CN103367469B - Solar cell - Google Patents

Abstract

A solar cell, comprising: the dielectric layer is arranged on the substrate, and the dielectric layer is arranged on the substrate. The electrode includes a first structure having a conductive segment and a second structure disposed in the through hole. The opening is located on the dielectric layer and surrounded by the electrode, wherein the conductive segment is configured in the opening. The first area is a total area of the opening orthographically projected to the substrate. The second area is a total area of the electrode orthographic projection to the substrate and is greater than the first area. The electrode can limit the diffusion of the substrate material, so that enough substrate material is mixed with the electrode material to form a back electric field structure with thick thickness and good quality, thereby improving the conversion efficiency of the battery.

Description

Solar battery

technical field

The invention relates to a solar cell, in particular to a silicon crystal solar cell with a local back electric field structure.

Background technique

Referring to Fig. 1, it is a known solar cell, which mainly includes: a substrate 11, an emitter layer 12 forming a p-n junction with the substrate 11, a dielectric layer 13, and a plurality of solar cells formed on local parts of the substrate 11. A back surface field (local back surface field, LBSF for short) 14 and a back electrode 15 .

The dielectric layer 13 is formed on a back surface 111 of the substrate 11 and has a plurality of circular through holes 131 spaced from front to back, left to right. The back electric field structure 14 is formed on the back surface 111 corresponding to the through hole 131 , and the carrier concentration of the back electric field structure 14 is higher than that of the p-type substrate, which can help improve carrier collection efficiency and photoelectric conversion efficiency. The production of the back electrode 15 is mainly to coat the aluminum paste on the surface of the dielectric layer 13 in a manner close to the entire surface by screen printing, and a part of the aluminum paste flows into the through hole 131 of the dielectric layer 13, The cured back electrode 15 can be formed by subsequent high-temperature sintering (firing), so the back electrode 15 has a surface layer portion 151 located on the surface of the dielectric layer 13 and is a continuous layer, and a plurality of through holes 131 and The contact portion 152 is in contact with the substrate 11 . In addition, during the sintering process, the aluminum paste filled in the through hole 131 will mix with the silicon of the substrate 11 to form an aluminum-silicon (Al-Si) mixed material, thereby forming the back electric field structure 14 .

However, it has been found in practice that the general sintering temperature is about 700°C to 800°C. At this high temperature, the diffusion of silicon in aluminum is good. In addition, the back electrode 15 made of aluminum is provided on the entire surface with a large area. Silicon does not have any limiting effect, so silicon is easy to diffuse from the contact portion 152 of the back electrode 15 to the direction of the surface portion 151, which will be unfavorable for the formation of the back electric field structure 14, because the back electric field structure 14 is formed by aluminum, silicon The materials are mixed and formed at the back surface 111 and the position corresponding to the through hole 131 . However, when the silicon continues to diffuse outwards, the silicon content that can be used to form the back electric field structure 14 is reduced, resulting in a reduction of the mixed aluminum-silicon alloy.

Referring to Fig. 2, a cavity (cavity) 10 will be formed between the back surface and the back electrode 15. The range formed by the cavity 10 does not contain any aluminum-silicon alloy. If the cavity ratio is too high, it will affect the conductivity of the electrode. In addition, the thickness and poor quality of the back electric field structure 14 formed by sintering will also result in poor performance, thereby affecting its performance.

In addition to the aforementioned batteries with circular perforations, there is currently another type of battery in which the dielectric layer is perforated in a linear shape extending in the long direction. Refer to Figures 3 and 4. The busbar (busbar) 153 and three conductive regions 154 separated by the busbar 153, each conductive region 154 has a layered surface portion 155 on the surface of the dielectric layer 13 and a plurality of layers from the surface portion 155 protrudes into a contact portion 156 in the through hole 131 . Since the back electrode 15 is also arranged continuously in a large area, the diffusion and movement of silicon cannot be restricted. Therefore, the high cavity ratio also affects the conductive performance of the back electrode 15 and the quality of the back electric field structure 14 .

Aiming at the absence of the linear perforated battery shown in Figure 3 and Figure 4, there is currently an improved design, as shown in Figure 5 and Figure 6, the back electrode 15 is no longer arranged continuously in a large area, but has multiple conductive segments at intervals 157 , each conductive segment 157 has a surface portion 158 and a contact portion 159 located in the through hole 131 . Since the conductive segments 157 are not directly connected to each other, the silicon diffused from the contact portion 159 to the surface portion 158 cannot continue to diffuse in all directions. On the other hand, because the size of the conductive segments 157 is limited, the The concentration of silicon will reach saturation quickly, so that silicon will no longer diffuse and move, so that more silicon material will remain at the back surface 111, and form an aluminum-silicon alloy with aluminum, thereby reducing the cavity ratio and improving the back electric field structure 14. quality.

Although the improved structure shown in Fig. 5 and Fig. 6 is helpful to reduce the cavity ratio of the linear through-hole battery, if the area occupied by the back electrode 15 is blindly reduced to limit the diffusion of silicon, the back electrode 15 will thus be reduced. If the area is too small, the conductivity of the electrode will be poor, and the series resistance of the battery will be too high. Moreover, in Figures 5 and 6, the current between two adjacent conductive segments 157 cannot be directly transmitted, but can only pass through the conductive segments 157 respectively. Conducted to the main grid line 153, so its current transmission is limited and its conductivity is poor.

To sum up, the point-shaped perforated battery shown in Figure 1 and the linear perforated battery shown in Figures 5 and 6 must solve the common problem of how to reduce the cavity ratio. How to strike a balance between the cavity rate and maintain the conductive performance is another problem to be solved.

Contents of the invention

The object of the present invention is to provide a solar cell with a good back electric field structure, good electrode conductivity and good conversion efficiency.

The solar cell of the present invention comprises: a substrate, a dielectric layer disposed on the substrate, at least one through hole disposed on the dielectric layer, and an electrode disposed on the dielectric layer, the electrode comprising at least A first structure located outside the through hole and at least one second structure disposed in the through hole and connected to the substrate, the first structure includes a body segment and at least one conductive segment connecting the body segment and the second structure; The solar cell also includes at least one opening located on the dielectric layer and surrounded by the electrode, and the conductive segment is disposed in the opening. The solar cell also includes a first area and a second area, the first area is the total area of the opening projected onto the substrate, the second area is the total area of the electrode projected onto the substrate, and the second The area is larger than the first area.

In the solar cell of the present invention, the materials of the second structure and the conductive segment are different from each other.

In the solar cell of the present invention, the material of the body segment of the first structure and the second structure are the same.

In the solar cell of the present invention, the material of the body section of the first structure and the conductive section are the same.

In the solar cell of the present invention, the conductive segment is filled in the entire space or a partial space of the opening.

In the solar cell of the present invention, the material of the conductive section includes any one selected from the group consisting of silver, zinc oxide and nickel.

According to the solar cell of the present invention, the conductive section is filled in the partial space of the opening, and the material of the conductive section and the second structure are the same.

In the solar cell of the present invention, the perforation includes any one selected from the group consisting of point perforation and linear perforation.

According to the solar cell of the present invention, the second structure is provided in the dotted hole, and the opening is located on the outer periphery of the second structure.

In the solar cell of the present invention, the second structure is provided in the linear perforation, and the opening is located on the outer periphery of the second structure.

The beneficial effect of the present invention is that: connecting the body segment of the first structure and the second structure through the conductive segment can limit the diffusion of the substrate material during the high-temperature sintering process, so that enough substrate material can be mixed with the electrode material, Furthermore, a thick and high-quality back electric field structure is formed, and the electrode has sufficient area and good conductivity. The above structural improvements can improve the conversion efficiency of the battery.

Description of drawings

Fig. 1 is a sectional view of a known solar cell;

Figure 2 is a photo taken with a scanning electron microscope (SEM), mainly showing a cavity in the solar cell;

Fig. 3 is the bottom view of another kind of known solar cell;

Fig. 4 is a sectional view taken along line 4-4 in Fig. 3;

Figure 5 is a bottom view of yet another known solar cell;

Fig. 6 is a sectional view taken along line 6-6 in Fig. 5;

Fig. 7 is a bottom view of a first preferred embodiment of the solar cell of the present invention;

Fig. 8 is an enlarged view of the part enclosed by the imaginary line taken from Fig. 7;

Fig. 9 is a sectional view taken along line 9-9 in Fig. 8;

Fig. 10 is a photo taken by SEM, showing the local parts of the first preferred embodiment;

Fig. 11 is a partial bottom view of a second preferred embodiment of the solar cell of the present invention;

Fig. 12 is a partial bottom view of a third preferred embodiment of the solar cell of the present invention;

Fig. 13 is a partial bottom view of a fourth preferred embodiment of the solar cell of the present invention;

Fig. 14 is a partial bottom view of a fifth preferred embodiment of the solar cell of the present invention;

Fig. 15 is a partial bottom view of a sixth preferred embodiment of the solar cell of the present invention;

Fig. 16 is a partial bottom view of a seventh preferred embodiment of the solar cell of the present invention;

Fig. 17 is a sectional view taken along line 17-17 in Fig. 16;

Fig. 18 is a partial bottom view of an eighth preferred embodiment of the solar cell of the present invention;

Fig. 19 is a partial bottom view of a ninth preferred embodiment of the solar cell of the present invention;

Fig. 20 is a cross-sectional view taken along line 20-20 in Fig. 19 .

detailed description

The present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that in the following description, similar elements are denoted by the same numerals.

Referring to Fig. 7, Fig. 8, Fig. 9, the first preferred embodiment of the solar cell of the present invention comprises: a substrate 2, a dielectric layer 3, at least one through hole 31, a plurality of back electric field structures 4, an electrode 5 and at least An opening 6 is formed on the dielectric layer 3 and surrounded by the electrode 5 .

The substrate 2 includes a first surface 21 and a second surface 22 opposite to each other. The first surface 21 is a back surface, and the second surface 22 is a light-incident surface, which can be roughened to increase the amount of light incident. The substrate 2 is also provided with an emitter layer 23 located at the second surface 22 and capable of forming a p-n junction with the substrate 2. In this embodiment, the substrate 2 is a p-type silicon substrate, and the emitter layer 23 It is an n-type semiconductor, but the conductivity types of the two can be interchanged during implementation, as long as a p-n junction can be formed.

In addition, an anti-reflection layer not shown in the figure can also be arranged on the emitter layer 23, and its material, such as silicon nitride (SiN _x ), is used to reduce the reflection of sunlight, increase the incidence rate of light, and reduce the surface area. Carrier recombination velocity (recombination velocity, SRV for short), but the present invention does not require the anti-reflection layer. There is also an unshown front electrode on the battery, which is used to cooperate with the electrode 5 to transmit the electric energy of the battery to the outside, but since the front electrode is not the focus of improvement of the present invention, it will not be described again.

The dielectric layer 3 is generally also called a passivation layer (passivation layer), and is arranged on the first surface 21 of the substrate 2, and is used to fill and reduce surface defects, thereby reducing the carrier on the first surface 21 of the substrate 2. The recombination rate at this point can improve the conversion efficiency of the battery. The material of the dielectric layer 3 is, for example, oxide, nitride, or a composite material of oxide and nitride.

The number of perforations 31 in this embodiment is multiple, and they are arranged at intervals on the dielectric layer 3, and then penetrate the upper and lower surfaces of the dielectric layer 3, and the perforations 31 are circular perforations (also known as circular perforations). perforation), but not limited thereto.

The back electric field structures 4 are respectively located on the first surface 21 of the substrate 2 corresponding to the through holes 31. The back electric field structures 4 in this embodiment are p-type semiconductors formed of aluminum-silicon (Al-Si) mixed materials, which The carrier concentration is higher than that of the substrate 2, and electrons are blocked from moving toward the first surface 21 by the electric field effect of the back electric field structure 4, so that electrons are collected in the emitter layer 23, so as to improve carrier collection efficiency and conversion efficiency.

The electrode 5 of the present invention is disposed on the dielectric layer 3 and includes at least one first structure 51 located outside the through hole 31 and at least one second structure 52 disposed on the through hole 31 and connected to the substrate 2 . Wherein, the first structure 51 includes a body segment 511 and at least one conductive segment 512 connecting the body segment 511 and the second structure 52 . Wherein the body segment 511 is entirely formed on the dielectric layer 3 .

Specifically, the electrode 5 of this embodiment is composed of a plurality of electrode units 50 arranged repeatedly, and each electrode unit 50 is configured corresponding to each through hole 31, and each electrode unit 50 includes a first structure 51 and a A circular second structure 52 located in the area surrounded by the first structure 51 . The first structure 51 of each electrode unit 50 includes a body segment 511 surrounded by a square (not limited to this) and spaced from the second structure 52, and two (not limited to this) spaced from each other and connected to the second structure 51. The body segment 511 and the conductive segment 512 of the second structure 52 .

Wherein, the material of the body section 511 of the first structure 51 and the second structure 52 can be the same or different. The materials of the body section 511 and the second structure 52 of this embodiment are both aluminum, and aluminum is printed by screen printing The area between the body section 511 and the second structure 52 is the area of the opening 6 , and the opening 6 is located on the outer periphery of the second structure 52 . The material of the conductive section 512 can be selected from any one of the group consisting of aluminum, silver, zinc oxide and nickel. Preferably, the material used in this embodiment is silver, and the silver paste is printed by screen printing. formed, and the conductive segment 512 is only filled in a partial space of the opening 6 , but does not completely fill the opening 6 .

The total area of the orthographic projection of the opening 6 onto the substrate 2 is defined as a first area, and the first area refers to the entire orthographic projection area of the entire opening 6 when the conductive segment 512 is not filled in the opening 6 . Define the total area of the positive projection of the electrode 5 onto the substrate 2 as a second area, and the second area is greater than the first area, because the electrode 5 is used as the back electrode of the battery, and the area of the back electrode is large enough to facilitate current conduction .

The present invention forms the back electric field structure 4 by screen printing the aluminum paste material and mixing the aluminum with the silicon of the substrate 2 through sintering when making the electrode 5 . Since the body section 511 of the first structure 51 of the electrode 5 is separated from the second structure 52 by the opening 6, and is only connected through the conductive section 512, and the area of the conductive section 512 is small, the electrode is formed during sintering. 5, even if the silicon in the substrate 2 diffuses toward the conductive segment 512 through the second structure 52, the diffusion path of silicon can be limited by the small-area conductive segment 512 on the one hand, and on the other hand, the conductive segment 512 The concentration of silicon will soon reach saturation, and silicon will no longer continue to diffuse toward the conductive segment 512, thereby effectively confining silicon to the first surface 21 of the substrate 2, so that enough silicon is mixed with the electrode 5. The aluminum is mixed to avoid cavities on the first surface 21, so a thick and high-quality back electric field structure 4 can be formed, thereby improving the conversion efficiency of the battery. And since the entire area of the electrode 5 is sufficient, good electrical conductivity can be maintained.

Referring to FIG. 10 , it is a photo taken by a scanning electron microscope (SEM), showing that the battery of the present invention forms a large amount of aluminum-silicon alloy at the back side of the substrate 2, almost without the existence of cavities, and the back electric field structure 4 The thickness is thick and the quality is good.

Referring to FIG. 11 , the structure of the second preferred embodiment of the solar cell of the present invention is substantially the same as that of the first preferred embodiment, except that the electrode unit 50 of this embodiment only includes one conductive segment 512 . This embodiment can also achieve the effects of the first preferred embodiment, which will not be described here again.

Referring to FIG. 12, the third preferred embodiment of the solar cell of the present invention has roughly the same structure as the first preferred embodiment, except that the left and right sides of the conductive segment 512 of this embodiment are slightly outward. Arc protrusion.

Referring to Fig. 13, the fourth preferred embodiment of the solar cell of the present invention has roughly the same structure as the first preferred embodiment, except that the electrode unit 50 of this embodiment includes four conductive segments 512, two of which are The first conductive segment 512 is flat and linear, and the other two conductive segments 512 gradually expand from the body segment 511 of the first structure 51 toward the direction of the second structure 52 .

Referring to Fig. 14, the fifth preferred embodiment of the solar cell of the present invention has roughly the same structure as the first preferred embodiment, except that the electrode unit 50 of this embodiment only includes one conductive segment 512, and the The conductive segment 512 extends in a spiral shape to connect the body segment 511 of the first structure 51 and the second structure 52 .

Referring to Fig. 15, the sixth preferred embodiment of the solar cell of the present invention has roughly the same structure as that of the first preferred embodiment, except that the body section 511 of the first structure 51 of this embodiment and the second structure The opening 6 defined by 52 is ring-shaped (not limited to this), and the first structure 51 only includes a conductive segment 512, which is also ring-shaped (not limited to this), and the space of the entire opening 6 is completely fill up. The material of the body section 511 and the second structure 52 is aluminum, and the material of the conductive section 512 is silver. The second structure 52 is surrounded by the conductive section 512 and separated from the body section 511, and is passed through the solar cell. During sintering, the poor diffusivity of silicon to silver can also avoid the diffusion of silicon in the substrate, thereby reducing the cavity ratio, so that the thickness of the subsequent back electric field structure is thick and the quality is good, and because the conductive segment 512 is the entire Surrounded in a ring shape, the contact area with the body section 511 of the first structure 51 and the second structure 52 is larger, which is conducive to current transmission and can improve the conversion efficiency of the battery.

As can be seen from the above description, the present invention does not need to limit the number and shape of the conductive segment 512, and when the conductive segment 512 does not fill the opening 6 as shown in the first to fifth embodiments of Figures 7 to 14, the conductive segment 512 The material can be the same as or different from the second structure 52, because the second structure 52 is separated from the body segment 511 of the first structure 51 by the opening 6, and is only connected by the conductive segment 512, so that the substrate 2 can be reduced. The diffusion path of silicon reduces the diffusion rate of silicon. Of course, if the material of the conductive segment 512 is different from that of the second structure 52, and the mutual solubility of the material of the conductive segment 512 and the material of the substrate 2 is smaller than that of the second structure 52 and the material of the substrate 2, the effect of the conductive segment 512 limiting silicon diffusion More preferably, for example, the miscibility of silver, zinc oxide or nickel with silicon is less than that of aluminum and silicon, so when the conductive segment 512 is made of silver, zinc oxide or nickel, the diffusion of silicon can be avoided more effectively. However, if the conductive section 512 is designed as shown in the sixth embodiment of FIG. 15 to fill the opening 6 , the material of the conductive section 512 must be different from that of the second structure 52 to avoid diffusion. As for the material of the body section 511 of the first structure 51 in various embodiments of the present invention, it may be the same as that of the conductive section 512 or may be different.

Referring to Fig. 16 and Fig. 17, the structure of the seventh preferred embodiment of the solar cell of the present invention is roughly the same as that of the first preferred embodiment, except that the perforation 31 of this embodiment is a line extending left and right. Perforation. The electrode 5 in this embodiment includes at least one first structure 51 located outside the perforation 31 and at least one second structure 52 disposed on the perforation 31 and extending along the perforation 31. The first structure 51 includes a body extending left and right segment 511 and a plurality of conductive segments 512 connecting the body segment 511 and the second structure 52 . At least one opening 6 is formed in the area between the body segment 511 and the second structure 52 , and the opening 6 is located on the outer periphery of the second structure 52 . It should be noted that the addition of fine dots in the area of the conductive segment 512 in FIG. 16 is for the convenience of identification, and the illustration is only for illustration and not for limitation.

The conductive segments 512 in this embodiment are spaced apart from each other, and are only filled in a partial space of the opening 6 . Both the body section 511 and the second structure 52 are made of aluminum, and the conductive section 512 is made of silver. In this way, the structure design of the electrode 5 pattern can also be matched with the material of the conductive section 512 to limit the size of the substrate 2. Silicon diffuses outward during the sintering process, so that a back electric field structure 4 with good quality and thick thickness can be formed. Moreover, the body segment 511 of the first structure 51 is connected to the second structure 52 through the conductive segment 512 , so that current can be directly transmitted between the body segment 511 and the second structure 52 , thus having good electrical conductivity.

Referring to FIG. 18 , the eighth preferred embodiment of the solar cell of the present invention has substantially the same structure as the seventh preferred embodiment, except that the conductive segment 512 of this embodiment will be originally formed on the first structure 51 The opening 6 between the body segment 511 and the second structure 52 is completely filled.

Referring to Fig. 19 and Fig. 20, the structure of the ninth preferred embodiment of the solar cell of the present invention is roughly the same as that of the eighth preferred embodiment, the difference lies in: the body section 511 of the first structure 51 and The material of the conductive segment 512 is silver (the boundary between the two is shown by a phantom line in FIG. 19 ), and the material of the second structure 52 is aluminum. This embodiment is the same as the previous embodiments. A portion of a structure 51 adjacent to and connected to the second structure 52 . In this embodiment, when the dielectric layer 3 and the through hole 31 are formed, firstly, the first structure 51 is formed by screen printing on the surface of the dielectric layer 3 and outside the through hole 31, so that the The main body segment 511 and the conductive segment 512 are integrally formed, and then the second structure 52 is screen-printed on the positions corresponding to the respective through holes 31 , and the fabrication is completed through sintering. Since the second structures 52 in this embodiment are separated by the first structures 51 made of silver, when the silicon of the substrate 2 diffuses out from the second structures 52, it can be blocked by the first structures 51, and Because the silicon concentration in the second structure 52 is easy to reach saturation, thereby limiting the diffusion of silicon.

Referring to Table 1, it shows the measurement results of series resistance (Rs) and cavity ratio of this embodiment and two comparative examples. The battery of Comparative Example 1 is shown in Figure 3 and Figure 4 of the prior art, and the battery of Comparative Example 2 is shown in Figure 5 and Figure 6 of the prior art. From the results of Table 1, it can be seen that the present embodiment is relatively 2 has a smaller series resistance, and has a lower cavity ratio compared with Comparative Example 1, so this embodiment is to achieve a balance between reducing the series resistance and reducing the cavity ratio. When the series resistance and the cavity ratio can be When taken into consideration, it helps to improve the overall conductivity and photoelectric conversion efficiency of the battery.

Rs (mΩ) Cavity ratio Comparative example 1 4.38 57.66% Comparative example 2 6.66 12.38% Embodiment 9 of the present invention 4.63 28.97%

Table 1

It should be noted that, in the embodiment of the linear perforation of the present invention, the selection concept of the material of the conductive segment is the same as that of the embodiment of the point perforation (embodiments 1 to 6), that is, if the conductive segment does not fill the opening, The material of the conductive segment can be the same as or different from that of the second structure. If the conductive segment fills the opening, the material of the conductive segment must be different from that of the second structure. As for the material of the body section of the first structure in each embodiment of the present invention, it may be the same as the conductive section, or may be different. It can also be known from the above description that the shape of the perforation in the present invention is not limited, and may include any one selected from the group consisting of point perforation and linear perforation, and of course other shapes may also be used. In addition, the electrode of the present invention may also include at least one busbar not shown in the figure, but since the busbar is not the focus of improvement of the present invention, it will not be described in detail.

Claims (10)

1. a solar cell, comprise: substrate, one is located at dielectric layer on this substrate, at least one is configured at perforation and the electrode be located on this dielectric layer of this dielectric layer, it is characterized in that, this electrode comprises at least one and is positioned at the first structure outside this perforation and at least one is located at this perforation and connects the second structure of this substrate, this first structure comprises a body section and at least one connects the conductive segment of this body section and this second structure, and wherein this body section is all formed on this dielectric layer; This solar cell also comprises at least one and to be positioned on this dielectric layer and by this electrode around the opening formed, this conductive segment is configured in this opening; This solar cell also comprises first area and a second area, and this first area is the gross area of this opening orthographic projection to this substrate, and this second area is the gross area of this electrode orthographic projection to this substrate, and this second area is greater than this first area.

2. solar cell according to claim 1, is characterized in that, the material of this second structure and this conductive segment is different from each other.

3. solar cell according to claim 2, is characterized in that, the body section of this first structure and the material of this second structure mutually the same.

4. solar cell according to claim 2, is characterized in that, the body section of this first structure and the material of this conductive segment mutually the same.

5. solar cell according to any one of claim 1 to 4, is characterized in that, this conductive segment is filled at whole space or the local space of this opening.

6. solar cell according to any one of claim 1 to 4, is characterized in that, the material of this conductive segment is selected from any one that silver, zinc oxide and nickel form group.

7. solar cell according to claim 1, is characterized in that, this conductive segment is filled at the local space of this opening, and the material of this conductive segment and this second structure is mutually the same.

8. solar cell according to claim 1, is characterized in that, this perforation is selected from point-like perforation and forms any one of group with linear perforation.

9. solar cell according to claim 8, is characterized in that, the perforation of this point-like is provided with this second structure, and this opening is positioned at the periphery of this second structure.

10. solar cell according to claim 8, is characterized in that, this linear perforation is provided with this second structure, and this opening is positioned at the periphery of this second structure.