DOI: 10.1002/adfm.200500881

The Use of Substituted Iridium Complexes in Doped PolymerElectrophosphorescent Devices: The Influence of Triplet Transferand Other Factors on Enhancing Device Performance

By Simon M. King, Hameed A. Al-Attar, Rebecca J. Evans, Aileen Congreve, Andrew Beeby, andAndrew P. Monkman*

1. Introduction

Electrophosphorescent devices may hold the key to unlock-ing the potential of polymeric light-emitting devices. By usingthe triplet states for light generation, the inefficiency caused bythe large number of nonemissive polymer triplet states gener-ated in conventional devices is overcome. The devices arebased on a blend of conjugated host polymer and a phospho-rescent dopant with a highly emissive triplet state.[1–6] This isparticularly important because triplet excitons are formed witha statistically high probability in electroluminescent devices,which, in neat singlet-emitting polymer devices, represents aconsiderable loss of efficiency.[7,8] Harnessing these can pro-duce highly efficient light-emitting devices once the correctcombination of host and dopant is found.[9,10]

One group of molecules used extensively as organic phos-phors is the green-light-emitting iridium(III) d6 complexes.[6] Aswith all phosphors, the presence of a heavy-metal atom in thecomplex provides a significant spin–orbit interaction that al-lows the normally spin-forbidden radiative triplet decay. In ad-dition to this, the same effect allows for rapid intersystemcrossing from the singlet to the lower-energy triplet state and,as a result, all emission is observed from the triplet state.[11]

Different complexes have different photophysical properties;in some, the most energetically favorable excitation for emis-sion is ligand-centered.[12] For example, in Ir[FlxPy]3 (Py: pyri-dine) the heavy-metal atom only has the effect of enhancingthe phosphorescence decay of the triplet state of the ligand.Other complexes, such as the Ir(ppy)3 (ppy: phenylpyridine)derivatives used in this investigation, have their principal exci-tation as an MLCT (MLCT: metal-to-ligand charge transfer)state that involves d-electron mixing from the iridium atom tothe ligand in order to facilitate the excitation, a d–p* transi-tion.[13] Naturally in MLCT states the heavy-atom effect fromthe iridium is still present and, once again, facilitates both trip-let-state formation and phosphorescence of the state.

There is, however, one significant problem with harnessingthe triplet state in electrophosphorescent devices, which lies inthe relationship between the complex’s triplet energy and thatof the polymer host. Polyfluorenes are one of the most com-mon classes of conjugated polymers suitable for use in light-emitting devices and, along with most others, they have tripletenergies lower than the triplet energy of the complexes usedfor green- and blue-light emission. As a result, any triplets gen-erated on the complex are able to efficiently undergo triplet-energy transfer back to the polymer host, where they are onceagain trapped as poorly emissive states. This energy-transferprocess is much faster than the phosphorescence of the com-plex, thus the efficiency of the device is considerably reduced.This is shown schematically for our system in the energy-leveldiagram in Figure 1. Developments have recently been madein the synthesis of new polymers with higher-energy tripletstates, with the aim of pushing the triplet of the polymer abovethat of the complex.[14] Devices have been made employing

Adv. Funct. Mater. 2006, 16, 1043–1050 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1043

–[*] Prof. A. P. Monkman, S. M. King, Dr. H. A. Al-Attar

Department of Physics, University of DurhamSouth Road, Durham, DH1 3LE (UK)E-mail: a.p.monkman@dur.ac.ukR. J. Evans, Dr. A. Congreve, Dr. A BeebyDepartment of Chemistry, University of DurhamSouth Road, Durham, DH1 3LE (UK)

The problem of phosphorescence quenching by the host polymer of a dopant in a polyfluorene-based electrophosphorescentdevice has been extensively studied. This paper concentrates on reduction of the rate of triplet-energy transfer from the dopantto the host by making inert t-butyl substitutions to the ligands of the well-understood fac-trisphenylpyridine iridium phospho-rescent dopant. These substitutions introduce steric bulk to the dopant that approximately halves the rate of energy transfercompared to the unsubstituted dopant, and a concomitant increase in device performance is observed. This is attributed to thestrong distance dependence of the Dexter-type energy transfer involved, the steric bulk of the t-butyl groups effectively pre-venting the energy transfer from emissive dopant to the host. In addition, through the use of specific substitutions on either thepyridyl or phenyl ring, the pathway of the energy transfer has been identified as being through the pyridyl ring of the ligand.Employing this technique of steric prevention of the triplet-energy transfer to the host reduces the need for development ofhosts with a high triplet level for electrophosphorescent devices.

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poly(N-vinylcarbazole) (PVK), an unconjugated polymer witha luminescent side group, as the host polymer, and the use ofmeta-coupled carbazole moieties in the polymer backbone hasbeen used to increase the triplet level of the polymer. Anotherpossibility that has been investigated in the literature is the useof shorter oligomers and small molecules, such as 4,4′-N,N′-di-carbazole diphenyl (CBP), which have higher triplet levelsthan their corresponding host polymers. These methods ener-getically inhibit triplet transfer to the host.[5,15]

The methods of excitation of the complex are crucial to ourunderstanding of the processes; there are three main possibili-ties for the method: The first, and simplest, is the formation oftriplet excitons on the polymer that undergo Dexter transfer tothe complex. This method is energetically impossible for poly-fluorene-based devices doped with blue and green phosphors.Another possibility is Förster energy transfer between thefluorescent polymer singlet state and the complex itself, ashas been shown to be the dominant process inPtOEP-doped (PtOEP: platinum octaethylporphy-rin) MEH-PPV devices[16] (MEH-PPV: poly(2-meth-oxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene)) and,indeed, there is the possibility that this is a contribu-tory process in polyfluorene devices. The final pro-cess is sequential electron and hole capture on thecomplex, whereby the complex forms a preferentialsite for charge recombination, currently believed tobe the most common dominant process in polyfluo-rene devices.[17,18] It is this process that is believed tobe the dominant process in our system, but we cannotrule out partial energy transfer by a Förster processdue to the emission spectrum of the host overlappingwith absorption spectrum of MLCT (Fig. 2).

Manipulating the triplet level of the polymer is notthe only way of improving the efficiency of electro-phosphorescent devices; another method would be tochange the rate of energy transfer between the com-plex and the host polymer. If the rate of triplet-ener-gy transfer were comparable too, or slower than, thephosphorescence decay time of the complex, then

the efficiency of the device would be significantly improved. Inorder to understand how to do this, it is necessary to under-stand the method of triplet-energy transfer between the com-plex and host. Dexter transfer allows for the transfer of tripletsby the exchange of two electrons between the guest and host;this requires not only an overlap of the absorption and emis-sion spectra of the host and guest as in Förster transfer, but aspatial overlap of the electronic orbitals occupied by the excita-tions is also necessary to facilitate the transfer of the elec-trons.[19] As we can simplify the molecular orbitals on the com-plex and polymer to have exponentially decaying wavefunctions, the probability of energy transfer decays exponen-tially with the separation distance between the guest and host.This manifests itself in an exponential dependence on the rateof energy transfer with distance. The rate of triplet-energytransfer is governed by Equation 1:[20]

kda � kdexp cda 1 � RR0

� �� ��1�

where kda is the rate of donor–acceptor triplet-energy transfer,kd is the natural lifetime of the donor (in a system in whichthere is no energy transfer taking place), R is the separation ofthe donor and acceptor, and R0 is the critical range for energytransfer, which is defined (in the same way as the Förster ra-dius) as the distance at which energy transfer competes equallywith radiative decay. The last component, cda = 2R0/L, where Lis called the localization Bohr radius and quantifies the rangeof the excitation, has been measured by Kalinowski et al. forIr(ppy)3 to be 1.1 Å.[17]

Using this theory, we can clearly see that, as the separationof the donor and acceptor increases, the rate of energy transferdecreases. This paper aims to demonstrate this theory by mea-suring the rate of triplet transfer directly for Ir(ppy)3 (1) and

1044 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1043–1050

PSBF Ir(ppy)3

2.9eV S1

2.1eV T1

1MLCT 2.7eV

3MLCT 2.5eV

khg

kgh

kisc

k h

kg

S 0

Figure 1. Energy-level diagram for the polymer–dopant system used in thestudy. khg: rate of host–guest singlet-energy (Förster) transfer; kisc: inter-system crossing rate of the polymer; kgh: rate of triplet-energy transfer tothe polymer triplet; kh: radiative decay rate of the host polymer; kg: rate ofphosphorescence of the complex; PSBF: poly(9,9′-spirobifluorene); S: sin-glet energy level; T: triplet energy level.

0.0

0.2

0.4

0.6

0.8

1.0

827 620 496 413 354 310

0.0

0.2

0.4

0.6

0.8

1.0

1.5 2.0 2.5 3.0 3.5 4.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.1

0.2

0.3

0.4

0.5

Wavelength (nm)

No

rma

lize

d F

luore

scence / P

hosp

hore

scence

Absorption of PSBF

Fluorescence of PSBF

Phosphorescence of PSBF

Energy (eV)

Phosphorescence of Ir(PPY)3

Absorption of Ir(PPY)3

Optica

l D

en

sity

Figure 2. Absorption, fluorescence, and phosphorescence of the PSBF host and thedopant, Ir(ppy)3, in a chlorobenzene solution.

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S. M. King et al./Substituted Iridium Complexes in Doped Polymer Electrophosphorescent Devices

three of its derivatives (2–4) in a poly(9,9′-spirobifluorene)(PSBF) host. Ir(ppy)3 is a complex widely used in electrophos-phorescent devices and many investigations have been carriedout to understand the nature of the excited states in the com-plex. The three derivatives have ligands incorporating tertiarybutyl substituents: two in complex 2, and one in complexes 3and 4, thus providing the extra steric bulk required to increasethe separation of the dopant and host and potentially reducingthe rate of energy transfer. The structures of the complexes areshown in Figure 3. The PSBF host was chosen because of itshigh molecular weight, approximately 770 000 g mol–1, which,corresponding to approximately 1000 repeat units, allows forthe production of high-quality films when spin-cast from a widerange of solvents.

It is interesting to compare the effect of the three differentsubstitutions on the molecule, especially when we consider thatthese are tris-chelated complexes and, as such, can form ineither the facial (fac) or meridional (mer) isomeric structures.It is known from the literature that the fac isomer is the moreefficient emitter[21] and, indeed, the mer isomer is known to un-dergo rearrangement to form the more stable fac isomer underUV irradiation. The effect of the two isomers becomes mostnoticeable in complexes that contain a singly substituted li-gand; the fac isomer used in this study projects all of the t-butylgroups out on the same side of the complex, effectively creat-

ing a hemisphere protecting one side of the complex. Conse-quentially in complexes 3 and 4 this presents steric hindrancearound either the phenyl ring or the pyridyl ring of the ligand.The doubly substituted version intuitively creates a completeshell around the complex, which would prevent close approachof the polymer to the aromatic rings from all directions.

Understanding the energy transfer between the statesrequires the kinetics of the build-up and decay of the states tobe understood. The energy-level diagram in Figure 1 shows thekinetic parameters that are relevant to the processes involvedin a doped polymer system. After excitation of the polymerhost the states are governed by the rate equations:

dSh

dt� G � �khg � kh � kic � kisc�Sh �2�

dNg

dt� khgSh � �kgh � kg � knr�Ng �3�

dTh

dt� kghNg � kiscSh �4�

In this scheme, G is the singlet-generation parameter determin-ing the number of singlets initially on the host, Sh is the numberof singlets on the host, Ng is the number of excitations on thedopant (guest), and t represents time. Although the energy istransferred as singlets, these rapidly become triplets because ofintersystem crossing and then undergo triplet-energy transferto the triplet level of the polymer. khg is the rate of host–guestsinglet-energy (Förster) transfer, kh is the radiative-decay rateof the host polymer, kic is the internal conversion rate of thepolymer, and kisc is the intersystem crossing rate of the poly-mer. On the dopant, kgh is the rate of triplet-energy transfer tothe polymer triplet, kg is the rate of phosphorescence of thecomplex, and knr is the sum of all the nonradiative decays ofthe complex. Finally, Th is the population of triplets on thehost; it is assumed that in the time regime of the build-up ofthe polymer triplets, there is no decay of the polymer trip-lets.[22]

2. Results and Discussion

2.1. Steady-State Measurements

Absorption, fluorescence, and phosphorescence of the PSBFhost and Ir(ppy)3 are shown in Figure 2. Figure 4 shows thenormalized absorption spectra of the complexes 1, 2, 3, and 4.The absorption band peaks appear at 380, 410, 457, and492 nm, respectively, with an almost identical small red-shift(< 10 nm) for complexes 2 and 4. This indicates that the t-butylsubstitutions do not disturb the p-conjugation system of theppy ligand. Theoretical calculations by Hay[13] allow us to as-sign the various features in the absorption spectrum. Thestrong absorption band that peaks at 280 nm (not shown) isassigned to ligand-based excitation (3p → p*) and the energy-

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N

NN

Ir

N

NN

Ir

N

NN

Ir

N

NN

Ir

Ir(PPY)3 (1)

(4)(3)

(2)

Figure 3. The structures of the three complexes used as dopants in thisstudy: Ir(ppy)3 (1) and three of its derivatives (2–4).

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absorption peaks from 380 to 500 nm are MLCT states. Theabsorptions at ∼ 380, 410, and 457 nm are attributed to thesinglet MLCT state (i.e., S5: d1 → p2*, S3: d1 → p2*, and S1:d1 → p1*), and that at 492 nm has been assigned by Hay tothe triplet MLCT state (i.e., T1(3A): d2 → p1*) of iridium com-plexes.[13]

The ratio of the intensity of the MLCT S1 and T1 transitionsare very close for 1, 2, and 3, suggesting that the T1 transition isstrongly allowed by singlet–triplet mixing because of spin–orbitcoupling.[23] The weak influence of the substituent groups onthe molecular structure shows that the electronic nature of thelowest excited 3MLCT state is very similar in all complexes(peaks around 492 nm). As a consequence of this, the phos-phorescence spectra for all complexes are virtually identicaland peak at 510 nm, as shown in Figure 5; the only slight ex-ceptions are complexes 2 and 4, whose spectra show a smallbut significant red-shift due to the presence of the slightly elec-tron-donating tBu group on the phenyl ring.[24] Additionally,compound 4 shows an enhanced absorption in the 1MLCTtransition, although the origins of this remain unclear. Thebroad and unresolved nature of the photoluminescence andelectroluminescence spectra confirm that emission is from theMLCT transition rather than the ligand-centered 3p → p* state,which would normally display a strong vibronic progression.[25]

Further bathochromic shifts can be induced by using ligandsthat have a longer p-conjugation sequence.[12]

The photoluminescence quantum yields (�PL) were mea-sured in a chlorobenzene solution relative to complex 1; (forIr(ppy)3, �PL= 0.4 taken from the literature[10]), the doublysubstituted complex 2 has a �PL of 0.2 ± 0.05, while both com-plexes 3 and 4 have a �PL value of 0.29 ± 0.05. All the measureddecay lifetimes for all complexes are approximately the samein deoxygenated dilute solution ∼ 1.4 ± 0.2 ls. From these mea-

surements we calculate the rates of radiative and nonradiativedeactivation of the excited states, and find that kf decreases bya factor of two between complexes 1 and 2, while knr remainsapproximately constant.

The emission spectra of films doped with the complex at adoping ratio of 7 % are shown in Figure 6. The spectra clearlyshow two separate emissions, one at ∼ 2.8 eV and the other at∼ 2.4 eV. The higher-energy peak originates from the fluores-cence of the host polymer, and the lower-energy one from thecomplex phosphorescence following energy transfer from thePSBF to the iridium complexes. The spectra have been normal-

1046 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1043–1050

Wavelength (nm)

Figure 4. The absorption spectra of the complexes. The inset shows the re-gion of the triplet and singlet MLCT bands in more detail. The data arenormalized for ease of comparison (1: dashed line; 2: dashed/dotted line;3: solid line; 4: dotted line).

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Energy (eV)

Figure 5. Photoluminescence spectra of the complexes used in the study(1: solid line; 2: dashed/dotted line; 3: dashed line; 4: dotted line). A verti-cal line at 2.44 eV is shown, and the data have been normalised and offsetfor ease of comparison.

Energy (eV)

Wavelength (nm)

Complex Phosphorescence

Figure 6. Room-temperature photoluminescence emission (excitationwavelength, kexc = 355 nm) of the blended films of PSBF and the variousphosphorescent dopants at a dopant concentration of 7 % (1: solid line; 2:long-dashed line; 3: short-dashed line; 4: dotted line).

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ized to the absorption of the films at the excitation wavelength,355 nm. However, comparing the relative magnitudes of thetwo components of the emission makes one thing clear: thedoubly substituted complex (2) shows considerably morecomplex phosphorescence than the other three. The singlysubstituted complexes show some complex phosphorescence,whereas the unsubstituted complex shows none. Thissignificant difference in phosphorescence quenching could bean indication of a slower rate of energy transfer from thecomplex to the host polymer, thus allowing a greater propor-tion of radiative decay to take place from the organometallicdopant.

2.2. Transient Photoinduced Absorption

Investigation into the rate of triplet transfer was made usingphotoinduced absorption (PA) measurements that were carriedusing the doped films. The magnitude of the triplet PA for thePSBF host was monitored following excitation of the complex.For this simplified case of exciting only the complex (which isthe case for the PA experiment and is analogous to deviceswhere charge trapping on the complex forms the triplet exci-tons) the kinetic scheme previously described can be simplifiedas follows:

dNg

dt� G � �kgh � kg�Ng �5�

dTh

dt� kghNg �6�

The build-up of triplets on the host polymer following excita-tion of the complex as described by Equation 6 is shown for allfour complexes in Figure 7. Additionally, there is build-up oftriplets on the host polymer following direct excitation of thePSBF and intersystem crossing. This is clearly a much morerapid process with a time constant for the build-up of 800 ps,which after verification that there is no PA of the dopant at theprobe wavelength (Fig. 8), allows us to establish that the build-up in the blends is definitely not from direct photoexcitation ofthe polymer. As expected from Equation 1, the fastest build-up is from energy transfer from the unsubstituted complex(time constant, s= 2.8 ns) and the slowest is from energy trans-fer from the doubly substituted complex (s = 7.1 ns). Thisconfirms that the bulky side groups on the complex seem to beinhibiting the energy transfer to the polymer in the way pre-dicted. From this data we can estimate R0 for the unsubstitutedcomplex to be 0.77 nm for the system, calculated using Equa-tion 1 and the mean radius of separation of the polymer chainsand dopant molecules derived from the concentration of thedopants in the polymer. Thus, more than halving of the energy-transfer rate by adding the protective shell of the t-butyl groups(which have the effect of increasing the molecular radius byca. 0.9 nm) is entirely plausible.

2.3. Phosphorescence Quenching

The dopant–host energy transfer can also be measured bymonitoring the decay of the triplets on the complex. The rela-tive efficiency of the energy transfer can be measured by com-paring the quenching of the phosphorescence of the four dopedfilms. The greater the rate of energy transfer, the more thephosphorescence quenching. As demonstrated previously, thelifetime of all the complexes is the same (∼ 1.4 ls). The data inTable 1 shows the lifetimes of the decay phosphorescence ofthe complex when doped into a PSBF host. The quenching of

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0.0 1.0x10-8

2.0x10-8

3.0x10-8

0

1

2

3

(2)

(3)

(4)

(1)

-dT

/T (

a.u

.)

Time (s)

(0)

Figure 7. The build-up of triplets on the host polymer following excitationof the complex, showing the effect of the substitutions on the complexes(complexes labeled as before) on the rate of energy transfer. Also shownfor comparison is the build-up of the triplets on the polymer followingdirect excitation of the polymer in an undoped system (0). The data arenormalized and offset by 0.5 for ease of comparison.

1.0 1.5 2.0 2.5 3.0 3.5

-3.0x10-5

-2.0x10-5

-1.0x10-5

0.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

1240 827 620 496 413 354

Wavelength (nm)

Ir(PPY)3

PSBF

-dT

/T (

a.u

.)

Energy (eV)

x 5

Figure 8. Photoinduced absorption spectra of films of the pure polymerand a film of Ir(ppy)3 in Zeonex measured at 80 K.

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the phosphorescence fit well to a biexponential decay model.The fast decay time may contain a contribution from quench-ing due to aggregation and other effects that have been ob-served in doped films. Triplet–triplet annihilation has also beenobserved in Ir(ppy)3 at high excitation densities.[10] However,when comparing the slow component of the two simplest com-plexes, we see that there is a clear change in the rate of quench-ing; adding the two t-butyl groups almost halves the quenchingrate, which, consistent with the PA data, confirms that the rateof energy transfer to the PSBF host is strongly reduced.

2.4. The Effect of Substituting at the Phenyl Ring Compared toat the Pyridyl Ring

If we now consider the more complicated case of the singlysubstituted ligands, complexes 3 and 4, we must remember thatthe effect of the fac isomerism of the complex is to shield eitherthe phenyl or pyridyl ring of the ligand. The data in Figure 6shows that when the t-butyl groups are substituted onto thepyridyl ring, as in 3, the energy transfer is slowed more thanwhen the substitution is made just on the phenyl part of the li-gand (time constant of 6.3 ns compared to 5.3 ns, respectively).If we return once again to the emission spectra in Figure 4, wecan see the same thing; there is more emission from the com-plex for dopant 3 which has the substitution on the pyridyl ring,than for complex 4. Furthermore, for phosphorescence quench-ing identical behavior is observed, with complex 3 showing thesame slower rate of quenching as the doubly substituted com-plex 2, and complex 4 the same as that of the unsubstitutedcomplex. Thus, forming a shell of inert groups around the halfof the molecule containing the pyridyl rings has the same effectas encasing the whole molecule in inert groups.

The implications for the electron-transfer route are clear: ifthe electron transfer is slowed more when the polymer is heldaway from the pyridyl ring, then we can deduce that the elec-trons transfer more favorably via the pyridyl ring rather thanthe phenyl. This confirms the theoretical calculations by Haythat the triplet MLCT state is more localized on the pyridylring. There are two factors which could contribute to this:firstly, the increased electronegativity of the nitrogen atomcompared to carbon, and, secondly, the fact that the nitrogenlone pair is not conjugated into the ring, leaving it electron de-ficient. Both these factors make the pyridyl ring more ready toaccept an electron during the MLCT excitation. When we nowconsider the nature of the energy transfer, the effect of substi-tuting at the pyridyl rather than at the phenyl ring is much

greater, as it holds the polymer chain away from the excited or-bital that acts as the donor in the energy-transfer process,rather than just generally increasing the complex–polymerspacing, reducing the efficiency of the triplet-energy transferfrom the complex to the polymer host.

2.5. Electrophosphorescent Device Characteristics

Device characteristics of all three dopants 2–4 in the samehost polymer (PSBF) indicate that the current–voltage (I–V)characteristics vary with the type of the dopant (Fig. 9). Currentstudies[26] of the electrical properties of phosphorescent organiclight-emitting diodes (OLEDs) as a function of dopant concen-tration indicated that higher dopant concentration induces high-

er electron conductivity, which fills the lowest trap state of theactive layer thereby shifting the Fermi level upwards, enhancingthe mobility of the carrier and reducing the turn-on voltage. Theeffect of electron conduction through the dopant is clearly seenin the I–V characteristics of our devices as the complexes forwhich the pyridyl sites were blocked (2, 3) show lower currentsdue to lower electron conduction through the device.

The external quantum efficiency and luminescence intensityare shown in Figures 10 and 11. Firstly, we must note that theefficiencies are low because of the problem of triplet transferfrom the phosphorescent dopant to the host. However, devicescontaining the doubly substituted complex have efficiencies twoto four times higher than the monosubstituted complexes, dueto a reduction of the triplet-energy transfer; correspondinglycomplex 3 shows greater efficiency then complex 4. When oneconsiders that the photoluminescence quantum yield of the dou-bly substituted complex is lower, the change in energy transferis further increased to six times on going from complex 4 to com-

1048 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1043–1050

Table 1. Phosphorescence quenching times (s1,2) for the various blends in-vestigated. A1,2 represent the comparative strengths of the two compo-nents.

Blend A1 s1 [ns] A2 s2 [ns]

Ir(ppy)3 (1):PSBF 0.83 2.3 0.17 5.4

4:PSBF 0.83 2.2 0.17 5.4

3:PSBF 0.89 2.5 0.11 9.7

2:PSBF 0.9 2.4 0.1 9.7

Figure 9. I–V characteristics of doped devices made with PSBF and the var-oius cyclometalating ligands.

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plex 2. Figure 12 shows the electroluminescence spectra of thedoped devices. At variance to the film photoluminescence spec-tra, these show that for the devices the emission originates al-most entirely from the dopants. This suggests that, in commonwith other polyfluorene-based devices and the generally ac-cepted view,[27–29] the dopant is acting as the primary emitter,i.e., charge trapping occurs on the dopant and so emission onlyoccurs on the host when all the dopant sites are saturated.

3. Conclusion

The data presented demonstrates some of the factors affect-ing the efficiency of electrophosphorescent devices in whichtriplet transfer from the phosphor to the host is energeticallyfavorable by using a phosphor with bulky side groups. Thebulky side groups were found to reduce the energy transfer byreducing the orbital overlap necessary for Dexter transfer to

take place. A halving of the rate of the energy transfer wasachieved by substituting a t-butyl group on to each ring in theligands of Ir(ppy)3. The substitutions form a shell, effectivelypreventing energy transfer from the 3MLCT state to the poly-mer. However, conversely, we have also shown that the substi-tution reduces the efficiency of the emission of the complex byincreasing the nonradiative-decay rate which, in turn, affectsthe overall efficiency of the device. When this is considered, weget three to six times improvement in device performancewhen two t-butyl substitutions are made on the ligand of thedopant compared to a single substitution. In addition to show-ing that the rate of energy transfer between Ir(ppy)3 and apolymer host can be controlled by the addition of bulky substi-tutions, the use of specific substitutions on the ligands has al-lowed the identification of the pathway of energy transfer be-tween host and guest as being through the pyridyl ring on theligand; this is shown by the change in the rate of energy trans-fer and the increase in the current through the complex mani-fold when the pyridyl ring is left free of substitution.

4. Experimental

The complexes were prepared by established routes. Iridium chlo-ride was reacted with two molar equivalents of the substituted phenyl-pyridine ligand in a mixture of 2-ethoxyethanol/water (2:1) at 160 °Covernight, and the yellow l-chlorodimer was isolated and purified. Thisdimer was reacted with a further equivalent of the ligand in the pres-ence of silver(I) triflate in ethylene glycol at 195 °C. The trimers wereisolated and purified by column chromatography.

The absorption spectra were obtained using a Perkin-Elmer Lambda-19 spectrophotometer with the complexes dissolved in chlorobenzene.Doped films, were prepared from a blend of the host polymer and each ofthe complexes containing the same concentration (5.3 × 10–3 M) of Iratoms to allow accurate comparison; the chosen molar ratio equates to ap-proximately 7 % doping by weight. Films were spun on sapphire sub-strates at 1200 rpm from solutions in chlorobenzene or toluene, yieldingfilms that were approximately 100 nm thick. Sapphire substrates wereused as they provide higher thermal conductivity than the more conven-tional quartz, which is preferable for low-temperature measurements. Itwas necessary to use chlorobenzene for the unsubstituted complex (1) be-cause it is insoluble in toluene. The high molecular weight of

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Figure 10. External quantum efficiencies (EQE) of the doped devices; theincrease in efficiency on addition of the side groups is clear.

Lum

inescence (

cd/m

2)

Figure 11. Luminescence characteristics of the doped devices.

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Wavelength (nm)

Figure 12. Electroluminescence spectra of the doped devices (from bot-tom, 1–4).

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770 000 g mol–1 results in high-quality amorphous films. The photolumi-nescence of the blend films was measured following excitation at 355 nmusing a Jobin–Yvon Fluoralog-3 fluorimeter.

PA spectra were measured using a conventional quasi-continuouswave experiment outlined in the literature [22]. Briefly, excitation wasprovided by a 400 nm pulsed diode laser, and the absorption of the ex-cited state was measured by a tungsten or xenon lamp and a scanningmonochromator and photodiode; the ground-state absorption was re-moved by using standard lock-in techniques.

The PSBF PA was monitored using the time-resolved pump–probeexperimental procedure described in detail in the literature and shownin Figure 13 [22,30]. Briefly, a dye laser containing coumarin-500pumped by a 160 ps Nd:YAG (YAG: yttrium aluminum garnet) laserwas used to excite the sample at 500 nm exclusively within the absorp-tion band of the MLCT states on the complex. This was done to ensurethat the PSBF host was not directly excited, such that any excitationswhich subsequently formed on the polymer host must be the result ofenergy transfer from the complex. The probe beam was a 785 nm diodelaser chosen to be at the peak of the PA of the triplet state of PSBF andwell away from the PA of the complex. As Figure 7 shows, the probewas monitored by a 2 GHz photodetector and transimpedance ampli-fier (Femto GmbH), and the trace was recorded on a 1 GHz digital os-cilloscope (Agilient Infiniuum), allowing the data to be averaged overmany pump pulses. This apparatus allows sub-nanosecond time resolu-tion, and the use of the diode laser as probe beam rather than the moreconventional white-light source removes the need for light-collectionoptics, which focus the fluorescence of the sample onto the detector,causing saturation. This reduces the time resolution of the experimentto outside the radiative lifetime of the sample. All the data presentedhere were obtained at 80 K in order to slow down triplet-migrationprocesses that result in triplet–triplet annihilation on the host and thatcould obscure the effects of the energy transfer. This experiment allowsus to monitor the build-up of the PSBF triplets directly as the energytransfer from the complex populates the PSBF.

Phosphorescence decays were measured using a time-gated intensi-fied CCD-camera-based (CCD: charge-coupled device) detection sys-tem [31]. The emission was focused onto the slit of a monochromator fol-lowing excitation by the third harmonic of a Nd:YAG laser at 355 nm.The light was dispersed through the monochromator onto a time-gatedCCD camera, and by varying the time period between excitation and ex-posure and the integration time of the CCD, the decay of the phosphor-escence could be recorded. So that the lifetimes could be compared withthe PA measurements, all of the data was recorded at 80 K.

Electrophosphorescent devices were constructed with the followingunoptimized configuration; ITO/PEDOT:PSS/PSBF:Ir[CN]3/Ca/Al

(ITO: indium tin oxide; PEDOT: poly(3,4-ethylenedioxythiophene);PSS: poly(styrene sulfonic acid)) with identical Ir concentrations(5.3 × 10–3

M), where (CN) is a monoanionic cyclometalating ligand(1–4). To ensure that the devices were tested with the minimum of deg-radation, they were tested at room temperature under vacuum on thesame day as manufacture.

Received: December 8, 2005Final version: January 27, 2006Published online: April 7, 2006

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Nd:Y

AG

laser

(160ps)

Trigger Diode

785nm CW Diode Laser

Photodetector

Si p-i-n photodiode

Digital Oscilloscope(1GHz)

Sample film in cryostat

Figure 13. Apparatus for time-resolved transient-absorption measure-ments, comprising a pulsed Nd:YAG (YAG: yttrium aluminum garnet) la-ser for excitation and a continuous-wave diode laser for probing.

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