1 OLLA Research Newsletter - CORDIS

OLLA Research Newsletter no 1 1

OLEDs are a new and attractive class ofsolid-state light sources, which are openingup completely new applications in large-areaillumination. OLEDs are produced bydepositing a stack of inorganic layers on aglass substrate. By carefully tuning the of the red, green and blue emitter materials,OLEDs can produce any light colour –including white – with excellent light properties.

Goal of this European funded integratedproject is to advance OLED technology and prove it for general lighting applications.By 2008, the consortium will be able to produce white OLEDs for illumination purposes (1000 Candela/m2) with an effi-ciency of 50 lumens per Watt and a lifetimeof 10.000 hours.This is In comparison withthe standard incandescent bulb 5 timesmore efficient and 10 times longer lasting.

A research sample of an Small Molecule OLED,substrate size 50x50 mm.

The project covers several important R&Dfields, including fundamental understandingof the materials and device aspects, electri-cal and optical properties of OLEDs, severalmanufacturing principles for making whitelight OLEDs and system integration. As newmaterials are the key for the advancementof OLED technology, about 40% of the project effort is put into the search anddevelopment of new materials systems.

Developments in this project are leading the way to use OLEDs as new light sourcesfor diffuse, large area illumination withexceptional design freedom.Applicationstudies will complement the researchefforts to match technology developmentwith consumer interests.

NEWSLETTER

The OLLA research newsletters are intend-ed to inform a larger audience, experts andnon-experts of this technology, on thenews, publications, events, and researchresults of the OLLA project. Its researcharticles will be thematically scheduled andare reviewed by a team of internal experts.

OLLA: the European way towardsOLED Lighting Applications

The OLLA Research Newsletter is part of thedissemination activities of the OLLA project.For electronic subscriptions:[email protected].

For more information please visit our website:www.OLLA-Project.org

High Brightness Organic LEDs for ICT & LightingApplications

Contract nr: IST-2002-004607

Contact:Ir. Peter VisserPhilips Lighting, OLED developmentTel: + 49 241 539 [email protected]

OLLA Partners CountryUniversities- Ecole Polytechnique Fédérale

de Lausanne (EPFL), CH- Katholieke Universiteit Leuven (KUL), B- Rijksuniversiteit Groningen (RUG), NL - Technische Universität Dresden (IAPP), D- Universität Kassel, D- Université Louis Pasteur (ULP), F- Universiteit Gent, B

Research Institutes:- Centre National de la

Recherche Scientifique (CNRS-IMN), F- Consiglio Nazionale delle

Ricerche (CNR-ISOF), I- Fraunhofer Institute for

Photonic Microsystems (IPMS), D - Institute of Physical Chemistry of

the Polish Academy of Science, PL - Interuniversitair Micro-Electronica

Centrum (IMEC), B - National Nanotechnology Lab

(NNL), Lecce, I - VTT Technical Research Centre, Fin

Industrial Partners:- Aixtron AG, D- Merch OLED Materials GmbH, D - H.C. Starck GmbH, D - Novaled GmbH, D - Osram Opto Semiconductors GmbH, D - Philips Electronics Nederland BV, NL- Philips Lighting GmbH, D- Philips GmbH Forschungslaboratorien, D- Sensient Imaging Technologies GmbH, D- Siemens AG, D

OLLA is a 45-month R&D project funded under the 6th EU-Framework programme (IST-2002-004607)

© OLLA consortium 2004-2008

Organic Light-Emitting Diode (OLED) technology has improved to the point

where it is now possible to envision white OLEDs as the next solid-state light

source.Therefore one year ago the OLLA project was formed. Coordinated

by Philips Lighting, the OLLA consortium covers more than 20 partners from

industry, universities and academia across Europe.

1

SIXTH FRAMEWORKPROGRAMME

Information SocietyTechnologies

OLLA Research Newsletter


Redoxstability and location of the energeticlevels of hole and electron transport materials as well as matrix materials are ofparticular importance for application inOLEDs. Electrochemistry and especiallycyclic voltammetry (CV) are powerfulmethods to obtain information about both parameters. [1]

Electrochemical processes usually take placeat the electrode-solution interface.A three-electrode cell (working, reference, auxiliary)is commonly used in controlled-potentialexperiments like CV.The reaction understudy occurs at the working electrode.Another inert conducting material (e.g. plati-num, graphite) is usually used as auxiliaryelectrode to close the electric circuit.Thereference electrode (e.g.Ag/AgCl) providesa stable potential to refer the potential ofthe working electrode. Electrochemical measurements are carried out in a solventcontaining the analyte and a supporting elec-trolyte.The choice of the solvent dependson the solubility and the redox activity ofthe analyte. Commonly used non aqueoussolvents for organic materials are acetonitrile,dichloromethane and tetrahydrofurane. Inertsupporting electrolytes are necessary to decrease the resistance of the solution, to eliminate electromigration effects and toensure a constant ionic strength. For organicmedia tetraalkylammonium hexafluorophos-phate (or tetrafluoroborate) are commonlyused electrolytes. [2]

Figure 1 applied triangular potential waveformand the corresponding cyclic voltammogram fora reversible oxidation process

CV measurements are based on the linearscan of the applied potential at the workingelectrode using a triangular potential wave-form and the current response is measuredand plotted versus the applied potential.A typical cyclic voltammogram of a reversible oxidation is shown in figure 1.

Initially the neutral compound A is presentin solution.As the applied voltage approa-ches the formal redox potential E0 a faradaic anodic current ia begins to rise dueto oxidation of this compound to its radicalcation (A•+).The growth of a diffusion layercauses the anodic current to reach a maxi-mum value ipa and decrease afterwards.After passing the oxidation peak on reachingthe switching potential Eλ the scan directionis reversed. During the reverse sweep thegenerated radical cations (A•+) are reducedback to the neutral form which leads to acorresponding cathodic peak current ipc.The cyclic voltammogram is characterisedby several important parameters.The ratioof the anodic and cathodic peak currentipc/ipa is 1 for a simple reversible couple.It strongly depends on chemical reactionscoupled to the redox process.The formalpotential E0 of a reversible redox couple is located symmetrically to the correspondingpeak potentials E0 = (Epa + Epc)/2.Thesepotentials are generally referenced againstferrocene/ferrocenium as internal redoxstandard which is assumed to have an energylevel of 4.8 eV below the vacuum level. [3]

A•+ + e- A

ipa

ipc

Epc

Epa

E λ

E λ

A A•+ + e-

E eE i

E i E 0

potential [V vs. Fc/Fc+]

curr

ent

[µA

]

pote

ntia

l

time

oxidation

reverse

forward

Electrochemical characterisationof materials for OLEDs by cyclic voltammetry

by Kirstin Bausch, Michael Arlt, Josef Salbeck

Macromolecular Chemistry and Molecular Materials, Department of Science, University of Kassel,

Heinrich-Plett-Strasse 40, D-34132 Kassel, Germany

OLLA RESEARCHU N I K A S S E LV E R S I T Ä T

The HOMO level of the investigated com-pound can therefore easily be calculatedfrom the oxidation potential (figure 1).The LUMO level of the compound can bededuced from the reduction potential (radi-cal anion formation) respectively.A completeoxidation and reduction potential determi-nation is given in figure 2. It is important torealise that such energy level investigationscontain solvatation effects in contrast tospectrocopic measurements in the gasphase.

Figure 2 complete potential waveform and cyclic voltammogram of oxidation and reductionprocesses

In the example are shown two reversibleoxdidation processes (radical cation anddication formation) and one reversiblereduction process (radical anion formation),the gap between the first oxidation waveand the reduction wave can be used to calculate the HOMO-LUMO energy difference.The potential scan used for the complete redox process is given on the left side of figure 2.As an example how CV can be used todetect electronic changes by structuralmodification, the following figure 3 shows the cyclic voltammograms of four triphenyl-amine[4] derivatives.TAD (a standard holetransport material) shows to reversible oxidation waves indicating the radical cationTAD•+ and dication TAD2+ formation,separated by 250 mV. If the conjugativeinteraction between the two electron donating nitrogen centres is reduced bystructural modification e.g. by introducing anadditional phenyl ring in between (ST1357)or a forced twisting by four methyl groups(ST16/30) the redox potentials change in acharacteristic way.The first oxidation signalis shifted to higher potentials due to thelower stabilisation of the positive charge in

the radical cation, and the potential separati-on between radical cation and dication for-mation becomes smaller due to the lowerinteraction of the two positive chargesdominantly located at the two nitrogen cen-tres. Introducing two additional phenyl ringsas shown with ST1576 the interaction isfurther reduced resulting in such a lowpotential separation that only one oxidationwave but again at higher potential appears.This wave represents a formal two electron

Figure 3 cyclic voltammograms of four triphenylamine derivatives

transfer leading immediately to the dication.The increase of the phenyl chain length ortwisting the central biphenyl unit of TADmakes the molecule harder to oxidise shifting the HOMO energy levels in thesame order.Consequently, with CV measurements small structurell changes in molecules canbe detected and therewith it can be aredundant method to support and estimate synthetic strategies to tune energy levels.

References:[1] J. Heinze, Angew. Chem., 1984, 96, 823.

B. Speiser, Chem. Unserer Zeit, 1981, 1, 21.B. Speiser, Chem. Unserer Zeit, 1981, 2, 65.

[2] P.T. Kissinger,W. R. Heinemann, LaboratoryTechniques in Electroanalytical Chemistry,1996, 2nd ed.H. Lund, O. Hammerich, OrganicElectrochemistry, 2001, 4th ed.

[3] J. Pommerehne, H.Vestweber,W. Guss, R. F.Mahrt, H. Bässler, M. Porsch, J. Daub, Adv.Mater. 1995, 7, 551.

[4] ST 1357, ST 1576 and ST 16/30 were syn-thesised and provided by Sensient ImagingTechnologies GmbH,Wolfen, Germany.

0,0 0,2 0,4 0,6 0,8

-100

0

100

200NN

NN

NN

NN

curr

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[µA

]


TAD

ST 1357

ST 1576

ST 16/30

-3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5-200

-100

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200

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reduction

oxidat ion

Epc3

Epa3

Epc1

Epc2

Epa1

Epa2

forward scan

reverse scan

oxidation

reduction

"HOMO-LUMO gap" E1/2(ox)E1 /2( re d )

time

EλEλ

radical cation

radical anion

Ei

Ee

curr

ent

[µA

]


po

ten

tial



the determination ofcharge carrier mobilitiesin materials for oled’sJeroen Ollevier, Mikhail Parchine and Mark Van der Auweraer

Lab of Photochemistry and Spectroscopy, Department of Chemistry, K.U. Leuven

Celestijnenlaan 200F, B-3001 Heverlee, Belgium

An important parameter of hole and electrontransport materials as well as matrix materialsused in OLED’s is the mobility of the chargecarriers. E.g. in order to decrease the Ohmicvoltage drop over the device and increase thepower efficiency it is important to have mate-rials with a large mobility. Electron and holemobilities can be determined in different waysbut the most direct information is obtainedwith the time of flight method (TOF).Thesample is placed between two blocking elec-trodes of which one is semitransparant (eg.ITO or a very thin metal electrode).A voltageis applied between the electrodes that createsa uniform field in the sample. Upon excitationof thin layer of the sample by a short mono-chromatic laser pulse, a thin sheet of excitonsis created close to one of the electrodes.Their dissociation yields electrons and holesthat will migrate to the anode and cathoderespectively.As long as the carriers migratethrough the sample, a current is observedwhich becomes zero when all carriers left thesample.While the mobility of the carriers canbe determined from the width of the currentpulse, corresponding to the transit time (ttr),the area under the pulse yields the amount ofcarriers created. Eventually, the mobility (μ)can be derived from ttr = L2/ μV, where L isthe sample thickness and V the applied voltage.

While electron and hole mobilities are wellknown in materials used in xerography, no orvery limited information is available for manymaterials used in OLEDS (CBP, BCP, metalcomplexes, conjugated polymers ).We willstress in our research the mobility in mixturesof matrix materials responsible for chargetransport and luminescent metal complexes.Field- and temperature dependencies of themobility will be studied over a wide range.These dependencies will then be explained by formerly described models (e.g. disorderformalism), which will be extend to take intoaccount the luminescent dopant, acting as atrapping center. Contrary to the studies ofmaterials used in xerography we will try toinvestigate the bipolar mobility of the materi-als, e.g. we are not only also interested in thehole mobility a hole transporting material butalso in its electron mobility and vice versa.The latter will allow us to correlate themobility of the “wrong” carrier with the merits of the material as electron and holeblocking material.

1) R.G. Kepler, Phys. Rev. 1960, 119, 4.2) O.H. Le Blanc, J. Chem. Phys.

1960, 33, 626.3) M.Van der Auweraer, F.C. De Schryver,

P. Borsenberger, H. Bässler, AdvancedMat., 1994, 6, 199.

semitransparentelectrode (Al), 30 nm

vapor deposited p-EFTP(100 nm)

Counterelectrode (SnO2)

Substratum

V (-)

V (+)

0 10000 20000 30000 40000 50000

Time (µs)

-20000

-10000

0

1000020000

30000

40000

5000060000

70000

8000090000

Photocurrent (a.u.)

TOF signal of a triphenylamine dispersed in polycarbonate3

TOF sample

OLLA RESEARCH

Related EU projects


Color appearance of our natural environ-ment in the absence of sunlight is always animportant issue in lighting applications.Therefore the Color Rendering Index (CRI)was defined.The CRI describes the color reproductionappearance of any light source. It rangesfrom 0 to 100 and is having no unit.A highcolor rendering is essential in settings wherenatural color appearance is important (e.g.in shops).

OLLA now published on its website areport on the CRI of white OLED lightsources.The scope of this report is to investigate whether the current state-of-the-art broadband emitting polymers arematching the CRI properties of usual lightbulbs or other illumination sources. So farthe polymers were developed for displayapplications, for which only the visual colorcoordinates are essential.

In the first part of the report the basics -definition and measurement – of the CRIare described.The second chapter dealswith the limitations of the CRI measure-ment. In the last section CRI measurementsof current polymeric OLED emitters areshown and are compared with other lightsources with which OLEDs have to competein future applications.

The document (PDF format) is available forfree from the download area of the OLLAwebsite. www.olla-project.org

OLLA CRIreport published

Also funded from within the EuropeanCommission’s IST program, the IntegratedProject FlexiDis, or "Flexible Displays", is tar-geting the fabrication of flexible active-matrixdisplays. Flexible displays cover a range ofmechanical properties and product formats –from robust/unbreakable, ultra-thin, non-rectangular, non-flat, conformal, bendable andeven rollable displays, but in all cases, there isin-depth research required to make thesedevices a commercial reality.

Goal of FlexiDis is to develop the novel process technologies required for flexibledisplays, along with a fundamental understan-ding of their mechanical behaviour and reliability.And, of course, manufacturingmethods must be developed that are suitablefor scale-up to commercialisation and pro-duct generation, in the FlexiDis targetedmarkets of mobile, wearable, automotive orother consumer displays.

“The research on novel materials, devices,handling and production methods for flexibledisplays will enable us to break through thecapital-intensive, high-investment factorymodel currently true today in the Far East.”,says Eliav Haskal, coordinator of FlexiDis.“This project is therefore aimed at advancingdisplay R&D in Europe, with the express purpose of strengthening display manu-facturing in Europe.”

The active-matrix backplane is composed of a high-resolution 2D array of thin-filmtransistor (TFT) switches prepared on aflexible plastic or steel foil, where each TFTis located behind a display pixel and used toturn on or off that particular pixel. The display principles, either organic light-emitting devices (OLEDs) or electrophoretic (EP)foils, are combined with the active-matrixbackplane to make the final display. With driving circuitry and interconnects, imagescan be produced on the displays.

Within the course of FlexiDis, several different types of active-matrix displays willbe fabricated, including • Full-colour organic light-emitting

displays (OLED) on bendable metal and/orplastic foils with silicon thin-film driving transistors, and

• electrophoretic (EP) monochrome displayson bendable and rollable plastic substrateswith organic thin-film driving transistors.

The goal is to show a series of displaydemonstrators over the period Q4 2005 out to 2007.For more information, and a copy of theFlexiDis newsletter, please see www.flexidis-project.org

Processing of low-temperature poly-silicon (LTPS)on metal foil (courtesy of CEA-LETI)

FlexiDis


OLEDSummerschool 2005

From 16-26 September, a decicate OLEDSummer School will take place in Krutyn,Poland.This 2005 edition is organised by theInstitute of Physical Chemistry of the PolishAcademy of Sciences in connection with theFP6 OLLA Project. Main topic will be, amongother more generic OLED topics,AdvancedLuminescent Materials based on LanthanideOrganic/Inorganic Complexes.About 35 participants and 15 speakers are expectedfor a deep scientific dive.

The Summer School site is located in theheart of Mazurian Lake District.This is oneof the most beautiful and undiscovered lakearea’s of Europe. Krutyn is accessible byshuttle bus or car within 3 hours fromWarsaw.

For Summer school program details, costand further information:http://ichf.edu.pl/summer_school2005/index.html

As part of the activities of the applicationrequirements taskforce, OLLA will organizea dedicated Signage ApplicationWorkshop in Frankfurt on 18 November 2005.Signage applications are seen as one of the first adopters of OLED lighting technology, as the requirements are different from general lighting applications.

Goal of the workshop is to get intocontact with the European Signageindustry, inform them on the OLEDtechnology status and discuss therequirements and application conditi-ons in more detail.These details arehelpful input for the project, and willbe feed into the project for furtherdevelopment of OLED materialsand devices.

If you are interested to join this workshop, please mail to:[email protected]

For the same reason the OLLA project haspublished a requirements survey.This survey,which is already filled in by many companies,is access able via the OLLA homepage.

Signage Application Workshop18 November 2005Frankfurt

OLED SignageApplication workshop, Frankfurt, 18 November

The participants of the OLLA Tutorial sessions, Pommersfelden, April 2005


Other EU initiatives: EPIC

EPIC, the European Photonics IndustryConsortium, is working to create opportuni-ties: growing business and new technologiesfor European photonics industries. Startingfrom our base of five Board members in2004, now more than 50 companies decidedto join. EPIC is now recognised throughoutEurope as one of the driving forces suppor-ting the creation of a photonics technologyplatform at the European level.Membership in EPIC is open to allEuropean companies, research institutes,universities, and financial partners (such asbanks or venture capital organisations).

ROADMAPEPIC works with the European Commissionto translate industry needs into researchand development programmes. EPIC ownsand maintains the Photonic TechnologiesRoadmap.Topics that have been addressed:Building OLED infrastructure in Europe,Photonics in the automobile, and Photonicsin the FP-6 R&D programmes.

Photonics PlatformAll in all, EPIC participated on behalf of itsmembers, in nine different actions.Among themost important, EPIC helped to organise twoimportant meetings with the EuropeanCommission to initiate a Photonics technolo-gy platform in FP-7.The initiative to create aPhotonics Technology Platform is a collabora-tive effort involving EPIC,The EuropeanCommission, the Association of GermanEngineers (VDI) and many members of theEuropean photonics community.

The EPIC Vision Photonics for the 21stCentury**, has successfully focussed attenti-on of the strategic importance of photonicsfor the healthy growth and development ofEuropean industry in 5 main sectors:Communications and display, Lighting, Healthand medicine, Manufacturing, and Defenceand security.We are now moving forward to create the operational structure for this Platform,and we welcome the participation of OLLA in building the strategic research agenda for FP-7.

Cover of the Vision document, Photonics for the21st Century

**You may download your copy of theVision document, Photonics for the 21stCentury on the EPIC web-site:www.epic-assoc.com

European Photonics IndustryConsortiumBy T.P. Pearsall, EPIC chairman

European Photonics Industry Consortium

OLLA board meeting at partner IAPP, December 2004, Dresden.


OLLAgenda & Events calendar

16-26 Sept 2005 IPC/OLLA summerschool onAdvanced Luminescent Materials based on Lanthanide Organic/Inorganic Complexes, Krutyn, Poland

27-30 Sept 2005 ECOER, European Conference on Organic Electronics & Related PhenomenaOLLA partner contribution Winterthur, Switserland www.ccp.zhwin.ch/ecoer

26-28 Sept 2005 Organic Semiconductor Conference’05OLLA presentationQueens' College, Cambridge, UKwww.cintelliq.com/conference.htm

4-5 Oct 2005 Plastic Electronics 2005, Conference & ShowcaseOLLA showcase and presentationMesse Frankfurt, Germanywww.plastictronic2005.org

17 Oct 2005 OLLA PhD-Day trainingIAPP, Dresden, Germany

18-19 Oct 2005 3rd OLLA-Wide-WorkshopNH hotel, Dresden, Germany

19 Oct 2005 Adria Project Roadmap meeting #3, Pariswww.adria-network.org

14-16 Nov 2005 OLEDs 2005San Diego, CA, USAOLLA partner papers and posterwww.intertechusa.com/conferences

14-15 Nov 2005 CER, Communicating European ResearchOLLA showcaseBrussels, Belgium

18 Nov 2005 OLED Signage Application Workshop,OLLA workshop for invited personsFrankfurt, Germany

29 Nov 2005 FP6- IST - Organic Cluster Event,OLLA presentationBrussel, Belgium

27 Nov - 2 Dec 2005 MRS-Fall, Symposium DBoston, MA, USAwww.mrs.org/meetings/fall2005/

21-26 Jan 2006 SPIE- Photonics West, OPTO conference,San Jose, USAwww.spie.org

3-7 Apr 2006 SPIE - Photonics EuropeStrasbourg, Francewww.spie.org

24-26 Apr 2006 OLEDs Asia 2006Singapore, Singapore

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1 OLLA Research Newsletter - CORDIS