Department of Physics, Chemistry and Biology
Linköping University
URL för elektronisk version
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-105629
ISBN
ISRN: LITH-IFM-A-EX--14/2858--SE
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Serietitel och serienummer ISSN
Title of series, numbering ______________________________
Språk Language Svenska/Swedish Engelska/English ________________ Rapporttyp Report category Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport _____________ Titel Title
Lamination of Organic Solar Modules
Författare
Author Sofie Kalldin
Nyckelord
Keyword
Organic solar modules, Lamination, PEDOT:PSS, Ethylene Glycol (EG), Diethylene Glycol (DEG), Polyethylene Glycol (PEG), Roll to roll process (R2R).
Sammanfattning
Abstract
As the Worlds energy demand is increasing we need more of our energy to be generated from resources that affect the climate as little as possible. Solar power could be the solution if there were solar panels with a less energy demanding production than the established silicon based solar modules.
Printable organic solar cells will enable a cheap production process, thus they are mainly made out of polymers in solution. However, to be able to decrease the total cost of the solar modules the commonly used indium tin oxide (ITO) for the transparent electrode needs to be replaced by a less expensive material. If the cheap, high conductive and transparent polymer
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) could replace ITO the cost of organic solar modules would significantly decrease.
For PEDOT:PSS to be able to replace ITO there are requirements that have to be met. The transparent electrode needs to be apart from transparent, highly conductive, have a low contact resistance to the other materials in the organic solar cell and be printable.
In this study it has been shown that the PEDOT:PSS film with Zonyl and Diethylene Glycol (DEG) as an secondary dopant, is capable of laminating to thin films made out of PEDOT:PSS, metal or a polymer fullerene blend. The contact resistances between two PEDOT:PSS films and PEDOT:PSS film and a metal film proved to be low. When laminating to a metal film an interlayer of Silver Nano Wires (AgNW) was needed to achieve a low contact resistance.
Master’s Thesis
Lamination of Organic Solar Modules
Author: Sofie Kalldin Supervisors: Jonas Bergqvist Anders Elfwing Examiner:
Prof. Olle Ingan¨as
Biomolecular and Organic Electronics
IFM, Department of Physics, Chemistry and Biology
Lamination of Organic Solar Modules by Sofie Kalldin
As the Worlds energy demand is increasing we need more of our energy to be generated from resources that affect the climate as little as possible. Solar power could be the solution if there were solar panels with a less energy demanding production than the established silicon based solar modules.
Printable organic solar cells will enable a cheap production process, thus they are mainly made out of polymers in solution. However, to be able to decrease the total cost of the solar modules the commonly used indium tin oxide (ITO) for the transparent elec-trode needs to be replaced by a less expensive material. If the cheap, high conduc-tive and transparent polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) could replace ITO the cost of organic solar modules would significantly decrease.
For PEDOT:PSS to be able to replace ITO there are requirements that have to be met. The transparent electrode needs to be apart from transparent, highly conductive, have a low contact resistance to the other materials in the organic solar cell and be printable. In this study it has been shown that the PEDOT:PSS film with Zonyl and Diethylene Glycol (DEG) as an secondary dopant, is capable of laminating to thin films made out of PEDOT:PSS, metal or a polymer fullerene blend. The contact resistances between two PEDOT:PSS films and PEDOT:PSS film and a metal film proved to be low. When laminating to a metal film an interlayer of Silver Nano Wires (AgNW) was needed to achieve a low contact resistance.
I would like to thank various people for their contribution to this project. I am grateful for all the help and support you have given me.
Prof. Olle Ingan¨as, examiner
Jonas Bergqvist and Anders Elfwing, tutors Laura Sammalisto, opponent
Dr. Mattias Andersson, for the material from the machine in Norrk¨oping Abeni Wickham, for performing the DSC scans
Co-workers at IFM Family and friends Emil Hellman
Abstract i Acknowledgements ii Contents iii Abbreviations v 1 Aim 1 2 Introduction 2 3 Background 4 3.1 Solar cells . . . 4
3.1.1 Organic solar cells . . . 5
3.2 Polymers . . . 6 3.2.1 PEDOT:PSS . . . 7 3.3 Additives . . . 9 3.3.1 Surfactants . . . 9 3.3.2 Conductivity enhancers . . . 9 3.3.3 Adhesives . . . 10 3.4 Roll-to-roll fabrication . . . 11 3.4.1 Blade coating . . . 11 3.5 Lamination of PEDOT:PSS . . . 12
4 Materials and Methods 14 4.1 Materials . . . 14
4.1.1 Equipment . . . 15
4.1.2 Chemicals . . . 16
4.1.3 Other Materials. . . 16
4.2 Methods . . . 17
4.2.1 Lamination of PEDOT:PSS thin films . . . 17
4.2.2 Plasma Patterning . . . 21
5 Results and Discussion 22 5.1 Lamination of two Identical Films . . . 22
5.1.1 Lamination Properties of the Film . . . 22 iii
5.1.2 Film Resistance. . . 26
5.1.3 DSC scan . . . 29
5.1.4 Transmission vs. Sheet Resistance . . . 29
5.2 Lamination of Films Containing DEG to Metal Thin Films . . . 30
5.2.1 Introduction of AgNW to Decrease the Contact Resistance . . . . 30
5.3 Lamination of Films Containing DEG to the Active Layer . . . 32
5.4 Plasma Patterning . . . 33 6 Conclusions 34 A Estimation 39 B Early experiments 40 C DSC scan 44 D Transmission 46
AgNW Silver Nano Wires Al/Cr-film Aluminum Crome film
CdTe Cadmium Telluride
CIGS CopperIndium Gallium (di)Selenide
DEG DiEthylene Glycol
DSC Differential Scanning Calorimetry
EDOT 3,4-Ethylene- DiOxyThiophene
EG Ethylene Glycol
HOMO Highest Occupied Molecular Orbital
ITO Indium Tin Oxide
LUMO Lowest Unoccupied Molecular Orbital
PDMS PolyDiMethylSiloxane
PEDOT Poly(3,4-Ethylene- DiOxyThiophene)
PEG PolyEthylene Glycol
PET Poly(Ethylene Terephthalate)
PSS Poly(StyreneSulfonate)
R2R Roll To Roll fabrication
SQ SilQuest
Tg Glass Transission
Aim
The aim of this project is to laminate a poly(3,4-ethylenedioxythiophene)- poly(styrene-sulfonate) (PEDOT:PSS) thin film to another thin film made out of PEDOT:PSS, metal or a polymer fullerene derivative blend. They should have a strong adhesion as well as a low contact resistance.
Introduction
According to the projections made by the United States Energy Information Adminis-tration (U.S. EIA) the world’s energy consumption will increase with 56% from 2010 to 2040.[1] Today most of the energy supply is based on resources giving rise to decay products that pollute the planet. In December 2011 the world’s use of energy produced from fossil fuels was as high as 81%. Only 13% of the energy used was produced from renewable resources and hydro-power.[2] According to the International Climate Change Partnership (ICCP) climate report from 2013 they present that there is a clear correla-tion between the humans’ pollucorrela-tion and the climate changes. But it is also stated that if the effort is made, it is possible to slow down the changes.[3] To meet the increasing demands for energy, several different resources needs to be used and the use of renewable resources has to increase, to maintain the planet as we know it today.
There are several different renewable resources that can be converted into electrical energy, such as wind, wave and solar power. The term renewable resource means that it cannot be used up. The techniques used to extract energy from these resources do not give rise to decay products during operation, thus not contributing to the climate changes during use. If the product is to be called environmental friendly the whole process “from cradle to grave” should be considered, meaning that the impact from the production as well as during usage and the dismantling of the product is summarized. This is why organic solar modules are a good choice; they are constructed of organic material that can be printed from solvent thus giving a low production impact in form of low energy usage.[4–6] During operation no decay products are created and since the
main part of the material is organic there is no problem to recycle or burn them when they are worn-out.
Not only are the organic solar cells environmentally friendly; they are cheap as well thanks to the printing ability and the low material cost. This will make the technique available to the greater public and thus spreading the usage. Hopefully this will lead to that a larger fraction of electricity derives from renewable resources.
Today the transparent electrodes in organic solar modules are most often made out of Indium Tin Oxide (ITO). ITO is a material with a limited resource and used in many electronic applications, thus making the price of the material increase.[7] When ITO is used as an electrode in organic solar cells it stands for 37-50% of the total material cost.[4] So if this material could be replaced by a cheaper alternative it would make a big impact on the total cost of the solar module. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is an electrically conductive and transparent poly-mer that can replace ITO in different applications.[4,7,8] This projects main goal is to study if it is possible to make an electrically conductive lamination with a PEDOT:PSS thin film to another thin film made out of PEDOT:PSS, a metal or a polymer fullerene blend. If this is possible new opportunities will arise in the production steps as well as keeping the material costs low for the solar module.
Background
In this chapter the background information needed to understand the project is found, such as the chemicals used and the main theories behind the project.
3.1
Solar cells
The sun radiates the planet Earth with around 1017W, i.e. the energy reaching the Earths surface in one hour is the same amount of energy that was consumed by the human population for the entire year of 2010.[9] Thus there is a great deal of unused solar energy. So why are there not more solar cells in use? An answer to that could be that the mature and efficient technique for converting solar radiation into energy today is silicon based solar modules. Silicon is expensive due to that it is produced from melted silica, which demands high energy to obtain the pure crystal form needed for the solar cells, thus making the final product expensive.
Besides silicon there are two types of thin film solar modules called Copper Indium Gal-lium (di)Selenide (CIGS) and Cadmium Telluride (CdTe). The CIGS module consists of the materials copper, indium, gallium and selenium, whereas the CdTe, as the name indicates, consists of cadmium and tellurium. The two thin film modules both contain materials that have a limited supply (indium and tellurium), making them expensive. CdTe also contains the toxic material cadmium. The advantage of the thin film solar modules is that they contain a smaller amount of material.
Silicon solar modules are by far the most installed type of solar cells, with CdTe owning a small share. Thanks to governmental subsidies in Germany and a reduced manufacturing cost on Chinese modules the solar installations has boomed in recent years. However, without subsidies the time to return the invested money is over 15 years (se Appendix A). There is a need of a cheaper production technique to make solar energy widely spread.
3.1.1 Organic solar cells
The advantage with organic solar cells compared to the already established and more efficient silica based cells is that they can be printed from solution, potentially giving a very low production cost. One way to decrease the material cost further is by re-placing Indium Tin Oxide (ITO) with the cheaper conductive polymer PEDOT:PSS as an electrode. For this to work there are some requirements that need to be met. The requirements are that the electrode must be transparent, highly conductive and connect to the other materials in the solar cell with a low contact resistance.
Figure 3.1: Schematic pictures of organic solar cell structures.
Organic solar cells are built up from different materials that are stacked on top of each other in a structure enabling harvesting of electrons. There are two main types; dye sensitized solar cells and polymer or small molecule solar cells. The later is the type used in this project, in figure 3.1 a schematic picture of the two typical structures for
polymer solar cells are shown. The harvesting of electrons from the active material is enabled by the materials different energy levels. What happens is that the light irradiates the active material and an electron is excited from the HOMO to the LUMO leaving a so-called “hole” in the HOMO state. Instead of letting the electron relax back and emit light the electron is driven out of the material. The hole enables a new electron to enter the material, making it possible to connect in series. Electrons are always trying to find a lower energy state, which is used to harvest the electrons by combining materials with the right energy levels. This is shown in figure3.2.
Figure 3.2: When the polymer fullerene derivative blend, active material, is hit by incoming light the energy is absorbed and an electron is excited from the highest occu-pied molecular orbital (HOMO) to the lowest unoccuoccu-pied molecular orbital (LUMO). Normally the electron would relax back to the HOMO and emit light to release the excessive energy. Since the point is to extract electrons the polymer fullerene derivative blend consists of materials were one of them (P1) has a lower LUMO than the other (P2) thus driving the electron from P1 to P2 instead of relaxing. When the electron reaches the Al/Cr it can be collected into a battery. As the electron leaves the first polymer (P1) a hole is left. Holes unlike electrons move towards higher energy levels
and will therefore move toward the PEDOT:PSS side.
3.2
Polymers
A polymer is a molecule that contains of several repeating units called monomers. Poly-mers consist mostly of carbon and hydrogen atoms. PolyPoly-mers in general are insulating and are commonly used as isolators in electronic devises.[10] However, it has been discov-ered that by doping a conjugated polymer it is possible to make it conductive. Doping
means that an electron is withdrawn or added to the molecule, by oxidization respec-tively reduction.[11] The polymers are able to become conducting due to their conjugated delocalized double bonds (π-bonds), suggesting that these charges can move along the polymer chain. However, when the polymer is oxidized its conductivity is increased significantly due to that one of the delocalized π-bonds is removed, enabling charge transport along and between by hoping polymer backbones.[10, 12] (Fig. 3.3) As the polymer is doped it becomes insoluble and gets poor mechanical properties narrowing their possible applications.[11]
Figure 3.3: A part of a polyacetylene polymer, when oxidized one of the π-bonds is delocalized, enabling charge transport along and between molecules.
3.2.1 PEDOT:PSS
PEDOT:PSS is a complex of the two polymers PEDOT and PSS. PEDOT is a polymer containing only 6 to 8 3,4-ethylene-dioxy thiophene (EDOT) monomers. It is so small that it can be considered to be an oligomer. The PEDOT can form π-stacking thanks to the thiophenes in the chain.[8] PSS is a longer polymer that is added in excess amounts to an EDOT solution when PEDOT:PSS is synthesized.[13] During the synthesis the longer PSS chain acts as a template for the EDOT monomers and form the PEDOT polymer.[7] In figure 3.4the PEDOT:PSS complex chemical structure is shown as well as a schematic picture of the complex. Two other reasons for why the PSS polymer is needed is that the polymer makes the PEDOT dispersible in water as well as doping the polymer. Even if the complex is not fully soluble in water the PSS polymer enables the formation of an easily processed blue dispersion that can be printed.[8,13]
When the doping counter ions are present they will lay in-between the π-stacks of the PEDOT molecules, thus making the distance between the PEDOT chains equal to the size of the PSS polymer. A likely model of the PEDOT:PSS is shown in figure3.4. In the complexes secondary structure the smaller PEDOT will attach itself to the longer PSS
Figure 3.4: a) The structural formula of PEDOT:PSS, b) a schematic picture of the PEDOT:PSS complex.[14]
polymer. (Fig. 3.5 a)) The tertiary structure of the complex is probably formed as a tangle. For this material to become conductive the polythiophene chains need to be in a stacked formation within and between the tangles. The conductivity is dependent on the size of the complex tangles; smaller tangles give a lower conductivity in the film. This indicates that the resistivity lies in the interlayers between the tangles and the PSS.[8] (Fig. 3.5b)) Pristine PEDOT:PSS has a film surface that contains of PEDOT:PSS grains surrounded by excessive thin PSS layers.[15] (Fig. 3.5 c)) The charges are thought to move along the backbone of the PEDOT chain as described for polyacetylene, but it is the hopping between the polymers that is thought to be the main charge transport.[4]
Figure 3.5: A schematic figure of how the structure of PEDOT:PSS. a) the secondary structure of the complex, a PSS-chain with PEDOT oligomers attached to it. b) The tertiary structure, a tangle of the complex kept together with π-bounds between PE-DOT oligomers. c) The tertiary structure in a film with the excessive PSS surrounding
3.3
Additives
As stated earlier the PEDOT:PSS complex has a low solubility and conductivity. In order to enhance both of these properties different additives are incorporated into the PEDOT:PSS solution.
3.3.1 Surfactants
PEDOT:PSS has a high surface tension making it impossible to wet a hydrophobic surface. If a surfactant is added to the dispersion the surface tension will decrease thus enabling wetting of the surface.[16] Zonyl FS-300 flourosurfactant (Zonyl) is a commonly used surfactant to lower the surface tension of PEDOT:PSS solutions.[7,16,17] (Fig.3.6) It has been shown that Zonyl even can increase the conductivity of PEDOT:PSS to a certain point by inducing a phase separation leading to longer connected networks of PEDOT:PSS. As Zonyl is an insulator and is left in the film after film formation there is a limitation to how much can be added before the conductivity decreases. It has been shown that films containing 0.001-0.1wt% Zonyl give the highest conductivity.[15]
Figure 3.6: The chemical structure of Zonyl FS-300.
3.3.2 Conductivity enhancers
To enhance the conductivity of the PEDOT:PSS film, secondary dopants, additives with high boiling point, can be added in small amounts to the solution. Such additives could be sugar alcohols, amides, sulfoxides or other polar compounds.[8,17]
There are several different studies made with diols, sugar alcohols, as the secondary dopants. Researchers seem to agree on that the diol induces a phase separation in the film when added to the solution pre film formation or onto the surface after film formation. What is thought to happen when the diol is added is that the diol interacts with the PSS
Figure 3.7: At the top of the figure the diols chemical structure are shown. Diols are thought to out compete the weaker binding between PEDOT and PSS with their stronger hydrogen bonds to the PSS molecule, resulting in bigger tangles of PEDOT
thus enhancing the conductivity in the film.
chains with hydrogen bonding thus competing with the weaker ion interactions between PEDOT and PSS. (Fig.3.7) This results in that the conductive PEDOT molecules gain higher crystallinity thus increased conductivity. The increase in conductivity is due to that the amount of conductive barriers is decreased, which enables more charge transport through hopping due to the increased order of PEDOT polymers.[4,8,13,17] The most commonly used diols are ethylene glycol (EG), diethylene glycol (DEG) and polyethylene glycol (PEG) with varying molecular weights. See figure3.7for their chemical structures.
3.3.3 Adhesives
Adhesives can be used to enhance the films hardness, wear resistance and adhesion to the substrate. Silanes and tetraalkylorthosilicates are often added to the PEDOT:PSS solutions for this purpose.[8] Silquest A187 (SQ) is an epoxy alkoxy silane, when added to PEDOT:PSS it enhances the adhesion to the substrate.[17] (Fig.3.8)
Figure 3.8: The chemical structure of Silquest A187 (Trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane).
3.4
Roll-to-roll fabrication
Figure 3.9: A schematic picture of how the slot die printing is done. The ink is added to the chamber and the printer head rakes the ink into a homogenous film. The picture on the right is the printing machine in Norrk¨oping. Photograph taken by Mattias
Andersson.
As the materials used for constructing an organic solar cell are in solution it is possible to simply print the structures. The printing method is often called roll-to-roll fabri-cation (R-2-R). There are several differing R-2-R techniques such as screen printing, flexographic printing and slot die printing just to mention a few. The technique used in the machine in Norrk¨oping is slot die printing. (Fig. 3.9) The goal is that the material and production technique developed in this project should be appliable on this machine.
3.4.1 Blade coating
Due to that it is expensive and large amounts of solution are needed it is not convenient to print with the big machine in Norrk¨oping. Therefore the cheaper and faster blade coater was used for the film formations during this project. The blade coater bar height
is adjustable giving the user the ability to control the film thickness. A blade coater has blade that simply disperses the solution over a substrate into a thin film (Fig.3.10), thus the same method as the printing machine in Norrk¨oping. The difference between the two methods is that in the case of the blade coater it is the blade that moves whereas in the printing machine it is the substrate that moves during printing.
Figure 3.10: A schematic picture of how the blade coating is done. The blade coater rakes the solution onto a substrate thus creating a film.
3.5
Lamination of PEDOT:PSS
Yang Yang and coworkers have done a study on how to laminate PEDOT:PSS. This study showed that if a layer of D-sorbitol is applied on top of the PEDOT:PSS film or added to the blend, the film gains properties that makes it possible to laminate the film to several different materials. Their study demonstrated that it is possible to laminate PEDOT:PSS without solvents when D-sorbitol is present. (Se figure3.11for the chemical structure of D-sorbitol.) The lamination was done by putting the D-sorbitol side of the film to another film and pressing them together from both sides with a mild applied force. As a second step the device was baked in an oven at 130◦C for 30 min. The lamination worked well and hade a conductivity of 100S/cm.[18]
Figure 3.11: Chemical structure of D-sorbitol. D-sorbitol is a sugar alcohol with several OH-groups.
The idea behind this project was that if lamination of PEDOT:PSS thin films to other thin films was possible, than this would open up the possibility to fold the structure on itself. (Fig. 3.12) This would create semi-transparent solar modules, since both sides of the module would be transparent to light. More importantly this technique would give new possibilities in the production process, e.g. as shown in figure 3.12.
Figure 3.12: A schematic and simplified picture of the organic solar modules structure if PEDOT:PSS was used as an electrode.
Figure 3.13: A schematic picture of how the electrons and holes move in the solar organic module when illuminated.
Materials and Methods
In this section the materials and methods used during this project are described.
4.1
Materials
Here the materials used in this research such as equipment and chemicals will be de-scribed.
4.1.1 Equipment
During the project different equipment was used to study the films. In table 4.1these are listed.
Table 4.1: A table over the equipment used in the project.
Usage Equipment Brand and Model
Measuring resistance in films Multimeter FLUKE
Creating films Blade coater ERICHSEN, coatmaster 510
Measuring film thickness DEktak 6M, Stylus
profiler VEECO
Measuring sheet resistance in films
Four-point probe KEITHLEY 4200
Transmission measurement UV/Vis Spectropho-tometer
PERKIN ELMER LAMBDA 950
Photographing Camera iPhone 4S
Material from Norrk¨oping Roll-to-roll printer R2R Solar X3 Coater
4.1.2 Chemicals
The chemicals used in this project are listed in table4.2.
Table 4.2: A table listing the chemicals used in the project.
Usage Name of Chemical Brand
Conductive polymer CLEVIOS PH1000,
PE-DOT:PSS
HERAEUS
Surfactant Zonyl FS-300 DUPONT
Secondary dopant Ethylene Glycol (EG) SIGMA ALRICH
” Dithylene Glycol (DEG) MERCK
” Poly(ethylene Glycol) (PEG) 200 MERCK
” PEG 400 KEBO LAB AB
” PEG 1000 ALDRICH
Adhesive Silquest 187A (SQ) MOMENTIVE
Transparent conductive ink
Silver Nanowires (Ag NW) Classified
Connection paste Agar Silver paint G302 (Ag paste) AGAR SCIENTIFIC
Active material Thiophene-bis(3- octyloxyphenyl)-quinoxaline polymer (TQ1) and c60-fulleren blend, 1:1 ratio 4.1.3 Other Materials
In table4.3 is the other materials used in this project listed.
Table 4.3: A table listing materials used.
Usage Name Brand
Substrate film Hostphan GN 100 4600, Polyethy-lene Terephthalate (PET) 100µm
MITSUBISHI polyester
film
“ PMX727, PET 50µm Hifi Industrial film
“
60nm vaporized aluminum film with 15nm sputtered chrome 50µm on 50µm PET (Al/Cr-film)
4.2
Methods
A general description of how the experiments were executed is presented in this section. More details such as concentrations are found under the Results and Discussion section.
4.2.1 Lamination of PEDOT:PSS thin films
Initially several different approaches to laminate the films together were investigated. The lamination technique chosen to proceed with is described below and the others are found in AppendixB.
Figure 4.1: A schematic picture of the bridging setup.
Putting one film up side down on top of the other and then laminating them together on a hotplate was the method used. (Fig. 4.1) A lamination has succeeded when the films have stuck to each other. These experiments were done in some varying combi-nations; PEDOT:PSS to PEDOT:PSS, PEDOT:PSS to Al/Cr-film and PEDOT:PSS to the Active material. All of the PEDOT:PSS materials were blade coated onto a PET film.
Solution preparation
The additives were pipetted into a 4 ml glass vial before the PEDOT:PSS was added. The solutions were subsequently vortexed for about 30 sec. A new blend was done each day, to eliminate the risk for reactions in the solution.
Film formation
Blade coating was used to make thin films of the prepared solutions. The blade coating bar was placed on films with the same thickness as the substrate used for the coating.
The height of the bar was set to 140µm, were nothing else is stated. A piece about 3x6 cm of substrate was cut out with a pair of scissors. On the rear side of the substrate small droplets of water were applied to make the films stick to the blade coater table. When everything was in place about 60µl of the required sample solution was applied onto the PET substrate and the blade coater bar raked the solution over the substrate. During the coating the temperature of the blade coater table was 50◦C to dry the films.
Annealing Study
An annealing series was done to study how the annealing temperature affects the adhe-sion of the material to the substrate. The materials where annealed on a hotplate. To study how well the films hade adhered to the substrate they were scraped with a Q-tip, paper towel, pipette tip, a fingernail and a pair of tweezers.
Experimental set up
The lamination set up was done as shown in figure 4.1. This set up will be called the bridging set up. The two strings of film on the same substrate will be referred to as “the double strings” and the film connecting them will be called “the bridge”. The double string was a bigger piece of film on which a thin line of film had been scraped away in the middle of the film. (Fig. 4.2and4.3) Measurements with the multimeter were done to control that the strings no longer had a conducting contact. After the films were assembled as shown in figure4.1, they were transferred to a hotplate for lamination.
Figure 4.2: A schematic picture of how the double strings were constructed by scrap-ing the film with a pair of tweezers, after blade coatscrap-ing of the PEDOT:PSS solution. The double string was about 3x4cm with a thin spacing in the middle of about 3mm
Figure 4.3: A picture of a PEDOT:PSS film that has been scraped with a pair of tweezers to gain a double string. A bridge lies to the right of the double string.
For the experiments were the double string was an Al/Cr-film, the film was cut with a scalpel to create the electrical isolation giving two separate strings on the same piece. A bridge made out of an PEDOT:PSS film on a PET substrate was put up side down across the scratch, as shown in figure 4.4. The same experiment was done on an Al/Cr-film with AgNW blade coated onto the film, se figure 4.5. After the films were laminated together the PET substrate was pealed off from the bridge with a pair of tweezers, leaving the PEDOT:PSS on the metal film. When the bridge-film was released from its PET and stuck to the double string the films were heated for a second time on the hot plate for annealing.
Figure 4.4: A schematic picture of how the Al/Cr-film was prepared before lamination of PEDOT:PSS and last the PET film is peeled off.
When studying the lamination of PEDOT:PSS to the active material, a PET substrate was blade coated with the polymer fullerene derivative blend before laminating the PEDOT:PSS film onto it on a hotplate. After the lamination the PET substrate was pealed off the PEDOT:PSS film and an annealing was done.
Figure 4.5: A schematic picture of how the Al/Cr-film with AgNW blade coated on top was prepared before lamination of PEDOT:PSS and after the lamination the PET
film is peeled off.
Film Resistance
To measure the resistance in films and over the bridge a multimeter was used as shown in figure 4.6. To study how the resistance over a bridge varies over time, samples were left in a petri dish for 20 days in the laboratory.
Figure 4.6: A schematic picture of how the resistance was measured with a multime-ter. The set up on the left is for measuring on one single film and the one on the right
is for measuring over a bridge.
DSC scan
A scan with a differential scanning calorimetry (DSC) was performed to determine the glass transmission (Tg) temperature of the materials. By using the Tg temperature of
the material, the films could be heated to this temperature thus making them sticky. This would lead to that the pieces could be put together for lamination when they are sticky and by cooling them down to room temperature they would become stiff and laminated. The sample is placed in one of the two pans and the other is left empty as a reference. Both of the pans are heated at the same rate and the difference in heat flow is measured directly in mW or J/s.[19]
The materials measured on were drop casted onto TL-1 washed glass slides and dried on a hot plate at 70◦C until they were dry. To ensure that there was enough material to run the scan, the glass slides were weighed before and after drop casting. The material was scraped off the glass slide, and placed in the sample chamber with a scalpel. The samples were heated from 20◦C to 219◦C at the rate of 10◦C/min and then cooled down to 20◦C before a second scan up to 219◦C was done. Abeni Wickham performed these scans.
Sheet Resistance vs. Transmission
Transmission measurements and four-point probe measurements were done on films with varying thickness, to determine the relation between transmission and sheet resistance. For these two analyzes the same samples were used.
4.2.2 Plasma Patterning
A study to make the PEDOT:PSS solution coat a substrate without surfactants into a desired pattern was done by cutting out polydimethylsiloxane (PDMS) pieces and placing them on the PET substrate. Then the substrate with the PDMS pieces was placed in a plasma chamber and treated. After the treatment the PMDS pieces were removed and PEDOT:PSS solution was blade coated onto the treated substrate.
Results and Discussion
Under this headline you will find the results and discussions from the experiments de-scribed in chapter 4. The sections in this chapter are ordered after what materials the PEDOT:PSS films are laminated to. First you will find the lamination of PEDOT:PSS to PEDOT:PSS and some evaluation experiments done on the films. Followed by PE-DOT:PSS lamination to Al/Cr-films and PEPE-DOT:PSS laminated to theactive material. The chapter ends with the results from the plasma patterning experiment.
5.1
Lamination of two Identical Films
The first goal was to create a conducting lamination of two PEDOT:PSS thin films blade coated on a Polyethylene Terephthalate (PET) substrate. Different additives were examined to study which solution would give a material that would laminate, have a low sheet resistance and hopefully even give a low contact resistance. All of the samples contained of PEDOT:PSS PH1000 with 0.5v/v% Zonyl, unless something else is stated, and the secondary dopants were added to the solution prior to film formation.
5.1.1 Lamination Properties of the Film
It has been stated in several reports that when a diol is added to a PEDOT:PSS solution prior to film formation the resistance in the material will decrease.[4,8,13,17] When the diols were added to the PEDOT:PSS solution the films became sticky, making the
lamination easier. Initially three different diols were examined; Ethylene Glycol (EG), Diethylene Glycol (DEG) and Polyethylene Glycol200 (PEG200). Depending on the diol added to the solution the film achieved different properties, the samples containing 8v/v% EG or DEG hade similar properties. Both films were very sticky thus making it difficult to get contact with the multimeter, due to that the films floated away. How-ever, when the diol concentrations were lowered to 2v/v% they became less sticky, still enough for enhancing the lamination. Even the resistance decreased sufficiently when decreasing the concentration from 8v/v% to 2v/v%, more about this in section 5.1.2. Films containing 8v/v% PEG200 were so sticky that the PEDOT:PSS film barely stuck to the substrate, hence making it impossible to make contact with the material due to that the films slipped around when trying to measure their resistance with a multimeter. (Fig. 5.1) When lowering the films concentration to 2v/v% PEG200 the films were still too sticky to be of interest.
Figure 5.1: A photograph of a film containing PH1000, Zonyl 0.5v/v% and 8v/v% PEG200. The scratch marks on the film were from an attempt to measure the resistance.
Due to these findings new films were done with higher weight PEGs. With the hypotheses that a polymer with a longer chain would give the material a firmer and drier texture, due to its higher melting point. 2v/v% PEG1000 respective PEG400 was added to the PEDOT:PSS solution, to study if the polymers length would make any difference. The film containing PEG1000 became less sticky, but still too sticky to be able to make reliable measurements of the resistance. Films with PEG400 were almost as sticky as those with PEG200.
Since it was not enough to increase the length of the PEG molecule and the EG and DEG films were still a bit too sticky, Silquest A187 (SQ) was introduced. According to earlier studies SQ enhance the PEDOT:PSS films ability to adhere to substrates and may even crosslink the molecules within the solution.[17] After adding 1v/v% of SQ to
the solutions containing PH1000, 0.5v/v% Zonyl and 2v/v% of one diol per film, the films became less floating. Thus showing that adding SQ made the films less sticky. To study how well the adding of SQ worked an annealing study was done on films containing PH1000, 0.5v/v% Zonyl and 6v/v% DEG with and without 1v/v% SQ.
Annealing Study
The annealing study comparing films with and without SQ was done to quantify at what temperatur the films were annealed properly. The experiment was done on films containing:
• PH1000, Zonyl 0.5v/v% and DEG 6v/v%
• PH1000, Zonyl 0.5v/v%, DEG 6v/v% and SQ 1v/v%
The results are compiled in figure 5.2and in table 5.1.
Figure 5.2: Samples within the blue area contain PH1000, Zonyl 0.5v/v%, DEG 6v/v% and SQ 1v/v% and those in the green area contain PH1000, Zonyl 0.5v/v% and DEG 6v/v%. The arrows point on the damaged done by the treatment represented by that color. Samples with a bar instead of an arrow are treated but did not get any
Table 5.1: Samples within the blue area contain PH1000, Zonyl 0.5v/v%, DEG 6v/v% and SQ 1v/v% and those in the green area contain PH1000, Zonyl 0.5v/v% and DEG 6v/v%. Each number represents a treatment used for examining the films stiffness after annealing at different temperatures. Treatment number 1 is a gentle stroke with a paper towel. Number 2 is a stroke with a Q-top. Number 3 is a hard stroke with a fingernail. Number 4 is a harsh stroke with a paper towel. Number 5 a stroke with a pipette tip. Number 6 a stroke with a pair of tweezers. NO stands for that the sample did not stand the treatment, YES that there were no marks and NEARLY when it
withstood the treatment better than NO but there were marks.
Treatment Temperature 1 2 3 4 5 6 None NO NO NO NO NO 70◦C NO NO NO NO NO 80◦C NEARLY NO NO NO NO 90◦C YES NEARLY NO NO NO 100◦C YES YES NO NO NO
110◦C YES YES NEARLY NO NO
120◦C YES YES NEARLY NEARLY NO
130◦C YES YES NEARLY YES NO
140◦C YES YES NEARLY YES NO
150◦C YES YES NEARLY YES NO
None NO NO NO NO
70◦C NO NO NO NO
80◦C NO NO NO NO
90◦C NEARLY NO NEARLY NO
100◦C NEARLY NO NEARLY NO
110◦C YES YES NO YES NO
120◦C YES YES NO YES NO
130◦C YES YES NEARLY YES NO
140◦C YES YES NEARLY YES NO
150◦C YES YES NEARLY YES NO
From these results it can be said that films containing SQ anneal at lower temperatures. None of them can stand sharp scrapings, so the films are somewhat sensitive to sharp items. Films with or without SQ were both properly annealed at 130◦C. As the films without SQ were dry and adhered properly after annealing at 130◦C, SQ is not needed
for these samples. This is a great result since films containing SQ showed to increase the resistance of the material. More about the films resistance studies are found in section 5.1.2.
5.1.2 Film Resistance
From the earlier tests it could be seen that the secondary dopants affected the stickiness of the films. When measuring the resistance of the films, the secondary dopants showed to also effect the resistance. PEDOT:PSS thin films containing only PH1000 gave re-sistances at the distance of 1.5cm in the MΩ range, which is too high for the solar cell application.
Figure 5.3: A boxplot of the resistances measured at 1.5cm distance on PEDOT:PSS films. All of the films contain PH1000 and Zonyl 0.5v/v% with the additives written
under each sample in the plot.
When adding diols to the solution the resistance in the films decreased significantly, particularly for EG and DEG. A trend can be seen in the box plot (Fig. 5.3), that shows that films containing DEG have lower resistance compared to the other diol additives. Even if the films containing PEG1000 and SQ became stiffer, their resistance increased considerable thus making them less suitable as transparent solar cell electrodes. Figure
5.4shows that films containing only PH1000, Zonyl 0.1v/v% and DEG 6v/v% have the lowest resistance at the distance of 1.5cm. Resulting in that this blend was the one chosen to proceed with.
Figure 5.4: A enlargement of the resistances measured at 1.5cm distance on PE-DOT:PSS films containing 6v/v% EG or DEG and 1v/v% SQ from the box plot in
figure5.3.
Contact Resistance
The contact resistance between the laminated films should be as low as possible this was done by measuring over a bridging, and on one film over the same distance. The results are shown in figure 5.5, where it is shown that the contact resistance is a few Ω. Since the data comes from only four samples these results should be considered an indicator, it should also be noted that the measurement over the bridge was done over a little longer distance than fore the measurement over the bridge. Which leads to that the resistance over the bridge is more likely to be lower than the results shown in figure 5.5. The results suggest that the material should be able to connect in series, due to the low contact resistance. To determine the actual contact resistance additional tests should be done.
Figure 5.5: A boxplot of the resistances measured at 1.5cm distance on one film and a bit longer distance over a bridging, four samples of each type.
Film Resistance Over Time
To analyze the films conductivity over time four samples made out of PH1000, Zonyl 0.1v/v% and DEG 6v/v% bridged films were left in the laboratory for 20 days in room temperature and about 50% humidity. They were first laminated together at 140◦C for 2 minutes and after pealing of the PET film they were annealed for 1 min at the same temperature. A drop of Ag-paste was placed onto the PEDOT:PSS film to make sure that the measuring was done on the same place on the film and lowering the contact resistance between the multimeter and the film. These results show that the films decreased their conductivity with 20-30% after 20 days. Worth noting is that these samples are not in a enclosed environment such as a solar cell is, thus giving the surrounding air the possibility to react with the samples. However, further studies should be done over a longer timespan to determine the materials behavior over time.
Table 5.2: Resistance measured at the distance of 1.5cm on day 0 and day 20. These results show that the films kept their conductivity within the same range at least for
20 days.
Sample number Resistance Day 0 [Ω] Resistance Day 20 [Ω]
1 39 52
2 40 54
3 54 70
4 50 64
5.1.3 DSC scan
An attempt to examine at what temperatures the polymers shift phases was done by running a DSC scan. Nothing more that the water evaporation during the first scan was seen, which was expected. [20] The result of the scan is shown in Appendix C.
5.1.4 Transmission vs. Sheet Resistance
The results shown in figure 5.6 where expected in terms of that the thicker films gave a lower sheet resistance and a lower transmission. In the plot the height of the blade coater bar is specified. Attempts for measuring the thickness of the films in the DEkTak were done but without any results due to the soft PET substrate.
Figure 5.6: Sheet resistance vs. transmission at the wavelength 500nm. The height specified in the legend is the height of the blade coater bar for each film.
When choosing a material for a certain application compromises are often to be made. In the case of solar cells both transparency and conductivity are of relevance if the material is to work as an electrode. From the data above the film with the blade coater hight of 60µ is chosen, due to its low sheet resistance of 40Ω/ and high transparency of 86%.
5.2
Lamination of Films Containing DEG to Metal Thin
Films
The PEDOT:PSS films laminated nicely to the Al/Cr-films and the PET substrate was easily removed from the bridge, showing that the adhering to the metal film was strong. (Fig. 5.7) However, when using PEDOT:PSS films directly on the Al/Cr-film the contact resistance was high, in the 500Ω range, between the laminated films. Therefore some different experiments were carried out to see if introduction of silver nanowires (AgNW) in different ways could decrease the contact resistance. AgNW was chosen due to that it is easy to coat the substrate with, forms transparent high conductive films and because silver creates a conductive contact to both metals and PEDOT:PSS. With this in mind the hypothesis was that a mixture of AgNW and PEDOT:PSS would give a highly conductive and transparent electrode with low contact resistance to the Al/Cr-film.
Figure 5.7: A photograph of an Al/Cr-film with a PEDOT:PSS film laminated to it. On the right of the Al/Cr-film the pealed off PET film with patches of PEDOT:PSS.
5.2.1 Introduction of AgNW to Decrease the Contact Resistance
By introducing AgNW into the PEDOT:PSS solution the conductivity should increase in the material and decrease the contact resistance to the Al/Cr-film, since the AgNW are
highly conductive and have low contact resistance to both PEDOT:PSS and the Al/Cr-film. However, this showed not to be the case. Instead a new experiment was performed where an interface of AgNW introduced by blade coated them onto the Al/Cr-film, before lamination of PEDOT:PSS. (Fig. 5.8)
Figure 5.8: A photograph of an Al/Cr-film with AgNW blade coated onto and a PE-DOT:PSS film laminated on to it. The pealed off PET film with patches of PEPE-DOT:PSS
lays to the right of the Al/Cr-film.
To compare how the different material combinations affect the contact resistance three different set ups were done as shown in figure 5.9. Figure 5.9 b) showed the lowest resistance of 20Ω, compared to a) and c) with the resistances of 550Ω and 320Ω respec-tively. The hopes were that c) with its few coating steps would have the lowest contact resistance. However, due the big difference in resistance c) is not of interest.
It would have been preferred to have c) as the best conductive material, due to when considering the production process this would have saved one coating step. But the difference in conductivity is too big for this method to be of interest.
Figure 5.9: These samples all have Al/Cr-film as a substrate and the PEDOT:PSS films are all laminated to the material beneath. Sample a) Al/Cr-film with a PE-DOT:PSS film laminated to it, b) a PEPE-DOT:PSS film laminated to a a layer of blade coated AgNW on an Al/Cr-film, c) a PEDOT:PSS film containing AgNW laminated
Film Resistance Over Time
The samples in figure5.9and four bridging samples as the one shown in figure5.8 were left for 20 days to study how the resistance changed over time. One of the bridging samples was destroyed already on the day 0, due to that the pealing of the PET did not succeed. The results are presented in table 5.3. Due to that these results do not seem to give a corresponding picture of how the materials resistance changes over time it is difficult to make any conclusions. There is a need for more of these experiments in order to evaluate the material properly.
Table 5.3: Sample number states which sample got which results. Number 1-3 are bridging samples and a) - c) are the samples shown in figure5.9.
Sample Number Resistance Day 0 [Ω] Resistance Day 20 [Ω]
1 230 1428 2 68 225 3 68 646 a) 545 2320 b) 20 60 c) 316 1970
5.3
Lamination of Films Containing DEG to the Active
Layer
A test was done to evaluate if the PEDOT:PSS films containing DEG would laminate to the active layer used in the solar cells. The active material was blade coated onto a PET substrate and the PEDOT:PSS film was laminated onto it. The lamination was done at 140◦C for 1 min. After the PET substrate was pealed off from the PEDOT:PSS, the film stayed on the active material and the films were annealed at 140◦C for 1 min. The lamination to the active material showed to work since the PET substrate was easily pealed off from the PEDOT:PSS film. This showint that the PEDOT:PSS film had stronger adhesion to the active layer than to the PET substrate. The active material did not seem to adhere well to the PET substrate, it seemed more rigid where the laminated and annealed PEDOT:PSS was. (Fig. 5.10)
Figure 5.10: The picture to the left is the active layer blade coated on to a PET film with a PEDOT:PSS film laminated to it. To the right of the film the pealed off PET
film with patches of PEDOT:PSS lays. The lamination was successful.
5.4
Plasma Patterning
When coating with only PH1000 the PEDOT:PSS arranged itself after the pattern done with the polydimethylsiloxane (PDMS), giving a self-assembled pattern, seen in figure 5.11. This preliminarily result showed that the methodology is promising, but the stripes are uneven thus the technique needs tuning.
Figure 5.11: The result of the plasma patterning. The drawn lines mark were the PDMS stripes where during the plasma treatment.
When coating with PH1000 and DEG 6v/v% on plasma treated PET the solution would not wet the treated substrate. Instead the solution formed droplets on top of the sub-strate. Apparently the addition of DEG changes the surface tension or other properties stopping it from wetting the surface. Further studies are needed to understand what happens.
Conclusions
The aim of this project was to find a way to make a PEDOT:PSS thin film laminate to another thin film made out of PEDOT:PSS, metal or a polymer fullerene blend. Laminating the films together on a hotplate fulfilled this goal. To enhance the lamination ability and reducing the resistance in the films different additives were added to the PEDOT:PSS solution. Of the additives studies in this research combination of PH1000 with Zonyl 0.1v/v% and Diethylene Glycol (DEG) 6v/v% gave films with the lowest resistance.
Another goal was for the laminated materials to have a low contact resistance. When laminating PEDOT:PSS to a film of the same material there is virtually no contact resistance measured. Good results, with only 20Ω contact resistance, were also achieved for the films laminated to Al/Cr-films that had Silver Nano Wires (AgNW) blade coated as an interlayer,. Both the PEDOT:PSS thin film and PEDOT:PSS with AgNW incor-porated into the film laminated nicely to the Al/Cr-film, but their contact resistance were both over 300Ω making them of low interest for the application of a transparent electrodes.
The PEDOT:PSS film can laminate to the different materials used in an organic solar cell. Films coated with 60µm height on the blade coater bar has a sheet resistance of 40Ω/ and 84% transparency when illuminated with light with the wavelength of 500nm. These results indicate that PEDOT:PSS with the mentioned additives is suitable for the application as a transparent electrode in organic solar cells. It may still be possible to decrease the resistance even more, mainly due to that it is still not stated
what happens in the material when the secondary dopants are added. If this were known it would be easier to optimize the concentrations used to enhance the materials conductivity. Also if metal contacts are used, further studies on decreasing the contact resistance between PEDOT and the metal is needed. Different types of low cost materials such as carbon paste, graphene paste or PSS-free PEDOT, such as PEDOT-S, should be investigated as interlayers between the metal and PEDOT to reduce the contact resistance.
The general conclusion from this study is that the PEDOT:PSS thin film containing PH1000, Zonyl and DEG is high conductive, transparent and can laminate to another thin film made out of the same solution, metal or a polymer fullerene derivate blend with a low contact resistance.
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Estimation
Here you will find the estimation of how many years it takes to return the invested money for an installed silicon solar cell.
The sun radiates Sweden with about 1000kWh/(m2 year). Say that the solar panel has an efficiency of 18%.
That will give 180kWh/(m2 year). 1kWh costs about 1SEK in Sweden.
Giving that one m2 generates about 180SEK/year.
A 1kW package from Norden Solar, including solar panels of 7m2, converter, cables and all other installation materials costs about 19 000SEK. [21].
19 000SEK / 7m2 = 2 714SEK/m2
2 714SEK/m2 / 180SEK/(m2 year) = 15 years
Early experiments
During the first weeks several experiment were done to study different possibilities of achieving a conductive connection between PEDOT:PSS films. Below is a list of the different approaches examined. These studies were all done on material from the machine in Norrk¨oping, 50µm PET and low conductive PEDOT:PSS. The samples were prepared as shown in figureB.1
Figure B.1: A schematic picture of how the material from Norrk¨oping was cut into pieces, and how the pieces were assembled.
1. Sewing the pieces together with sewing thread 2. Sewing the pieces together with copper thread
3. Taping the films together with conductive carbon tape 4. Pressing the films together between to glass slides
Figure B.2: The films made contact when pressing them together with a finger as the arrow shows.
Experiment 1
Sewing the pieces together with sewing thread the theory behind this experiment was based on the fact that when pressing the films together with a finger a contact resistance was measurable. (Fig. B.2) This implies that it would be enough to get the films close enough together, to create a contact between the films. Sewing is also a well-known technique in the industry, which makes it a promising method to use in the production. Unfortunately it turned out that this was not quite enough to enable a good conducting contact. There were still a measurable contact between the films, but it was in the MΩ range. Maybe it would have worked out better if the needle used was thinner and/or if the sewing was done on a sewing machine, because now the holes became a lot bigger that the thread. (Fig. B.3)
Figure B.3: A Picture of the experiment were the films are sewn together with ordi-nary sewing thread. The copper tape is there to give a better contact between the film
Experiment 2
Sewing the pieces together with copper thread was done to study if the copper tread could reduce the resistivity. Unfortunate the results were in the same range as the sewing done with sewing thread. (Fig. B.4) These experiments show too poor results to be continued with.
Figure B.4: A Picture of the experiment were the films are sewn together with a copper thread. The copper tape is there to give a better contact between the film and
the voltmeter.
Experiment 3
Taping the films together with conductive carbon tape also gave poor results, and was thus not continued with. (Fig. B.5)
Figure B.5: A Picture of the experiment with conductive carbon tape keeping the two double strings together. The copper tape is there to give a better contact between
the film and the voltmeter.
Experiment 4
The idea with pressing the films together between two glass slides was that the pressure might be enough to make the films laminate. The sample was left under pressure over night. (Fig. B.6 and B.7) When the glass slides were withdrawn from each other the PET films were stuck on the glass slides instead of to each other.
Figure B.6: A Picture of the experiment were the films are pressed together between two glass slides. Picture taken from the side.
Figure B.7: A Picture of the experiment were the films are pressed together between two glass slides. A close up from above.
DSC scan
Data from the DSC scan is presented in the graph in figure C.1. The samples were heated from 20◦C to 219◦C with a 10◦C/min increase, and then cooled down to 20◦C for a second scan up to 219◦C again. The Tg temperature was not found in the graphs. What can be seen is the water from the samples that evaporate around 100◦C during the first heating cycle.
Figure C.1: The results from the DSC scan.
Transmission
Figure D.1: Transmission graph of four different films. The thickness written in the legend corresponds to the height of the blade coater bar.
