 75 mm and perpendicular to the bus bar electrode 1 (in the horizontal direction of the substrate) and symmetrical with respect to the vertical center line of the substrate (the bus bar crossing in the middle) 149 mm), the width of the finger electrode 2 was 160 μm, and the average distance between the center lines of adjacent finger electrodes 2 was 2.4 mm. Table 1 shows the results of measuring various characteristics of this solar cell element.

  In addition, the bus bar electrode 1 arranged in a total of three, one in the vertical center line of the substrate and two symmetrical with respect to it, has a length of 148.8 mm, a width of the bus bar electrode 1 of 1.3 mm, and two bus bar electrodes 1 The distance from one end of the substrate to the other end of the finger electrode 2 arranged perpendicularly to the bus bar electrode 1 (in the horizontal direction of the substrate) and symmetrical with respect to the vertical center line of the substrate is 50 mm. 149 mm), the width of the finger electrode 2 is 80 μm, and the average distance between the center lines of adjacent finger electrodes 2 is 2.4 mm. In addition, the average width of the finger electrode 2 is obtained by dividing the length from one end connected to the bus bar electrode 1 to the other end by 10 and obtaining the width at each division position (9 points). Asked. Table 2 shows the results of measuring various characteristics of this solar cell element.

The value of Sb was changed by making the portion corresponding to the lower part of the electrode formation position rough by reactive ion etching. At this time, Cl ₂ was supplied at 0.1 slm, O ₂ at 0.6 slm, SF ₆ at 0.4 slm, and the RF power was 5 kW. Moreover, in order to change the value of Sb, the reaction pressure was appropriately changed.

  Moreover, the value of Sb measured the surface area about the site | part in which this surface electrode was removed by immersing in aqua regia after measuring the output characteristic of a solar cell element. The surface area was measured using an AFM (Atomic Force Microscope, Digital Instruments, Nanoscope III a), and using a cantilever with a tip diameter of 5 nm, a 1 μm square was measured at 512 × 1024 points at 0.2 Hz. It was. Moreover, the measurement position was performed at the same 9 points as the width measurement of the finger electrode 2, and the value was obtained by a simple average.

  About the value of Sb / Sa, it rounded off to the 3rd decimal place and compared whether it became the range of this invention.

  In determining the width of the finger electrode and the surface area of the front electrode, each measured value has been validated by performing a t-test with a significance level of 0.05 as a rejection test.

  As the characteristics of the solar cell, the short-circuit current value (Isc) and the fill factor (FF) defined in JIS C 8913 (1998) were measured based on this standard.

Furthermore, as a reference comparison, the measurement results of the solar cell element produced under the conventional conditions are shown as Sample No. It is described as 12. This method uses reactive ion etching, but is formed under conditions prior to finding out such a condition that the Sb / Sa value falls within the scope of the present invention.

As can be seen from Table 1, the range of 1.10 ≦ Sb / Sa ≦ 2.10. 3 to 10 is a short-circuit current of 35.3 mA / cm ² or more, and FF also shows a high value exceeding 0.747. On the other hand, sample No. Sb / Sa is in a range smaller than 1.10. In 1 and 2, although the short circuit current showed the same value, FF fell to 0.731 or less. Sample No. Sb / Sa exceeded 2.10. In the case of 11, FF also decreased to 0.732. Thereby, when 1.10 ≦ Sb / Sa ≦ 2.10, it was possible to obtain a solar cell element having a high conversion efficiency exceeding 16%.

Also in the solar cell element provided with three 1.3 mm-wide bus bars shown in Table 2, the range of 1.10 ≦ Sa / Sb ≦ 2.10, that is, sample No. 3-10 showed the high value whose short circuit current density exceeded 36.6 mA / cm < ² >, and all the FF became the high value exceeding 0.75. Thereby, in the range of 1.10 ≦ Sb / Sa ≦ 2.10, a solar cell element having a high conversion efficiency exceeding 17% could be obtained.

  In both cases of Tables 1 and 2, Sample Nos. Produced under conventional conditions were used. As for the solar cell element of 12, the tendency for FF to become low compared with the solar cell element of this invention was seen.

  When the solar cell module shown in FIG. 3 was produced using the solar cell element of the present invention produced as described above, good results were obtained.

(A) is a figure which shows an example of the electrode shape by the side of the light-receiving surface (surface side) of the solar cell element of this invention, and is the elements on larger scale of the A section of FIG.2 (b). (B) is the elements on larger scale when cut in the cross section of the BB direction of (a). (A) is a figure which shows the cross-section of the solar cell element of this invention, (b) is a figure which shows an example of the electrode shape of the light-receiving surface side (surface side) of the solar cell element of this invention, (c ) Is a diagram showing an example of the electrode shape on the non-light-receiving surface side (back surface side). (A) is a figure which shows the cross-section of the solar cell module of this invention, (b) is the elements on larger scale which show the connection state of the solar cell element in a solar cell module. The general structure of a bulk type silicon solar cell is shown, (a) is a cross-sectional structure diagram, (b) is a top view of the front side electrode from the light incident surface (surface) side, and (c) is ( It is the elements on larger scale of the C section of b).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Bus-bar electrode 2 ... Finger electrode 3 ... Semiconductor area | region 4 ... Reverse conductivity type area | region 5 ... p-type bulk area | region 6 ... Antireflection film 7 ... p ⁺ type | mold area | region 8 ... Back collector electrode 9 ... Back output electrode 10 ... Solar cell element 11 ... Tab 12 ... Translucent panel 13 ... Filler 14 ... Back protection material 15 ... Output wiring 16 ... Terminal box 17 ... Solar cell module 101 ... Bus bar electrode 102 ... Finger electrode 103 ... Semiconductor region 104 ... Reverse conductivity type region 105 ... P-type bulk region 106 ... Antireflection film 107 ... p ⁺ type region 108 ... Back collector electrode 109 ... Back output electrode 110 ... Bulk silicon solar cell

Claims (3)

A solar cell element having a front side electrode on a light receiving surface side of a semiconductor substrate of polycrystalline silicon,
The area when the front side electrode is viewed in plan from the vertical direction on the light receiving surface side is Sa (cm ² ), and the surface area of the region of the light receiving surface of the solar cell element where the front side electrode is provided is Sb (cm ² ). And when
1.10 ≦ Sb / Sa ≦ 2.10
A solar cell element that satisfies the above relationship. The solar cell element according to claim 1, wherein the front side electrode includes a substantially straight bus bar electrode and a plurality of finger electrodes having at least one end connected to the bus bar electrode. A solar cell module having a plurality of plate-like solar cell elements arranged at predetermined intervals and electrically connected to each other,
The plurality of solar cell elements are solar cell modules including the solar cell element according to claim 1.

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