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Metallization of crystalline silicon solar cells for shingled photovoltaic module application
Solar Energy 195 (2020) 527–535
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Metallization of crystalline silicon solar cells for shingled photovoltaic module application
T
Wonje Oha, Jisu Parka, Sima Dimitrijevb, Eung Kwon Kimc, Yong Seob Parkd, Jaehyeong Leea,
⁎
a
Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Griffith School of Engineering, Queensland Micro- and Nanotechnology Centre, Queensland 4111, Australia c Digital Broadcasting Examination, Korean Intellectual Property Office, Daejeon 35208, Republic of Korea d Department of Electronics, Chosun College of Science and Technology, Gwangju 61453, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Shingled solar cell Gird Metallizaion Back surface field Screen printing Firing
The shingled photovoltaic (PV) module is a high-power PV module technology that is manufactured cells dividing and bonding with an electrically conductive adhesive (ECA). Here, the ECA is composed of 70–80% of silver, an acrylic, and a solvent. We focused on the formation of solar cell metallization suitable for the shingled PV module. Generally, the Ag pads on the rear side of the crystalline Si solar cells don’t printed on the Al surface because of the peeling problem. Therefore, a back surface field (BSF) layer is not formed at this area. The characteristics of the solar cell can be improved by applying a rear Ag pad together with the BSF layer in the entire area of the back surface of the solar cell. The formation of the Al-BSF layer and the peeling of the Ag pad off from the interface between Al-BSF and Al were analyzed using by a scanning electron microscope. The peeling phenomenon could be removed by the adjustment of the elevation rate to the peak temperature during co-firing process. The external quantum efficiency (EQE) analysis revealed the influence of the full Al-BSF layer on the rear side of the solar cells showing higher values in the long-wavelength region. Additionally, the divided cell strips were interconnected via ECA bonding, and the characteristics were investigated. The interconnected cell exhibited the efficiency of 18.302% while the efficiencies of two cell strips were 18.039% and 18.162%, respectively.
1. Introduction As the efficiency of Si solar cells has been greatly improved through research and development, the output of the photovoltaic (PV) module is increasing as well. In addition to improving the efficiency of solar cells, new technologies are being developed for manufacturing highpower modules (Gue et al., 2013; Mittage et al., 2017; Walter et al., 2014; Wöhrle et al., 2017). Among the various manufacturing technologies, the shingled PV module can yield a high power in a limited area. This technology differs from conventional PV modules with regard to the manner in which solar cells are boned to each other with an electrically conductive adhesive (ECA). Fig. 1 shows the solar cells interconnection methods for the conventional module and the shingled module. In common crystalline silicon modules, the front contacts of each cell are soldered with the back contacts of the next cell through a metal ribbon, resulting in a connection in series (cell stringing) (Deutsche Gesellschaft für Sonnenenergie, 2013; Esfahani et al., 2017; Jeong et al., 2012). In this method, the output of the solar cell is lost
⁎
because the light is reflected without passing through the metal ribbon and busbar contact (Braun et al., 2012; Zhao et al., 1997). However, the interconnection of the divided cell strips through the ECA eliminates the optical loss of busbar shading owing to the overlap of the cell strips, where the shingled string appears to be without busbars. Thus, highdensity and high-power modules can be manufactured. Additionally, the solar cells must have a suitable metallization pattern for fabricating the shingled PV module. Fig. 2 shows how the solar cell with the metallization pattern for a division is cleaved. For most Si solar cells, the metallization is formed via screen printing and firing (Neuhaus and Münzer, 2007). During firing of the metallization process, the organic binder remaining after drying is completely removed and the metal contact is contacted with Si (Huljic et al., 2000). In addition, the aluminum used for the rear contact is diffused into Si to form a back surface field (BSF). The BSF layer is a heavily doped region with Al near the rear surface of the p-type wafer. This significantly reduces rear recombination as electrons cannot move backward, owing to the internal potential difference. (Kaminski et al.,
Corresponding author. E-mail address: [email protected] (J. Lee).
https://doi.org/10.1016/j.solener.2019.11.095 Received 17 March 2019; Received in revised form 13 October 2019; Accepted 26 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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were used. The Ag pads were required to produce a shingled string through divided cell strips bonding with an ECA. The Ag busbars on the front side of the cell strip are in contact with the Ag pads on the back side of the other cell strip. However, the Ag pads are not usually printed on the Al contact surface because they may peel off during co-firing when applied on top of each other. Consequently, no Al-BSF layer is formed under the Ag pad. In this study, we improved the efficiency of crystalline Si solar cells by increasing the BSF area. We changed the screen-printing layouts to increase the BSF layer region. After introducing modified printing layouts, the peeling phenomenon of Ag pad was alleviated by the adjustment of the elevation rate to the peak temperature during co-firing process. Additionally, we compared the characteristics of the cell fabricated by the conventional rear-printing method. By performing the thermomechanical analysis of the paste, the method for the adhesion improvement between the rear Al contact and Ag pad was developed. Consequently, the BSF layer was formed in the entire rear region, improving the efficiency of the solar cell. We also applied the dividing and ECA bonding technology to the cell with the full Al-BSF layer on the rear side. The efficiency of interconnected or shingled cells was improved by eliminating the shading loss due to the front busbar. 2. Experimental details 2.1. Preparation of metallization In this study, we used the p-type multicrystalline Si wafers doped with a boron. The wafers with the resistivity ranging from 0.5 to 5.0 Ωcm had the size of 156.75 × 156.75 mm2 and the thickness of 200 μm. To evaluate the effect of the screen-printing layouts to the characteristics of the solar cells, the wafers were processed with the standard fabrication procedure: a saw damage removal, a POCl3 diffusion for an emitter layer formation (the sheet resistance of 80 Ω/sq.), a phosphorus-silicate-glass (PSG) removal and a plasma edge isolation, a SiNx layer coating on the front side for anti-reflection and passivation (Zhang et al., 2016). The metallization designs of the solar cells for the shingled string are shown in Fig. 3. We made the different screen-printing layouts, especially back side patterns for Al contact. Fig. 3(a) and (b) represents the screen mask patterns for a front grid and rear Ag pad, respectively. Fig. 3(c) and (d) present the rear Al contact patterns that has the opening area for rear Ag pad printing and the full area Al back contact pattern to increase the BSF layer region.
Fig. 1. Solar cell interconnection methods for PV module manufacturing: (a) conventional string; (b) shingled string.
2.2. Screen printing and firing The metallization of the multi-crystalline silicon solar cells without the contacts was performed by a screen printing process. The fabric of the screen mask with the contact patterns was made of a stainless steel with 350 mesh, and had the tensile strength 34 N/cm. For the metallization, Ag paste was used for the front grid and the rear pad, Al paste was used for the back contact. After printing, the paste should be dried to volatilize its organic material and then fired to chemically bond Si and metal (Lin et al., 2011). In the conventional screen-printing method, the front grid is firstly printed. It is dried quickly to prevent widening of the line width of the printed grid. The drying temperature and duration were set at 265 °C and 30 s, respectively, and the same conditions were used for all drying steps. Fig. 4 shows the two different printing procedures for the rear side metallization. For a conventional Al contact having uncovered area on the back surface, the Ag pad was printed on the back side and then dried. After that, the Al contact was screen-printed on the rear surface except the region here the Ag pad was printed, and was dried. On the other hand, in the case of the second printing procedure, the Al paste
Fig. 2. Diagram of the metallization pattern of the solar cells for division.
2002). Consequently, the efficiency of the solar cell is improved by increasing the open-circuit voltage (Voc). We fabricated the rear Ag pad of the solar cell by the screen printing. At the rear of the solar cell, the full area Al contact and Ag pad 528
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Fig. 3. Metallization designs of the solar cells for the shingled string: (a) Ag grid pattern on the front side; (b) Ag pad pattern on the rear side; (c) Al contact having uncovered area on the rear; (c) full area Al contact on the rear.
Fig. 4. Two different printing procedures for rear side metallization. 529
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Fig. 5. Structures of the of solar cells structures of the solar cells fabricated through the different rear side printing procedure.
was printed over the entire area of the rear surface to increase the BSF layer region and dried. Next, the Ag pad was printed on the Al contact and then dried. The final step is firing or sintering. Fig. 5 shows the resulting structures of the solar cells fabricated through the different rear side printing procedure. The screen-printing conditions, such as the distance between the screen mask and the wafer substrate, printing pressure angle, and printing speed, are important, and the drying and firing processes are equally crucial. Even if the printing conditions are completely optimized, the characteristics of the solar cells depend on the temperature and time of the drying and firing processes (Eom et al., 2017; Hilali et al., 2006). The Ag paste used for the front contact contains a glass frit and is bonded to the emitter layer by etching the antireflective layer of the solar cells. The etch rate depends on the firing temperature and duration. (Hilali et al., 2006; Cooper et al., 2010; Huster et al., 2004). Fig. 6 shows the various firing temperature profiles for the metallization of the solar cells with the Ag pad on the Al contact. These three temperature profiles were used to confirm the detachment state of the rear Ag pad of the solar cells by applying different ramp-up and rampdown rates. In this study, more than 10 specimens were prepared for each experimental condition and their characteristics were examined. A thermomechanical analysis (TMA) of the Al paste and Ag paste was performed to investigate the primary cause of the peeling. The principle of TMA is to heat the sample to be measured while applying a load close to zero. Thus, the change in the length of the sample according to the temperature can be confirmed.
Firing temperature (oC)
1000 Temp-1 Temp-2 Temp-3
800 600 400 200 0 0
10
20
30
40
50
60
70
Process time (sec) Fig. 6. Various firing temperature profiles for the metallization of the solar cells with the Ag pad on the Al contact. The firing experiments for each profile were performed on at least 10 samples.
shingled) cells. The characteristics of the divided and interconnected cells were determined by the solar simulator (WXS-155S-LS, AM1.5GM, WACOM, Japan) and a PV I–V analyzer (DKSCT-3 T, Denken, Japan) under AM 1.5 conditions. 3. Results and discussion
2.3. Cell dividing and ECA bonding
In the case of conventional rear screen printing layouts as shown in Fig. 3(b) and (c), there was no peeling of the Ag pad when the firing temperature profile of Temp-1 was applied to the metallization of the solar cells. However, the different result was obtained when the rear Ag pad was printed on the Al contact with the printing layouts in Fig. 3(b) and (d) (corresponded to the second printing procedure in Fig. 4) and then was fired with the Temp-1 profile. The Ag pad was peeled off from the rear Al contact during the firing process. Fig. 8 show the thermomechanical analysis (TMA) of Al and Ag pastes. The Ag paste and Al paste exhibited different relative length changes. The organic binders and solvents in the paste are evaporated through the firing process. The glass frit melts at a specific temperature and attaches the conductive metal powder to the substrate. However, Ag paste screen-printed on the back Al contact is not intended to be fired on the Al, thus the peeling is expected to occur. To eliminate this phenomenon, we applied the different firing temperature profiles such as Temp-2 and Temp-3 in Fig. 6.
We divided the solar cells with the metallization for the shingled module, as seen in Fig. 2. The scribing equipment with the green laser (MAATRIX 532-7-30, Coherent Inc., USA) was used to cleave the solar cell. The laser power and frequency were 10 W and 50 kHz. The scan rate and the repetition number were of 1300 mm/s and 30 times. These cleaved cell strips were bonded by the shingling equipment (Genesem Inc., South Korea). The system consists of an ECA dispenser, a positioning robot, and a heating table. The ECA (ABLESTIK CA 3556 HF, Henkel AG & Co. KGaA, Germany) was applied to the front busbar of the cell strips by means of the dispenser (Beaucarne, 2016). The ECA is cured at 150 °C for 5 s in air ambient. Through this process, the solvent in the ECA evaporated and cured to bond the cell strips (Kim and Shi, 2001; Yim et al., 2012). More details about the cell scribing and bonding were described in detail elsewhere (Park et al., 2019). Fig. 7 illustrates the photographs of the divided cell strips and boned (or
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Fig. 7. Photographs of the divided cells and the boned (or shingled) cells: (a) and (c) front side; (b) and (d) rear side.
images, there was little Al and/or Ag layer on the Si wafer owing to striping off both rear Ag pad and Al contact, and only small particles remained. For the Temp-2 profile, the SEM images confirmed that the Ag pad was unstable and still peeled off in many cells (about 70% among 10 specimens), although this problem appeared to improve somewhat. In the case of Temp-3 condition, the ramp-up speed to the maximum firing temperature (940 °C in this work) was adjusted to prevent a sudden temperature change. The Temp-3 condition prevent the peeling of most cells, even though 10% of the cells still appear the striping off problem. Table 1 and Fig. 10 shows the photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. The cells with the rear Ag pad on the Al contact showed the larger power-conversion efficiency of 18.109% (average efficiency of ten specimens) while the cells with the Ag pad on the opened Al contact region (corresponding to the conventional metallization procedure) exhibited the lower average efficiency of 17.864%. The efficiency improvement of the solar cells with the Ag pad on the Al contact is attributed to the lager BSF area. The rear surface of the Si wafers diffused with Al paste during the firing process was heavily doped to the p-type, generating a BSF layer, as shown in Fig. 11(a). However, the Si region under the Ag pad without the Al contact doesn’t make the BSF layer, as shown in Fig. 11(b). Therefore, applying Al paste to the uncovered region after printing the Ag pad yields smaller BSF area, as shown in Fig. 5. Fig. 12 shows the external quantum efficiency (EQE) of the solar cells metallized through different printing procedures. The solar cell with rear Ag pad on the Al contact region (corresponded to the closed squares in the figure) appeared the improvement of EQE, especially in
Fig. 8. Thermomechanical analysis (TMA) of Al and Ag pastes.
Fig. 9 shows the rear surface images and cross-sectional micrographs of the solar cells fabricated under different firing temperature profiles. For the measurement of cross-sectional morphologies, the solar cells were cleaved with the green laser to prevent applying force. Ten specimens were prepared and analyzed to confirm that the improvement of the peeling phenomenon resulted from the firing temperature profile. Peeling of a large area occurred in the rear Ag pad region of all samples when the solar cell was fired with the Temp-1, as shown in Fig. 9(a). As indicated by the scanning electron microscopy (SEM)
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Fig. 9. Rear surface images and cross-sectional micrographs of the solar cells fabricated under different firing temperature profiles: (a) Temp-1; (b) Temp-2; (c) Temp-3.
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The open-circuit voltage (Voc) of 1.291 V was observed after the ECA bonding, nearly the same compared to the sum of Voc of two cell strips. The short-circuit current (Isc) of the shingled cell was 2.171 A, which was smaller than those (2.174 and 2.172 A) of the cleaved cells. The efficiency of the solar cells was increased after the ECA bonding. This is because the bonding reduced the optical loss of the solar cell. The front grids (fingers and busbars) of the solar cells correct the photo-current (or photo-generated carriers) but caused the optical loss by the light reflection. For the shingled cell, however, the front busbar was disappeared by the overlap of two cell strips, as shown in Fig. 1(b). Thus, the shading loss owing to the busbar was removed. Consequently, the shading ratio of the front grid was reduced from 6.9% (each cell strip) to 5.1% (the shingled cell), resulting in the improvement of the efficiency.
Table 1 Photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. For the conventional metallization procedure, the Ag pad was printed on the back side region uncovered with Al contact. In the case of the second printing procedure, the Ag pad was formed on the Al contact to make larger BSF layer on the rear side. The characteristics are the average values taken from the ten samples.
Cell Size Isc (A) Jsc (A/cm2) Voc (mV) FF (%) Eff (%) Pm (W)
Rear Ag pad on the opened Al contact region
Rear Ag pad on the Al contact region
245.7 8.571 34.883 644.286 79.702 17.864 4.389
8.646 35.188 650.671 79.090 18.109 4.449
4. Conclusion
the long-wavelength region, when compared to the cell with the rear Ag pad on the opened Al contact region (corresponded to the closed circles). This is attributed to the reduction of the carrier recombination at the rear surface by the larger BSF region (Brendel et al., 1996; Schaper et al., 2005). Consequently, the solar cell with rear Ag pad on the Al contact region yielded the larger Isc (or Jsc) and Voc, and thus the higher efficiency was achieved. After the metallization, the solar cells were divided and bonded with the ECA. Fig. 13 shows the photo current–voltage characteristics of the cleaved cell strips and the interconnected (or shingled) cell in series.
We introduced the rear side metallization procedure of the solar cells suitable for the shingled PV module application. We printed the rear Ag pad on the Al contact to obtain a larger BSF layer, but the rear contact including the Ag pad was peeled off during the firing process. To eliminate this problem, we adjusted the firing temperature profile by changing the ramp-up rate. The power-conversion efficiency of the solar cells with the rear Ag pad on the Al contact exhibited the higher efficiency compared to the cells with the Ag pad printed on the region without Al contact. This was attributed to the reduction of carrier recombination at the back side 10
660 651.67
Isc (A)
Voc (V)
630
615
600
8.549
Ag pad on Al contact
Ag pad without Al contact
8
644.29
645
8.646
6 4 2
Ag pad on Al contact
0
Ag pad without Al contact
(a) 80
(b) 20
90
79.701
79.09
17.864
Ag pad on Al contact
Ag pad without Al contact
15
60
Efficiency (%)
Fill Factor (%)
70
18.109
50 40 30 20
10
5
10 0
Ag pad on Al contact
0
Ag pad without Al contact
(d)
(c)
Fig. 10. Photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. All data for both cell types were averaged from 10 specimens.
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Fig. 13. Photo current–voltage characteristics of the cell strips and the shingled cell.
shingled strings can be made by bonding dozens of cell strips, it is expected that the area of the PV module will be decreased to obtain the same output compared to the conventional PV module and the efficiency of the module will be further increased. Acknowledgements This research was supported by Korea Electric Power Corporation (Grant number: R17XA05-1) and the paper was prepared when the author was staying at the Queensland Micro- and Nanotechnology Centre, Griffith University in Brisbane, Australia. References Beaucarne, G., 2016. Materials challenge for shingled cells interconnection. Energ. Proc. 98, 115–124. Braun, S., Micard, G., Hahn, G., 2012. Solar cell improvement by using a multi busbar design as front electrode. Energ. Proc. 27, 227–233. Brendel, R., Hirsch, M., Plieninger, R., Werner, J.H., 1996. Quantum efficiency analysis of thin-layer silicon solar cells with back surface fields and optical confinement. I. Tran. Elec. Dev. 43 (7), 1104–1113. Cooper, I.B., Ebong, A., Renshaw, J.S., Reedy, R., 2010. Understanding and use of IR belt furnace for rapid thermal firing of screen-printed contacts to Si solar cells. I. Elec. Dev. Let. 31 (5), 461–463. Deutsche Gesellschaft für Sonnenenergie (DGS), 2013. Planning and Installing Photovoltaic Systems, third ed. Routledge, Abingdon Oxon, pp. 73. Eom, T.W., Lee, S.H., Song, C.M., Park, S.Y., Lim, D.G., 2017. Screen printing electrode formation process for crystalline silicon solar cell. Cur. Photo. Res. 5, 9–14. Esfahani, S.N., Asghari, S., Rashid-Nadimi, S., 2017. A numerical model for soldering process in silicon solar cells. Sol. Energ. 148, 49–56. Gue, S., Walsh, T.M., Peters, M., 2013. Vertically mounted bifacial photovoltaic modules: a global analysis. Energ. 61, 447–454. Hilali, M.M., Sridharan, S., Khadilkar, C., Shaikh, A., Rohatgi, A., Kim, S., 2006a. Effect of glass frit chemistry on the physical and electrical properties of thick-film Ag contacts for silicon solar cells. J. Elec. Mater. 35 (11), 2041–2047. Hilali, M.M., Nakayashiki, K., Khadilkar, C., Reedy, R.C., Rohatgi, A., Shaikh, A., Kim, S., Sridharan, S., 2006b. Effect of Ag particle size in thick-film Ag paste on the electrical and physical properties of screen printed contacts and silicon solar cells. J. Elect. Soci. 153 (1), 5–11. Huljic, D.M., Biro, D., Preu, R., Castillo, C.C., Ludemann, R., 2000. Rapid thermal firing of screen printed contacts for large area crystalline silicon solar cells. In: Conf. Reco. I. Photo. Spec. Conf. sept 15–22, Anchorage, USA, pp. 379–382. Huster, F., Seren, S., Schubert, G., Kaes, M., Hahn, G., Breitenstein, O., 2004. Shunts in silicon solar cells below screen-printed silver contacts. Nine. Euro. Photo. Sol. Energ. Conf 832–835. Jeong, J.S., Park, N.C., Han, C., 2012. Field Failure mechansim study of solder interconnection for crystalline silicon photovoltaic module. Micro. Reliab. 52 (9–10), 2326–2330. Kaminski, A., Vandelle, B., Fave, A., Boyeaux, J.P., Nam, L.Q., Monna, R., Sarti, D., 2002. Aluminium BSF in silicon solar cells. Sol. Energ. Mater. Sol. Cel. 72 (1–4), 373–379. Kim, H.K., Shi, F.G., 2001. Electrical reliability of electrically conductive adhesive joints: dependence on curing condition and current density. Micro. J. 32 (4), 315–321. Lin, C.H., Hsu, S.P., Hsu, W.C., 2011. Silicon Solar Cells: Structural Properties of AgContacts/Si-Substrate. Kosyachenko, InTech, Shanghai. Mittage, M., Zech, T., Wiese, M., Bläsi, D., Ebert, M., Wirth, H., 2017. Cell-to-Module (CTM) Analysis for Photovoltaic Modules with Shingled Solar Cells. In: Conf. PVSC.
External quantum efficiency (%)
Fig. 11. Cross-sectional morphologies of the solar cells metallized through different printing procedures for the rear side: (a) the solar cells with the Ag pad on the Al contact; (b) the solar cells with the Ag pad printed on the open region.
90 80 70 60 50 40 30 20 10 200
Ag pad without Al contact Ag pad on Al contact
400
600
800
1000
1200
Wavelength (nm) Fig. 12. External quantum efficiency (EQE) of the solar cells metallized through different printing procedures: Rear Ag pad on the opened Al contact region (closed circles) and rear Ag pad on the Al contact region (closed squares).
confirmed from the EQE analysis. The interconnected or shingled cells through the cells dividing and ECA bonding technique showed the higher efficiencies than those of cleaved cell strips. If this technique is further developed and very long 534
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W. Oh, et al. June 25–30, Washington. D.C, USA. Neuhaus, D.H., Münzer, A., 2007. Industrial Silicon Wafer Solar cell. Advan. OptoElec. OptoElectronics.(2007). https://doi.org/10.1155/2007/24521. Park, J., Oh, W., Park, H., Jeong, C., Choi, B., Lee, J., 2019. Analysis of solar cells interconnected by electrically conductive adhesives for high-density photovoltaic modules. Appl. Surf. Sci. 484, 732–739. Schaper, M., Schmidt, J., Plagwitz, H., Brendel, R., 2005. 20.1%-efficient crystalline silicon solar cell with amorphous silicon rear-surface passivation. Pro. Photo. 13 (5), 381–386. Walter, J., Tranitz, M., Volk, M., Ebert, C., Eitner, U., 2014. Multi-wire interconnection of busbar-free solar cells. Energ. Pro. 55, 380–388. Wöhrle, N., Fellmeth, T., Lohmüller, E., Baliozian, P., Fell, A., Preu, R., 2017. The SPEER
solar Cell-Simulation study of shingled bifacial PERC-Technology-Based Stripe cells. In: Conf. Euro. Photo. Sol. Energ. Conf. Ex. Sept 25–29, Amsterdam, Nederland, pp. 844–848. Yim, M.J., Li, Y., Moon, K.S., Paik, K.W., Wong, C.P., 2012. Review of recent advances in electrically conductive adhesive materials and technologies in electronic packaging. J. Adhe. Sci. Tec. 22 (14), 1593–1630. Zhao, J., Wang, A., Abbaspour-Sani, E., 1997. Improved efficiency silicon solar cell module. IEEE Elec. Dev. Lett. 18 (2), 48–50. Zhang, Y., Tao, J., Chen, Y., Xiong, Z., Zhong, M., Feng, Z., Yang, P., Chu, J., 2016. A large-volume manufacturing of multi-crystalline silicon solar cells with 18.8% efficiency incorporating practical advanced technologies. RSC Adv. 6, 58046–58054.
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Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Metallization of crystalline silicon solar cells for shingled photovoltaic module application
T
Wonje Oha, Jisu Parka, Sima Dimitrijevb, Eung Kwon Kimc, Yong Seob Parkd, Jaehyeong Leea,
⁎
a
Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Griffith School of Engineering, Queensland Micro- and Nanotechnology Centre, Queensland 4111, Australia c Digital Broadcasting Examination, Korean Intellectual Property Office, Daejeon 35208, Republic of Korea d Department of Electronics, Chosun College of Science and Technology, Gwangju 61453, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Shingled solar cell Gird Metallizaion Back surface field Screen printing Firing
The shingled photovoltaic (PV) module is a high-power PV module technology that is manufactured cells dividing and bonding with an electrically conductive adhesive (ECA). Here, the ECA is composed of 70–80% of silver, an acrylic, and a solvent. We focused on the formation of solar cell metallization suitable for the shingled PV module. Generally, the Ag pads on the rear side of the crystalline Si solar cells don’t printed on the Al surface because of the peeling problem. Therefore, a back surface field (BSF) layer is not formed at this area. The characteristics of the solar cell can be improved by applying a rear Ag pad together with the BSF layer in the entire area of the back surface of the solar cell. The formation of the Al-BSF layer and the peeling of the Ag pad off from the interface between Al-BSF and Al were analyzed using by a scanning electron microscope. The peeling phenomenon could be removed by the adjustment of the elevation rate to the peak temperature during co-firing process. The external quantum efficiency (EQE) analysis revealed the influence of the full Al-BSF layer on the rear side of the solar cells showing higher values in the long-wavelength region. Additionally, the divided cell strips were interconnected via ECA bonding, and the characteristics were investigated. The interconnected cell exhibited the efficiency of 18.302% while the efficiencies of two cell strips were 18.039% and 18.162%, respectively.
1. Introduction As the efficiency of Si solar cells has been greatly improved through research and development, the output of the photovoltaic (PV) module is increasing as well. In addition to improving the efficiency of solar cells, new technologies are being developed for manufacturing highpower modules (Gue et al., 2013; Mittage et al., 2017; Walter et al., 2014; Wöhrle et al., 2017). Among the various manufacturing technologies, the shingled PV module can yield a high power in a limited area. This technology differs from conventional PV modules with regard to the manner in which solar cells are boned to each other with an electrically conductive adhesive (ECA). Fig. 1 shows the solar cells interconnection methods for the conventional module and the shingled module. In common crystalline silicon modules, the front contacts of each cell are soldered with the back contacts of the next cell through a metal ribbon, resulting in a connection in series (cell stringing) (Deutsche Gesellschaft für Sonnenenergie, 2013; Esfahani et al., 2017; Jeong et al., 2012). In this method, the output of the solar cell is lost
⁎
because the light is reflected without passing through the metal ribbon and busbar contact (Braun et al., 2012; Zhao et al., 1997). However, the interconnection of the divided cell strips through the ECA eliminates the optical loss of busbar shading owing to the overlap of the cell strips, where the shingled string appears to be without busbars. Thus, highdensity and high-power modules can be manufactured. Additionally, the solar cells must have a suitable metallization pattern for fabricating the shingled PV module. Fig. 2 shows how the solar cell with the metallization pattern for a division is cleaved. For most Si solar cells, the metallization is formed via screen printing and firing (Neuhaus and Münzer, 2007). During firing of the metallization process, the organic binder remaining after drying is completely removed and the metal contact is contacted with Si (Huljic et al., 2000). In addition, the aluminum used for the rear contact is diffused into Si to form a back surface field (BSF). The BSF layer is a heavily doped region with Al near the rear surface of the p-type wafer. This significantly reduces rear recombination as electrons cannot move backward, owing to the internal potential difference. (Kaminski et al.,
Corresponding author. E-mail address: [email protected] (J. Lee).
https://doi.org/10.1016/j.solener.2019.11.095 Received 17 March 2019; Received in revised form 13 October 2019; Accepted 26 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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were used. The Ag pads were required to produce a shingled string through divided cell strips bonding with an ECA. The Ag busbars on the front side of the cell strip are in contact with the Ag pads on the back side of the other cell strip. However, the Ag pads are not usually printed on the Al contact surface because they may peel off during co-firing when applied on top of each other. Consequently, no Al-BSF layer is formed under the Ag pad. In this study, we improved the efficiency of crystalline Si solar cells by increasing the BSF area. We changed the screen-printing layouts to increase the BSF layer region. After introducing modified printing layouts, the peeling phenomenon of Ag pad was alleviated by the adjustment of the elevation rate to the peak temperature during co-firing process. Additionally, we compared the characteristics of the cell fabricated by the conventional rear-printing method. By performing the thermomechanical analysis of the paste, the method for the adhesion improvement between the rear Al contact and Ag pad was developed. Consequently, the BSF layer was formed in the entire rear region, improving the efficiency of the solar cell. We also applied the dividing and ECA bonding technology to the cell with the full Al-BSF layer on the rear side. The efficiency of interconnected or shingled cells was improved by eliminating the shading loss due to the front busbar. 2. Experimental details 2.1. Preparation of metallization In this study, we used the p-type multicrystalline Si wafers doped with a boron. The wafers with the resistivity ranging from 0.5 to 5.0 Ωcm had the size of 156.75 × 156.75 mm2 and the thickness of 200 μm. To evaluate the effect of the screen-printing layouts to the characteristics of the solar cells, the wafers were processed with the standard fabrication procedure: a saw damage removal, a POCl3 diffusion for an emitter layer formation (the sheet resistance of 80 Ω/sq.), a phosphorus-silicate-glass (PSG) removal and a plasma edge isolation, a SiNx layer coating on the front side for anti-reflection and passivation (Zhang et al., 2016). The metallization designs of the solar cells for the shingled string are shown in Fig. 3. We made the different screen-printing layouts, especially back side patterns for Al contact. Fig. 3(a) and (b) represents the screen mask patterns for a front grid and rear Ag pad, respectively. Fig. 3(c) and (d) present the rear Al contact patterns that has the opening area for rear Ag pad printing and the full area Al back contact pattern to increase the BSF layer region.
Fig. 1. Solar cell interconnection methods for PV module manufacturing: (a) conventional string; (b) shingled string.
2.2. Screen printing and firing The metallization of the multi-crystalline silicon solar cells without the contacts was performed by a screen printing process. The fabric of the screen mask with the contact patterns was made of a stainless steel with 350 mesh, and had the tensile strength 34 N/cm. For the metallization, Ag paste was used for the front grid and the rear pad, Al paste was used for the back contact. After printing, the paste should be dried to volatilize its organic material and then fired to chemically bond Si and metal (Lin et al., 2011). In the conventional screen-printing method, the front grid is firstly printed. It is dried quickly to prevent widening of the line width of the printed grid. The drying temperature and duration were set at 265 °C and 30 s, respectively, and the same conditions were used for all drying steps. Fig. 4 shows the two different printing procedures for the rear side metallization. For a conventional Al contact having uncovered area on the back surface, the Ag pad was printed on the back side and then dried. After that, the Al contact was screen-printed on the rear surface except the region here the Ag pad was printed, and was dried. On the other hand, in the case of the second printing procedure, the Al paste
Fig. 2. Diagram of the metallization pattern of the solar cells for division.
2002). Consequently, the efficiency of the solar cell is improved by increasing the open-circuit voltage (Voc). We fabricated the rear Ag pad of the solar cell by the screen printing. At the rear of the solar cell, the full area Al contact and Ag pad 528
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Fig. 3. Metallization designs of the solar cells for the shingled string: (a) Ag grid pattern on the front side; (b) Ag pad pattern on the rear side; (c) Al contact having uncovered area on the rear; (c) full area Al contact on the rear.
Fig. 4. Two different printing procedures for rear side metallization. 529
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Fig. 5. Structures of the of solar cells structures of the solar cells fabricated through the different rear side printing procedure.
was printed over the entire area of the rear surface to increase the BSF layer region and dried. Next, the Ag pad was printed on the Al contact and then dried. The final step is firing or sintering. Fig. 5 shows the resulting structures of the solar cells fabricated through the different rear side printing procedure. The screen-printing conditions, such as the distance between the screen mask and the wafer substrate, printing pressure angle, and printing speed, are important, and the drying and firing processes are equally crucial. Even if the printing conditions are completely optimized, the characteristics of the solar cells depend on the temperature and time of the drying and firing processes (Eom et al., 2017; Hilali et al., 2006). The Ag paste used for the front contact contains a glass frit and is bonded to the emitter layer by etching the antireflective layer of the solar cells. The etch rate depends on the firing temperature and duration. (Hilali et al., 2006; Cooper et al., 2010; Huster et al., 2004). Fig. 6 shows the various firing temperature profiles for the metallization of the solar cells with the Ag pad on the Al contact. These three temperature profiles were used to confirm the detachment state of the rear Ag pad of the solar cells by applying different ramp-up and rampdown rates. In this study, more than 10 specimens were prepared for each experimental condition and their characteristics were examined. A thermomechanical analysis (TMA) of the Al paste and Ag paste was performed to investigate the primary cause of the peeling. The principle of TMA is to heat the sample to be measured while applying a load close to zero. Thus, the change in the length of the sample according to the temperature can be confirmed.
Firing temperature (oC)
1000 Temp-1 Temp-2 Temp-3
800 600 400 200 0 0
10
20
30
40
50
60
70
Process time (sec) Fig. 6. Various firing temperature profiles for the metallization of the solar cells with the Ag pad on the Al contact. The firing experiments for each profile were performed on at least 10 samples.
shingled) cells. The characteristics of the divided and interconnected cells were determined by the solar simulator (WXS-155S-LS, AM1.5GM, WACOM, Japan) and a PV I–V analyzer (DKSCT-3 T, Denken, Japan) under AM 1.5 conditions. 3. Results and discussion
2.3. Cell dividing and ECA bonding
In the case of conventional rear screen printing layouts as shown in Fig. 3(b) and (c), there was no peeling of the Ag pad when the firing temperature profile of Temp-1 was applied to the metallization of the solar cells. However, the different result was obtained when the rear Ag pad was printed on the Al contact with the printing layouts in Fig. 3(b) and (d) (corresponded to the second printing procedure in Fig. 4) and then was fired with the Temp-1 profile. The Ag pad was peeled off from the rear Al contact during the firing process. Fig. 8 show the thermomechanical analysis (TMA) of Al and Ag pastes. The Ag paste and Al paste exhibited different relative length changes. The organic binders and solvents in the paste are evaporated through the firing process. The glass frit melts at a specific temperature and attaches the conductive metal powder to the substrate. However, Ag paste screen-printed on the back Al contact is not intended to be fired on the Al, thus the peeling is expected to occur. To eliminate this phenomenon, we applied the different firing temperature profiles such as Temp-2 and Temp-3 in Fig. 6.
We divided the solar cells with the metallization for the shingled module, as seen in Fig. 2. The scribing equipment with the green laser (MAATRIX 532-7-30, Coherent Inc., USA) was used to cleave the solar cell. The laser power and frequency were 10 W and 50 kHz. The scan rate and the repetition number were of 1300 mm/s and 30 times. These cleaved cell strips were bonded by the shingling equipment (Genesem Inc., South Korea). The system consists of an ECA dispenser, a positioning robot, and a heating table. The ECA (ABLESTIK CA 3556 HF, Henkel AG & Co. KGaA, Germany) was applied to the front busbar of the cell strips by means of the dispenser (Beaucarne, 2016). The ECA is cured at 150 °C for 5 s in air ambient. Through this process, the solvent in the ECA evaporated and cured to bond the cell strips (Kim and Shi, 2001; Yim et al., 2012). More details about the cell scribing and bonding were described in detail elsewhere (Park et al., 2019). Fig. 7 illustrates the photographs of the divided cell strips and boned (or
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Fig. 7. Photographs of the divided cells and the boned (or shingled) cells: (a) and (c) front side; (b) and (d) rear side.
images, there was little Al and/or Ag layer on the Si wafer owing to striping off both rear Ag pad and Al contact, and only small particles remained. For the Temp-2 profile, the SEM images confirmed that the Ag pad was unstable and still peeled off in many cells (about 70% among 10 specimens), although this problem appeared to improve somewhat. In the case of Temp-3 condition, the ramp-up speed to the maximum firing temperature (940 °C in this work) was adjusted to prevent a sudden temperature change. The Temp-3 condition prevent the peeling of most cells, even though 10% of the cells still appear the striping off problem. Table 1 and Fig. 10 shows the photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. The cells with the rear Ag pad on the Al contact showed the larger power-conversion efficiency of 18.109% (average efficiency of ten specimens) while the cells with the Ag pad on the opened Al contact region (corresponding to the conventional metallization procedure) exhibited the lower average efficiency of 17.864%. The efficiency improvement of the solar cells with the Ag pad on the Al contact is attributed to the lager BSF area. The rear surface of the Si wafers diffused with Al paste during the firing process was heavily doped to the p-type, generating a BSF layer, as shown in Fig. 11(a). However, the Si region under the Ag pad without the Al contact doesn’t make the BSF layer, as shown in Fig. 11(b). Therefore, applying Al paste to the uncovered region after printing the Ag pad yields smaller BSF area, as shown in Fig. 5. Fig. 12 shows the external quantum efficiency (EQE) of the solar cells metallized through different printing procedures. The solar cell with rear Ag pad on the Al contact region (corresponded to the closed squares in the figure) appeared the improvement of EQE, especially in
Fig. 8. Thermomechanical analysis (TMA) of Al and Ag pastes.
Fig. 9 shows the rear surface images and cross-sectional micrographs of the solar cells fabricated under different firing temperature profiles. For the measurement of cross-sectional morphologies, the solar cells were cleaved with the green laser to prevent applying force. Ten specimens were prepared and analyzed to confirm that the improvement of the peeling phenomenon resulted from the firing temperature profile. Peeling of a large area occurred in the rear Ag pad region of all samples when the solar cell was fired with the Temp-1, as shown in Fig. 9(a). As indicated by the scanning electron microscopy (SEM)
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Fig. 9. Rear surface images and cross-sectional micrographs of the solar cells fabricated under different firing temperature profiles: (a) Temp-1; (b) Temp-2; (c) Temp-3.
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The open-circuit voltage (Voc) of 1.291 V was observed after the ECA bonding, nearly the same compared to the sum of Voc of two cell strips. The short-circuit current (Isc) of the shingled cell was 2.171 A, which was smaller than those (2.174 and 2.172 A) of the cleaved cells. The efficiency of the solar cells was increased after the ECA bonding. This is because the bonding reduced the optical loss of the solar cell. The front grids (fingers and busbars) of the solar cells correct the photo-current (or photo-generated carriers) but caused the optical loss by the light reflection. For the shingled cell, however, the front busbar was disappeared by the overlap of two cell strips, as shown in Fig. 1(b). Thus, the shading loss owing to the busbar was removed. Consequently, the shading ratio of the front grid was reduced from 6.9% (each cell strip) to 5.1% (the shingled cell), resulting in the improvement of the efficiency.
Table 1 Photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. For the conventional metallization procedure, the Ag pad was printed on the back side region uncovered with Al contact. In the case of the second printing procedure, the Ag pad was formed on the Al contact to make larger BSF layer on the rear side. The characteristics are the average values taken from the ten samples.
Cell Size Isc (A) Jsc (A/cm2) Voc (mV) FF (%) Eff (%) Pm (W)
Rear Ag pad on the opened Al contact region
Rear Ag pad on the Al contact region
245.7 8.571 34.883 644.286 79.702 17.864 4.389
8.646 35.188 650.671 79.090 18.109 4.449
4. Conclusion
the long-wavelength region, when compared to the cell with the rear Ag pad on the opened Al contact region (corresponded to the closed circles). This is attributed to the reduction of the carrier recombination at the rear surface by the larger BSF region (Brendel et al., 1996; Schaper et al., 2005). Consequently, the solar cell with rear Ag pad on the Al contact region yielded the larger Isc (or Jsc) and Voc, and thus the higher efficiency was achieved. After the metallization, the solar cells were divided and bonded with the ECA. Fig. 13 shows the photo current–voltage characteristics of the cleaved cell strips and the interconnected (or shingled) cell in series.
We introduced the rear side metallization procedure of the solar cells suitable for the shingled PV module application. We printed the rear Ag pad on the Al contact to obtain a larger BSF layer, but the rear contact including the Ag pad was peeled off during the firing process. To eliminate this problem, we adjusted the firing temperature profile by changing the ramp-up rate. The power-conversion efficiency of the solar cells with the rear Ag pad on the Al contact exhibited the higher efficiency compared to the cells with the Ag pad printed on the region without Al contact. This was attributed to the reduction of carrier recombination at the back side 10
660 651.67
Isc (A)
Voc (V)
630
615
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8.549
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Ag pad without Al contact
8
644.29
645
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6 4 2
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0
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(b) 20
90
79.701
79.09
17.864
Ag pad on Al contact
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15
60
Efficiency (%)
Fill Factor (%)
70
18.109
50 40 30 20
10
5
10 0
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0
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(d)
(c)
Fig. 10. Photovoltaic characteristics of the solar cells fabricated with two different printing procedures for the rear side metallization. All data for both cell types were averaged from 10 specimens.
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Fig. 13. Photo current–voltage characteristics of the cell strips and the shingled cell.
shingled strings can be made by bonding dozens of cell strips, it is expected that the area of the PV module will be decreased to obtain the same output compared to the conventional PV module and the efficiency of the module will be further increased. Acknowledgements This research was supported by Korea Electric Power Corporation (Grant number: R17XA05-1) and the paper was prepared when the author was staying at the Queensland Micro- and Nanotechnology Centre, Griffith University in Brisbane, Australia. References Beaucarne, G., 2016. Materials challenge for shingled cells interconnection. Energ. Proc. 98, 115–124. Braun, S., Micard, G., Hahn, G., 2012. Solar cell improvement by using a multi busbar design as front electrode. Energ. Proc. 27, 227–233. Brendel, R., Hirsch, M., Plieninger, R., Werner, J.H., 1996. Quantum efficiency analysis of thin-layer silicon solar cells with back surface fields and optical confinement. I. Tran. Elec. Dev. 43 (7), 1104–1113. Cooper, I.B., Ebong, A., Renshaw, J.S., Reedy, R., 2010. Understanding and use of IR belt furnace for rapid thermal firing of screen-printed contacts to Si solar cells. I. Elec. Dev. Let. 31 (5), 461–463. Deutsche Gesellschaft für Sonnenenergie (DGS), 2013. Planning and Installing Photovoltaic Systems, third ed. Routledge, Abingdon Oxon, pp. 73. Eom, T.W., Lee, S.H., Song, C.M., Park, S.Y., Lim, D.G., 2017. Screen printing electrode formation process for crystalline silicon solar cell. Cur. Photo. Res. 5, 9–14. Esfahani, S.N., Asghari, S., Rashid-Nadimi, S., 2017. A numerical model for soldering process in silicon solar cells. Sol. Energ. 148, 49–56. Gue, S., Walsh, T.M., Peters, M., 2013. Vertically mounted bifacial photovoltaic modules: a global analysis. Energ. 61, 447–454. Hilali, M.M., Sridharan, S., Khadilkar, C., Shaikh, A., Rohatgi, A., Kim, S., 2006a. Effect of glass frit chemistry on the physical and electrical properties of thick-film Ag contacts for silicon solar cells. J. Elec. Mater. 35 (11), 2041–2047. Hilali, M.M., Nakayashiki, K., Khadilkar, C., Reedy, R.C., Rohatgi, A., Shaikh, A., Kim, S., Sridharan, S., 2006b. Effect of Ag particle size in thick-film Ag paste on the electrical and physical properties of screen printed contacts and silicon solar cells. J. Elect. Soci. 153 (1), 5–11. Huljic, D.M., Biro, D., Preu, R., Castillo, C.C., Ludemann, R., 2000. Rapid thermal firing of screen printed contacts for large area crystalline silicon solar cells. In: Conf. Reco. I. Photo. Spec. Conf. sept 15–22, Anchorage, USA, pp. 379–382. Huster, F., Seren, S., Schubert, G., Kaes, M., Hahn, G., Breitenstein, O., 2004. Shunts in silicon solar cells below screen-printed silver contacts. Nine. Euro. Photo. Sol. Energ. Conf 832–835. Jeong, J.S., Park, N.C., Han, C., 2012. Field Failure mechansim study of solder interconnection for crystalline silicon photovoltaic module. Micro. Reliab. 52 (9–10), 2326–2330. Kaminski, A., Vandelle, B., Fave, A., Boyeaux, J.P., Nam, L.Q., Monna, R., Sarti, D., 2002. Aluminium BSF in silicon solar cells. Sol. Energ. Mater. Sol. Cel. 72 (1–4), 373–379. Kim, H.K., Shi, F.G., 2001. Electrical reliability of electrically conductive adhesive joints: dependence on curing condition and current density. Micro. J. 32 (4), 315–321. Lin, C.H., Hsu, S.P., Hsu, W.C., 2011. Silicon Solar Cells: Structural Properties of AgContacts/Si-Substrate. Kosyachenko, InTech, Shanghai. Mittage, M., Zech, T., Wiese, M., Bläsi, D., Ebert, M., Wirth, H., 2017. Cell-to-Module (CTM) Analysis for Photovoltaic Modules with Shingled Solar Cells. In: Conf. PVSC.
External quantum efficiency (%)
Fig. 11. Cross-sectional morphologies of the solar cells metallized through different printing procedures for the rear side: (a) the solar cells with the Ag pad on the Al contact; (b) the solar cells with the Ag pad printed on the open region.
90 80 70 60 50 40 30 20 10 200
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Wavelength (nm) Fig. 12. External quantum efficiency (EQE) of the solar cells metallized through different printing procedures: Rear Ag pad on the opened Al contact region (closed circles) and rear Ag pad on the Al contact region (closed squares).
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