JP5618465B2 - Thin film solar cell module - Google Patents

Description

  The present invention relates to a thin film solar cell module.

In recent years, production of solar cell modules using thin film solar cell elements is increasing. In this solar cell module, incident sunlight is photoelectrically converted by a thin film solar cell element to produce electric power. The thin film solar cell element includes an electrode and a photoelectric conversion layer having a semiconductor junction. A solar cell module having such a solar cell element is expected to be highly efficient.
Japanese Patent Laid-Open No. 2-106077 JP 2002-299661 A

  In the thin film solar cell module described above, it is important to improve the conversion efficiency while further spread is expected. In thin-film solar cells, a light confinement structure for efficiently absorbing light incident on the module surface into a thin photoelectric conversion layer is a very important factor for improving conversion efficiency.

  The present invention has been devised in view of such problems, and an object thereof is to improve the conversion efficiency of a solar cell module.

The solar cell module of the present invention includes a substrate including a first surface that receives sunlight and a second surface on the back side of the first surface, and a transparent formed on the second surface of the substrate sequentially. A thin-film solar cell element having a photoconductive first electrode, a photoelectric conversion layer having a tandem structure of amorphous silicon and microcrystalline silicon, and a translucent second electrode, and the second electrode of the thin-film solar cell element A translucent insulating material provided on the surface, and a reflective material provided on a surface of the translucent insulating material, and the interface between the translucent insulating material and the reflective material has an uneven shape over the entire surface. There, the interface serves to reflect the light of 700~1200nm of the light that has passed through the photoelectric conversion layer into the photoelectric conversion layer, the irregularities angle is 125 ° and surface between the forming the interface 140 degrees and the horizontal direction of the unevenness The average value of the interval is greater than 1400nm.

  In the present invention, the photoelectric conversion layer is configured such that light transmitted to the back side of the thin film solar cell element among the light incident on the solar cell module is reflected on the back side of the thin film solar cell element by having the above-described configuration. By making the light incident again, the light confinement effect is enhanced, and the power generation efficiency of the solar cell module can be increased.

Embodiments of the thin-film solar cell module of the present invention will be described in detail with reference to the drawings.
(Embodiment 1 )

  In the following description, as a typical structure, a p-i-n junction (hereinafter referred to as a unit) of hydrogenated amorphous silicon (hereinafter referred to as a-Si: H or a-Si) formed by plasma CVD as the photoelectric conversion layer 3 is used. A-Si / .mu.c-Si thin film tandem solar cell comprising unit cells of hydrogenated microcrystalline silicon (hereinafter referred to as .mu.c-Si: H or .mu.c-Si) formed by plasma CVD. Suppose it is a structure.

  This embodiment will be described with reference to FIGS. FIG. 1 is a cross-sectional view partially showing the solar cell module of the present embodiment. FIG. 2 schematically shows an optical path inside the thin-film solar cell in the present embodiment.

  The thin film solar cell module shown in FIG. 1 is a thin film solar cell in which a translucent first electrode 2, a photoelectric conversion layer 3, and a translucent second electrode 4 are sequentially formed on a translucent substrate 1. A thin film solar cell element comprising an element M1, a translucent insulating material 5 provided on the second electrode 4 of the thin film solar cell element M1, and a reflector 6 provided on the surface of the translucent insulating material 5. And a light reflection structure M2 that reflects the light transmitted through M1. Such a thin film solar cell module is generally called a super straight type.

In the light reflecting structure portion M2 of the present embodiment, the interface 5a between the translucent insulating material 5 and the reflecting material 6 has an uneven shape composed of a plurality of surfaces over the entire surface.

Hereinafter, each part will be described in more detail. The translucent electrode as the first electrode 2 is formed by forming an oxide transparent conductive film (for example, SnO ₂ ) on the glass substrate 1 by a thermal CVD method, for example. The interface 2a between the first electrode 2 and the photoelectric conversion layer 3 has a concavo-convex shape generated at the time of film formation by a thermal CVD method. Or the average value of the interval between the valleys is about 200 to 400 nm.

  Next, an a-Si / μc-Si thin film tandem solar cell element structure comprising an a-Si: H unit cell and a μc-Si: H unit cell is formed on the first electrode 2 by plasma CVD. . Here, the thickness of the photoelectric conversion layer 3 is about 0.2 to 0.3 μm for the a-Si: H unit cell and about 1.5 to 2.5 μm for the μc-Si: H unit cell.

Next, a thin film solar cell element M1 is formed on the photoelectric conversion layer 3 by forming an oxide conductive film such as a ZnO film, a SnO ₂ film, or an ITO film as the second electrode 4 by sputtering. In particular, the use of a ZnO film can reduce the light absorption loss of long wavelength light.

  Furthermore, a translucent insulating material 5 is provided on the second electrode 4 of the thin-film solar cell element M1, a reflective material 6 is provided on the translucent insulating material 5, and the translucent insulating material 5 and the reflective material 6 are provided. The interface should be uneven. Further, it is desirable to use a white material for the reflector 6 in order to increase the reflectance of light.

  The power generation principle of the solar cell element is based on the fact that a pair of electrons and holes called carriers are generated inside the photoelectric conversion layer 3 by light entering the photoelectric conversion layer 3, and these are separated and taken out as a photocurrent.

  In FIG. 1, sunlight enters from the translucent substrate 1 side (upper side in FIG. 1).

  In FIG. 2, among the light 7 incident perpendicularly to the solar cell module, a wavelength sufficiently larger than the average size (height, pitch) of the unevenness of the interface 2a between the first electrode 2 and the photoelectric conversion layer 3, for example, 700 nm or more The light travels almost straight without being scattered. That is, it enters the photoelectric conversion layer 3 perpendicularly.

  When the photoelectric conversion layer 3 has an a-Si / μc-Si thin film tandem structure, light of about 700 nm or more out of the light 7 incident on the solar cell module is transmitted through the solar cell element portion M1 and the solar cell element portion M1. On the back side (light reflecting structure M2) (details of the reason will be described later).

  In the present embodiment, such transmitted light 7 is reflected at an interface 5a (hereinafter referred to as a back-surface reflective interface) between the translucent insulating material 5 and the reflecting material 6 of the light reflecting structure portion M2, and the solar cell element. It proceeds toward the portion M1 (reflected light 8 in FIG. 2). As described above, the back surface reflecting interface 5a has an uneven shape.

  The reflected light 8 makes an incident angle 9 with respect to the film surface (plane) of the photoelectric conversion layer 3 according to the inclination of each surface constituting the concavo-convex structure.

  Here, the back surface reflecting interface 5a has a concavo-convex structure in which the average value of the interval in the horizontal direction of the concavo-convex and the inclination angle of the concavo-convex are controlled.

  On the other hand, when the slopes of the surfaces constituting the concavo-convex structure vary irregularly, the reflected light 8 reflected by the surfaces constituting the concavo-convex structure is the film surface (plane) of the photoelectric conversion layer 3. There is no regularity, and various incident angles 9 are formed.

  In either case, the reflected light 8 travels obliquely through the photoelectric conversion layer 3. As a result, the optical path length in the photoelectric conversion layer 3 increases, the amount of light absorption into the photoelectric conversion layer 3 can be increased, and the photocurrent density Jph of the solar cell element can be improved.

  The light confinement effect in such a thin film solar cell module will be described in more detail. In FIG. 3, the dependence with respect to the optical wavelength of each characteristic of such a thin film solar cell module is shown. The horizontal axis represents the light wavelength [nm], and the vertical axis represents each characteristic described below as a relative value when the total intensity of light irradiated to the thin film solar cell module is 1.

Curves 47 to 49 indicate the wavelength dependence of the external quantum efficiency of the thin film solar cell module in which the photoelectric conversion layer 3 has an a-Si / μc-Si thin film tandem structure. Curve 47 is for the a-Si: H unit cell, curve 48 is for the μc-Si: H unit cell, and curve 49 is the sum of both, that is, the external quantum efficiency of the entire thin film solar cell. Here, “external quantum efficiency” is effective for generating light carriers of each wavelength by absorbing them in the photoelectric conversion layer 3 out of the total amount of light (number of photons) N ₀ irradiated to the thin film solar cell module. The ratio N ₁ / N ₀ of the amount of light extracted as current (number of photons) N ₁ is shown.

  From this figure, it can be seen that when the photoelectric conversion layer 3 has an a-Si / μc-Si thin film tandem structure, the wavelength of light contributing to power generation is approximately 300 to 1200 nm.

  A curve 46 indicates the ratio 1-R (R is reflectance) of light taken into the module. At a wavelength of 700 nm or less, 1-R has a substantially constant value of 0.9, which indicates that the reflectance R of the surface glass is 0.1. Further, at a wavelength greater than 700 nm, the value of 1-R decreases, that is, the reflection R from the surface of the solar cell module increases. This is because light having a wavelength shorter than about 700 nm is almost absorbed in the photoelectric conversion layer 3, but light having a wavelength longer than about 700 nm is transmitted through the photoelectric conversion layer 3 according to the wavelength, and from the solar cell element. It means that it is emitted. That is, it shows that a part of the light reflected by the back surface of the solar cell module is re-emitted from the surface.

  Therefore, it can be seen that among the light transmitted through the photoelectric conversion layer 3, it is necessary to effectively confine light having a wavelength of 700 to 1200 nm inside the element to contribute to photovoltaic power generation.

  In the present embodiment, since the back surface reflection interface 5a is uneven, the incident angle 9 of the reflected light 8 at the back surface reflection interface 5a to the photoelectric conversion layer 3 is when the back surface reflection interface 5a is optically flat ( (Incident angle 9 is zero). The “optical flat” means that the average value 16 of the unevenness in the horizontal direction (the average value of the distance between the apexes or the average value of the interval between the valleys) is compared with the wavelength λ of the incident light 10. That is less than the same level. In such a case, returning to Huygens's principle and examining the behavior of the reflected light, it can be seen that this is almost the same as the case of reflection on a surface having no irregularities.

Hereinafter, the case of optical flatness and the case of non-optical flatness will be described in more detail. In FIG. 4A, the light reflecting surface 13 corresponds to the back surface reflecting interface 5a in FIG. Further, the upper part of the interface 13 in FIG. 4A corresponds to the translucent insulating material 5 in FIG. Therefore, in the following description, if the wavelength of the incident light 10 in vacuum is λ ₀ and the refractive index of the translucent insulating material 5 is n ₅ , the wavelength of the incident light 10 inside the translucent insulating material 5 is in vacuum. Λ ₁₂ = λ ₀ / n ₅ which is a value smaller than the wavelength of.

  4 (b) and 4 (c) are enlarged views of the unevenness of FIG. 4 (a). When light is irradiated vertically from above, light is scattered at each point on the slope according to Huygens' principle. This is an illustration of the situation.

  FIG. 4B shows a case where the average value 16 in the horizontal direction of the unevenness of the interface 13 is larger than the wavelength of the light, that is, not an optical flat. Here, the light 10 incident on each point on the uneven slope is scattered as a spherical wave from each point, and forms a scattered light wavefront having the same phase at each point on the slope indicated by the dotted line 44. Further, the envelope surface of the scattered light wavefront 44 is shown as a dotted line 45, and the envelope surface 45 forms a wavefront of reflected light. At this time, the reflected light 11 is in a direction perpendicular to the wavefront 45 of the reflected light.

  FIG. 4C shows the case where the average value 16 in the horizontal direction of the unevenness of the interface 13 is smaller than the wavelength of light, that is, in the case of optical flatness. Here, the envelope surface of the scattered light wavefront 44 due to the light 10 incident on each point on the uneven slope has an approximately flat surface with some unevenness, as is apparent from the figure. In other words, the wavefront of the reflected light is a plane parallel to a surface that is obtained by averaging the unevenness of the interface 13.

In FIGS. 4D to 4F, the direction in which the direction of the energy of the light reflected from the interface 13 having unevenness as shown in FIGS. 4B and 4C is directed. It shows the result of simulation calculation for the property. That is, it is shown by a curve 43 in polar coordinate display by an angle φ indicating the direction of reflected light and a point (φ, I) having an intensity I in that direction. Here, the angles (first angles) θ of the concavo-convex portions of the interface 13 are all 120 °, and FIGS. 4D, 4E, and 4F show the horizontal intervals of the concavo-convex portions. The average value 16 is 0.1 times, 1 time, and 3 times the optical wavelength λ _{12 in} the medium, respectively.

  In this figure, a dotted line 39 indicates the inclination of the uneven slope in FIG. 4B, a dashed line 40 indicates a direction perpendicular to the uneven slope, an arrow 41 indicates the direction of incident light toward the slope, It shows that light is incident from directly above. An arrow 42 represents the direction of the reflected light from the inclined surface that reflects according to the incident angle = reflection angle.

In FIG. 4F, it can be seen that energy is reflected with a strong directivity in the reflection direction where the incident angle from the central point to the inclined surface = the reflection angle. This is due to the fact that the average interval 16 in the horizontal direction of the irregularities is three times the optical wavelength λ _{12 in} the medium.

On the other hand, FIG. 4D has no particular directivity. This is due to the fact the mean value 16 of the spacing in the horizontal direction of the irregularities is 0.1 times the optical wavelength lambda ₁₂ in the medium.

Also, FIG. 4E shows that the directivity is inferior to that of FIG. That is, it can be seen that the effect of controlling the directivity of the reflected light is insufficient when the average value 16 of the uneven spacing in the horizontal direction is one time the light wavelength λ _{12 in} the medium.

  The back surface reflecting interface 5a of the present embodiment has a regular uneven structure as shown in cross section in FIG. Here, the “regular concavo-convex structure” means that all the slopes constituting the concavo-convex have substantially the same inclination with respect to the horizontal plane of the thin film solar cell module, that is, the plane on which the photoelectric conversion layer 3 spreads. By having such a structure, the inclination of the reflected light 11 from any reflection surface with respect to the thin film solar cell module plane is substantially the same, and the approach angle to the photoelectric conversion layer 3 is also substantially the same. Here, the optical path length can be maximized by adjusting the angle (first angle) θ of each vertex of the unevenness.

  Examples of the uneven shape of the back surface reflecting interface 5a include a V-groove shape and a pyramid structure surrounded by four surfaces.

Next, the case where the back surface reflecting interface 5a is a surface 14 having irregularities such that the reflected light as shown in FIG. 5 takes a random incident angle with respect to the photoelectric conversion layer 3 will be described. As described above, if the average value 15 in the horizontal direction of the unevenness sufficiently larger than the wavelength λ _{12 within} the translucent insulating material 5 of the incident light 10 is given to each inclined surface, As shown in FIG. 5, light reflection occurs in such a relationship that the incident angle and the reflection angle with respect to the inclined surface are equal. Here, the slopes of the uneven slopes are randomly distributed, and the angle of the reflected light 11 is also randomly distributed. Also in this case, the reflected light does not enter the photoelectric conversion layer 3 perpendicularly, and the optical path length increases as in the case of the regular uneven structure, but the reflection angle cannot be controlled. In many cases, the optical path length becomes shorter than in the case of the regular uneven structure in which the angle is controlled. That is, the photoelectric conversion efficiency is slightly inferior.

  Here, the relationship between the incident angle of the reflected light to the photoelectric conversion layer 3 from the back surface reflection interface 5a in the “regular uneven structure” and the light absorption amount of the reflected light inside the photoelectric conversion layer 3 will be described.

  First, the incident angle of the reflected light from the light reflecting structure portion M2 to the photoelectric conversion layer 3 corresponding to the first angle θ which will be supplementarily described with reference to FIGS. It has already been mentioned above that the angle is determined.

  However, the reflected light from the light reflecting structure part M2 has two ways of reflection to the back surface reflection interface 5a and two cases of reflection to the back surface reflection interface 5a depending on the first angle θ. There are cases.

  For example, as shown in FIG. 8, when the first angle θ is θ ≧ 120 °, the back surface reflecting interface can be applied to any point on the slope forming the irregularities as is apparent from the figure. After being reflected only once at 5a, it goes to the second electrode 4.

  Also, as shown in FIGS. 9 and 10, when the first angle θ is 120 °> θ> 90 ° (both in both figures, the first angle θ is 110 °), the slopes forming the irregularities Depending on the position of the upper point, the light is reflected once at the back surface reflecting interface 5a and then directed to the second electrode 4 as shown in FIG. There is a case as shown in FIG.

  Further, when the first angle θ is 90 ° ≧ θ ≧ 72 °, the light is always reflected twice on the back surface reflecting interface 5a and then directed to the second electrode 4 (not shown), and the first angle θ is 72. In the case of °> θ> 60 °, depending on the position of the point on the slope forming the unevenness, the light is reflected twice on the back surface reflecting interface 5a and then directed to the second electrode 4; In some cases, the light is reflected back toward the second electrode 4.

  Such light having different numbers of reflections has different incident angles to the photoelectric conversion layer 3. That is, the optical path length in the photoelectric conversion layer 3 is different, and as a result, the light absorption amount is different.

  In the following discussion, the light absorption amount ΔPt of the reflected light in the photoelectric conversion layer 3 corresponding to the entire uneven slope of the back surface reflecting interface 5a when two types of light having different numbers of reflections are mixed is shown in FIGS. Based on the explanation.

  When there are two refraction angles (γ and γ1) of the reflected light in the photoelectric conversion layer 3 with respect to the first angle θ, the slope region corresponding to the case where the refraction angle in the photoelectric conversion layer 3 is γ. And Sa is the area of the slope region corresponding to the case where the refraction angle at the photoelectric conversion layer 3 is γ1, and the amount of light absorption is proportional to the ratio Sa1: s2 of the area Sa to the area Sb. It is assumed that the light absorption amount ΔPt of the reflected light in the photoelectric conversion layer 3 corresponding to the entire uneven slope of the back surface reflection interface 5a with respect to the first angle θ with the distributed value.

Here, assuming that the light absorption amounts when the refraction angles in the photoelectric conversion layer 3 are γ and γ1 are ΔP ₁ and ΔP ₂ , respectively, the photoelectric of the reflected light corresponding to the entire uneven slope of the back surface reflecting interface 5a. The light absorption amount ΔPt in the conversion layer 3 is expressed by the following equation.
s1 = Sa / (Sa + Sb)
s2 = Sb / (Sa + Sb)
ΔPt = ΔP ₁ · s1 + ΔP ₂ · s2 (1)

Here, the area ratio of the slope region will be described with reference to FIG.
FIG. 11 shows a boundary between the case where the light is reflected once at the back surface reflecting interface 5a (FIG. 9) and the case where the light is reflected twice at the back surface reflecting interface 5a (FIG. 10). Lights 19 and 20 shown in FIG. 11 are incident and reflected in such a boundary state. In FIG. 11, the light 19 traveling toward the back surface reflecting interface 5a is incident on a point 27 that internally divides the apex 26 and the adjacent valley point 28 into s1: s2 in the uneven slope. When the light 19 traveling toward the back reflecting interface 5a is incident between the apex 26 and the point 27 in the uneven slope, it is reflected only once at the back reflecting interface 5a and enters between the point 27 and the valley point 28. In this case, the light is reflected twice at the back surface reflecting interface 5a. Such ratios s1 and s2 have the following relationship with the first angle θ because of the geometric relationship.
s1 = −2 cos θ, s2 = 1−s1
The first angle θ at which this equation is established is in the range of 120 °>θ> 90 ° where both single reflection and double reflection are mixed, but in the case of only one reflection (θ ≧ 120 °). ,
s1 = 1, s2 = 0
In the case of only two reflections (90 ° ≧ θ ≧ 72 °),
s1 = 0, s2 = 1
Thus, the formula (1) is established in the range of θ ≧ 72 °.

In addition, when the first angle θ is 72 °>θ> 60 ° (not shown), the boundary between the case where the light is reflected twice by the back surface reflecting interface 5a and the case where the light is reflected three times by the back surface reflecting interface 5a. As for the area ratio of the slope region, there is a fixed relationship with the first angle θ, and this corresponds to the case where the back surface reflection interface 5a reflects twice as in the case of θ ≧ 72 °. If the area of the slope is Sb, the light absorption amount ΔP ₂ at that time, the area of the slope that is reflected three times by the back reflection interface 5a is Sc, and the light absorption amount ΔP ₃ at that time is the entire uneven slope of the back reflection interface 5a The light absorption amount ΔPt of the reflected light in the photoelectric conversion layer 3 corresponding to is expressed by the following equation.
s2 = Sb / (Sb + Sc)
s3 = Sc / (Sb + Sc)
ΔPt = ΔP ₂ · s ₂ + ΔP ₃ · s ₃ (2)
In this case, s2 and s3 are not described in detail, and only the result of simulation calculation is shown in FIG. 14A and FIG. 14B for the light absorption amount ΔPt.

Subsequently, the light absorption amount in the photoelectric conversion layer 3 of the light reflected by the back surface reflection interface 5a will be described. After the light reaches the photoelectric conversion layer 3, the intensity P of the light traveling inside is P _{0 as} the energy intensity of the light immediately after entering the photoelectric conversion layer 3, and the distance traveled through the photoelectric conversion layer 3 is X, When the light absorption coefficient of the photoelectric conversion layer 3 is α, P = P ₀ exp (−αX). Therefore, if the optical path length of the light in the photoelectric conversion layer 3 is L, the light absorption amount ΔP is ΔP = P _{(X = 0)} −P _{(X = L)} = P ₀ (1-exp (−αL) (3)
It becomes.

Here, the optical path length L of the light in the photoelectric conversion layer 3 is set such that the thickness of the photoelectric conversion layer 3 is D, and the refraction angle in the photoelectric conversion layer 3 is γ ₀ .
L = D / cosγ ₀ (4)
There is a relationship.

Next, the light energy intensity P ₀ immediately after entering the photoelectric conversion layer 3 and the refraction angle γ ₀ in the photoelectric conversion layer 3 will be described. First, a case where light is reflected only once to the back surface reflecting interface 5a will be described with reference to FIG.

Incident light 19 on the back reflecting interface 5a is reflected on the uneven slope (light ray 20), further passes through the second electrode layer (light ray 21), and further travels through the photoelectric conversion layer 3 (light ray 22).
The incident angle of the light beam 20 passing through the translucent insulating material 5 to the second electrode 4 is α, the incident angle of the light beam 21 passing through the second electrode 4 to the photoelectric conversion layer 3 is β, and the light beam passes through the photoelectric conversion layer 3. When the refractive angle of 22 is γ, the refractive index of the translucent insulating material 5 is n ₅ , the refractive index of the second electrode 4 is n ₄ , and the refractive index of the photoelectric conversion layer 3 is n ₃ , these follow Snell's equation. .

  Here, with respect to each refractive index, in the present embodiment, the extinction coefficient k is neglected because it has a small value.

On the other hand, between the incident angle α of the light ray 20 with respect to the second electrode 4 and the first angle θ, simply α = π−θ from a geometrical relationship,
sinγ = n ₅ / n ₃ · sin (π−θ)
It becomes. Therefore, the refraction angle γ of the light beam 22 passing through the photoelectric conversion layer 3 can be controlled by the first angle θ.

Next, the light energy intensity P ₀ immediately after entering the photoelectric conversion layer 3 will be described.

  In FIG. 9, since the reflectance at the back surface reflecting interface 5a is very high, energy loss can be ignored. Therefore, in the following discussion, the intensity of the light beam 20 reflected by the back surface reflecting interface 5a is assumed to be 1. Proceed. First, regarding the light beam 21 passing through the second electrode 4, a part of the energy of the light beam 20 is reflected as the light beam 39 at the interface between the translucent insulating material 5 and the second electrode 4, and thus the reflectance at the interface is Assuming r1, the intensity of the light beam 21 is 1-r1.

Further, with respect to the light beam 22, a part of the energy of the light beam 21 is reflected as the light beam 40 at the interface between the second electrode 4 and the photoelectric conversion layer 3. The intensity P ₀ immediately after entering 3 is determined by considering the intensity (1-r1) of the light beam 21 as follows: P ₀ = (1-r1) · (1-r2)
It becomes.

Strictly speaking, there is absorption of light inside the second electrode layer 4, but the transparent conductive film, which is a material originally used for this layer, has a small light loss. There is no problem. As described above, the light absorption amount of the light beam 22 in the photoelectric conversion layer 3 in the case of a single reflection is expressed by ΔP ₁ = (1-r1) · (1-r2) (1-exp from Equations (3) and (4). (−α · D / cos γ)) (5)
It becomes. Next, a case where the light is reflected twice to the back surface reflection interface 5a will be described with reference to FIG.

Also in this case, the refraction angle γ1 of the light beam 25 passing through the photoelectric conversion layer 3 is related to the first angle θ from the Snell equation and the geometric relationship with the first angle θ as described above. . That means
α1 = θ / 2−χ
χ = π−3θ / 2
sin γ1 = n ₅ / n ₃ · sin (2θ−π)
It becomes.

Also, the reflectance at the back surface reflecting interface 5a is 1, the reflectance at the interface between the translucent insulating material 5 and the second electrode 4 is r3, and the reflectance at the interface between the second electrode 4 and the photoelectric conversion layer 3 is r4. In the case of twice reflection, the light absorption amount ΔP ₂ of the light ray 25 in the photoelectric conversion layer 3 is the same as in the case of single reflection.
^ΔP ₂ = (1−r3) · (1−r4) · (1−e− ^αL1 ), L1 = D / cos γ1 (6)
It becomes.

As described above, the light absorption amount ΔPt of the reflected light to the photoelectric conversion layer 3 corresponding to the entire uneven slope of the back surface reflecting interface 5a is obtained using ΔP ₁ and ΔP ₂ of the equations (5) and (6). It is represented by Formula (1).

  As a simulation condition, an a-Si / μc-Si thin film tandem structure is used as the photoelectric conversion layer 3, a ZnO film is used as the translucent second electrode 4, and a transparent ethylene vinyl acetate resin is used as the translucent insulating material 5. Using.

Table 1 shows the optical constants of the respective materials used in this simulation.
Here, each optical constant is a value when the wavelength λ ₀ of light in a vacuum is 1000 nm.

In the case of the thin film solar cell element of the a-Si / μc-Si thin film tandem structure in this example, the light that needs to be reflected by the back surface reflecting interface 5a out of the light transmitted through the photoelectric conversion layer 3 as described above. The wavelength is λ ₀ = 700 to 1200 nm. Therefore, here, λ = 1000 nm is selected as the representative wavelength. When a material other than silicon is used for the photoelectric conversion layer 3, a representative value may be selected and considered in the same manner within a wavelength band range of light transmitted through the material.

  In FIG. 12, the secondary reflected light by the light reflected by the uneven slope of the back surface reflecting interface 5a being further reflected by each interface of the translucent insulating material 5, the second electrode 4, and the photoelectric conversion layer 3, and The higher order reflected light is drawn with a dotted line. Since these high-order reflected light also repeats reflection and finally enters the photoelectric conversion layer 3, more accurately calculating the intensity of each of these high-order reflected light and absorbing the light. Although it is necessary to calculate the contribution to the amount ΔPt, the contribution due to the higher-order reflected light is not so large, and there is no essential influence in the discussion of the first angle dependency of the light absorption amount. .

  In this simulation, it is assumed that no light absorption loss occurs when the light is reflected by the back surface reflecting interface 5a. In the reflectance calculation, the light is circularly polarized with a deflection angle ξ = 45 °, and the reflectances of P wave and S wave are combined.

FIGS. 14A and 14B show the light absorption amount ΔPt of the reflected light in the photoelectric conversion layer 3 corresponding to the entire uneven slope of the back surface reflecting interface 5a by the light reflecting structure M2 with respect to the first angle θ. An example of the simulation results is shown. In both graphs, the light absorption amount ΔPt is plotted on the vertical axis and the first angle θ is plotted on the horizontal axis. In FIG. 14 (a), the plot by the point □ in the graph shows the contribution ΔPt ₁ · s1 when the light is reflected only once on the back surface reflection interface. △ Pt ₂ · s2 is shown for the case of the reflection at the degree of reflection, and the plot by the point ◇ shows the contribution ΔPt ₃ · s3 for the case of the reflection at the back reflecting interface three times. The plot by shows the sum ΔPt of these. Here, ΔPt ₁ · s1, ΔPt ₂ · s2, ΔPt ₃ · s3, and ΔPt are the ones that appear in the explanations of the above-described equations (1) and (2). .

FIG. 14B is a plot of black dots in FIG. 14A, that is, the amount of light absorption Δ to the photoelectric conversion layer 3 of the reflected light corresponding to the entire uneven slope of the back surface reflecting interface 5a by the light reflecting structure M2. A graph in which Pt is enlarged is shown.
In the graph, a dotted line is displayed in parallel with the horizontal axis at the value of light absorption ΔPt when the first angle θ is 90 °.

Here, when the first angle θ is 90 °, the reflected light is always reflected twice to the back surface reflecting interface 5a and reflected in the direction perpendicular to the photoelectric conversion layer 3 as shown in FIG.
Therefore, this case is a case where the optical path length increase in the photoelectric conversion layer 3 is the smallest with respect to the light reflection from the light reflection structure M2, and the light absorption amount at this time is ΔPt = 4.86%. The dotted line in FIG. 14B corresponds to this light absorption amount ΔPt.

  Therefore, the desirable first angle range θ is 60 to 90 ° or 115 ° or more as a range in which the light absorption amount is ΔPt> 4.86% from the graph of FIG.

  In particular, a concavo-convex member having a first angle θ greater than 90 ° can be manufactured at low cost because it is easy to manufacture. Therefore, it is desirable that the first angle θ is larger than 90 ° because of the merit in producing the concavo-convex member.

  Further, a local peak exists in a range where the first angle range is smaller than 90 °, and the light absorption amount is ΔPt = 4.99% at θ = 70 °. In addition, a local peak also exists in the range where the first angle range is larger than 90 °, and the largest light absorption amount ΔPt = 5.03% is shown at θ = 130 °.

  That is, the first angle range θ is 125 ° to 140 ° as a range satisfying the condition that the first angle range θ is larger than 90 ° and takes a value near the maximum value of the light absorption amount ΔPt. More desirable compared to the angular range.

  The optimum range of the first angle θ is that the photoelectric conversion layer 3 is an a-Si / μc-Si thin film tandem structure, the translucent second electrode 4 is a ZnO film, and the translucent insulating material 5 is transparent ethylene. The case where a vinyl acetate resin is used is described.

  In addition to the ZnO film, an ITO film, a SnO2 film, or the like can be used as the translucent second electrode 4, and a polyvinyl alcohol resin (other than ethylene vinyl acetate resin (EVA)) can be used as the translucent insulating material 5. A highly transparent resin such as PVA) or optical glass having a high transmittance around 1000 nm may be used. Also in this case, since there is almost no difference from the optical constants of the materials used in the simulation shown in FIG. 14, the optimum range of the first angle θ is almost the same.

Here, the average value of the intervals in the horizontal direction of the unevenness of the back surface reflecting interface 5a is, as described above, assuming that the wavelength of the incident light 10 in vacuum is λ ₀ and the refractive index of the translucent insulating material 5 is n ₅ . The wavelength of the incident light 10 inside the translucent insulating material 5 is λ ₁₂ = λ ₀ / n ₅ . For this reason, when the material of the translucent insulating material 5 is, for example, a transparent ethylene vinyl acetate resin (transparent EVA), the refractive index of the substance in the vicinity of a wavelength of 700 to 1200 nm is approximately n ₅ = 1.5. The wavelength inside the translucent insulating material 5 is λ ₁₂ = 470 to 800 nm. Therefore, when the translucent insulating material 5 is a transparent ethylene vinyl acetate resin and the photoelectric conversion layer 3 has an a-Si / μc-Si thin film tandem structure, light having a wavelength of 700 to 1200 nm is incident angle = reflection angle. The average value 16 in the horizontal direction of the unevenness of the back surface reflecting interface 5a to be reflected is set to be larger than at least 470 to 800 nm, so that an optically flat surface is not formed.

Also, as more preferred it said in the description of the reflected light directivity characteristics of the light reflection in a plane having irregularity, 3 times the average value 16 of the spacing in the horizontal direction of the unevenness of the back surface reflective interface 5a is the lambda _12, i.e. 1400 It should be larger than ˜2400 nm.

  In addition, this embodiment uses a super straight type solar cell module, whereby moisture transmitted from the light incident side is reduced and internal corrosion hardly occurs, and reliability is improved. In addition, an element integration process (laser scribing) is easy to perform in module manufacture.

  Next, the manufacturing method of the thin film solar cell module of this embodiment is demonstrated. Hereinafter, the case where the photoelectric conversion layer 3 has an a-Si / μc-Si thin film tandem structure will be described as an example. The present invention is not limited to the a-Si / μc-Si thin film tandem structure, but can be applied to an a-Si single cell structure and a multi-tandem structure of three or more tandem structures.

  First, as shown in FIG. 1, a first electrode 2 made of a transparent conductive material is formed on a translucent substrate 1 such as glass. Here, when the glass used as the translucent substrate 1 is one that transmits a wavelength of 600 nm or more with high efficiency, the power generation efficiency is improved. That is, the material of μc-Si: H contained in the photoelectric conversion layer 3 is currently silicon, and in this case, the upper limit of the wavelength of light contributing to power generation is up to around 1200 nm. A glass material (white glass) for a solar cell produced by melting a raw material having a low iron content and having a high transmittance is used.

As the transparent conductive material for the first electrode 2, an oxide-based transparent conductive film such as SnO ₂ , ITO, or ZnO can be used. As a film forming method, methods such as sputtering, thermal CVD, and LPCVD are possible. At this time, it is performed under the condition that the surface of the formed film forms an uneven shape. In the thermal CVD method, the surface is naturally uneven, but in the case of sputtering and LPCVD, the unevenness is formed by etching after film formation as necessary.

  Next, the photoelectric conversion layer 3 is formed on the uneven surface of the first electrode 15 by a technique such as plasma CVD. In the photoelectric conversion layer 3, as described above, silicon-based materials are mainstream. In this case, the a-Si / μc-Si thin film tandem structure is formed in the order of the a-Si unit cell and the μc-Si unit cell from the front side. Then, it is possible to widen the width of the light wavelength band that can be absorbed.

  A second electrode 4 made of a transparent conductive material is formed on the photoelectric conversion layer 3 by sputtering or LPCVD. When forming the second electrode, in order not to damage the quality of the already formed photoelectric conversion layer 3, a sputtering method or an LPCVD method that enables film formation at a substrate temperature as low as about 200 ° C. is used. Is preferred. In this case, the film formed on the photoelectric conversion layer 3 as the base is formed in a form that reflects the surface shape of the base as it is, and the unevenness is hardly emphasized. In addition, spontaneous irregularities are generated on the surface of the μc-Si: H film, but the size is not necessarily sufficient for light confinement. Therefore, a considerable amount of light that is transmitted through the photoelectric conversion layer 3 and having a wavelength of 700 nm or longer is transmitted straight without being sufficiently scattered.

As such a transparent conductive material for the second electrode 4, an oxide-based transparent conductive film such as SnO ₂ , ITO, ZnO or the like can be used as in the first electrode. Among these, ZnO is particularly excellent in the transmittance of long wavelength light.

  As a material of the translucent insulating material 5, a transparent body having a small absorption coefficient with respect to light, such as resin and glass, can be used. As the resin material, ethylene vinyl acetate (EVA) resin, polyvinyl alcohol (PVA) resin, or the like can be used. Ethylene vinyl acetate (EVA) resin is excellent in terms of cost and reliability such as water resistance.

  The reflective material 6 may be any material that reflects light in contact with the transparent insulating material 5 on the base side, such as a white resin added with a white dye, such as an ethylene vinyl acetate resin or a fluorine-based resin, TiO2, or the like. It is possible to use a solidified inorganic white pigment or a material dispersed in a resin.

  Hereinafter, a method for manufacturing the light reflecting structure portion M2 in which the interface between the translucent insulating material 5 and the reflecting material 6 in the present embodiment has an uneven shape of the back surface reflecting interface 5a will be described.

  First, a method for independently forming the translucent insulating material 5 having an uneven shape will be described.

  In FIG. 16, the translucent insulating material is composed of a plate-like member 30 and a removal portion 29. The surface of the plate-like material 30 can be processed into a V-groove shape to form a translucent insulating material having an uneven shape. As the processing method, when the plate-like material 30 is a thermosetting resin, as shown in FIG. 17, an uneven shape can be formed by pressing the shape transfer member 31 against the heated plate-like member 30.

  When the plate-like member 30 is made of glass, as shown in FIG. 18, it is possible to form an uneven shape by grinding the V-groove from the surface of the plate-like member 30 with a processing grindstone 32.

  As another method of forming the translucent insulating material 5 having an uneven shape alone, as shown in FIG. 19, the resin uncured material is not formed from a plate-like member but is formed in the mold 33 by the resin supply nozzle 34. It is also possible to employ an injection molding method in which the molding die 33 is removed as shown in FIG. In this case, a thermoplastic resin heated and melted may be used as the resin uncured product 35, and a metal mold 33 may be used. Alternatively, a photocurable resin may be used as the resin uncured product 35, and a glass mold 33 may be used. It may be used after being poured into the mold 33 and exposed and cured from the outside of the glass mold 33 to be taken out.

  As shown in FIG. 21, the translucent insulating material 5 having the uneven shape formed by the above method is adhered to the second electrode 4 of the thin film solar cell element portion M1 by the resin adhesive 36. When the fine wire electrode 17 is formed on the second electrode 4, the resin-based adhesive 36 is carefully made so that the thickness of the resin-based adhesive 36 is about the height from the surface of the second electrode 4 of the fine wire electrode 17. Adhesive 36 is applied.

  In order to form the reflector 6 on the back side of the concavo-convex shape of the translucent insulating material 5, as shown in FIG. 22, the thin film solar cell element portion M1 to which the translucent insulating material 5 having the concavo-convex shape is bonded is a laminator. The thin film solar cell module of this embodiment is obtained by injecting a heated melt of a thermoplastic resin such as ethylene vinyl acetate resin containing white paint from the resin supply port 38, which is placed in the apparatus 37. I can do it.

  Next, a method of directly forming the translucent insulating material 5 having the uneven shape on the thin film solar cell element portion M1 will be described.

  In this case, first, as shown in FIG. 23, an uncured resin 35 is applied on the second electrode 4 of the thin-film solar cell element portion M1. When the resin uncured product 35 is a hot melt of a thermoplastic resin, it is applied to the second electrode 4 and then cooled and hardened before the shape transfer member 31 is pressed as shown in FIG. As shown in FIG. 24, it is possible to form an uneven shape of the translucent insulating material 5 on the second electrode 4 of the thin film solar cell element portion M1 as shown in FIG.

  In addition, as the resin uncured product 35, when using a thermoplastic resin, a photocurable resin, or a two-component mixed curable resin in which a main agent and a curing agent are mixed and cured, after curing the resin, As shown in FIG. 18, a light-transmitting insulating material 5 on the second electrode 4 of the thin-film solar cell element portion M1 as shown in FIG. Can be formed.

  As another method for directly forming the light-transmitting insulating material 5 having an uneven shape directly on the thin film solar cell element M1, as shown in FIG. 25, a molding die is formed above the second electrode 4 of the thin film solar cell element M1. The thin film solar cell element portion M1 as shown in FIG. 24 is obtained by placing the 33 at an appropriate height, injecting the resin uncured material 35 with the resin supply nozzle 34 and curing it, and then removing the molding die 33. It is possible to form a concavo-convex shape of the translucent insulating material 5 on the second electrode 4.

  After the concavo-convex shape of the translucent insulating material 5 is formed on the second electrode 4 of the thin film solar cell element portion M1 as described above, the translucent insulating material 5 having the concavo-convex shape is singly used. In the same manner as in the case of the formation, it is placed in a laminator apparatus as shown in FIG. 26, and a heated melt of a thermoplastic resin such as ethylene vinyl acetate resin containing white paint is injected from the resin supply port 38. By doing so, the thin film solar cell module of this Embodiment can be obtained.

As described above, according to the present embodiment, the light transmitted to the back surface side of the thin-film solar cell module is reflected at the back surface reflection interface 5 in which the first angle θ is 60 to 90 °, or 115 ° or more. Therefore, it is possible to increase the amount of absorption ΔPt of the reflected light into the photoelectric conversion layer 3 as compared with the case where the photoelectric conversion layer 3 is incident vertically, thereby improving the photoelectric conversion efficiency of the thin film solar cell module. It becomes possible. In addition, the angle at which the reflected light is incident on the photoelectric conversion layer 3 again can be controlled by reflecting at the back surface reflecting interface 5 having a regular uneven shape.
In addition, by setting the first angle θ to 125 ° to 140 °, it is possible to maximize the amount of absorption ΔPt of reflected light in the photoelectric conversion layer 3 and to further increase the photoelectric conversion efficiency of the thin-film solar cell module. It becomes possible to improve.

  (Embodiment 2)

  As another embodiment, the transparent conductive film generally has a higher electrical resistivity (ρb) than a metal thin film. Therefore, when the thickness of the second electrode 4 is reduced, the sheet resistance (ρs = ρb / Film thickness) is increased, the fill factor (FF) as the solar cell characteristic is decreased, and the photoelectric conversion efficiency η is decreased. On the other hand, as shown in FIG. 15, if the thin wire electrode 17 is formed in close contact with the second electrode 4 between the second electrode 4 and the translucent insulating material 5, the entire back electrode including the second electrode 4 is formed. It is possible to reduce the resistance, and it is possible to prevent the photoelectric conversion efficiency η from being lowered. At this time, if the pitch of the thin wire electrode 17 is brought to the apex position of the back surface reflecting interface 5a as shown in FIG. 15, the optical loss (light absorption loss on the collector surface) by the thin wire collector electrode 17 is minimized. It can be.

  (Embodiment 3)

  As another embodiment, magnesium hydroxide is contained as an acid-accepting material component in the ethylene vinyl acetate resin of the translucent insulating material 5 in contact with the second electrode 4. With such a configuration, it is possible to reduce the production of acetic acid due to moisture that permeates and enters the translucent insulating material 5. In particular, when a material that is easily attacked by an acid such as ZnO is used for the second electrode 4, it is possible to reduce the problem that the transparent conductive film is altered and the light transmittance and electrical resistivity are deteriorated.

  As a result, by using ZnO as the second electrode 4, in addition to improving the photoelectric conversion efficiency of the thin-film solar cell due to the high light transmittance of the ZnO layer, it is possible to improve the reliability.

  (Embodiment 4)

As another embodiment, when a ZnO film is used for the second electrode 4 and a resin material is used for the translucent insulating material 5, the second electrode 4 and the translucent insulating material 5 are used as shown in FIG. It is also possible to improve moisture resistance by inserting an insulating layer 50 of an insulating oxide layer such as TiO ₂ or SiO ₂ or an SiN-based insulating nitride layer between them. This is because moisture that has passed through the translucent insulating material 5, which is a resin material, is blocked by the insulating layer 50. In particular, combining the configuration employing the ethylene vinyl acetate resin containing the above acid-accepting material component as the translucent insulating material 5 and the configuration providing an insulating layer between the second electrode 4 and the translucent insulating material 5. Thus, the moisture resistance performance of the second electrode 4 can be remarkably improved.

  Further, when the thin wire collecting electrode 17 is formed between the second electrode 4 and the translucent insulating material 5, the thin wire collecting electrode 17 is formed on the second electrode 4 as shown in FIG. The insulating layer 50 can be formed from above.

  As a result, as in the case of the third embodiment, by using ZnO as the second electrode 4, in addition to improving the photoelectric conversion efficiency of the thin film solar cell due to the high light transmittance of the ZnO layer, the reliability is also improved. It becomes possible.

Explanation of symbols

M1. Thin film solar cell element M2. 1. Light reflecting structure First electrode 3. 3. Photoelectric conversion layer Second electrode (translucent electrode)
5a. 4. Interface between reflector and translucent insulating material Translucent insulating material6. Reflective material

Sectional schematic diagram which partially shows the thin film solar cell module structure of Embodiment 1 The schematic diagram which shows typically the optical path in the thin film solar cell of one Embodiment of Embodiment 1 Graph showing light wavelength dependence of various characteristics of thin film solar cell module (A) Schematic diagram showing the incidence / reflection of light with respect to a flat surface having irregularities, (b) Schematic diagram showing how light is scattered according to Huygens's principle with respect to a planar surface having irregularities when not optically flat, (C) Schematic diagram showing how light is scattered according to Huygens's principle with respect to a flat surface having irregularities when it is an optical flat, and (d) to (f) average of horizontal intervals of different irregularities Graph of reflected light intensity directivity drawn against value Schematic diagram showing light reflection for a plane with a concavo-convex structure with random slope inclination Schematic diagram showing light reflection on a plane with a regular uneven structure Schematic diagram showing the optical path of the reflected light when the first angle θ is 120 ° Schematic diagram showing the optical path of the reflected light when the first angle θ is larger than 120 ° Schematic diagram showing the optical path when the reflected light when the first angle θ is larger than 120 ° is reflected once by the back surface reflecting interface 5a. Schematic diagram showing the optical path when the reflected light when the first angle θ is larger than 120 ° is reflected twice by the back surface reflecting interface 5a Schematic diagram showing the boundary between the case where the reflected light when the first angle θ is larger than 120 ° is reflected once at the back surface reflecting interface 5a and the case where it is reflected twice. The schematic diagram which shows the optical path about the secondary or more reflected light of the light reflected in the back surface reflective interface 5a about the reflected light in case the 1st angle (theta) is larger than 120 degrees. The schematic diagram which shows the optical path about reflected light in case the 1st angle (theta) is 90 degrees. (A), (b) The graph which shows the relationship between the light absorption amount Pt and 1st angle (theta). (A) Schematic cross-sectional view showing the thin-film solar cell module structure of Embodiment 2, (b) Schematic cross-sectional view showing the thin-film solar cell module structure of Embodiment 4, (c) Embodiment 2 and Embodiment 4 is a schematic cross-sectional view showing the structure of a thin-film solar cell module combining 4 The figure which shows processing the surface of a plate-like material into V-groove shape, and forming unevenness The figure which shows one Example which processes the surface of a plate-shaped member in V groove shape The figure which shows another Example which processes the surface of a plate-shaped member into V groove shape The figure which shows one Example which performs translucent insulation material by resin injection molding The figure which shows the other Example which performs translucent insulation material by resin injection molding The figure which shows one Example which forms the light reflection structure part M2 on the thin film solar cell element part M1 The figure which shows the other Example which forms the light reflection structure part M2 on the thin film solar cell element part M1. The figure which shows one Example which forms the translucent insulating material 5 directly on the thin film solar cell element part M1 The figure which shows the other Example which directly forms the translucent insulating material 5 on the thin film solar cell element part M1. The figure which shows another Example which directly forms the translucent insulating material 5 on the thin film solar cell element part M1. The figure which shows the Example which forms the reflecting material 6 on the translucent insulating material 5

Claims (8)

A substrate including a first surface for receiving sunlight and a second surface on the back side of the first surface; a translucent first electrode sequentially formed on the second surface of the substrate; A thin film solar cell element having a photoelectric conversion layer having a tandem structure of amorphous silicon and microcrystalline silicon, and a translucent second electrode;
A translucent insulating material provided on the second electrode of the thin film solar cell element;
A reflector provided on the surface of the translucent insulating material,
The interface between the translucent insulating material and the reflective material is an uneven shape over the entire surface, and the interface reflects 700 to 1200 nm of light that has passed through the photoelectric conversion layer to the photoelectric conversion layer, the uneven with angle face each other shape is 125 ° to 140 °, the thin-film solar cell module is greater than the average value of the interval in the horizontal direction 1400nm of irregularities forming the interface.   The thin film solar cell module according to claim 1, wherein an insulating layer is provided between the second electrode and the translucent insulating material.   The thin film solar cell module according to claim 1, wherein the interface includes a plurality of planes having the same inclination with respect to the first surface.   The thin film solar cell module according to any one of claims 1 to 3, wherein the translucent insulating material is a polyvinyl alcohol resin.   The thin-film solar cell module according to any one of claims 1 to 3, wherein the translucent insulating material is an ethylene vinyl acetate resin.   The thin film solar cell module according to claim 1, wherein the second electrode is made of zinc oxide, and the translucent insulating material is an ethylene vinyl acetate resin containing an acid acceptor.   The thin film solar cell module according to claim 1, wherein the reflective material is white.   The thin film solar cell module according to claim 1, further comprising a thin wire collecting electrode provided on the second electrode.