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Design of a solar cell electrode for a shingled photovoltaic module application
Journal Pre-proofs Full Length Article Design of a solar cell electrode for a shingled photovoltaic module application Wonje Oh, Jisu Park, Chaehwan Jeong, Jinhong Park, Junsin Yi, Jaehyeong Lee PII: DOI: Reference:
S0169-4332(20)30176-8 https://doi.org/10.1016/j.apsusc.2020.145420 APSUSC 145420
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Applied Surface Science
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30 November 2018 27 November 2019 14 January 2020
Please cite this article as: W. Oh, J. Park, C. Jeong, J. Park, J. Yi, J. Lee, Design of a solar cell electrode for a shingled photovoltaic module application, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc. 2020.145420
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Design of a solar cell electrode for a shingled photovoltaic module application
Wonje Oha, Jisu Parka, Chaehwan Jeongb, Jinhong Parka, Junsin Yia, Jaehyeong Leea*
a
Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
b
Applied Optics & Energy Research Group, Korea Institute of Industrial Technology, Gwangju 61012, Republic of Korea
*E-mail: [email protected], tel: +82-31-299-4950, fax: +82-31-290-7170
Abstract1 New technologies to fabricate high-output power photovoltaic (PV) modules include a cell dividing and bonding technique. This technique divides and interconnects cells into a string arranged in series and in parallel to produce a module. Therefore, we designed a 3–6 dividing front electrode structure that is suitable for the shingled module. Thus, power loss was calculated based on the number of cell divisions and the number of fingers. The simulation results indicated that increases in the number of cells to be divided decreased the number of fingers exhibiting the maximum efficiency. The number of fingers optimized for division in to 5 cells was 128. Additionally, the power conversion efficiency was 17.346%, and this corresponded to the highest efficiency among various electrode structures for division from
1
Abbreviations: CTM, cell-to-module; ECA, electrically conductive adhesive; FF, fill factor; PV, photovoltaic; RTP, rapid thermal processing.
three to six solar cells. The optimized finger number for division into 3 cells was 171, and this corresponded to the lowest efficiency of 16.855%. The multi-crystalline silicon solar cells exhibiting a finger number of 100 were fabricated to compare with the simulation results. We analyzed the characteristics and obtained results that were nearly similar to those of the simulation. For application to a shingled module, a solar cell with an appropriate electrode structure was divided into 5 cells via the laser scribing system, subsequently bonded with an electrically conductive adhesive (ECA), and the characteristics were analyzed.
Keywords: solar cell, electrode pattern, shingled photovoltaic module, high power and density module, cell dividing and bonding, electrically conductive adhesive (ECA)
1. INTRODUCTION Photovoltaic (PV) systems composed of large-scale power plants are commonly used. However, the importance of distributed energy source technology is emphasized recently to secure stable energy through diversification of energy sources. In order to actively promote this, developed countries support the use of distributed energy source technology under various government initiatives [1]. When compared to large-scale PV systems, small-scale solar power generation or rooftop solar power generation systems exhibit a limited installation area. Specifically, rooftop solar systems require technologies that allow high power generation within the limited space of the roof area. A technology that produces high power in a limited area corresponds to a shingled PV module [2,3]. In the case of a general PV module, a metal ribbon is soldered on the busbar of a solar cell and connected to other cells [4,5]. Thus, the busbar corresponds to a shading area, thereby resulting in the loss of the light-receiving active area [6,7]. Additionally, stress and microcracks can occur due to local heat transfer and pressure
during soldering to the cell [8-13]. Current losses due to ribbon resistance can occur when the collected current moves through the ribbon [14,15]. Therefore, a cell-to-module (CTM) loss in a cell interconnection process is inevitable [16-19]. The shingled PV modules were fabricated via a dividing and bonding technology for solar cells. Firstly, the full size (typically 6 inches) cells are divided into several cell strips (usually 3 to 6 cell strips) using by a laser. The divided cell strips are bonded together to form a shingled string. The bonding process uses an electrically conductive adhesive (ECA) to connect the cell strips together. The shingled strings are interconnected through a metal ribbon to fabricate a high power and high density photovoltaic module. Therefore, the solar cell should exhibit the electrode structure suitable for dividing and bonding. A string created by a dividing and bonding technique corresponds to a busbar-less structure since the divided solar cell overlaps the bus bars. Therefore, the shadowing loss caused by the bus bar is eliminated, and the cell is arranged in the removed bus bar region, thereby generating a high-density string. Additionally, each cell is not connected for the ribbon soldering process, and thus it eliminates damage to the solar cell and reduces electrical loss resulting from the resistance of the ribbon [20,21]. Furthermore, there is no space between the cells, and thus it is possible to fabricate a module with high density such that a module exhibiting higher output than that of a conventional module is manufactured in the same area. There is no space between the cells, and thus it is possible to manufacture a highdensity module as a module with an output exceeding that of the existing module with the same area. In the study, we designed an electrode structure of a solar cell that can be divided and bonded. The number of fingers in the electrode structure was optimized in a 3 – 6 dividing front electrode structure. We also fabricated cells with divided patterns. Subsequently, we analyzed characteristics before and after dividing and bonding.
2. EXPERIMENT DETAILS 2.1. Design of electrode structure The shingled PV module differs from the general module manufacturing method. The module is fabricated by arranging strings fabricated by dividing and bonding techniques in series and parallel as shown in Figure 1. Thus, it must exhibit an electrode pattern suitable for the technology. The characteristics of the solar cell change is based on the electrode structure of the division pattern. Thus, there is a difference in the output of the string or module fabricated using it. We designed 3–6 divided electrode patterns as shown in Figures 2, 3, 4, and 5 and simulated the efficiency change of each based on the number of fingers. The red-dotted lines indicate lines cut through the laser scribing process. The busbar of the divided electrode structure exhibits a width of 1.5 mm and finger width of 50 μm. The positions of the cutting lines are different in the 3, 4, 5, and 6 divided electrode patterns, and thus the number and positions of the front and rear bus bars were designed as different from each other.
2.2. Model of solar cell electrode Generally, a conversion efficiency of solar cell is different based on the manner in which the electrode structure is designed. In the study, the efficiency change in the solar cells exhibiting various electrode patterns was calculated. Based on Meier’s study [22,23], a total electrical power loss of a solar cell is calculated by summing their losses due to various causes. The power loss is related to the current traveling through the diffused emitter, across the contact interface, along the grid fingers and bus bar, and through the base of the cell. These power loss factors appear as a series resistance component of the solar cell. Furthermore, shadows from
electrode patterns result in additional loss of solar cells [24]. The total power loss per unit area, Ptotal, is summarized in (1) as follows:
Ptotal = Pemitter + Pcontact + Pfinger + Pbus + Pbase + Pshading
(1),
where Pemitter denotes the power loss associated with the movement of the current or carrier through the resistive emitter layer, and the power loss resulting from the contact resistance component between the front electrode and surface is expressed as Pcontact. Specifically, Pfinger denotes the loss resulting from the finger of the front electrode, and Pbusbar denotes the loss due to the busbar. Additionally, the power loss of the device base is expressed as Pbase, and the power loss resulting from shadowing of the electrode pattern composed of the busbar and the finger is expressed as Pshading. Thus, the cell efficiency, effective, is estimated using by Equation (2) as follows:
effective = ((PL) - Ptotal) / PL
(2),
where PL denotes the power density of incident light and denotes the energy conversion efficiency of the ideal cell without any power loss. The input parameters to calculate the effective efficiency simulation are summarized in Table 1. The values of the resistivity and thickness of the multicrystalline silicon wafer are the specifications provided by the manufacturer. The sheet resistance of emitter after phosphorus diffusion doping was measured as 80 ohm/sq. The thickness of finger and busbar, busbar half width, finger width, ideal cell efficiency, and light-generated current density are common parameters for all division patterns. The finger length, half spacing, finger count, and unit cell area are input parameters that depend
on the number of cell division and fingers. In addition, the contact resistivity and the finger material resistivity were cited from the literature. [25-27].
2.3. Fabrication of solar cells with a divided electrode structure A screen printing process was used for metallization, and a 6-inch multicrystalline blue wafer without electrodes was used. A multicrystalline silicon solar cell with an electrode pattern for division was fabricated to verify the simulation results. The wafer corresponded to p-type, and boron was used as a dopant. The wafer thickness was 200 μm, and the sheet resistance was 80 ohm/sq. The fabrication steps of the wafer are shown in Figure 6. In the metallization step, the electrode pattern was printed on a wafer by using a mesh mask and a screen printer. The front electrode of the solar cell was dried at 265 °C for 30 s to remove the solvent after printing, and the rear electrode was also then processed in the same manner. Finally, the front and back electrodes were fired by using rapid thermal processing (RTP) equipment. The front and back paste were sintered through the process, and the electrode was formed via contact with the silicon surface [28-32]. Figure 7 shows the cell photograph after electrode printing. Thus, fabrication of solar cells with 3–6 divided electrode structures was completed.
2.4. Dividing and bonding process of a solar cell It is important to optimize the electrode structure of the dividing and bonding cell applicable to the shingled PV module, and thus it is necessary to check the characteristics of the process of dividing and bonding the cell by using a real electrode structure. Therefore, we divided and bonded the solar cell with the divided electrode structure to confirm the subsequent characteristic changes. First, the process of the division of a solar cell was performed via a combination of a laser scribing (Coherent, USA) and mechanical breaking. [33] The laser
power used for scribing corresponded to 10 W with a frequency of 50 kHz, a scan rate of 1300 mm/s, and repetition frequency of 30. Figure 8 shows the multicrystalline silicon solar cells divided into three to five cells. The divided cells were bonded by using shingling system equipment (Genesem Inc., South Korea). An electrically conductive adhesive (ECA) was applied to the front bus bar of the divided cell [34]. Subsequently, a curing process was performed at 150 °C for 5 s in air ambient. The solvent in the ECA evaporated and cured to bond the cells through the process [35-37]. Figure 9 shows the interconnection of two cells divided into five cells.
3. RESULTS AND DISCUSSION
3.1. Effects of the number of cell divisions and fingers in the electrode The total power loss that was caused by various sources and expressed in Eq. (1) was calculated. In the study, the number of cell divisions and fingers for a simulation varied from 3 to 6 and 75 to 250, respectively. Figure 10 shows the dependence of simulated efficiency on the number of cell divisions and fingers of the front electrode pattern. Regardless of the number of cells to be divided, the efficiency gradually increased with the number of finger, appearing the maximum values. However, further increase of the fingers decreased the cell efficiency. Among the various metallization patterns, the pattern for five cells division exhibited the highest efficiency of 17.411% at 128 fingers. The power loss (Pemitter in Eq. (1) caused by the emitter resistance decreases with respect to the number of fingers (n) since the carrier photogenerated at the active cell area flows through a shorter distance to each finger (b) as shown in Eq. (3). [22]
Pemitter (2 / 3) J L2nab3 Rs
(3)
In above equation, JL denotes light-generated current density, n denotes the finger number, a denotes finger length, and 2b denotes the spacing between two fingers including the finger width. The finger numbers exhibiting the maximum efficiency corresponded to 171 for threecell division, 142 for four-cell division, 128 for five-cell division, and 120 for six-cell division, respectively. However, further increases in the finger number inversely decreased the efficiency because photo-current loss due to the finger shading was more dominant when compared to Pemitter loss. As shown in Eq. (4) [22], increases in finger number decreases 2b value, thereby increasing Pshading. This is expressed as follows:
Pshading = PL 𝜂(𝑤𝑓 /2𝑏 + 𝑤𝑏 /𝑎)
(4)
Additionally, the optimum number of fingers decreased when the full cell (before division) was cut into smaller size cells. When the number of cells to be divided increased, the active area between the busbars became narrower, and thus the amount of photo-generated current to be collected by a finger in the area decreased since the busbar collected the current through the fingers. Therefore, the gap between the fingers was widened to collect smaller photo-current, thereby implying a decrease in the number of fingers. In order to verify the simulation results, multicrystalline silicon solar cells with various electrode patterns were fabricated, and their characteristics were investigated. Table 2 shows the simulated and measured solar cell efficiencies for various electrode patterns. The number of fingers was fixed as 100 for all samples. It was observed that the two values were almost the same irrespective of the electrode pattern type to be divided. Conversely, the efficiency of the
cell with the optimal number of fingers improved with respect to the number of cells to be divided, thereby exhibiting a maximum efficiency at a division number corresponding to five. Increases in the number of cells to be divided increases the number of busbars, and thus the spacing between the busbars becomes narrower and the finger length to the busbars also decreases as shown in Fig. 8. The amount of power loss related to the current flowing through the finger per unit area is given in Eq. (5) [22] as follows:
Pfinger (2 / 3)( J L2a2b f ) /(t f wf )
(5),
where 𝑎 denotes the length of the finger, 2b denotes the width of current collection area assigned to a finger, 𝜌𝑓 denotes finger material resistivity, and 𝑡𝑓 and 𝑤𝑓 denote the thickness and width of the finger, respectively. Using Eq. (3), the power loss per unit area due to the resistance of the finger, Pfinger, at the optimum number of fingers for each pattern is as follows: 1.466 mW/cm2 for 3-cell division, 0.824 mW/cm2 for 4-cell division, 0.528 mW/cm2 for 5-cell division, and 0.366 mW/cm2 for 6-cell division. Therefore, Pfinger decreased with respect to the number of cells to be divided. Additionally, increases in the number of cell division increased the number of busbars in the electrode pattern. When the number of bus bars increased, the current collected through the fingers was dispersed by various bus bars, and thus reduced the amount of current flowing through one busbar and decreased the amount of the power loss in the busbar (Pbusbar).
Specifically, Pbusbar is expressed in Eq. (6) [22] as follows:
Pbusbar (1 / 3)( J L2an2b2 b ) /(t f wb )
(6).
The calculated values of Pbusbar corresponded to 0.149 mW/cm2 for the 3-cell division pattern, 0.112 mW/cm2 for the 4-cell division pattern, 0.090 mW/cm2 for the 5-cell division pattern, and 0.075 mW/cm2 for the 6-cell division pattern. As shown in the aforementioned results, the power loss due to the busbar decreased with respect to the number of busbars or the cell division. However, further increases in the number of busbars and/or cells increased shading loss, thereby exhibiting smaller conversion efficiency. The power loss related to shading is calculated using by Eq. (4) as follows: 1.2339 mW/cm2 in the three-cell division pattern, 1.4287 mW/cm2 in the four-cell division pattern, 1.6235 mW/cm2 in the five-cell division pattern, and 1.8183 mW/cm2 in the six-cell division pattern.
3.2 Characteristics of the solar cell after division and interconnection Table 3 shows the characteristics before and after the division of the solar cells exhibiting various electrode patterns. The open-circuit voltage (Voc) and fill factor (FF) decreased after solar cell division irrespective of the type of electrode pattern. The cell area around the scribing line was thermally damaged since the scribing used a laser. Figure 11 shows the electroluminescence image of the divided cells. The green color denotes the area where the electroluminescence phenomenon is less, and it indicates that the area is damaged by the laser. The thermal damage of the front passivation around the scribing line region increased the reverse saturation current (I0), which degraded the cell characteristics [38]. Therefore, decreases in Voc and FF are attributed to increases in I0 as given in Eqs. (7) and (8) as follows:
Voc
nkT I L ln 1 q I0
FF
voc ln( voc 0.72) q , where voc Voc voc 1 nkT
(7)
(8)
In the above Eqs., n denotes the diode ideal or quality factor, k denotes the Boltzmann constant, q denotes the electron charge, and IL denotes the light-generated current. However, the efficiency after division did not decrease despite decreases in Voc and FF after division since the short-circuit current density (Jsc) increased. The Voc of the junction cell almost doubled since the two divided cells were connected in series. Conversely, Jsc was more than half of that before bonding. The active area ratio for current generation after interconnection exceeded that of the divided cells since the cells were connected by overlapping the busbars, thereby exhibiting a busbar-less structure. The role of a busbar in the conventional electrode pattern involves collecting the current flowing through the fingers although the busbar is a major cause of the shading loss. However, in the case of the interconnected cell, the busbar of each divide cell disappeared, and thus that the shading loss due to the busbar was eliminated. Therefore, the efficiency of the solar cell after bonding improved although the FF deteriorated due to increases in the series resistance (Rseries). The Rseries was 26.315 mΩ before bonding, although it increased to 61.657 mΩ after interconnection, and was more than twice as high as before bonding. When the divided cells were bonded with ECA, a new resistance component should be added into the Rseries of the solar cell since the cells were connected in series. The resistance due to ECA was extracted from the measured Rseries values of divided and boned cells and corresponded to 9.027 mΩ. Hence, FF decreased. Table 4 compares the characteristics before and after the bonding of two cell strips. The full size (6 inches) cells were divided into the cells divided into five cell strips. Two cell strips were bonded together using by an ECA and the characteristics of the five bonded cells were averaged. After ECA bonding, the efficiency increased by 0.2216%, although the FF decreased slightly due to ECA resistance. When two cell strips were interconnected in series, the busbar of one cell strip overlapped with the rear Ag pad of the other cell strip, disappearing the front busbar,
as shown in Fig. 9. This leads to a larger active area for light absorbing, producing larger photo current density. Double value of Jsc after bonding was larger than that of cell strip before bonding.
4. CONCLUSION This study involved the optimization of electrode design for dividing and bonding applicable to a shingled PV module. The metallization patterns, particularly the finger numbers, depended on the number of the cell strips to be divided. The metallization pattern for five cells division exhibited the highest efficiency and the optimum finger number was 128. In addition, the results of the simulation were similar to experimental values of the multicrystalline silicon solar cell with cell division patterns. The efficiency of the divided solar cell was almost the same or slightly exceeded that before cutting and increased after interconnection. In this study, only two cells were combined and analyzed. However, if more cells are bonded to form a PV module, the efficiency and output are expected to increase further, as shading losses are eliminated over a wider area.
Acknowledgements: The study was supported by the Korean Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea. (No. 20163030014070)
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List of figure captions:
Figure 1. Concept of cell division and bonding technology for the shingled PV module. Figure 2. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into three cells. The red dotted lines denote the scribing and breaking lines by the laser singulation system. Figure 3. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into four cells. Figure 4. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into five cells. Figure 5. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into six cells. Figure 6. Fabrication process of a multi-crystalline silicon solar cell. Figure 7. Photograph of multicrystalline silicon solar cells with the electrode structures of cell division and interconnection: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division. Figure 8. Images of a multicrystalline silicon solar cell cut by a laser: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division. Figure 9. Interconnection of two divided cells. The divided cells used here are cut into five from the full size (6 inches) of the multicrystalline silicon solar cell. Figure 10. Dependence of simulated cell efficiency on the number of cell divisions and fingers in the front electrode pattern. Figure 11. Electroluminescence analysis for the divided cell. The full cell with a size of 6 inches is cut into 5 smaller cells.
Figure 1. Concept of cell division and bonding technology for the shingled PV module.
(a)
(b)
Figure 2. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into three cells. The red dotted lines denote the scribing and breaking lines by the laser singulation system.
(a)
(b)
Figure 3. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into four cells.
(a)
(b)
Figure 4. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into five cells.
(a)
(b)
Figure 5. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into six cells.
SDR/Texture Doping PSG Removal SiNx Deposition and Removal Edge-isolation Anti Reflection Coating Figure 6. Fabrication process of a multi-crystalline silicon solar cell.
(a)
(b)
(c)
(d)
Figure 7. Photographs of multicrystalline silicon solar cells with the electrode structures of cell division and interconnection: (1) three-cell division, (2) four-cell division, (3) five-cell division, and (4) six-cell division.
(a)
(b)
(c)
(d)
Figure 8. Images of a multicrystalline silicon solar cell cut by a laser: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division.
Figure 9. Interconnection of two divided cells. The divided cells used here are cut into five from the full size (6 inches) of the multicrystalline silicon solar cell.
Figure 10. Dependence of simulated cell efficiency on the number of cell divisions and fingers in the front electrode pattern.
Figure 11. Electroluminescence analysis for the divided cell. The full cell of with a size of 6 inches is cut into 5 smaller cells.
Table 1. Simulation input parameters
Symbol Substance Value Unit Rs Emitter sheet resistance 80 /sq.2 c Contact resistivity 0.003 cm f Finger material resistivity 0.00006 mm tf Finger thickness 0.002 cm tb Busbar thickness 0.05 cm b Wafer resistivity 1.5 cm l 1) Wafer thickness 0.02 cm Ideal cell efficiency 20 % a Finger length2) variable cm b Half spacing variable cm n Finger count variable ea A Unit cell area variable cm2 wf Finger width 0.005 cm wb Half bus bar width 0.15 cm JL Light-generated current density 42.5 mA/cm22 PL Incident light power density 100 mW/cm 1) JL, denotes the ideal efficiency considering the recombination loss. In the study, we assumed that this corresponds to 20% for the multicrystalline silicon solar cell. 2) b corresponds to the half spacing between fingers including finger width.
Table 2. Comparison of simulated and measured solar cell efficiency for various electrode patterns. The number of fingers is fixed as 100 for all samples.
3-Division 4-Division 5 -Division 6-Division Eff_Simulated (%) 16.73 17.22 17.35 17.33 Eff_measured (%) 16.85 17.20 17.34 17.22 Voc (V) 0.6383 0.6372 0.6427 0.6400 FF (%) 76.7390 78.8716 79.2833 80.0228 Eff (%) 16.8549 17.1982 17.3427 17.2145 Rs (mΩ) 4.5239 5.1195 5.0283 4.6686 Rsh (Ω) 44.0783 36.4341 68.7042 112.1476
Table 3. Comparison of solar cell characteristics before and after division.
Jsc (A/cm2) Voc (V) FF (%) Eff (%) Rs (m) Rsh (Ω)
3-division pattern 4-division pattern 5-division pattern 6-division pattern before after before after before after before after division division division division division division division division 34.399 34.797 34.202 34.630 34.036 34.418 33.610 34.175 0.6383 76.739 16.855 4.524 44.08
0.6358 76.193 16.855 19.996 129.56
0.6372 78.872 17.198 5.120 36.43
0.6331 78.646 17.098 21.925 78.65
0.6427 79.283 17.343 5.028 68.70
0.6395 78.738 17.330 26.883 525.83
0.6400 80.023 17.215 4.669 112.15
Table 4 Comparison of cell characteristics before and after bonding of two cell strips.
5- divided cell Before bonding After Bonding Cell size 48.67 94.38 Isc 16.67043 16.4304 Jsc (A/cm2) 34.3204 17.4089 Voc (V) 0.64008 1.28222 FF (%) 79.2467 78.9814 Eff (%) 17.4092 17.6308
0.6365 79.583 17.312 30.495 895.83
Laser
Finger
Busbar
Highlights Design of a divided electrode structure suitable for a shingled PV module. Simulation of the designed divided electrode pattern. Fabrication of real cells with the designed divided electrode pattern Comparison and analysis of characteristics before and after dividing and bonding.
S0169-4332(20)30176-8 https://doi.org/10.1016/j.apsusc.2020.145420 APSUSC 145420
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
30 November 2018 27 November 2019 14 January 2020
Please cite this article as: W. Oh, J. Park, C. Jeong, J. Park, J. Yi, J. Lee, Design of a solar cell electrode for a shingled photovoltaic module application, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc. 2020.145420
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© 2020 Published by Elsevier B.V.
Design of a solar cell electrode for a shingled photovoltaic module application
Wonje Oha, Jisu Parka, Chaehwan Jeongb, Jinhong Parka, Junsin Yia, Jaehyeong Leea*
a
Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
b
Applied Optics & Energy Research Group, Korea Institute of Industrial Technology, Gwangju 61012, Republic of Korea
*E-mail: [email protected], tel: +82-31-299-4950, fax: +82-31-290-7170
Abstract1 New technologies to fabricate high-output power photovoltaic (PV) modules include a cell dividing and bonding technique. This technique divides and interconnects cells into a string arranged in series and in parallel to produce a module. Therefore, we designed a 3–6 dividing front electrode structure that is suitable for the shingled module. Thus, power loss was calculated based on the number of cell divisions and the number of fingers. The simulation results indicated that increases in the number of cells to be divided decreased the number of fingers exhibiting the maximum efficiency. The number of fingers optimized for division in to 5 cells was 128. Additionally, the power conversion efficiency was 17.346%, and this corresponded to the highest efficiency among various electrode structures for division from
1
Abbreviations: CTM, cell-to-module; ECA, electrically conductive adhesive; FF, fill factor; PV, photovoltaic; RTP, rapid thermal processing.
three to six solar cells. The optimized finger number for division into 3 cells was 171, and this corresponded to the lowest efficiency of 16.855%. The multi-crystalline silicon solar cells exhibiting a finger number of 100 were fabricated to compare with the simulation results. We analyzed the characteristics and obtained results that were nearly similar to those of the simulation. For application to a shingled module, a solar cell with an appropriate electrode structure was divided into 5 cells via the laser scribing system, subsequently bonded with an electrically conductive adhesive (ECA), and the characteristics were analyzed.
Keywords: solar cell, electrode pattern, shingled photovoltaic module, high power and density module, cell dividing and bonding, electrically conductive adhesive (ECA)
1. INTRODUCTION Photovoltaic (PV) systems composed of large-scale power plants are commonly used. However, the importance of distributed energy source technology is emphasized recently to secure stable energy through diversification of energy sources. In order to actively promote this, developed countries support the use of distributed energy source technology under various government initiatives [1]. When compared to large-scale PV systems, small-scale solar power generation or rooftop solar power generation systems exhibit a limited installation area. Specifically, rooftop solar systems require technologies that allow high power generation within the limited space of the roof area. A technology that produces high power in a limited area corresponds to a shingled PV module [2,3]. In the case of a general PV module, a metal ribbon is soldered on the busbar of a solar cell and connected to other cells [4,5]. Thus, the busbar corresponds to a shading area, thereby resulting in the loss of the light-receiving active area [6,7]. Additionally, stress and microcracks can occur due to local heat transfer and pressure
during soldering to the cell [8-13]. Current losses due to ribbon resistance can occur when the collected current moves through the ribbon [14,15]. Therefore, a cell-to-module (CTM) loss in a cell interconnection process is inevitable [16-19]. The shingled PV modules were fabricated via a dividing and bonding technology for solar cells. Firstly, the full size (typically 6 inches) cells are divided into several cell strips (usually 3 to 6 cell strips) using by a laser. The divided cell strips are bonded together to form a shingled string. The bonding process uses an electrically conductive adhesive (ECA) to connect the cell strips together. The shingled strings are interconnected through a metal ribbon to fabricate a high power and high density photovoltaic module. Therefore, the solar cell should exhibit the electrode structure suitable for dividing and bonding. A string created by a dividing and bonding technique corresponds to a busbar-less structure since the divided solar cell overlaps the bus bars. Therefore, the shadowing loss caused by the bus bar is eliminated, and the cell is arranged in the removed bus bar region, thereby generating a high-density string. Additionally, each cell is not connected for the ribbon soldering process, and thus it eliminates damage to the solar cell and reduces electrical loss resulting from the resistance of the ribbon [20,21]. Furthermore, there is no space between the cells, and thus it is possible to fabricate a module with high density such that a module exhibiting higher output than that of a conventional module is manufactured in the same area. There is no space between the cells, and thus it is possible to manufacture a highdensity module as a module with an output exceeding that of the existing module with the same area. In the study, we designed an electrode structure of a solar cell that can be divided and bonded. The number of fingers in the electrode structure was optimized in a 3 – 6 dividing front electrode structure. We also fabricated cells with divided patterns. Subsequently, we analyzed characteristics before and after dividing and bonding.
2. EXPERIMENT DETAILS 2.1. Design of electrode structure The shingled PV module differs from the general module manufacturing method. The module is fabricated by arranging strings fabricated by dividing and bonding techniques in series and parallel as shown in Figure 1. Thus, it must exhibit an electrode pattern suitable for the technology. The characteristics of the solar cell change is based on the electrode structure of the division pattern. Thus, there is a difference in the output of the string or module fabricated using it. We designed 3–6 divided electrode patterns as shown in Figures 2, 3, 4, and 5 and simulated the efficiency change of each based on the number of fingers. The red-dotted lines indicate lines cut through the laser scribing process. The busbar of the divided electrode structure exhibits a width of 1.5 mm and finger width of 50 μm. The positions of the cutting lines are different in the 3, 4, 5, and 6 divided electrode patterns, and thus the number and positions of the front and rear bus bars were designed as different from each other.
2.2. Model of solar cell electrode Generally, a conversion efficiency of solar cell is different based on the manner in which the electrode structure is designed. In the study, the efficiency change in the solar cells exhibiting various electrode patterns was calculated. Based on Meier’s study [22,23], a total electrical power loss of a solar cell is calculated by summing their losses due to various causes. The power loss is related to the current traveling through the diffused emitter, across the contact interface, along the grid fingers and bus bar, and through the base of the cell. These power loss factors appear as a series resistance component of the solar cell. Furthermore, shadows from
electrode patterns result in additional loss of solar cells [24]. The total power loss per unit area, Ptotal, is summarized in (1) as follows:
Ptotal = Pemitter + Pcontact + Pfinger + Pbus + Pbase + Pshading
(1),
where Pemitter denotes the power loss associated with the movement of the current or carrier through the resistive emitter layer, and the power loss resulting from the contact resistance component between the front electrode and surface is expressed as Pcontact. Specifically, Pfinger denotes the loss resulting from the finger of the front electrode, and Pbusbar denotes the loss due to the busbar. Additionally, the power loss of the device base is expressed as Pbase, and the power loss resulting from shadowing of the electrode pattern composed of the busbar and the finger is expressed as Pshading. Thus, the cell efficiency, effective, is estimated using by Equation (2) as follows:
effective = ((PL) - Ptotal) / PL
(2),
where PL denotes the power density of incident light and denotes the energy conversion efficiency of the ideal cell without any power loss. The input parameters to calculate the effective efficiency simulation are summarized in Table 1. The values of the resistivity and thickness of the multicrystalline silicon wafer are the specifications provided by the manufacturer. The sheet resistance of emitter after phosphorus diffusion doping was measured as 80 ohm/sq. The thickness of finger and busbar, busbar half width, finger width, ideal cell efficiency, and light-generated current density are common parameters for all division patterns. The finger length, half spacing, finger count, and unit cell area are input parameters that depend
on the number of cell division and fingers. In addition, the contact resistivity and the finger material resistivity were cited from the literature. [25-27].
2.3. Fabrication of solar cells with a divided electrode structure A screen printing process was used for metallization, and a 6-inch multicrystalline blue wafer without electrodes was used. A multicrystalline silicon solar cell with an electrode pattern for division was fabricated to verify the simulation results. The wafer corresponded to p-type, and boron was used as a dopant. The wafer thickness was 200 μm, and the sheet resistance was 80 ohm/sq. The fabrication steps of the wafer are shown in Figure 6. In the metallization step, the electrode pattern was printed on a wafer by using a mesh mask and a screen printer. The front electrode of the solar cell was dried at 265 °C for 30 s to remove the solvent after printing, and the rear electrode was also then processed in the same manner. Finally, the front and back electrodes were fired by using rapid thermal processing (RTP) equipment. The front and back paste were sintered through the process, and the electrode was formed via contact with the silicon surface [28-32]. Figure 7 shows the cell photograph after electrode printing. Thus, fabrication of solar cells with 3–6 divided electrode structures was completed.
2.4. Dividing and bonding process of a solar cell It is important to optimize the electrode structure of the dividing and bonding cell applicable to the shingled PV module, and thus it is necessary to check the characteristics of the process of dividing and bonding the cell by using a real electrode structure. Therefore, we divided and bonded the solar cell with the divided electrode structure to confirm the subsequent characteristic changes. First, the process of the division of a solar cell was performed via a combination of a laser scribing (Coherent, USA) and mechanical breaking. [33] The laser
power used for scribing corresponded to 10 W with a frequency of 50 kHz, a scan rate of 1300 mm/s, and repetition frequency of 30. Figure 8 shows the multicrystalline silicon solar cells divided into three to five cells. The divided cells were bonded by using shingling system equipment (Genesem Inc., South Korea). An electrically conductive adhesive (ECA) was applied to the front bus bar of the divided cell [34]. Subsequently, a curing process was performed at 150 °C for 5 s in air ambient. The solvent in the ECA evaporated and cured to bond the cells through the process [35-37]. Figure 9 shows the interconnection of two cells divided into five cells.
3. RESULTS AND DISCUSSION
3.1. Effects of the number of cell divisions and fingers in the electrode The total power loss that was caused by various sources and expressed in Eq. (1) was calculated. In the study, the number of cell divisions and fingers for a simulation varied from 3 to 6 and 75 to 250, respectively. Figure 10 shows the dependence of simulated efficiency on the number of cell divisions and fingers of the front electrode pattern. Regardless of the number of cells to be divided, the efficiency gradually increased with the number of finger, appearing the maximum values. However, further increase of the fingers decreased the cell efficiency. Among the various metallization patterns, the pattern for five cells division exhibited the highest efficiency of 17.411% at 128 fingers. The power loss (Pemitter in Eq. (1) caused by the emitter resistance decreases with respect to the number of fingers (n) since the carrier photogenerated at the active cell area flows through a shorter distance to each finger (b) as shown in Eq. (3). [22]
Pemitter (2 / 3) J L2nab3 Rs
(3)
In above equation, JL denotes light-generated current density, n denotes the finger number, a denotes finger length, and 2b denotes the spacing between two fingers including the finger width. The finger numbers exhibiting the maximum efficiency corresponded to 171 for threecell division, 142 for four-cell division, 128 for five-cell division, and 120 for six-cell division, respectively. However, further increases in the finger number inversely decreased the efficiency because photo-current loss due to the finger shading was more dominant when compared to Pemitter loss. As shown in Eq. (4) [22], increases in finger number decreases 2b value, thereby increasing Pshading. This is expressed as follows:
Pshading = PL 𝜂(𝑤𝑓 /2𝑏 + 𝑤𝑏 /𝑎)
(4)
Additionally, the optimum number of fingers decreased when the full cell (before division) was cut into smaller size cells. When the number of cells to be divided increased, the active area between the busbars became narrower, and thus the amount of photo-generated current to be collected by a finger in the area decreased since the busbar collected the current through the fingers. Therefore, the gap between the fingers was widened to collect smaller photo-current, thereby implying a decrease in the number of fingers. In order to verify the simulation results, multicrystalline silicon solar cells with various electrode patterns were fabricated, and their characteristics were investigated. Table 2 shows the simulated and measured solar cell efficiencies for various electrode patterns. The number of fingers was fixed as 100 for all samples. It was observed that the two values were almost the same irrespective of the electrode pattern type to be divided. Conversely, the efficiency of the
cell with the optimal number of fingers improved with respect to the number of cells to be divided, thereby exhibiting a maximum efficiency at a division number corresponding to five. Increases in the number of cells to be divided increases the number of busbars, and thus the spacing between the busbars becomes narrower and the finger length to the busbars also decreases as shown in Fig. 8. The amount of power loss related to the current flowing through the finger per unit area is given in Eq. (5) [22] as follows:
Pfinger (2 / 3)( J L2a2b f ) /(t f wf )
(5),
where 𝑎 denotes the length of the finger, 2b denotes the width of current collection area assigned to a finger, 𝜌𝑓 denotes finger material resistivity, and 𝑡𝑓 and 𝑤𝑓 denote the thickness and width of the finger, respectively. Using Eq. (3), the power loss per unit area due to the resistance of the finger, Pfinger, at the optimum number of fingers for each pattern is as follows: 1.466 mW/cm2 for 3-cell division, 0.824 mW/cm2 for 4-cell division, 0.528 mW/cm2 for 5-cell division, and 0.366 mW/cm2 for 6-cell division. Therefore, Pfinger decreased with respect to the number of cells to be divided. Additionally, increases in the number of cell division increased the number of busbars in the electrode pattern. When the number of bus bars increased, the current collected through the fingers was dispersed by various bus bars, and thus reduced the amount of current flowing through one busbar and decreased the amount of the power loss in the busbar (Pbusbar).
Specifically, Pbusbar is expressed in Eq. (6) [22] as follows:
Pbusbar (1 / 3)( J L2an2b2 b ) /(t f wb )
(6).
The calculated values of Pbusbar corresponded to 0.149 mW/cm2 for the 3-cell division pattern, 0.112 mW/cm2 for the 4-cell division pattern, 0.090 mW/cm2 for the 5-cell division pattern, and 0.075 mW/cm2 for the 6-cell division pattern. As shown in the aforementioned results, the power loss due to the busbar decreased with respect to the number of busbars or the cell division. However, further increases in the number of busbars and/or cells increased shading loss, thereby exhibiting smaller conversion efficiency. The power loss related to shading is calculated using by Eq. (4) as follows: 1.2339 mW/cm2 in the three-cell division pattern, 1.4287 mW/cm2 in the four-cell division pattern, 1.6235 mW/cm2 in the five-cell division pattern, and 1.8183 mW/cm2 in the six-cell division pattern.
3.2 Characteristics of the solar cell after division and interconnection Table 3 shows the characteristics before and after the division of the solar cells exhibiting various electrode patterns. The open-circuit voltage (Voc) and fill factor (FF) decreased after solar cell division irrespective of the type of electrode pattern. The cell area around the scribing line was thermally damaged since the scribing used a laser. Figure 11 shows the electroluminescence image of the divided cells. The green color denotes the area where the electroluminescence phenomenon is less, and it indicates that the area is damaged by the laser. The thermal damage of the front passivation around the scribing line region increased the reverse saturation current (I0), which degraded the cell characteristics [38]. Therefore, decreases in Voc and FF are attributed to increases in I0 as given in Eqs. (7) and (8) as follows:
Voc
nkT I L ln 1 q I0
FF
voc ln( voc 0.72) q , where voc Voc voc 1 nkT
(7)
(8)
In the above Eqs., n denotes the diode ideal or quality factor, k denotes the Boltzmann constant, q denotes the electron charge, and IL denotes the light-generated current. However, the efficiency after division did not decrease despite decreases in Voc and FF after division since the short-circuit current density (Jsc) increased. The Voc of the junction cell almost doubled since the two divided cells were connected in series. Conversely, Jsc was more than half of that before bonding. The active area ratio for current generation after interconnection exceeded that of the divided cells since the cells were connected by overlapping the busbars, thereby exhibiting a busbar-less structure. The role of a busbar in the conventional electrode pattern involves collecting the current flowing through the fingers although the busbar is a major cause of the shading loss. However, in the case of the interconnected cell, the busbar of each divide cell disappeared, and thus that the shading loss due to the busbar was eliminated. Therefore, the efficiency of the solar cell after bonding improved although the FF deteriorated due to increases in the series resistance (Rseries). The Rseries was 26.315 mΩ before bonding, although it increased to 61.657 mΩ after interconnection, and was more than twice as high as before bonding. When the divided cells were bonded with ECA, a new resistance component should be added into the Rseries of the solar cell since the cells were connected in series. The resistance due to ECA was extracted from the measured Rseries values of divided and boned cells and corresponded to 9.027 mΩ. Hence, FF decreased. Table 4 compares the characteristics before and after the bonding of two cell strips. The full size (6 inches) cells were divided into the cells divided into five cell strips. Two cell strips were bonded together using by an ECA and the characteristics of the five bonded cells were averaged. After ECA bonding, the efficiency increased by 0.2216%, although the FF decreased slightly due to ECA resistance. When two cell strips were interconnected in series, the busbar of one cell strip overlapped with the rear Ag pad of the other cell strip, disappearing the front busbar,
as shown in Fig. 9. This leads to a larger active area for light absorbing, producing larger photo current density. Double value of Jsc after bonding was larger than that of cell strip before bonding.
4. CONCLUSION This study involved the optimization of electrode design for dividing and bonding applicable to a shingled PV module. The metallization patterns, particularly the finger numbers, depended on the number of the cell strips to be divided. The metallization pattern for five cells division exhibited the highest efficiency and the optimum finger number was 128. In addition, the results of the simulation were similar to experimental values of the multicrystalline silicon solar cell with cell division patterns. The efficiency of the divided solar cell was almost the same or slightly exceeded that before cutting and increased after interconnection. In this study, only two cells were combined and analyzed. However, if more cells are bonded to form a PV module, the efficiency and output are expected to increase further, as shading losses are eliminated over a wider area.
Acknowledgements: The study was supported by the Korean Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea. (No. 20163030014070)
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List of figure captions:
Figure 1. Concept of cell division and bonding technology for the shingled PV module. Figure 2. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into three cells. The red dotted lines denote the scribing and breaking lines by the laser singulation system. Figure 3. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into four cells. Figure 4. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into five cells. Figure 5. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into six cells. Figure 6. Fabrication process of a multi-crystalline silicon solar cell. Figure 7. Photograph of multicrystalline silicon solar cells with the electrode structures of cell division and interconnection: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division. Figure 8. Images of a multicrystalline silicon solar cell cut by a laser: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division. Figure 9. Interconnection of two divided cells. The divided cells used here are cut into five from the full size (6 inches) of the multicrystalline silicon solar cell. Figure 10. Dependence of simulated cell efficiency on the number of cell divisions and fingers in the front electrode pattern. Figure 11. Electroluminescence analysis for the divided cell. The full cell with a size of 6 inches is cut into 5 smaller cells.
Figure 1. Concept of cell division and bonding technology for the shingled PV module.
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Figure 2. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into three cells. The red dotted lines denote the scribing and breaking lines by the laser singulation system.
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Figure 3. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into four cells.
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Figure 4. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into five cells.
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Figure 5. Front (a) and rear (b) electrode patterns of a multicrystalline silicon solar cell for division into six cells.
SDR/Texture Doping PSG Removal SiNx Deposition and Removal Edge-isolation Anti Reflection Coating Figure 6. Fabrication process of a multi-crystalline silicon solar cell.
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Figure 7. Photographs of multicrystalline silicon solar cells with the electrode structures of cell division and interconnection: (1) three-cell division, (2) four-cell division, (3) five-cell division, and (4) six-cell division.
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Figure 8. Images of a multicrystalline silicon solar cell cut by a laser: (a) three-cell division, (b) four-cell division, (c) five-cell division, and (d) six-cell division.
Figure 9. Interconnection of two divided cells. The divided cells used here are cut into five from the full size (6 inches) of the multicrystalline silicon solar cell.
Figure 10. Dependence of simulated cell efficiency on the number of cell divisions and fingers in the front electrode pattern.
Figure 11. Electroluminescence analysis for the divided cell. The full cell of with a size of 6 inches is cut into 5 smaller cells.
Table 1. Simulation input parameters
Symbol Substance Value Unit Rs Emitter sheet resistance 80 /sq.2 c Contact resistivity 0.003 cm f Finger material resistivity 0.00006 mm tf Finger thickness 0.002 cm tb Busbar thickness 0.05 cm b Wafer resistivity 1.5 cm l 1) Wafer thickness 0.02 cm Ideal cell efficiency 20 % a Finger length2) variable cm b Half spacing variable cm n Finger count variable ea A Unit cell area variable cm2 wf Finger width 0.005 cm wb Half bus bar width 0.15 cm JL Light-generated current density 42.5 mA/cm22 PL Incident light power density 100 mW/cm 1) JL, denotes the ideal efficiency considering the recombination loss. In the study, we assumed that this corresponds to 20% for the multicrystalline silicon solar cell. 2) b corresponds to the half spacing between fingers including finger width.
Table 2. Comparison of simulated and measured solar cell efficiency for various electrode patterns. The number of fingers is fixed as 100 for all samples.
3-Division 4-Division 5 -Division 6-Division Eff_Simulated (%) 16.73 17.22 17.35 17.33 Eff_measured (%) 16.85 17.20 17.34 17.22 Voc (V) 0.6383 0.6372 0.6427 0.6400 FF (%) 76.7390 78.8716 79.2833 80.0228 Eff (%) 16.8549 17.1982 17.3427 17.2145 Rs (mΩ) 4.5239 5.1195 5.0283 4.6686 Rsh (Ω) 44.0783 36.4341 68.7042 112.1476
Table 3. Comparison of solar cell characteristics before and after division.
Jsc (A/cm2) Voc (V) FF (%) Eff (%) Rs (m) Rsh (Ω)
3-division pattern 4-division pattern 5-division pattern 6-division pattern before after before after before after before after division division division division division division division division 34.399 34.797 34.202 34.630 34.036 34.418 33.610 34.175 0.6383 76.739 16.855 4.524 44.08
0.6358 76.193 16.855 19.996 129.56
0.6372 78.872 17.198 5.120 36.43
0.6331 78.646 17.098 21.925 78.65
0.6427 79.283 17.343 5.028 68.70
0.6395 78.738 17.330 26.883 525.83
0.6400 80.023 17.215 4.669 112.15
Table 4 Comparison of cell characteristics before and after bonding of two cell strips.
5- divided cell Before bonding After Bonding Cell size 48.67 94.38 Isc 16.67043 16.4304 Jsc (A/cm2) 34.3204 17.4089 Voc (V) 0.64008 1.28222 FF (%) 79.2467 78.9814 Eff (%) 17.4092 17.6308
0.6365 79.583 17.312 30.495 895.83
Laser
Finger
Busbar
Highlights Design of a divided electrode structure suitable for a shingled PV module. Simulation of the designed divided electrode pattern. Fabrication of real cells with the designed divided electrode pattern Comparison and analysis of characteristics before and after dividing and bonding.