Công nghệ tính toán tạo ra Multipe busbar

Một electron nối được gọi là dây dẫn đã được áp dụng cho các mô-đun thay vì dây nối giữa chúng. Dây dẫn có nhiều sợi và có thể đạt được cấu trúc multibusbar bằng cách tạo ra các liên lạc ohmic với các điện cực của tế bào. Các vấn đề sau đã được nghiên cứu với các phương pháp sáng tạo để tối ưu hóa các mô-đun multibusbar: tỷ lệ che phủ và điện trở tiếp xúc của dây dẫn, mối quan hệ giữa điện trở chuỗi của dây và số lượng sợi, và ảnh hưởng của biến thiên điện trở chuỗi đối với công suất đầu ra tối đa. Hơn nữa, số lượng và đường kính sợi được tối ưu hóa theo các điều kiện sau: kích thước tế bào là đầy, một nửa và một phần ba, và trọng lượng ẩm của một tế bào đầy là 80 mg, 40 mg và 20 mg. Kết quả cho thấy rằng cấu trúc multibusbar và nửa tế bào có thể đạt được công suất đầu ra tối đa, số lượng sợi là 16 và đường kính sợi là 200 μm, và trọng lượng ẩm của các ngón tay đã giảm xuống còn 20 mg. Cuối cùng, độ tin cậy của các mô-đun được làm bằng dây dẫn đã được kiểm tra và đạt theo tiêu chuẩn của Tổ chức Điện kỹ thuật Quốc tế.

1. Introduction

Adding more busbars can shorten the effective finger length and reduce the series resistance () of the module Lỗi giao diện: file 'snippets/shortcode-

To date, the busbar number of the conventional modules has grown from three to four or five, and the width of the interconnection ribbons has decreased to 0.9 mm. Making interconnection ribbons even narrower is difficult, and if more busbars are added, the module efficiency would drop because of the increase in shading loss. Using copper wires instead of interconnection ribbons can reduce the busbar width to less than 0.3 mm; thus, the busbar number can be increased. On the other hand, copper wire can reflect a part of the incident light back to the cell, which will decrease its shading loss Lỗi giao diện: file 'snippets/shortcode-

Soldering copper wires onto the silver pads of the cells with Sn alloy can achieve an MBB structure. Increasing the pad area Lỗi giao diện: file 'snippets/shortcode-

The laminated grid cell (LGCell) is mainly suitable for heterojunction solar cells with transparent conducting oxide (TCO) layers on the surface Lỗi giao diện: file 'snippets/shortcode-

Half-cell design can decrease 50% of module current and reduce the resistance power loss of interconnection ribbons by 75%. The added cell gaps can reflect a part of the incident light back to cells and increase the output power of modules Lỗi giao diện: file 'snippets/shortcode-

In this study, an interconnecting electrode called conductive belt was applied instead of interconnection ribbons. The conductive belt had a certain number of wires and could achieve an MBB structure. To optimize the number and the diameter of the wires, several experiments were designed to test the following parameters: the resistance and the shading loss of different parts of the module electrode and the influence of the series resistance variation on the power output. The module electrode was optimized according to the current technological levels in photovoltaic industry development, and a 16-wire structure with a diameter of 200 μm combined with half cell was proved to achieve the most power output.

2. Materials and Feasibility Test

2.1. Introduction to Conductive Belts

The conductive belt has three layers: wires, composite films, and support films. The composite films have several separate parts and are used to press the wires onto cells or string connectors. The support films are used to load the wires and the composite films. Figure 1 shows two types of conductive belts. The A type is used to connect cells to string connectors, where the two contact surfaces are on the same side, and the support film is successive. The B type, which connects the adjacent solar cells, has two parts, and both parts have three layers. In the left part, from top to bottom are wires, composite films, and support films. The wires extend to the right part. The composite films and the support films are ordered upon the right part of the wires. As shown in Figure 1, the cells, conductive belts, and string connectors are laid successively, and after laminating the wires form ohmic contacts with the cells or the string connectors.

Figure 1 

The structure and the connect mode of the conductive belts.

The cross section of a conductive belt combined with a cell is shown in Figure 2. Compared to SmartWire Connection Technology, the structure of the conductive belt is more complex, but it can reduce the cost and increase the transmittance. The support film is made of the same material with the same thickness as the module-encapsulated adhesive film, that is, ethylene-vinyl acetate copolymer (EVA) with 0.3 mm thickness. Through laminating, the support film is integrated into the encapsulated adhesive film. The total EVA usage in the modules does not need to be increased. Thus, the support film will neither increase the cost of the module nor increase the shading of the incident light. The composite film is composed of a substrate layer and an adhesive layer. The substrate layer is used to separate the support film and the wires, and it must have a high sunlight transmittance and a strong anti-ultraviolet aging ability. Ethylene tetrafluoroethylene (ETFET) is the most suitable material, but the cost is too high. Therefore, the thickness of ETFET is reduced to less than 40 μm, making its cost more acceptable. The adhesive layer is used to fill the gap between the substrate layer and the cell surface. Its main component is EVA, and the thickness is 20 μm. In order to further reduce the cost and shading losses, the composite film is designed as a hollow structure and only covers the copper wire.

Figure 2 

Cross section of the combined structure of a conductive belt and a cell.

As the busbar is removed, SiN cannot be soldered well; that is, traditional soldering reliability will be greatly reduced. Therefore, the conductive adhesive not only is used to coat the copper wire instead of tin-bismuth alloy but also can effectively bond with the fingers and SiN. The curing temperature is lower than 150°C, and the curing time is less than 30 s. During the lamination process, it forms a reliable contact with the cells.

2.2. Transmittance Test

The composite film and the support film of the conductive belt remain unchanged, but the wire number and wire diameter will be adjusted to match the cell size and the conductivity of the fingers. And a part of the incident light on the wires can be reflected back into the cell Lỗi giao diện: file 'snippets/shortcode-∗156 mm². For each cell, the interconnection ribbons were soldered onto the front and back busbars and extended out of the cells, as shown in Figure 3. The extended interconnection ribbons of the two surfaces were isolated by electric tape. The current–voltage (I–V) curves of the soldered cells were tested by an I–V tester. The front current and voltage test leads for the I–V tester were I⁻ and V⁻, and the back current and voltage test leads for the I–V tester were I⁺ and V⁺. As shown in Figure 3, the test leads of the I–V tester were connected to different ends of the interconnection ribbons. The  tested by this connect mode only include the cell series resistance and the contact resistance () of the interconnection ribbons to the cell. This test mode could reduce the test error from the resistance of the interconnection ribbons and the contact resistance of the interconnection ribbons to the test leads.

Figure 3 

Schematic of the connect mode of the I–V tester to the cells soldered with interconnection ribbons.

The soldered cells were divided into five groups numbered 1 to 5, and each group had five cells. The cells of group 1 were encapsulated into conventional one-cell modules directly. The other cells were encapsulated into one-cell modules with A-type conductive belts on the front of the cells. Shading loss mainly affected photogenerated current. To test the shading loss of the conductive belt precisely and minimize its influence on the module resistance, the wires of the conductive belts did not contact with the cell electrode. The wire numbers and the wire diameters were changed by groups: group 2 had no wires; groups 3, 4, and 5 had 15 wires, and the wire diameters of each group were 150 μm, 200 μm, and 250 μm, respectively. The I–V curves of the modules were tested with the connect mode unchanged. The light source was 1000 W/m² and AM 1.5, and the test temperature was 25°C. For group x,  to 5. The average short current () of the soldered cells before and after encapsulating was named  and . The shading rate of the conductive belt () could be expressed as

The average shading rates tested for each group are shown in Figure 4. The total shading rate of the composite film and the support film was . As could be calculated from Figure 4, . The shading rate of the conductive belt with 150 μm wire is 1.08%, so the shading rate of the wires is about 1%. The wire coverage rate is the wire diameter multiplied by the wire number and divided by the cell width. As could be calculated, the shading rate is 70% of the coverage rate. The same conclusion can also be obtained for 200 μm and 250 μm wires. Thus, the shading rate of the wires () could be expressed as

Figure 4 

The shading rate of the conductive belts for groups 2 to 5.

 is the width (perpendicular to the cell-connecting direction) of the cell.  and  are the diameter and the number of the wires, respectively. The power loss of the conductive belts caused by shading the incident light () could be expressed as

 is the module maximum power output. As can be seen from the experiment results, the main shading rate of the conductive belt is from the wires, and since the wire coating has a high reflectivity, 30% of the wire incident light can be reflected back into the cells.

2.3. Contact Resistance Test

The wire contact resistances to fingers were measured by the following experiments. Ten one-cell modules were made with conductive belts. The front structure of the modules is shown in Figure 5(a). There were two wires at the right and left quarter positions of the cell. The wire diameter was 250 μm, and the distance between the adjacent two wires was about 2 mm. The back electrode and back connect mode were the same as those in Figure 3. Each module was tested as follows. We kept the lower terminals of the two left wires connected to the I⁻. First, we connected the upper terminal of the two left wires to the V⁻ and tested the I–V curve, and the module series resistance tested was . The test method was comparing two I–V curves measured at 500 W and 1000 W illumination intensities Lỗi giao diện: file 'snippets/shortcode-− and tested the I–V curve, and the module series resistance tested was . In the one-cell module, all the 4 wires were 160 mm long. The 3 ribbons which connect to the wires had the same length of 165 mm. Therefore, the wire resistance could hardly have contribution to . The first test mode tested the voltage of the wires, and the second test mode tested the voltage of the fingers. Thus,  is equal to the sum of the contact resistance of one wire to the fingers and the resistance of the fingers between the adjacent two wires.

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Figure 5 

Schematic of the test modes to analyze the wire contact resistance to different parts of the cell electrode. (a) Fingers, (b) back busbars, and (c) Al-BSF.

The wire contact resistances to the back busbars and to the Al-BSF were also tested by the same method, and 10 one-cell modules were made, respectively. The front electrode and the front test mode of the modules were the same as those in Figure 3. The back electrode and the back test modes are shown in Figures 5(b) and 5(c). In Figure 5(b),  is equal to the sum of the contact resistance of one wire to the back busbars and the resistances of the back busbars between the adjacent two wires. In Figure 5(c),  is equal to the sum of the contact resistance of one wire to the Al-BSF and the resistances of the Al-BSF between the adjacent two wires.

The  tested from the three experiments are shown in Figure 6. All the  tested are less than 0.45 mΩ. Thus, the contact resistance of one wire to the fingers and Al-BSF was less than 0.9 mΩ and the contact resistance of one wire to the back busbars was less than 1.8 mΩ. Contact resistance is inversely proportional to the wire number. If the wire number increases to more than eight, the wire contact resistances to fingers and Al-BSF can be reduced to less than 0.1 mΩ, and the resulting power loss can be ignored. In addition, the wires can contact with fingers and Al-BSF instead of front and back busbars. Thus, front and back busbars can be removed, which will improve open circuit voltage () and reduce the cost of the silver electrode.

Figure 6 

The  tested from the fingers, back busbars, and Al-BSF.

3. Analysis of Finger Series Resistance

3.1. Finger Series Resistance Measured by a 6½-Digit Multimeter

The series resistance of the fingers () can be expressed as Lỗi giao diện: file 'snippets/shortcode- is the resistance of a whole finger,  is the finger number, and  is the busbar number. Thirty conventional four-busbar crystalline solar cells produced by screen printing currently were chosen as samples. The average width and thickness of their fingers were tested and were 55 μm and 15 μm, respectively. The finger number was 105. The finger resistance was measured by the following experiment: the cells were separated by laser scribing and subsequent cleaving. The separated part of the cell had one full finger, and the four busbar areas of the finger were numbered 1, 2, 3, and 4. A 6½-digit multimeter was used to contact with the busbar areas and to test the finger resistance. Suppose , 2, 3, 4, the tested resistance between the busbar areas of number  and number  was , the contact resistance of the 6½-digit multimeter probe to the busbar area of number  was , the finger line resistivity between the busbar areas of number  and number  was .  and  could be represented by the following equations:

In (5), , 3. In (6), , and  is the distance between the adjacent two busbar areas. For conventional 156 mm cells, . The resistivity of the diffusion layer under the fingers was about 1000 times higher than that of the fingers, and the impact of its shunting could be ignored. For each cell, finger line resistivity of different positions was tested by the above method, and the average linear resistivity was about 58 mΩ/mm, and the resistance of a whole finger was 9 Ω. According to (4), the series resistance of 105 fingers with four busbars was 0.45 mΩ.

3.2. Finger Series Resistance Measured by I–V Tester

Two four-busbar monocrystalline solar cells with similar I–V curves were chosen to make two one-cell modules by applying conductive belts. Before encapsulation, I–V curves of the two cells were tested by the I–V tester with four rows of probes, and the cell series resistances tested were  and . The front electrode structures of the modules are shown in Figure 7(a). The left structure had two wires on the middle position of the cell, and the distance between the two wires was 2 mm. The right structure had two wires, and the wires were located on the two quarter positions of the cell, respectively. Two string connecters contacted with the two ends of the wires, respectively, and extended from the modules. The V⁻ and I⁻ test leads of the I–V tester connected to the upper and the lower string connecters of the front electrode as shown in Figure 7(a). The irrelevant resistance includes the contact resistance of the wires to the string connecters, and the contact resistances of the test leads to the string connecters; the wire resistance and the string connecter resistance could thus be avoided in the test result. The back structure and the back test mode of the two modules were the same as those in Figure 3, and the series resistances of the two modules tested were  and . The difference of the series resistance () caused by the different wire positions of the two modules could be expressed as

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Figure 7 

Four groups of module electrode structures were designed to test the finger series resistance; each group had two module electrode structures with different busbar numbers. (a) one and two busbars, (b) two and four busbars, (c) four and eight busbars, and (d) eight and sixteen busbars.

In Figure 7(a), the left module structure had two wires on the middle position. The distance between the two wires was so small that the left module was equivalent to have one busbar. Similarly, the right module structure had two busbars. The two modules both had two wires, and the shading loss and the resistance of the conductive belt were the same. Thus, the difference of the series resistance was from the finger series resistance caused by different busbar numbers. Suppose that the series resistance of the fingers with four busbars was ; according to (4), the  of the left and right module structures should be equal to . Another three groups of modules were made and tested in the same way, and the structures of each group are shown in Figures 7(b)–7(d). The  of each group was also calculated with (4). In Figure 7(b),  should be equal to  due to the difference of two and four busbars. In Figure 7(c),  should be equal to  due to the difference of four and eight busbars. In Figure 7(d),  should be equal to  due to the difference of eight and sixteen busbars.

The above four experiments were repeated four times, and 32 one-cell modules were made and tested. For the four groups of module structures, the  resulting from the four repeated experiments are shown in Figure 8. The  calculated with (4) are also shown in Figure 8. The  that got through the two ways were approximately identical. The result proves that (4) is accurate for calculating the series resistance of the fingers.

Figure 8 

 of different busbar numbers tested in two ways: (1) the series resistance of the four groups of modules with different busbar numbers was tested with the I–V tester, and the experiment was repeated four times, and (2) the series resistance of the fingers with different busbar numbers was calculated with (4).

3.3. Relationship between Series Resistance and Power Output

Series resistance power loss () can be expressed as Lỗi giao diện: file 'snippets/shortcode- is the current at maximum power point. The relationship between the current (I) and the voltage (V) of the module is Lỗi giao diện: file 'snippets/shortcode-, , and  are the short circuit current, shunt resistance, and temperature of the module, respectively,  is the Boltzmann constant, and  is the ideality factor.

A standard one-cell module was chosen, and its electrical parameters were tested. The  and  of the module were 9.221 A and 0.641 V, respectively.  is the voltage at maximum power point; the , , and  of the module (, , and ) were 8.660 A, 0.520 V, and 4.503 W, respectively, and . The I–V curve and the power–voltage (P–V) curve of the module were simulated, and the results are shown in Figure 9. As could be calculated with (9), the series resistance of the original module () was 4.4 mΩ and . Suppose that the module electrode was optimized, and the optimization mainly decreased the module series resistance. The increment of the series resistance () was −2 mΩ. The I–V and P–V curves of the optimized module were calculated again, and the results are also shown in Figure 9. The , , and  of the optimized module (, , and ) were 8.691 A, 0.535 V, and 4.650 W.

Figure 9 

The I–V and P–V curves of the original and optimized modules.

For the original and optimized modules, the increment of  () was calculated with (8) and can be defined as

, and . As could be calculated,

And (10) could be simplified to

 calculated with (10) and (12) were −0.149 W and −0.150 W, respectively. , and ΔP_m = 0.151 W. So .

Based on the above analysis, the influence of the series resistance variation on  was found: if ,  can be calculated with (12), and  was approximate to .

The following experiment was done to prove the above conclusion. A one-cell module was made. The front structure is shown in Figure 10, where the front interconnection ribbon extended from the lower side of the cell. Five points of the interconnection ribbon named A, B, C, D, and E were chosen, and all the resistances between the adjacent points (A and B, B and C, C and D, and D and E) of the sting ribbon were 0.5 mΩ. The back connect mode was the same as that in Figure 3. The I⁻ and V⁻ test leads of the I–V tester contact to the same point and test the I–V curve. This test was carried out five times, and the contact points were A, B, C, D, and E. The electrical parameters of each point were compared, and the comparison mode includes A versus B, B versus C, C versus D, and D versus E. The difference of , , and  (, , and ) for each comparison mode is shown in Table 1. The  for each comparison mode was approximate to the resistance between the adjacent points, which was 0.5 mΩ. So the rangeability of series resistance tested by the I–V tester was accurate. For each comparison mode,  mA, thus , which was consistent with (11). So  could be calculated with (12). The  of each comparison mode can be seen in Table 1. For each comparison mode, the absolute difference between  and  was less than 6%. Thus,  was approximate to , and the above conclusion was proved.

Figure 10 

Sketch of the module electrode structure and test mode used to analyze the influence of the series resistance variation on .

Table 1 

, , , and  between adjacent two points: , , and  were tested by I–V tester;  was calculated with (12).

4. Optimization of the Multibusbar Module Electrode

4.1. Relationship between the Optimized Wire Diameter and the Wire Number

The primary optimization of the MBB module electrode is in reducing the total power loss caused by the shading and the series resistance of the wires and fingers. MBB mainly reduces finger series resistance, and this decrement is less than 2 mΩ per cell; hence,  caused by the decrement in series resistance is equal to , which can be calculated with (12). Suppose the wire electrical resistivity was , the power loss caused by the shading and the resistance of the conductive belt is , according to (3) and (8); the  of a module whose cell area is equal to a full cell can be obtained by

 is the length of a full cell, which at present is about 156 mm.  is sheet resistance of Al-BSF. Al-BSF can share back current with the back wires. The Al-BSF sheet resistance of conventional cells was tested and was 20 mΩ. For different wire numbers, the wire diameters to achieve the lowest power losses were calculated, and the cell sizes were full, half, and one-third. The results are shown in Figure 11. The wire diameter could be reduced by decreasing the cell size or by adding more wires.

Figure 11 

Relationship between the optimized wire diameters and the wire numbers for hull-, half-, and one-third-cell designs.

4.2. Power Loss Analysis of Multibusbar Combined with Different Cell Sizes

The power loss caused by the finger resistance () can be defined as

 can be obtained by (4), and in (4),  is inversely proportional to the wet weight of the fingers. Braun et al. Lỗi giao diện: file 'snippets/shortcode-14). The results are shown in Figure 12. To reduce the  to less than 0.01 W per cell, the 80 mg finger wet weight requires at least 8 busbars, the 40 mg finger wet weight requires at least 11 busbars, and the 20 mg finger wet weight requires at least 15 busbars.

Figure 12 

Relationship between the number of the wires and the following power losses of a module whose cell area is equal to a full cell: resistance power losses of fingers with wet weights of 80 mg, 40 mg, and 20 mg per full cell; power losses of conductive belts with wire diameters of 100 μm, 200 μm, and 300 μm, and with cell sizes of full, half, and one-third.

The half-cell or one-third-cell designs will lead to a sharp decrease in the production rate of the soldering process and increase the module costs. However, the conductive belt technology can remove the soldering process. Therefore, the costs for half-cell or one-third-cell modules with conductive belt are acceptable.

In order to achieve an acceptable cell breakage rate, the wires cannot be too thick. For those cells with a conventional thickness of about 180 μm, the present acceptable wire diameter is no more than 200 μm. The conductive belt technology will be developed continuously to apply thicker wires. On the other hand, cells will become thinner, and the wires had better be finer to maintain the cell breakage rate. For full, half, and one-third cell designs, and for the maximum diameters of 100 μm, 200 μm, and 300 μm, the  of a module with cell area equal to a full cell was calculated, and the results are shown in Figure 12. As the wires become finer,  increases sharply, and the wire number at the lowest  also increases. If the maximum diameter is 200 μm or 300 μm, the lowest  of half-cell design has a great advantage compared to full-cell design. One-third-cell design can decrease  by less than 0.043 W compared to the half-cell design. One-third-cell design needs one more cell separation per cell compared with half-cell design, and the power loss caused by a cell separation is more than 0.02 W. On the other hand, one-third-cell design also needs a more production process. Thus, one-third-cell design is not so economical. However, if the wire diameter decreases to less than 100 μm,  would increase sharply, and one-third-cell would be a better choice.

Define . As calculated above, for the cell thickness and the conductive belts at current technologies, half cells combined with 200 μm-thick wire can achieve the lowest . Moreover, for the finger wet weights of 80 mg, 40 mg, and 20 mg, the  for different numbers of wires was calculated. The results are shown in Figure 13. The shading rate of the interconnection ribbons with a width of 1.2 mm was tested by the same method shown in Section 2.2.  for a conventional four-busbar one-full-cell module with a finger wet weight of 80 mg was calculated, and the result is also shown in Figure 13.

Figure 13 

The  of three MBBHC modules and a conventional module: (1) multibusbar modules with two half cells, respectively, the wire numbers range from 4 to 40, and the finger wet weights are 80 mg, 40 mg, and 20 mg; (2) a four-busbar module with a full cell, whose finger wet weight is 80 mg.

For full-cell and multibusbar structures, the most relevant contribution to  is that of wires, while the finger resistance has a minor impact Lỗi giao diện: file 'snippets/shortcode-. We can get the same conclusion from Figure 13. However, as the cell cut to 1/2 or 1/3,  will drop sharply. For the finger wet weights of 80 mg, 40 mg, and 20 mg, the wire numbers for the lowest  are 13, 14, and 16, and the differences between these lowest  are less than 0.01 W. Therefore, if the busbar number increases to more than 13, the 20 mg wet weight will barely decrease the module power output. As shown in Figure 13, compared with a conventional four-busbar module with a full cell, the  of a 16-busbar module with two half-cells can be decreased by at least 0.19 W and can save 75% of the front finger silver paste. In other words, compared with the conventional four-busbar module with 72 full cells, the multibusbar and half-cell (MBBHC) module, which has 16 busbars and 144 half cells, can increase  by 13.68 W.

In addition, the MBBHC structure achieved with conductive belts can further increase  in the following ways: (1) The silver paste on the front and back busbar areas is removed, which will reduce the surface recombination and improve the  of the cell Lỗi giao diện: file 'snippets/shortcode-μm by conventional screen printing, which will increase  by at least 0.05 W per cell.

5. Reliability Analysis of Multibusbar Modules Made with Conductive Belts

The materials applied in the modules except the cells and the conductive belts are the same as conventional modules. The cell metallization pattern will be optimized by applying conductive belt technology, but this may not reduce the cell reliability. Thus, the module reliability analysis should focus on the reliability of the conductive belt. As the conductive belts are encapsulated inside the module, the main relevant factors that may affect them are the module temperature and the incident ultraviolet, and the reliability tests of the conductive belts mainly include two aspects: (1) the wire contact resistance to fingers and Al-BSF and (2) the shading loss of the conductive belt.

Twenty conventional four-busbar monocrystalline solar cells were made into one-cell modules, respectively, by applying conductive belts, and the front and the back structure were the same as those in Figures 5(a) and 5(c). The wire contact resistance to fingers and Al-BSF were tested using the same method described in Section 2.3, and the results are shown in Figure 14. The following aging tests for the 20 modules were conducted: (1) ultraviolet aging test, which required 85°C and 200 W/m² ultraviolet power for 1000 hours, (2) thermal cycling test, which required temperature cycling between 85°C and −40°C for 200 cycles, and (3) damp heat test, which required 85°C and 85% relative humidity for 1000 hours. After aging tests, the wire contact resistance to fingers and to Al-BSF were tested again. As shown in Figure 14, after aging tests, the average wire contact resistance to fingers and Al-BSF were between 0.4 mΩ and 0.6 mΩ and were almost unchanged. The module series resistance included three parts: (1) the series resistance of the cell, (2) the wire contact resistance to fingers and Al-BSF, and (3) the series resistance of the wires. The wire resistance tested initially and after aging tests were also unchanged. Thus, the series resistance of the modules with conductive belts will not increase after aging tests.

Figure 14 

The wire contact resistances to fingers and Al-BSF, which were tested initially and after aging tests.

Another two groups of modules were made to test the degradations of the electrical properties caused by the aging tests. Each group had four modules, which were composed of four conventional half cells, respectively. The half cells were chosen from the same batch and were not used before this experiment.  and  of the half cells were tested and were nearly equal. In group 1, the half cells were serially connected by interconnection ribbons, and in group 2, the half cells were serially connected by conductive belts. The conductive belts had 12 wires, respectively, and the diameter of the wires was 200 μm. The other materials applied in the two groups of modules were the same.

The aging tests shown above were made to these modules again, and after aging tests, there were almost no changes in the appearances of the modules. The appearance and electroluminescence (EL) images of a second group module after aging tests are shown in Figures 15(a) and 15(b). As shown in Figure 15(b), the wires and the fingers kept a uniform contact effect after aging tests. Thus, the wires could collect the cell current at each contact point.

(b)

(a)

(b)

(a)

(b)

• 
  (a)

• 
  (b)

Figure 15 

The appearance (a) and EL image (b) of a module made with conductive belts and four half cells after aging tests.

Before and after aging tests, the I–V curves of the modules were tested. For the two groups, the average degradations of , , , ,  (, , , , and  were calculated, and the results are shown in Table 2. The degradations in the modules made with conductive belts were slightly less than in the conventional modules and were below the 5% maximum degradation prescribed by International Electrotechnical Commission (IEC) standards. The shading rate of the conductive belt may affect . The  degradation of group 2 was less than that of group 1 and was less than 5%. Thus, the shading loss of the conductive belt did not increase after aging tests, or its incensement was acceptable.

Table 2 

The average degradations of the module electric performance after aging tests: group 1, conventional four-busbar modules; group 2, modules made with conductive belts.

6. Conclusion and Outlook

In this paper, we prepared an interconnect electrode called conductive belt to achieve a multibusbar structure and the conductive belt can form ohmic contacts with its wires to the cell electrode. The practical shading area of the conductive belt tested was about 70% of the wire covering area. The wire contact resistance to fingers and Al-BSF was so small that it could be ignored. Thus, the conductive belts could contact with fingers and Al-BSF directly, and the front and back busbars could be removed. For fingers with different numbers of wires, we tested the finger series resistance and found the relationship between the series resistance variation and the maximum power output in two ways. For the full-cell, half-cell, and one-third-cell designs, and for the finger wet weights of 80 mg, 40 mg, and 20 mg per full cell, we optimized the diameter and the number of the wires and got the lowest power loss of the module electrode. Because the capacity reduction brought about by the half cell could be relieved by applying conductive belts, half cells combined with sixteen 200 μm thick wires could achieve the maximum profit and could save 75% of the finger wet weight. We made aging tests and found that the reliability of the MBBHC modules made with conductive belts could meet IEC standards.

We have carried out an outdoor test for MBBHC modules for three months and will continue the test for more than one year. MBBHC modules will be prepared with optimized cell electrode patterns, and their electrical parameters will be investigated in practice.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the financial support of the National Nature Science Foundation of China (Grant nos. 11404253 and 11404260).

https://www.hindawi.com/journals/ijp/2018/7630390/