Solar cell
By setting a protective layer at the junction of the transparent conductive layer in the photoelectric conversion unit, the problems of damage to the transparent conductive layer during plating and the blank area in heterojunction cells are solved, thereby improving photoelectric conversion efficiency and cell weather resistance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- TRINA SOLAR CO LTD
- Filing Date
- 2025-06-26
- Publication Date
- 2026-07-21
Smart Images
Figure CN224538657U_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of solar panel technology, and in particular to solar cells. Background Technology
[0002] Hetero-junction technology (HJT) was first developed by Panasonic (Sanyo) in 1990. Through continuous technological breakthroughs and innovations, its highest conversion efficiency has now reached over 27%. The heterojunction back contact (HBC) solar cell, based on a heterojunction structure, has even set a new world record of 27.30% efficiency for single-crystal silicon solar cells, making heterojunction cells even more prominent. A heterojunction cell consists of a photoelectric conversion unit and transparent conductive layers disposed on the front and back sides of the photoelectric conversion unit. However, during the fabrication of the transparent conductive layer on the front side, a plating-around phenomenon occurs. This means that the transparent conductive layer, which should be deposited on the front side, is deposited simultaneously on both the front and side sides due to process or equipment limitations. This causes the transparent conductive layers on the front and back sides to come into contact, leading to direct leakage of current generated inside the cell and a lower photoelectric conversion efficiency of the photoelectric conversion unit.
[0003] In related technologies, when depositing a transparent conductive layer on the back of the photoelectric conversion unit, the width of the transparent conductive layer is generally made smaller than the width of the back of the photoelectric conversion unit, that is, a blank area is formed on the back of the cell, so that the photoelectric conversion unit is directly exposed to avoid leakage.
[0004] However, the photoelectric conversion unit is directly exposed, and the blank area without a transparent conductive layer for protection is easily damaged and susceptible to water and oxygen intrusion. Utility Model Content
[0005] Therefore, it is necessary to provide a solar cell that addresses the problems of easy damage to the blank area and water and oxygen damage caused by directly exposing the photoelectric conversion unit.
[0006] A solar cell, the solar cell comprising:
[0007] Photoelectric conversion unit;
[0008] A first transparent conductive layer is disposed on the front side of the photoelectric conversion unit;
[0009] A second transparent conductive layer is disposed on the back side of the photoelectric conversion unit. The size of the second transparent conductive layer is smaller than the size of the back side of the photoelectric conversion unit, so that there is a blank area on the back side of the photoelectric conversion unit.
[0010] The first protective layer covers part of the first transparent conductive layer, the blank area, and part of the second transparent conductive layer.
[0011] In one embodiment, the first transparent conductive layer is further disposed on at least a portion of the side region of the photoelectric conversion unit; the first protective layer simultaneously covers both the side of the photoelectric conversion unit and a portion of the front of the photoelectric conversion unit.
[0012] In one embodiment, the width of the first protective layer covering the front surface of the photoelectric conversion unit is W3, and the range of W3 is: 0mm < W3 < 1mm.
[0013] In one embodiment, the width of the first protective layer covering the back of the photoelectric conversion unit is W2, and the range of W2 is: 0mm < W2 < 1mm.
[0014] In one embodiment, the blank areas are respectively provided at both ends of the second transparent conductive layer along the length direction and / or the width direction.
[0015] In one embodiment, the photoelectric conversion unit includes:
[0016] Silicon substrate;
[0017] A first intrinsic silicon layer and a second intrinsic silicon layer are respectively disposed on the front and back sides of the silicon substrate; and
[0018] A first doped layer disposed on the side of the first intrinsic silicon layer away from the silicon substrate, and a second doped layer disposed on the side of the second intrinsic silicon layer away from the silicon substrate;
[0019] The first doped layer is either N-type or P-type doped, and the second doped layer is either N-type or P-type doped, with the doping types of the first and second doped layers being opposite.
[0020] In one embodiment, the first intrinsic silicon layer extends to the side of the silicon substrate, the second intrinsic silicon layer extends to the side of the silicon substrate, and the portions of the first intrinsic silicon layer extending to the side of the silicon substrate and the portions of the second intrinsic silicon layer extending to the side of the silicon substrate overlap.
[0021] The first doped layer extends to the side of the silicon substrate, the second doped layer extends to the side of the silicon substrate, and the portions of the first doped layer extending to the side of the silicon substrate and the portions of the second doped layer extending to the side of the silicon substrate overlap.
[0022] In one embodiment, the blank area is provided with a second protective layer, and the first protective layer covers the second protective layer.
[0023] In one embodiment, the second protective layer is formed by oxidizing the second doped layer exposed in the blank area.
[0024] In one embodiment, the size of the first transparent conductive layer is smaller than the front size of the photoelectric conversion unit, so that there is a blank area on the front of the photoelectric conversion unit, and the size of the blank area on the front is smaller than the size of the blank area on the back.
[0025] In the aforementioned solar cell, the first protective layer covers the blank area, preventing damage to the blank area and reducing water and oxygen ingress. Simultaneously, the first end of the first protective layer extends beyond the first transparent conductive layer, and the second end extends beyond the second transparent conductive layer. That is, one end of the first protective layer partially overlaps with the first transparent conductive layer, and the other end partially overlaps with the second transparent conductive layer. This means the first protective layer effectively protects both the weak areas where the blank area meets the first and second transparent conductive layers, preventing water and oxygen ingress into these vulnerable areas. Attached Figure Description
[0026] Figure 1 This is a schematic diagram of the structure of a solar cell in one embodiment.
[0027] Figure 2 This is a schematic diagram of the structure of a solar cell in another embodiment.
[0028] Reference numerals: 100, photoelectric conversion unit; 110, silicon substrate; 120, first intrinsic silicon layer; 130, second intrinsic silicon layer; 140, first doped layer; 150, second doped layer; 151, second protective layer; 160, blank area; 200, first transparent conductive layer; 300, second transparent conductive layer; 400, first protective layer. Detailed Implementation
[0029] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.
[0030] In the description of this application, it should be understood that if the terms "center", "longitudinal", "lateral", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. appear, these terms indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.
[0031] Furthermore, where the terms "first" and "second" appear, these terms are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, where the term "multiple" appears, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.
[0032] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.
[0033] In this application, unless otherwise expressly specified and limited, the use of descriptions such as "above" or "below" the second feature indicates that the first and second features are in direct contact or indirect contact via an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. Similarly, "below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.
[0034] It should be noted that if an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. If an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. If so, the terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this application are for illustrative purposes only and do not represent the only possible implementation.
[0035] See Figure 1 An embodiment of this application provides a solar cell comprising a photoelectric conversion unit 100, a first transparent conductive layer 200, a second transparent conductive layer 300, and a first protective layer 400. The first transparent conductive layer 200 is disposed on the front side of the photoelectric conversion unit 100; the second transparent conductive layer 300 is disposed on the back side of the photoelectric conversion unit 100, and the size of the second transparent conductive layer 300 is smaller than the size of the back side of the photoelectric conversion unit 100, so that a blank area 160 exists on the back side of the photoelectric conversion unit 100; the first protective layer 400 covers a portion of the first transparent conductive layer 200, the blank area 160, and a portion of the second transparent conductive layer 300.
[0036] In this embodiment, the first protective layer 400 covers the blank area 160, which can prevent damage to the blank area 160 and reduce water and oxygen damage to the blank area 160. Furthermore, the first protective layer 400 covering the blank area 160 provides a certain anti-reflection effect, thereby improving the photoelectric conversion efficiency of the solar cell. Simultaneously, the first protective layer 400 also covers a portion of the first transparent conductive layer 200 and a portion of the second transparent conductive layer 300, such that one end of the first protective layer 400 partially overlaps with the first transparent conductive layer 200, and the other end partially overlaps with the second transparent conductive layer 300. That is, the first protective layer 400 can effectively protect the weak area at the junction of the blank area 160 and the first transparent conductive layer 200, as well as the weak area at the junction of the blank area 160 and the second transparent conductive layer 300, preventing water and oxygen damage to the weak areas.
[0037] Specifically, the first protective layer 400 is an organic protective layer, an inorganic protective layer, or an organic-inorganic hybrid or composite protective layer. The organic protective layer is epoxy resin, polyurethane resin, or acrylic resin. The inorganic protective layer is silicon oxide, titanium oxide, or aluminum oxide. Furthermore, when the first protective layer 400 is silicon oxide, titanium oxide, or aluminum oxide, it also has an anti-reflection effect, which can improve the photocurrent in the blank area 160 and increase the conversion efficiency of the battery.
[0038] The specific material of the first protective layer 400 can be changed according to the actual needs of the solar cell. For example, if it is necessary to ensure the light transmittance of the solar cell, a silicon dioxide thin film layer with a light transmittance greater than 90% or fluorine-doped silicon oxide with a light transmittance greater than 90% can be used; if it is necessary to ensure the water-blocking performance of the solar cell, fluorine-doped silicon oxide can be used.
[0039] In some embodiments, the first transparent conductive layer 200 is also disposed on at least a portion of the side region of the photoelectric conversion unit 100, and the first protective layer 400 simultaneously covers the side of the photoelectric conversion unit 100 and a portion of the front of the photoelectric conversion unit 100.
[0040] Specifically, the photoelectric conversion unit 100 has a rectangular block structure and has four sides, with the first protective layer 400 covering all four sides.
[0041] In this embodiment, after the first transparent conductive layer 200 and the second transparent conductive layer 300 are fabricated, electrodes need to be electroplated on the first transparent conductive layer 200 and the second transparent conductive layer 300. After electroplating, the electroplating solution needs to be cleaned with alkaline solution to remove the mask. However, alkaline solution can damage the first transparent conductive layer 200 and the second transparent conductive layer 300. Therefore, this application first sets a first protective layer 400 before cleaning, so that the first end of the first protective layer 400 extends from the side of the photoelectric conversion unit 100 to the front of the photoelectric conversion unit 100, thereby protecting the side and part of the front area of the photoelectric conversion unit 100 from being damaged by alkaline solution.
[0042] Furthermore, the width of the first protective layer 400 covering the front surface of the photoelectric conversion unit 100 is W3, and the range of W3 is: 0mm < W3 < 1mm.
[0043] In this embodiment, the first protective layer 400 covers the front edge from 0mm to 1mm along the first direction to protect the front edge and make the process window on the front larger, thus minimizing the optical impact on the battery. However, when the size of W3 is greater than 1mm, the first protective layer 400 easily covers the grid lines, thereby affecting current collection. Therefore, the range of W3 is: 0mm < W3 < 1mm.
[0044] In some embodiments, the width of the first protective layer 400 covering the back surface of the photoelectric conversion unit 100 is W2, and the dimension range of W2 along the first direction is: 0mm < W2 < 1mm.
[0045] In this embodiment, the first protective layer 400 covers the back edge from 0mm to 1mm to protect the edge of the second transparent conductive layer 300 and the blank area 160 on the back. When the size of W2 is greater than 1mm, the first protective layer 400 may easily cover the gate lines, thus affecting current collection. Therefore, the range of W2 is: 0mm < W2 < 1mm.
[0046] Furthermore, the width of the blank area 160 is W1, and the range of W1 is: W1 < W3.
[0047] In some embodiments, the blank area 160 is located at both ends of the second transparent conductive layer 300 along the length direction and / or width direction.
[0048] In some embodiments, the blank areas 160 are located at both ends of the second transparent conductive layer 300 along the length direction, and correspondingly, the first protective layer 400 is located on the two sides that are opposite to each other along the length direction.
[0049] In some other embodiments, the blank area 160 is located at both ends of the second transparent conductive layer 300 along the width direction, and correspondingly, the first protective layer 400 is located outside the two sides that are opposite to each other along the width direction.
[0050] In some other embodiments, blank areas 160 are respectively provided at both ends of the second transparent conductive layer 300 along the width direction and at both ends of the length direction, and correspondingly, the first protective layer 400 covers the four sides.
[0051] The first transparent conductive layer 200 covers all sides of the photoelectric conversion unit 100 to protect the entire side of the photoelectric conversion unit 100. The blank area 160 is located at both ends of the second transparent conductive layer 300 along the length and / or width directions to prevent the periphery of the second transparent conductive layer 300 from contacting the first transparent conductive layer 200 and causing leakage.
[0052] The first transparent conductive layer 200 and the second transparent conductive layer 300 comprise one or more transparent conductive layers of different types. For example, the transparent conductive layers may be selected from one or more composites of indium tin oxide (ITO), zinc aluminum oxide (AZO), indium tungsten oxide (IWO), and indium cerium oxide (ICO).
[0053] In some embodiments, the photoelectric conversion unit 100 includes a silicon substrate 110, a first intrinsic silicon layer 120 and a second intrinsic silicon layer 130 respectively disposed on the front and back sides of the silicon substrate 110; a first doped layer 140 disposed on the side of the first intrinsic silicon layer 120 away from the silicon substrate 110, and a second doped layer 150 disposed on the side of the second intrinsic silicon layer 130 away from the silicon substrate 110; wherein the first doped layer 140 is N-type doped or P-type doped, the second doped layer is N-type doped or P-type doped, and the doping types of the first doped layer 140 and the second doped layer 150 are opposite.
[0054] In this embodiment, the solar cell of this application is a heterojunction solar cell. The silicon substrate 110 can be N-type doped or P-type doped. The upper surface of the silicon substrate 110 is the front side, used to receive light; the lower surface of the silicon substrate 110 is the back side. The first intrinsic silicon layer 120 and the second intrinsic silicon layer 130 contain elements such as oxygen, carbon, and nitrogen, and are amorphous silicon or microcrystalline silicon; preferably, the first intrinsic silicon layer 120 and the second intrinsic silicon layer 130 are amorphous silicon. The first doped layer 140 and the second doped layer 150 contain elements such as oxygen, carbon, and nitrogen, and are amorphous silicon or microcrystalline silicon; preferably, the first doped layer 140 and the second doped layer 150 are microcrystalline silicon.
[0055] In another embodiment, the solar cell of this application may also be a tunnel oxide passivated contact cell, which is not limited here.
[0056] Furthermore, the first intrinsic silicon layer 120 extends to the side of the silicon substrate 110, the second intrinsic silicon layer 130 extends to the side of the silicon substrate 110, and the portion of the first intrinsic silicon layer 120 extending to the side of the silicon substrate 110 and the portion of the second intrinsic silicon layer 130 extending to the side of the silicon substrate 110 overlap; the first doped layer 140 extends to the side of the silicon substrate 110, the second doped layer 150 extends to the side of the silicon substrate 110, and the portion of the first doped layer 140 extending to the side of the silicon substrate 110 and the portion of the second doped layer 150 extending to the side of the silicon substrate 110 overlap.
[0057] In this embodiment, the side overlap can act as a physical barrier to isolate the external environment from the corrosion of the silicon substrate 110 side (such as moisture and chemical contaminants), thereby improving the weather resistance and lifespan of the battery; and in terms of manufacturing process, it can reduce complex steps such as edge etching and simplify the production process.
[0058] Combination Figure 2 In some embodiments, the blank area 160 is provided with a second protective layer 151, and the first protective layer 400 covers the second protective layer 151.
[0059] In this embodiment, the second protective layer 151 is a silicon oxide layer, and the blank area 160 is protected by both the first protective layer 400 and the second protective layer 151, i.e., a double protective layer, which can better protect the photoelectric conversion unit 100 in subsequent electroplating and wet etching.
[0060] Specifically, the first protective layer 400 includes a middle section located in the middle and edge sections located on both sides of the middle section. The middle section is made by applying liquid paint and drying and curing, while the edge sections are made by spraying. The manufacturing process is simple and easy to mass-produce.
[0061] In some embodiments, the second protective layer 151 is formed by oxidizing the second doped layer 150 exposed in the blank area 160.
[0062] In this embodiment, the second protective layer 151 is not a separate protective layer outside the second doped layer 150, but is formed by oxidation directly on the second doped layer 150 in the blank area 160. The specific oxidation method can be formed directly during the annealing of the second transparent conductive layer 300, or by soaking in hydrogen peroxide or by ozone oxidation. The operation process is simple.
[0063] In some embodiments, the thickness of the first transparent conductive layer 200 is less than the thickness of the second transparent conductive layer 300.
[0064] In this embodiment, the thickness of the first transparent conductive layer 200 is less than the thickness of the second transparent conductive layer 300, that is, the transparent conductive layer on the front side is thinner, which is used to reduce the loss of light on the front side and improve the photoelectric conversion efficiency of the solar cell.
[0065] In other embodiments, the thickness of the first transparent conductive layer 200 may also be greater than or equal to the thickness of the second transparent conductive layer 300.
[0066] In some embodiments, the size of the first transparent conductive layer 200 is smaller than the front size of the photoelectric conversion unit 100, so that there is a blank area on the front of the photoelectric conversion unit 100, and the size of the blank area on the front is smaller than the size of the blank area on the back. The blank area on the front is provided to further prevent leakage of the photoelectric conversion unit, and the smaller size of the blank area on the front is provided to increase the anti-reflection effect of the front.
[0067] To illustrate the beneficial effects of the solar cells provided in the embodiments of this application, several types of heterojunction cells are listed below.
[0068] Example 1 (First Protective Layer 400): The HJT wafer size is 210mm × 105mm. The process sequence is as follows: cleaning and texturing, plasma-enhanced chemical vapor deposition of intrinsic silicon layers (including first intrinsic silicon layer 120 and second intrinsic silicon layer 130) and doped layers (including first doped layer 140 and second doped layer 150), physical vapor deposition of transparent conductive layers (including first transparent conductive layer 200 and second transparent conductive layer 300), spraying of the first protective layer 400, and electrode fabrication. The first transparent conductive layer 200 and the second transparent conductive layer 300 are both ITO (indium tin oxide) targets. The thickness of the first transparent conductive layer 200 is 100nm, the second transparent conductive layer 300 is 110nm, W1 is 0.5mm, the thickness of the first protective layer 400 is 100nm, W2 is 0.8mm, and W3 is 0.2mm.
[0069] Example 2 (First protective layer 400 and second protective layer 151): Unlike Example 1, after the electrode is fabricated, the first protective layer 400 is sprayed. When fabricating the electrode, the second doped layer 150 in the blank area 160 can be oxidized to form the second protective layer 151. That is, Example 2 is the same as Example 1.
[0070] Comparative Example (No Protective Layer): Unlike Example 1, no protective layer was applied; otherwise, it was the same as Example 1.
[0071] The comparison data for the three are shown in the table below:
[0072]
[0073] The alkaline washing test conditions are: 4% KOH + silica protective additive, room temperature for 10 minutes. DH40 test refers to a 40-hour damp heat test. Voc is the open-circuit voltage, Isc is the short-circuit current, FF is the fill factor, and ETA is the photoelectric conversion efficiency.
[0074] As can be seen from the table above, the photoelectric conversion efficiency (ETA) of HJT cells with edge protection is 0.04% higher than that of HJT cells without protection.
[0075] After DH40 testing, compared to HJT cells without a protective layer, HJT cells with edge protection showed a smaller decrease in photoelectric conversion efficiency (ETA). This is because the unprotected HJT cells are more susceptible to damage and water and oxygen ingress in the unprotected areas.
[0076] After alkaline washing, HJT batteries with two protective layers can maintain higher performance, such as higher open-circuit voltage, higher short-circuit current, higher fill factor, and higher photoelectric conversion efficiency.
[0077] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.
[0078] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.
Claims
1. A solar cell, characterized in that, The solar cell includes: Photoelectric conversion unit; A first transparent conductive layer is disposed on the front side of the photoelectric conversion unit; A second transparent conductive layer is disposed on the back side of the photoelectric conversion unit. The size of the second transparent conductive layer is smaller than the size of the back side of the photoelectric conversion unit, so that there is a blank area on the back side of the photoelectric conversion unit. The first protective layer covers part of the first transparent conductive layer, the blank area, and part of the second transparent conductive layer.
2. The solar cell according to claim 1, characterized in that, The first transparent conductive layer is also disposed in at least a portion of the side region of the photoelectric conversion unit; The first protective layer covers both the side of the photoelectric conversion unit and part of the front of the photoelectric conversion unit.
3. The solar cell according to claim 2, characterized in that, The width of the first protective layer covering the front surface of the photoelectric conversion unit is W3, and the range of W3 is: 0mm < W3 < 1mm.
4. The solar cell according to claim 1, characterized in that, The first protective layer covers the back of the photoelectric conversion unit with a width of W2, and the range of W2 is: 0mm < W2 < 1mm.
5. The solar cell according to claim 1, characterized in that, The second transparent conductive layer has blank areas at both ends along its length and / or width.
6. The solar cell according to claim 1, characterized in that, The photoelectric conversion unit includes: Silicon substrate; A first intrinsic silicon layer and a second intrinsic silicon layer respectively disposed on the front and back sides of the silicon substrate; and A first doped layer disposed on the side of the first intrinsic silicon layer away from the silicon substrate, and a second doped layer disposed on the side of the second intrinsic silicon layer away from the silicon substrate; The first doped layer is either N-type or P-type doped, and the second doped layer is either N-type or P-type doped, with the doping types of the first and second doped layers being opposite.
7. The solar cell according to claim 6, characterized in that, The first intrinsic silicon layer extends to the side of the silicon substrate, the second intrinsic silicon layer extends to the side of the silicon substrate, and the portions of the first intrinsic silicon layer extending to the side of the silicon substrate and the portions of the second intrinsic silicon layer extending to the side of the silicon substrate overlap. The first doped layer extends to the side of the silicon substrate, the second doped layer extends to the side of the silicon substrate, and the portions of the first doped layer extending to the side of the silicon substrate and the portions of the second doped layer extending to the side of the silicon substrate overlap.
8. The solar cell according to claim 6, characterized in that, The blank area is provided with a second protective layer, and the first protective layer covers the second protective layer.
9. The solar cell according to claim 8, characterized in that, The second protective layer is formed by oxidizing the second doped layer exposed in the blank area.
10. The solar cell according to claim 1, characterized in that, The size of the first transparent conductive layer is smaller than the front size of the photoelectric conversion unit, so that there is a blank area on the front of the photoelectric conversion unit, and the size of the blank area on the front is smaller than the size of the blank area on the back.