Master of Science in Mechanical Engineering June 2020
Performance evaluation and development of contact solutions for flexible organic solar cells
Bastiaan Hamer
This thesis is submitted to the Faculty of Engineering at Blekinge Institute of Technology in partial fulfilment of the requirements for the degree of Master of Science in Mechanical Engineering. The thesis is equivalent to 20 weeks of full time studies.
The authors declare that they are the sole authors of this thesis and that they have not used any sources other than those listed in the bibliography and identified as references. They further declare that they have not submitted this thesis at any other institution to obtain a degree.
Contact Information:
Author:
Bastiaan Hamer
E-mail: baha14@student.bth.se
University advisor:
Alessandro Bertoni
Department of Mechanical Engineering
A BSTRACT
In today’s society many non-renewable and environmentally harming energy sources are used to facilitate people’s everyday energy demands. This causes ecosystems to break down, global temperatures to rise, pollution and many more critical long lasting problems. By replacing non- renewable energy sources and taking advantage of the 100% renewable energy source, light, these problems will diminish. This project has been in collaboration with a company called Epishine who develop indoor organic solar cell devices to be able to replace conventual battery driven electrical devices with solar power harvested from indoor light. Since there is no good existing contacting solution, for Epishine to be able to enter the market, a contact solution between their solar cell device and the electrical devices it will power has to be developed. This thesis focuses on developing, designing, testing and evaluating the performance of new contact solutions for encapsulated flexible organic printed solar cells with the feasibility, viability, scalability and durability in focus.
This project was conducted by first performing a literature study, thereafter, establishing a baseline for future referencing of new contact solutions and the main part, developing new concepts and evaluating them. By using the design thinking method, an iterative process could take place, allowing for a constant flow of new ideas whilst testing concepts throughout the project.
The baseline tests were successful and the hypothesis of organic materials degrading over time was confirmed. From the many sub-concepts and production methods for a new contacting solution, two concepts showed promising results and were merged into one main concept. Two devices were created with the new concept, one functional device and one showing the design.
To conclude, the thesis resulted in a functional solar cell device with a new contact solution which shows great potential and a new production method which enables all organic printed electronics to be design and developed in a more compact and component dense design. This production method is beneficial to not only Epishine, but everywhere where printed electronics are used and need to be optimized due to restrictions such as space and weight.
Keywords: Organic electronics, printed electronics, OPV, solar cell, renewable energy.
S AMMANFATTNING
I dagens samhälle används många icke-förnybara energikällor för att underlätta människans vardagliga behov men skadar samtidigt miljön. Detta leder till att hela ekosystem fallerar, den globala temperaturen stiger, giftiga ämnen släpps fria och flera kritiska, långvariga problem skapas. Genom att byta ut icke-förnybara energikällor och istället dra nytta av den 100 % förnybara energikällan, ljus, kommer dessa ovanstående problem att minska. Detta projekt har varit i samarbete med ett företag vid namn Epishine som utvecklar organiska solcellsenheter för inomhusbruk, för att kunna ersätta konventionella batteridrivna elektriska apparater med solenergi tillvaratagen av inomhusbelysning. I dagsläget finns det ingen bra kontaktlösning mellan solcellsenheten och den apparat den ska driva, vilket är ett av Epishines större problem i nuläget, som hindrar dem från att kunna slå igenom på marknaden.
Denna avhandling fokuserar på att utveckla, designa, testa och utvärdera prestandan av nya kontaktlösningar för inkapslade flexibla organiska solceller.
Projektet började med en litteraturstudie, därefter etablerades en ”baseline” för att kunna jämföra de nya kontaktlösningarna. Största delen av rapporten handlar om att utveckla och testa nya kontaktlösningar för att sedan utvärdera dem. Genom att använda ”Design thinking” processen, kunde en iterativ process äga rum, vilket möjliggjorde ett konstant flöde med nya idéer som genererades samtidigt som koncept och prototyper utvecklades och utvärderades.
Resultaten av ”baseline”-testerna var framgångsrika och hypotesen om att de konduktiva egenskaperna av organiska material försämras med tiden bekräftades. Från alla delkoncept och potentiella produktionsmetoderna för en ny kontaktlösning visade två koncept lovande resultat och slogs därför samman till ett huvudkoncept. Två olika solcellsenheter skapades med den nya kontaktlösningen implementerad. En funktionell enhet skapades och en enhet som visar layouten och designen.
Sammanfattningsvis resulterade avhandlingen i en funktionell solcellsenhet med en ny kontaktlösning som visar stor potential samt en ny produktionsmetod som gör att all organisk tryckt elektronik kan designas och tillverkas i en mer kompakt och komponenttät design. Denna produktionsmetod är en fördel inte bara för Epishine utan också överallt där tryckt elektronik används och behöver optimeras i form av utrymme och vikt.
Nyckelord: Organisk elektronik, tryckt elektronik, OPV, solcell, förnybar energi.
Preface
Due to a non-disclosure agreement, the majority of the concepts are classified, hence they can’t be disclosed in this report. Therefore, certain steps in the production process and idea generation can’t fully be revealed as well as the technology used in the final prototype.
I would like to thank Epishine for letting me write my master thesis at their company and use their equipment. Thank you to all employees for helping and answering questions and a special thank you to my supervisors, Luis Aguirre, Hassan Abdalla and Alessandro Bertoni for always helping and giving valuable feedback. I am very thankful to Epishine for giving me the chance to finish the thesis although the circumstances with the COVID-19 situation.
Bastiaan Hamer 14-06-2020
Abbreviations and explanations
Mm Millimeter = 10-3 m
µm Micrometer = 10-6 m
nm Nanometer= 10-9 m
OPV Organic Photovoltaic solar cell
PET Polyethylene terephthalate. UV-resistant plastic
Substrate Plastic sheet made from PET
ETL Electron Transport Layer. A layer which
transports electrons in an OPV
HTL Hole Transport Layer. A layer which transports
holes in an OPV
AL Active layer. The part of the solar cell which
harvest light
Barrier Plastic sheet used by Epishine to encapsulate
devices.
PEDOT A conductive polymer used in OPVs
Stack A word used to refer to all layers in an OPV
R2R Roll to Roll. A manufacturing process used when
producing OPVs.
C ONTENTS
ABSTRACT ... III SAMMANFATTNING ... IV CONTENTS ... VII
LIST OF FIGURES ... 0
1 INTRODUCTION ... 1
1.1 BACKGROUND CONTACT SOLUTION FOR PRINTED ELECTRONICS ... 1
1.2 BACKGROUND EPISHINE ... 2
1.3 IMPORTANT FACTORS FOR A CONTACTING SOLUTION ... 4
1.4 SCOPE ... 4
1.4.1 Problem statement ... 4
1.4.2 Aim ... 4
1.4.3 Thesis question ... 4
1.5 THEORY ... 5
1.5.1 Organic photovoltaic solar cells ... 5
1.5.2 Organic printing techniques for electronics ... 5
1.5.3 Basic electronics ... 8
1.6 LIMITATIONS ... 10
2 RELATED WORK ... 11
2.1 CURRENT CONTACTING SOLUTION ... 11
2.2 PCB CONNECTOR ... 11
2.3 PCBVIAS ... 11
2.4 EXISTING SOLUTIONS BY ALL FLEX INC ... 12
3 METHOD ... 13
3.1 DESIGN THINKING ... 13
3.1.1 Initiation ... 13
3.1.2 Inspiration ... 14
3.1.3 Ideation ... 14
3.1.4 Implementation ... 14
3.2 BASELINE ... 15
3.3 INTRODUCTORY LITERATURE STUDY ... 16
3.4 IDEA GENERATION AND TECHWATCHING ... 16
4 RESULTS AND ANALYSE OF THE ITERATIVE DEVELOPMENT ... 17
4.1 LITERATURE STUDY AND IDEA GENERATION ... 17
4.2 RELOCATED NICOMATIC CONTACTS ... 18
4.3 INTERCONNECTING LAYERS CONCEPTS ... 18
4.3.1 Layout design concepts ... 19
4.3.2 Interconnecting layer contact solutions... 21
4.4 INTERCONNECTING LAYER CONCEPT PRODUCTION METHODS ... 22
4.4.1 Production concept 1 ... 22
4.4.2 Production concept 2 ... 26
4.4.3 Production concept 3 ... 28
4.4.4 Production concept 4 ... 30
5 RESULTS AND ANALYSIS ... 31
5.1 BASELINE ... 31
5.1.1 Probe ... 31
5.1.2 Nicomatics ... 33
5.1.3 Comparison probe vs Nicomatic ... 36
5.2 MAIN CONTACT SOLUTION CONCEPT ... 37
6 DISCUSSION ... 38
6.1 BASELINE ... 38
6.1.1 Potential measurement errors ... 38
6.1.2 Discussion on baseline results ... 38
6.2 DESIGN THINKING PROCESS ... 39
6.3 RELOCATED CONTACTING POINTS ... 39
6.4 INTERCONNECTING LAYER CONCEPT DESIGNS ... 40
6.4.1 Layout design concepts ... 40
6.4.2 Interconnecting layer contact solutions... 41
6.5 INTERCONNECTING LAYER CONCEPT PRODUCTION METHODS ... 41
6.5.1 Production concept 1 ... 42
6.5.2 Production concept 2 ... 43
6.5.3 Production concept 3 ... 43
6.5.4 Production process 4 ... 44
6.6 MAIN CONTACT SOLUTION CONCEPT ... 44
6.7 INTERCONNECTING LAYERS CONCEPT APPLICATION POSSIBILITIES ... 45
7 CONCLUSION AND FUTURE WORK ... 46
7.1 CONCLUSION ... 46
7.2 THE THESIS’S CONTRIBUTION ... 46
7.2.1 Contribution to the flexible printed electronics industry ... 46
7.2.2 Contribution to Epishines contact solution problem ... 46
7.3 FUTURE WORK ... 46
REFERENCES ... 47
APPENDIX ... 49
APPENDIX A ... 49
List of figures
Figure 1. Cross section of a general Epishine solar cell stack. 2 Figure 2. Picture (left) and drawing (right) of Epishines current solar cell design. 3
Figure 3. Nicomatic standard male tab [2]. 3
Figure 4. Screen printing process illustration [3]. 6
Figure 5. Illustration of a blade coating process [4]. 7
Figure 6. Illustration of a slot-die coating process [5]. 7
Figure 7. Illustration of all measured resistances when measuring an object with a multimeter. 9 Figure 8. Illustration of a four-probe resistance measurement of an object. 9 Figure 9. Illustrational graph of the design thinking process used. 13
Figure 10. Carbon ear solar cell design. 18
Figure 11. Solar cell stack with double-sided, interconnecting carbon layers the lower substrate. The connection is bridged in this illustration due to the real method being classified. 19 Figure 12. Design idea with the interconnecting layer technology implemented. 19 Figure 13. Design idea with interconnecting layer technology implemented. 20 Figure 14. Contact solution concept combined with interconnecting layer technology. 21 Figure 15. Contact solution concept combined with interconnecting layer technology. 21 Figure 16. Contact solution concept combined with interconnecting layer technology. 22 Figure 17. Cross section illustration of C1O1 and C1O2 by screen printing on both sides of a pretreated
substrate. 23
Figure 18. Cross section illustration of four-probe planar measurement (left) and between-layer measurement (right). The bridge is the connection between the two layers. 24
Figure 19. Top-view of the same sample as shown in figure 18. 24
Figure 20. Graph of between-layer resistances measured with different treatments. 24 Figure 21. Planar resistance measurements on different t1 combinations. 25 Figure 22. Between-hole resistance measurements on different t1 combinations. 25 Figure 23. Normalized values for the different treatment combinations. 26 Figure 24. Normalized values for different pretreatments on blade coated samples. 27
Figure 25. Cross section view of production concept 3. 28
Figure 26. Normalized values for different between-layer treatment combinations. 29 Figure 27. Normalized values for different between-layer treatment combinations. 29 Figure 28. Average decay of resistance of six different material combinations aged in an ambient
environment measured with probes. 31
Figure 29. Average decay of resistance of six different material combinations aged in a climate chamber
measured with probes. 32
Figure 30. Average decay of resistance of six different material combinations aged in a glove box
measured with probes. 32
Figure 31. Average decay of resistance of six different material combinations aged in an oven and
measured with probes. 33
Figure 32. Average decay of resistance of six different material combinations aged in an ambient
environment with Nicomatics as contact points. 34
Figure 33. Average decay of resistance of six different material combinations aged in a climate chamber
with Nicomatics as contact points. 34
Figure 34. Average decay of resistance of six different material combinations aged in a glove box with
Nicomatics as contact points. 35
Figure 35. Average decay of resistance of six different material combinations aged in an oven with
Nicomatics as contact points. 36
Figure 36. Comparison graph of average resistance over time between probe and Nicomatics. 36 Figure 37. Layout design prototype. Frontside (left) and backside (right) 37
Figure 38. Functional prototype. 37
1 I NTRODUCTION
1.1 Background contact solution for printed electronics
The first printed circuit board was produced by Paul Eisler in 1936 [1, p. 1], 33 years after Albert Hansson coined the idea of printing wires [1, p. 10]. Since then, there has been a constant development of printed technology and it is used in the majority of electrical devices. During recent years, printed electronics have been used to develop and produce photovoltaic solar cells. The year 2011 [2], researchers from MIT created the first printed photovoltaic solar cell and the first organic solar cell was created by researchers at Rice University in 2018 [3]. Since then, several companies have started producing printed solar cells and organic photovoltaic (OPV) cells have reached an efficiency of 17,4 % [4], [5]. Even though an organic solar cell is not as efficient as other technologies [5], it shows a lot of potential because of the price, flexible design and ease of production. Because OPVs are printed onto a flexible plastic polyethylene terephthalate (PET) sheet, called a substrate, it can be produced in a so- called roll-to-roll (R2R) process. A R2R process lets a substrate unroll from a roll into a series of machines who use different additive printing techniques to apply conductive materials to the substrate.
After all the steps are completed in the R2R process, a giant solar cell of several kilometers in length is rolled up on another roll. In the last stage, the giant solar cell is cut into smaller solar cells called devices.
These devices are most likely to be encapsulated to prohibit oxygen and moisture to enter the solar cell.
The last step is to make contacting points between the solar cell and e.g. a pair of wires to connect to an electrical device in need of a current supply. It is this step, the contacting between the device and the outside world which is an unsolved problem within the flexible encapsulated printed electronics industry in present day.
There are many good contacting solutions for printed hardboard circuits on the market but not for flexible encapsulated printed electronics. The problematic part of creating a contact on a flexible organic printed electronic device is the fact that the connection must be flexible in itself. If it is not, after a certain time, the connection will fatigue and break due to mechanical failure. A second issue which this thesis is faced with is the complexity that the printed devices must be encapsulated. This complicates the contacting solution drastically because an electrical connection must be made by penetrating the encapsulation of the device, without compromising it, and reach the conductive layer within the device without shorting it, while being flexible.
1.2 Background Epishine
This project has been conducted together with a start-up company called Epishine who are specialized in printing encapsulated OPVs for inside use. This means the solar cells are specifically developed to be able to harvest low energy light typically emitted by lamps and ambient light in an environmentally friendly and sustainable way. Epishine is able to replace conventional batteries with wireless solar power and thereby reduce maintenance costs with 80% and one’s battery related climate impact by up to 99%. Development is also ongoing for outside-use solar cells for incorporation into e.g.
buildings [6].
The solar cells are constructed of several layers of printed material and plastic sheets. All layers combined are called a stack which the solar cell consists of. Epishines solar cells generally look like the stack displayed in figure 1.
Figure 1. Cross section of a general Epishine solar cell stack.
The solar cell consists of two substrates which sandwich the layers which harvest the solar power and transport it to the anode and cathode. The barriers are plastic sheets which encapsulate the solar cell fully to prevent oxygen or moisture to enter. The active layer (AL) is the layer which harvest the light and extracts electrical power from it. Electron Transport Layer (ETL) and Hole Transport Layer (HTL) are the layers which transport the “holes” and “electrons” to the carbon transport layer which also act as anode and cathode. The PEDOT layer is made from a polymer mixture and allows the electricity to flow from the HTL and ETL to the anode and cathode of the device. A more in depth explanation of the different layers can be found in chapter 1.5.1.
Just as all other companies within the OPV industry, Epishine has yet to find a contacting solution for their solar cells. The inside-use devices are currently in their final stages of development before hitting the market, and therefore, finding a contacting solution is of high importance for Epishine. Their current solution is using a metallic stamp-connection from Nicomatic [7] which pierces through the encapsulation layer called the barrier into a conductive carbon layer which acts as a transportation layer.
Figure 2. Picture (left) and drawing (right) of Epishines current solar cell design.
The carbon layer is closely located to the right and left of the solar cells affective area (see figure 2) which causes problems when a stamp connection is installed. Due to the stamp piercing through the barrier, it lets oxygen and moisture enter underneath the barrier over time and deteriorates the organic materials and potentially short circuits the solar cell. More on why this happens can be found in chapter 1.5.1.1. As seen in figure 3, the sharp edges are pierced through the complete stack and then bent for a secure mechanical contact. This gives the contact solution, which is the Nicomatic stamp, a relatively good connection to the carbon layer and establishes a secure and reliable connection to the solar cell.
Figure 3. Nicomatic standard male tab (adapted from [7, p. 4]).
The Nicomatic stamps create problems in the manufacturing process as well. To be able to incorporate a stamping machine into the R2R process would introduce even bigger difficulties and secondary steps would need to be incorporated to be able to seal the device from water and oxygen. Therefore, a new contacting solution has to be developed which does not compromise the lifetime, performance or stability of the solar cell while being feasible and viable enough to incorporate into the R2R process.
1.3 Important factors for a contacting solution
There are many different factors which affect the desired outcome for a contact solution. The main factors which dictate the desired outcome are performance, layout/design, applicability, feasibility and usability. First of all, the connection must be able to transfer electrical power from the solar cell to an electrical device in a as efficient way as possible with minimal losses. If a contact solution does not transfer the electrical energy in an efficient way, a lot of energy harvested by the solar cell is wasted.
Even if the solar cell is able to achieve an high efficiency, if half of the energy dissipates due to transportation, the performance of the complete device will be inadequate. The layout and design affect all other factors such as performance, applicability, feasibility and usability. By moving the connections further from the active area, shown in figure 2 in chapter 1.2., which would solve the problem of moisture and oxygen interacting with the solar cell, it would decrease the performance of the contact solution due to added transportation length (the theory behind this is explained in chapter 1.5.3.1).
Depending on how the layout of the contacting points are designed, the applicability and usability are affected. E.g. if the contact solution requires a lot of area, either the active area will be compromised, or the size of the device has to be increased. In either of these cases, the performance is decreased in regard to mV/cm2, because the active area vs total area is decreased. This affects the applicability and usability in a negative way because the device will have to be bigger and output the same power or have the same size but less power. Another important factor which affect the applicability is the modularity of the device. If the device has a predetermined fixed place for the contacting points to be situated, potential customers may choose not to use Epishines devices. By designing the device’s contacting points to be modular, the applicability increases due to the increased options. The feasibility is a factor which takes the production process in consideration. If the contact solution is too intricate and difficult to incorporate into a R2R process, it will not be feasible nor viable to consider as a potential solution.
1.4 Scope
1.4.1 Problem statement
As explained in chapter 1.1, 1.2 and 1.3, there is a need within the printed electronics industry which Epishine also has, to develop a durable and efficient contact solution for encapsulated printed electronics. More specifically, Epishine is in need for a contact solution for encapsulated flexible organic printed electronics for there solar cells. In this thesis, a
1.4.2 Aim
The aim with this thesis is to develop, design, fabricate and evaluate potential electrical contact solutions for encapsulated printed electronic devices. By testing different contacting solutions for encapsulated printed solar cells, Epishine will know which concepts are worth pursuing.
1.4.3 Thesis question
To be able to achieve the desired results based on the problem statement and aim of the thesis, the following thesis question has been formulated:
1. How can a durable and efficient electrical contact for encapsulated printed electronics be assured without damaging or compromising the solar cell’s active area?
To be able to answer the thesis question, the definition of durable and efficient need to be set into context. Therefore, a sub-question is formulated:
2. What is considered durable and efficient regarding a contact solution for encapsulated printed electronics?
1.5 Theory
1.5.1 Organic photovoltaic solar cells
An organic photovoltaic solar cell (OPV) can convert light into electrical energy in a more environmentally friendly way than conventional solar cells such as silicon based solar cells. Due to the different production processes, the OPV technology is cheaper to produce [8].
An organic photovoltaic solar cell contains a stack of materials which are printed on top of each other. The harvesting of light through solar cell technology is done by choosing specific materials for the AL. The AL consists of a donor layer and acceptor layer. When light with a higher energy than the band gap reaches the donor material, it will excite an electron and create a “hole”. The band gap is the minimum energy needed to free an electron from its bond in a specific material. The electron is negatively charged and will travel to the acceptor. The electron will then travel through the electron transport layer (ETL) to the cathode. The hole is positively charged and will travel through the hole transport layer (HTL) and reach the anode of the solar cell. The anode and cathode are the carbon strips seen in figure 1 and 2. When e.g. a lamp is connected to the anode and cathode, the electrons will travel from the cathode to the anode and pair up with a hole. As long as light is exciting electrons from the donor layer, this process will continue, and electricity is created [8].
The combination of donor and acceptor material compositions is important when designing solar cells for different environments. The band gap between the two materials has to be designed in a way so that the light which hits the donor material will have enough energy to excite the electron and let it jump down to the acceptor material [8]. This is where Epishine has chosen to produce an AL which is able to harvest low intensity light from e.g. indoor light instead of sunlight.
1.5.1.1 Aging of OPVs
Because OPVs consist of organic materials, they lose performance over time and when in contact with oxygen and/or water, they decay even faster [9, p. 430]. This is one of two reasons for OPVs to be encapsulated, the other reason being prevention of external damage. There are two types of encapsulation methods for flexible organic printed electronics widely used. The first one is encapsulating the device between two glass lids and sealing of the edges with UV glue. The other method, used in this thesis and by Epishine, is to encapsulate the device between two thin-film plastic sheets, called barriers. By applying UV glue inside of the encapsulation, the glue will prevent moisture and oxygen to enter the device and degrade the organic materials. There are two types of aging of organic materials, the first one being natural aging which is the natural decay of material properties and structure of the organic materials. The decay of the organic materials leads to diminished conductivity and performance. All other aging is classified as unnatural aging e.g. when water or oxygen reacts with the materials or changes in temperature. Different organic material combinations decay at different rates due to the structure and chemical composition of the material. When an organic material ages from natural causes, the material’s conductivity is aging at a faster rate at the start of the process and stabilizes after a certain time. This means that the material stabilizes after a certain period of time and will perform in a predictable manner, if no unnatural decay occurs. If an unnatural decay occurs through e.g. oxygen and water chemically reacting with the organic materials, the aging process will accelerate [9, pp. 430- 431]. Therefore, it is of great importance for a long lasting device to be sealed of properly with a durable encapsulation method.
1.5.2 Organic printing techniques for electronics
Organic printed electronics is a technique used to create electrical devices. By printing a conductive organic material such as carbon on to a substrate, an electrical connection can be made between electrical components. There are many different printing techniques which enable printing of different organic materials. Examples of printing techniques are: Screen printing, slot-die coating, blade coating, flexography, inkjet and gravure. In this thesis, the printing techniques used are screen printing,
1.5.2.1 Screen printing
Screen printing is a printing technique where in a conductive ink is pressed through a screen and applied to a substrate. The screen is made out of a mesh of silk, plastic or metal threads where on a stencil is applied on top of it to acts like a template. The screen is covered by a stencil everywhere but where the ink is meant to pass through. The mesh only allows a certain amount of ink to pass through, resulting in a thin layer of printed ink on the substrate [10, p. 55]. Depending on which mesh is used, a thinner or thicker layer can be applied. A screen printer is constructed of a frame where in the screen is stretched. The frame is put above the substrate in a way in which the screen is suspended above the substrate and does not touch it. A thick past of conductive material/ink placed upon the screen is moved from one side to the other by the blade/squeegee. When the pressure from the blade is enough, the screen will touch the substrate and deposit a layer of ink, where the stencil is cut out. This results in a thin layer of ink deposited on the substrate according to the template on the stencil [10, pp. 55-56]. If the screen isn’t touching the substrate, it will not deposit any ink through the mesh.
Figure 4. Screen printing process illustration (adapted from [11]).
The thickness of the ink, pressure and angel of the blade and the properties of the mesh are all factors which determine the final result of the printed ink [10, pp. 55-58].
At Epishine, a variant of screen printing is used in the R2R production called rotary screen printing. It is the same principle as the screen printing process explained above, but instead of using a blade/squeegee which moves across the screen, the substrate is moving linearly past the screen rotor which is rotating [10, p. 57]. While the screen is in contact with the substrate is rotates at the same rate as the substrate is moving, resulting in ink deposited on the substrate through the screen. The ink deposited by the screen rotor as well as the manual screen used in this thesis is 10 µm thick, but when dried about 4-5 µm.
1.5.2.2 Blade coating
Blade coating is a printing technique which is able to spread a paste of conductive material/ink on a substrate with great precision. The machine used in this thesis consists of a base plate, a motor and drivetrain which move a blade [12]. The blade is adjustable in height, and when pushed from one side of the substrate to the other, will spread material in an even layer. The height adjustability ensures that a thin film of ink is deposited on the substrate. The parameters which affect the coating is the speed in which the blade travels, the height of the blade and the viscosity of the ink [12]. By increasing or decreasing these parameters a thinner or thicker coating can be printed depending on how the machine
is set up. Some blade coaters are equipped with a heating bed as a base plate where on the substrate is positioned. This allows the newly coated ink to be cured faster.
Figure 5. Illustration of a blade coating process (adapted from [13]).
In Figure 5 the fabric/substrate is moving, which usually is the case when using a blade coating process in a R2R process. When using a blade coater on a smaller scale, as done in this thesis, the blade moves instead of the substrate.
Although the blade coating process can be applied in a R2R process, Epishine uses the blade coating technique in their testing phases while prototyping.
1.5.2.3 Slot-die coating
Slot die coating is a coating method used when coating printed electronics which lets the substrate, where the ink is deposited, move relative to the slot-die head. The machine has a so-called slot-die head which has two die lips positioned very close to each other as well as to the substrate. Between the die lips there is a shim also called a lip land or meniscus guide. A fluid pump is connected to the inlet of the slot-die head and feeds the head with ink. The ink is forced through the head and pressed out onto the substrate [13, p. 2508]. This ensures a uniform coating of ink on the substrate. The thickness of the coating can be varied by changing the viscosity of the ink, increasing or decreasing the substrate travelling speed as well as changing the ink feed rate by increasing or decreasing the pressure of the fluid pump [14].
Figure 6. Illustration of a slot-die coating process (adapted from [15]).
Epishine uses the slot-die production technique in their R2R process when applying the AL as well as the PEDOT layers.
1.5.3 Basic electronics
To be able to understand which parameters are important for an electrical contacting solution, some basic electronic knowledge must be understood. Therefore, a brief explanation of different important terms, parameters, formulas and measuring methods are given.
1.5.3.1 Resistance and resistivity
The resistance of a substance is defined as how difficult it is for current to pass through an object or substance. It is a physical property which can be described by Ohm’s law, formula 1 as well as formula (2):
Formula (1): 𝑅 =𝑈𝐼
Where R is resistance measured in Ohm (Ω), U is the voltage measured in Volt (V) and I is the current which is measured in amperes (A).
Formula (2): R=⍴×𝐿𝐴
Where ⍴ is resistivity measured in Ohm-meter (Ωm), A is the cross sectional area (m2) and L is the length of the object measured in meter (m).
When the resistance is high in a substance, the conductivity is low and vice versa. This is because they are the inverse of each other. As seen in formula X1, if the length of a object is increased, so does the resistance.
Resistivity is depending on the dimensions of an object and its substance and can be described by the following formula.
Formula (3): ⍴=𝑅×𝐴𝐿
Where ⍴ is resistivity measured in Ohm-meter (Ωm), R is resistance measured in Ohm (Ω), A is the cross-sectional area (m2) and L is the length of the object measured in meter (m).
Formula (2) shows that the resistance is dependent on the cross-sectional area as well as length of the object being measured. When the area is increased on a conductive substance, the resistance decreases and if the length increases, so does the resistance and vice versa. The resistivity of the object on the other hand, will remain the same.
1.5.3.2 Four-probe method
A four-probe method is used when a resistance measurement is performed and must be more precise than measuring with a multimeter. With a multimeter, the resistance of the contact points from the probes touching the material is too great to neglect when performing precise measurements.
Therefore, a four-probe method is used where in the resistance of the contact points between the surface and the probes are not measured.
When the resistance is measured with a traditional multimeter, there will be a certain resistance for the object, R5 in figure 7, which is measured but also a resistance in the wires of the probes, R1 and R2 and the contact points between the probes and the object, R3 and R4. Because the resistance is dependent of the area, as shown in formula 2, the minimal surface area between the probes and the object result in a high resistance. To conclude, if the resistance of an object is to be measured with a multimeter with probes, the resulting resistance shown on the display is not only the resistance of the object, but a collective resistance of the complete circuit created when measuring as seen in figure 7.
Figure 7. Illustration of all measured resistances when measuring an object with a multimeter.
As seen in figure 7, the resulting resistance displayed on the multimeter will be the sum of all resistances, R1 + R2 + R3+ R4+ R5 = Rtotal. Even though R5 will result in the biggest resistance, all the other resistances will give a contribution and therefore give a false reading. Because the only resistances which is interesting is R5, all other resistances will need to be cancelled out which is done by using the four-probe measuring method.
The four-probe method uses a source of current which is applied on the outside probes and a separate pair of probes which measure the voltage drop between the inner probes as seen in figure 8.
By separating the current applying and voltage sensing probes into two separate pairs, the contact and wire resistance is eliminated.
Figure 8. Illustration of a four-probe resistance measurement of an object.
As seen in figure 8, the only existing resistance is R1, which is the resistance of the carbon layer measured between the inner probes. The even spacing, S, between the probes is not necessary but it simplifies the comparison process between different sample measurements. Because the resistance is dependent on the length between the two measuring points, as seen in formula 2, it will affect the resistance measured. If the length between the two inner probes between two samples are not the same, the resistance has to be recalculated with the length difference in consideration.
1.6 Limitations
In this thesis several limitations had to be set due to the time and economical constraints. Only a prototype(s) of a contact solution(s) will be developed. This means there will be no real-life implementations of the contact solution(s) in the existing R2R process.
Several technical limitations were made for this project to ensure a reasonable number of parameters to keep track of. E.g. limiting the tests of different contacting solutions to only one substrate thickness (75 µm). Epishine is working with substrates ranging from 50-125 µm and if all of them would have to be tested, there would be no time for several contacting concepts. Therefore, a limitation is made to only test on a 75 µm thick substrate. Another technical limitation is to only print material with three different printing techniques: blade coating, screen printing and slot die coating. This is because these are the printing techniques which Epishine use and have machines for in-house. When it comes to printing material, only one carbon ink will be used due to the endless variables as well as complexity it would bring to the project. The decision was also made to only experiment with one carbon ink viscosity for the same reason as the limiting the conductive materials used. During the prototyping phase, the need for cutting the substrate occurred, which was limited to a Cricut Maker due to equipment limitations.
There are also limitations set in the design thinking process which was used to generate new ideas.
In the inspiration phase, only informal discussions were conducted at Epishine with Epishine employees.
There were no formal interviews held within the company nor with outside companies and only informal discussions were held through mail, or telephone calls with external concerns.
2 R ELATED W ORK
In this chapter related works as well as similar solutions or techniques which may be interesting and potentially can be used in this thesis are presented.
2.1 Current contacting solution
The current contacting solution uses connections from Nicomatics and is explained in chapter 1.2.
Nicomatic is a company which specializes in electrical contacting solutions in many different industries e.g. aerospace, transportation, medical and printed electronics [16]. In the printed electronic sector, they are known for their CRIMPFLEX® connectors which are explained in chapter 1.2. Since Epishine uses the CRIMPFLEX® stamp connectors as their current contacting solution, it is highly relevant to include in this chapter.
2.2 PCB connector
PCB is an abbreviation for printed circuit board and is used to connect electrical components by printed conductive paths. They are used in the most basic electronic devices such as a light dimmer but also in more complicated systems such as computers. A PCB consists of a solid board upon where electrical components are mounted and interconnected by conductive paths. The principle of a PCB is the same as printing on flexible electronics, to interconnect electrical components with each other by printing electrical paths on a substrate. The difference is that the substrate used in a PCB is called a board and is rigid unlike the flexible plastic substrate used in flexible electronics. A PCB is able to house rigid electrical components and acts like an interconnecting link at the same time as it allows the components to mount to it. PCBs are often connected to other PCBs or electrical components which is mostly done by wire. This means a PCB has to have a durable and efficient contact solution from the board to a wire. There are many different wire connections for PCBs but the most common one is a pin header which consist of a metal pins which are fitted into premade holes in the board. The pins can be soldered into the board, creating a secure bond at the same time as the solder creates an electrical connection to the paths connected to the holes. The pins are of the male part of the connector which accept a female socket to connect. The male pins slide into the female socket, which is connected to a wire, resulting in a contact solution from the board to a wire [17].
Since a PCB generally has the same purpose as a flexible printed substrate and has several solutions for a similar problem as the one described in this thesis, seeing if there is a technology which can be applied to flexible substrates is desirable. The difference is the connection made in the PCB case is from a rigid substrate to a rigid connector, where in this thesis’s case it is flexible to rigid, which complicates the solution.
2.3 PCB VIAS
To be able to increase the component density on a PCB, a technique called vertical interconnect access (VIA) is used to gain access to the underside of the board, enabling components to be mounted on both sides of the PCB. This is done by a hole in the board which is either plated or has a metal rivet in it, allowing an electrical current to pass through. The VIA technique allows PCBs to be built in several layers and thereby creating a so called multilayered PCB [18]. This technique is desirable in the application of encapsulated flexible printed electronics due to the compact stack of materials and the potential possibility to move the contact solution to the other side of the substrate. Since the PCB has a solid hard board as substrate, a hole can easily be made and plated and or be riveted. This is not possible with a flexible substrate such as the one used in OPVs due to the lack of flexibility in the VIA as well as the minimal thickness of the substrate compared to a PCB. Therefore, a different technique must be used to interconnect the two sides of the flexible substrate.
The principal of the VIA technique can potentially be used in this thesis to relocate the contacting
2.4 Existing solutions by All Flex Inc
All flex Inc. is an American based company which manufactures flexible electrical circuits. They are able to manufacture single-sided, double-sided as well as multi-layered flexible circuits [19]. Since All flex is using non-organic materials such as copper, they are able to solder rigid connections on to the substrates allowing wires to connect into them. This can’t be done on in Epishine’s devices since only organic materials are used which can’t soldered on. All flex also has an advantage of not needing to encapsulate the electrical circuit to prevent unnatural aging to occur due to the materials used not being affected by moisture or oxygen like the ones used by Epishine. They do use a similar encapsulation with, what they call a cover layer, which can be compared to the barrier, but it only protects the electronics from external physical damage. Since the intrusion of oxygen and moisture does not affect the performance, the surrounding area of the connector can be partly exposed, as long as there is no danger for external damaging. This can’t be done in the case with encapsulated OPVs since if there is an opening in the barrier, moisture and oxygen will penetrate the organic materials and accelerate the aging process or potentially short circuit a cell.
All flex is also able to produce double-sided and multi-layered flexible circuits which use VIAs.
They are able to use similar VIAs as in PCBs due to the possibility of plating, riveting and soldering, which are enabled by the metal materials used as conductive paths.
Although the contact solution used by All flex can’t be implemented into an OPV, their products are similar to Epishines devices and they have found a way to print on both sides of a substrate as well as created a multilayered interconnected circuit.
3 M ETHOD
3.1 Design thinking
Design thinking is a tool used for working towards finding a solution for a certain problem which can be solved by creating a product which provides value to the customer. The method consists of four overlapping phases, Initiation, Inspiration, Ideation and Implementation which allows for an iterative proses for problem solving [20]. Throughout the design thinking process and its phases, three parameters were used to measure the potentials of the solutions generated by the process. These parameters are [20]:
- Desirability: will people want/need the product?
- Feasibility: is it possible to produce?
- Viability: is there any economical profit?
Figure 9. Illustrational graph of the design thinking process used (altered from [20]).
The Initiation phase is about getting familiarized with the problem at hand and gather the needed information and knowledge to be able to take the following step. The outlines of the project and a general plan is created in this stage. Most often this step is used to find and define problems or getting introduced to a problem which needs to be solved. The second step consists of getting inspired by previous works, existing products, other industries and so on. This initiates the third step which is the ideation phase, where in the inspirations lead to conceptual ideas. It is in this stage where the idea generating tools are used, and workshops can be held. If the ideation phase results in potential concepts which need to be tested, it is in the implementation phase where this is done. Both simple and intricate prototypes can be created in this stage of the project. If a prototype is tested and does not perform as desired, the design thinking process is repeated, this time with more valuable knowledge than last time. This iterative process benefits from dead ends, failures and partially functioning prototypes because the collected data and knowledge from those concepts will be profitable when going back to the inspiration and ideation phase to develop the next prototype [20], [21], [22], [23].
3.1.1 Initiation
The design thinking method with the iterative process was widely used throughout the project.
Many ideas were founded from previously tested concepts who resulted in insights which were considered when generating new concepts. At the start of the project, during the initiation phase, the
combined with creating a baseline upon all future concepts will be compared to in the implementation phase. Additionally, testing of different material combinations and the difference of encapsulated and non-encapsulated were added to the baseline. More about the baseline and the different combinations can be found in chapter 3.2.
3.1.2 Inspiration
An introductory literature study was conducted in the inspiration phase which gave a steady base to work from. Alongside the literature study, a techwatching study was performed by informal discussions with Epishine employees, searching for industries with similar problems and observing how other industries have solved similar problems. More about this can be found in chapter 3.4. The inspiration phase was active until far into the project when the final concepts were tried out. Every week, research and techwatching was conducted to find new potential ideas and implemented into new concepts. When an idea had gone through the inspiration, ideation and implementation phase and it failed the last step, the data gathered through the testing and the lessons learned were brought back to the inspiration phase to fuel new ideas.
3.1.3 Ideation
The ideation phase was, just as the inspiration phase, active until the very end due to the iterative nature of the project. After an idea had been sparked through either the inspiration phase or lessons learned from a failed concept in the implementation phase, the ideation phase was used to generate, screen and select which concepts to move on with towards the implementation phase. Every week, a meeting was held with the supervisors from Epishine where the results from the previous week were shared and new ideas were discussed, which often resulted in a small brainstorming session. A plan had to be created for the next upcoming week which meant another iteration of the initiation phase had to be done.
3.1.4 Implementation
The implementation phase started the same week as the initiation process by planning and producing samples for the baseline experiment. From that week and onwards, the baseline was an active process which was revisited every week. Alongside the baseline tests, concepts which had made it past the ideation phase were moved to the implementation phase. First of all, a practical way of measuring the performance of the concept must be determined as well as how to evaluate the samples compared to the previous concepts and baseline. Thereafter, the design and layout of the samples can be determined and manufactured. The previously determined measuring method can then be applied to evaluate the performance of the concept. After the measuring stage, the data is collected and compiled into usable plots to be able to analyse the results in an organized structure. A conclusion is made which most often results in new ideas, which entails moving back to the inspiration and ideation phase, thus starting a new iteration.
3.2 Baseline
For the main thesis question to be answered, the sub-question has to be answered. Therefore, a so- called baseline is created to evaluate what is durable and efficient in the context of a contact solution for organic printed electronics. To be able to evaluate if a new contact solution is performing efficiently enough, a baseline was created to be able to compare to when testing new ideas for contact solutions. If a new idea is tested and has more or less the same values or better, the contacting method is worth further developing. The resistance of different material combinations and already existing contact solutions was measured with a four-probe method to ensure a valid result. The baseline is also a test to evaluate how the conductivity of different material combinations and contacting solutions deteriorate over time in different environments. The hypothesis is that the material will decay at a faster rate in the beginning and over time stabilize and reach a stable resistance [9, pp. 430-431]. Low resistance is an inverse for good conductivity which is sought after when designing new solutions. There are two different solutions being tested in the baseline. The first one is directly on the conductive surface with probes and the second one is with a Nicomatic contact (a punch-through contact used in the printed electronics industry). The different material combinations tested for the baseline are listed below:
Probe:
- Single Carbon layer Not encapsulated (Ne) and encapsulated (Enc).
- Single Carbon layer on top of PEDOT layer (Ne and Enc).
- Single PEDOT layer (Ne and Enc).
Nicomatic contact:
- Single Carbon layer (Ne and Enc).
- Single Carbon layer on top of PEDOT layer (Ne and Enc).
- Single PEDOT layer (Ne and Enc).
- Double layer Carbon (Ne and Enc).
The probe tests will show the direct resistance of the different materials while the Nicomatic contact tests will show what impact a punch-through contact solution has on the conductivity. To be able to evaluate how the different samples deteriorate over time in different environments, all combinations were tested in four different environments.
- Ambient (Amb): Room temperature, approximately 20 degrees Celsius, and humidity 40%.
- Climate chamber (CC): 55 degree Celsius, 85% humidity.
- Glove Box (GB): No oxygen only a pure inert gas, 20 degree Celsius, 0% humidity.
- Oven (Temp): 55 degree Celsius, 40% humidity.
For a more accurate test result, three samples are tested simultaneously for every material combination as well as environment. This results in a total of 144 samples. Pictures of the samples can be seen in the appendix A, P1-P7.
To be able to take accurate measurements on the probe-samples with the four-probe method, the distance between the measuring points must be the same on all samples. Therefore, a template was made to ensure the right position for the crocodile clips with the exact same distance between them. This way, all the different sample measurements can be compared to each other in an easy way. If the distance varies between the measuring points, the resistance will read higher if the points are further away and lower when the points are closer together. With the same distance on every sample, the distance between the separate samples does not need to be accounted for. The samples with Nicomatic contacts were made in a similar way, with the contacts pressed into the sample with the same length between the points. On several samples, the standard template could not be used, therefore other dimensions were used. To be able to compare samples with different probe spacings, the measurements were rescaled in regards to the different lengths.
From previous studies and experiences which Epishine has had in the past, the degradation of the conductivity of the samples is thought to be degrading with a faster rate in the beginning of the test and after a certain time it is thought to stabilise. Therefore, tighter measurement intervals were taken in the beginning of the test, to be able to observe how the degradation is behaving in an as accurate way
measurement to be as precise as possible. The Keithley 2401 directly calculates the resistance between the inner probes, eliminating the chance for human calculation error.
After all measurements were taken, they were normalized to be able to compare the different samples with each other. Due to various distances between the measuring probes on the different samples, they could not be compared. The normalisation was done by dividing all measurements of the same sample with the first measurement taken. By normalizing the measurements, the size of the resistance measured is not relevant anymore but how the different measurements taken over time relate to each other is easily analysable.
3.3 Introductory literature study
A literature study was conducted in the beginning of the thesis by reading previous related work, articles and books related to printed solar cells, printed electronics, encapsulated electronics and contact solutions. This was done to get a better understanding of how the previously listed topics function, what had been done and tested as well as how production processes look like. BTH Summon has been used as main search engine for information gathering as well as finding reliable sources. Google Scholar has also been used as well as Google for the most basic information. A technique called snowballing has been used to find as much related content inform of articles and reports as possible. This entails finding related content by looking through the references of a related article.
3.4 Idea generation and techwatching
Every week a meeting was held with two Epishine employees who acted as supervisors. During these meetings potential new ideas which originated from previously evaluated concepts and techwatching were discussed. This resulted in a mini brainstorm session each week to generate new ideas and since it was an iterative process, it suited the design thinking method. Throughout the weeks, informal discussions with different employees at Epishine were held to identify the different needs which different departments had regarding the contact solution. E.g. the sales department had different views and ideas compared to the production team. These ideas had to be taken in consideration when designing new concepts together with the three main parameters: Desirability, feasibility and viability.
During the idea generating a constant flow of inspiration came from the techwatching activities performed weekly. Techwatching is a way to familiarize with the existing technologies, upcoming technologies as well as the market. There are many different tools to use when techwatching, but the main purpose is to find and identify potential solutions for the given problem. The techwatching performed in this thesis was conducted by searching the internet for companies which face the same problems or potentially solved it, new upcoming technologies in the printed electronics market, but also looking at nature how it has solved similar problems e.g. how flexible leaves are connected to a rigid branch and so on.
All the idea generation sessions, informal discussions and techwatching activities resulted in many ideas, but only the most promising ideas made it to the implementation stage of the design thinking process. These ideas evolved into concepts and can be seen in chapter 4.
4 R ESULTS AND ANALYSE OF THE ITERATIVE DEVELOPMENT
The first part of this chapter shows the results of the literature study, techwatching and idea generation sessions. The remaining sub-chapters show the results of the brainstorming sessions, informal interviews and the iterative process which was used when performing the design thinking process. The different concepts are explained, the manufacturing methods are explained as well as the results are presented and analysed. Only the most relevant and successful concepts which lead up to the main prototype are shown. The result of the baseline and main contacting solution can be found in chapter 5.
4.1 Literature study and Idea generation
As mentioned in the chapter 3.3, a study was made on relevant work which can be related to this thesis and its content. The most relevant content to this thesis can be found in chapter 2 and has been a base where upon some ideas have been founded.
The results of the literature study, techwatching, brainstorming sessions and informal interviews are shown in a list of desired features which the contacting solution has to achieve.
Must have:
• Low or no increase in resistance.
• As component dense as possible (Active area vs total area ratio as big as possible).
• Limit or eliminate the risk of oxygen and moisture degrading the organic materials.
• Scalable and incorporable into a R2R process.
• It should be viable enough from a practical and economic standpoint for Epishine to incorporate the process into their manufacturing process.
Optional:
• Modularity.
• Not piercing the barrier.
• Minimal added production steps into the R2R process.
The following sub-chapters in chapter 4 are the most relevant concepts which are based upon the previously mentioned desired features.
4.2 Relocated Nicomatic contacts
The first concept originates from the initial literature study, initial informal discussions and first iteration of the design thinking process. The main problem with the current solution, as discussed in chapter 1.2 is the location of the Nicomatic contacts. If the contacting locations were to be relocated further away from the AL to a position where the moisture and oxygen have a longer way to travel to be able to damage the AL, the Nicomatics might work. The problem with this is related to formula 2 in chapter 1.5.3.1. which shows that the resistance will increase as the distance which the electricity travels increases. One design which was pitched, but did not get through the implementation phase, was a relocation concept.
Figure 10. Carbon ear solar cell design.
This concept did not reach the testing stage due to two main problems. The first problem was the increased length of the transportation layer (see carbon strips in figure 10) which would increase the resistance. The second, more problematic part of this design, is the unequal length of the carbon strips.
This causes a problem in the solar cell itself because there is a difference in resistance between the anode and cathode.
After discussing the carbon ear concept, the decision was made to focus on another approach which did not include Nicomatics.
4.3 Interconnecting layers concepts
This concept is based on the thought that relocating the contacting points to the backside of the device would create many more opportunities e.g. increased modularity, applicability and component density. A major advantage to this concept the possibility to develop a slim design, where in the contacting points are close to the solar cell itself, minimizing the size of the device. The design of the device should have a as high percentage of active area as possible resulting in a high mV/cm2, minimizing unused area. Multilayered PCBs are able to reach a higher component density due to multiple interconnected layers. The VIAs on PCBs are usually manufactured through interconnecting plated holes in different layers with a metal tube or rivet [19]. Although metal rivets can’t be used on 75µm flexible substrates, other concepts to achieve the same outcome are shown in this chapter.
As shown in figure 1, chapter 1.2, the carbon layer is easier to access from the outside (backside) than from the inside through the HTL, ETL, AL and two PEDOT layers. Making a connection through all these layers creates an even bigger problem due to risk of short circuiting the solar cell. The main idea of this concept is to create a contacting possibility on the outside (backside) of the lower substrate and create a bridge between the inside and outside of the lower substrate, thereby being able to transfer the electrical power to the outside of the lower substrate. If this is possible, a kiss-cut can be made in the barrier to expose the carbon for an easy contact solution as shown in figure 11.
Figure 11. Solar cell stack with double-sided, interconnecting carbon layers the lower substrate. The connection is bridged in this illustration due to the real method being
classified.
The bridge between the inside and outside of the lower substrate can be created in different ways.
These different methods are displayed in 4.3. The interconnecting layers concept enables various new slim designs which have been developed through the iterative design thinking process with the three main factors in mind: Desirability, feasibility and viability.
4.3.1 Layout design concepts
Figure 12. Design idea with the interconnecting layer technology implemented.
carbon strips on the backside have a bigger surface area for a contacting solution to connect to. When the barrier is applied to the device in the R2R process, it will encapsulate the whole device including the carbon contacting patches on the backside. To be able to contact the contact patches, a laser cutting technology can be used to cut precise cuts, called kiss-cuts, into the barrier. These cuts will only go through the barrier and not the carbon layer or the substrate. When the kiss-cuts are made, a connection can be made with the carbon layer. Examples of these connection called contact solutions can be found in chapter 4.2.2.
Figure 13. Design idea with interconnecting layer technology implemented.
The second interconnecting layer design can be seen in figure 13. This is basically the same principle as Interconnecting layer concept design 1, but instead of limiting the connection possibilities to one small patch, the connections can be made everywhere on the backside. With the same size active area as in design 1 but the possibility to connect everywhere on the backside, this design increases the applicability and usability due to the increased modularity. This design follows the same production process as design 1 with the possibility for the customer to decide the location of the connections.
From a feasibility perceptive, both these concept designs have the potential to become a final design of the contact solutions due to their slim and compact design as well as modularity.
4.3.2 Interconnecting layer contact solutions
In this chapter, different concepts on how to establish a contact with the carbon layer on the backside of the devices are shown. See figure 14-16 for potential interconnecting layer contact solutions.
Figure 14. Contact solution concept combined with interconnecting layer technology.
Figure 14 shows a potential contact solution concept which uses glue to secure the connection.
To be able to connect to the carbon layer without stamping through the whole stack, like a Nicomatic stamp does, something must hold the wire pressed against the surface. In this instance, a Nicomatic stamp would short circuit the solar cells. By using a generic flat cramp-on connection on an electrical wire, it can be placed flat against the carbon surface, creating a low-resistance connection due to the large surface area. To be able to keep the connection from moving, a durable, non-conductive, fast- curing glue is used. This concept is showed in figure 14.
Figure 15. Contact solution concept combined with interconnecting layer technology.
Instead of securing the wire to the carbon surface with glue, a piece of conductive tape is taped
