Review Article Singlet Generation from Triplet Excitons in Fluorescent Organic …downloads.hindawi.com/archive/2013/670130.pdf · 2019-07-31 · Review Article Singlet Generation

Hindawi Publishing CorporationISRNMaterials ScienceVolume 2013 Article ID 670130 19 pageshttpdxdoiorg1011552013670130

Review ArticleSinglet Generation from Triplet Excitons inFluorescent Organic Light-Emitting Diodes

A P Monkman

Organic Electroactive Materials Research Group Department of Physics University of Durham Durham DH1 3LE UK

Correspondence should be addressed to A P Monkman apmonkmandurhamacuk

Received 22 August 2012 Accepted 4 October 2012

Academic Editors I A Hummelgen and E J Nassar

Copyright copy 2013 A P Monkman This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

A potential major drawback with organic light-emitting devices (OLEDs) is the limit of 25 singlet exciton production throughspin-dependent charge recombination Recent device results however show that this limit does not hold and far higher efficienciescan be achieved in purely fluorescent-based systems (Wohlgenannt et al (2001) Dhoot et al (2002) Lin et al (2003) Wilson etal (2001) Cao et al (1999) Baldo et al (1999) and Kim et al (2000)) Thus the question arises is recombination spin dependent(Tandon et al (2003)) or are singlet excitons generated in secondary processes Directmeasurement of the singlet generation rate inworking devices of 44has been shown (Rothe et al (2006)) which have been verified as being part due to direct singlets formed onrecombination and part from triplet fusion singlets produced during triplet annihilation (Kondakov et al (2009) King et al (2011)and Zhang and Forrest (2012)) Here the various routes by which triplet excitons can generate singlet states are discussed and theirrelative contributions to the overall electroluminescence yield are given The materials requirements to obtain maximum singletproduction from triplet states are discussed These triplet contributions can give very high device yields for fluorescent emitterswhich in the case of blue devices can be highly advantageous Further new devices architectures open up which are simple andhave intrinsically low turn on voltages ideal for large-area OLED lighting applications

1 Introduction

Current state-of-the-art OLED and PLED devices have beenoptimised for use in displays having small-area pixelsyielding high efficiency and good individual colour fromeach pixel Active matrix displays using these are now foundin the latest smart phones (ca 2012) including Samsungrsquosfastest selling smart phone to date theGalaxy S2Thedisplaysemploy red phosphorescent emitters but blue and green flu-orescent emitters In the latest generation of high-efficiency(gt60 lmW) organic solid-state lighting (OSSL) panels fromNovaled Osram and Konica Minolta-Philips use of allphosphorescent emitters yields very warm white colourswith poor colour temperatures of 2600K and poor lifetimesespecially for the blue component [1ndash3] The phosphorescentemitters lack good saturated colour but more importantlythe blue metal organic complexes used are unstable For themost common blue (aqua) phosphor FIrpic [4] vacuumdeposition causes partial loss of fluorine substituents fromthe ligands and partial decomplexation of the picolinate (pic)

ancillary ligand [5 6] this then causes further degradationover a short period of running leaving the blue pixels with amuch shorter lifetime than both the red and green Secondlybecause of the necessity of having such a high triplet energyto yield the correct blue colour problems arise with findinga host material with a triplet energy higher than that of theemitter but which can also act as a good transport materialfor electrons and holes [7ndash11] Currently the best state-of-the-art blue phosphorescent material is that demonstrated byBASF and the group of Kido at Yamagata University [12] whoreport a ldquobluerdquo phosphorescent device from a nonfluorinecontaining Ir complex having peak emission at 454 nm withhigh quantum efficiency 36 lmW 29 cdA EQE 186 butwhich suffers from serious efficiency roll off As yet onlypoor lifetimes have been achieved which suggest that it toohas degradation problems probably arising from the tripletenergy being close to that required to break the ligand bondsthat is a fundamental limitation For PLED systems theproblem with blue phosphors is the fact that only few layerdevices can be fabricated by sequentially solution deposition

2 ISRNMaterials Science

[13ndash18] so the high triplet energy host materials requiredcause serious charge injection problems Although manygroups are currently trying to tackle this problem devel-oping new materials sets for blue phosphorescent emittersusing simple few layer device structures alternatives to bluephosphorescent OLEDs are being sort There is still greatpotential to carry on using fluorescent blue emitters for bothdisplay and white light applications where lifetime is themost demanding and critical parameter for the devices butwe would still require a way to make use of (75) tripletexcitons formed in the fluorescent emitters Several ideashave emerged on triplet harvesting from a blue fluorescentemitter into red and green phosphors [19ndash21] however thedevice structures required to achieve this are very complexand involve exciton blocking layers of order 2 nm thicktotally impractical to manufacture at high yield over the largeareas required for lighting These complex device structuresare not used in the current generation lighting panels ordisplays Thus a new triplet harvesting approach is requiredIt is well known that triplet excitons can be used to deriveuseful single excitons from triplet-triplet annihilation (TTA)by the process of triplet fusion (TF) [22ndash26] When twotriplet excitons interact and depending on the subsequentspin configuration of the resultant ldquointeraction pairrdquo theycan produce a singlet exciton (TF) further triplet excitonsor quintet states [27] Historically a pure spin statisticallyargument has been used to imply that only 1 singlet exciton isproduced from 18 initially interacting triplets [27ndash29] but inmost molecules the quintet states are energetically untenable[30ndash32] and in a few specific molecules the upper excitedtriplet state 119879

119899 cannot be reached either as with rubrene [33

34] given the sum of the energy of the two triplet excitonsThus the number of singlet excitons produced via TF canbe much higher than 118 allowing fluorescent dopants tobe effectively used as triplet ldquoharvestingrdquo emitters yieldinglonger device lifetimes and removing the stringent require-ments of high triplet energy host materials required withblue phosphorescent dopants In this paper the processesand various different ways in which singlet excitons can begenerated from triplet states are explained

The factors which control the overall quantum efficiencyof an organic light-emitting diode are rather different thanthose that govern inorganic LEDs efficiency most notably theratio of singlet to triplet excitons formed as a result of chargerecombinationwithin the organic emitter layer In the organicsemiconductors the singlet and triplet excitons are very wellresolved with little intermixing [35] This difference comesabout because of the excitonic nature of the fundamentalexcited states of the organic emitters and the large electroncorrelation energies of the molecules [36 37] Since theelectron and hole are tightly boundwith large exciton bindingenergy [38ndash40] singlet excitons are spatially localized and sotheir exchange energy is large whereas for the triplet excitonsthe electron and hole are in orthogonal orbitalrsquos so the tripletexcitons have a zero exchange term [41 42] This manifestsitself in the very large difference in the energies of the lowestsinglet (119878

1) and triplet (119879

1) excited states (excitons) experi-

mentally found to be1198791= (113times119878

1minus143) plusmn 015 eV (typically

of order 07 eV) and conjugation length dependent [37 43]

The singlet excitons are also strongly coupled to the groundstate giving rise to high photoluminescence quantum yieldsFurther as organic semiconductors typically contain onlylow atomic mass elements spin orbit coupling the dominantmechanism for triplet formation [35 44] is weak and sothe interconversion of singlet excitons into triplet excitonsthat is intersystem crossing is very inefficient [45 46]again helping to achieve high photoluminescence quantumyieldsThus to understand the charge recombination processin OLEDs we must also take into account these excitonicproperties

2 Exciton Formation in an OLED

Charges of opposite sign injected into the organic semi-conductor form dressed states not free electrons and holesthe charge perturbs the covalent 120587 bonding structure ofthe organic semiconductor creating a localized distortionwhich traps the charge that is a polaron (P) [47] andit is these polarons (positive and negative) that migratethrough the organic layer to meet in a thin recombinationzone [48 49] These polarons are spin 12 particles Whenoppositely charge polarons capture they initially form someintermediate state held by their mutual columbic potentialwell [50] before relaxing into an exciton (which can thenemit) Before capture the polarons are uncorrelated and sotheir spin states have random orientation with respect toone another Only at the point of recombination do thetwo spins of the P+ and Pminus become correlated and singletor triplet character can be associated to the intermediatestate [51] The intermediate state can thus be best describedas either a singlet or triplet charge transfer (CT) state (orcharge transfer exciton) [52] If this recombination processis thus independent of the spin states (darr) of the initial(P+Pminus) pair then quantummechanical spin statistics dictatesthat there are four ways in which the spin wavefunctions ofthe individual polarons can combine when the CT excitonicstate is formed that is uarr + uarr (triplet) darr + darr (triplet)1radic2[uarrdarr + uarrdarr] spins precessing in-phase (triplet) 1radic2[uarrdarrminus uarrdarr] spins precessing out of phase (singlet) From this itcan be seen that only 25 of the excitons thus created willhave singlet character and be emissive which puts an upperlimit of 025 on the internal quantum efficiency of an OLEDHowever this limit only arises if the recombination processis independent of spin If at any stage the recombination iseffected directly or indirectly by the spin configuration ofthe intermediate CT states [53] or the polaron capture isaffected by their spins [54] then it can follow that the 25limit is broken and possibly more singlets could be produced[21]

Over the past few years an increasing number of experi-mental reports make it clear that the 25 limit is broken [55ndash59] In small-molecule-based OLEDs original experimentalevidence pointed to the 25 limit being obeyed [60 61]whereas in polymers this was not so however more recentreports especially those from theOLED group inKodak haveshown that the 25 limit in small-molecule devices was alsogreatly exceeded [62 63] Many theories were put forward as

ISRNMaterials Science 3

to why there might be such a difference in the fundamentalphysics of these two different materials based systems

From the first experimental results showing greater than025 singlet yield in polymers models of the charge recombi-nation process which predicted spin-dependent recombina-tion were developed The earliest model was put forward byTandon et al [64] modelling the initial polymer based resultsof Wohlgenannt et al [55 65] Their model assumes that therecombination process initially occurs on two neighbouringchains forming an interchain CT exciton [66] before a finalexciton forms on one chain They show that in a conjugatedpolymer both the ground state and the lowest excited tripletstate (exciton wavefunction) are covalent whereas the singletexcited state wavefunction is made from a combination ofcovalent and charge separated (ionic) configurations that isdoubly occupied sites as originally put forward in Simpsonrsquosldquovalance bondrdquo model [67 68] Thus when a P+ and Pminus(on neighbouring chains) initially capture and form anintermediate ldquocharge transferrdquo (CT) state this too must haveionic character so there should be a higher probability toform the singlet exciton than the covalent triplet excitonfrom the intermediate CT state This stems from FermirsquosGolden rule the bigger the overlap of the initial and finalstate wavefunction the greater the rate of the transitionThus because of the large singlet electron exchange energythe ionic singlet wavefunction is more delocalized that isthe two electrons cannot occupy the same site than thelocalized triplet and so the diffuse (ionic) intermediate CTstate will more readily decay into the singlet channel thanthe triplet This idea then readily explains why in the small-molecule systems the spin statistical limit is preserved as herethere is very little difference between the spatial extent ofthe singlet and triplet wavefunctions However recent in-depth analysis of CT states in luminescent polymers [5469 70] shows that the decay channels for CT states followthe inverse of this behaviour which is outlined later in thepaper

At the same time as this Karabunarliev and Bittner[52 71] proposed a different model again assuming anintermediate CT state but they considered the whole recom-bination process to occur on a single chain In this modelthe polarons capture on a single chain the wavefunctionof the CT state initially formed is a 1 1 superposition ofcovalent and ionic configurations yielding 1 1 singlet andtriplet CT states Because the energies of both the CT statesare roughly equal (true in certain cases) the relaxationfrom a singlet CT state to a singlet exciton state goes at afaster rate because less energy (phonons) need to be givenoff noting the exchange interaction yields triplet excitons07ndash1 eV [37] lower in energy than singlet excitons Thussinglets are formed at a faster rate than triplets From theircalculations the ratio of formation rates (singlet to triplet)is chain-length-dependent and for short chains the ratesof exciton formation become equal as the singlet tripletenergy gap equalize [42 43] yielding a 25 singlet excitongeneration fraction as seen in small molecules Thus givenan initial 1 1 singlet to triplet CT formation rate on polaronrecombination the relaxation to excitons is controlled bythe number of phonons emitted in relaxing down to the

respective exciton and in polymers because the triplet stateis at least 07 eV below that of the singlet the singlets aremade faster This theory faces two major problems howeverfirstly in small molecules the triplet exciton is again between05 and 08 eV below that of the singlet exciton [72 73]hence the model cannot predict the correct small moleculebehaviour and second it again does not take into account thefact that the triplet state of the polymer must be below thatof the CT triplet state which gives a very rapid quenchingchannel for the CT states to the triplet exciton of the polymerespecially when the CT singlet and triplet states are closein energy as required by the theory and noting the CTsinglet must be energetic enough to create a polymer singletstate

Many others have followed on from Karabunarliev andBittnerrsquos work using their model as a starting point Yin etal [74] have suggested that an applied electric field suchas we have in a working device would lower the energy ofthe CT states This would bring the singlet CT state closerinto resonance with the singlet exciton and thus enhance thesinglet channel over the triplet channel Barford [50] cameto a very similar conclusion but also showed that becausethe Frank-Condon overlap between the phonon modes inthe triplet manifold are smaller than for the singlets therelaxation rate of the triplet CT into the exciton is slowed evenmore Das et al [75] Beljonne et al [76 77] and Chen et al[78] come to similar conclusion as that of Karabunarliev andBittner

As already mentioned recent studies on CT states inluminescent polymers and small molecules casts severedoubts on these models based on an intermediate CT stateFirstly as with all CT systems the CT3 rapidly decays toa lower lying triplet exciton of the donor-acceptor pair(forming the CT state) [79] whereas CT1 has much morecomplicated decay channels CT1 can cross to CT3 withtypically an enhanced ISC (compared to the donor) [46]Those CT1 which decay to the 1198781 can also suffer quenchingto the 1198791 state via normal ISC The key parameters dictating1198781 formation are thus the competition between the CT1 rarrCT3 interconversion rate the CT1 rarr 119878

1 formation rate and1198781

rarr 1198791ISC rate We know that for most emissive polymers

ISC is rather slow of order 106 sminus1 [80] so radiative decay willdominate but ISC within the CT state is higher than this andso someCT1will cross toCT3Thus 25 singlet yieldmust beconsidered the theoretical maximum and in practice a lowersinglet yield would be found Furthermore inmost materialsthe CT states are lower in energy than the 120587 states so can notform ldquosinglet statesrdquo

3 Experimental Observations

The majority of early 119878 119879 ratio measurements on polymericsemiconductors were based on photophysical measurementson the emissive materials not devices and have givenratios ranging from 33 to 62 depending on the polymerbackbone structure [55 56 65] The validity of assumptionsmade or estimated absolute values of many quantities that


are difficult to obtain are questionable for example thebasis of the magnetic resonance measurements used byWohlgenannt et al [55] which initially attracted consider-able attention have recently been seriously challenged [8182] The nature of excited state species and interactionsthat optically detected magnetic resonance (ODMR) andphotoluminescence detected magnetic resonance (PLDMR)actually detect is not clear For example Segal et al [8283] have proposed the importance of PL quenching arisingfrom singlet (and triplet) exciton polaron interactions inthese polymers a process that we have shown to be efficientin working PLED devices [84 85] Further studies havetried to determine the relative singlet to triplet yield byemploying emissive (phosphorescent) acceptors [60 86]These however are not measurements on the pristine poly-mer and uncertainties remain as to what extent the inter-system-crossing yield 120581ISC in such doped systems is still ameaningful constant [87] as it has been shown that the heavyatom dopants seriously perturb the spin orbit coupling ofthe polymer backbone greatly enhancing the polymer ISCrate Furthermore the implied equivalence of optically andelectrically excited excitons in these experiments may nothold true [88] given the fact that common phosphorescentacceptors are electron traps and thus act as recombinationcentres only during electrical excitation [89] Apart fromsuch indirect materials approaches experiments that directlyprobe the number of singlet and triplet excitons formed fromcharge carrier recombination are generally more convincingand transparent [90] Attempts to determine absolute valuesare certainly unrealistic [56 91 92] given the experimentaldifficulties in collecting all the emission from a deviceespecially waveguided light and the difficulty in measuringthe true current which yields light generation rather thanIR losses in the ITO and capacitance effects are unknownInstead relative measurements are more appropriate meth-ods and the general approach we have taken is outlined asfollows

Generally we can define two parameters 119878 and 119879 whichrepresent the experimentally measurable signals that areproportional to the singlet and triplet generation rates andeach 119888

119894denotes appropriate constant of proportionality to

account for the collected signal that is representing theexperimental collection cone The superscripts el and optdistinguish electrical and optical excitation Given that acertain electrical (optical) excitation causes exciton forma-tion 119868 then the notionally observable signals are givenas

119878opt

= 119888opt119878119868opt119878

(1 minus 120581ISC)

119879opt

= 119888opt119879119868opt119879120581ISC

119878el= 119888

el119878119868el119878120594 (1 minus 120581

ISC)

119879el= 119888

el119879119868el119879(1 minus 120594 (1 minus 120581

ISC))

(1)

with 120594 being the singlet generation yield By using the sameexcitation conditions for the singlet and triplet measure-ments that is 119868opt

119878= 119868

opt119879

and 119868el119878= 119868

el119879 one can measure 119879

relative to 119878

119879opt

119878opt =

119888opt119879

119888opt119878

120581ISC

(1 minus 120581ISC)

119879el

119878el =

119888el119879

119888el119878

1 minus 120594 (1 minus 120581ISC)

120594 (1 minus 120581ISC)

(2)

In this case neither the driving current including darkcurrents nor the optical excitation dose (intensity actuallyabsorbed by the semiconductor) need to be known Fur-thermore if 119878opt (or 119879opt resp) is probed under the sameexperimental conditions (geometry) as 119878

el (or 119879el) then

119888opt119878

= 119888el119878(119888

opt119879

= 119888el119879) and one measures the electrically excited

signals relative to the corresponding optical ones

119879el119878

el

119879opt119878

opt =1 minus 120594 (1 minus 120581

ISC)

120594120581ISC (3)

Here the absolute value of only one parameter theintersystem-crossing yield 120581ISC is required in order to cal-culate the singlet formation yield 120594 Equation (3) is true forany optical excitation combined with any electrical one aslong as (i) all signals depend linearly on excitation (ii) thecorresponding 119879 and 119878 signals are excited the same way and(iii) the corresponding el and opt signals are measured thesame way The latter two points are satisfied using a singleexperimental optical layout as shown schematically for ourexperiment in Figure 1This also removes complications withcollection of emitted light for example the light outcouplingefficiency is the same for all measurements [93] Also thiscondition accounts for all exciton quenching mechanism aslong as they apply in the same way for optically and electri-cally excited excitons For example quenching at the anodeor impurity sites including the well-known (photooxidative)keto defect [94 95] It is known that keto defects act as chargetraps for electrical excitation but this is still not a problemsince it reduces the singlet and triplet density proportionallysimilar to a dark current

By way of example we have studied polyspirobifluorenein detail this was synthesized by Merck OLED GmbH[96] Spirofluorene derivatives are chemically inert againstbackbone oxidation which otherwise causes the formation ofketo defects [94 95] State-of-the-art diodes were fabricatedat Philips Laboratories Eindhoven using ITO and BaAl aselectrode materials A hermetically sealed metal cap wasapplied as well in order to protect the cathode fromoxidationdetails are given in [97] All measurements were performed at20K using an optically and electrically accessible closed cyclehelium cryostatThe triplet population was determined usingtriplet transient absorption as a function of the time duringa one millisecond excitation pulse Without any alternationsin the geometry of the spectrometer the latter could eitherbe electrically applied using a 100W current pulse generatoror optically using a 405 nm laser diode A 780 nmprobe beam


1 GHz digitaloscilloscope

plus PC

Trip

let s

igna

l

signlet signalTrigger

Optical excitation

Electricpulse generator

10 ns 1 s 2A

Electrical excitationPhotodiode

with 20 MHztransimpedance

amplifierNDfilterwheel

Laser diodemodule 24 mW

400 nm

Dispersivelens


780 nm

Focuslens filter 720 nm

Sample insidedisplex heliumcryostat

min 15 K

filter 435 nm

Photodiodewith 20 MHz

transimpedanceamplifier

Focuslens

Cutoff

Cutoff

Figure 1 Diagrammatic optical layout of the experiment used to measure the singlet generation yield from an OLED Simultaneousmeasurement of fluorescence electroluminescence and triplet-induced absorption both through optical and electrical excitation of thedevice are made whilst operating at 20K or below

0 2 4 6

0

002

004

006

008

Time (ms)

opticalexcitation

Electricalexcitation

0075

0029

0046

Fluo

resc

ence

inte

nsity

(V)

Figure 2 Demonstration of the additive nature of the simultaneous signals and lack of excitation-dependent quenching mechanisms forthese measurements Here the optical electrical and combined excitations of fluorescence from a polyspirobifluorene device are compared

was focused onto the active area of the device reflected by thecathode passed through an appropriate cutoff filter detectedby a 200MHz transimpedance amplifier and monitored bya 1GHz Oscilloscope Up to 1000 excitations were averagedfor a single dataset with a repetition frequency of only 03Hzin order to allow for sufficient triplet decay between theexcitation pulses The fluorescence level was simultaneouslyobserved using a second photodiode

Equation (3) holds for any pair of signals 119879 and 119878 aslong as both depend linearly on excitation dose Fluores-cence and electroluminescence intensities obviously satisfythese requirements For both kinds of excitation (opticalor electrical) these signals are truly time-independent anddirectly proportional to the singlet generation rates at normalexcitation densities only at high laser fluences do nonlineareffects start to emerge [98] This can be seen in Figure 2

where the individual optically and electrically excited signalsperfectly add up to the simultaneously excited one that is theoptically excited fluorescence contribution in the presenceof the electrical excitation is unchanged This also showsthat for singlet excitons quenching by the electric field[99] or by polarons [100] is negligible under the conditionsused in our experiments For the triplet signal either triplettransient absorption [90 101] or phosphorescence [85] couldbe used However both require large excitation densities toyield appropriate signal-to-noise ratios which also causesmigration activated TTA a major quenching channel [102103] The situation becomes (relatively) worse at highertemperature because triplet mobility increasesmdashat roomtemperature triplet excitons decay essentially only by TTAwithout any phosphorescence emission [49 104] Given thatboth emission and transient absorption detection are not


12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C

Figure 3 Spectroscopic properties of polyspirobifluorene indicating the optical features used in the measurement of the singlet generationratios Absorption (a) electroluminescence (b) electrophosphorescence (c) and transient triplet absorption spectra (d) The vertical linesindicate the energies of the optical excitation and the transient absorption probe respectively and the inset shows the repeat unit of thepolyspirobifluorene

ideal measurements for the triplet population we still haveto rely on them because there are no convincing alternatives

Basic optical properties of the blue-emitting polyspirobi-fluorene polymer see inset of Figure 3 for chemical structure[96] are shown in Figure 3 All experiments were performedat 20K as this reduces the triplet migration to quenchingsites but does prevent TTA [103] The time-dependent tripletpopulation density was monitored by its transient absorption[101] and Figure 4 shows two typical datasets for opticaland electrical excitation Here we observe the build-in ofthe triplet excitons as a function of time during a onemillisecond electrical excitation pulse TTA gives rise toboth the saturation of the induced absorption signal and therapid decay of the triplet density in the absence of excitationcompared to the long radiative lifetime of the triplet excitonsim1 s [102 103]Therefore the observed data correspond to theaccumulated (integrated) triplet density limited only byTTAFitting these curves then yields the triplet generation rate andpopulations The starting point for this is the rate equationfor the triplet accumulation As the triplet exciton lifetimesim1 s within the time frame of the measurements (1ms)we can ignore monomolecular decay (phosphorescence) Ifunder certain continuous excitation conditions the tripletgeneration rate is 119868

119879then the measured signal intensity given

by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879

being the TTA constantA further important consideration that also has to be

taken into account both for electrical and optical excitationsis the fact that the excitons are not homogeneously formedthroughout the organic semiconductor This is obvious for

optical excitation but also for electrical excitation the dissim-ilar mobility of the charge carriers creates a recombinationzone near to the electrode that injects the slower movingcharges [105] Given that the triplet excitons are nearly immo-bile at 10 K [103] they cannot rapidly compensate for thisinhomogeneity by migration and so the triplet populationdensity is far higher within this narrow recombination zoneThis has an immediate effect on TTA as the TTA rate dependson the triplet exciton density and so forming them withina very thin recombination zone in the active material willgive much higher TTA rates than one would expect for thesame triplet population distributed throughout the whole ofthe organic semiconductor The narrow recombination zonepersists even at room temperature and so this picture holdsat elevated temperatures as well Modelling of such simplepolymer devices shows that both the recombination zoneand the optical absorption occur in similar positions closeto the anode This means that the optical cavity effects onemission from the device structure are very similar for bothtypes of excitationmechanismTherefore we account for thisinhomogeneous excitation profile using a simple exponentialexcitation profiles with characteristic inverse thickness ofrecombination zone 120582OPT and 120582el respectively

119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886

ln cosh(119905radic119868119879119886) with 119886 = 120574

119879119879120582

(5)

This model only relies on two free fitting parameters1119888119879119886 and 119868

119879119886 with 119868

119879119888119879

= 119889119879119889119905 for 119905 = 0 being thedesired quantity Indeed accounting for the inhomogeneousexciton generation results in a much improved fit of theexperimental data which can only really be appreciated in thesemilogarithmical presentation of the inset of Figure 4 Theslope obtained according to (5) is shown as a dashed line Wehave also used this to give a more detailed analysis the charge


0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)

Figure 4 Transient triplet absorption data set measured during and after a 1ms optical (black) and electrical (green) electrical pulse as afunction of time The red and blue solid lines are least square fits according to (5) the cyan line according to (4) The dashed lines indicatethe slopes obtained for this particular datasets In the inset the same data are replotted with a logarithmic time scale

10minus2

10minus1

100

101

102

103

10minus3 10minus2 10minus1 100 101

ElectricalOptical

Figure 5 Dependency of the rise of the transient triplet absorption signal (119879) on its corresponding fluorescence level (119878) for electrical andoptical excitation on a double logarithmical scale The two solid lines are least square fits of the offset of a strictly linear dependency Data aregiven in volts from the original measurement from the oscilloscope

carrier recombination layer 120582el in working devices showingthat the recombination zone is only some 5ndash10 nm thick [101]Slopes were then measured for the optically electrically andsimultaneously excited transient triplet absorption signalsFor small driving currents we find that the individual slopesdo approximately add up to the simultaneous signal (as wefound for the singlet signals) which implies that the tripletpopulation is also not significantly affected by polaron or theelectric field quenching under our experimental conditionsThe absence of electric field quenching has also been recentlyshown by us using an alternative method [106]

In Figure 5 the experimentally measured triplet signalsunder various excitation intensities are plotted versus theircorresponding singlet levels for both excitation modes Theplots are limited by the maximum laser excitation powersand device drive voltages that can be used The graph showsthat at the same singlet density about ten times more tripletsare generated for electrical as compared to optical excitationCritically both datasets arewell described by a linear functionwith slope of +1 in a double logarithmical plot which impliesthat both electrically and optically excited triplet densi-ties depend linearly on the corresponding singlet densities


Thus the singlet generation yield is a true constant and incontrast to previous reports [90 91] we find no electric fielddependence with the drive voltages we have used From theseplots both 119879el

119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn

00002 for electrical and optical excitations respectively andconsequently the elopt ratio can be obtained 119 plusmn 08

In order to obtain an absolute singlet generation yieldwe need to know the absolute triplet formation yield foroptical excitation importantly under the same experimentalconditions that is low temperature and solid state We havealso developed a methodology for measuring the ISC yieldin thin films by observation of the ground-state recoveryof the photobleaching [80 107] Using this method withsubpicosecond time resolution and very low excitation dosesa yield of 120581ISC = 012 plusmn 002 is found for polyspirobifluorenein solid state at low temperature which compares verywell with the fluorescence emission quantum yield of thepolyspirobifluorene in solution 082 plusmn 003 Given this (3)yields an absolute singlet formation yield of 120594 = 044 plusmn 004It is clear that by successfully accounting for many possibleexperimental nonlinearities we still observe higher singletexciton production than predicted by spin-independentcharge recombination Following a very similarmethodologyWallikewitz et al [108] also find similar singlet generationratios in other luminescent polymers The question thusarises do these results verify that charge recombination isspin dependent or are singlet excitons being generated insubsequent processes after the recombination step which givethe false impression that more than 25 singlets are createddirectly from recombination

To further expand on these results we have made a seriesof measurements on different polymers and as a function oftemperature In Figure 6 is shown temperature-dependentresults from the polymer poly(991015840-dioctylfluorene-co-bis-NN1015840-(4-butylphenyl)-bis-NN1015840-phenyl-14-phenylenediam-ine) (95 5mol) (F8-PFB random copolymer) To accu-rately calculate the singlet yield from devices as a function oftemperature the effects on device performance as a functionof temperature were first carefully characterized and theelectroluminescence output normalized with respect todrive current as a function of temperature Clearly it canbe seen that at low temperature 130 K one measures a farhigher triplet population than at 250K in the quasisteadystate as determined from the triplet induced absorptionfrom a device More importantly a key measurement isthe observation of the effect of temperature on the ldquodelayelectroluminescence (DE)rdquo from the devices As firstshown by Sinha et al there is a considerable long-livedemission from devices after the drive current is turned off[85 109] Spectrally resolving this DE shows that it has thesame spectrum as the delayed emission seen with opticalexcitation arising from TF As a function of temperatureit is found that this DE is very strong and long lived at130K but at room temperature it is quenched very rapidlyThis correlates with the temperature-dependent tripletpopulation measured in the devices (Figure 6) This generalbehaviour can be directly attributed to the temperaturedependence of triplet exciton migration [103 104 110]

at high temperature the triplets can rapidly diffuse out of therecombination layer reducing triplet density and also findquenching sites before meeting another triplet to annihilatewith As will be shown in detail from these results it is clearthat the singlet yield will include a contribution from TFand that this will be strongly temperature dependent arisingfrom the temperature dependence of TTA not the chargerecombination mechanism In Figure 7 is given a graph ofthe measured temperature dependence of the singlet yieldfor F8-PFB-based devices At low temperatures we finda very high singlet yield which decreases as temperatureincreases and approaches ca 035 at room temperatureKondakov et al [30 63] have reported similar findings fromsmall-molecule-based devices again a strong DE signal isobserved indicating a large contribution from TF to theoverall singlet population in devices which clearly break the25 singlet generation rule Given that the recombinationprocess does only produce 25 singlets the results of Baldoet al [60] can be reconciled with those of Kondakov et aland ourselves

4 Is Charge RecombinationDependent on Spin

Theevidence given above clearly indicates that TF contributesto the electroluminescence yield but by how much and doesthis fit with the classical spin statistical production rate ofsinglets from TTA In collaboration with Cambridge DisplayTechnology we have combined experimental measurementwith detailed device modelling to put a quantitative measureon the contribution of TF to overall EL yieldWe find at roomtemperature a voltage-dependent (max 035) TF contributionto the total EL signal [111] (Figure 8) We also show thatas devices age it is the TF contribution which is quenchedcausing the initial rapid reduction of EL yield before astable plateau is reached when only the 025 singlets directlyproduced by recombination contribute to EL Some questionshave been raised about the outcoupling ratios assumed byKondakov when calculating internal quantum efficiency butcertainly their small-molecule devices must be producingmuch more than 025 singlets as well

5 Historical Perspective on TTA

The first observations of delayed fluorescence from triplet-triplet annihilation in organic conjugated hydrocarbon solu-tions (including anthracene) were reported by Parker andHatchard [23 112] Soon after TF was also observed inmolecular crystals of anthracene by Kepler et al [22] Jort-ner et al developed a theory of singlet production duringTTA using simple quantum mechanical spin statistics oftwo uncorrelated interacting triplet excitons to form anintermediate pair state postulated that nine possible spinconfigurations could result [113] as shown in Scheme 1 Asa result of annihilation one excited singlet state is formedgiving rise to delayed fluorescence for every 18 annihilatingtriplet excitons (9 pairs) giving a maximum singlet yield of01 (accounting for triplet recycling that is the triplet statesformed during TTA go through further annihilation steps


300 250 200 150 100 505

10

15

20

25

30

Temperature (K) Temperature (K)

Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K

Triplet decay after electrical excitationprobe808 nm

(b)

Figure 6 Data collected for the determination of the temperature dependence of the singlet generation yield (a) Show how the electricalcharacteristics of the devices change as a function of temperature which enables the optical output from the devices to be scaled to take intoaccount changes in the electrical characteristics (b) Show how the triplet-population (measured by the triplet induced absorption) changesstrongly with temperature and also how the delayed electroluminescence is greatly quenched at high temperatures

until all triplets are depleted) Experimentally this purelytheoretically scheme was questioned mainly in the natureand decay channels of the quintuplet states One can thinkof the intermediate pair (or encounter complex) as a highlyexcited excimer of various spin multiplicities in the ratio1 3 5 In this view Birks explained delayed fluorescencefrom pyrene solutions [24 114] as the result of the followingchannels 5(AA)lowastlowast rarr

5(AA)lowast rarr1(AA)lowast and (3Alowast + 1Alowast)

the latter by a temperature-dependent disproportionation3(AA)lowastlowast rarr 1Alowast + 1A and 1(AA)lowastlowast rarr 1(AA)lowast where (AA)represents an excimer Thus assuming the quintuplet yields 3triplet states to 2 singlet states then some sim40 of tripletsfuse yielding singlet states (note any triplet produced cango on to annihilate again until all triplets are used up giventheir very long monomolecular lifetime) Saltiel et al [115116] made in-depth kinetic studies of TTA and proposed amodification of Birksrsquo picture whereby 1(AA)lowastlowast rarr 1(AA)lowast3(AA)lowastlowast rarr

3Alowast + 1A 5(AA)lowastlowast rarr 110 (1Alowast + 1A) + 910

(3Alowast + 3Alowast) Current work on the opposite process to TFsinglet fission (a singlet forming two triplets) which couldbe very important in solar cells generating two excitons perphoton to maximise charge production seems to indicatethat a coherent superposition of singlet and triplet pairexcited state wavefunctions are initially photocreated when119879119899

cong 1198781such as in the case of tetracene and rubrene

Spin dipole-dipole interactions may then be responsible forforming the (TT) intermediate pair which can again bethought of as an ldquoexcimerrdquo Smith and Michl have writtenan excellent in-depth review on this subject [117] A moresimple and elegant argument though is that the quintets inmost systems energetically cannot be created as the highenergy quintuplet state cannot be formed with only twice theavailable triplet energy [30 32] For C

60 it has been calculated

that a quintet state would physically break a CndashC bond [118]and calculations for DPA diphenylanthracene indicate thatthe quintet state is too energetic to form from two triplets


1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020

Figure 7 Calculated singlet yield for F8-PFB devices measured as a function of temperatureThe two dashed red vertical lines give the upperand lower limits on our best determinations of the ISC yield of this polymer Clearly it is seen that the singlet yield depend strongly ontemperature At room temperature the yield is ca 033 still appreciably above the 025 limit of spin-independent recombination

0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960

Figure 8 Delayed electroluminescence after glow as a function of device ageing showing both the 35 contribution of delayed fluorescenceto the total EL output and the loss of the DF with device age 119879

60implies that the devices has been run until its output has fallen to 60 of its

initial light output

[21] in this scenario we would thus gain 02 singlets fromTF as shown in Scheme 1 From Kondakov et alrsquos work onhighly efficient fluorescent OLEDs using devices based onanthracene derivatives such as DPA as a host for blue flu-orescence dyes (gt095 quantum yield) sandwiched betweenhole and electron transport layers very simple devices theydemonstrated better than 8 external quantum efficiencyfor these devices Calculating back this implies an internalquantum yield of gt04 way beyond the theoretical spin-independent recombination limit of 025 singlet generationFrom studies of the DE from these devices they concludethat TF is contributing strongly (asymp02) to the total singletyield in these devices However Kondakov et al has made

further claims of devices that exceed this value indicatingthat TF in DPA approach levels of singlet generation thatexceeds the 02 TTA singlet yield [30] How is it possible toachieve higher singlet production yields than 02 from TTAThe energy diagram in Figure 9 shows schematically how thiscould be possible on the right-hand sidewe have the situationwhere 2119864

1198791

gt 119864119879119899

and 21198641198791

gt 1198781 in which case TTA can

produce both 1198781and 119879

119899excited states On the right we have a

slightly different scenario now 21198641198791

gt 1198781but 2119864

1198791

lt 119864119879119899

inthis case it is now energetically not possible to form 119879

119899states

only 1198781states giving a 05 singlet yield In this case we could

have in the best case a total ELQY= 025 + (075times 05) = 0625with TF contributing 57 to the total EL This then reaches


Triplet recycling

Singlet per tripletyield


01 02

EL yield 025 + (075 01) = 0325or DF contributes 23 of the total EL


3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1

Scheme 1 TTA decay channels

into the realm of phosphorescent-based devices but froma fluorescent emitter gaining all the benefits a fluorescentemitter gives to an OLED especially in the blue Thereforethere is a huge potential step change in OLED performanceespecially for blue emission in finding fluorescent emittersthat fulfil this latter criterion

In an attempt to reach this goal Zhang and Forrest haverevisited the potential of rubrene as an emitter which mightyield 05 TF yield [33] Rubrene has been studied for manyyears as an emissive material [34] however it has an ill-defined triplet energy in solid state as no phosphorescencehas been observed the long-time-delayed emission is dom-inated by DF and so triplet energies are only known frompulsed radiolysis energy transfer measurements [119] withan error of order plusmn02 eV These measurements are madein solution and so gauging the degree to which the tripletenergy relaxes in solid state is difficult [120] therefore insolid state the error on the triplet energy has to be of orderplusmn02ndash04 eV Thus it is not clear if 2119864

1198791

= 1198781in the solid

state yielding resonant singlet fission (SF) [121] or if 21198641198791

lt

119864119879119899

such that TTA can only proceed via the singlet decaychannel to give 05 TF yield Zhang like Kondakov before [30]used a highly fluorescent dopant (1) in their rubrene deviceswhich is populated by Forster transfer from the rubrene inan effort to avoid rubrene quenching by SF These devicesgive 67 EQE at low drive current clearly breaking the 25singlet generation rule At higher currents severe roll-off isobserved which the authors attribute to onset of efficientsinglet triplet annihilation (STA) [122] However they alsosee the emergence of weak rubrene emission at high currentswhich suggests saturation of the dopant emitters [123] whichmay be the cause of the excessive STA STA can be preventedby ensuring that Forster transfer from 119878

1to 1198791(causing 119879

1

to 119879119899absorption) cannot occur A further complication with

rubrene comes from the fact that 21198641198791

asymp 1198781causing the SF If

SF outcompetes the FRET to the fluorescent dopant deviceefficiency will be reduced Because of SF the efficiency ofpure rubrene devices is very poor If the dopant sites saturate(especially at low doping levels) then SF will become aneffective quenchingmechanism thus there aremany possible

causes of loss of efficiency at high currents as shown inthis work The design rules for emitters for high TF devicesbecome complexes when aiming for 05 TF yield because ofthese problems associated with SF and STA

Further support for this picture of device operation andthe key role of TTA in enhancing the electroluminescent yieldwas given by Iwasaki et al who have made an interestingobservation on the role of triplet triplet annihilation [124]From magnetic field dependencies of the TTA process theyconclude that in fact TTAcontributes substantially to the finalsinglet yield We have also shown that at room temperaturethe typical triplet exciton lifetime reduces to below 250 nscompared to gt1 s at 10 K [125] this in part is due to TTA butalso other nonradiative pathways may becomemore efficientthis gives rise to the substantial temperature dependence ofthe ELQY we find in fluorescence devices as discussed above

It is thus clear from this body of work that TTA via TFcontributes substantially to device efficiency and given opti-mal materials properties fluorescent devices having 625internal efficiency are theoretically possible For this werequire materials with triplet energies such that 2119864

1198791

lt

119864119879119899

with all triplets contributing to TTA so reducing tripletmobility as much as possible by confinement with excitonblocking layers is important as is the possible prevention ofsinglet fission by ensuring 2119864

1198791

= 1198781 Last the photophysics

of singlet triplet annihilation needs to be understood moreclearly in order to design systems and device architecturesthat minimize this loss mechanism

6 Alternative Ways to Produce SingletExcitons from Triplet States

There are several known mechanisms for generating singletemission long after all initially formed singlet states havedecayed Emission which involves triplet states can be cate-gorised as either ldquoP-typerdquo or ldquoE-typerdquo emission The formeris singlet emission generated as a result of triplet fusionas discussed above E-type emission (or eosin-type) is avery different process being a thermally activated long-lived


2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )

Current density (mA cmminus2)

(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)

Figure 9 PBD TPBI (50 50 blend emitter layer) exciplex OLED device characteristics EQE versus voltage (a) current efficiency versuscurrent density (b) power efficiency versus voltage (c) and current density versus voltage (d)

emission first observed by Perrin in 1929 [126] It was notuntil Lewis and Kasha identified the triplet states of organicmolecules that it was realised that the activation causedbackcrossing of triplet states into the singlet manifold that isa process of thermal activation of a triplet state to a higherlying vibronic state followed by ISC to a resonant singletvibronic state [127]This state can then decay radiatively againwith the normal fluorescence quantum yield Since this isan activated process the rate is determined by a Boltzmann-type energy barrier proportional to exp(minusΔ119864ST119896119879) whereΔ119864ST is the singlet triplet energy gap and the ISC rateThus as Δ119864ST decreases the rate of back transfer approachesthe ISC rate Given that for most materials the singletfluorescence lifetime is many orders of magnitude shorterthan the triplet lifetime most triplet excitons will decayvia the singlet channel as long as there is enough thermalactivation even with many recycling steps between singletand triplet manifold E-type emission also yields a commonlifetime for both delayed emission and phosphorescencewhere 119896TA is the rate of thermally activated ISC Hence at lowtemperature phosphorescence dominates (radiative) tripletdecay but at high temperature delayed singlet emission dom-inates Theoretical modelling of the phosphine-supportedCu2(120583-NAr

2)2diamond core complex of Deaton et al [128

129] shows a very small exchange splitting of 750 cmminus1 andclearly identifies the HOMO on the Cu

2ndashN2core with the

LUMO on the peripheral aryl bringing units The lowestexcited state is found to comprise gt90 of this HOMO-LUMO transition and hence nearly pure CTThis then yieldsa very small 2-electron exchange integral and the very lowsinglet triplet splitting required [130 131] In terms of devicesmade form E-type emitters one needs to have a high totalluminescence quantum yield and a small Δ119864STThe diamondcore complex reported by Deaton yields green devices withexternal quantum efficiency of 161 close to that obtainablewith Ir-based phosphors [128] Further there are also reportsof all organic emitters which have low Δ119864ST and appreciableE-type contribution [132] Endo et al have reported devicesbased on this mechanisms giving 53 EQE in the bluegreen[133 134] Moreover the key advantage here is increasedstability of the (rigid) emitter giving better device lifetimeand more importantly potentially easier ways to obtain bluesince all the blue light is generated from the singlet state notthe triplet This avoids emitters with large HOMO-LUMOgaps which cause problems with electron or hole blocking(into the recombination layer) so fewer layers are required tomatch the emitter levels This makes device design simplerrequiring fewer layers increasing yield and reducing cost


Negligible Stokes losses also improve device power efficiencybecause electrical energy is not wasted converting singlets totriplets as in phosphorescence losing some 03 to 07 V Thisis very important for maintaining high lmW values Thususing E-type DF hasmany advantages over phosphorescencebut requires the special criterion of very small singlet tripletgap but most importantly this can give devices with atheoretical yield of 100 This triplet harvesting paradigm isessentially unexplored in OLED research but can give a realstep change in both performance and lifetime

Endorsquos original work used an emitter that had stronginternal charge transfer character to produce very low elec-tron exchange energy the natural extension of this is theintermolecular exciplex An exciplex is an intermolecularcharge-transfer state formed under photo- or electrical exci-tation by the interaction of an electron donor (D) andan electron acceptor (A) [135] The wavelength of exciplexemission depends on the ionisation potential and electronaffinity of the donor and acceptor molecules respectivelyThere is literature dating back to the early days of OLEDresearch discussing the pros but mostly the cons of exci-plexes [136] This early work focused on exciplexes formedunintentionally at the interface between a transport layer andthe emitter layer usually seen only in electroluminescence(EL) and not in photoluminescence (PL) The first reportof interfacial exciplex emission was in 1998 by Itano et al[137] and then in a blended exciplex device by Cocchi et al[138] The latter devices were inefficient as they incorporatedthe emitter molecules in a polycarbonate matrix and theexciplex had low photoluminescence quantum yield (PLQY)(ΦPL 17) The authors clearly described exciplex evolutionfrom a tightly bound |DAgtlowast exciplex to an ionic |D+Aminus gtlowastion pair and the effect of Coulomb relaxation which yieldslarge red shifts thereby explaining the previously observedldquoelectroplexrdquo [139] Palilis et al [140] were the first to reporttrue blend devices using an exciplex system of high PLQY(ΦPL 62) between a triarylamine hole transporter (the Dunit) and a highly fluorescent (ΦPL 85) silole-based emitterand electron transporter (the A unit) Devices with externalquantumefficiency (EQE) of 34were reported which at thetimewas excellentThese results clearly show that it is possibleto engineer exciplexes with strong ground-state coupling andthus high luminescence efficiency

Usually E-type emission is an inefficient process as mostmaterials have large electron exchange energies Frederichsand Staerk [141] were the first to show experimentally theassertion of Beens and Weller [142] that thermally assistedISC from an exciplex triplet to singlet manifold can occurand that certain exciplexes have very small exchange energies(lt01 eV) with clear E-type emission from the exciplex Theyalso showed the importance of coupling to theD ground stateto achieve high luminescence yields These solution studiesalso showed the role of environment polarity in stabilising thedegree of charge separation in the exciplex Only for highlypolar environments is the radical ion pair stabilised Giventhat in the solid state the environment will be only weaklyor moderately polar there will be a driving force to stabilisethe more excitonic-like |DAgtlowast exciplex rather than the fullion pair |D+Aminus gtlowast This will have the benefits of enhancing

both the ground-state coupling and the luminescence yieldsand will limit the electric field quenching of the exciplex inthe device The importance in the context of OLEDs of thefact that in principle exciplexes can have vanishingly smallexchange energies [143] was first highlighted by Cocchi et alin 2006 [144] who discussed the possibilities of electrophos-phorescence from exciplexes However their system (donorTPD acceptor BCP in a polycarbonate matrix) has a ratherlarge singlet-triplet gap Δ119864 119888119886 04 eV More interestinghowever is the fact that in this system the electron-hole pairenergy 119864

ℎ= 119868119863minus 119860119860 is less than that required to form

an excited donor singlet However as shown by Morteaniet al [145] direct injection into the exciplex is possible andthis gives the benefit of low drive voltages which is a criticalfinding

The first report of an intentional exciplex-based devicegiving E-type exciplex emission was by Goushi et al in2012 [146] The donor molecule is a triarylamine and theacceptor a triarylborane derivative these were coevaporatedin a 50 50 blend emission layer OLEDs with EQE of 54were realised from an exciplex system having an PLQY ofonly 26 indicating that far more than 25 singlets werebeing generated in the device Subsequently Goushi hasreported a device giving up to 10 EQE 47 lmWminus1 for greenemission [147] The device structures in both types of deviceare extremely simple consisting of only three organic layersan HTL of the donor the 50 50 emitter layer and an ETLlayer consisting of the acceptor A very important addedbenefit of such a very simple device structure is that it gives avery low working voltages ca 25 V This is vitally importantfor high luminance efficacy lighting and good compatibilitywith CMOS backplanes in mobile devices The high EQEand luminance power efficiencies derive from the efficientdirect electron-hole capture at the exciplex There are novoltage drops associated with charge injection and transportthrough additional layers and the usual necessity of forcingthe electron and hole onto a single molecular emitter siteis overcome [145] Thus E-type exciplex devices have manyadvantages over phosphorescence based devices notably avery simple device structure (two materials in three layers)and very high power efficiency In Figure 9 results froma deep blue exciplex device (structure given in Figure 10)based on PBD (D) and TPBI (A) show far higher than 25total singlet production in electroluminescenceThe exciplexhas a PLQY of 26 and EQE of 26 (unoptimized) at450 nm For this low PLQY of the emitter the theoreticalmaximumdevice EQE is 13 [148] However in this exciplexsystem the D (NPB) has a triplet level [149] lower than thatof the exciplex triplet which we believe strongly quenchesthe exciplex triplet state but the subsequent high tripletpopulation gives rise to strong TF which enhances the deviceefficiency This quenching route can be overcome by carefulmaterials design making sure that both the D and A havehigher triplet levels than the exciplex Bittner et al [150]calculated for a mixed TFBF8BT monomer system that thisbacktransfer mechanism is possible however they assumedthermally activated singlet transfer in line with much of thework from Morteani et al [151] on exciton regeneration at


LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex

Donor (D) Acceptor (D)

Figure 10 Schematic energy level diagram for a simple low turn on volatge exciplex OLED Comparing to the device used in Figure 9 thedonor layer is PBD the acceptor layer is TPBI and the emitter layer a 50 50 blend of NPB and TPBI Here the NPB is a good hole transportlayer and TPBI is a good electron transport layer Layers are deposited by vacuum sublimation

interfaces This behaviour can also readily be explained asbacktransfer via the triplet channel and that the regeneratedexcitons arise from TF within theDmanifold

Polymer-based exciplex systems have been reported [152153] but they have an extra complexity in that many exciplexsystems give rise to efficient charge production as usedin photovoltaic applications As opposed to the polymericTFBF8BT system which can yield OLEDs with gt19 lmWminus1PFBF8BT (PFB has one extra phenylamine unit per polymerrepeat unit than TFB) efficiently forms charge pairs with noemission but also has a high triplet exciton yield [54] Thiscould indicate differences in exciplex stabilisation if an ionpair is the initial species created by photoexcitation as field-dependent quenching would suggest [151] The result impliesrapid singlet exciplex ionisation as well as quenching of thetriplet exciplex to the triplet state of the PFB The PFBF8BTexciplex has the larger charge separation radius leading togreater ion pair character in the exciplex

E-type emission thus offers great potential for highlypower-efficient fluorescent OLEDs ideal for both lightingapplications and micro-OLEDs fabricated on CMOS chipsCompared to devices exploiting TF E-type emitters offera theoretical maximum 100 internal efficiency withoutthe drawback of requiring a high triplet host material theproblem which bedevils high efficiency blue phosphorescentemitters Moreover they allow very simple device architec-ture to be used whilst maintaining extremely high opticaland electrical efficiency This is very important for large-area lighting applications and the use of solution processingldquoExciplex blendrdquo devices thus opens a new chapter in OLEDdevices one which has great promise for many applicationsrequiring large area or high yield fabrication

References

[1] G He C Rothe S Murano A Werner O Zeika and J Birn-stock ldquoWhite stacked OLED with 38 lmW and 100000-hourlifetime at 1000 cdm 2 for display and lighting applicationsrdquoJournal of the Society for Information Display vol 17 no 2 pp159ndash165 2009

[2] N Ide H Tsuji N Ito Y Matsuhisa S Houzumi and TNishimori ldquoWhite OLED devices and processes for lightingapplicationsrdquo in Organic Photonics Iv P L Heremans RCoehoorn and C Adachi Eds vol 7722 Spie-Int Soc OpticalEngineering Bellingham Wash USA 2010

[3] Y S Tyan Y Q Rao X F Ren et al Tandem Hybrid WhiteOLED Devices With Improved Light Extraction CampbellSociety For Information Display 2009

[4] C Adachi R C Kwong P Djurovich et al ldquoEndothermicenergy transfer amechanism for generating very efficient high-energy phosphorescent emission in organic materialsrdquo AppliedPhysics Letters vol 79 no 13 pp 2082ndash2084 2001

[5] V Sivasubramaniam F Brodkorb S Hanning et al ldquoFluorinecleavage of the light blue heteroleptic triplet emitter FIrpicrdquoJournal of Fluorine Chemistry vol 130 no 7 pp 640ndash649 2009

[6] V Sivasubramaniam F Brodkorb S Hanning et al ldquoInvesti-gation of FIrpic in PhOLEDs via LCMS techniquerdquo CentralEuropean Journal of Chemistry vol 7 no 4 pp 836ndash845 2009

[7] K T Kamtekar A P Monkman and M R Bryce ldquoRecentadvances in white organic light-emitting materials and devices(WOLEDS)rdquo Advanced Materials vol 22 no 5 pp 572ndash5822010

[8] A Van Dijken J J A M Bastiaansen N M M Kiggenet al ldquoCarbazole compounds as host materials for tripletemitters in organic light-emitting diodes polymer hosts forhigh-efficiency light-emitting diodesrdquo Journal of the AmericanChemical Society vol 126 no 24 pp 7718ndash7727 2004

[9] K Brunner A VanDijken H Borner J J AM Bastiaansen NM M Kiggen and B MW Langeveld ldquoCarbazole compoundsas host materials for triplet emitters in organic light-emittingdiodes tuning the HOMO level without influencing the tripletenergy in small moleculesrdquo Journal of the American ChemicalSociety vol 126 no 19 pp 6035ndash6042 2004

[10] Y C Chen G S Huang C C Hsiao and S A Chen ldquoHightriplet energy polymer as host for electrophosphorescence withhigh efficiencyrdquo Journal of the American Chemical Society vol128 no 26 pp 8549ndash8558 2006

[11] S O Jeon K S Yook C W Joo and J Y Lee ldquoHigh-efficiencydeep-blue-phosphorescent organic light-emitting diodes usinga phosphine oxide and a phosphine sulfide high-triplet-energy host material with bipolar charge-transport propertiesrdquoAdvanced Materials vol 22 no 16 pp 1872ndash1876 2010


[12] H Sasabe J I Takamatsu T Motoyama et al ldquoHigh-efficiencyblue and white organic light-emitting devices incorporating ablue iridium carbene complexrdquoAdvancedMaterials vol 22 no44 pp 5003ndash5007 2010

[13] H A Al-Attar G C Griffiths T N Moore et al ldquoHighly effi-cient solution-processed single-layer electrophosphorescentdiodes and the effect of molecular dipole momentrdquo AdvancedFunctional Materials vol 21 no 12 pp 2376ndash2382 2011

[14] H A Al-Attar and A P Monkman ldquoErratum solution pro-cessed multilayer polymer light-emitting diodes based on dif-ferent molecular weight host (Journal of Applied Physics (2011)109 (074516))rdquo Journal of Applied Physics vol 110 no 2 ArticleID 029905 2011

[15] N Tian D Lenkeit S Pelz et al ldquoScreening structure-propertycorrelations and device performance of Ir(III) complexes inmulti-layer PhOLEDsrdquo Dalton Transactions vol 40 pp 11629ndash11635 2011

[16] K S Yook and J Y Lee ldquoSolution processed multilayer deepblue and white phosphorescent organic light-emitting diodesusing an alcohol soluble bipolar host and phosphorescentdopant materialsrdquo Journal of Materials Chemistry vol 22 pp14546ndash14550 2012

[17] J S Kim RH Friend I Grizzi and JH Burroughes ldquoSpin-castthin semiconducting polymer interlayer for improving deviceefficiency of polymer light-emitting diodesrdquo Applied PhysicsLetters vol 87 no 2 pp 1ndash3 2005

[18] X Gong S Wang D Moses G C Bazan and A J HeegerldquoMultilayer polymer light-emitting diodes white-light emissionwith high efficiencyrdquo Advanced Materials vol 17 no 17 pp2053ndash2058 2005

[19] Y Sun N C Giebink H Kanno B Ma M E Thompson andS R Forrest ldquoManagement of singlet and triplet excitons forefficient white organic light-emitting devicesrdquo Nature vol 440no 7086 pp 908ndash912 2006

[20] S Reineke F Lindner G Schwartz et al ldquoWhite organic light-emitting diodes with fluorescent tube efficiencyrdquo Nature vol459 no 7244 pp 234ndash238 2009

[21] M E Kondakova J C Deaton T D Pawlik et al ldquoHighlyefficient fluorescent-phosphorescent triplet-harvesting hybridorganic light-emitting diodesrdquo Journal of Applied Physics vol107 no 1 Article ID 014515 2010

[22] R G Kepler J C Caris P Avakian and E Abramson ldquoTripletexcitons and delayed fluorescence in anthracene crystalsrdquo Phys-ical Review Letters vol 10 no 9 pp 400ndash402 1963

[23] C A Parker and C G Hatchard ldquoDelayed fluorescence fromsolutions of anthracene and phenanthrenerdquo in Proceedings ofthe Royal Society of London Series a-Mathematical and PhysicalSciences vol 269 p 574 1962

[24] J B Birks ldquoOn the delayed fluorescence of pyrene solutionsrdquoJournal of Physical Chemistry vol 67 no 10 pp 2199ndash22001963

[25] R P Groff R E Merrifield and P Avakian ldquoSinglet and tripletchannels for triplet-exciton fusion in anthracene crystalsrdquoChemical Physics Letters vol 5 no 3 pp 168ndash170 1970

[26] M Pope Geacinto Ne and F Vogel ldquoSinglet exciton fission andtriplet-triplet exciton fusion in crystalline tetracenerdquoMolecularCrystals and Liquid Crystals vol 6 p 83 1969

[27] J Jortner S A Rice J L Katz and S I L Choi ldquoTriplet excitonsin crystals of aromatic moleculesrdquo The Journal of ChemicalPhysics vol 42 no 1 pp 309ndash323 1965

[28] R S Knox and C E Swenberg ldquoDirect radiative Exciton-exciton annihilationrdquo The Journal of Chemical Physics vol 44no 7 pp 2577ndash2580 1966

[29] C E Swenberg ldquoTheory of triplet exciton annihilation inpolyacene crystalsrdquoThe Journal of Chemical Physics vol 51 no5 pp 1753ndash1764 1969

[30] D Y Kondakov T D Pawlik T K Hatwar and J P SpindlerldquoTriplet annihilation exceeding spin statistical limit in highlyefficient fluorescent organic light-emitting diodesrdquo Journal ofApplied Physics vol 106 no 12 Article ID 124510 2009

[31] B Dick ldquoAM1 and INDOS calculations on electronic singletand triplet states involved in excited-state intramolecular pro-ton transfer of 3-hydroxyflavonerdquo Journal of Physical Chemistryvol 94 no 15 pp 5752ndash5756 1990

[32] B Dick and B Nickel ldquoAccessibility of the lowest quintet state oforganic molecules through triplet-triplet annihilation an indoci studyrdquo Chemical Physics vol 78 no 1 pp 1ndash16 1983

[33] Y Zhang and S R Forrest ldquoTriplets Contribute to Bothan Increase and Loss in Fluorescent Yield in Organic LightEmitting Diodesrdquo Physical Review Letters vol 108 Article ID267404 5 pages 2012

[34] RW T Higgins A PMonkmanH G Nothofer andU ScherfldquoEffects of singlet and triplet energy transfer to moleculardopants in polymer light-emitting diodes and their usefulnessin chromaticity tuningrdquo Applied Physics Letters vol 79 no 6pp 857ndash859 2001

[35] A Kohler andH Bassler ldquoTriplet states in organic semiconduc-torsrdquo Materials Science and Engineering R vol 66 no 4ndash6 pp71ndash109 2009

[36] A P Monkman H D Burrows M D Miguel I Hamblettand S Navaratnam ldquoMeasurement of the S0-T1 energy gap inpoly(2-methoxy5-(21015840-ethyl-hexoxy)-p-phenylenevinylene) bytriplet-triplet energy transferrdquoChemical Physics Letters vol 307no 5-6 pp 303ndash309 1999

[37] A P Monkman H D Burrows L J Hartwell L E Hors-burgh I Hamblett and S Navaratnam ldquoTriplet energies of 120587-conjugated polymersrdquo Physical Review Letters vol 86 no 7 pp1358ndash1361 2001

[38] M Knupfer ldquoExciton binding energies in organic semiconduc-torsrdquo Applied Physics A vol 77 no 5 pp 623ndash626 2003

[39] S F Alvarado P F Seidler D G Lidzey and D D CBradley ldquoDirect determination of the exciton binding energy ofconjugated polymers using a scanning tunneling microscoperdquoPhysical Review Letters vol 81 no 5 pp 1082ndash1085 1998

[40] M Rohlfing and S G Louie ldquoOptical Excitations in ConjugatedPolymersrdquo Physical Review Letters vol 82 no 9 pp 1959ndash19621999

[41] S M King H L Vaughan and A P Monkman ldquoOrientation oftriplet and singlet transition dipole moments in polyfluorenestudied by polarised spectroscopiesrdquo Chemical Physics Lettersvol 440 no 4ndash6 pp 268ndash272 2007

[42] A Monkman and H D Burrows ldquoBackbone planarity effectson triplet energies and electron-electron correlation in lumines-cent conjugated polymersrdquo Synthetic Metals vol 141 no 1-2 pp81ndash86 2004

[43] A P Monkman H D Burrows I Hamblett S NavarathnamM Svensson and M R Andersson ldquoThe effect of conjugationlength on triplet energies electron delocalization and electron-electron correlation in soluble polythiophenesrdquo Journal ofChemical Physics vol 115 no 19 pp 9046ndash9049 2001


[44] S King C Rothe and A Monkman ldquoTriplet build in anddecay of isolated polyspirobifluorene chains in dilute solutionrdquoJournal of Chemical Physics vol 121 no 21 pp 10803ndash108082004

[45] J S De Melo H D Burrows M Svensson M R Anderssonand A P Monkman ldquoPhotophysics of thiophene based polym-ers in solution the role of nonradiative decay processesrdquo Journalof Chemical Physics vol 118 no 3 pp 1550ndash1556 2003

[46] S M King R Matheson F B Dias and A P MonkmanldquoEnhanced triplet formation by twisted intramolecular charge-transfer excited states in conjugated oligomers and polymersrdquoJournal of Physical Chemistry B vol 112 no 27 pp 8010ndash80162008

[47] Z H Kafafi Organic Electroluminescence Marcel Dekker NewYork NY USA 2005

[48] J Kalinowski L C Palilis W H Kim and Z H KafafildquoDetermination of the width of the carrier recombination zonein organic light-emitting diodesrdquo Journal of Applied Physics vol94 no 12 pp 7764ndash7767 2003

[49] C Rothe H A Al Attar and A P Monkman ldquoAbsolute mea-surements of the triplet-triplet annihilation rate and the charge-carrier recombination layer thickness inworking polymer light-emitting diodes based on polyspirobifluorenerdquo Physical ReviewB vol 72 no 15 Article ID 155330 9 pages 2005

[50] W Barford ldquoTheory of singlet exciton yield in light-emittingpolymersrdquo Physical Review B vol 70 no 20 Article ID 2052048 pages 2004

[51] M ReuferM JWalter P G Lagoudakis et al ldquoSpin-conservingcarrier recombination in conjugated polymersrdquo Nature Materi-als vol 4 no 4 pp 340ndash346 2005

[52] S Karabunarliev and E R Bittner ldquoSpin-dependent electron-hole capture kinetics in luminescent conjugated polymersrdquoPhysical Review Letters vol 90 no 5 Article ID 057402 4 pages2003

[53] M Segal M Singh K Rivoire S Difley T Van Voorhis andM A Baldo ldquoExtrafluorescent electroluminescence in organiclight-emitting devicesrdquo Nature Materials vol 6 no 5 pp 374ndash378 2007

[54] T A Ford H Ohkita S Cook J R Durrant and N CGreenham ldquoDirect observation of intersystem crossing incharge-pair states in polyfluorene polymer blendsrdquo ChemicalPhysics Letters vol 454 no 4ndash6 pp 237ndash241 2008

[55] M Wohlgenannt K Tandon S Mazumdar S Ramasesha andZ V Vardeny ldquoFormation cross-sections of singlet and tripletexcitons in 120587-conjugated polymersrdquo Nature vol 409 no 6819pp 494ndash497 2001

[56] J S Kim P K H Ho N C Greenham and R H Friend ldquoElec-troluminescence emission pattern of organic light-emittingdiodes implications for device efficiency calculationsrdquo Journalof Applied Physics vol 88 no 2 pp 1073ndash1081 2000

[57] C Rothe SMKing andA PMonkman ldquoDirectmeasurementof the singlet generation yield in polymer light-emitting diodesrdquoPhysical Review Letters vol 97 no 7 Article ID 076602 2006

[58] A P Monkman C Rothe and S M King ldquoSinglet generationyields in organic light-emitting diodesrdquo Proceedings of the IEEEvol 97 no 9 pp 1597ndash1605 2009

[59] L C Lin H F Meng J T Shy et al ldquoTriplet-to-singlet exci-ton formation in poly(p-phenylene-vinylene) light-emittingdiodesrdquoPhysical Review Letters vol 90 no 3 Article ID 0366014 pages 2003

[60] M A Baldo D F OrsquoBrien M E Thompson and S R ForrestldquoExcitonic singlet-triplet ratio in a semiconducting organic thinfilmrdquo Physical Review B vol 60 no 20 pp 14422ndash14428 1999

[61] M SegalMA Baldo R J Holmes S R Forrest andZG SoosldquoExcitonic singlet-triplet ratios in molecular and polymericorganic materialsrdquo Physical Review B vol 68 no 7 Article ID075211 14 pages 2003

[62] D Y Kondakov ldquoRole of triplet-triplet annihilation in highlyefficient fluorescent devicesrdquo Journal of The Society for Informa-tion Display vol 17 no 2 pp 137ndash144

[63] D Y Kondakov ldquoCharacterization of triplet-triplet annihilationin organic light-emitting diodes based on anthracene deriva-tivesrdquo Journal of Applied Physics vol 102 no 11 Article ID114504 5 pages 2007

[64] K Tandon S Ramasesha and S Mazumdar ldquoElectron corre-lation effects in electron-hole recombination in organic light-emitting diodesrdquo Physical Review B vol 67 no 4 Article ID045109 19 pages 2003

[65] MWohlgenannt XM Jiang ZVVardeny andRA J JanssenldquoConjugation-length dependence of spin-dependent excitonformation rates in Π-conjugated oligomers and polymersrdquoPhysical Review Letters vol 88 no 19 pp 1974011ndash19740142002

[66] F Feller and A P Monkman ldquoElectroabsorption studies ofpoly(25-pyridinediyl)rdquo Physical Review B vol 60 no 11 pp8111ndash8116 1999

[67] W T Simpson ldquoResonance force theory of carotenoid pig-mentsrdquo Journal of the American Chemical Society vol 77 pp6164ndash6168 1955

[68] W T Simpson ldquoInternal dispersion forces The polyenesrdquoJournal of the American Chemical Society vol 73 no 11 pp5363ndash5367 1951

[69] E W Snedden A P Monkman and F B Dias ldquoPhoto-physics of charge generation in organic photovoltaic materialskinetic studies of geminate and free polarons in a modeldonoracceptor systemrdquo Journal of Physical Chemistry C vol116 pp 86ndash97 2012

[70] E W Snedden A P Monkman and F B Dias ldquoKineticstudies of geminate polaron pair recombination dissociationand efficient triplet exciton formation in PCPCBM organicphotovoltaic blendsrdquo Journal of Physical Chemistry C vol 116pp 4390ndash4398 2012

[71] S Karabunarliev and E R Bittner ldquoDissipative dynamics ofspin-dependent electron-hole capture in conjugated polymersrdquoJournal of Chemical Physics vol 119 no 7 pp 3988ndash3995 2003

[72] V Jankus CWinscom and A PMonkman ldquoThe photophysicsof singlet triplet and degradation trap states in 44- N N1015840 -dicarbazolyl- 1 11015840 -biphenylrdquo Journal of Chemical Physics vol130 no 7 Article ID 074501 2009

[73] V Jankus C Winscom and A P Monkman ldquoCritical role oftriplet exciton interface trap states in bilayer films of NPB andIr(piq)3rdquo Advanced Functional Materials vol 21 no 13 pp2522ndash2526 2011

[74] S Yin L Chen P Xuan K Q Chen and Z ShuaildquoField effect on the singlet and triplet exciton formation inorganicpolymeric light-emitting diodesrdquo Journal of PhysicalChemistry B vol 108 no 28 pp 9608ndash9613 2004

[75] M Das S Ramasesha and S Mazumdar ldquoRole of electron-electron interactions on spin effects in electron-hole recombi-nation in organic light emitting diodesrdquo Synthetic Metals vol155 no 2 pp 270ndash273 2005


[76] S Difley D Beljonne and T V Voorhis ldquoOn the singlet-tripletsplitting of geminate electron-hole pairs in organic semicon-ductorsrdquo Journal of the American Chemical Society vol 130 no11 pp 3420ndash3427 2008

[77] D Beljonne Z Shuai A Ye and J L Bredas ldquoCharge-recombination processes in oligomer- and polymer-based light-emitting diodes a molecular picturerdquo Journal of the Society forInformation Display vol 13 no 5 pp 419ndash427 2005

[78] L Chen L Zhu and Z Shuai ldquoSingletmdashtriplet splittingsand their relevance to the spin-dependent exciton formationin light-emitting polymers an EOMCCSD studyrdquo Journal ofPhysical Chemistry A vol 110 no 50 pp 13349ndash13354 2006

[79] M Gordon andW RWare EdsTheExciplex Academic PressNew York NY USA 1975

[80] S M King C Rothe D Dai and A P Monkman ldquoFemtosec-ond ground state recovery measuring the intersystem crossingyield of polyspirobifluorenerdquo Journal of Chemical Physics vol124 no 23 Article ID 234903 2006

[81] MK LeeM Segal Z G Soos J Shinar andMA Baldo ldquoYieldof singlet excitons in organic light-emitting devices a doublemodulation photoluminescence-detected magnetic resonancestudyrdquo Physical Review Letters vol 94 no 13 Article ID 1374032005

[82] M Segal M A Baldo M K Lee J Shinar and Z G Soos ldquoFre-quency response and origin of the spin-12 photoluminescence-detected magnetic resonance in a 120587-conjugated polymerrdquo Phys-ical Review B vol 71 no 24 pp 1ndash11 2005


[84] S Sinha and A P Monkman ldquoDelayed recombination ofdetrapped space-charge carriers in poly[2-methoxy-5- (21015840 -ethyl-hexyloxy)-14-phenylene vinylene]-based light-emittingdioderdquo Journal of Applied Physics vol 97 no 11 Article ID114505 pp 1ndash7 2005

[85] S Sinha C Rothe R Guntner U Scherf and A PMonkman ldquoElectrophosphorescence and delayed electrolumi-nescence from pristine polyfluorene thin-film devices at lowtemperaturerdquo Physical Review Letters vol 90 no 12 Article ID127402 4 pages 2003

[86] J S Wilson A S Dhoot A J A B Seeley M S Khan AKohler andRH Friend ldquoSpin-dependent exciton formation in120587-conjugated compoundsrdquo Nature vol 413 no 6858 pp 828ndash831 2001

[87] C Rothe S King and A Monkman ldquoLong-range resonantlyenhanced triplet formation in luminescent polymers dopedwith iridiumcomplexesrdquoNatureMaterials vol 5 no 6 pp 463ndash466 2006

[88] P A Lane L C Palilis D F OrsquoBrien et al ldquoOrigin ofelectrophosphorescence from a doped polymer light emittingdioderdquo Physical Review B vol 63 no 23 Article ID 235206 8pages 2001

[89] H A Al Attar andA PMonkman ldquoDopant effect on the chargeinjection transport and device efficiency of an electrophospho-rescent polymeric light-emitting devicerdquo Advanced FunctionalMaterials vol 16 no 17 pp 2231ndash2242 2006


[91] A S Dhoot D S Ginger D Beljonne Z Shuai and N CGreenham ldquoTriplet formation and decay in conjugated polymerdevicesrdquo Chemical Physics Letters vol 360 no 3-4 pp 195ndash2012002

[92] Y Cao I D Parker G Yu C Zhang and A J HeegerldquoImproved quantumefficiency for electroluminescence in semi-conducting polymersrdquo Nature vol 397 no 6718 pp 414ndash4151999

[93] MTammer RWTHiggins andA PMonkman ldquoHigh opticalanisotropy in thin films of polyfluorene and its affect on theoutcoupling of light in typical polymer light emitting diodestructuresrdquo Journal of Applied Physics vol 91 no 7 Article ID4010 p 4 2002

[94] E J W List R Guentner P S de Freitas and U ScherfldquoThe effect of keto defect sites on the emission properties ofpolyfluorene-type materialsrdquo Advanced Materials vol 14 pp374ndash378 2002

[95] S I Hintschich C Rothe S Sinha A P Monkman PScandiucci de Freitas and U Scherf ldquoPopulation and decay ofketo states in conjugated polymersrdquo Journal of Chemical Physicsvol 119 no 22 pp 12017ndash12022 2003

[96] H Spreitzer H Becker E Breuning et al ldquoLight emittingpolymer materials for full-color displaysrdquo in Organic Light-Emitting Materials and Devices VI pp 16ndash25 usa July 2002

[97] A Van Dijken A Perro E A Meulenkamp and K BrunnerldquoThe influence of a PEDOTPSS layer on the efficiency of apolymer light-emitting dioderdquo Organic Electronics vol 4 no2-3 pp 131ndash141 2003

[98] S M King D Dai C Rothe and A P Monkman ldquoExcitonannihilation in a polyfluorene low threshold for singlet-singletannihilation and the absence of singlet-triplet annihilationrdquoPhysical Review B vol 76 no 8 Article ID 085204 2007

[99] M Deussen M Scheidler and H Bassler ldquoElectric field-induced photoluminescence quenching in thin-film light-emitting diodes based on poly(phenyl-p-phenylene vinylene)rdquoSynthetic Metals vol 73 no 2 pp 123ndash129 1995

[100] E J W List C H Kim A K Naik et al ldquoInteractionof singlet excitons with polarons in wide band-gap organicsemiconductors a quantitative studyrdquo Physical Review B vol64 no 15 Article ID 155204 pp 1552041ndash15520411 2001

[101] C Rothe H A Al Attar and A P Monkman ldquoAbsolute mea-surements of the triplet-triplet annihilation rate and the charge-carrier recombination layer thickness inworking polymer light-emitting diodes based on polyspirobifluorenerdquo Physical ReviewB vol 72 no 15 pp 1ndash9 2005

[102] DHertel H Bassler R Guentner andU Schert ldquoTriplet-tripletannihilation in a poly(fluorene)-derivativerdquo Journal of ChemicalPhysics vol 115 no 21 pp 10007ndash10013 2001

[103] C Rothe and A P Monkman ldquoTriplet exciton migration ina conjugated polyfluorenerdquo Physical Review B vol 68 no 7Article ID 075208 pp 752081ndash7520811 2003

[104] C Rothe and A Monkman ldquoDynamics and trap-depth distri-bution of triplet excited states in thin films of the light-emittingpolymer poly(99-di(ethylhexyl)fluorene)rdquo Physical Review Bvol 65 no 7 Article ID 073201 pp 0732011ndash0732014 2002

[105] P W M Blom M J M De Jong and J J M VleggaarldquoElectron and hole transport in poly(p-phenylene vinylene)devicesrdquo Applied Physics Letters vol 68 no 23 pp 3308ndash33101996

[106] C Rothe S M King and A P Monkman ldquoElectric-field-induced singlet and triplet exciton quenching in films of the


conjugated polymer polyspirobifluorenerdquo Physical Review Bvol 72 no 8 Article ID 085220 2005

[107] H E Lessing A Von Jena and M Reichert ldquoTriplet yielddetermination and heavy-atom effect from ground-state repop-ulation kineticsrdquoChemical Physics Letters vol 42 no 2 pp 218ndash222 1976

[108] B H Wallikewitz D Kabra S Gelinas and R H FriendldquoTriplet dynamics in fluorescent polymer light-emittingdiodesrdquo Physical Review B vol 85 Article ID 045209 15 pages2012

[109] S Sinha and A P Monkman ldquoDelayed electroluminescencevia triplet-triplet annihilation in light emitting diodes basedon poly[2-methoxy-5-(21015840-ethyl-hexyloxy)-14-phenylene viny-lene]rdquo Applied Physics Letters vol 82 no 26 pp 4651ndash46532003

[110] C Rothe andAMonkman ldquoRegarding the origin of the delayedfluorescence of conjugated polymersrdquo Journal of ChemicalPhysics vol 123 no 24 Article ID 244904 pp 1ndash6 2005

[111] S M King M Cass M Pintani et al ldquoThe contributionof triplet-triplet annihilation to the lifetime and efficiency offluorescent polymer organic light emitting diodesrdquo Journal ofApplied Physics vol 109 no 7 Article ID 074502 2011

[112] C A Parker and C G Hatchard ldquoDelayed fluorescence ofpyrene in ethanolrdquo Transactions of the Faraday Society vol 59pp 284ndash295 1963

[113] J Jortner S I Choi J L Katz and S A Rice ldquoTriplet energytransfer and triplet-triplet interaction in aromatic crystalsrdquoPhysical Review Letters vol 11 no 7 pp 323ndash326 1963

[114] J B Birks ldquoThe quintet state of the pyrene excimerrdquo PhysicsLetters A vol 24 no 9 pp 479ndash480 1967

[115] J Saltiel ldquoSpin-statistical factors in reactions of free-radicalsand triplet-statesrdquo Abstracts of Papers of the American ChemicalSociety vol 182 p 65 1981

[116] J Saltiel G R Marchand W K Smothers S A Stout andJ L Charlton ldquoConcerning the spin-statistical factor in thetriplet-triplet annihilation of anthracene tripletsrdquo Journal of theAmerican Chemical Society vol 103 no 24 pp 7159ndash7164 1981

[117] M B Smith and J Michl ldquoSinglet fissionrdquo Chemical Reviewsvol 110 no 11 pp 6891ndash6936 2010

[118] R Froese and K Morokuma ldquoAccurate calculations of bond-breaking energies in C

60using the three-layered ONIOM

methodrdquo Chemical Physics Letters vol 305305 no 5-6 pp 419ndash424 1999

[119] W G Herkstroeter and P B Merkel ldquoThe triplet state energiesof rubrene and diphenylisobenzofuranrdquo Journal of Photochem-istry vol 16 no 4 pp 331ndash341 1981

[120] H D Burrows J Seixas de Melo C Serpa et al ldquoTriplet statedynamics on isolated conjugated polymer chainsrdquo ChemicalPhysics vol 285 no 1 pp 3ndash11 2002

[121] L Ma K K Zhang C Kloc H D Sun M E Michel-Beyerleand G G Gurzadyan ldquoSinglet fission in rubrene single crystaldirect observation by femtosecond pump-probe spectroscopyrdquoPhysical Chemistry Chemical Physics vol 14 pp 8307ndash83122012

[122] Y Zhang M Whited M E Thompson and S R ForrestldquoSinglet-triplet quenching in high intensity fluorescent organiclight emitting diodesrdquoChemical Physics Letters vol 495 no 4-6pp 161ndash165 2010

[123] RW T Higgins A PMonkmanH G Nothofer andU ScherfldquoEnergy transfer to porphyrin derivative dopants in polymerlight-emitting diodesrdquo Journal of Applied Physics vol 91 no 1pp 99ndash105 2002

[124] Y Iwasaki TOsasaMAsahiMMatsumura Y Sakaguchi andT Suzuki ldquoFractions of singlet and triplet excitons generated inorganic light-emitting devices based on a polyphenyleneviny-lene derivativerdquo Physical Review B vol 74 no 19 Article ID195209 2006

[125] C Rothe K Brunner I Bach S Heun and A P MonkmanldquoEffects of triplet exciton confinement induced by reducedconjugation length in polyspirobifluorene copolymersrdquo Journalof Chemical Physics vol 122 no 8 Article ID 084706 pp 1ndash62005

[126] F Perrin ldquoLa fluorescence des solutionsrdquo Annals of Physics vol12 pp 169ndash275 1929

[127] G N Lewis and M Kasha ldquoPhosphorescence and the tripletstaterdquo Journal of the American Chemical Society vol 66 no 12pp 2100ndash2116 1944

[128] J C Deaton S C Switalski D Y Kondakov et al ldquoE-typedelayed fluorescence of a phosphine-supported cu 2(120583-nar 2)2 diamond core harvesting singlet and triplet excitons inOLEDsrdquo Journal of the American Chemical Society vol 132 no27 pp 9499ndash9508 2010

[129] A J M Miller J L Dempsey and J C Peters ldquoLong-livedand efficient emission from mononuclear amidophosphinecomplexes of copperrdquo Inorganic Chemistry vol 46 no 18 pp7244ndash7246 2007

[130] H C Longuet-Higgins and J N Murrell ldquoThe electronic spec-tra of aromatic molecules V the interaction of two conjugatedsystemsrdquo Proceedings of the Physical Society Section A vol 68no 7 article no 308 pp 601ndash611 1955

[131] J N Murrell ldquoRelative importance of exciton delocalizationand electron delocalization in polyene spectrardquo The Journal ofChemical Physics vol 37 no 5 pp 1162ndash1163 1962

[132] D Chaudhuri HWettach K J Van Schooten et al ldquoTuning thesinglet-triplet gap in metal-free phosphorescent 120587-conjugatedpolymersrdquo Angewandte Chemie vol 49 no 42 pp 7714ndash77172010

[133] A EndoK Sato K Yoshimura et al ldquoEfficient up-conversion oftriplet excitons into a singlet state and its application for organiclight emitting diodesrdquo Applied Physics Letters vol 98 no 8Article ID 083302 2011

[134] A Endo M Ogasawara A Takahashi D Yokoyama Y Katoand C Adachi ldquoThermally activated delayed fluorescence fromSn4+-porphyrin complexes and their application to organiclight-emitting diodes -A novel mechanism for electrolumines-cencerdquoAdvancedMaterials vol 21 no 47 pp 4802ndash4806 2009

[135] J Kalinowski ldquoExcimers and exciplexes in organic electrolumi-nescencerdquoMaterials Science- Poland vol 27 no 3 pp 735ndash7562009

[136] S A Jenekhe and J A Osaheni ldquoExcimers and exciplexes ofconjugated polymersrdquo Science vol 265 no 5173 pp 765ndash7681994

[137] K Itano H Ogawa and Y Shirota ldquoExciplex formation atthe organic solid-state interface yellow emission in organiclight-emitting diodes using green-fluorescent tris(8-quinolinol-ato)aluminum and hole-transporting molecular materials withlow ionization potentialsrdquo Applied Physics Letters vol 72 no 6pp 636ndash638 1998

[138] M Cocchi D Virgili G Giro et al ldquoEfficient exciplex emittingorganic electroluminescent devicesrdquoApplied Physics Letters vol80 no 13 pp 2401ndash2403 2002


[139] J Kalinowski M Cocchi P DiMarcoW Stampor G Giro andV Fattori ldquoImpact of high electric fields on the charge recom-bination process in organic light-emitting diodesrdquo Journal ofPhysics D vol 33 no 19 pp 2379ndash2387 2000

[140] L C Palilis A J Makinen M Uchida and Z H KafafildquoHighly efficient molecular organic light-emitting diodes basedon exciplex emissionrdquoApplied Physics Letters vol 82 no 14 pp2209ndash2211 2003

[141] B Frederichs and H Staerk ldquoEnergy splitting between tripletand singlet exciplex states determined with E-type delayedfluorescencerdquoChemical Physics Letters vol 460 no 1-3 pp 116ndash118 2008

[142] H Beens and A Weller ldquoApplication of the tyablikov-bogol-yubov diagonalization method to magnetic thin filmsrdquo ActaPhysica Polonica vol 34 pp 539ndash541 1968

[143] A Wellar The Exciplex Academic Press New York NY USA1975

[144] M Cocchi D Virgili C Sabatini and J Kalinowski ldquoOrganicelectroluminescence from singlet and triplet exciplexes exci-plex electrophosphorescent dioderdquo Chemical Physics Lettersvol 421 no 4-6 pp 351ndash355 2006

[145] A C Morteani A S Dhoot J S Kim et al ldquoBarrier-FreeElectron-Hole Capture in Polymer BlendHeterojunction Light-Emitting Diodesrdquo Advanced Materials vol 15 no 20 pp 1708ndash1712 2003

[146] K Goushi K Yoshida K Sato and C Adachi ldquoOrganic light-emitting diodes employing efficient reverse intersystem cross-ing for triplet-to-singlet state conversionrdquoNature Photonics vol6 pp 253ndash258 2012

[147] K Goushi and C Adachi ldquoEfficient organic light-emittingdiodes through up-conversion from triplet to singlet excitedstates of exciplexesrdquo Applied Physics Letters vol 101 Article ID023306 4 pages 2012

[148] V Jankus C Chiang F B Dias and A Monkman ldquoDeep blueexciplex organic light emitting diodes with enhanced efficiencythrough triplet fusionrdquo Advanced Materials In press

[149] V Jankus C Winscom and A P Monkman ldquoDynamicsof triplet migration in films of N N1015840-diphenyl-N N1015840-bis(1-naphthyl)-1 11015840-biphenyl-4 410158401015840-diaminerdquo Journal of Physics Con-densed Matter vol 22 no 18 Article ID 185802 2010

[150] E R Bittner I Burghardt and R H Friend ldquoDoes interchainstacking morphology contribute to the singlet-triplet inter-conversion dynamics in polymer heterojunctionsrdquo ChemicalPhysics vol 357 no 1ndash3 pp 159ndash162 2009

[151] A C Morteani P Sreearunothai L M Herz R H Friendand C Silva ldquoExciton regeneration at polymeric semiconductorheterojunctionsrdquo Physical Review Letters vol 92 no 24 ArticleID 247402 1 pages 2004

[152] A C Morteani R H Friend and C Silva ldquoEndothermicexciplex-exciton energy-transfer in a blue-emitting polymericheterojunction systemrdquo Chemical Physics Letters vol 391 no 1ndash3 pp 81ndash84 2004

[153] D D Gebler Y Z Wang J W Blatchford et al ldquoExciplexemission in bilayer polymer light-emitting devicesrdquo AppliedPhysics Letters vol 70 no 13 pp 1644ndash1646 1997

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


[13ndash18] so the high triplet energy host materials requiredcause serious charge injection problems Although manygroups are currently trying to tackle this problem devel-oping new materials sets for blue phosphorescent emittersusing simple few layer device structures alternatives to bluephosphorescent OLEDs are being sort There is still greatpotential to carry on using fluorescent blue emitters for bothdisplay and white light applications where lifetime is themost demanding and critical parameter for the devices butwe would still require a way to make use of (75) tripletexcitons formed in the fluorescent emitters Several ideashave emerged on triplet harvesting from a blue fluorescentemitter into red and green phosphors [19ndash21] however thedevice structures required to achieve this are very complexand involve exciton blocking layers of order 2 nm thicktotally impractical to manufacture at high yield over the largeareas required for lighting These complex device structuresare not used in the current generation lighting panels ordisplays Thus a new triplet harvesting approach is requiredIt is well known that triplet excitons can be used to deriveuseful single excitons from triplet-triplet annihilation (TTA)by the process of triplet fusion (TF) [22ndash26] When twotriplet excitons interact and depending on the subsequentspin configuration of the resultant ldquointeraction pairrdquo theycan produce a singlet exciton (TF) further triplet excitonsor quintet states [27] Historically a pure spin statisticallyargument has been used to imply that only 1 singlet exciton isproduced from 18 initially interacting triplets [27ndash29] but inmost molecules the quintet states are energetically untenable[30ndash32] and in a few specific molecules the upper excitedtriplet state 119879

119899 cannot be reached either as with rubrene [33

34] given the sum of the energy of the two triplet excitonsThus the number of singlet excitons produced via TF canbe much higher than 118 allowing fluorescent dopants tobe effectively used as triplet ldquoharvestingrdquo emitters yieldinglonger device lifetimes and removing the stringent require-ments of high triplet energy host materials required withblue phosphorescent dopants In this paper the processesand various different ways in which singlet excitons can begenerated from triplet states are explained

The factors which control the overall quantum efficiencyof an organic light-emitting diode are rather different thanthose that govern inorganic LEDs efficiency most notably theratio of singlet to triplet excitons formed as a result of chargerecombinationwithin the organic emitter layer In the organicsemiconductors the singlet and triplet excitons are very wellresolved with little intermixing [35] This difference comesabout because of the excitonic nature of the fundamentalexcited states of the organic emitters and the large electroncorrelation energies of the molecules [36 37] Since theelectron and hole are tightly boundwith large exciton bindingenergy [38ndash40] singlet excitons are spatially localized and sotheir exchange energy is large whereas for the triplet excitonsthe electron and hole are in orthogonal orbitalrsquos so the tripletexcitons have a zero exchange term [41 42] This manifestsitself in the very large difference in the energies of the lowestsinglet (119878

1) and triplet (119879

1) excited states (excitons) experi-

mentally found to be1198791= (113times119878

1minus143) plusmn 015 eV (typically

of order 07 eV) and conjugation length dependent [37 43]

The singlet excitons are also strongly coupled to the groundstate giving rise to high photoluminescence quantum yieldsFurther as organic semiconductors typically contain onlylow atomic mass elements spin orbit coupling the dominantmechanism for triplet formation [35 44] is weak and sothe interconversion of singlet excitons into triplet excitonsthat is intersystem crossing is very inefficient [45 46]again helping to achieve high photoluminescence quantumyieldsThus to understand the charge recombination processin OLEDs we must also take into account these excitonicproperties

2 Exciton Formation in an OLED

Charges of opposite sign injected into the organic semi-conductor form dressed states not free electrons and holesthe charge perturbs the covalent 120587 bonding structure ofthe organic semiconductor creating a localized distortionwhich traps the charge that is a polaron (P) [47] andit is these polarons (positive and negative) that migratethrough the organic layer to meet in a thin recombinationzone [48 49] These polarons are spin 12 particles Whenoppositely charge polarons capture they initially form someintermediate state held by their mutual columbic potentialwell [50] before relaxing into an exciton (which can thenemit) Before capture the polarons are uncorrelated and sotheir spin states have random orientation with respect toone another Only at the point of recombination do thetwo spins of the P+ and Pminus become correlated and singletor triplet character can be associated to the intermediatestate [51] The intermediate state can thus be best describedas either a singlet or triplet charge transfer (CT) state (orcharge transfer exciton) [52] If this recombination processis thus independent of the spin states (darr) of the initial(P+Pminus) pair then quantummechanical spin statistics dictatesthat there are four ways in which the spin wavefunctions ofthe individual polarons can combine when the CT excitonicstate is formed that is uarr + uarr (triplet) darr + darr (triplet)1radic2[uarrdarr + uarrdarr] spins precessing in-phase (triplet) 1radic2[uarrdarrminus uarrdarr] spins precessing out of phase (singlet) From this itcan be seen that only 25 of the excitons thus created willhave singlet character and be emissive which puts an upperlimit of 025 on the internal quantum efficiency of an OLEDHowever this limit only arises if the recombination processis independent of spin If at any stage the recombination iseffected directly or indirectly by the spin configuration ofthe intermediate CT states [53] or the polaron capture isaffected by their spins [54] then it can follow that the 25limit is broken and possibly more singlets could be produced[21]

Over the past few years an increasing number of experi-mental reports make it clear that the 25 limit is broken [55ndash59] In small-molecule-based OLEDs original experimentalevidence pointed to the 25 limit being obeyed [60 61]whereas in polymers this was not so however more recentreports especially those from theOLED group inKodak haveshown that the 25 limit in small-molecule devices was alsogreatly exceeded [62 63] Many theories were put forward as


















119878opt

= 119888opt119878119868opt119878

(1 minus 120581ISC)

119879opt

= 119888opt119879119868opt119879120581ISC

119878el= 119888

el119878119868el119878120594 (1 minus 120581

ISC)

119879el= 119888

el119879119868el119879(1 minus 120594 (1 minus 120581

ISC))

(1)


119878= 119868

opt119879

and 119868el119878= 119868


relative to 119878

119879opt

119878opt =

119888opt119879

119888opt119878

120581ISC

(1 minus 120581ISC)

119879el

119878el =

119888el119879

119888el119878

1 minus 120594 (1 minus 120581ISC)

120594 (1 minus 120581ISC)

(2)



119888opt119878

= 119888el119878(119888

opt119879



119879el119878

el

119879opt119878

opt =1 minus 120594 (1 minus 120581

ISC)

120594120581ISC (3)





plus PC

Trip

let s

igna

l


Optical excitation


10 ns 1 s 2A





400 nm

Dispersivelens


780 nm



min 15 K

filter 435 nm



Focuslens

Cutoff

Cutoff


0 2 4 6

0

002

004

006

008

Time (ms)

opticalexcitation


0075

0029

0046

Fluo

resc

ence

inte

nsity

(V)






12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C





by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879




119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886


119879119879120582

(5)


119879119886 with 119868

119879119888119879



0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




















119878opt

= 119888opt119878119868opt119878

(1 minus 120581ISC)

119879opt

= 119888opt119879119868opt119879120581ISC

119878el= 119888

el119878119868el119878120594 (1 minus 120581

ISC)

119879el= 119888

el119879119868el119879(1 minus 120594 (1 minus 120581

ISC))

(1)


119878= 119868

opt119879

and 119868el119878= 119868


relative to 119878

119879opt

119878opt =

119888opt119879

119888opt119878

120581ISC

(1 minus 120581ISC)

119879el

119878el =

119888el119879

119888el119878

1 minus 120594 (1 minus 120581ISC)

120594 (1 minus 120581ISC)

(2)



119888opt119878

= 119888el119878(119888

opt119879



119879el119878

el

119879opt119878

opt =1 minus 120594 (1 minus 120581

ISC)

120594120581ISC (3)





plus PC

Trip

let s

igna

l


Optical excitation


10 ns 1 s 2A





400 nm

Dispersivelens


780 nm



min 15 K

filter 435 nm



Focuslens

Cutoff

Cutoff


0 2 4 6

0

002

004

006

008

Time (ms)

opticalexcitation


0075

0029

0046

Fluo

resc

ence

inte

nsity

(V)






12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C





by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879




119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886


119879119879120582

(5)


119879119886 with 119868

119879119888119879



0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








119878opt

= 119888opt119878119868opt119878

(1 minus 120581ISC)

119879opt

= 119888opt119879119868opt119879120581ISC

119878el= 119888

el119878119868el119878120594 (1 minus 120581

ISC)

119879el= 119888

el119879119868el119879(1 minus 120594 (1 minus 120581

ISC))

(1)


119878= 119868

opt119879

and 119868el119878= 119868


relative to 119878

119879opt

119878opt =

119888opt119879

119888opt119878

120581ISC

(1 minus 120581ISC)

119879el

119878el =

119888el119879

119888el119878

1 minus 120594 (1 minus 120581ISC)

120594 (1 minus 120581ISC)

(2)



119888opt119878

= 119888el119878(119888

opt119879



119879el119878

el

119879opt119878

opt =1 minus 120594 (1 minus 120581

ISC)

120594120581ISC (3)





plus PC

Trip

let s

igna

l


Optical excitation


10 ns 1 s 2A





400 nm

Dispersivelens


780 nm



min 15 K

filter 435 nm



Focuslens

Cutoff

Cutoff


0 2 4 6

0

002

004

006

008

Time (ms)

opticalexcitation


0075

0029

0046

Fluo

resc

ence

inte

nsity

(V)






12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C





by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879




119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886


119879119879120582

(5)


119879119886 with 119868

119879119888119879



0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





plus PC

Trip

let s

igna

l


Optical excitation


10 ns 1 s 2A





400 nm

Dispersivelens


780 nm



min 15 K

filter 435 nm



Focuslens

Cutoff

Cutoff


0 2 4 6

0

002

004

006

008

Time (ms)

opticalexcitation


0075

0029

0046

Fluo

resc

ence

inte

nsity

(V)






12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C





by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879




119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886


119879119879120582

(5)


119879119886 with 119868

119879119888119879



0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




12 16 2 24 28 32 36 4 44 48

1000 800 700 600 500 400 300

(d) (c) (b)

Energy (eV)

(a)

Wavelength (nm)

O

O

O

O

CH3

CH3

CH3

H3C

H3C

H3C

CH3H3C





by

119889119879 (119905)

119889119905

=

1

119888119879

[119868119879minus 1205741198791198791198882

119879(119879 (119905))

2

]

119879 (119905) =

1

119888119879

radic119868119879

120574119879119879

tanh(119905radic119868119879120574119879119879)

(4)

with 120574119879119879




119889119879 (119909 119905)

119889119905

=

1

119888119879

[119868119879120582119890minus120582119909

minus 1205741198791198791198882

119879(119879 (119909 119905))

2

]

119879 (119905) =

1

119888119879

2

119905119886


119879119879120582

(5)


119879119886 with 119868

119879119888119879



0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

1

2

3

4

5

6

7

0 05 1 15

0

3

6

001 01 1

Time (ms)


10minus2

10minus1

100

101

102

103


ElectricalOptical






119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





119878el= 0069 plusmn 0002 and 119879opt

119878opt

= 00058 plusmn










300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




300 250 200 150 100 505

10

15

20

25

30


Curr

ent (

mA

)48

44

4

36

32300 280 260 240 220 200 180 160 140 120Cu

rren

t nor

m e

miss

ion

inte

gral

106

(Vs

mA

)

(a)

3

25

2

15

1

05

0

0 4 8 12 16 20

20

Probe808 nm

130 K 134 mA

290 K 234 mA

Curr

ent (

mA

)

40

30

20

100 4 8 12 16

290 K

130 K

1

08

06

04

02

0

0 10 20 30 40

290 K 130 K


(b)












1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




1

08

06

04

02

0RT

250 K200 K

150 K100 K

1080604020


0 2 4 6 8 10001

01

1

Nor

mal

ised

lum

inan

ce

Time (s)

0 05 1 15 2

Lum

inan

ceT

TA ra

tio

Time

06

08

1

10minus7

119879100

11987990

11987980

11987970

11987960






1198791

gt 119864119879119899

and 21198641198791





gt 1198781but 2119864

1198791

lt 119864119879119899


119899states




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Triplet recycling



01 02



3A + 3A

191(AA)

595(AA)

1A + 1A

3A + 1A

3A + 1A

133(AA)

5(AA) (3A + 3A )5(AA) 3A + 0

=0

818 +1

=0

1318 +1




1198791



lt

119864119879119899


1to 1198791(causing 119879

1








1198791

lt

119864119879119899


1198791






2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




2 3 4 5 6 7 8

0

04

08

12

16

2

24

28

0

04

08

12

16

2

24

28E

QE

()

Voltage (V)

(a)

0 20 40 60 80 100

0

05

1

15

2

25

3

0

05

1

15

2

25

3

Curr

ent e

ffici

ency

(cd

Aminus1 )


(b)

0 1 2 3 4 5 6 7 8 9

0

05

1

15

2

25

3

0

05

1

15

2

25

3

minus1

Voltage (V)

Pow

er effi

cien

cy (l

m W

minus1 )

(c)

2 3 4 5 6 7 8 9

001

01

1

10

100

001

01

1

10

100

Curr

ent d

ensit

y (m

A cm

minus2 )

Voltage (V)

(d)
















LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials












LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




LUMO

LUMO

HOMO

HOMO

ITO

Al Ag AuEg

Exci

plex






References









































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
































































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials































































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






























































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials


























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Review Article Singlet Generation from Triplet Excitons in Fluorescent Organic …downloads.hindawi.com/archive/2013/670130.pdf · 2019-07-31 · Review Article Singlet Generation