Connection techniques of textile wires to the solid and flexible solar cells

MASTER FINAL THESIS Connection techniques of textile wires to the solid and flexible solar cells Author: Li, Zhuo Director /Co-director: Ilén, Elina Emilia / Ardanuy Raso, Monica Degree: Master in Textile Design and Technology Examination session: Autumn, 2023 Document: Report Connection techniques of textile wires to the solid and flexible solar cells Foreword Thanks to Foxa Ltd. for providing fabrics and Aalto University project Sun Powered Textiles for providing solar cells for the study. Also thanks to Dr. Janne Halme from Aalto University, about providing knowledge of measuring the capacity of solar cells, who gave inspiration when stabilizing the conditions with a light source and light intensity meter. Connection techniques of textile wires to the solid and flexible solar cells 1 Abstract In order to make better use of clean energy such as solar energy, textile on earth have the potential to be made into solar textiles. The commercial solar cells can be embedded between the textile layers by laminating to harvest energy for e-textile applications. Research about solar textiles is not mature enough, and many aspects of the problem need to be solved. However, connecting techniques of conductive textile wires to flexible and solid solar cells are not in-deep studied. In addition, solar textile products with these connection technologies must meet washable requirements. To solve the problem of connecting textile wires to flexible and solid solar cells in the production of solar textiles, this study proposes three connection techniques for solar textiles, which are tape, adhesive and stitching based on literature and experimental validation. The feasibility of tape and adhesive methods was analyzed by literature review and the feasibility of stitching method was verified by experiments. The stitching is unapplicable method for solid solar cells that are difficult to penetrate, but applicable for flexible solar cells. The machine-washing durability of solar textile which composed of solar cells with stitch-connected conductive textile wires was verified by experiments. First, the textile wires were joined to flexible solar cells by stitching with the sewing machine and then embedded between fabric layers with TPU-lamination to simulate real set up in e-textile application. The humidity stickers were attached to the surface of solar cells, and it was expected to present solar textile samples with or without water inside. After the machine-washing process is completed, the individual parts of the solar textile sample are disassembled by delamination. For stitching method, after 15 machine wash tests by household washing machines, the performance of the solar cells was hardly affected, and the internal water resistance of the samples was good. The delamination process verifies that the components of the solar textile can be disassembled and have the potential to be recycled. The feasibility of tape connections has been proven by previous studies. Flexible adhesives that have the potential to connect textile wires to solar cells are listed. Connection techniques of textile wires to the solid and flexible solar cells 2 Table of contents ABSTRACT ................................................................................................................................................... 1 TABLE OF CONTENTS ................................................................................................................................... 2 LIST OF TABLES ............................................................................................................................................ 4 LIST OF FIGURES .......................................................................................................................................... 5 LIST OF ABBREVIATIONS / GLOSSARY .......................................................................................................... 6 1. INTRODUCTION ....................................................................................................................................... 7 1.1 OBJECT ............................................................................................................................................... 7 1.2 SCOPE................................................................................................................................................. 7 1.3 REQUIREMENTS .................................................................................................................................... 8 1.4 RATIONALE .......................................................................................................................................... 8 2 BACKGROUND AND/OR REVIEW OF THE STATE OF THE ART ............................................................. 10 2.1 E-TEXTILE SYSTEM AND CHARACTERISTICS ................................................................................................. 10 2.2 SOLAR TEXTILES .................................................................................................................................. 15 2.3 CONNECTION TECHNIQUES IN E-TEXTILES ................................................................................................. 16 2.3.1 Sewing and embroidery ............................................................................................................. 17 2.3.2 Adhesives ................................................................................................................................... 18 2.3.3 Crimping .................................................................................................................................... 20 2.3.4 Soldering .................................................................................................................................... 21 2.3.5 Welding ...................................................................................................................................... 21 2.3.6 Tape ........................................................................................................................................... 23 2.3.7 Conclusion of connection techniques ......................................................................................... 24 2.4 EVALUATION OF WASHING DURABILITY OF E-TEXTILES AND ENCAPSULATION ..................................................... 24 3 MATERIALS SELECTION OF EXPERIMENTS FOR STITCHING METHOD ................................................. 26 3.1 SELECTION OF SOLAR CELLS ................................................................................................................... 26 3.2 SELECTION OF TEXTILE WIRES ................................................................................................................. 29 3.3 SELECTION OF FABRICS ......................................................................................................................... 30 4 EXPERIMENTS FOR STITCHING METHOD ........................................................................................... 32 4.1 FIRST ROUND OF MACHINE-WASHING EXPERIMENTS ................................................................................... 32 4.1.1 Samples for the first round of experiments ................................................................................ 34 4.1.2 Waterproof property test ........................................................................................................... 37 4.1.3 Electrical measurements ............................................................................................................ 37 4.1.4 Visual observation ..................................................................................................................... 39 4.2 SECOND ROUND OF MACHINE-WASHING EXPERIMENTS ............................................................................... 39 4.2.1 Samples for the second round of experiments ........................................................................... 40 4.2.2 Waterproof property test ........................................................................................................... 41 4.2.3 Electrical measurements ............................................................................................................ 42 4.2.4 Visual observation ..................................................................................................................... 43 4.3 INFLUENCE OF HUMIDITY TO THE HARVESTING CAPABILITY EXPERIMENTS ......................................................... 43 4.3.1 Fabric moisture and washing effect test .................................................................................... 44 4.3.2 Solar cell penetration humidity impact test ............................................................................... 44 4.3.3 Textile wire humidity impact test ............................................................................................... 45 5 EXPERIMENT RESULTS AND ANALYSIS FOR STITCHING METHOD ....................................................... 46 5.1 FIRST ROUND OF MACHINE-WASHING EXPERIMENTS ................................................................................... 46 5.1.1 Waterproof property test ........................................................................................................... 46 5.1.2 Electrical measurements ............................................................................................................ 46 Connection techniques of textile wires to the solid and flexible solar cells 3 5.1.3 Visual observation ..................................................................................................................... 48 5.2 SECOND ROUND MACHINE WASH EXPERIMENTS ......................................................................................... 48 5.2.1 Waterproof property test ........................................................................................................... 48 5.2.2 Electrical measurements ............................................................................................................ 48 5.2.3 Visual observation ..................................................................................................................... 49 5.3 INFLUENCE OF HUMIDITY ON HARVESTING CAPACITY EXPERIMENTS ................................................................ 51 5.3.1 Fabric moisture and washing effect test .................................................................................... 51 5.3.2 Solar cell penetration humidity impact test ............................................................................... 53 5.3.3 Textile wire humidity impact test ............................................................................................... 53 6 RESULTS AND ANALYSIS FOR TAPE AND ADHESIVE METHOD ............................................................ 54 6.1 TAPE ................................................................................................................................................ 54 6.2 POTENTIAL ADHESIVES FOR SOLAR CELLS .................................................................................................. 54 7 BUDGET SUMMARY .......................................................................................................................... 56 8 ANALYSIS AND ASSESSMENT OF ENVIRONMENTAL AND SOCIAL IMPLICATIONS ............................... 57 9 CONCLUSIONS .................................................................................................................................. 58 10 REFERENCES ..................................................................................................................................... 59 11 APPENDIX ......................................................................................................................................... 62 Connection techniques of textile wires to the solid and flexible solar cells 4 List of tables TABLE 1. EXPERIMENTAL FABRIC DETAILS ................................................................................................................... 31 TABLE 2. FABRICS FOR SOLAR TEXTILE SAMPLES (FIRST ROUND) ...................................................................................... 34 TABLE 3. COLOR CHANGE OF HUMIDITY STICKERS (FIRST ROUND). .................................................................................. 46 TABLE 4. COLOR CHANGE OF HUMIDITY STICKERS (SECOND ROUND) ............................................................................... 48 TABLE 5. ADHESIVES FOR CONNECTING TEXTILE WIRES AND SOLAR CELLS ......................................................................... 55 Connection techniques of textile wires to the solid and flexible solar cells 5 List of figures FIGURE 1. DEVELOPMENT OF THE PROJECT’S TIMING ..................................................................................................... 9 FIGURE 2. THE XIAOMI MIJIA CARDIOGRAM T-SHIRT FOR ECG MONITORING. SOURCE: HTTPS://XIAOMIPLANETS.COM/XIAOMIMIJIA-CARDIOGRAM-T-SHIRT-2/ ..................................................................................................................... 10 FIGURE 3. THE DEVICE CONFIGURATION AND WORKING PRINCIPLE OF THE TRIBOELECTRIC INTERACTING PATCH. SOURCE: [6] ..... 11 FIGURE 4. A TEXTILE-BASED TACTILE LEARNING PLATFORM. SOURCE: [7] ......................................................................... 12 FIGURE 5. THE FOUR-MODE CONTROLLER GRIPPING MOTION DETECTED BY THE PVDF MEMS. SOURCE: [8] ........................ 13 FIGURE 6. E-TEXTILE SYSTEM FRAMEWORK (SOURCE: [2]) ............................................................................................ 14 FIGURE 7. LIGHT PASSES THROUGH THE FABRIC TO REACH THE SOLAR CELLS. SOURCE: [2] ................................................... 15 FIGURE 8. CONNECTION METHODS APPLIED TO E-TEXTILE. ............................................................................................ 16 FIGURE 9. CHIP COMPONENTS, PCBS AND SENSORS PLACED ON THE SUBSTRATE ARE CONNECTED BY SEWING. SOURCE: [19] .... 17 FIGURE 10. EMBROIDERY METHODS: (A)STANDARD EMBROIDERY FORMING DOUBLE LOCK STITCH AND (B) THE TAILORED FIBER PLACEMENT METHOD. SOURCE: [20] ............................................................................................................... 18 FIGURE 11. EXAMPLE OF CONDUCTIVE ADHESIVE. SOURCE: HTTPS://MGCHEMICALS.COM/PRODUCTS/ADHESIVES/THERMALLYCONDUCTIVE-ADHESIVES/THERMALLY-CONDUCTIVE-EPOXY-ADHESIVE/ ................................................................... 19 FIGURE 12. ILLUSTRATION OF THE ADHESIVE BONDING PROCESS ADOPTED FOR ELECTRONICS-IN-TEXTILES. SOURCE: [22] .......... 19 FIGURE 13. DETAILED ILLUSTRATION OF ICA AND ACA METHODS. SOURCE: [18] ............................................................. 20 FIGURE 14. EXAMPLE OF CRIMPING. SOURCE: HTTPS://WWW.HOMESIMPROVEMENTS.NET/2021/03/10/BEST-CRIMPING-TOOLGET-IT-FROM-WIREFYSHOP/ .......................................................................................................................... 20 FIGURE 15. EXAMPLE OF SOLDERING FOR CIRCUITS. SOURCE: HTTPS://TECHNIMARK-INC.COM/OUR-BLOG/POST/THE-ULTIMATEGUIDE-TO-ELECTRONIC-SOLDERING.................................................................................................................. 21 FIGURE 16. WELDING IN THE PRODUCTION OF FABRIC. SOURCE: HTTPS://WWW.LEISTER.COM/EN/SOLUTIONS/TECHNICALTEXTILES .................................................................................................................................................... 22 FIGURE 17. HOT AIR WEDGE WELDING DIAGRAM. SOURCE: HTTPS://LIGHTWEIGHTMANUFACTURING.COM/UNCATEGORIZED/TYPES-OF-INDUSTRIAL-FABRIC-WELDS/ ...................... 22 FIGURE 18. CONDUCTIVE TAPE. SOURCE: HTTP://WWW.MALAYSIA3MTAPE.DIECUT.COM.MY/WPCONTENT/UPLOADS/2017/03/3M-363-ALUMI_E-1.JPG .................................................................................. 23 FIGURE 19. LAYER STRUCTURE OF THE TEXTILE–SOLAR CELL MODULE. SOURCE: [3] ........................................................... 24 FIGURE 20. THE DETACHABLE PART OF XIAOMI MIJIA CARDIOGRAM T-SHIRT. SOURCE: HTTPS://XIAOMIPLANETS.COM/XIAOMIMIJIA-CARDIOGRAM-T-SHIRT-2/ ..................................................................................................................... 25 FIGURE 21. SOLAR CELLS CONVERT LIGHT INTO ELECTRICITY. SOURCE: [2] ....................................................................... 26 FIGURE 22. EXAMPLES OF SOLAR CELLS FOR SOLAR TEXTILE. SOURCE: [2] ........................................................................ 27 FIGURE 23. FLEXIBLE SOLAR CELL (A) AND SOLID SOLAR CELL (B). SOURCE: HTTPS://SCITECHDAILY.COM/SOLAR-CELL-EFFICIENCYINCREASED-WITH-INNOVATIVE-TWO-DIMENSIONAL-MATERIALS/ ........................................................................... 28 FIGURE 24. EXAMPLES OF CONDUCTIVE YARNS FOR SOLAR TEXTILE. SOURCE: [2] .............................................................. 29 FIGURE 25. DETAILED IMAGE OF THREE MAIN TEXTILE STRUCTURES: A). WOVEN, B). KNITTING, C). NON-WOVEN. SOURCE: [32] ................................................................................................................................................................ 30 FIGURE 26. LIGHT PASSING THROUGH THE FABRIC. SOURCE: [2] .................................................................................... 30 FIGURE 27. EXPERIMENT PROCEDURE....................................................................................................................... 32 FIGURE 28. DOMESTIC WASHING MACHINE. .............................................................................................................. 33 FIGURE 29. DOMESTIC DETERGENT. ......................................................................................................................... 33 FIGURE 30. THE EXPERIMENT PROCESS OF WASHING TEST (FIRST ROUND) ........................................................................ 34 FIGURE 31. STITCHING SEWING FOR FLEXIBLE SOLAR CELLS. .......................................................................................... 35 FIGURE 32. STITCHING DETAILS ON THE FRONT AND BACK SIDE OF CONTACT AREA ON FLEXIBLE SOLAR CELL ............................. 35 FIGURE 33. LAYER STRUCTURE OF THE SOLAR TEXTILE MODULE (FIRST ROUND). ................................................................ 36 FIGURE 34. SOLAR CELL WITH HUMIDITY STICKER (FIRST ROUND). .................................................................................. 36 FIGURE 35. SAMPLE BEFORE AND AFTER LAMINATION (FIRST ROUND). ............................................................................ 36 FIGURE 36. THE MEASUREMENT PROCESS OF TEXTILE SOLAR SAMPLES (FIRST ROUND). ....................................................... 37 FIGURE 37. MULTIMETER. ..................................................................................................................................... 38 FIGURE 38. SCHEMATIC DIAGRAM OF ELECTRIC TEST. ................................................................................................... 38 FIGURE 39. THE EXPERIMENT PROCESS OF WASHING TEST FOR SAMPLES WITH STITCHES (SECOND ROUND). ........................... 39 FIGURE 40. THE EXPERIMENT PROCESS OF WASHING TEST FOR SAMPLES WITHOUT YARNS (SECOND ROUND). .......................... 40 FIGURE 41. LAYER STRUCTURE OF THE SOLAR TEXTILE MODULE (SECOND ROUND). ............................................................ 40 Connection techniques of textile wires to the solid and flexible solar cells 6 FIGURE 42. FLEXIBLE SOLAR CELL WITH HOLES AT CONTACT AREA. .................................................................................. 41 FIGURE 43. SOLAR CELL WITH HUMIDITY STICKERS (SECOND ROUND). ............................................................................. 41 FIGURE 44. THE MEASUREMENT PROCESS OF TEXTILE SOLAR SAMPLES (SECOND ROUND). ................................................... 42 FIGURE 45. STABLE IKEA TABLE LAMP. ..................................................................................................................... 42 FIGURE 46. LUXMETER. SOURCE: HTTPS://ES.RS-ONLINE.COM/WEB/P/LUXOMETROS/1232360?GB=S .............................. 43 FIGURE 47. THE PROCESS OF INFLUENCE OF HUMIDITY EXPERIMENTS. ............................................................................. 43 FIGURE 48. THE MEASUREMENT OF SOLAR CELL COVERED WITH FABRIC 3 WASHED 15 TIMES. ............................................. 44 FIGURE 49. SOLAR CELL WITH PENETRATIONS WITH WATER DROPLETS ............................................................................. 45 FIGURE 50.TEXTILE WIRES OF SAMPLE WITH WATER DROPLETS ....................................................................................... 45 FIGURE 51. EVOLUTION OF THE RELATIVE OUTPUT VOLTAGE IN THE WASHING TEST (FIRST ROUND). ....................................... 46 FIGURE 52. CHANGES IN PERFORMANCE OF SAMPLES AFTER 15 WASHES AND DELAMINATION (FIRST ROUND). ........................ 47 FIGURE 53. DAMAGED SOLAR CELL IN SAMPLE 6. ........................................................................................................ 47 FIGURE 54. EVOLUTION OF THE RELATIVE OUTPUT VOLTAGE IN THE WASHING TEST (SECOND ROUND). ................................... 49 FIGURE 55. VISUAL OBSERVATION OF SEPARATED SOLAR CELL FROM DELAMINATION (WITH STITCHING). ................................. 50 FIGURE 56. VISUAL OBSERVATION OF SEPARATED SOLAR CELL FROM DELAMINATION (WITHOUT STITCHING). ........................... 50 FIGURE 57. EVOLUTION OF THE EFFECT OF WET AND DRY COVERING FABRICS ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON (FABRIC 1) ............................................................................................................................... 51 FIGURE 58. EVOLUTION OF THE EFFECT OF WET AND DRY COVERING FABRICS ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON (FABRIC 3) ............................................................................................................................... 51 FIGURE 59. EVOLUTION OF THE EFFECT OF WASHING INFLUENCE OF COVERING FABRICS ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON (FABRIC 1) ............................................................................................................... 52 FIGURE 60. EVOLUTION OF THE EFFECT OF WASHING INFLUENCE OF COVERING FABRICS ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON (FABRIC 3) ............................................................................................................... 52 FIGURE 61. EVOLUTION OF THE EFFECT OF WET AND DRY OF PERFORATED SOLAR CELL ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON. ............................................................................................................................. 53 FIGURE 62. EVOLUTION OF THE EFFECT OF WET AND DRY TEXTILE WIRES ON THE PERFORMANCE OF SOLAR CELL SAMPLES IN COMPARISON. ............................................................................................................................................. 53 List of abbreviations / Glossary ACA Anisotropic conductive adhesive CNC Computer numerically controlled CPU Central processing unit DC Direct current ECA Electrically conductive adhesive ECG Electrocardiogram FPCBs Flexible printed circuit boards ICA Isotropic conductive adhesive IoTs Internet of Things MCEYs Metal composite embroidered yarns NCA Non-conductive adhesive bonding PEDOT Poly(3,4-ethylene dioxythiophene) PSS Poly(styrenesulfonate) SIPN Semi-interpenetrating network SoT System on textile TFE Tailed fiber placement TPU Thermoplastic polyurethanes Connection techniques of textile wires to the solid and flexible solar cells 7 1. Introduction 1.1 Object The general objective of this study is to provide feasible and producible connection techniques of textile wires to the solid and flexible solar cells in solar textile. The secondary objectives of this study are presented in the form of questions. Questions: 1. What are potential flexible adhesives commercially available for solar textile based on literature review? 2. Is there an effect of some humidity on solar cells on the harvesting capability? 3. Are there any losses of light harvesting capacity in samples with stitching connection method after machine washing? Is harvesting capacity reduced? 4. Which method is most effective for mass production based on literature review and experiments? 1.2 Scope Through a literature review, tapes, adhesive and stitching connection techniques were considered to connect textile threads and solar cells in solar textiles. The feasibility of both tape and adhesive methods were analyzed by literature review method when the stitching was experimentally validated. In experiments for stitching method, the scope is below. 1. Solar cell. Only flexible solar was involved in the experiment. Solid solar cell was not involved since it cannot be penetrated by sewing machine. 2. Encapsulation. Encapsulation was applied to make waterproof solar textile samples in the experiments. This technique was not developed in this study. 3. Fabric. Different kinds of fabrics were not studied in the experiment, although 3 fabrics were involved in the experiment. These fabrics were made from different technology: woven and knitted, and one of them was with waterproof coating. To ensure the light passes smoothly through the fabric, white fabrics was selected and color of fabrics was comparably same. 4. Textile wire. Only one commercial conductive yarn was selected as the textile wire. 5. Durability test method. Only washing durability was selected in the experiment. 6. Solar cell performance test method. The energy harvest capacity of solar cell was tested by electric tests. Voltage or current between solar cell contact area was tested by multimeter and a luxmeter showed the intensity of light. Connection techniques of textile wires to the solid and flexible solar cells 14 (5). Computer or Central Processing Unit (CPU). The computer or central processing unit "CPU" is the brain of the system. Computers operate the control system, process information, and store data inside and outside the garment. The e-textile system framework is shown in Figure 6. The e-textile system always includes signal inputs (e.g., fabric electrodes on the human body), output devices (e.g., a cell phone), a CPU and an energy source for data processing and transmission between all these components. The sensors collect the input signals are transmitted to the CPU for data processing and then the output devices display the processed data [2]. Figure 6. E-textile system framework (source: [2]) Power is necessary for the operation of the components in the system. E-textile requires the use of batteries or energy generators to power electronic components throughout the life cycle of the device. To meet the design of e-textile, the batteries are often in the same enclosure as the electronics always replaceable and rechargeable, which makes them unreliable in terms of protection and safety [10] [11]. In addition, traditional batteries often are clumsy, robust, non-washable and do not meet the basic requirements for textiles. Harvestable energy sources including light, heat, piezoelectricity or friction and wind have the potential to solve this problem. Hybrid energy generators can increase and stabilize the output of a constant power source. Using inductive coupling, electricity can be transmitted wirelessly to a woven polyester glove through a flat spiral coil embroidered with conductive wires [12]. The energy of human movement can be collected by triboelectric or piezoelectric methods, for example, a simple and low-cost 3D-printing process for preparing a flexible PVDF-TrFE copolymer with multiple thin layers, and a Polydimethylsiloxane (PDMS) rugby ball structure was studied [13]. Of certainly, it cannot be ignored that solar energy is also a good option for the e-textile energy source, which will be introduced in 2.2. Connection techniques of textile wires to the solid and flexible solar cells 15 2.2 Solar textiles Theoretically, e-textiles need an energy source, such as a battery, to fulfill its function. Batteries always need to be charged or replaced, and they need to be removed during etextile washing. Solar textiles are designed with the idea of removing e-textile's dependence on traditional power sources and providing a clean, unnecessarily removable and washable power source. The principle of solar textiles: solar cells are used as the energy source of electronic textiles to provide energy for electronic textiles [2]. There are some studies that have involved solar textiles. The latest research reports on different types of textile solar cells, including details of their manufacturing technologies, are presented [14]. The paper designed textiles that combine solar power and triboelectricity to power electronic devices [15]. A biomorphic textile actuator that can be manufactured at scale by conventional textile routes and triggered autonomously by sunlight has been investigated, in which the active and passive layers of the bimorph are composed of polypropylene tapes and MXene-enhanced polyamide filaments [16]. The solar cells were embedded into the textile to make solar textiles, which were machine washed and tested [3]. Among the many studies on solar textile, the solar textile model that best fits the idea of commercial production is the one shown in Figure 7. Commercial solar cells are designed to be placed under the textile, replacing the solar cell integrated above the textile, and the principle of light passing through the fabric to reach the solar cell [2]. The fabric is covered with solar cells, and light hitting the fabric is reflected, refracted and absorbed. The reflected light reflects the color of the fabric, and the absorbed light becomes thermal radiation. The part of the light that passes through the fabric is converted into usable energy by the solar cells. Although some energy harvesting is sacrificed, this scheme improves aesthetics. And to a certain extent, the solar cell is protected from damage caused by impact and scratches, making it more durable. Fiber material, textile structure, density, color, and post-treatment all affect the optical properties of textiles and thus the energy harvesting capacity of solar cells. Optimizing the design of these textile properties is crucial for textile solar modules. From a functional application point of view, many factors need to be considered, such as the choice of solar cells, the choice of fabric, and the choice of connection methods, among others. Figure 7. Light passes through the fabric to reach the solar cells. Source: [2] Connection techniques of textile wires to the solid and flexible solar cells 16 According to visioning the applications for solar textiles, solar energy can be designed for lightning, body sensing, cooling or heating and mechanical movements. Beautiful and durable textile solar cell energy harvesting modules can be widely used in smart textiles and wearable technology solutions. They are potentially useful in the following fields and industries: agriculture, sports, medicine and building. Textiles which frequently exposed to sunlight, can be developed as a brilliant use of solar energy. For example, cargo covers have a large surface and exposed to sun but have no mains power to be utilized added with the that truck brands have launched the fully electric trucks such as BYD T10ZT 31T 8X4 5.6M Full Electric Dumper (BYD3310EH9BEV). A high potential for solar power can be found from interiors of passenger cars, as the glass roofs of cars are become common, textile solar panels could be integrated to the seat, hat rack or shades for instance. Similarly, in agriculture and farming the large areas of fabrics are used to protect plants from unfavorable weather conditions or wild animals. Depending on location and season of the year the solar power is available even more than 12 hours per day. In robotics, especially in the area of person assistive robotics used in personal care where robot look is softer being associated as more emphatic product in comparison to industrial robots, the sun powered textile could be an attractive solution in the future [2]. The above usage ideas are for reference only. Careful technical analysis is required in order to confirm the feasibility of the idea of solar textile applications. 2.3 Connection techniques in E-textiles This section introduces several connection methods commonly used in e-textile through extensive literature research. Without the use of connection technology, wires and electronic components would come into poor contact and even make it impossible to connect the circuit. In addition, without the fixing effect of the connection technology, wires or electronic components may lose performance due to friction. E-textiles require the connection of components and traditional connection techniques include sewing and embroidery, adhesives, crimping, soldering, welding and tape, as shown in Figure 8. The connection method written here refers to the connection method commonly used in smart textiles and only some of them are suitable to apply to solar textile. Figure 8. Connection methods applied to e-textile. Connection techniques of textile wires to the solid and flexible solar cells 17 2.3.1 Sewing and embroidery Sewing and embroidery techniques, which are the conventional methods of interconnection of electronic components, create designs by stitching strands of certain materials onto an appropriate substrate. Embroidery is a connect method that is also often used e-textiles [17]. In general, sewing and embroidery connection with conductive yarn on insulating fabrics with sewing machines or embroidery machines or even by hand [18]. For instance, chip elements, Printed Circuit Board (PCBs), and sensors are sewn together by sewing method (as shown in Figure 9), providing a connection between the circuit elements and fabric [19]. In addition, conductive yarns for touch sensing can be used as user interfaces for smart clothing by building embroidery circuits [20]. Embroidery machines are expected to be put into the production of smart textiles, which can be adjusted to embroider beautiful patterns. Figure 9. Chip components, PCBs and sensors placed on the substrate are connected by sewing. Source: [19] From a textile point of view, the vast majority of fabrics can be used, woven, non-woven or knitted. This embroidery circuit formation process ensures to a large extent that the conductive threads are embroidered onto the fabric to fit any shape [20]. However, it must be considered that ordinary machines do not perfectly achieve conductive yarn embroidery and sewing. Because of the stress caused by bending and shearing, as well as the difficulty in forming strong stitches and the special properties of conductive yarns, custom machinery may be required to produce the perfect finished product, or threads with high strength and flexibility may be used [18]. The study "Conductive Yarn Embroidered Circuits for System on Textiles" introduces new conductive embroidery yarns for touch sensing and signal transmission in textile systems (SoT). Conductive yarns for touch sensing can be used as user interfaces for smart clothing by building embroidery circuits. Conductive yarns for signal transmission can be embroidered on smart clothing and used as transmission lines to transmit power and signals. Connection techniques of textile wires to the solid and flexible solar cells 18 New metal composite embroidered yarns (MCEYs) were introduced for touch sensing, textile-based interconnects, signal communication, and power transmission for smart wearable devices. Robust and reliable MCEY embroidery circuits are used for sensing, interconnect, and signal and power transmission [20]. Two computer numerically controlled (CNC) embroidery methods (Figure 10) are introduced: a). In the standard embroidery method, the stitch and bottom thread form a double-lock stitch that creates a technically recognizable appearance on the upper and lower sides, respectively (Figure 10-a). The CNC standard embroidery process can undertake complex work and connect flexible printed circuit boards (FPCBs) or small electronic components and embroidery circuits during the embroidery process. b). The tailed fiber placement (TFP) method, on the other hand, is a three-threaded system. The TFP method is used when the thread is very hard, inelastic, and very thick like fiberglass or carbon fiber and cannot work on a standard embroidery machine (Figure 10-b). A set of top and bottom threads is used to fix the substrate fabric, and the coarse conductive yarn is fixed to the substrate fabric, forming a zigzag stitch. In the TPF method, it is not possible to connect small electronic components directly to the embroidery circuit during the embroidery process. Figure 10. Embroidery methods: (a)standard embroidery forming double lock stitch and (b) the tailored fiber placement method. Source: [20] The design of the needle thread and bottom thread in standard embroidery forming double lock stitch is not restricted. Conductive yarn can act as needle thread or bottom yarn or even both. 2.3.2 Adhesives The adhesive method has a lower curing temperature, making it suitable for a wider range of e-textile applications. There are many types of conductive adhesives, and several types of adhesives are used in e-textile: non-conductive adhesive bonding (NCA), isotropic conductive adhesive (ICA), and anisotropic conductive adhesive (ACA). The method of adhesive bonding to be applied to e-textile can be done on an industrial scale using special machines or to provide technological developments, such as the control of the process and Connection techniques of textile wires to the solid and flexible solar cells 19 its variables (temperature, pressure, etc.) [21]. One example of conductive adhesive is shown in Figure 11. Figure 11. Example of conductive adhesive. Source: https://mgchemicals.com/products/adhesives/thermally-conductiveadhesives/thermally-conductive-epoxy-adhesive/ In NCA, a layer of thermoplastic adhesive is applied to the parts in contact. Subsequently, the components are pressed together to remove the adhesive out of the contact area, and the part is then cured at the required temperature. And the schematic of this NCA bonding method by the example of an encapsulated electronic module and a fabric circuit made from embroidered insulated conductive yarn is shown in Figure 12 [22]. Figure 12. Illustration of the adhesive bonding process adopted for electronics-in-textiles. source: [22] ICA and ACA are both electrically conductive adhesive (ECA) that make contacts to be connected conductive. The major differentiating factor between ICA and ECA is that ICA bonding involves the addition of conductive fillers to the adhesive material, while ACA has a much lower concentration of conductive fillers. In Figure 13, the differences between these two methods are obvious [18]. Connection techniques of textile wires to the solid and flexible solar cells 20 Figure 13. Detailed illustration of ICA and ACA methods. Source: [18] Applications for ACA include flat panel displays containing thin-film ACA, glass flip chips, smart cards, and flip chip boards where soldering cannot be applied due to the thermal sensitivity of the substrate. The limitation of NCA adhesives is that they can only be used in high bond strength applications where electrical conductivity is not required [23]. Moreover, there are some drawbacks to ECA, such as sensitivity to the type and quality of component and board metallization, curing time requirements, and lack of durability in various climates [19]. 2.3.3 Crimping In electronics, crimping is the process of deforming a metal casing around a conductor to form a gas-tight permanent connection. Also known as cold soldering, crimping is a common method used in the automotive industry. Crimping is a fixed connection technique theoretically, but there are many detachable connector products that can be called crimp connectors. In this condition, a crimp terminal is used at one end to form a permanent contact with a flexible substrate and a connector such as a header at the other end [18]. The example of crimping is shown in Figure 14. Figure 14. Example of crimping. Source: https://www.homesimprovements.net/2021/03/10/best-crimping-tool-get-it-fromwirefyshop/ Connection techniques of textile wires to the solid and flexible solar cells 21 The crimping approach uses standard manufacturing techniques and durable, inexpensive, and low-temperature interconnect technologies, providing opportunities for the manufacture of e-textiles. The strengths of this method are high reliability and simple-fast processing. The use of this technology in SoT has the potential to reduce the cost of producing e-textile products and thus make them competitively priced [24]. The idea of the crimping connection technique approach in e-textile is still relatively new and remains to be investigated. 2.3.4 Soldering Soldering is a technique used to join two or more electrical or conductor contacts. It works by melting a metal, usually an alloy with a melting point lower than the melting point of the material of the contacts to be joined and applying it between the contacts. Soldering is a viable method of manufacturing e-textile circuits, and some of the conductive materials in e-textiles can be soldered. And the Figure 15 shows the example of soldering for circuits. When it is used in e-textiles, the components are connected to a flexible substrate such as conductive wire, flexible copper wire or polyimide. Soldered connections have low contact resistance but are mechanically brittle, and in textile applications any bent or stretched connections must be reinforced to avoid breakage [18]. Figure 15. Example of soldering for circuits. Source: https://technimark-inc.com/ourblog/post/the-ultimate-guide-to-electronic-soldering This connection technique is not suitable for temperature-sensitive textiles, as standard soldering typically requires temperatures above 200 ℃. This may mean that it is not suitable for use as a connection method for most e-textile products, as this high temperature can damage e-textile components. In addition, excessive reverse seams on trace and thread trimming can pose a challenge, and these issues can lead to electrical short circuits [19]. 2.3.5 Welding Welding is a process similar to soldering, but in welding, the metal is heated but without melted, bonded by a third alloy. Welding is known as a joining technique that achieves continuity of materials by applying heat or pressure, with or without any additional joining material. Of the various welding methods that exist today, the most common is industrialscale welding, because the entire process can be easily automated. Among the various welding processes for the production of sensors and actuators for textile applications, Connection techniques of textile wires to the solid and flexible solar cells 22 ultrasonic welding, laser welding and resistance welding have the greatest potential for automation. Of these, ultrasonic welding in particular has great opportunities in the development of electronic textiles, as this welding process does not damage textiles and produces very reliable contact points [25]. Figure 16 shows how welding is applied to the production of textiles. Figure 16. Welding in the production of fabric. Source: https://www.leister.com/en/Solutions/Technical-Textiles In earlier studies, e-textile transfer lines were manufactured using hot air welding technology, and the potential possibility of textile transfer lines was obtained by adding conductive yarns to the fabric through a hot air welding process. The demonstrated use of hot air welding as a convenient technique for producing reliable and durable transmission lines while maintaining the properties of textiles poses a great challenge to the field of e-textile research [26]. For instance, the hot air wedge welding diagram is shown in Figure 17. Figure 17. Hot Air wedge welding diagram. Source: https://lightweightmanufacturing.com/uncategorized/types-of-industrial-fabric-welds/ Connection techniques of textile wires to the solid and flexible solar cells 23 The resistance welding method offers the possibility to draw conductive patterns of contact embroidery. In the available research investigation, a series of samples were tested for the optimization of resistance welding parameters for hybrid conductive wires crossed on textile substrates and optimized for 3 different design variables. It was shown that the resistance welding technique can achieve high quality contact structures on textile substrates. Moreover, it has been validated that, in an alternative to embroidering conductive threads directly on the fabric, the method is also applicable to embroidered contact pads for connecting conventional conductors (wires or cables) to a fabric substrate [27]. In summary, the advantage of welding connection method is that they have comparably high mechanical strength. Some welding processes do not require any filler material, and the running cost of the whole process is very low. However, temperature is a factor that cannot be ignored in the application of welding technology. When metals must be handled, welding is not a commonly used connection technique in e-textiles due to various problems associated. In the future, welding technology needs to be researched and developed to be more applicable to e-textile. 2.3.6 Tape Tape, either conductive tape or conventional tape, can be used for the connection of etextile components. It works well in textiles because of its flexibility to change with the movement of the textile. An example of conductive tape is shown in Figure 18, which is 3M™ 363 Conductive Aluminium Tape. Figure 18. Conductive tape. Source: http://www.malaysia3mtape.diecut.com.my/wpcontent/uploads/2017/03/3M-363-alumi_e-1.jpg Connection techniques of textile wires to the solid and flexible solar cells 30 3.3 Selection of fabrics Fabric is described as “a 2-dimensional textile which is produced by knitting, weaving or non-woven technology” [2]. The numerous combinations of fibers, yarns and structures bring about the diversity of fabrics. The materials that make up a fabric are called fibers and are often classified as natural fibers and chemical fibers. Among them, natural fibers include plant fibers, such as cotton fibers, animal fibers, such as wool fibers and mineral fibers. Chemical fibers include recycled fibers, synthetic fibers and inorganic fibers. The fibers themselves are also different, even if they are made from the same raw materials. For example, polyester filaments can be woven together by a dozen or dozens of single filaments to form a fabric, while polyester fiber short yarns must be spun to form a continuous yarn after twisting and hugging between fibers to form a continuous yarn for weaving [31]. Fabrics are divided into weaving, knitting and nonwovens according to their weaving methods, as shown in Figure 25. Figure 25. Detailed image of three main textile structures: a). Woven, b). Knitting, c). Nonwoven. Source: [32] When designing a solar textile, the fabric acts as a covering on the solar cell, which affects the performance of the solar cell. The light reaches the surface of the fabric, and after reflection, absorption and refraction, the refracted light can reach the solar cell covered by the fabric [2], which is shown in Figure 26. Finding a balance between transparency and coverage of textiles is a challenge in designing textiles to cover solar cells. To harvest as much energy as possible, textiles need to be transparent enough to allow light to penetrate it. But to ensure that electronic components are well concealed, textiles need to be opaque enough to cover the electronics underneath them. Figure 26. Light passing through the fabric. Source: [2] Connection techniques of textile wires to the solid and flexible solar cells 31 In this study, the experimental part does not address the effect of fabric selection on aesthetics and light transmittance, and the fabric only serves as an outer covering for solar textile. Mainly research on the connection technology of solar cells and textile wires, the fabric used in the experiment is required to be as consistent as possible, and the impact on the light received by the solar cell is minimal, and white fabric is the first choice. The white fabric has high light transmittance and has little effect on the performance of solar cells. Non-woven fabrics are generally not suitable for machine washing, so only two kinds of fabrics, woven and knitted, and non-woven fabrics are not selected. In order to verify the effect of the fabric waterproof coating on the joining effect, coated and uncoated woven fabrics were selected, and the effect of coating on solar samples could be judged by comparing experimental data. Based on the discussion above, 3 white fabrics were selected for the experiment in this study, as shown in Table 1. Table 1. Experimental fabric details Fabric Company Product Type Finishing Structure Composite Weight 1 Geisa fabrics NANDO Warp knitting Uncoated plain 84 % PES / 16 % EA 220 g/m2 2 FOXA OY Action Mistral Woven Coated - 80 % PES / 20 % PU 145 g/m2 3 CARRING TON VARESE Woven Uncoated 2/1 twill 64% Polyester / 33% TencelTM / 3% EOL (XLANCE®) 205gsm Connection techniques of textile wires to the solid and flexible solar cells 32 4 Experiments for stitching method The experiment procedure is shown in Figure 27. Figure 27. Experiment procedure The flexible solar cells used in the experiment were PowerFilm Solar company's Flexible amorphous silicon solar cells PowerFilm SP 3-37. Textile wire is Madeira HC 12 from Shieldex. Sewing machine is PFAFF selectTM 3.2 sewing machine, the stitch selected is 301 lock stitch. The heat press used in the heat-press process is Plancha Beinsen Semiautomática from ESPIRAL. 4.1 First round of machine-washing experiments Machine washing conditions: 40°C, 800 rmp, 52min. Detergent: 47 g / 75 ml of household laundry detergent Washing machine: LG Direct Drive™ F4J609WN 9Kg 1400 Spin Washing Machine The solar textile sample was packed in a polyester laundry bag and washed with 2 kg of cotton fabric. The washing machine and detergent for machine washing experiments are shown in Figure 28 and Figure 29 separately. Connection techniques of textile wires to the solid and flexible solar cells 33 Figure 28. Domestic washing machine. Figure 29. Domestic detergent. Connection techniques of textile wires to the solid and flexible solar cells 34 The flow of the first round of machine wash testing is shown in Figure 30. Figure 30. The experiment process of washing test (first round) 4.1.1 Samples for the first round of experiments In this experiment, 10 solar textile samples were produced, which is shown in Table 2. In this experiment, 4 pieces of fabric 1, 3 pieces of fabric 2 and 3 pieces of fabric 3 are prepared as parallel samples for trail separately. Table 2. Fabrics for solar textile samples (first round) According to the sewing and embroidery method researched in 2.3.1, the flexible solar cell and the textile wire are connected by a sewing machine with cotton spun yarn, as shown in Figure 31. Different from the stitching in Figure 10, the needle yarn is normal yarn and the No. Description Composite 1 Knitted 84 % PES / 16 % EA 2 3 4 5 Woven coated 80 % PES / 20 % PU 6 7 8 Woven 64% Polyester / 33% TencelTM / 3% EOL 9 10 Connection techniques of textile wires to the solid and flexible solar cells 35 bottom yarn is conductive yarn. To be detailed, the front and back of contact area on flexible solar cell with stitching are shown in Figure 32. Figure 31. Stitching sewing for flexible solar cells. Figure 32. Stitching details on the front and back side of contact area on flexible solar cell Connection techniques of textile wires to the solid and flexible solar cells 36 The layer structure of the solar textile module (first round) is shown in Figure 33. The textile wire is partially exposed outside the encapsulation for easy measurement during the experiment. A humidity sticker is applied diagonally on the back of the flexible solar cell after stitching (Figure 34) and the fabric and TPU adhesive film are hot-pressed together for encapsulation (145°C, 20s, pressure: 3.6 bar) (Figure 35). Figure 33. Layer structure of the solar textile module (first round). Figure 34. Solar cell with humidity sticker (first round). Figure 35. Sample before and after lamination (first round). Connection techniques of textile wires to the solid and flexible solar cells 37 4.1.2 Waterproof property test The surface of the humidity sticker is white, and when it encounters water, the surface of the sticker turns red. Stick a humidity sticker on the bottom left and top right of the back of the solar cell. By observing the reddening of the humidity sticker after washing, the degree of waterproof of the sample was judged. 4.1.3 Electrical measurements The electrical measurement process for the first round of machine-washing test is illustrated in Figure 36. The direct current (DC) voltage of each sample is tested using a multimeter AMPROBE (Figure 37) after stitching and the schematic diagram of electric test is illustrated in Figure 38. Then the flexible solar cell after stitching is packaged, and the DC voltage is tested on the packaged fabric sample. The solar textile sample is tested for DC voltage before machine washing, dried at room temperature after every 5 washes, observed for damage in appearance, whether the sticker turns red, and detects the DC voltage between textile wires exposed to the fabric. Until after 15 washes, disassemble the fabric and observe if the sticker turns red and the solar cells, wires and connections are not broken. Next, solar textile samples were delamination, where TPU and fabric were removed from the surface of the solar cell and the voltage across the textile wire was tested. Test the DC voltage between the wires that were once inside the package and measure the DC voltage across the exposed wires outside the package and compare it. After the solar cell ends are stitched with textile yarn, the measurement is between two conductive yarns. It is important to note that simply testing the yarn exposed outside the encapsulation is not enough to demonstrate a change in solar cell performance. After all, yarns exposed outside the encapsulation may be damaged by the washing process. Therefore, after the washing experiment is completed, the encapsulated sample needs to be disassembled and the ends of the yarn wrapped in the encapsulation measured. Figure 36. The measurement process of textile solar samples (first round). Connection techniques of textile wires to the solid and flexible solar cells 38 Figure 37. Multimeter. Figure 38. Schematic diagram of electric test. Connection techniques of textile wires to the solid and flexible solar cells 39 4.1.4 Visual observation Contrast the visual changes at all stages of the solar textile. To check if the solar cell crack. 4.2 Second round of machine-washing experiments After the first round of testing, the experimental results were scattered and no reliable conclusions could be drawn. Therefore, a second round of experiments was designed to test the conjecture based on the results of the first round (Figure 39). The sample production and testing methods of the first round of experiments were optimized, and the test of the influence of solar cell pores and humidity on the performance of conductive yarns was supplemented. In the machine wash test portion of the second round of experiments, solar textile samples were made in a similar way to the first round of experiments, but the conductive yarns were designed not to be exposed outside the fabric, and an experimental group of no wires was added to verify the effect of solar cell penetration on performance. Figure 39. The experiment process of washing test for samples with stitches (second round). The machine wash conditions of the second machine wash test remain the same. The covering fabric used in the second round of machine wash experiments was uncoated knitted fabric (84 % PES / 16 % EA). After encapsulation, the solar cells and yarn are completely wrapped inside the encapsulation and no longer have textile wires exposed to the fabric. It cannot be tested after packaging, nor after 5 washes, only after 15 washes. So, it is not known if the sample after encapsulation is damaged until delamination. Connection techniques of textile wires to the solid and flexible solar cells 46 5 Experiment results and analysis for stitching method 5.1 First round of machine-washing experiments 5.1.1 Waterproof property test The color variations of the humidity sticker are shown in Table 3. From the experimental results, samples 2, 9 and 10 have poor water resistance, samples 4 and 8 have poor water resistance, and there is no problem with the water resistance of other samples. Table 3. Color change of humidity stickers (first round). Sample Left-down Right-top 1 Knitted 2 3 4 Woven 5 coated 6 7 8 Woven 9 10 moist slightly moist dry 5.1.2 Electrical measurements The voltage test results for the washing phase are shown in Figure 51. The test results for the voltage across the sample performance change after 15 washes and delamination are shown in Figure 52. Figure 51. Evolution of the relative output voltage in the washing test (first round). Connection techniques of textile wires to the solid and flexible solar cells 47 Figure 52. Changes in performance of samples after 15 washes and delamination (first round). Electrical measurements of the first round of experiments cannot be verified. Because the test conditions are not standardized, and the lighting conditions may be different for different tests. Nevertheless, the solar textile samples with fabric 2 maintain good performance after 15 washes compared to the general degradation in performance with other samples. Fabric 2 is a coated fabric, and the solar textile samples produced have stable performance, and the solar textile designed with this fabric has the potential to have high durability. But because this sample is too strong, it is difficult to separate the fabric, TPU, textile wires or solar cells when delamination. Consequently, sample 6 is scrapped after delamination, as shown in Figure 53. Although samples made from this fabric remain performing well, it is not suitable for solar textile product recycling. Figure 53. Damaged solar cell in sample 6. Connection techniques of textile wires to the solid and flexible solar cells 48 In view of the fact that the reliability of stitching connection technology cannot be verified by experimental data, a second round of experiments was designed to optimize the sample production process and test process. Through the failure of the first round of experiments, 3 factors, the humidity and washing of the fabric, the humidity of textile wires and the influence of humidity on perforated solar cells, were suspected to affect the experimental results. To test this conjecture, a group of influence factor tests were designed to verify the correlation of the three factors of fabric humidity and washing, textile wire humidity and humidity on perforated solar cells, respectively. 5.1.3 Visual observation The solar textile samples produced were not visible before and after washing. After delamination, the solar cells are difficult to strip from the TPU film, and obviously cannot be compared with the original solar cells. When the first machine wash test sample was made, the pressure of the heat press was 3.6 bar. Considering that too much pressure could make it difficult to peel off the solar cells, the pressure of the heat press was reduced to 3.5 bar during the second round of experimental sample production. 5.2 Second round machine wash experiments 5.2.1 Waterproof property test According to the results of the humidity sticker experiment in Table 4, each wired sample had a red sticker, and the wireless sample did not turn red. This indicates that there is water ingress inside each wired sample, which may also be the reason for the reduced performance of the wired sample after washing. Even if the preliminary experimental judgment of humidity has little effect on performance, laundry detergent solution may have an impact on the performance of solar cells or yarns. Table 4. Color change of humidity stickers (second round) moist slightly moist dry 5.2.2 Electrical measurements The cause of water ingress inside the wired sample remains to be studied. At present, it is speculated that it may be due to the presence of yarn, which makes the surface of the solar cell uneven, which in turn leads to gaps in the TPU package. During the washing process, the laundry detergent solution enters the wired sample through the gap, corroding the yarn Solar cell Condition Front 1 Front 2 Front 3 Front 4 Front 5 Back 1 Back 2 Back 3 Back 4 Back 5 1 2 no wire 3 4 5 with wire 6 Connection techniques of textile wires to the solid and flexible solar cells 49 and solar cells. In order to optimize this problem, finer yarns should be selected, or flat sewing stitches should be selected to make the surface of the solar cell after sewing flatter. Figure 54. Evolution of the relative output voltage in the washing test (second round). As far as the results of the current second round of 15 machine washes (Figure 54) are concerned, although the performance of solar cells is reduced, the proportion of decline is very low, and the performance is still good. Compared with the results of the first round of 15 machine washes, which also used fabric 1 knit as the covering fabric, the performance of the second round sample was much better than that of the first round. This shows that textile wires exposed to the package are indeed more susceptible to corrosion by washing powder. It may even be possible because of the wicking phenomenon unique to textiles entering the package and corroding the inside of the sample. In the first round of experiments, the end of the textile wire was designed to be exposed outside the package so that the sample could be measured after 5 washes. If the textile wire is fully encapsulated, as in the second round of experiments, it cannot be tested before delamination after lamination. If the sample is damaged, it is not known where the problem occurred. There was no sample damage in this round of experiments. 5.2.3 Visual observation After the machine wash test, the sample is delamination, and the solar cells are manually removed from the package. For operational reasons, some solar cells are crimped and even have creases. It is more difficult to peel off solar cells with wires than in groups without wires. The solar cell is perforated at both ends and may have been damaged by the yarn force. In addition, the combination of yarn and TPU is more difficult to separate. Therefore, when the solar cell is stripped, it is easy to damage the solar cell. The visual compare of separated solar cell from delamination of solar textile samples is taken, and the results for samples with stitching and without stitching are illustrated in Figure 55 and Figure 56 separately. Connection techniques of textile wires to the solid and flexible solar cells 50 Figure 55. Visual observation of separated solar cell from delamination (with stitching). Figure 56. Visual observation of separated solar cell from delamination (without stitching). Connection techniques of textile wires to the solid and flexible solar cells 51 5.3 Influence of humidity on harvesting capacity experiments The results show that the humidity of the holes on the solar cells and the humidity of the textile wires do not affect the performance of the solar cells. However, the humidity and washing of the fabric have an effect on the performance of the solar cell. Compared to the unwashed fabric, the fabric with 15 washes made the performance of the solar cell slightly degraded. The wet fabric gives increased performance compared to the dry fabric. Fabric 3 (uncoated woven fabric) was affected to a greater extent than fabric 1 (uncoated knitted fabric). 5.3.1 Fabric moisture and washing effect test Figure 57 and Figure 58 show the humidity influence of the covered fabric 1 and fabric 3 to the performance of the solar cell samples separately. The wet fabric gives increased performance compared to the dry fabric. Fabric 3 (uncoated woven fabric) was affected to a greater extent than fabric 1 (uncoated knitted fabric). Figure 57. Evolution of the effect of wet and dry covering fabrics on the performance of solar cell samples in comparison (fabric 1) Figure 58. Evolution of the effect of wet and dry covering fabrics on the performance of solar cell samples in comparison (fabric 3) Connection techniques of textile wires to the solid and flexible solar cells 52 As shown in Figure 59 and Figure 60, the difference in test results is not significant compared to unwashed fabric 1 and fabric 3 after 15 washes separately. Compared to the unwashed fabric, the fabric with 15 washes made the performance of the solar cell slightly degraded. This means that the fabric after 15 washes has light effect on the energy harvested by the solar cell. Figure 59. Evolution of the effect of washing influence of covering fabrics on the performance of solar cell samples in comparison (fabric 1) Figure 60. Evolution of the effect of washing influence of covering fabrics on the performance of solar cell samples in comparison (fabric 3) Connection techniques of textile wires to the solid and flexible solar cells 53 5.3.2 Solar cell penetration humidity impact test As shown in Figure 61, humidity has little effect on solar cell performance after perforation. Figure 61. Evolution of the effect of wet and dry of perforated solar cell on the performance of solar cell samples in comparison. 5.3.3 Textile wire humidity impact test As shown in Figure 62, the humidity of the textile wire has little effect on the performance of the sample. Figure 62. Evolution of the effect of wet and dry textile wires on the performance of solar cell samples in comparison. Connection techniques of textile wires to the solid and flexible solar cells 54 6 Results and analysis for tape and adhesive method 6.1 Tape Single-sided adhesive tapes are used to attach textile wires to the connection area of solar cells and have been shown to maintain good performance. In order to have a better conductive effect, the contacts were made by soldering low-resistance copper tape to the solar module contacts [3]. This method is applicable to both flexible and rigid solar cells. The conductivity of the tape is not necessary, after all, the tape does not actually touch the connection area, but fixes the textile wire in the appropriate position to play a connecting role. Although this method does not require high requirements for joining materials, it requires manual and delicate operation, which means that when mass production, errors are easy to generate. If the process of tape application can be optimized and automated production can be realized, it is also a connection technology that can be put into mass production. 6.2 Potential adhesives for solar cells Through the method of adhesives as a connection technique, laboratory synthetic adhesives and the purchase of commercial adhesives are considered. In e-textile, in order to bond electronic components and textiles, many studies have attempted to synthesize flexible conductive adhesives. A polyurethane based ECA is developed to meet all the requirements of flexible interconnects, including an ultralow bulk resistivity that is maintained during bending, rolling, and compressing, good adhesion to various flexible substrates, and facile processing [33]. Transparent, adhesive, stretchable and tough hydrogels via semi-interpenetrating network (SIPN) strategy, which consists of linear poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and chemically cross-linked poly(acrylamide-co-methacrylic acid), was developed [34]. A stretchable conductive adhesive consisting of silver particles with carbon nanotube as an auxiliary filler in silicone adhesives was proposed. Based on strong adhesion to a stretchable substrate, the gel-free dry adhesive printed on elastic bandages for ECG monitoring showed extremely stable performance during subject movement, even after multiple separation-bonding and machine washing [35]. In addition, there are many conductive adhesives on the market that can be applied to electronic products, but few are suitable for combining electronic components with textiles. Most conductive adhesives for electronic components can be applied if flexibility is not a concern. However, because textiles are soft and foldable, flexible conductive adhesives are often required when connected to electronic components. This is also the difficulty of finding a suitable conductive adhesive, soft conductive adhesive has a higher price. In addition, the use of conductive adhesives usually requires heating. Since (flexible) solar cells are not resistant to high temperatures, the maximum temperature is only 150°C, which makes there are fewer flexible conductive adhesives available on the market. The available conductive adhesives on the market are shown in Table 5. Connection techniques of textile wires to the solid and flexible solar cells 55 Table 5. Adhesives for connecting textile wires and solar cells NO. Description Type Company Product Name Assessment of flexibility Hardness Curing Condition 1 Electrically Conductive Adhesive (ECA) silver epoxy ECA SunRay Scientific S-CEP7SF5 flexible - Less than 2 min @ 130°C down/10 min @ 110°C 2 Two component, room temperature curing epoxy for bonding, sealing and coating ECA Master Bond EP79 stiff - overnight at room temperature followed by a post cure of 1-2 hours at 150-200°F 3 bonding heat-sensitive components, bonding flexible circuits ICA Panacol Elecolit® 3036 Medium flexible Hardness shore D: 70-80 at room temperature or more rapidly with heat （25 ℃ - 150 ℃） 4 LCD bonding, bonding flexible conductors ACA Panacol Elecolit® 3063 High flexibility Hardness shore A: 60-70 UV + pressure, VIS + pressure 5 Flexible Electrically Conductive Silver Epoxy, Thermally Conductive Adhesive Solder Replace ECA ConductiveX ElectroBond 16 High flexibility Hardness shore D: 64 24 hours @ Room Temp/ 2 to 4 hours @ 60°C 6 Flexible Highly Conductive Pure Silver Epoxy Elecrically Conductive Adhesive 1 to 1 Mix D ECA ConductiveX ElectroBond 17 High flexibility Hardness shore D: 63 24 hours @ Room Temp/ 2 to 4 hours @ 60°C 7 Flexible Nickel Epoxy Electrically Condutive Conductivity Nickel Filled Adhesive EMI RFI ECA ConductiveX ElectroBond F3 High flexibility Hardness shore D: 65 24 hours @ Room Temp/ 2 to 4 hours @ 60°C Despite the high price, according to testing studies, SunRay Scientific's ECA conductive adhesive is considered to be the most suitable flexible conductive adhesive product. It is proven to be suitable for the combination of textiles and electronic components, with low operating temperatures, good connection quality and very flexible [36]. Forms are optional flexible conductive adhesives. In principle, these glues can be used as a connection technology to connect solar cells (whether flexible or rigid) and textile wires. However, this study does not confirm whether any conductive adhesive is suitable for solar textile. However, this method of conductive adhesive has a disadvantage, that is, adhesive as a chemical reagent remains a problem for recycle without sacrificing the environment. Whether or not the use of the adhesive connection technique for textile wires to solar cells will affect the environment needs to be followed up and investigated to determine. Connection techniques of textile wires to the solid and flexible solar cells 62 11 Appendix The raw data for the experimental part of the experiment are as follows. First round of machine-washing test Table 1. Solar cell performance at every stage (1) Sample Voltage（V） After stitching After encapsulation Before washing Washed 5 Washed 10 Washed 15 After delamination Knitted 1 3.560 2.309 1.989 0.626 0.628 0.145 1.756 2 3.019 2.953 2.489 0.478 0.157 0.335 1.747 3 3.638 3.319 2.932 2.257 0.798 0.054 1.717 4 3.627 3.345 2.962 2.375 1.281 0.659 2.293 Woven coated 5 3.617 3.107 2.236 1.472 1.515 1.236 - 6 3.652 2.974 0.322 0.407 0.655 0.033 0.776 7 3.557 2.034 1.429 1.510 1.486 1.518 - Woven 8 3.631 3.390 2.866 0.005 0.001 0.001 0.128 9 3.610 3.297 2.309 2.286 2.159 0.001 2.107 10 3.515 3.088 2.643 1.997 0.173 0.053 0.564 Reference R1 3.209 3.310 2.118 2.215 2.101 2.118 3.308 R2 3.632 Table 2. Solar cell performance at beginning and end Sample Voltage（V） After stitching After delamination Yarn out Yarn inner 1 3.560 1.023 1.019 Knitted 2 3.019 0.367 0.374 3 3.638 1.139 1.133 4 3.627 0.998 1.002 Woven 5 3.617 0.675 0.002 coated 6 3.652 0.328 0.336 7 3.557 0.004 0.005 8 3.631 0.000 0.010 Woven 9 3.610 0.000 1.184 10 3.515 0.096 0.101 Reference 3.209 1.545 1.561 Connection techniques of textile wires to the solid and flexible solar cells 63 Second round of machine-washing test Table 3. Solar cell performance at every stage (2) Condition Current (μA) Solar cell Before stitching After stitching and sticker After washing 15 Solar textile sample without wire 1 354.8 347.9 375.8 2 351.8 345.8 352.1 3 359.2 353.2 351.3 Reference solar cell without wire 4 362.7 346.8 354.8 Solar textile sample with wire 5 351.6 332.1 292.7 6 363.1 328.4 295.5 7 357.2 350.4 330.4 Reference solar cell with wire 8 354.5 328.6 357.6 Light condition (lux) 1247 1250 1271 Influence factor experiments Table 4. Solar cell performance with washed / unwashed fabric Sample Washed fabric Unwashed fabric 1 Reference without fabric Fabric 1 Piece 1 Piece 2 Piece 3 Piece 4 Current (μA) 91.6 87.7 93.9 90.7 112.2 363.8 Lux 1274 1277 1278 1280 1281 1281 Sample Washed fabric Unwashed fabric 3 Reference without fabric Fabric 3 Piece 8 Piece 9 Piece 10 Current (μA) 104.4 110.9 112.2 121.1 363.5 Lux 1301 1302 1301 1300 1299 Table 5. Solar cell performance with wet / dry fabric Fabric 1 (84 % PES / 16 % EA knitted) Sample 1 2 3 Reference without fabric Dry Wet Dry Wet Dry Wet Weight (g) 0.78 1.66 0.78 1.77 0.78 1.82 - Water content (%) 112.8205 126.9231 133.3333 - Current (μA) 94.5 118.6 100.1 129.7 95.1 133.9 357.3 Lux 1306 1305 1304 1305 1304 1304 1302 Connection techniques of textile wires to the solid and flexible solar cells 64 Fabric 3 (64% Polyester / 33% TencelTM / 3% EOL woven uncoated) Sample 1 2 3 Reference without fabric Dry Wet Dry Wet Dry Wet Weight (g) 0.8 1.48 0.68 1.31 0.8 1.55 - Water content (%) 85 92.64706 93.75 - Current (μA) 116.4 205.7 117.9 194.2 117.5 192.8 359.1 Lux 1306 1303 1305 1305 1306 1305 1303 Table 6. Solar cell performance with humidity on perforated solar cell Solar cell Current (μA) Before perforation After perforation 1 drop 5 drops 10 drops Lux 1 368.7 360.5 356.6 357.3 355.1 1310 2 372.1 357.5 357.4 358.9 359.7 1308 3 376.4 370.1 371.3 373.4 375.7 1309 Reference 368.9 - Lux 1298 - Table 7. Solar cell performance with humidity on textile wires Solar cell Current (μA) Before stitching After stitching 1 drop 5 drops 10 drops Lux 1 366.8 365.4 364.4 363.5 364.7 1302 2 369.1 354.4 358.1 355.8 354.8 1301 3 367.4 361.2 357.9 359.1 358.2 1300 4 368.9 365.1 365.3 366.4 368.1 1299 Reference 369.4 - Lux 1303 -