Table Of ContentRubén D. Costa Editor
Light-Emitting
Electrochemical
Cells
Concepts, Advances and Challenges
Light-Emitting Electrochemical Cells
é
Rub n D. Costa
Editor
Light-Emitting
Electrochemical Cells
Concepts, Advances and Challenges
123
Editor
Rubén D.Costa
IMDEA Materiales
Parque Científicoy
Tecnológico-Tecnogetafe
Getafe (Madrid)
Spain
ISBN978-3-319-58612-0 ISBN978-3-319-58613-7 (eBook)
DOI 10.1007/978-3-319-58613-7
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Foreword
Light-Emitting Electrochemical Cells: organic semiconductor
devices augmented by ions
Polymer light-emitting electrochemical cells (LEC) was invented in 1994 to
facilitate the injections of charge carriers into luminescent conjugated polymers in
light emitting diodes. Initially, the benefits of adding a solid electrolyte into the
organicsemiconductordeviceswaseminent:electronscouldbeeffectivelyinjected
from a stable metal like aluminum into the lowest unoccupied molecular orbitals
whichare1eVhigherthanaluminum’sworkfunction;andthedrivingvoltagewas
substantially reduced. Also, the emissive layer could be much thicker than that
allowable for tunneling charge injection interfaces. Michael Rubner at MIT
obtained similar results with conjugated small molecules such as a soluble
(cid:1) (cid:3) (cid:1) (cid:3)
derivative of RuðbpyÞ 2þ PF(cid:2) , a compound that had been studied for eletro-
3 6 2
generatedchemiluminescence long before LECwas first reported.The presenceof
mobile ions in the LECs and consequently overpotential could induce degradation
which has been partially addressed over the years. Karl Leo and Junji Kido, et al.
introduced immobile ions or “doped” the organic semiconductor via organic
donor-acceptor complex, which have leap frogged the operational stability of
OLEDs.
Works by Richard Friend, Ian Parker and others in the early 1990s showed
that the conjugated polymer/electrode interfaces could be modeled as tunnelling
barriers. The work function difference between ITO, a commonly used transparent
anode, and calcium is around 1.8 eV. Low electron and hole injection barriers are
feasible when using a small band gap polymer such as MEH-PPV to produce
orange light emission. For blue light emitting polymers, large barriers are inevi-
table. The emissive layer has to be made as thin as processing can produce a
defect-free layer, typically around 100 nm for spin coating. Even so, the driving
voltagewasoftentensofvolts,andthebluepolymerOLEDsfabricatedatthetime
v
vi Foreword
wereveryshortlived,eventhoughthequantumefficiencywasdecent.Infindinga
solution to this challenge, the doping propagation model that Olle Inganäs and me
usedtosimulatethepolypyrrole/polyethylenebilayerbendingbeamsinspiredmeto
introduceelectrochemicaldopingintothepolymerOLEDs.Thus,acommonlyused
ionicallyconductivepolymer,polyethyleneoxidepluslithiumtriflate,wasselected
tosupplythe mobile dopants. The Wessling precursor ofPPVwas selectedthanks
to its compatibility with the PEO-lithium salt. The resulting blend of an ionically
conductive and an electronically conductive polymers showed remarkably
improved electroluminescent performance compared to control OLEDs based on
ITO/PPV/Al. The driving voltage was lower, the quantum efficiency was higher,
and the operational stability was also enhanced.
Alan Heeger, Jun Gao, Ludvig Edman, and others separately confirmed the
formation of p-i-n junction in the polymer LECs by optical beam induced current
measurements, direct imaging under microscope, and measurement of electrostatic
potential distribution by scanning Kelvin probe microscopy. Light emission and
major potential drop were observed to occur at the junction. This p-i-n junction
model may not rule out other mechanisms, particularly when the junction is not
formed to exhibit the ideal doping profiles at the electrode interfaces. Electrical
double layers could dominate at driving voltage well below the band gap of the
conjugated polymer, i.e. the onset of simultaneous p- and n-doping.
The junction model essentially guides the material selection to fabricate high
performanceLECs.Threeelectronic/ionicpolymerblendsystemswereexaminedin
the early years include (MEH-PPV + PEO-lithium salt) and (polyfluorene with
ethylene oxide side chains + lithium salt), in addition to the (PPV + PEO-LiTf)
system used in the very first LEC device. Morphological control was a critical
factor in the device performance: one had to consider the tradeoffs among carrier
transport,ionicmobility,luminescencequenching,andaccessibilityofdopingions
into the low-polarity conjugated polymer domain. Furthermore, mobile ions could
lead to electrochemical over-reaction or degradation at high driving voltages.
Freezingtheionsaftertheformationofthep-i-njunctionappearstobeeffectiveto
slow down such degradation.
The added freedom in electrode selection for LECs allows the fabrication of
devices without the use of high vacuum: Sue Carter printed silver paste as the
cathode;carbon nanotubecoatingcould alsobeusedasthecathode,aswellasthe
anode.LECsformedbysandwichingtheemissivepolymerlayerbetweenapairof
carbon nanotube electrodes were flexible, even stretchable if the nanotubes were
coated on elastomeric substrate.
TheLECisnowintertwinedwithmanyotherfields.Itisexcitingtowitnessthe
latest progress in LEC performance and exploration of unique applications.
HereImerelystatemypersonalviewonwhatoccurredinthepastthathelpedshape
the fieldasit istoday. More historyandexciting developments arecovered by the
authorswhowrotethechaptersofthisbook.Myhat’sofftotheseactiveresearchers
Foreword vii
who have made critical contributions to the field. I am indebted to Dr. Chi Zhang,
Dr.YangYang,Dr.YongCaoforhelpingfabricatethefirstgenerationofpolymer
LECs, Prof. Alan Heeger for polishing the junction model, and Dr. Gang Yu for
suggesting the planar LEC structure to image the junction.
Qibing Pei
Department of Materials Science and Engineering
Henry Samueli School of Engineering and Applied Science
University of California,
Los Angeles, CA, USA
Preface
Theorigins oftheorganic-basedsolid-statelighting(SSL)datebackto1953,butit
was only in the 90s when the organic-based light-emitting diode (OLED) and
light-emittingelectrochemicalcell(LEC)technologiesstartedtoflourish.Although
OLEDs have made all the way from laboratory to commercial products, the LEC
technology is considered as the simplest SSL device. The two pillars of LECs are
the type of emitter that holds charge injection, charge transport, and emission and
the ionic additive that assists charge injection at low applied voltages. As intro-
ducedbyProf. Peiintheforeword,theLECrevolutionisbased ontheuseofions
toreduce the turn-onvoltage.After 15years of research, we havegained a mature
understandingofthedevicemechanism.Thishas,inaddition,beenachievedalong
with the optimization of the two traditional emitters (luminescent conjugated
polymersandionictransitionmetalcomplexes),theionicadditivesforeachtypeof
emitters, and the type of poling modes. After having fully understood the device
limitations, we have achieved several breakthroughs with respect to the efficiency
using multilayered architectures (cascade and/or tandem), frozen junctions, color
converting layers, etc. and low-cost and up-scalable fabrication protocols using, in
addition,unconventionalconductivesubstrates.Asthemostrecentresearchaction,
we have focused on investigating different types of emitters like small molecules,
nanoparticles, quantum dots, etc. Hence, the last two decades have been a suc-
cessful test-bed time for LECs, reaching both a high industrial relevance and an
always-rising research interests, as LECs are an easy set-up to investigate the
electroluminescence features of the emitters and the device physics of ionic-based
optoelectronics.
Overall,Ifeltthatitwasnowtherighttimetobringtogetheralltheeffortsofthe
LECcommunityinthisfirstbookdevotedtotheLECtechnology.Theintentionof
this book is to provide to young students a general description of the LEC tech-
nology with a focus on the device mechanism and the different techniques to
elucidatetheroleofmobileanions(PartI).Afterthisgeneralview,theywillenjoy
twosectionsspecializedonthedefinitionandroleoftheionicadditives(PartII),as
well as the last advances in traditional and new electroluminescent materials (Part
III). Part II is divided into five chapters that describe in-depth the type of ionic
ix
x Preface
additives and the different techniques to study the effect of the mobile ions on the
devicemechanism(Chaps.2to5),aswellashowtheionicelectrolytesarecrucial
forthefabricationofLECsusingdepositiontoolsofindustrialrelevance(Chap.6).
PartIIIconsistsofsevenchapterssummarizingi)theprogressindesigningiridium
(III) complexes (Chap. 7), in general, and blue-emitting iridium(III) complexes
(Chap. 8), in particular, ii) the studies on new materials with thermally activated
delayedfluorescencefeatures (Chap. 9)aswellasexciplex emissioninconjugated
polymers(Chap.10),andiii)thelastadvancesinnewelectroluminescentmaterials,
suchascopper(I)complexes(Chap.11),small-molecules(Chap.12),andquantum
dots (Chap. 13).
My intention is to provide a comprehensive vision of the past and present
developments intheLEC technology asinsights for future advances coveringnew
device designs, industrial progress, and novel types of emitters.
Erlangen, Germany Rubén D. Costa
Contents
Part I Introduction to the Light-Emitting Electrochemical
Cell Technology
1 Light-Emitting Electrochemical Cells: Mechanisms
and Formal Description . ..... .... .... .... .... .... ..... .... 3
Stephan van Reenen and Martijn Kemerink
Part II Definition and Role of the Ionic Additives
2 Optical-Beam-Induced-Current Imaging of Planar Polymer
Light-Emitting Electrochemical Cells.... .... .... .... ..... .... 49
Faleh AlTal and Jun Gao
3 Optical Engineering of Light-Emitting Electrochemical
Cells Including Microcavity Effect and Outcoupling
Extraction Technologies . ..... .... .... .... .... .... ..... .... 77
Hai-Ching Su
4 The Use of Additives in Ionic Transition Metal Complex
Light-Emitting Electrochemical Cells.... .... .... .... ..... .... 93
Lyndon D. Bastatas and Jason D. Slinker
5 Improving Charge Carrier Balance by Incorporating
Additives in the Active Layer.. .... .... .... .... .... ..... .... 121
Hai-Ching Su
6 Morphology Engineering and Industrial Relevant Device
Processing of Light-Emitting Electrochemical Cells .... ..... .... 139
G. Hernandez-Sosa, A.J. Morfa, N. Jürgensen, S. Tekoglu
and J. Zimmermann
xi
