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Rafal Sliz

ANALYSIS OF WETTING AND OPTICAL PROPERTIES OF MATERIALS DEVELOPED FOR NOVEL PRINTED SOLAR CELLS

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING,DEPARTMENT OF ELECTRICAL ENGINEERING;INFOTECH OULU

C 492

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Rafal Sliz

D492etukansi.kesken.fm Page 1 Tuesday, May 27, 2014 12:33 PM

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 4 9 2

RAFAL SLIZ

ANALYSIS OF WETTING AND OPTICAL PROPERTIES OF MATERIALS DEVELOPED FOR NOVEL PRINTED SOLAR CELLS

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in Auditorium TS101, Linnanmaa, on 4 July 2014,at 12 noon

UNIVERSITY OF OULU, OULU 2014

Copyright © 2014Acta Univ. Oul. C 492, 2014

Supervised byProfessor Risto Myllylä

Reviewed byProfessor Kai PeiponenProfessor Poopathy Kathirgamanathan

ISBN 978-952-62-0487-1 (Paperback)ISBN 978-952-62-0488-8 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2014

OpponentsProfessor Kai PeiponenProfessor Ryszard Jachowicz

Sliz, Rafal, Analysis of wetting and optical properties of materials developed fornovel printed solar cells. University of Oulu Graduate School; University of Oulu, Faculty of Information Technologyand Electrical Engineering, Department of Electrical Engineering; Infotech OuluActa Univ. Oul. C 492, 2014University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Printed electronics offer unique possibilities for the development of devices and manufacturingmethods. A prime example of printed electronics where the production volume can besignificantly increased are solution-processed organic solar cells. Roll-to-roll (R2R) technologyhas made it possible to print solar cells almost as fast as newspaper.

Unfortunately, the performance of printed devices depends strongly on film morphology,which is affected by the behaviour of the used ink on the confining surface - wetting. Keyparameters that influence the wetting behaviour include surface energy, ink formulation, surfaceroughness, solvent properties, processing temperature and pre/post-treatments (heat, acid orplasma) and chemical heterogeneity. Importantly, a precise control of wetting and, consequently,film morphology is emphasized by many authors as an important factor for the commercializationof printed solar cells.

This research focuses on measuring and analysing the influence of substrate processingtemperature as well as plasma and UV pre-treatments on the wettability of various inks andsubstrates used in Organic Solar Cell (OSC) fabrication. It also explores the application ofinteresting novel materials, such as nanocellulose, in solar cell manufacture. The main tool appliedhere is the contact angle measurement method, since it is commonly used to obtain quantitativedata describing the behaviour of ink droplets on substrate surfaces.

Chief among the achieved results is the finding that the three factors mentioned abovesignificantly influence ink-substrate interactions. Therefore, manipulation of plasma and UVtreatments as well as substrate processing temperature, allow us to control wetting properties and,in consequence, the printing process. Another important result shows that the degree of control isstrongly dependent on ink formulation and material composition and must, therefore, be taken intoaccount in process development. These findings will contribute to a faster development of printedsolar cells and their manufacturing conditions and requirements.

Keywords: contact angle, organic solar cells, printed electronics, printing, wetting

Sliz, Rafal, Uudentyyppisiä painettavia aurinkokennoja varten kehitettyjenmateriaalien kostumis- ja optisten ominaisuuksien analysointi. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Tieto- ja sähkötekniikan tiedekunta,Sähkötekniikan osasto; Infotech OuluActa Univ. Oul. C 492, 2014Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Painettava elektroniikka tarjoaa uusia mahdollisuuksia elektronisten laitteiden ja niiden valmis-tusmenetelmien kehittämiseen. Liuoskäsitellyt orgaaniset aurinkokennot ovat hyvä esimerkkipainetun elektroniikan tuotteesta, jonka tuotantomäärää on voitu kasvattaa merkittävästi. Rullal-ta rullalle (engl. Roll-to-roll, R2R) -menetelmän avulla aurinkokennoja voidaan painaa lähessamalla nopeudella kuin sanomalehteä. Painettujen laitteiden suorituskyky riippuu suuresti tuo-tetun kalvon morfologiasta, johon vaikuttavat tuotantoprosessissa käytettyjen materiaalien kos-tumisominaisuudet. Tärkeimmät kostumiskäyttäytymiseen vaikuttavat parametrit ovat pintaener-gia, pinnan karheus, musteen koostumus, liuotinominaisuudet, käsittelylämpötila, esi- ja jälkikä-sittely (lämpö, happo tai plasma) sekä kemiallinen heterogeenisyys. Kostumisen, ja sitä kauttakalvon morfologian, tarkka säätely on tärkeää painettujen aurinkokennojen kaupallisen hyödyn-tämisen kannalta.

Tässä väitöskirjatyössä mitataan ja analysoidaan käsittelylämpötilan sekä plasma- ja UV-esi-käsittelyiden vaikutuksia orgaanisten aurinkokennojen valmistuksessa käytettyjen musteiden jaalustojen kostumisominaisuuksiin sekä tarkastellaan aurinkokennoliuoksissa käytettäviä uusia,mielenkiintoisia materiaaleja, kuten nanoselluloosaa. Työssä eniten hyödynnetty menetelmä onkontaktikulman mittaus, joka on yleisesti käytetty tapa hankkia kvantitatiivista tietoa mustepisa-roiden käyttäytymisestä erilaisilla pinnoilla.

Keskeisin saavutettu tutkimustulos on se, että kaikilla yllämainituilla kolmella käsittelyllä onhuomattava merkitys musteen ja alustan vuorovaikutuksiin. Näin ollen plasma- ja UV-käsitte-lyillä sekä alustan käsittelylämpötilan säätelyllä voidaan hallita kostumisominaisuuksia ja sitäkautta koko painatusprosessia. Toinen tärkeä löydös on, että musteen koostumus ja alustan mate-riaali vaikuttavat siihen, kuinka voimakkaasti kostumista voidaan hallita. Näin ollen ne täytyyottaa huomioon painatusprosessin suunnittelussa. Työssä saavutettuja tuloksia voidaan käyttääpainettujen aurinkokennojen sekä niiden tuotantomenetelmien kehittämiseen.

Asiasanat: kontaktikulma, kostumisilmiö, orgaaniset aurinkokennot, painaminen,painettava elektroniikka

To my family, toall those who supported me along the wayand to those who fell asleep while readingthis document. Finally, to all those whosmile with no reason.

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Acknowledgements

This study is based on research carried out in the Optoelectronics and MeasurementTechniques Laboratory, University of Oulu, during the years 2008–2013.

I would like to express my deepest gratitude to my supervisor, Professor RistoMyllylä, for his guidance, understanding and patience during these years. Then I wouldlike to thank Professor Ghassan Jabbour for his time and invaluable guidance. I wouldalso like to acknowledge my friends and colleagues at the OEM laboratory for makingthis research possible and pleasant, especially Adjunct Professor Tapio Fabritius for hisunfailing support. I am grateful to the friends and colleagues I have met during myresearch visits at ASU and LCN – you guys made my time prodigious.

I’d like to recognize Professor Kai-Erik Peiponen and Professor KathirgamanathanPoopathy for serving as official reviewers of this thesis.

I must also acknowledge all my friends and colleagues that very successfully andpatiently distracted me from work so I could live longer as a PhD student...

Naturally, the crew of IEEE members deserve recognition as well: you boys and girlsturned my volunteering into pure fun and joy. Especially (in a random, alphabeticallyreversed, order): Karoliina Jokinen, Janne Lauri, Jakub Czajkowski, Kalle von derGeest Blomberg, Arttu Ollikkala and many others, who provided invaluable experiencesduring IEEE technical events.

I highly appreciate the financial support of the Infotech Oulu Graduate School andFinnish Cultural Foundation. Without their generous support, this research would havetaken even more time. Finally, I would like to express my love to my family, for simplybeing My Family.

Oulu, 11th of November 2013

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List of abbreviations

AFM Atomic force microscopyAM-0 Airmass coefficient on top of Earth’s atmosphereAM-1.5G Airmass coefficient for 48.2 from zenithASTM American society for testing and materialsAl AluminiumAr ArgonCHB P3HT:PCBM dissolved in chlorobenzeneCIGS Copper indium gallium selenideCdS Cadmium sulfideCdTe Cadmium tellurideDCB P3HT:PCBM dissolved in 1,2-dichlorobenzeneDI-water Deionized waterDSSC Dye-sensitized solar cellDoD Drop on demandECD Electrochemical depositionEQE External quantum efficiencyGaAs Gallium arsenideGaN Gallium nitrideGe GermaniumHTL Hole transport layerI-V Current-voltage curveIEC International electrotechnical commissionIPA Isopropyl alcoholIQE Internal quantum efficiencyITO Indium tin oxideJIS Japanese industrial standardsLED Light emitting diodeLiF Lithium FluorideO2 DioxygenOLED Organic light emitting deviceOSC Organic solar cell

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P3HT Poly(3-hexylthiophene-2,5-diyl)PC PolycarbonatePCBM Phenyl-C61-butyric acid methyl esterPCBM70 Phenyl-C71-butyric acid methyl esterPCDTBT Poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-

2’,1’,3’-benzothiadiazole)PCE Power conversion efficiencyPECVD Plasma-enhanced chemical vapour depositionPEDOT:PSS Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)PEN Polyethylene naphthalatePES PolyethersulfonePET Polyethylene terephthalatePTB7 Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-

2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]R2R Roll-to-Roll processingSCE Standard calomel electrodeSEM Scanning electron microscopySi SiliconTiO2 Titanium dioxideUV UltavioletZn(NO3)2 Zinc nitrateZnO Zinc oxideZnO NR ZnO nanorodsa-Si Amorphous siliconc-Si Crystalline siliconnc-Si Nanocrystalline siliconE Energy of the photonFF Fill FactorI CurrentISC Short circuit currentImax Maximum currentJSC Short circuit current densityPmax Maximum power pointPs Incident light power densityRL Load resistance

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Rav Average roughnessRq Root mean square roughnessRsh Shunt resistanceRs Series resistanceV VoltageVOC Open circuit voltageVmax Maximum voltagec Speed of light in vacuumh Planck’s constantΘ Contact angleη Solar cell efficiencyγLV Interfacial tension between the liquid and the vapourγD

LV Disperse component of the interfacial tension between the liquidand the vapour

γPLV Polar component of the interfacial tension between the liquid and

the vapourγSL Interfacial tension between the solid and the liquidγSV Interfacial tension between the solid and the vapourγD

SV Disperse component of the interfacial tension between the solidand the vapour

γPSV Polar component of the interfacial tension between the solid and

the vapourλ Wavelength of the photon

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List of original articles

This template is based on the following articles, which are referred to in the text by theirRoman numerals (I–VII):

I Sliz R, Suzuki Y, Nathan A, Myllylä R & Jabbour GE (2012) Organic solvent wettingproperties of UV and plasma treated ZnO nanorods - printed electronics approach. SPIEOptics+Photonics 2012, 12 - 16 August 2012, San Diego, USA.

II Sliz R, Suzuki Y, Fabritius T & Myllylä R (2014) Influence of temperature on wettingproperties of thin films in organic solar cells applications. Colloids and Surfaces A:Physicochemical and Engineering Aspects 443: 182–187.

III Sliz R, Ahnood A, Nathan A, Myllylä R & Jabbour G (2012) Characterization of microcrys-talline I-layer for solar cells prepared in low temperature - plastic compatible process. SPIEPhotonics Europe 2012, 16–19 April 2012, Brussels, Belgium.

IV Kopola P, Aernouts T, Sliz R, Guillerez S, Ylikunnari M, Cheyns D, Välimäki M, TuomikoskiM, Hast J, Jabbour G, Myllylä R & Maaninen A (2011) Gravure printed flexible organicphotovoltaic modules. Solar Energy Materials & Solar Cells 95(5): 1344–1347.

V García V, Valkama H, Sliz R, King AWT, Myllylä R, Kilpeläinen I & Keiski RL (2013)Pervaporation recovery of [AMIM]Cl during wood dissolution; effect of [AMIM]Clproperties on the membrane performance. Journal of Membrane Science 444: 9–15.

VI Liimatainen H, Ezekiel N, Sliz R, Ohenoja K, Sirviö JA, Berglund L, Hormi O & NiinimäkiJ (2013) High strength nanocellulose-talc hybrid barrier films. ACS Applied Materials &Interfaces 5: 13412–13418.

VII Visanko M, Liimatainen H, Sirviö JA, Haapala A, Sliz R, Niinimäki J & Hormi O (2014)Porous thin film barrier layers from 2,3-dicarboxylic acid cellulose nanofibrils for membranestructures. Carbohydrate Polymers 102: 584–589.

In publications (I-III), the author developed and performed all experiments. Moreover,he performed the literature review and wrote the manuscripts. The co-authors thenreviewed and improved the manuscripts before publication. The ZnO nanorods werefabricated by Yuji Suzuki. In publication (IV), the author conducted the describedperformance measurements on printed solar cells. In publications (V-VII), the authorperformed the contact angle measurements and carried out the wettability analysis ofmembranes and barriers.

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Contents

AbstractTiivistelmäAcknowledgements 9List of abbreviations 11List of original articles 15Contents 171 Introduction 192 Solar cells 25

2.1 Solar cells types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.1.1 Inorganic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.1.2 Organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2 Solar cells operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3 Solar cells characterization methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3.1 Electrical characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3.2 Solar cell spectral response characterization . . . . . . . . . . . . . . . . . . . . . . 35

2.3.3 Morphology characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.3.4 Lifetime characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.4 Solar cells manufacturing technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.4.1 Conventional technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

2.4.2 Printed solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Printing process 413.1 Printing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.1 Conventional printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.1.2 Digital printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2 Interface interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.1 Inks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.2 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

3.2.3 Surface tension and surface energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.2.4 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3 Printed electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

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4 Printing process manipulation 594.1 Effect of oxygen, argon plasma and UV pre-treatment on the

wettability of ZnO-nanostructured thin-films . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.2 Influence of temperature on wetting properties in OSC. . . . . . . . . . . . . . . . . . . 654.3 Temperature dependable material distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5 Summary 81References 83Original articles 93

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1 Introduction

While the majority of the world’s current electricity supply is generated from fossilfuels, such as coal, oil and natural gas, these traditional energy sources face a number ofchallenges, including rising prices, security concerns over dependence on imports froma limited number of countries and growing environmental concerns [1]. As a result ofthese and other challenges posed by traditional energy sources, funding institutionsand industry are increasingly supporting the development of alternative energy sourcesand new technologies for electricity generation. Renewable energy sources, such assolar, biomass, geothermal, hydroelectric and wind power generation have emerged aspotential alternatives, which address some of the issues [2]. In opposition to fossil fuels,which draw on finite resources that may eventually become too expensive to retrieve,renewable energy sources are generally unlimited in availability [3–5]. One potentialcandidate for future environmental-friendly energy production is solar power, since theenergy of solar radiation reaching the earth is many times greater than the current globalpower consumption [6].

Solar power generation is seen as one of the most rapidly growing renewable sourcesof electricity. Although a variety of technologies exist for making solar energy usable,the only way of converting sunlight directly into electrical energy is solar cells [7–9].Primary costs in solar cell manufacture are determined by materials and manufacturingprocesses. Most solar cells in use today use silicon as the semiconductor that enables thephotoelectric effect. Despite the fact that ramped-up production has brought along asignificant drop in the cost of silicon and, consequently, solar cells, it is still expensiveto process silicon from raw material to solar cell semiconductor. For that reason,streamlining the production process has become a top goal for solar industrialists, andwhile advancements have certainly been made, production costs remain high.

A variety of techniques have been developed to improve the cost-efficiency ofsolar cells, which is important for the strive towards carbon-free technologies [10, 11].Low-cost large-area thin film photovoltaic solar cells (Amorphous silicon (a-Si),Nanocrystalline silicon (nc-Si), Copper-Indium-Gallium-Selenide (CIGS), OSC andDye-Sensitized Solar Cell (DSSC)) have been developed to compliment their high-cost,high-efficiency crystalline silicon (c-Si) counterparts and are paving the way for low-cost

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renewable electricity [12]. Optimal function of the solar cell also requires making basicoptical measurements (Paper III) of the active medium in order to optimize its operation.

In recent years, the problem of manufacturing costs has been circumvented bycreating solar cells from organic components that can be processed as easily as plastics.Due to low manufacturing costs, OSCs have a high potential for becoming an essentialrenewable and environmentally-friendly energy production technology of the future. Afundamental difference between solar cells based on organic materials and conventionalinorganic photovoltaic cells is that light absorption results in the formation of excitonsin molecular materials, rather than in free electrons and holes [13]. Organic moleculesand polymers have the remarkable advantage of allowing facile, chemical tailoringto alter their properties, such as the optical band gap [14]. For instance, conjugatedpolymers combine the electronic properties of traditional semiconductors with the easeof processing and mechanical flexibility of plastics. Since the discovery of conductivepolymers, this new class of materials has attracted considerable attention owing toits potential of providing environmentally safe, flexible, lightweight and inexpensiveelectronics [15]. As a disadvantage, OSCs still suffer from reduced lifetime and arelatively low energy-conversion efficiency, which is currently about 10 % [11, 16–18].

Nevertheless, thin-film inorganic, organic and organic-inorganic hybrid solar cellscreated on flexible substrates using high-throughput fabrication technologies can beintegrated with various passive and active components, allowing the development ofendless applications containing resistors, solar cells, transistors, Light Emitting Diodes(LEDs), etc., with the additional benefits of light weight and low cost [19, 20].

Even though the terms printed electronics and organic electronics are sometimesused interchangeably, they do not necessarily refer to the same group of technologies.Printed electronics refers to the application of printing techniques, both conventionaland digital, to the fabrication of electronic structures, devices and circuits, regardless ofthe materials used [21]. The only prerequisite is that the functional material must beprocessable from the liquid phase [22, 23]. Organic electronics, on the other hand, dealswith the application of organic materials to the manufacture of electronic structures,devices and circuits on rigid or flexible substrates. The field of flexible electronics,which is being intensively researched at the moment, is based on bendable substratesmade of plastic or paper. Recent advances in printed electronics has opened up a newapproach, leading to a quantity of new concepts, designs, fabrication methods, packagingtechniques and applications of electronic devices and circuits. A remarkable exampleof applied printed electronics is mass production of organic-based photovoltaic cells

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processed at low temperature using R2R and inkjet printing (Paper IV) [24, 25]. Also,Organic Light Emitting Devices (OLEDs) are an important example of the successfulapplication of printed electronics [26, 27]. Printing allows depositing active materialson specific areas of a substrate, contrary to other methods, such as spin coating [28].Offset, gravure, screen and flexographic printing are the major types of conventionalprinting technologies. They typically require a printing master or a printing plate.Digital printing, in contrast, works without a physical, pre-manufactured master printingplate and prints without a significant impact force on the substrate.

The interaction of a liquid with a solid is referred to as wettability and, togetherwith surface chemistry, plays a crucial role in many aspects of engineering and science,including printing, development of coating materials and biochemistry, to name justa few [29, 30]. Controlling the hydrophobicity or hydrophilicity of a surface is animportant aspect for numerous technical applications. In everyday life applications, itis desirable to have highly hydrophobic surfaces to avoid the adhesion of snow andraindrops or in order to form self-cleaning surfaces. Hydrophilic surfaces, on the otherhand, are desirable in biomedical applications, such as tissue reconstruction. Wettabilityis an important factor in the evaluation of nano-enhanced membranes and barriers.For instance, it can be used to evaluate the increase in component flux caused by achange in the hydrophilicity of a membrane once it comes in contact with an ionic liquid(Paper V). This study involved TiO2, which has been a subject of continuous researchas a potential solar cell medium. Some recently published papers have described atransparent and conductive paper made from nanocellulose fibers and even demonstrateda novel solar cell based on it [31]. Thus, another important aspect of this thesis concernsnanocellulose-talc barriers, the wettability of which can be tuned by adjusting eithertheir talc content or surface roughness (Paper VI, VII). In ink-jet printing, the interfacebetween the substrate and solution (ink) drop is much studied, as it is considered themost critical factor for achieving high printing quality [32–34]. The contact anglemeasurement is a commonly used method that provides quantitative data describing thebehaviour of ink droplets on substrate surfaces. At a low contact angle, ink dropletsspread well and distribute material over a large surface area. In contrast, at a largecontact angle, droplets with an equal volume preserve their drop shape at the surface,resulting in small area coverage, while forming a thicker layer after drying [35]. Thus,contact angle and ink drop volume are values that need to be calculated to determine thearea covered by a drop on a substrate surface and, consequently, to predict the printedfeature size and morphology [36, 37].

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Wettability depends strongly on the properties (curvature, roughness, chemistry,pre-treatments, and chemical heterogeneity) of the confining surface and the ink (content,viscosity, surface energy, solvents boiling point, etc.) [38, 39]. Surface pre-treatmentwith UV illumination, oxygen and argon plasma have been proven to improve thehydrophilicity of treated surfaces and, thus, printing properties (Paper I) [40–43]. Duringthe past few years, several groups have studied the effect of temperature and contactangle and a number of theories have been put forward (Paper II) [44–52]. Nonetheless,since experimental results in many cases differ from the proposed theoretical models,further development is required. And although many groups have researched thewettability of commonly used liquids and solids, certain applications, solar cells inparticular, require a separate approach and methodology, mostly due to the use ofnovel materials and techniques. To give an example, nano-structured thin films andnanoparticles exhibit unique physical and chemical properties, which have a significanteffect on wettability [53, 54].

This research presumes that wettability and, consequently, the printing of OSCs, canbe precisely controlled by applying UV and plasma pre-treatments and by manipulatingthe substrate processing temperature. Therefore, the influence of the following aspectsrelating to the wettability of OSC materials will be investigated:

– Physical and chemical substrate pre-treatments (UV irradiation, argon and oxygenplasma treatments),

– Various substrate temperatures,– Substrate surface morphology.

In addition, the influence of substrate temperature on solar cell performance andmaterial distribution will be presented. The materials used in this research include:Poly(3-hexylthiophene-2,5-diyl : Phenyl-C61-Butyric Acid Methyl Ester (P3HT:PCBM)blends, Zinc Oxide (ZnO) nano-structures and conductive polymers. Importantly, thisresearch uses commercially available solar cell materials and involves no materialsdevelopment work. Ink-substrate interactions were measured using the contact anglemethod. Additionally, morphological characterizations were made with Atomic ForceMicroscopy (AFM), Scanning Electron Microscopy (SEM) and optical profilometry.The collected data and experience concerning the wetting processes of inks and thinfilms used in this investigation will result in a better control of the printing process andallow faster research and development of printed solar cells and their manufacturingconditions.

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This thesis is organised as follows:

– Section 2 describes different types of solar cells, their operating principles, manufac-turing methods and characterization.

– Section 3 concentrates on printing technologies and requirements of printed electron-ics.

– Section 4 introduces methods of manipulating wettability, including various pre-treatments, and the influence of substrate temperature on wettability and deviceperformance. In addition, the section presents evidence on the influence of temperatureon material distribution.

– Section 5 sums up the results achieved during this investigation, highlighting theimportance of the performed research and identifying potential further work to becarried out in this field.

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2 Solar cells

Indirectly, the sun is the source of most of the energy on Earth, not only that of fossilfuels and biomass, but also the wind, the waves, as well as in other sources. Sun was"build" about 4.6 billion years ago and chemical reactions within are projected to last forabout 5 billion years. Therefore, compared to the age of our civilization, we can assumesun as an infinite source of energy. At a distance of 150 million km, our planet receivesabout 1366 W/m2 (1 W = 1 J·s) irradiance from the sun. This value that is reaching thesurface is reduced by processes occurring in the Earth’s atmosphere: reflection andabsorption reduce the ordinance to about 1000 W/m2. Although Earth receives only asmall portion of the radiation emitted by the sun, equal to 1,7x1014 kW, it has beenestimated that 84 min of solar radiation approaching the surface is equal to the worldenergy demand for one year [55]. Solar radiation is therefore a promising source ofenergy and interest in direct photons-electrons conversion has been gradually increasing.

The foundations for the modern use of solar energy were laid in 1839, when Frenchphysicist Edmond Becquerel observed increase in electrical conductivity in certainmaterials when they were exposed to light: the phenomenon is now known as thephotovoltaic effect [56, 57]. This phenomenon can be used in solar cells to collectsolar energy and convert it into electricity. Since the first discovery of photovoltaiceffect, solar cells have been continuously developed resulting in various structures andimplemented materials, high power conversion efficiency, and numerous applications.

2.1 Solar cells types

Currently, there are several types of solar cell structures. Therefore, there are alsoseveral different criteria for their distribution and classification, e.g. by the base materialused, material structure, architecture, etc. [58]. For the purpose of this work, solarcells are firstly classified by the material used for manufacturing. Hence, materials thatproduce the photovoltaic effect can be divided into two groups: inorganic and organic.In addition, the hybrid solar cells group can be formed by combining inorganic andorganic components together into one structure. In addition, solar cells can be dividedby the type of semiconductors connection. We can distinguish cells with homo andheterojunction and those based on the p-i-n junction (typically amorphous silicon).

25

Homojunction p-n is composed of two suitably doped regions of the same material,the width of the energy gaps for p-type and n-type layers are the same here. In theheterojunction the combination of two different semiconductor materials with differentenergy gaps width, is implemented. The p-i-n (n-i-p) junction is created by sandwichingthe middle intrinsic layer between n-type and p-type materials. The intrinsic layer isused to generate electrons and holes, which are consequently separated by the electricfield. Finally, a separate group of solar cells are the multijunction structures often calledtandem solar cells. In the most common tandem solar cell, individual cells with differentbandgaps are stacked together. The individual cells are stacked that sunlight first reachesthe material that has the largest bandgap. Lower energy photons that are not absorbed inthe first cell are passing to the second cell, which absorbs the remaining portion of thesolar radiation. Accordingly to the amount of stacked cells this selective absorptionprocesses continue through to the bottom cell, which has the smallest bandgap.

2.1.1 Inorganic solar cells

Inorganic solar cells are the largest group mostly due to silicon which absorptioncharacteristics fairly match the solar spectrum, and fabrication technology which is welldeveloped as a result of its pervasiveness in the semiconductor electronics industry. Thiswell represented group of inorganic solar cells can be further divided accordingly to thestructure of used material. There are main three subgroups: monocrystal, polycrystalineand amorphous solar cells. Although monocrystalline solar cells (Silicon (Si), GalliumArsenide (GaAs)) have the highest efficiency reaching 25 %, the production costs andthe ability to deposit on a variety of substrates increase significantly the popularityand the range of applications of polycrystalline solar cells [58]. The most popularpolycrystalline cells are made of polycrystalline Si, polycrystalline GaAs, CIGS, andCadmium Telluride/Sulfide (CdTe/CdS).

Amorphous silicon is a material with atoms not arranged in any particular order.Solar cells that utilize the amorphous silicon are extremely cheap in manufacturingand low temperature requirements allow them to be implemented in flexible substrates.Unfortunately, a low cost of production and unique applications are significantlycompromised by their low efficiency reaching approximately 10 %. Quantum dots solarcells are an emerging field of research which utilizes quantum dots as photovoltaicsmaterial [59]. The main advantage of quantum dots is the possibility of tuning theirband-gap by changing their size (larger the size of the dot, the smaller the band gap,

26

and the less energy needed to excite the dot). Therefore, different size quantum dotsimplemented in one device might absorb the solar spectrum totally. Besides the band-gaptuning, quantum dots solar cells open new avenues in energy generation, for instancemultiple exciton generation [60, 61].

2.1.2 Organic solar cells

The organic solar cells devices are made from organic compounds that contain in theircomposition carbon atoms. They were first introduced by Tang in 1986 [62]. Theorganic solar cells are built from polymers and small organic molecules that are able toabsorb solar radiation and conduct the electricity. In their structure and physicochemicalproperties they resemble the widely used plastic and consequently their manufacturingprocess is fast and cheap. The molecular engineering of organic compounds can easilychange the properties of the polymers to current needs. Additionally, organic solar cellscan be processed at temperatures below 200 C, printed, and manufactured on flexiblesubstrates, so they are ideal candidates for the plastic and portable energy harvestingsystems. Unfortunately, despite numerous advantages, currently produced organiccells suffer mainly from low efficiency and short lifetime. Presently, depending ofthe organic semiconducting material, three types of organic solar cells are fabricated:polymer-based, small molecules and dye-sensitised (Gratzel cell) [63].

– Polymer based solar cells. Although most of polymer solar cells are based on twosemiconducting materials - donor and acceptor, the way of connection of the aforemen-tioned materials distinguish bilayer and bulk heterojunction. Similarly to inorganicp-n junction, in the bilayer structure donor (p-type semiconductor) and acceptor(n-type semiconductor) materials form a stacked structure. For bulk heterojunctionarchitecture, both donor and acceptor materials form the uniform and homogeneousregion of active material. By implementing novel low-bandgap polymers the efficiencyhas been increased to over 10 %. The most common materials used as a donormaterial on organic solar cells are: P3HT, Poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) or Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) to name a few. Among the accep-tor materials used in organic solar cells industry the most popular are: PCBM orPhenyl-C71-Butyric Acid Methyl Ester (PCBM70). Although solar cells based on the

27

P3HT:PCBM blend suffer from rather low performance and short lifetime, manyresearchers utilize them due to their low manufacturing costs [64]. Moreover, theirperformance can be stepped up. For instance, devices with an additional titaniumoxide layer show significantly improved performance: Isc = 11.1 mA/cm2, Voc = 0.61V, FF = 66%, and PCE = 5% [65]. Another method of improving the efficiency ofP3HT:PCBM solar cells is the thermal annealing process. Solar cells with thermallyannealed P3HT:PCBM layers have the following parameters: Isc = 10.6 mA/cm2, Voc

= 0.61 V, FF = 67%, and PCE = 4.37% [66].– Small Molecules. The main difference between small molecular weight materials and

polymer materials is the way of manufacturing thin films. In case of small molecules,mostly the vacuum thermal evaporation techniques are implemented. The polymermaterials are usually solution processed in the inert gas atmosphere [67]. For smallmolecular weight materials, phthalocyanine and pentacene are typical donor materials,and perylene derivatives and fullerene are typical acceptor materials [68].

– Dye Sensitized Solar Cells. A dye-sensitized solar cell consists of organic dyeadsorbed distributed at the surface of an inorganic wide-band gap semiconductor,which is used to absorb the photons and photoexcite electrons into the conductionband. A Gratzel group as a first used nanoporous Titanium Dioxide (TiO2) to improvethe interfacial area between organic donor and inorganic acceptor [63]. Currently, thebest DSSCs reach a power conversion efficiency of about 14 %.

2.2 Solar cells operation

For the purpose of this work only the essential concepts of the physics of solar cellsare described. A more complete and advanced explanation of solid-state physics andorganic semiconductors is available from a number of sources [13, 14, 69–72].

The idea of a solar cell is the effective absorption of electromagnetic radiation,mainly in the visible light and convert that energy into electricity. Such an effect canbe obtained with different mechanisms and materials, but all devices are based onsemiconductors. The conductivity of semiconductors can vary over a wide range (10−8

to 103 S/cm) depending on doping, temperature, light illumination and other factors.Typical semiconductor conductivity is between the conductivity of metals and dielectrics.Semiconductors have a band gap between the valence band and the conduction band inthe range of 0 - 6 eV (eg. Germanium (Ge) 0.7 eV and Si 1.1 eV, GaAs 1.4 eV andGallium Nitride (GaN) 3.4 eV). Recently, the intense development of carbon-based

28

organic semiconductors resulted in polymer materials and implementations that are notavailable for their inorganic counterparts.

A well studied p-n junction applied in most common silicon devices will be used toexplain the working principle of solar cell. The essential part of the structure of thesolar cell is the p-n junction, which consists of n-type semiconductor in contact withp-type semiconductor (Figure 1). In thermal equilibrium, since there is a concentrationdifference of holes and electrons between the two types of connected semiconductors,holes diffuse from the p-type region into the n-type region and, similarly, electrons fromthe n-type material diffuse into the p-type region [7].

n-type p-type

+ ++ ++ ++ +

Depletion region

p-n junction region

- -- -- -- -

Fig 1. Simplified version of p-n junction and charges location.

The transition region between the p-type and n-type semiconductors is called thespace-charge or depletion region, since it is completely depleted of electrons and holes.The positive space-charge on the n-type side must be equal in magnitude to the negativespace-charge on the p-type side. The adjacent positive and negative space-charge causean electric field which tends to resist additional diffusion of electrons and holes towardsthe p-side and n-side respectively. This electric field is called the built-in electric field,and the potential difference between the front and the back of the device is called thebuilt-in voltage [73].

The incident photons can interact with a solar cell in the following ways: they can beabsorbed, reflected or can go through the material without any interaction. These effectsdepend mainly on the reflection ratio of the surface and the energy of photons. From theviewpoint of energy conversion, the most important is the phenomenon of absorption. Inthe solar cell, the absorbed photon excites an electron from the valence band to the levelof the conduction band. Hence, the pair of charge carriers is formed: electrons in theconduction band and holes in the valence band. If the photogeneration occurred withinthe depletion region, the electric field extracts the electron towards the n-side of the

29

junction and the hole towards the p-side of the junction. The principle of p-n junctionphotogeneration undergone intensive development and has been significantly improvedresulting in complex architectures and devices i.e. p-i-n, tandem, etc.

The fundamental difference between the working principles of organic and inorganicsolar cells is the direct generation of free charge carriers in the inorganic solar cells.In organic materials the light absorption results primarily in creation of excitons witha typical binding energy of 0.3 - 0.5 eV rather than free electron-hole pairs. Sincethe necessary electric field to overcome this coulomb-interaction binding energy isunavailable in an organic solar cell, the electrically neutral excitons are usually separatedat the interface between two different organic layers (heterojunction) (Figure 2), wherethey dissociate into an electron in one phase and a hole in the other [74].

Exciton dissociation

Cathode Anode+-

Fig 2. Typical representation of the organic solar cell structure.

The energy alignment of these two blended materials has to be optimised, toefficiently separate excitons and avoid energy losses in this process. The example ofwell aligned energy levels is depicted in the figure below (Figure 3).

Cathode

(Al)

Anode

(ITO) Donor

(P3HT) Acceptor

(PCBM)

+

-

HOMO

LUMO

-4,7 eV

-5,1 eV

-6 eV

-4,3 eV

-4,2 eV

-3,2 eV

Fig 3. Potential energy levels representation for organic solar cells materials.

30

An important factor in solar cells considerations is the spectrum of the light thatapproaches Earth’s surface. Spectral distribution of solar radiation is close to blackbodyradiation at a temperature of about 5800 K. About half of the solar radiation is within therange of visible light and the remaining is divided to ultraviolet and infrared. Figure 4depicts the difference in the spectrum of solar light at the earth’s surface (AM-1.5G) andon top of Earth’s atmosphere (AM-0).

500 750 1000 1250 1500 1750 2000 2250 2500 2750 30000,00

0,25

0,50

0,75

1,00

1,25

1,50

1,75

2,00

2,25

Spec

tral I

rradi

ance

[Wm

-2nm

-1]

Wavelength [nm]

AM-0 AM-1.5G

Fig 4. Comparison of the solar spectrum reaching the Earth’s surface and at topof the Earth’s atmosphere.

The spectral difference is caused by the Earth’s atmosphere that attenuates the solarradiation mostly due to absorption and scattering processes. In addition, the location’slatitude, day and year time, and the weather conditions have the major impact on thetotal amount of solar irradiance reaching surface. Based on the Figure 4 the amount ofirradiance is different for different wavelengths. Therefore, the exact amount of energyfrom solar radiation approaching Earth depends of the energy of the photons at particularwavelengths. The energy of the photon can be calculated from the simple equation (1):

E =h · cλ

, (1)

where E is the energy of the photon, h is Planck’s constant (6,62 ·10−34m2 ·kg/s), c is thespeed of light in vacuum (299792 km/s) and λ is the wavelength of the photon. Althoughlight propagates slower in the Earth’s atmosphere than in vacuum, the difference is

31

negligible and c can be assumed to be constant. Therefore, the energy of the photondepends only on its wavelength and can be calculated from equation (2):

E(eV ) =1,24

λ (µm). (2)

Current solar cells materials due to their distinct optical properties characterised bytheir band gap energy, make possible to convert into electric energy only limited part ofthe solar spectrum. In general, only photons with energy higher than the band gap energyof the absorbing material generate electron-hole pairs. Nevertheless, new architecturesand configurations of solar cells try to overcome material band gap limitations.

2.3 Solar cells characterization methods

Although solar cells behave differently according to technologies used in manufacturing,the main goal of solar cells is to generate energy. Therefore in order to properly evaluatefabricated cells, aside dissimilarities, the unified characterisation methods are needed.There are several ways to characterize solar cells, however the most important are theelectrical, spectral, morphology and lifetime characterizations.

2.3.1 Electrical characterization

From the electrical point of view, the solar cell works like a battery. In the situationwhen the positive and negative terminal are disconnected, the open circuit voltage (VOC)is developed (Figure 5a). When the solar cell’s terminals are connected together, theflowing current is called short circuit current (ISC) (Figure 5b). For any intermediate loadresistance (RL) the solar cell develops a voltage (V) between 0 and VOC and delivers acurrent I according to equation (3) and (Figure 5c):

I =VRL

. (3)

In case of electrical performance the most important parameter is the maximumpower point (Pmax) which can be determined by applying forward bias voltage acrossthe illuminated device under test. The measurement results with the Current-Voltage(I-V) curve. The example of the I-V curve is depicted in the Figure 6. Also, Papers IIIand IV present I-V curves of the investigated solar cells.

32

Voc

(a) Open circuit voltage VOC

Isc(b) Short circuit current ISC

I

RL

V(c) Load resistance RL

Fig 5. Simplified representation of a solar cell and its main electrical parameters.

The applied potential is in the forward bias direction. The curve shows the turn-onand the buildup of the forward bias current in the diode. Without illumination (blueline), no current flows through the diode unless there is external potential applied. Underillumination (red line), the I-V curve shifts down and indicates that there is externalcurrent flow from the solar cell to a passive load.

In addition, the I-V curve makes possible to extract several parameters needed forconvenient evaluation of solar cell. For instance, based on the Pmax point, it is possibleto determine Vmax and Imax points, which can be used further to calculate the fill factor(FF) defined as a ratio (4):

FF =Imax ·Vmax

Isc ·Voc. (4)

The efficiency η of the solar cell is defined by the relation of the energy delivered inthe form of incident light and electric power extracted from the cell (5):

η =Imax ·Vmax

Ps, (5)

where Ps is the incident light power density. Consequently, implementation of the FFinto equation (5) results in the equation (6):

33

η =Isc ·Voc ·FF

Ps. (6)

Importantly, since the current is closely proportional to the illuminated area, insteadof using short circuit current (Isc), the short circuit current density Jsc over definedarea should be used [9]. For more complex determination of the solar cells electricalproperties like shunt resistance (Rsh) and series resistance (Rs), the circuit presented inthe Figure 7 is used. In addition to resistances, circuit contains a semiconductor diode,which represents the behaviour of not illuminated solar cell.

To keep the repeatability and fulfil the measurement standards all the measurementsmust be done in a very precisely defined conditions [75, 76]. Especially, the sourceof illumination plays an important role in the cell characterization. According to IEC60904-9 Edition 2 (2007), JIS C 8912 , and ASTM E 927-05 standards there arethree main factors defining the class of simulated solar illumination: Spectral Match,Non-Uniformity of Irradiance, and Temporal Instability of Irradiance. In addition,simulated illumination must be adjusted to specified atmospheric conditions, whichcould be defined as Air Mass (AM) spectra. In principle air mass is the measure of howfar light passes through the earth’s atmosphere. The difference between different air

Voc Voltage

Current

Isc

Pmax

Dark

Illuminated

Imax

Vmax

Fig 6. I-V curve of the solar cell in the light and the dark.

34

Rs

Rsh

Id

Isc

+

-

V

Fig 7. Equivalent circuit including shunt and series resistances.

masses is depicted in Figure 8. Moreover, two different spectra for AM-0 and AM-1.5Gconditions were depicted previously in Figure 4.

Zenith

AM-0

AM-1.0Atmosphere

48.2°

AM-1.5

60.1°

AM-2.0

Fig 8. Designation of solar air masses.

2.3.2 Solar cell spectral response characterization

To acquire data on how efficiently solar cell converts various wavelengths light intocurrent, the solar cell spectral response measurement system is utilized. The mentionedsystem makes possible to measure solar cell quantum efficiency, which is defined as theratio of the number of electrons in the external circuit produced by an incident photonof a given wavelength. Basic measurement systems are capable to measure ExternalQuantum Efficiency (EQE) only. More advanced systems equipped with additional

35

sensors are capable to measure both internal and external quantum efficiencies. Themajor difference between systems comes from the treatment of the photons reflectedfrom the solar cell. In advanced systems the light reflected from the cell is analysed. Bysubtracting the reflected light from known incident light to the cell the measurementsystem is able to calculate the Internal Quantum Efficiency (IQE) [77]. Finally, thequantum efficiency system delivers detailed information about device performance,material applicability, device design, etc. The example EQE and IQE curves for PTB7material are depicted in the Figure 9.

300 400 500 600 700 800 9000

10

20

30

40

50

60

70

Qua

ntum

Effi

cien

cy [%

]

Wavelength (nm)

EQE IQE

Fig 9. Exemplary EQE and IQE curve for PTB7 polymer solar cell.

2.3.3 Morphology characterization

Besides the electrical properties, the morphology of fabricated layers plays an importantrole in the solar cells performance and consequently in the evaluation process, especiallyat the development stage. Importantly, different solar cells and applications requiredifferent processing, material thicknesses, layers’ structures, etc. For instance, insilicon solar cells to increase the light absorption the top layers consist a nanostructuredantireflection layer which significantly increases the light trapping. Extremely sensitive tomorphology modifications are organic solar cells, which contain nanometre-thick layers.Also the surface characteristics of substrates used to fabricate solar cells might vary andtherefore need characterisation. Several surface characterization techniques have beeninvented and developed in order to perform accurate characterization accordingly to the

36

needs and thin-film properties, however, the most common are the surface profilometry,AFM and SEM. Due to technical limitations, mentioned methods are suitable forparticular applications and surfaces, hence, need to be chosen adequately. Four examplesof different characterization techniques used to characterize the ZnO Nanorods (ZnONR) layers are presented in the Figure 10. In this experiment, ZnO NRs were selectedfor their high application potential for optoelectronic devices and various methods ofproduction [78–81].

(a) Scanning electron microscopy image(JEOL JCM-5000)

(b) AFM 3-Dimensional data representa-tion (Veeco D3100)

(c) Optical profiler 3-D data representa-tion (Bruker Contour GT-K0)

(d) Stylus profilometer 3-D data representation(Bruker Dektak 8)

Fig 10. Different techniques of surface morphology characterisation of ZnOnanorods layer used in the solar cells industry.

37

2.3.4 Lifetime characterization

As aforementioned the morphology characterization is important during the developmentprocess and electrical characterisation is critical in evaluating the solar cell. Nonetheless,solar cells are devices that are exposed to the extreme conditions in terms of temperature,radiation, humidities, stress, etc. Within time these factors significantly contributeto the degradation of materials used to produce solar cells and consequently in theperformance of solar cells. Therefore additional lifetime measurements of the cells invarious conditions, provide valuable evidence needed for evaluation of the usability oftechnology in particular applications and conditions. For instance, for organic solar cells,the lifetime has undoubtedly improved over the last two decades from hours or days toeven years [82–85]. Nonetheless, the lifetime remains low, compared to silicon-basedsolar cells, which present astonishing lifetime values reaching over 25 years [86–88].

2.4 Solar cells manufacturing technologies

2.4.1 Conventional technologies

A solar cell fabrication process strongly depends on materials used. For instancesilicon-based monocrystalline and multicrystalline solar cells require high temperaturesand clean-room conditions during processing. The monocrystalline solar cells aremanufactured from thin silicon wafers made in the Czochralski method [89]. Theamorphous silicon solar cells are made in the Plasma-Enhanced Chemical VapourDeposition (PECVD) process, where a thin layer of amorphous silicon is formed onthe substrate. Application of certain conditions during the process makes possible tomanufacture nano-crystalline silicon layers or implementation of flexible substrates(Paper III) [90]. A small molecule solar cells processed in vacuum vapour physicaldeposition process is another example of fabrication of solar cells. Although this processrequires vacuum conditions, the temperature during the process remains below 100 Callowing deposition on the flexible substrates.

Recent achievements in materials science allowed the manufacturing of materialsthat can be deposited at various rigid and flexible substrates at low temperatures. Oneof the well know method of deposition fluid-phase materials is spin-coating, wheredroplet containing material is placed on the rotating substrate. The adjustment ofrotating speed and time, results in the repeatable fabrication of thin films of desired

38

thicknesses. This method is especially suitable for research and development of organicand organic/inorganic solutions used to manufacture solar cells. Unfortunately, duringspin-coating deposition most of the fluid is wasted and therefore, more efficient methodswere needed.

2.4.2 Printed solar cells

To overcome aforesaid difficulties related to manufacturing, a new branch of electronics,so called printed electronics, has emerged. Printed electronics include all methods ofdeposition material-containing inks on a various substrates. This revolutionary conceptallows implementation of new ideas that have been unavailable for other technologies. Ingeneral printed electronics refer to technology that allows deposition or material that canbe processable from the liquid phase. Recent developments prove that printed electronicsmight be an answer for growing demand for cheap and flexible solar cells [91–93]. Moredetailed consideration of printing of solar cells is presented in the following chapter.

39

40

3 Printing process

Since Gutenberg, printing has been used for presenting information on paper. But at theend of the 20th century, researchers took an interest in utilizing printing techniquesto the fabrication of functional electronic devices [34, 94–96]. A major differencebetween traditional printing and printing of electronic devices involves the materialsand substrates used. While the requirements that traditional printing imposes on inksmostly concern attributes related to sight perception, printed electronics demandselectrical functionality from inks, such as resistance, conductivity, semi-conductivity,light emission or absorption. Printed electronics is also referred to as organic or flexibleelectronics. Of these terms, the first one originates from the utilization of organic-basedmaterials, i.e., conductive polymers, during processing. The second term - flexibleelectronics - originates from the use of flexible substrates in fabrication, includingplastic foil or thin steal sheets.

3.1 Printing techniques

Since the fifteenth century, printing technology has been continuously developed, basedon the achievements of engineers, physicists and chemists, and consists nowadays of avariety of techniques that can be applied to achieve desired goals. Modern methodsof printing can be divided into two main groups: conventional and digital printing(non-impact printing). The first group, comprising techniques that require a printingplate or a printing master, includes screen, offset, gravure and flexographic printing. Thesecond group includes techniques that work without a prefabricated master printingplate and do not apply a significant force on the substrate beneath. A detailed divisionof printing methods is presented in Figure 11 [97]. Although the description of mainprinting methods refers to printing information on paper and plastics, the techniquesused in printed electronics utilize the same printing principles.

41

Printing Technologies

Powder toner Liquid tonerMagnetic

tonerLiquid ink Hot-melt ink

Color donor

(ribbon/foil)

Color sensitive

coatingInk (liquid)

Sheet or Web

(substrate)

Prepress Press Postpress

Sublimation Transfer

ContinuousDrop on

DemandOffset

Waterless

offset

Screen Printing Flexography Litography Gravure Electrophotography Ionography Magnetography Ink-Jet Thermography Photography

Digital Printing

(masterless)

Conventional Printing

(with master)

Fig 11. Detailed presentation of printing methods.

3.1.1 Conventional printing

Conventional printing technologies are also referred to as printing technologies withprinting master, which is the information carrying medium. For the purpose of this work,only the four main manufacturing methods used in printed electronics will be described:screen, flexography, lithography (offset) and gravure printing.

Screen printing

Screen printing is a technique in which ink or paste is forced through a screen made ofmetal, plastic or synthetic fabric. During printing, ink is transferred through openings inthe screen that are not covered by a stencil, i.e., the non-ink permeable part of the screen(stencil). A blade moves across the screen, spreading and forcing the ink through theopenings. One popular screen printing method is rotary printing, where the screen iscylindrical in shape and the blade is fixed in the middle of it. Figure 12 depicts theprinting principle. The precision and resolution of the print is highly dependent on thedensity of the screen and is usually significantly lower than in offset printing. Screenprinting can be used to print on a variety of substrates, including paper, plastic, glass,metal and fabric.

42

Screen with stencil Blade

Stencil covers

the screen

Residual

ink

Ink preamble

area

Substrate

(e.g., paper)

Ink film

Printing master

(screen)

Frame

Base plate

(stationary)

Fig 12. Screen printing method.

Flexographic printing

The most characteristic feature of flexographic printing is the presence of flexibleprinting plates attached to a rotating plate cylinder. These printing plates are mostlymade of polymer or, less often, of rubber. During printing, the application of rollersallows ink-coating of printing elements of the same height and a consequent transfer ofink to the substrate. By implementing flexible printing plates, this technique can besuccessfully utilized to print on various substrates: paper sacks, plastic bags, milk andbeverage cartons, folding cartons, disposable cups and containers, adhesive tapes, labels,envelopes, newspapers and wrappers. Figure 13 illustrates a flexographic printing unit.

43

Plate cylinder

Printing plate

(soft) Impression

cylinder (hard)

Printing substrate

Anilox roller

Ink supply

(chambered doctor

blade system)

Cells of the anilox

roller filled with inkInked up image

element

Elastic printing plate

with raised image

elements

Fig 13. Flexographic printing method.

Lithography/Offset printing

Offset printing is a lithographic method, used widely in producing large volume printouts,such as newspapers. Being a lithographic printing method, offset printing is sometimesreferred to as offset lithography. The most remarkable feature of offset printing isthat the plate cylinder is not in physical contact with the printed medium. The platecylinder consists of lithographically manufactured printing areas that are ink-acceptingand non-printing areas that are ink-repellent. A balance between hydrophilic andhydrophobic inks and wetting fluids is essential for the printing process and, therefore,control of interfacial surface phenomena is critical. A schematic of this printing processis depicted in Figure 14.

44

Inking unit

Plate cylinder

Dampening unit

Blanket cylinder

Impression cylinder with substrate

Inkig

Printing plate

Ink-accepting area

Ink-repellent area

Resplit inkDampening

Fig 14. Lithography/Offset printing method.

Gravure printing

Gravure printing is a technique, in which the components of an image are engraved intothe surface of a cylinder made of copper, with a thin layer of chromium. Figure 15presents the main elements of a gravure system, with the image-containing rotatingcylinder immersed in ink. During rotation, ink covers the cylinder. A doctor blade thenremoves all excessive ink, leaving only the gravured cells filled. Both the thicknessand colour intensity of the printed layer can be controlled by adjusting the deepness ofgravured structures. Deeper cells produce thicker layers and more intensive colours.Finally, the ink is transferred from the cells to the substrate by applying immersioncylinder pressure and utilizing the adhesive forces between the ink and substrate. Typicalgravure printed products include food packaging, wrapping paper, wall papers, furniturelaminates, panelling and magazines. Also, the gravure printing techniques can besuccessfully utilized to manufacture solar cells (Paper IV).

45

Gravure

cylinder

Ink fountain

Impression

cylinder/roller

Blade

Image elements are equally

spaced but differ in area and

volume (variable depth and

variable area)

Fig 15. Gravure printing method.

More detailed printing conditions for conventional printing methods are shown inTable 1 [97, 98].

3.1.2 Digital printing

Unlike conventional printing, digital printing produces printouts without a pre-manufa-ctured physical master plate and without imparting a substantial impact force on thesubstrate. The main feature of digital printing is accurate positioning of the printedmedium on the substrate. The printing system consists of a precise printing areanavigation subsystem accompanied by information on whether a particular positionshould be filled/covered with the printing medium. Digital printing does not require pre-processing steps, allowing printing in a very small volume. Thus, it offers an economicalway of depositing materials. The two most common digital printing techniques areelectrophotography and ink-jet printing.

46

Table 1. Characteristics of selected printing methods.

Printing Method Printing Pattern Layer Thickness[µm]

Printing Resolution[µm]

Ink DynamicViscosity [Pa·s]

Offset Printing Defined bydifference in

wetting of a planesurface

0.5–1.5 <20 40–100

Gravure Printing Defined by surfacerelief (recesses) of

master

0.8–8 <20 0.05–0.2

FlexographicPrinting

Defined by surfacerelief (raised

features) of master

0.8–2.5 ∼20 0.05–0.5

Screen Printing Defined byopenings in printing

master

>10 >100 Broad variety0.02–100

Inkjet Printing Master-less,droplet size

determined bynozzle diameterand waveform

0.1 5 0.01–0.03

Electrophotography

Electrophotography represents a ‘direct to plate’ printing method, where informationis first transferred onto a printing plate and then onto a substrate. The most commonexample of electrophotography is the laser printer, in which a photoconductive drum,charged with a laser light source, is used to transfer a toner to the substrate. Since adifferent image can be recorded on the photoconductive drum at each rotation, thismethod makes it possible to generate an entirely new print image with each rotation. Aschematic of the electrophotographic printing process is depicted in Figure 16.

47

Delivery FeederSubstrate

--

--

-

---

-

-

-

-

-

-

-

-

-

-

-

-

--

--

-

-

-

--- - -

++

+

+++

++

++

+

++

+

++++++

+

++

+++

-

--

-

-

-

+

++

++

++

++ +

+ + + + +

+

+

Toner fixing/fusing

(heat, pressure)

Corona (+)

Toner transfer

Electrostatic forces

hold the toner

Inking

(developing unit)

Latent,

electrostatic image

Photoconductor

Light

(Laser, LED)

Corona

discharge

Light

discharge

Brush and

suction

Cleaning

Imaging

Fig 16. Electrophotographic printing method.

Ink-jet

Ink-jet is essentially a ‘direct to substrate’ printing method, where information istransferred directly onto a substrate. During ink-jet printing, ink drops measuring 1 - 10pl are directly ejected from a nozzle to the substrate surface. Ink-jet printing methods fallinto two categories: Drop on demand (DoD) and continuous ink. In the former method,a drop is only generated when necessary. This contrasts with the continuous ink method,which generates a constant stream of droplets. Charged droplets are deflected, whileuncharged drops fall on the substrate. DoD ink-jet printing can also be classified by theprocess that causes drop ejection. This ejection process can be induced by thermal, piezoor electrostatic processes occurring in a nozzle. Ink-jet printers with a piezoelectricnozzle are the most common type, and in this setup, an imaging signal causes thepiezoceramic element to bend, thereby increasing ink pressure, which leads to theejection of a drop (Figure 17b). A thermal ink-jet nozzle, in turn, consists of a thermalelement that heats up and forms a bubble that pushes out an ink drop (Figure 17c). Inan electrostatic system, the drop of ink is forced out of the nozzle by an electrostaticfield applied between the nozzle and the substrate or a conductive material beneath(Figure 17d).

48

SubstrateGutter

Deflector

Imagining signal

Charge

electrode

Piezoelectric

crystal

Pump

Nozzle

(Ø approx. 12 µm)

(a) Continuous

Imagining signal

Substrate

Piezoelectric

ceramics

Nozzle

Ink

(b) DoD - Piezoelectric

Imagining signal

Substrate

Nozzle

Ink

Bubble

Heat source

(c) DoD - Thermal

Imagining signal

Electrical

field

Meniscus

Ink

Switching element

E

Substrate

(d) DoD - Electrostatic

Fig 17. Ink-Jet printing technologies.

3.2 Interface interactions

Though printing quality depends on several factors, wettability is the main attribute thataffects the printing process. As a result, the most critical aspect of printing involves theinterface formed between the substrate and the drop, and a deep understanding of dropbehaviour at the substrate surface is crucial. Moreover, different printing techniquesrequire different degrees of wettability. Among the numerous aspects that influencewettability, the most important are the properties of the ink and the substrate.

3.2.1 Inks

In principle, inks comprise several different constituents, which can be divided into fourgroups: solvents, binders, colourants and additives. Solvents are used to dissolve thecomponents of the ink and to reduce viscosity. Binders are used to provide structure andto keep the dye or polymer in place once it is printed. Colorants, usually a pigment or a

49

dye, are used to provide a colour to the printed surface. Finally, additives complete therequired functionality of the mixture, according to current needs.

Properties of the layer of ink on the substrate depend on two physical features:rheology and surface tension. An appropriate variation of these factors during the inkformulation process makes it possible to produce inks for a range of applications.

Rheology relates to the deformation and flow of matter - primarily fluids [99]. Inconcept, rheology is similar to viscosity, but takes into account a much broader range offorces, temperatures, rates and deformation processes [100]. Surface tension, in turn, isdefined by the energy associated with intermolecular forces at the interface between twomedia - for liquids, surface tension equals surface energy per unit area. For instance, adroplet of grain alcohol, which has a lower surface energy than water, will spread andcover a larger area of the substrate than a droplet of water. A more detailed explanationof surface tension and surface energy will be presented in the measurement methodssection. By manipulating ink composition, we can control the behaviour of ink drops onthe substrate surface. Researchers are currently investing a lot of effort in micro- andnano-materials, which, when added to inks, greatly influence their wetting properties[101, 102]. As seen in Figure 18, the properties of the used ink have a great effect onboth the wet and dry properties of manufactured films.

Rheology

Surface Energy

Application Print Method

Ink

Composition

”Wet”

properties

”Dry”

properties

Viscosity

Printability

Substrate

Structure

Adhesion/Cohesion

Surface Properties

Function

Fig 18. Factors affecting ink development.

50

3.2.2 Substrates

Another major factor affecting the printing process is the substrate beneath the printedlayer. One factor that has a significant effect on the wetting process is the surface energyof the substrate. For instance, a droplet of water will spread out across a high energysubstrate, such as a metal or a ceramic, whereas a polyethylene film, which has a lowersurface energy, reduces the contact surface (increases the contact angle) between thedrop and the substrate, resulting in poor wetting. Figure 19 presents an example of thewetting behaviour of substrates with different surface energy values using the sameliquid.

Poor wetting(substrate surface energy low)

Moderate wettingGood wetting

(substrate surface energy high)

Fig 19. Wetting behaviour of various substrates.

Although surface energy is an important factor, most substrates are not perfectlysmooth and, therefore, additional considerations need to be taken into account. Variationsin surface uniformity, rigidity or chemical homogeneity, or in pre/post processing, resultin significantly different behaviour of the fluid, due to different phenomena governingwetting. For instance, implementation of micro- or nano-structures results in uniquewetting properties that differ greatly from those that we get by utilizing the same materialin bulk form [103]. Two models have been proposed to explain wettability on texturedsurfaces: Wenzel and Cassie-Baxter [104–106].

There is an ongoing debate centring on the question, whether these models aresufficient to describe nanostructured surface phenomena precisely enough [107, 108].Some researchers are in fact putting forward more precise and significantly morecomplex methods [109, 110]. Nonetheless, the wettability of nanostructured surfacesremains a very complicated issue, owing to the large amount of factors that affect it(such as size, shape, materials and conditions).

51

3.2.3 Surface tension and surface energy

From the energy point of view, a molecule in contact with a neighbouring molecule is ina lower state of energy than a molecule on its own. Interior molecules within a dropletare completely surrounded by other molecules of the same kind, whereas boundarymolecules miss some of their neighbours and have a consequently higher energy rate. Tominimize its energy state, a liquid must minimize the number of high energy boundarymolecules. This, in turn, results in a minimized surface area [111].

As all systems tend towards a state of lowest energy, they strive to reduce theirsurface area to minimize surface energy. The shape with the smallest surface area for agiven volume is the sphere, which explains why drops of fluid tend to be spherical inshape [112]. Known as surface tension, the force that tries to minimize the surface areaof a liquid is caused by attractions of particles within the surface layer. Surface tensionand surface energy are used interchangeably and are expressed as force per unit length(N/m) and force times distance per unit area (N ·m/m2 = J/m2), respectively.

Water, for example, has a surface tension of 72.7 mJ/m2 at 20 C, while the organicsolvents used in printed electronics, such as benzene and alcohols, have a lower surfacetension. An increase in liquid temperature reduces the net force of attraction amongits molecules and, therefore, decreases its surface tension. Surface energy arises fromvarious independent phenomena and interactions, such as dispersive (van der Waals)forces and hydrogen bonding [113, 114]. Van der Waals forces are a general term usedto define the attraction of intermolecular forces between molecules and can be dividedinto two types of force: London Dispersion Forces and dipole - dipole forces [115].Hydrogen bonding is a type of dipole - dipole attraction, which occurs when a hydrogenatom is strongly bonded to an electronegative atom, i.e., nitrogen, oxygen or fluorine[116, 117].

3.2.4 Measurement methods

In 1805, Young determined the angle formed between a liquid and a solid in equilibrium(3-phase contact line), and the surface energy difference between these two [118].Figure 20 depicts Young’s findings, while Formula (7) enables estimating the surfaceenergy of an unknown surface by monitoring the contact angles of different liquids withknown surface tension. Even today, the contact angle measurement method is the mostcommon one used to determine the interfacial tension between a solid and a liquid [119].

52

The contact angle measurement method can be utilized to characterize varioussurfaces, starting from materials used in everyday life applications and printed electronicsand ending at membranes, which are mostly used to separate two fluids with differentchemical properties or different particle compositions (Paper V). This method can alsobe successfully applied to measuring the wetting properties of nanocellulose-basedmaterials, which are considered highly promising for such applications as light-weightpackaging, coatings and films (Paper VI, VII).

γLV

ϴ

γSL

γSV

Fig 20. Graphical representation of force balance as proposed by Young.

The most common goniometric contact angle measurement method comes in twovarieties: static and dynamic. The static method involves measurements, where a drop isplaced on a horizontal substrate and has a constant volume during the measurement.The dynamic method can be separated into two sub-methods. Of these, the first onecomprises a tilted substrate, along which a drop moves slowly, forming a series ofadvancing and receding contact angles. The second method involves measurements,where the drop is enlarged by adding a fluid and then reduced by drawing it out. Boththese dynamic methods produce advancing and receding contact angles, forming aso-called contact angle hysteresis, which can be attributed to interface properties [120].

γSV − γSL = γLV · cosΘ, (7)

where theta Θ is the contact angle, γSV the interfacial tension between the solid andthe vapour, γLV the interfacial tension between the liquid and the vapour and γSL theinterfacial tension between the solid and the liquid.

Several calculation models have been developed to estimate surface energy from thecontact angles of fluids. Among the most common are Zisman (recently disregarded dueto low accuracy), Equation of the State, Fowkes, Owens-Wendt-Rabel-Kaelble, Wu andvan Oss [44, 121–126].

This thesis discusses the Owens-Wendt-Rabel-Kaelble method in more detail. Intheir theory, the surface energy of a solid consists of two components, a dispersive and apolar component:

53

γLV = γPLV + γ

DLV , (8)

γSV = γPSV + γ

DSV , (9)

where γLV is the interfacial tension between the liquid and the vapour, γPLV the polar

component of the interfacial tension between the liquid and the vapour, γDLV the disperse

component of the interfacial tension between the liquid and the vapour, γSV the interfacialtension between the solid and the vapour, γP

SV the polar component of the interfacialtension between the solid and the vapour, and γD

SV the disperse component of theinterfacial tension between the solid and the vapour.

Based on this assumption, Owens and Wendt developed a model for describingsurface interactions with two parameters. The authors combined Good’s equation (10)with Young’s equation (11) [127]:

γSL = γSV + γLV −2√

γDLV · γD

SV −2√

γPLV · γP

SV , (10)

γSV − γSL = γLV · cosΘ, (11)

where γSL is the interfacial tension between the solid and the liquid, and theta Θ is thecontact angle. By combining these two equations, Owens and Wendt got:

γLV (cosΘ+1)

2√

γDLV

=√

γPSV ·

√γP

LV√γD

LV

+√

γDSV . (12)

Equation (12) has a linear form y = mx+b, where:

y =γLV (cosΘ+1)

2√

γDLV

, (13)

m =√

γPSV , (14)

x =

√γP

LV√γD

LV

, (15)

b =√

γDSV . (16)

54

By using two liquids of known polar and dispersive components, Rabel was able tocalculate the dispersive and polar components and, consequently, the surface energy ofthe solid. In order to calculate the surface energy values, Rabel used a single linearregression from the contact angle values of various liquids with known dispersive andpolar components Figure 21.

= γ

γ

=

γ

·

(1+

ϴ

)

2γ

= γ

= γ

Fig 21. Owen-Wendt-Rabel-Kaelble theory, used to determine the polar and dis-perse fraction of the surface energy of a solid.

3.3 Printed electronics

In printed electronics, traditional deposition processes, such as sputtering or thermaldeposition, are replaced by a solution or suspension of material, which is printedto achieve properties similar to traditionally deposited films. Inks used to presentinformation in the traditional form are replaced by functional inks that provide electricalor electronic capabilities. Main differences between traditional electronics manufacturingand printed electronics are described in Table 2 [22]. Use of appropriate materials allowsthe printing of electronic devices, starting from basic passive components like resistorsand ending up at complex structures, such as organic light emitting transistors [128].

55

Table 2. Comparison of conventional and printed electronic processes

Solid-State (Conventional) Printed ElectronicsProcess Batch ContinuousProduction Speed Slow Potentially FastCapital Cost Extremely High Low to ModerateMaterials Well Defined DevelopmentalCost Moderate in High Volume Low to ModerateSubstrate Rigid Silicon Various, FlexibleEnvironmental Acceptable FriendlyEconomic Run Length Large Small to Very Large

Two factors are crucial for printed electronics: inks and substrates. Inks fall into threemain groups: conductive inks, dielectric inks and active inks.

– Conductive inks usually consist of silver, gold, copper, alloys and carbon-basedmaterials [129–132]. It is important to formulate the ink such that the conductiveparticles bind together after printing, creating a continuous conductive path. Inmost cases, increasing the thickness of the printed layers increases conductivity.However, thicker layers are less transparent, which is an undesired side-effect in someapplications.

– Dielectric inks, containing a dielectric material that prohibits the flow of electriccurrent within the structure, are mostly used in capacitor and transistor devices[133, 134]. Current dielectric inks are largely based on polymers, although newapproaches, such as ion gels, are gaining ground [135, 136].

– Active inks have the capability to perform a particular function after printing,constituting the active material in solar cells, light emitting devices, transistorsand sensors, for instance. Inks used to print the active layer in a solar cell shoulddemonstrate a high light absorption coefficient, a low level of degradation againstenvironmental conditions and facile printing on various substrates [91].

Substrates used in printed electronics can be divided into two groups: rigid andflexible. Although rigid substrates based on silicon and silicon oxides are very com-mon, this thesis concentrates on flexible substrates, which have a significantly widerapplication range. Different applications based on printed electronics require differ-ent substrate properties, including optical properties, thermal properties, solvent andchemical resistance, surface quality and mechanical properties.

56

Optical properties are especially important for such devices as light emitting andlight collecting devices, i.e., LEDs and solar cells. Therefore, the selected substratesneed to be as transparent as possible in the required light spectrum. Thermal properties,on the other hand, are important from the processing point of view, where dimensionaland thermal stability are critical. Figure 22 presents the upper operating temperature ofsubstrates used in printed electronics [137, 138].

Polyeth

ylene

terep

hthala

te

Polyeth

ylene

naph

thalat

e

Polyca

rbona

te

Polyeth

ersulf

one

Polyary

late

Polyam

ide0

50

100

150

200

250

300

350

400

Upp

er p

roce

ssin

g te

mpe

ratu

re [

C]

Fig 22. Upper processing temperatures for flexible substrates used in printedelectronics.

Solution processing in printed electronics requires that the substrate must presenthigh solvent resistance. Importantly, compared to amorphous substrates: Polycarbonate(PC) and Polyethersulfone (PES), semi-crystalline polymers: Polyethylene Terephthalate(PET) and Polyethylene Naphthalate (PEN) demonstrate this ability. Another keyproperty is surface smoothness, which is essential to guarantee the integrity of theprinted layers. Applications with a very thin layer require a surface smoothness of lessthan 1 nm, which is provided by PET and PEN-based substrates. Mechanical properties,i.e., substrate stiffness, need be considered in designing the printed device. The designprocess must also consider the conditions in which the device will be used. All theseproperties are strongly connected with both substrate thickness and substrate materials.

57

For example, amorphous substrates are less stiff than PET or PEN films of the samethickness. Importantly, these factors create a complex matrix of properties that influenceeach other and, therefore, need to be adjusted according to the requirements of theapplication at hand [139].

In addition to polymer-based substrates, applications in which an opaque substrate isacceptable often use stainless-steel foils. Although opaque and heavier than polymer-based substrates, foils are can withstand much higher processing temperatures and theydemonstrate exceptional dimensional stability.

58

4 Printing process manipulation

Several techniques can be used to control wettability, and this research concentrates onthree factors that are easily applicable to printed electronics: argon and oxygen plasmapre-treatments, UV treatments and processing temperature. A full explanation of theexperiments conducted during this study, together with the results, will be presented inthe following subsections.

4.1 Effect of oxygen, argon plasma and UV pre-treatmenton the wettability of ZnO-nanostructured thin-films

Intensive research effort has been directed toward the study of photoactive materials withthe aim of increasing their power output to better exploit their comparative advantages[140]. One popular candidate is ZnO, due to its physical and chemical properties, whichare suited to thin-film device applications. Its functionality in terms of improving solarcell performance by increasing the optical path has also been demonstrated, and ithas been used for such applications as photodiodes, thin-film transistors and sensors[102, 141–143]. However, the interface between ZnO nanostructured substrates andink solutions needs more investigation, being of utmost importance for achieving highquality thin-films [32, 53]. So far, UV, oxygen and argon plasma treatments have beenshown to improve the hydrophilicity of treated surfaces and, thus, printing properties[40–42].

This thesis analyses the wettability properties of ZnO NRs treated with UV illumina-tion, oxygen and argon plasma for varying periods of time. It employs the sessile dropcontact angle measurement method to characterize the ink-substrate interface. Althoughrecent research has illustrated the usability of pre-treatment methods for water solutions,a more in-depth investigation is necessary to understand the behaviour of solvents,particularly organic solvents on different substrates [43, 144–147]. Since this work con-centrates on solar cell applications, four of the most common solutions used in organicsolar cell manufacture were tested here: P3HT:PCBM dissolved in 1,2-dichlorobenzene(DCB), P3HT:PCBM dissolved in chlorobenzene (CHB), PEDOT:PSS and DI-water. Tocharacterize the morphology of ZnO NR layers, AFM and SEM devices were used.

59

ZnO NRs were electrochemically deposited (ECD) from Zinc Nitrate (Zn(NO3)2)solutions on Indium Tin Oxide (ITO) -covered glass substrates (resistivity of 20 Ω/).These ZnO NRs were grown using a galvanostatic deposition method adapted fromSeipel [148]. Deposition was set for a period of about 300 seconds, with an appliedcurrent density of 450 µA·cm−2 (cathodic current). Growth parameters were computercontrolled and monitored using a PAR 270 potentiostat/galvanostat. In all experiments,50 mM of aqueous zinc nitrate (Sigma-Aldrich, 99.999 %) was used as a precursor.The ZnO film was grown in an undivided three-electrode cell arrangement, with apre-cleaned ITO as the working electrode, 1 cm2 platinum foil as the counter and aStandard Calomel Electrode (SCE) as the reference electrode. During synthesis, thesolution, with a measured pH of 5.3, was maintained at 80C. Importantly, ZnO growthwas performed by Dr. Yuji Suzuki and supervised by Professor KathirgamanathanPoopathy at Brunel University, London. The surface morphology of the ZnO film wascharacterised by a Carl Zeiss Evo MA10 SEM and a Veeco Dimension 3100 AFM.

Sample surfaces were treated by UV, Argon (Ar) plasma, oxygen (O2) plasma, and amixture of UV and Ar plasma. Three ZnO NR-covered ITO samples were preparedfor each method and treated for 1, 2 or 3 minutes. The corresponding UV wavelengthwas 254 nm (Jetlight UVO cleaner 42-220), and the O2 and Ar plasma were generatedat Plasma-Preen II 862 (frequency 2.4 GHz and power 60 W). Lastly, the UV+Arplasma treatment was carried out in two steps: first, the samples were treated with UVillumination for 2 minutes and then with argon plasma for 1, 2 and 3 minutes. A samplecovered with untreated ZnO NR was used as reference.

A Kruss DSA100 drop shape analyser was used to measure and extract the surfacecontact angles of the different samples. Four solutions were prepared for this test:DI-water, PEDOT:PSS, P3HT:PCBM CHB and P3HT:PCBM DCB at 30 mg/ml wt. Allmaterials were supplied by Sigma-Aldrich. In these measurements, performed at roomtemperature, the surface contact angles of droplets at two different locations on the samesample were averaged out and standard deviations were calculated.

Morphology characterizations by AFM and SEM are presented in Figure 23. Theused ZnO nanorods were typically 150 nm in height and 100 nm in diameter. Figure 24presents a water droplet on a treated and untreated ZnO NR surface.

60

(a) AFM image of ZnO NR depositedat ITO substrate

(b) SEM image of ZnO NR deposited at ITO substrate

Fig 23. Morphology characterization of ZnO NRs deposited with the ECD methodat an ITO substrate.

(a) Water droplet at untreated ZnO NR substrate -large contact angle

(b) Water droplet at ZnO NR substrate treatedwith argon plasma for 3 minutes - small contactangle

Fig 24. Example of the influence of surface treatment on the contact angle.

Figure 25 and Figure 26 show the measured contact angles for each treatment, alongwith the corresponding treatment times for all solutions. As can be seen, the largestcontact angle had the reference (untreated) sample.

The difference between treated samples and the reference is particularly noticeablein water-based solutions, i.e., DI-water and PEDOT:PSS, where the contact angle was110 for untreated samples and significantly lower for treated samples.

61

Referen

ce

Plazma -

Argo

n 1 m

in

Plazma -

Argo

n 2 m

in

Plazma -

Argo

n 3 m

in

Plazma -

Oxy

gen 1

min

Plazma -

Oxy

gen 2

min

Plazma -

Oxy

gen 3

min

UV - 1 m

in

UV - 2 m

in

UV - 3 m

in

UV + Plaz

ma Argo

n 1 m

in

UV + Plaz

ma Argo

n 2 m

in

UV + Plaz

ma Argo

n 3 m

in0

20

40

60

80

100

120

Con

tact

ang

le fo

r DI W

ater

[]

Treatment method

(a) DI-water contact angle results

Referen

ce

Plazma -

Argo

n 1 m

in

Plazma -

Argo

n 2 m

in

Plazma -

Argo

n 3 m

in

Plazma -

Oxy

gen 1

min

Plazma -

Oxy

gen 2

min

Plazma -

Oxy

gen 3

min

UV - 1 m

in

UV - 2 m

in

UV - 3 m

in

UV + Plaz

ma Argo

n 1 m

in

UV + Plaz

ma Argo

n 2 m

in

UV + Plaz

ma Argo

n 3 m

in0

20

40

60

80

100

120

Con

tact

ang

le fo

r PE

DO

T:P

SS

[]

Treatment method

(b) PEDOT:PSS contact angle measurements results

Fig 25. Contact angle measurements of water-based solvents for different ZnO NRtreatments.

62

Referen

ce

Plazma -

Argo

n 1 m

in

Plazma -

Argo

n 2 m

in

Plazma -

Argo

n 3 m

in

Plazma -

Oxy

gen 1

min

Plazma -

Oxy

gen 2

min

Plazma -

Oxy

gen 3

min

UV - 1 m

in

UV - 2 m

in

UV - 3 m

in

UV + Plaz

ma Argo

n 1 m

in

UV + Plaz

ma Argo

n 2 m

in

UV + Plaz

ma Argo

n 3 m

in0

2

4

6

8

10

12

14

16

18

20

22

24

Con

tact

ang

le fo

r Chl

orob

enze

ne [

]

Treatment method

(a) Contact angle for P3HT:PCBM dissolved in chlorobenzene

Referen

ce

Plazma -

Argo

n 1 m

in

Plazma -

Argo

n 2 m

in

Plazma -

Argo

n 3 m

in

Plazma -

Oxy

gen 1

min

Plazma -

Oxy

gen 2

min

Plazma -

Oxy

gen 3

min

UV - 1 m

in

UV - 2 m

in

UV - 3 m

in

UV + Plaz

ma Argo

n 1 m

in

UV + Plaz

ma Argo

n 2 m

in

UV + Plaz

ma Argo

n 3 m

in0

2

4

6

8

10

12

14

16

18

20

Con

tact

ang

le fo

r 1,2

-Dic

hlor

oben

zene

[]

Treatment method

(b) Contact angle for P3HT:PCBM dissolved in 1,2-dichlorobenzene

Fig 26. Contact angle measurements for P3HT:PCBM materials dissolved in or-ganic solvents for different ZnO NR treatments.

63

-0,2 0,0 0,2 0,4 0,6 0,8-10

-5

0

5

10

Cur

rent

Den

sity

[mA/

cm2 ]

Voltage [V]

UV treatment Argon treatment O2 treatment

Fig 27. J-V curves of solar cells for differently treated ITO substrates.

All treatments reduced the contact angle to around 20 for DI-water and 30 forPEDOT:PSS. In addition, there was a visible time-dependable trend: a longer treatmentinvariably resulted in a lower contact angle value. Importantly, UV treatment provedmore time-dependant than plasma treatment and had less influence on the contact angle.Results from the UV+Plasma treatments do not differ from those achieved with plasmatreatment only. DCB and CHB based inks interact differently with ZnO NRs than withwater solutions, forming drops with a low contact angle of approximately 17, evenwithout treatment. Although treatment reduces the contact angle by approximately 5

and is time-dependable, the change is not as substantial as in water-based solutions.Pre-treated samples of water-based solutions had significantly reduced contact

angles. This implies that their surface properties had undergone modifications, makingthem more hydrophilic. Some of the underlying mechanisms have been demonstrated byprevious studies. For UV irradiation, for example, electron-hole pairs are generatedwithin the ZnO surface, and some of the holes react with the lattice oxygen to formsurface oxygen vacancies. Meanwhile, water and oxygen may compete to dissociativelyadsorb on them. As defective sites are kinetically more favourable for hydroxyladsorption than oxygen adsorption, surface hydrophilicity is improved [149, 150].In plasma treatments, ZnO NR films are etched anisotropically, which generatesrough, more hydrophilic, surfaces [43, 151]. The impact of treatment duration is moreprominent in UV that in plasma treatment. However, even after 3 minutes, the actualeffect on the contact angle is lower in UV than in plasma treatment, probably due to

64

differences in the energy level delivered during treatment and the subsequent surfacealteration. A comparison between the used gases - argon and oxygen - for plasmatreatment produced no significant difference in the measured contact angle, making gasselection a non-critical issue for the plasma process.

Differences in the measured contact angles observed with DCB and CHB wereless significant than those for water. Since these changes are mostly affected by thesurface tension of organic solvents, which is lower than that of water-based solutions,pre-treatment has less effect on contact angle. Additionally, the results for DCB andCHB are almost identical, because their surface tension values are similar, approximately33 mN·m−1. Finally, Figure 27 presents the J-V curves of solar cells with differentlytreated ITO substrates. As seen, UV treated ITO-solar cells have a higher performancethan cells treated with argon and oxygen plasma.

4.2 Influence of temperature on wetting properties inOSC

Many groups have researched the influence of temperature on the wettability of theliquids and solids that are commonly used in industry [44–52, 152, 153]. However,certain applications, including solar cells, require a separate approach and a differenttype of investigation, mostly due to the implementation of polymers and novel materialswith a significant effect on wettability [53, 154, 155]. Therefore, this part of theresearch focuses on measuring the influence of temperature on interactions betweenthe inks and substrates used in the organic solar cell industry. From the engineeringviewpoint, it is rather easy to implement an organic solar cell fabrication systemwith temperature-variation capability. As this work concentrates on organic solarcell applications, it studies four of the most common solutions (inks) used in organicsolar cell manufacture: P3HT:PCBM dissolved in 1,2-dichlorobenzene, P3HT:PCBMdissolved in chlorobenzene, PEDOT:PSS 4083 and DI-water, to provide a basis forcomparison [156]. Furthermore, five different materials (ITO, O2 plasma-treated ITO,PEDOT:PSS thin film, P3HT:PCBM thin-film and ZnO NR) are used as substrates. Tomeasure the ink-substrate interface, this research employs the sessile-drop contact-anglemethod. In order to guarantee the compatibility of the materials and flexible substrates,temperature was varied from 0 to 120 C. It is worth noting that, in this research, thecontact angle thermal coefficient defines the degree of contact angle change () per one

65

degree Celsius (C). Finally, to analyse the influence of substrate temperature on deviceperformance, organic solar cells were manufactured at various temperatures and theirperformance was examined.

All contact angle measurements were performed immediately after the formation ofdrops at the substrate, using a Kruss DSA100 system under nitrogen atmosphere. Thissystem was equipped with a high speed camera (360 fps), analysis software and anenvironmental chamber that allows precise heating and cooling of substrates. During theexperiments, substrate temperature was varied from 0 to 120 C with 10 C intervals and10 min delays for temperature stabilization. To avoid gravitational flattening, drop sizewas limited to 1.5 µl. To enhance the usability of achieved results, conditions duringcontact angle testing were identical to those during solar-cell fabrication, includinginert gas atmosphere, ink processing steps, layer thicknesses and type of the layerbeneath the investigated layer [157]. Surface contact angles of droplets at three differentlocations on the same pre-heated sample were averaged out and standard deviations werecalculated. Table 3 and Figure 28 present exact ink formulation data and the examinedinks-substrate configurations.

Table 3. Properties of the solvents used in the experiment.

Chemical material Solvent Ratio Concentration

P3HT:PCBM 1,2-Dichlorobenzene 1:0.8 30 mg ml−1

P3HT:PCBM Chlorobenzene 1:0.8 30 mg ml−1

PEDOT:PSS 4083 Water

DI-water NA

Prior to the experiment, ITO-covered glass substrates (sheet resistance 20 Ω/) werecleaned with acetone,(IPA) and methanol, and selected samples were plasma-treatedwith oxygen plasma for 5 min at 60 W. ZnO nanorods were grown using the galvano-static deposition method, adapted from Seipel et al. and D’Alkaine et al.[148, 158].Importantly, ZnO growth was performed by dr Yuji Suzuki and supervised by ProfessorKathirgamanathan Poopathy at the Brunel University, London.

Thin films (ITO, O2 plasma-treated ITO, PEDOT:PSS, and P3HT:PCBM) wereanalysed with an optical profilometer (Bruker Contour GT-K) to extract their surfacetopology, as it may have an influence on wetting properties. Moreover, the ZnO NR thinfilm was examined with AFM (Veeco D310).

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Finally, organic solar cells were manufactured to verify the influence of temperatureon device performance. A blend solution (P3HT:PCBM 1:0.8 wt.) was dissolved in1,2-dichlorobenzene at a concentration of 30 mg ml−1 and steered for 24 hours at 60C. Solar cell devices were fabricated on ITO-coated glass substrates and solvent-cleaned before testing. Next, samples were oxygen plasma-treated and a 50 nm layer ofPEDOT:PSS was spin-coated onto them. They were then transferred to a nitrogen-filledglove box and annealed in 120 C for 1 hour. Each sample was heated/cooled for 10minutes to a defined temperature, and a 110 nm thick layer of a P3HT:PCBM blendwas spin-coated onto them. After spin-coating, the samples were dried for 1 hour atroom temperature and annealed in 140 C for 15 min. The devices were completedby the thermal evaporation of lithium fluoride (LiF) and Aluminium (Al) layers witha thickness of 1 nm and 150 nm, respectively. Eight solar cells, measuring 15 mm2,were fabricated at each temperature. Finally, PCE measurements were conducted in anitrogen-filled glove box under AM-1.5G irradiation (100 mW/cm2) (Oriel Sol3A),calibrated with an NREL-certified reference silicon solar cell with a KG5 filter. The

(a)

(b)

(c)

(d)

(e)

Fig 28. Configurations of the inks and substrates used in the experiment.

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following plots (Figure 29 - Figure 33) show the behaviour of the investigated inks onvarious substrates. In addition, the above mentioned plots contain error bars representingstandard deviations.

0 10 20 30 40 50 60 70 80 90 100 110 12065,0

67,5

70,0

72,5

75,0

77,5

80,0

Con

tact

Ang

le [

]

Temperature [ C]

DI Water PEDOT:PSS

Fig 29. Contact angle values at different temperatures for DI-water and PE-DOT:PSS deposited on ITO.

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0 10 20 30 40 50 60 70 80 90 100 110 12020

25

30

35

40

45

50

55

60

65

70

Con

tact

Ang

le [

]

Temperature [ C]

PEDOT:PSS DI Water

Fig 30. Contact angle values at different temperatures for DI-water and PE-DOT:PSS deposited on O2 plasma-treated ITO.

0 10 20 30 40 50 60 70 80 90 100 110 12010

15

20

25

30

35

40

45

50

55

60

65

Con

tact

Ang

le [

]

Temperature [ C]

Chlorobenzene 1,2-Dichlorobenzene DI Water

Fig 31. Contact angle values at different temperatures for DI-water, P3HT:PCBMdissolved in CHB and DCB, deposited on a PEDOT:PSS thin film.

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0 10 20 30 40 50 60 70 80 90 100 110 12010

20

30

40

50

60

70

80

90

100

Con

tact

Ang

le [

]

Temperature [ C]

Chlorobenzene 1,2-Dichlorobenzene DI Water

Fig 32. Contact angle values at different temperatures for DI-water, P3HT:PCBMdissolved in CHB and DCB, deposited on a ZnO nanorod film.

0 10 20 30 40 50 60 70 80 90 100 110 120100,0

102,5

105,0

107,5

110,0

112,5

115,0

Con

tact

Ang

le [

]

Temperature [ C]

PEDOT:PSS DI Water

Fig 33. Contact angle values at different temperatures for DI-water and PE-DOT:PSS deposited on aP3HT:PCBM thin film.

Morphology characterization performed with an optical profilometer and AFM dataon thin-film roughness (Table 4 and surface topology (Figure 34).

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(a) ZnO NR thin film

(b) PEDOT:PSS/P3HT:PCBM thin film (c) PEDOT:PSS thin film

Fig 34. Morphology AFM characterisation of selected layers used in the experi-ment.

Table 4. Roughnesses of different thin films used in the experiment.

Thin filmRoughness

Rq [nm] Rav [nm]ITO 0,56 0,45Plasma-treated ITO 0,46 0,36PEDOT:PSS 0,63 0,41P3HT:PCBM 5,47 4,39ZnO NR 42 34

Lastly, Figure 36 presents the Power Conversion Efficiency (PCE) of organic solar cells(Figure 35) manufactured at different substrate temperatures. It is worth noting thatefficiency remains stable in the temperature range of 0 - 90 C. Above 90 C, however,the PCE decreases drastically to almost 0 % at 120 C.

71

Glass

ITO

PEDOT:PSS PH500

P3HT:PCBM

LiF

Al

50 nm

110 nm

1 nm

150 nm

Fig 35. Solar cell architecture used in solar cells efficiency measurements.

0 10 20 30 40 50 60 70 80 90 100 110 1200,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

Pow

er C

onve

rsio

n Ef

ficie

ncy

[%]

Temperature [ C]

Fig 36. Power conversion efficiency of organic solar cells manufactured at differ-ent substrate temperatures.

Table 5 combines all contact angle thermal coefficients obtained during this study.

Table 5. Thermal coefficients of the contact angle of various inks and substrates.(-) represents a configuration not used in organic solar cells and, therefore, notanalysed

InkSubstrate films

ITO Treated ITO PEDOT ZnO NR P3HT:PCBMDI-Water 0,04 0,23 0,12 -0,24 NegligiblePEDOT:PSS 0,04 0,27 - - NegligibleP3HT:PCBM DCB - - 0,18 0,20 -P3HT:PCBM CHB - - 0,15 0,08 -

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Results achieved during the experiments show that contact angle thermal coefficientsare almost linear in the temperature range 0 - 120 C. Rather small error bars (in mostcases below 3) represents good repeatability of the measurements. Linear behaviourreveals that no structural change occurred at the surface of the analysed substrates withinthe measured temperature range [46]. The positive contact angle thermal coefficientis in agreement with the theory proposed by Budziak et al., where solid substratesare considered similar to liquids, whose surface energy decreases with increasingtemperature [45]. However, DI-water on a ZnO NR surface proved an exception, as thecontact angle thermal coefficient was negative, most probably due to high roughness ofthe surface.

For DI-water and PEDOT:PSS on an ITO surface (Figure 29), temperature has only amarginal influence on contact angle values. Although the angle tends to increase linearly,it has a low positive temperature coefficient and, hence, the effect of temperature oncontact angle can be neglected.

The contact angles of DI-water and PEDOT:PSS are significantly lower for plasma-treated ITO than for those with untreated ITO, due to the following factors. Firstly,oxygen plasma reduces the roughness of the surface (Table 4), while decontaminatingthe surface of carbon [144, 159]. Secondly, the treatment also increases the bindingenergy of atoms, reducing oxygen vacancies in the film and significantly enhancingthe polar component of the surface [145–147]. The accumulated effect of all this isthat the contact angle value decreases and the surface hydrophilicity of both DI-waterand PEDOT:PSS increases. For DI-water and PEDOT:PSS on ITO plasma-treatedsamples, temperature has a major influence on the contact angle - when the temperatureis elevated to 120C, the contact angle doubles for both samples.

This phenomenon is caused by the time reversibility of the effects of plasma-treatment: the contact angle of a liquid deposited on a plasma-treated ITO increaseswith time, as the surface reverts back to its initial (pre-treatment) state after severalhours [147, 160, 161]. Most probably, increased temperature accelerates this decayingprocess, producing a higher contact angle value. Nonetheless, more investigation isneeded to unambiguously explain this behaviour.

For a PEDOT:PSS thin-film (Figure 31), variation between the contact angle oforganic solvents and that of DI-water is mostly caused by surface energy differencesbetween CHB, DCB and DI-water [162]. The temperature of a thin-film PEDOT:PSSaffects the contact angle of organic solvents in a similar fashion, producing a comparablepositive thermal coefficient.

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Notably, organic solvents behave similarly on a surface covered with ZnO nanorods,with both CHB and DCB exhibiting a positive temperature coefficient. The high contactangle (100) of DI-water on a ZnO NR surface at room temperature is caused by highmean surface roughness and is in good agreement with literature [163]. Nonetheless,increasing the substrate temperature reduces the contact angle considerably. A furtherdetailed analysis is necessary to accurately explain this negative contact angle thermalcoefficient.

For a P3HT:PCBM thin film, the contact angle for both DI-water and PEDOT:PSSremains steadily high (>105) in the investigated temperature range. This behaviouris mostly caused by P3HT, which during film formation distributes more favourablytowards the surface than the more hydrophilic PCBM [164, 165].

The temperature of the substrate during P3HT:PCBM deposition plays an importantrole for solar cell performance, as substrate temperatures above 90 C cause performancedeterioration. A slower drying process at a low substrate temperature allows P3HTto segregate towards the underlying PEDOT:PSS layer, resulting in vertical phasesegregation. This forms a p-n heterojunction, which directs charges towards therespective electrodes more efficiently, leading to improved photocurrent generation[130]. A high temperature process (above 90 C) excessively increases the crystallizationof P3HT and forces the aggregation of PCBM. As a result, a large extent of phaseseparation will be observed, which reduces the bi-continuous phases present in acomposite thin film and significantly decreases its efficiency [166].

4.3 Temperature dependable material distribution

This part of research investigates the influence of temperature on the distribution ofmaterial. To my knowledge, only a few reports exists that demonstrate the connectionbetween substrate temperature and material distribution [167–170]. According toSoltman et al., temperature can be used to control the coffee ring effect during ink-jetprinting. A previous section demonstrated the influence of substrate temperature on thewetting of inks used to fabricate solar cells. Nonetheless, a more systematic evaluationof deposited drops is needed. This experiment concentrates on ink-jet applicationsand on materials used to fabricate organic solar cells. It investigates the behaviour ofPEDOT:PSS droplets on the surface of an ITO-covered substrate. In printed electronics,PEDOT:PSS is widely used as an Hole Transport Layer (HTL) and conductive electrodeand, therefore, increased control of PEDOT:PSS printing is desirable [171, 172].

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During the experiments, the temperature of solvent-cleaned ITO-covered glasssubstrates varied from 0 to 120 C with 10 C intervals. All samples were heated/cooledto the designated temperature and, after 10 min for temperature stabilization, a dropletof PEDOT:PSS was deposited on the surface. The droplet was exposed to the assignedtemperature until it dried up. This procedure was repeated for temperatures within the0-120 C range at 10 C intervals, and the profile of the droplets were measured with anoptical profilometer (Bruker Contour GT-K). Next, the raw data were analysed and usedto plot a 3D profile and a cross-section profile. Figure 37 presents material distributionat various temperatures.

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0

6

4

2

6.4

(a) 0C - 3D representation

Hei

ght

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

Width profile

Length profile

(b) 0C - Droplet profile

0

8

6

4

2

9

(c) 10C - 3D representation

Hei

ght

0

1

2

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8

9

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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(d) 10C - Droplet profile

0

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7.5

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2.5

10.6

(e) 20C - 3D representation

Hei

ght

0

1

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3

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7

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9

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11

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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Length profile

(f) 20C - Droplet profile

0

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14.4

(g) 30C - 3D representation

Hei

ght

0

1

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9

10

11

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13

14

15

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

Width profile

Length profile

(h) 30C - Droplet profile

Fig 37. Material distribution of dried droplets of PEDOT:PSS deposited at 0 - 30C.

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0

12

8

4

14

(i) 40C - 3D representation

Hei

ght

0

1

2

3

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Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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(j) 40C - Droplet profile

0

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7.5

5

2.5

10.4

(k) 50C - 3D representation

Hei

ght

0

1

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4

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Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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(l) 50C - Droplet profile

0

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7.5

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2.5

11.9

(m) 60C - 3D representation

Hei

ght

0

1

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3

4

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12

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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(n) 60C - Droplet profile

0

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13

(o) 70C - 3D representation

Hei

ght

0

1

2

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4

5

6

7

8

9

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11

12

13

Lateral size0 100 200 300 400 500 600 700 800 900 1000 1100

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Length profile

(p) 70C - Droplet profile

Fig 37. Material distribution of dried droplets of PEDOT:PSS deposited at 40 -70C.

77

0

12

8

4

13.9

(q) 80C - 3D representation

Hei

ght

0

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(r) 80C - Droplet profile

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(s) 90C - 3D representation

Hei

ght

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(t) 90C - Droplet profile

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(u) 100C - 3D representation

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(v) 100C - Droplet profile

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(w) 110C - 3D representation

Hei

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(x) 110C - Droplet profile

Fig 37. Material distribution of dried droplets of PEDOT:PSS deposited at 80 -110C.

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0

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13.8

(y) 120C - 3D representation

Hei

ght

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(z) 120C - Droplet profile

Fig 37. Material distribution of dried droplets of PEDOT:PSS deposited at 120C.

As the figure shows, temperature has a noticeable effect on material distributionin a droplet. At room temperature (below 30C), the droplet is not uniform, but theheight difference between the edges and the middle of the drop is not dramatic. Fortemperatures above 30C, the difference between the height of the central point and theedges increases, forming a ‘batman shape’. Notably, the external walls of structuresformed at higher temperatures are almost perpendicular to the substrate surface. Thisphenomena may be utilized in development of new methods of thin-film patterning.However, the governing principles and exact applications of this behaviour still requiresmore research.

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5 Summary

Solar cells will inevitably be one of the major energy sources in future. Although theirmanufacturing costs are still high, intense research and development will graduallyreduce the cost of fabrication and increase the number of possible applications. Onehighly promising method of fabrication is printing, which allows fabricating cheap solarcells at a high volume.

The aim of this thesis was to provide additional data that will help to develop optimalconditions for organic solar cell production. To that end, it investigated the wettability ofvarious inks and substrates used in organic solar cell fabrication. In an effort to increaseour understanding of the underlying concepts and of commonly used techniques andmaterials, the thesis also sought to describe the operating principles and productionmethods of solar cells. In addition, it provided a detailed review and description ofvarious printing methods, together with information about the materials and inks used inprinted electronics. This work also presented information on the fundamentals of thewetting process.

Measurements of the wettability of the investigated materials were based on thecontact angle measurement method. Importantly, besides being well-suited to wettabilitymeasurements on solar cell materials, the contact angle method proved highly suitablefor measuring the wetting properties of barriers and membranes. Astonishingly, theequation developed by Young in 1805 and the simple contact angle measurement methodprovide valuable data that can be used in a variety of fields. This research concentratedon solar cells and only briefly described the diversity of applications to which the contactangle measurement method can be applied. Thus, this thesis provides just a modestexample how powerful this method is and how it can be successfully applied in scienceand engineering.

Experiments conducted during this research verified that wettability and, conse-quently, the printing of OSCs can be controlled by applying UV and plasma pre-treatments and by manipulating the substrate processing temperature. Since this researchrelated to organic solar cells, all the used materials and processing conditions werereproduced. The achieved results allow drawing the conclusion that the aforementionedmanipulation methods can be successfully applied to controlling the wettability of avariety of materials and substrates. In addition, within a particular range, wetting can be

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controlled by manipulating substrate temperature without deteriorating the efficiency ofthe thus fabricated solar cells.

To better understand the processes that govern the fluid/surface interactions ofmaterials developed for organic solar cells, additional measurements need to be doneand surface energies calculated. Also, further work needs to be carried out to calculatethe polar and disperse components of the surface energy of the used layers. Additionaldata and evidence will definitely help to better understand and predict the behaviour oforganic solar cell materials. Preliminary results of temperature-dependent materialdistribution present a very interesting and promising concept for a maskless patterningprocess. However, a detailed explanation of this phenomenon for various materialsand temperatures necessitates further investigation. Also experiments focusing onthe behaviour of picolitre ink-jet drops need to be performed and the results must becompared to those achieved during this research.

To conclude, the results of this study can be used to increase control over thesurface morphology of printed layers and, consequently, to improve the printing process.Although this study focused on organic solar cells, it can serve as a framework forfurther research related to printed electronics without restriction to a particular structureor device.

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Original articles

I Sliz R, Suzuki Y, Nathan A, Myllylä R & Jabbour GE (2012) Organic solvent wettingproperties of UV and plasma treated ZnO nanorods - printed electronics approach. SPIEOptics+Photonics 2012, 12 - 16 August 2012, San Diego, USA.

II Sliz R, Suzuki Y, Fabritius T & Myllylä R (2014) Influence of temperature on wettingproperties of thin films in organic solar cells applications. Colloids and Surfaces A:Physicochemical and Engineering Aspects 443: 182–187.

III Sliz R, Ahnood A, Nathan A, Myllylä R & Jabbour G (2012) Characterization of microcrys-talline I-layer for solar cells prepared in low temperature - plastic compatible process. SPIEPhotonics Europe 2012, 16–19 April 2012, Brussels, Belgium.

IV Kopola P, Aernouts T, Sliz R, Guillerez S, Ylikunnari M, Cheyns D, Välimäki M, TuomikoskiM, Hast J, Jabbour G, Myllylä R & Maaninen A (2011) Gravure printed flexible organicphotovoltaic modules. Solar Energy Materials & Solar Cells 95(5): 1344–1347.

V García V, Valkama H, Sliz R, King AWT, Myllylä R, Kilpeläinen I & Keiski RL (2013)Pervaporation recovery of [AMIM]Cl during wood dissolution; effect of [AMIM]Clproperties on the membrane performance. Journal of Membrane Science 444: 9–15.

VI Liimatainen H, Ezekiel N, Sliz R, Ohenoja K, Sirviö JA, Berglund L, Hormi O & NiinimäkiJ (2013) High strength nanocellulose-talc hybrid barrier films. ACS Applied Materials &Interfaces 5: 13412–13418.

VII Visanko M, Liimatainen H, Sirviö JA, Haapala A, Sliz R, Niinimäki J & Hormi O (2014)Porous thin film barrier layers from 2,3-dicarboxylic acid cellulose nanofibrils for membranestructures. Carbohydrate Polymers 102: 584–589.

Reprinted with permission from SPIE (I,III), Elsevier (II,IV, V, VII), ACS (VI).

Original publications are not included in the electronic version of the dissertation.

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