The project »Rock-Star« The evolution of rotary printing for solar cell metallization

AIP Conference Proceedings 2367, 020008 2021; https//doi.org/10.1063/5.0056060 2367, 020008 2021 Authors. The project Rock-Star The evolution of rotary printing for solar cell metallization Cite as AIP Conference Proceedings 2367, 020008 2021; https//doi.org/10.1063/5.0056060 Published Online 01 June 2021 Andreas Lorenz, M. Klawitter, M. Linse, S. Tepner, J. Rth, N. Wirth, R. Greutmann, M. Lehner, A. Senne, D. Reukauf, M. Drews, S. Gombert, H. Brocker, A. Mette, J. Rohde, E. Drsam, and F. Clement ARTICLES YOU MAY BE INTERESTED IN Advancement in screen printed fire through silver paste metallisation of polysilicon based passivating contacts AIP Conference Proceedings 2367, 020003 2021; https//doi.org/10.1063/5.0055978 Effects of constituents in paste on low light performance of silicon solar cells A case study of aluminum AIP Conference Proceedings 2367, 020002 2021; https//doi.org/10.1063/5.0056075 Fast screen printing and curing process for silicon heterojunction solar cells AIP Conference Proceedings 2367, 020006 2021; https//doi.org/10.1063/5.0056429 The Project Rock-Star The Evolution of Rotary Printing for Solar Cell Metallization Andreas Lorenz 1,a , M. Klawitter 1 , M. Linse 1 , S. Tepner 1 , J. Rth 2 , N.Wirth 3 , R. Greutmann 3 , M. Lehner 4 , A. Senne 5 , D. Reukauf 5 , M. Drews 2 , S. Gombert 3 , H. Brocker 3 , A. Mette 6 , J. Rohde 7 , E. Drsam 8 and F. Clement 1 1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrae 2, 79110 Freiburg, Germany 2 ASYS Automatisierungssysteme GmbH, Benzstr. 10, 89160 Dornstadt, Germany 3 Gallus Ferd. Resch AG, Harzbchelstrasse 34, 9016 St. Gallen, Switzerland 4 Lehner Engineering GmbH, Ebnetstrasse 18, 9032 Engelburg, Switzerland 5 ContiTechElastomer-BeschichtungenGmbH, BreslauerStr.14, 7154 Northeim, Germany 6 Hanhwa Q Cells GmbH, Sonnenallee 17-21, 06766 Bitterfeld-Wolfen, Germany 7 Kurt Zecher GmbH, Grlitzer Str. 2, 33098 Paderborn, Germany 8 Technische Universitt Darmstadt, Magdalenenstrae 2, 64289 Darmstadt, Germany a Corresponding author andreas.lorenzise.fraunhofer.de Abstract. Within this work, we provide a comprehensive overview about research activities and current status with respect to rotary printed metallization for silicon solar cells. We will present the major results of the research project Rock-Star and previous activities which focused on the metallization of Si solar cells using flexographic printing and rotary screen printing. We demonstrate that the rear side metallization of passivated emitter and rear cells PERC can be realized with rotary screen printing on the same quality level as state-of-the-art flatbed screen printing. Furthermore, it is shown that both rotary screen printing and flexographic printing are able to realize the fine line front side metallization. Fine line front side contacts down to approx. 40 μm for rotary screen printing and around 30 μm for flexographic printing are demonstrated. However, further optimization is required to reduce finger width, increase finger height and thus decrease the mean lateral finger resistance. A major result of project Rock-Star is the development of an innovative demonstrator platform to enable rotary printed solar cell metallization with high printing speed and low cycle time per wafer. Newly developed transport, alignment and printing concept enables a cycle time of down to 0.45 s/wafer. The concept and features of the demonstrator machine are presented within this paper. INTRODUCTION Flatbed screen printing FSP is currently the dominating technology for the metallization of crystalline Silicon c-Si solar cells. Within the last two decades, great progress has been done in optimizing process, materials and machinery of the FSP metallization process, leading to remarkable results with respect to the printing quality reduction of front side finger width [1] and productivity. However, despite of new backend lines which allow a reduction of the cycle time per wafer around 1s [2], the FSP process reaches the technical limitation with respect to a further increase in throughput. Thus, new concepts to boost the throughput of the metallization step will be required to cope with the rapidly growing global PV production capacity [3,4]. The throughput of the FSP process is limited by several factors the rheological properties of the high-viscous screen printing paste requires a sequential flooding and printing sequence. Despite of high velocities of up to 1000 mm/s for the flooding and approx. 500 mm/s for the printing step in modern production, the printing sequence has to be carried out at standstill of the wafer. Furthermore, certain transport and alignment procedures have to be carried out sequentially which further increases the cycle time per wafer. Proceedings of the 9th Workshop on Metallization and Interconnection for Crystalline Silicon Solar Cells AIP Conf. Proc. 2367, 020008-1–020008-9; https//doi.org/10.1063/5.0056060 Published by AIP Publishing. 978-0-7354-4101-9/30.00 020008-1 A very promising approach to overcome this general limitation is the transformation to a continuous instead of a sequential process. As printing technologies have proven their ability to apply the metallization pattern with excellent, reliable quality and high cost-efficiency, it is a natural choice to continue this success story in the future. Appropriate printing technologies for high-volume mass production of all kinds of printed matter exist since decades. Rotary printing machines are highly developed machines which enable highest precision at very high printing speed. First activities to apply this technology for the metallization of Si solar cells have been started more than 10 years ago [5]. Within the funded research project Rock-Star contract no. 13N13512, a project consortium of industry partners and research institutes has set itself the ambitious goal to develop a rotary printing demonstrator machine which is able to realize the metallization of solar cells with a printing speed of up to 600 mm/s. Such a machine would enable a gross throughput of up to 8000 wafers per hour on a single line. Two printing methods have been evaluated within the project rotary screen printing RSP and flexograhic printing FXP. Within this work, a comprehensive review of the major results within the project Rock-Star including the concept and capability of the demonstrator machine is provided. ROTARY PRINTING METHODS ROTARY SCREEN PRINTING AND FLEXOGRAPHIC PRINTING Rotary Screen Printing Rotary screen printing RSP is commonly applied in the field of textile or label printing using highly developed high-speed printing machines based on a roll-to-roll concept [6]. Web-fed machines with rotary screen printing units i.e. for label printing can realize a printing speed of up to 160 m/min. 2.7 m/s [7]. Similar to FSP, RSP is able to apply precise thick film patterns on various substrates. However, the capability of RSP to apply fine structures like fine line front side contacts for Si solar cells has also been demonstrated [1,8]. The working principle of the rotary screen printing unit within the demonstrator machine is shown in Fig. 1 a. The paste is pressed by a fixed squeegee through the openings of the rotating cylinder screen. The Silicon wafer is guided by a carrier with a vacuum fixation system through the printing unit. The printing pressure during the rotary printing process is comparable to flatbed screen printing thus enabling a printing process on thin and fragile wafer material. FSP requires a sequential flooding and printing step, the RSP metallization process is carried out in a continuous process, meaning that the paste is constantly pressed through the open areas during the rotation of the screen. Thus, the time-consuming two- step process of FSP can thus be replaced by a substantially faster continuous metallization process. While this is a clear advantage with respect to throughput, RSP also carries some drawbacks compared to FSP. Firstly, RSP cylinder screens require a considerably higher stability of the mesh than FSP flatbed screens. To ensure this stability, rotary screen meshes have significantly thicker wires of d wire | 30 – 50 μm compared to flatbed screens with a typical wire thickness d wire | 11-25 μm for metallization purposes. Thicker wires obviously reduce the open area of the mesh and thus the paste transfer capacity per unit area. They could also increase the impact of so-called “mesh marks” on the printed finger geometry [9]. A second characteristic of RSP is the necessity of a lower paste viscosity compared to FSP pastes which can be explained by the continuous printing process and the missing pre-filling of the screen. Reducing the paste viscosity substantially affects important rheological parameters for fine line printing like yield stress and wall slip behaviour which are important to obtain ultra-fine contact fingers [10,11]. Thus, printing very fine front side contacts using RSP is challenging and requires further optimization of pastes and screens. A further relevant topic is the alignment precision of the printed image on silicon wafers. It is assumed that a comparable alignment precision to FSP can be achieved using state-of-the-art machine technology. However, this assumption has to be investigated and confirmed. First attempts to use this technology for solar cell metallization already date back to the year 1999 [12], however no published results are known from these activities. Flexographic Printing Flexographic printing FXP is a well-known and widely used printing technology which is commonly applied for graphic arts printing on substrates like cardboard, paper or foil. FXP machines with roll-to-roll principle obtain 020008-2 printing speeds up to 800 m/min on web-based materials. This is obviously unrealistic for printing processes on single-item substrates like Silicon solar cells. However, applying FXP for the front side metallization opens the potential to increase the throughput of solar cell metallization considerably compared to the state-of-the-art flatbed screen printing method. Fig. 1b illustrates the working principle of the flexographic printing unit within the demonstrator machine A flexible relief printing plate or sleeve with elevated printing areas serves as image carrier. Inks with a low to medium viscosity can be used depending on the properties of the anilox roll and the requirements of the printing subject. The ink is transferred from the ink chamber onto the so-called anilox roll, a steel cylinder with a finely textured chromium or ceramic surface. The anilox roll continuously wets the elevated areas of the printing plate with a uniform layer thickness which is theoretically defined by the so-called dip volume per unit area [cm/m]. Excessive ink is removed from the surface of the anilox roll by doctor blades incorporated into the ink chamber. The relatively low printing pressure and the flexibility of the plate enable printing fine structures even on very rough substrates like textured silicon wafers. Critical parameters within the FXP process are printing pressure, anilox roller properties, ink properties and material tolerances. The applicable layer thickness by FXP is typically in a range of approx. 1 - 10 μm depending on the paste properties and the anilox roller specification. Thus, printing structures with high layer thickness is challenging and somehow limited. Beside graphic arts and package printing, FXP has been successfully applied for various printed electronics applications like micro-scale conductive networks [13], circuitry [14,15], roll-to-roll polymer solar cell modules are usually fabricated on roll-to-roll machines. a b FIGURE 1. a Working principle of rotary screen printing unit for solar cell metallization A mesh-based cylinder screen with a partly opened emulsion layer serves as printing form. During the printing process, the cylinder screen rotates. The paste is printed through the open areas of the rotating screen by a pneumatically onset polyurethane PU squeegee. The substrate Silicon wafer is transported through the printing unit on a carrier with vacuum fixation images of rotary screen and mesh with courtesy of Gallus Group. b Working principle of a flexographic printing unit for solar cell metallization The anilox roller with a finely textured surface is filled with a defined amount of ink from the ink chamber. Excessive ink is removed by blade knifes of the ink chamber system. Elevated areas of the flexo plate/sleeve are inked by the anilox roller and directly printed onto the substrate Silicon wafer. Image of flexo printing plate sample with courtesy of ContiTech Elastomer-Coatings GmbH. Flexo plate/ sleeve Anilox roller Shuttle Ink chamber Flexography Recessed areas non-printing Elevated areas Printing Flexo SleeveRotary screen Squeegee Shuttle Cylinder Screen Rotary Screen Printing Flexography 020008-3 Rotary SP 2019 d 26 μm Rotary Screen Printing Rear Al Flatbed Screen Printing Rear Al d 24 μm FROM ROCK’N’ROLL TO ROCK-STAR Rotary Screen Printed Front and Rear Side Metallization Within project Rock-Star, several experiments have been carried out to evaluate the applicability of RSP for the rear and front side metallization of aluminum back surface field Al BSF and passivated emitter and rear cells PERC. First principal tests have been carried out on a Gallus EM 280 label printing machine [16,1,8]. This machine is designed to print on continuous web materials like foil or paper web. For the experiments, every wafer has been fixed individually on the foil web before each print run to enable the transport of the wafers through the rotary screen printing unit with printing speed up to v max 330 mm/s. Due to existing challenges with the alignment of the printing layout, a smaller solar cell layout with 125 mm edge length has been printed on Cz-Si precursors with 156 mm edge length. The smaller solar cells 125 mm x 125 mm have subsequently been cut out along the edge of the printed rear/front side metallization by a laser cutting process. The results of these feasibility studies demonstrated that the rear side metallization of Al BSF and PERC solar cells can be applied with the same quality and electrical performance compared to the FSP reference Fig. 2 a and b. The thickness of the printed and fired Al layer mostly depends on the rotary screen specification mesh count, wire thickness in combination with the properties of the paste [8]. Depending on the screen mesh, an Al layer thickness between 20 and 40 μm could be achieved after contact firing [16]. The use of commercial FSP Al paste is possible if the viscosity is modified to a suitable lower value to enable a stable paste transfer within the RSP process. Within the experiments, the modification has been realized by diluting the p