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In-situ formation of indium seed layer for copper metallization of silicon heterojunction solar cells
Solar Energy Materials & Solar Cells 204 (2020) 110243
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
In-situ formation of indium seed layer for copper metallization of silicon heterojunction solar cells Junjun Li, Jian Yu *, Tao Chen **, Haichuan Zhang, Qiyun Wang, Pu Wang, Yuelong Huang Institute of Photovoltaics, Southwest Petroleum University, No. 8 Xindu Road, Chengdu, 610500, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Indium seed layer In-situ reduction Copper metallization Silicon heterojunction solar cells
Low production cost and simplified process are the prerequisites for large-scale commercialization of highly efficient silicon heterojunction (SHJ) solar cells. In this paper, an innovative method of plating process with insitu seed layer technique is proposed for the metallization of SHJ solar cells. As indicated by the measurements of cyclic voltammetry curves and scanning electron microscopy, electrochemical reduction reaction occurs and results in metal particles randomly distributed on the surface of tungsten doped indium oxide (IWO) layer. Given the measurement of Energy-dispersive X-ray spectroscopy elemental mapping and X-ray photoelectron spec troscopy, the metal particles are indium and are electrochemically reduced from the IWO film. These indium particles can serve as the seed layer facilitating the subsequent copper plating process. In order to prevent copper diffusion and enhance the adhesion, nickel plating and alkaline copper plating was applied. As a result, the maximum peeling force of plated copper busbar reaches 4.23 N. This is higher than that of the screen-printed silver busbar (2.31 N). By adopting the in-situ indium-seed-layer based plating process, a cell efficiency of 22.01% was achieved with significantly improved short circuit current density and fill factor. Taking advantage of the in-situ seed layer technique, the all-solution based plating process is simplified and effective without full area seed layer deposition and subsequent etch back step, showing great potential for the copper metallization of SHJ solar cells with reduced cost.
1. Introduction N-type crystalline silicon solar cell is a promising candidate for photovoltaic market due to its high electronic tolerance for defects and impurities, long effective minority carrier lifetime, as well as no-light induced degradation [1–3]. By applying advanced interface passiv ation technology, silicon heterojunction (SHJ) solar cell can achieve high circuit voltage (Voc ¼ 750 mV), high fill factor (FF ¼ 84.6%) and improved temperature coefficient [4–8]. A cell conversion efficiency of 25.1% has also been reported on bifacial SHJ solar cell [9]. The world record cell conversion efficiency of 26.6% has been achieved by combining SHJ technology and interdigitated back-contact structure [5]. Currently the most pressing issue hindering the industrialization of SHJ solar cell is the relatively high production costs. It is calculated that 30% of the cell processing cost is consisted by the large amount of expensive low temperature silver paste for screen printing process [10]. Thus, a substitutable metallization technology that maintain good cell
performance with low cost is of highly technical interest. Copper is regarded as an ideal alternative metal contact for silicon solar cell due to its high conductivity and low price. It has been reported that copper metallization technology can overcome the limitations of screen-printing technology [11,12]. However, the copper metallization for SHJ solar cell suffers from the complex of selective-plating process, and poor adhesion at the interface between the transparent conductive oxides (TCO) and the metal contact. The patterning process is essential for selective electroplating due to the high conductivity of TCO film. One patterning method is nonphotolithography process such as inkjet printing [13,14], or screen-printing resist [15]. Yet it is difficult to prepare the finger with opening width less than 30 μm. Another method of higher precision is photolithography-based process such as spin-coating liquid resist [16] or laminating dry film [12]. It also has been reported that laser ablated Al2O3/a-Si (amorphous silicon) stack could be employed as the plating mask [17]. Such double-mask layer can optically and thermally protect the SHJ substrate from the laser damage.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yu), [email protected] (T. Chen). https://doi.org/10.1016/j.solmat.2019.110243 Received 11 June 2019; Received in revised form 21 October 2019; Accepted 21 October 2019 Available online 24 October 2019 0927-0248/© 2019 Published by Elsevier B.V.
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Solar Energy Materials and Solar Cells 204 (2020) 110243
Besides the multi-step plating process, the poor adhesion between the plated copper and TCO is one of the main obstacles affecting the reliability of solar cell device. A seed layer is usually required for the plating of copper on TCO layer [16,18,19]. The seed layer can be pre pared by physical vapor deposition, magnetron sputtering, or electron-beam evaporation combined with thermal evaporation [9,11, 20], which makes the process more complicated and costly. Rodofili et al. has reported a laser-based method which utilizes the plating mask of Al2O3/TCO [21]. In this method, the NiV layer is transferred forward by laser and fired through the dielectric layer to form the contact, where serves as the seed layer. One can also combine the seed layer of screen-printed silver paste with SiOx/SiNx stack film serving both the anti-reflection coating and plating mask [22]. Yet the expensive silver paste is still required. In this paper, an innovative in-situ plating seed layer technique based on electrochemical reduction is proposed for the copper plating. The plating process is remarkably simplified and the influence of plating solution during the chemical process is investigated. Finally, the cell performance of SHJ solar cells with copper contact using in-situ seed layer technique is studied.
Fig. 1. Process flow for SHJ solar metallization with copper electroplating.
mapping. The cross-sectional images of the SHJ cells with the copper contact were investigated by SEM. To analyze the redox reaction, cyclic voltammetry (CV) curves were depicted using the electrochemical workstation. The composition of indium seed layer was analyzed by using high-resolution X-ray photoelectron spectroscopy (XPS). The adhesion test was performed using 45� peeling force. The ribbon was soldered to the tin-coated busbar at 230 oC for the adhesion test. The current-voltage (I–V) parameters of SHJ solar cells were measured under AM1.5 illumination and at room temperature.
2. Experimental details 2.1. Preparation of SHJ substrates N-type c-Si (100) wafers were immersed in a 4.5 wt% KOH solution at 80 � C for 20 min to remove the saw damage and form a random pyramid surface texture. The textured Si wafers were processed with a standard RCA cleaning, dipped in diluted HF (1%), and rinsed by the deionized water, subsequently. Intrinsic and phosphorus-doped (nþ)/ boron-doped (pþ) hydrogenated amorphous silicon (a-Si:H) layers were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. Finally, both sides of the substrate were deposited with tungsten doped indium oxide (IWO) films by reactive plasma deposition (RPD) using the target of 1 wt% WO3-doped In2O3.
3. Results and discussion 3.1. In-situ formation of indium plating seed layer The schematic drawing of in-situ indium plating seed layer technique is shown in Fig. 2(a). The patterned SHJ substrate with two electric contact points contacting to busbars is connected to the cathode. The anode is platinum and the chemical solution is sodium citrate aqueous solution (0.04 mol/L). The redox reaction occurs when the DC power is connected. Fig. 2(b) shows the first cyclic voltammetry (CV) curves in the potential range of 5 V–0 V at a scanning rate of 50 mV/s (vs. In3þ/ In). A calomel electrode is used as a reference electrode. The electrolysis reaction of water molecules appeared at 1.5 V, due to the changes of the electrolysis potential of water molecules in an alkaline solution [23]. The signal at 2.8 V can be ascribed to the cathodic peak of indium seed layer reduced from IWO film. It should be noted that the reversibility of the cathodic reaction (in the potential range of 0 V to 5 V) is poor according to the CV curves, indicating the irreversible formation of in dium. The possible reduction reaction process is illustrated in Fig. 2(c). The In–O bond is broken with sufficient energy. The In3þ ions get the electrons supplied from the DC power, and being reduced simulta neously to indium on the surface of IWO layer. The thickness of the indium seed layer can be precisely controlled by changing parameters such as plating time, current density, or solution composition. The involved electrochemical reactions occurring in sodium citrate aqueous solution at the cathode are as following:
2.2. Formation of indium seed layer and copper plating process The samples were divided into two groups. In group A, the surface of IWO film was patterned by photolithography. Then the SHJ substrate was connected to the DC power source by two electric contact points contacting the busbars (1 mm). Indium was electrochemically reduced from the IWO film at the finger opening under the condition of 4 V for 80 s. The current decreased from 17 mA to around 2 mA. These indium particles, serving as the seed layer, is designed to facilitate the subse quent plating process. The nickel layer was plated in a nickel sulfamate solution (Ni(NH2SO3).24H2O) under the conditions of 35 mA (~1 ASD) for 3 min. The copper layer was plated in a basic copper carbonate so lution (Cu2(OH)2CO3) under the conditions of 200 mA (~6 ASD) for 17 min with phosphorous-copper anode (0.02–0.1% phosphorous con tent). Finally, the copper layer was covered by plated tin coating (tinmethane-sulfonate solution) as soldering layer by tin anode at 60 mA (~1.9 ASD) for 5 min. The Ni/Cu/Sn plating solution was provided by Macdermid. The solution temperature was maintained at 30 � C. Group A were firstly annealed at 200 oC for 30 min in air to improve the photo electric properties of the IWO film, and then annealed at 180 oC for 10 min in N2 or mixed N2/H2 atmosphere after plating. The detailed copper plating process is illustrated in Fig. 1. As a comparison, the samples in Group B were metalized by screen-printed silver grids annealed at 200 � C for 30 min in air.
In2O3 þ 3H2O þ 6e ¼ 2In þ 6OH
(1)
2H2O þ 2e ¼ H2 ↑ þ 2OH
(2)
To investigate the surface topography and the elemental composi tion, scanning transmission electron microscope (SEM) with energydispersive X-ray spectroscopy (EDS) elemental mapping was per formed (Fig. 3). The surface of as-deposited IWO film on textured silicon wafer is smooth (Fig. 3(a)). The Si signal originates from the silicon substrate as the probe depth of EDS is about 1 μm. The O and In signals are distributed evenly originating from the IWO layer. There is no tungsten (W) signal detected due to its low doping concentration. After the electrochemical treatment of IWO coated SHJ substrates, an amount
2.3. Characterization The surface morphologies and element distributions of indium seed layer were investigated by scanning electron microscope (SEM) com bined with energy-dispersive X-ray spectroscopy (EDS) elemental 2
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Fig. 2. (a) In-situ reduction of indium seed layer for copper metallization; (b) C–V curves of the redox reaction; (c) the mechanism of forming indium seed layer.
Fig. 3. Surface morphologies and element analysis of (a) as-deposited IWO film; (b) electrochemically reduced indium seed layer after; (c) local enlarged image of In particles.
of indium particles with the size from dozens of nanometers to hundreds of nanometers are randomly distributed on the textured substrate, as shown in Fig. 3(b). Given the EDS measurement only Si and In signals are detected indicating that these particles consist of In element. It is highly possible that these In particles were electrochemically reduced from the IWO film. Fig. 3(c) shows the detailed EDS element mapping of the nanoparticle. This result further proves the randomly distributed particles on the SHJ substrate are In particles. The chemical composition of the IWO film and indium seed layer
after electrochemical reduction were further analyzed by highresolution X-ray photoelectron spectroscopy (XPS). Fig. 4(a) and Fig. 4 (b) show the XPS core-level spectra of In 3d and O 1s of the as-deposited IWO film on textured silicon wafer. The In 3d core level is fitted with one doublets (In 3d5/2-In 3d3/2) (shown in Fig. 4(a)). The In 3d5/2 compo nent is located at the bonding energy of 444.8 eV and the In 3d3/2 component is located at the bonding energy of 452.3 eV. In Fig. 4(b), the O 1s core lever peaks located at 530.0 eV, which can be ascribed to In2O3. The pink peak on the left is attributed to the surface-adsorbed 3
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Fig. 4. XPS spectra of (a) In 3d and (b) O 1s of as-deposited IWO film; (c) In 3d, (d) O 1s and (e) Si 2p of the film after electrochemical reduction.
oxygen. In Fig. 4(c), the In 3d5/2 components are located at the bonding energy of 443.6 and 444.7 eV, while the In 3d3/2 components are located at the bonding energy of 451.3 and 452.4 eV. The main In 3d peaks at the 443.6 and 451.2 eV are associated with In–In bonding, and the In 3d subcomponents centered at 444.7 and 452.4 eV are referred to the In–O
bonding. As compared with Fig. 4(b), the signal of O 1s peak at the bonding energy of 530.3 eV decreases. This change further confirms the existence of In metal. As concluded from the analysis of SEM, EDS, and XPS results, indium particles are electrochemically reduced from IWO film. It is noteworthy that Si 2p signal was detected. Because the In seed
Fig. 5. The cross-sectional (a) and top-view (b) SEM images of screen-printed silver finger, and the cross-sectional (c) and top-view (d) SEM images of plated copper finger. 4
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layer is not dense and silicon is partly exposed. Therefore, a barrier preventing the copper diffusion is necessary. 3.2. Preparation of copper plated SHJ solar cell with indium seed layer Indium particles are electrochemically reduced from IWO film and can be served as the seed layer facilitating the subsequent copper plating process. However, the prevention of copper diffusion and the adhesion of copper contact needs to be considered for the cell application. Nickle is widely applied as light induced plating seed layer for conventional solar cell, which is not only a good corrosion resistant material, but also an excellent adhesive material for substrate/Ni/Cu stack [24,25]. Therefore, the nickel coating is introduced as the metal bonding layer for the electroplated copper contact in this work. For protecting the indium seed layer from etching, Cu2(OH)2CO3 is used as the plating solution with PH > 9. Finally, a thin layer of tin is plated to prevent the oxidation of copper contact. The photo-resist mask is etched away by acetone and the whole cell is annealed at 180 � C for 10 min in N2 atmosphere. Overall, neither full area seed layer deposition or subsequent etch back step is needed for such all-solution based plating process. As a com parison, SHJ solar cells with screen-printed silver finger were also prepared. Fig. 5 shows the cross-sectional and top-view SEM images of solar cells with screen-printed silver finger and plated copper finger. The screen-printed Ag is porous and its edge is irregular as shown in Fig. 5(a) and Fig. 5(b), The Ag finger width is around 43 μm. Its width expands to 74 μm at the edge, which can cause the increasing of shading loss. For cells with plated copper fingers (in Fig. 5(c) and 5(d)), the copper con tact is conformably deposited on the textured SHJ surface, and no void is observed at the interface in the measured area. The copper finger width is about 40 μm with the aspect ratio greater than 0.5. This is beneficial for the carrier collection and the reduction of optical losses. The insets in Fig. 5(a) and Fig. 5(c) show the interfaces of Ag/IWO and plated grids/ IWO, respectively. With screen-printed low temperature silver paste, the electrodes are bonded by nano-size silver particles. The Ag/IWO inter face is porous leading to poor adhesion. While the plated grids/IWO interface is condensed. The 45� peeling measurement (details not shown here) was performed. The maximum peeling force of screen-printed silver contact is 2.31 N, which is lower than that of plated copper bus bar (In/Ni/Cu stacks, 4.23 N). The current-voltage parameters of SHJ solar cells with screenprinted or plated copper fingers are shown in Fig. 6. The short circuit current density (Jsc) gain of 1 mA/cm2 is observed compared with the screen-printed SHJ solar cell. This is mostly due to the significantly reduced optical shading loss by fine obtainable finger openings and high aspect ratio with electroplated copper grids. The fill factor (FF) increases from 78% to 78.8%. which could be caused by the significantly decreased bulk resistivity of plated finger, as compared to the screenprinted low temperature silver paste [22]. There was no significant change in open circuit voltage. Overall an improved cell conversion efficiency of 22.01% was obtained by using plated copper fingers. Although this cell performance could be further optimized, this all-solution plating method shows the feasibility of in-situ electro chemically reduced indium as copper plating seed layer and the possi bility to achieve high efficiency and low cost SHJ solar cell.
Fig. 6. J–V parameters including short-circuit current density (Jsc), opencircuit voltage (Voc), fill factor (FF), and cell efficiency (Eff) of silicon hetero junction solar cell with plating Cu and screen plating Ag contacts. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
printed silver busbar. By implementing the indium seed layer based copper plating process, a cell conversion efficiency of 22.01% was achieved, mainly due to the improved fill factor and short circuit current density. Such all-solution based plating process without full area seed layer deposition and subsequent etch back step shows a great potential for low cost copper metallization of SHJ solar cells. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements This work was supported in part by National Natural Science Foun dation of China (61904154), scientific research starting project of SWPU (2018QHZ022), National Natural Science Foundation of China (51702269). The authors would like to thank Dr. Jiale Xie, Dr. Changtao Peng and colleagues for their assistance. References [1] E. Kobayashi, Y. Watabe, R. Hao, T.S. Ravi, High efficiency heterojunction solar cells on n-type kerfless mono crystalline silicon wafers by epitaxial growth, Appl. Phys. Lett. 106 (2015) 223504. [2] K. Masuko, M. Shigematsu, T. Hashiguchi, D. Fujishima, M. Kai, N. Yoshimura, T. Yamaguchi, Y. Ichihashi, T. Mishima, N. Matsubara, T. Yamanishi, T. Takahama, M. Taguchi, E. Maruyama, S. Okamoto, Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell, IEEE J. Photovolt. 4 (2014) 1433–1435. [3] D. Macdonald, L.J. Geerligs, Recombination activity of interstitial iron and other transition metal point defects in p- and n-type crystalline silicon, Appl. Phys. Lett. 85 (2004) 4061–4063. [4] M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura, T. Nishiwaki, K. Fujita, E. Maruyama, 24.7% record efficiency HIT solar cell on thin silicon wafer, IEEE J. Photovolt. 4 (2014) 96–99. [5] K. Yoshikawa, W. Yoshida, T. Irie, H. Kawasaki, K. Konishi, H. Ishibashi, T. Asatani, D. Adachi, M. Kanematsu, H. Uzu, K. Yamamoto, Exceeding conversion efficiency of 26% by heterojunction interdigitated back contact solar cell with thin film Si technology, Sol. Energy Mater. Sol. Cells 173 (2017) 37–42. [6] J.C. Stang, T. Franssen, J. Haschke, M. Mews, A. Merkle, R. Peibst, B. Rech, L. Korte, Optimized metallization for interdigitated back contact silicon heterojunction solar cells, Sol. RRL 1 (2017) 1700021.
4. Conclusions In this paper, an innovative electrochemically reducing method and plating process for copper plating is developed, featuring an in-situ in dium seed layer technique. By optimizing the electrochemical solution conditions, the indium particles are reduced from the IWO layer where the fingers selectively open and served as the seed layer for copper plating. The copper metallization process was developed combined with nickel plating and alkaline copper plating. The maximum peel force of copper plated busbar reaches 4.23 N, which is higher than that of screen5
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
In-situ formation of indium seed layer for copper metallization of silicon heterojunction solar cells Junjun Li, Jian Yu *, Tao Chen **, Haichuan Zhang, Qiyun Wang, Pu Wang, Yuelong Huang Institute of Photovoltaics, Southwest Petroleum University, No. 8 Xindu Road, Chengdu, 610500, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Indium seed layer In-situ reduction Copper metallization Silicon heterojunction solar cells
Low production cost and simplified process are the prerequisites for large-scale commercialization of highly efficient silicon heterojunction (SHJ) solar cells. In this paper, an innovative method of plating process with insitu seed layer technique is proposed for the metallization of SHJ solar cells. As indicated by the measurements of cyclic voltammetry curves and scanning electron microscopy, electrochemical reduction reaction occurs and results in metal particles randomly distributed on the surface of tungsten doped indium oxide (IWO) layer. Given the measurement of Energy-dispersive X-ray spectroscopy elemental mapping and X-ray photoelectron spec troscopy, the metal particles are indium and are electrochemically reduced from the IWO film. These indium particles can serve as the seed layer facilitating the subsequent copper plating process. In order to prevent copper diffusion and enhance the adhesion, nickel plating and alkaline copper plating was applied. As a result, the maximum peeling force of plated copper busbar reaches 4.23 N. This is higher than that of the screen-printed silver busbar (2.31 N). By adopting the in-situ indium-seed-layer based plating process, a cell efficiency of 22.01% was achieved with significantly improved short circuit current density and fill factor. Taking advantage of the in-situ seed layer technique, the all-solution based plating process is simplified and effective without full area seed layer deposition and subsequent etch back step, showing great potential for the copper metallization of SHJ solar cells with reduced cost.
1. Introduction N-type crystalline silicon solar cell is a promising candidate for photovoltaic market due to its high electronic tolerance for defects and impurities, long effective minority carrier lifetime, as well as no-light induced degradation [1–3]. By applying advanced interface passiv ation technology, silicon heterojunction (SHJ) solar cell can achieve high circuit voltage (Voc ¼ 750 mV), high fill factor (FF ¼ 84.6%) and improved temperature coefficient [4–8]. A cell conversion efficiency of 25.1% has also been reported on bifacial SHJ solar cell [9]. The world record cell conversion efficiency of 26.6% has been achieved by combining SHJ technology and interdigitated back-contact structure [5]. Currently the most pressing issue hindering the industrialization of SHJ solar cell is the relatively high production costs. It is calculated that 30% of the cell processing cost is consisted by the large amount of expensive low temperature silver paste for screen printing process [10]. Thus, a substitutable metallization technology that maintain good cell
performance with low cost is of highly technical interest. Copper is regarded as an ideal alternative metal contact for silicon solar cell due to its high conductivity and low price. It has been reported that copper metallization technology can overcome the limitations of screen-printing technology [11,12]. However, the copper metallization for SHJ solar cell suffers from the complex of selective-plating process, and poor adhesion at the interface between the transparent conductive oxides (TCO) and the metal contact. The patterning process is essential for selective electroplating due to the high conductivity of TCO film. One patterning method is nonphotolithography process such as inkjet printing [13,14], or screen-printing resist [15]. Yet it is difficult to prepare the finger with opening width less than 30 μm. Another method of higher precision is photolithography-based process such as spin-coating liquid resist [16] or laminating dry film [12]. It also has been reported that laser ablated Al2O3/a-Si (amorphous silicon) stack could be employed as the plating mask [17]. Such double-mask layer can optically and thermally protect the SHJ substrate from the laser damage.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yu), [email protected] (T. Chen). https://doi.org/10.1016/j.solmat.2019.110243 Received 11 June 2019; Received in revised form 21 October 2019; Accepted 21 October 2019 Available online 24 October 2019 0927-0248/© 2019 Published by Elsevier B.V.
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Solar Energy Materials and Solar Cells 204 (2020) 110243
Besides the multi-step plating process, the poor adhesion between the plated copper and TCO is one of the main obstacles affecting the reliability of solar cell device. A seed layer is usually required for the plating of copper on TCO layer [16,18,19]. The seed layer can be pre pared by physical vapor deposition, magnetron sputtering, or electron-beam evaporation combined with thermal evaporation [9,11, 20], which makes the process more complicated and costly. Rodofili et al. has reported a laser-based method which utilizes the plating mask of Al2O3/TCO [21]. In this method, the NiV layer is transferred forward by laser and fired through the dielectric layer to form the contact, where serves as the seed layer. One can also combine the seed layer of screen-printed silver paste with SiOx/SiNx stack film serving both the anti-reflection coating and plating mask [22]. Yet the expensive silver paste is still required. In this paper, an innovative in-situ plating seed layer technique based on electrochemical reduction is proposed for the copper plating. The plating process is remarkably simplified and the influence of plating solution during the chemical process is investigated. Finally, the cell performance of SHJ solar cells with copper contact using in-situ seed layer technique is studied.
Fig. 1. Process flow for SHJ solar metallization with copper electroplating.
mapping. The cross-sectional images of the SHJ cells with the copper contact were investigated by SEM. To analyze the redox reaction, cyclic voltammetry (CV) curves were depicted using the electrochemical workstation. The composition of indium seed layer was analyzed by using high-resolution X-ray photoelectron spectroscopy (XPS). The adhesion test was performed using 45� peeling force. The ribbon was soldered to the tin-coated busbar at 230 oC for the adhesion test. The current-voltage (I–V) parameters of SHJ solar cells were measured under AM1.5 illumination and at room temperature.
2. Experimental details 2.1. Preparation of SHJ substrates N-type c-Si (100) wafers were immersed in a 4.5 wt% KOH solution at 80 � C for 20 min to remove the saw damage and form a random pyramid surface texture. The textured Si wafers were processed with a standard RCA cleaning, dipped in diluted HF (1%), and rinsed by the deionized water, subsequently. Intrinsic and phosphorus-doped (nþ)/ boron-doped (pþ) hydrogenated amorphous silicon (a-Si:H) layers were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. Finally, both sides of the substrate were deposited with tungsten doped indium oxide (IWO) films by reactive plasma deposition (RPD) using the target of 1 wt% WO3-doped In2O3.
3. Results and discussion 3.1. In-situ formation of indium plating seed layer The schematic drawing of in-situ indium plating seed layer technique is shown in Fig. 2(a). The patterned SHJ substrate with two electric contact points contacting to busbars is connected to the cathode. The anode is platinum and the chemical solution is sodium citrate aqueous solution (0.04 mol/L). The redox reaction occurs when the DC power is connected. Fig. 2(b) shows the first cyclic voltammetry (CV) curves in the potential range of 5 V–0 V at a scanning rate of 50 mV/s (vs. In3þ/ In). A calomel electrode is used as a reference electrode. The electrolysis reaction of water molecules appeared at 1.5 V, due to the changes of the electrolysis potential of water molecules in an alkaline solution [23]. The signal at 2.8 V can be ascribed to the cathodic peak of indium seed layer reduced from IWO film. It should be noted that the reversibility of the cathodic reaction (in the potential range of 0 V to 5 V) is poor according to the CV curves, indicating the irreversible formation of in dium. The possible reduction reaction process is illustrated in Fig. 2(c). The In–O bond is broken with sufficient energy. The In3þ ions get the electrons supplied from the DC power, and being reduced simulta neously to indium on the surface of IWO layer. The thickness of the indium seed layer can be precisely controlled by changing parameters such as plating time, current density, or solution composition. The involved electrochemical reactions occurring in sodium citrate aqueous solution at the cathode are as following:
2.2. Formation of indium seed layer and copper plating process The samples were divided into two groups. In group A, the surface of IWO film was patterned by photolithography. Then the SHJ substrate was connected to the DC power source by two electric contact points contacting the busbars (1 mm). Indium was electrochemically reduced from the IWO film at the finger opening under the condition of 4 V for 80 s. The current decreased from 17 mA to around 2 mA. These indium particles, serving as the seed layer, is designed to facilitate the subse quent plating process. The nickel layer was plated in a nickel sulfamate solution (Ni(NH2SO3).24H2O) under the conditions of 35 mA (~1 ASD) for 3 min. The copper layer was plated in a basic copper carbonate so lution (Cu2(OH)2CO3) under the conditions of 200 mA (~6 ASD) for 17 min with phosphorous-copper anode (0.02–0.1% phosphorous con tent). Finally, the copper layer was covered by plated tin coating (tinmethane-sulfonate solution) as soldering layer by tin anode at 60 mA (~1.9 ASD) for 5 min. The Ni/Cu/Sn plating solution was provided by Macdermid. The solution temperature was maintained at 30 � C. Group A were firstly annealed at 200 oC for 30 min in air to improve the photo electric properties of the IWO film, and then annealed at 180 oC for 10 min in N2 or mixed N2/H2 atmosphere after plating. The detailed copper plating process is illustrated in Fig. 1. As a comparison, the samples in Group B were metalized by screen-printed silver grids annealed at 200 � C for 30 min in air.
In2O3 þ 3H2O þ 6e ¼ 2In þ 6OH
(1)
2H2O þ 2e ¼ H2 ↑ þ 2OH
(2)
To investigate the surface topography and the elemental composi tion, scanning transmission electron microscope (SEM) with energydispersive X-ray spectroscopy (EDS) elemental mapping was per formed (Fig. 3). The surface of as-deposited IWO film on textured silicon wafer is smooth (Fig. 3(a)). The Si signal originates from the silicon substrate as the probe depth of EDS is about 1 μm. The O and In signals are distributed evenly originating from the IWO layer. There is no tungsten (W) signal detected due to its low doping concentration. After the electrochemical treatment of IWO coated SHJ substrates, an amount
2.3. Characterization The surface morphologies and element distributions of indium seed layer were investigated by scanning electron microscope (SEM) com bined with energy-dispersive X-ray spectroscopy (EDS) elemental 2
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Fig. 2. (a) In-situ reduction of indium seed layer for copper metallization; (b) C–V curves of the redox reaction; (c) the mechanism of forming indium seed layer.
Fig. 3. Surface morphologies and element analysis of (a) as-deposited IWO film; (b) electrochemically reduced indium seed layer after; (c) local enlarged image of In particles.
of indium particles with the size from dozens of nanometers to hundreds of nanometers are randomly distributed on the textured substrate, as shown in Fig. 3(b). Given the EDS measurement only Si and In signals are detected indicating that these particles consist of In element. It is highly possible that these In particles were electrochemically reduced from the IWO film. Fig. 3(c) shows the detailed EDS element mapping of the nanoparticle. This result further proves the randomly distributed particles on the SHJ substrate are In particles. The chemical composition of the IWO film and indium seed layer
after electrochemical reduction were further analyzed by highresolution X-ray photoelectron spectroscopy (XPS). Fig. 4(a) and Fig. 4 (b) show the XPS core-level spectra of In 3d and O 1s of the as-deposited IWO film on textured silicon wafer. The In 3d core level is fitted with one doublets (In 3d5/2-In 3d3/2) (shown in Fig. 4(a)). The In 3d5/2 compo nent is located at the bonding energy of 444.8 eV and the In 3d3/2 component is located at the bonding energy of 452.3 eV. In Fig. 4(b), the O 1s core lever peaks located at 530.0 eV, which can be ascribed to In2O3. The pink peak on the left is attributed to the surface-adsorbed 3
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Fig. 4. XPS spectra of (a) In 3d and (b) O 1s of as-deposited IWO film; (c) In 3d, (d) O 1s and (e) Si 2p of the film after electrochemical reduction.
oxygen. In Fig. 4(c), the In 3d5/2 components are located at the bonding energy of 443.6 and 444.7 eV, while the In 3d3/2 components are located at the bonding energy of 451.3 and 452.4 eV. The main In 3d peaks at the 443.6 and 451.2 eV are associated with In–In bonding, and the In 3d subcomponents centered at 444.7 and 452.4 eV are referred to the In–O
bonding. As compared with Fig. 4(b), the signal of O 1s peak at the bonding energy of 530.3 eV decreases. This change further confirms the existence of In metal. As concluded from the analysis of SEM, EDS, and XPS results, indium particles are electrochemically reduced from IWO film. It is noteworthy that Si 2p signal was detected. Because the In seed
Fig. 5. The cross-sectional (a) and top-view (b) SEM images of screen-printed silver finger, and the cross-sectional (c) and top-view (d) SEM images of plated copper finger. 4
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layer is not dense and silicon is partly exposed. Therefore, a barrier preventing the copper diffusion is necessary. 3.2. Preparation of copper plated SHJ solar cell with indium seed layer Indium particles are electrochemically reduced from IWO film and can be served as the seed layer facilitating the subsequent copper plating process. However, the prevention of copper diffusion and the adhesion of copper contact needs to be considered for the cell application. Nickle is widely applied as light induced plating seed layer for conventional solar cell, which is not only a good corrosion resistant material, but also an excellent adhesive material for substrate/Ni/Cu stack [24,25]. Therefore, the nickel coating is introduced as the metal bonding layer for the electroplated copper contact in this work. For protecting the indium seed layer from etching, Cu2(OH)2CO3 is used as the plating solution with PH > 9. Finally, a thin layer of tin is plated to prevent the oxidation of copper contact. The photo-resist mask is etched away by acetone and the whole cell is annealed at 180 � C for 10 min in N2 atmosphere. Overall, neither full area seed layer deposition or subsequent etch back step is needed for such all-solution based plating process. As a com parison, SHJ solar cells with screen-printed silver finger were also prepared. Fig. 5 shows the cross-sectional and top-view SEM images of solar cells with screen-printed silver finger and plated copper finger. The screen-printed Ag is porous and its edge is irregular as shown in Fig. 5(a) and Fig. 5(b), The Ag finger width is around 43 μm. Its width expands to 74 μm at the edge, which can cause the increasing of shading loss. For cells with plated copper fingers (in Fig. 5(c) and 5(d)), the copper con tact is conformably deposited on the textured SHJ surface, and no void is observed at the interface in the measured area. The copper finger width is about 40 μm with the aspect ratio greater than 0.5. This is beneficial for the carrier collection and the reduction of optical losses. The insets in Fig. 5(a) and Fig. 5(c) show the interfaces of Ag/IWO and plated grids/ IWO, respectively. With screen-printed low temperature silver paste, the electrodes are bonded by nano-size silver particles. The Ag/IWO inter face is porous leading to poor adhesion. While the plated grids/IWO interface is condensed. The 45� peeling measurement (details not shown here) was performed. The maximum peeling force of screen-printed silver contact is 2.31 N, which is lower than that of plated copper bus bar (In/Ni/Cu stacks, 4.23 N). The current-voltage parameters of SHJ solar cells with screenprinted or plated copper fingers are shown in Fig. 6. The short circuit current density (Jsc) gain of 1 mA/cm2 is observed compared with the screen-printed SHJ solar cell. This is mostly due to the significantly reduced optical shading loss by fine obtainable finger openings and high aspect ratio with electroplated copper grids. The fill factor (FF) increases from 78% to 78.8%. which could be caused by the significantly decreased bulk resistivity of plated finger, as compared to the screenprinted low temperature silver paste [22]. There was no significant change in open circuit voltage. Overall an improved cell conversion efficiency of 22.01% was obtained by using plated copper fingers. Although this cell performance could be further optimized, this all-solution plating method shows the feasibility of in-situ electro chemically reduced indium as copper plating seed layer and the possi bility to achieve high efficiency and low cost SHJ solar cell.
Fig. 6. J–V parameters including short-circuit current density (Jsc), opencircuit voltage (Voc), fill factor (FF), and cell efficiency (Eff) of silicon hetero junction solar cell with plating Cu and screen plating Ag contacts. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
printed silver busbar. By implementing the indium seed layer based copper plating process, a cell conversion efficiency of 22.01% was achieved, mainly due to the improved fill factor and short circuit current density. Such all-solution based plating process without full area seed layer deposition and subsequent etch back step shows a great potential for low cost copper metallization of SHJ solar cells. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements This work was supported in part by National Natural Science Foun dation of China (61904154), scientific research starting project of SWPU (2018QHZ022), National Natural Science Foundation of China (51702269). The authors would like to thank Dr. Jiale Xie, Dr. Changtao Peng and colleagues for their assistance. References [1] E. Kobayashi, Y. Watabe, R. Hao, T.S. Ravi, High efficiency heterojunction solar cells on n-type kerfless mono crystalline silicon wafers by epitaxial growth, Appl. Phys. Lett. 106 (2015) 223504. [2] K. Masuko, M. Shigematsu, T. Hashiguchi, D. Fujishima, M. Kai, N. Yoshimura, T. Yamaguchi, Y. Ichihashi, T. Mishima, N. Matsubara, T. Yamanishi, T. Takahama, M. Taguchi, E. Maruyama, S. Okamoto, Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell, IEEE J. Photovolt. 4 (2014) 1433–1435. [3] D. Macdonald, L.J. Geerligs, Recombination activity of interstitial iron and other transition metal point defects in p- and n-type crystalline silicon, Appl. Phys. Lett. 85 (2004) 4061–4063. [4] M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura, T. Nishiwaki, K. Fujita, E. Maruyama, 24.7% record efficiency HIT solar cell on thin silicon wafer, IEEE J. Photovolt. 4 (2014) 96–99. [5] K. Yoshikawa, W. Yoshida, T. Irie, H. Kawasaki, K. Konishi, H. Ishibashi, T. Asatani, D. Adachi, M. Kanematsu, H. Uzu, K. Yamamoto, Exceeding conversion efficiency of 26% by heterojunction interdigitated back contact solar cell with thin film Si technology, Sol. Energy Mater. Sol. Cells 173 (2017) 37–42. [6] J.C. Stang, T. Franssen, J. Haschke, M. Mews, A. Merkle, R. Peibst, B. Rech, L. Korte, Optimized metallization for interdigitated back contact silicon heterojunction solar cells, Sol. RRL 1 (2017) 1700021.
4. Conclusions In this paper, an innovative electrochemically reducing method and plating process for copper plating is developed, featuring an in-situ in dium seed layer technique. By optimizing the electrochemical solution conditions, the indium particles are reduced from the IWO layer where the fingers selectively open and served as the seed layer for copper plating. The copper metallization process was developed combined with nickel plating and alkaline copper plating. The maximum peel force of copper plated busbar reaches 4.23 N, which is higher than that of screen5
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