Phosphorescent-sensitized triplet-triplet annihilation in tris„8 ...

Phosphorescent-sensitized triplet-triplet annihilation 

in tris„8-hydroxyquinoline… aluminum 

Isao Tanaka a and Shizuo Tokito 

Nippon Hoso Kyokai (NHK) Science and Technical Research Laboratories, Kinuta, Setagaya-ku, 

Tokyo 157-8510, Japan 

Received 25 February 2005; accepted 31 March 2005; published online 3 June 2005 

We characterized the photoluminescence properties of an amorphous tris8-hydroxyquinoline 

aluminum Alq3 thin film heavily doped with fac tris2-phenylpyridine iridium Irppy3 at8K. 

Not only green fluorescence but also red phosphorescence from Alq3 was clearly observed, where 

Irppy3 plays the important role as a phosphorescent sensitizer for Alq3. The triplet energy of Alq3 was estimated to be 2.03 eV from the highest energy peak of the phosphorescence spectrum. The 

fluorescence intensity was proportional to the excitation power. On the other hand, the deviation 

from the linearity of the phosphorescence intensity to the excitation power was observed above 

0.01 W/cm2 . This nonlinear phosphorescence behavior is well explained by the simple 

triplet-triplet annihilation theory. It was demonstrated that the efficient triplet energy transfer from 

Irppy3 enables us to observe triplet-triplet annihilation in Alq3.©2005 American Institute of 

Physics. DOI: 10.1063/1.1925764 

I. INTRODUCTION 

Tris8-hydroxyquinoline aluminum Alq 3 is a representative 

fluorescent material in the research field of organic 

light-emitting devices OLEDs, and has been widely used as 

the emitting layer or the electron-transporting layer since it 

was discovered to produce efficient electroluminescence 

EL by Tang and VanSlyke. 1 However, until recently, there 

were only a few reports on triplet excitons in Alq 3 because 

attempts to observe the phosphorescence proved 

unsuccessful. 2,3 The phosphorescence quantum yield of Alq 3 

has been reported to be extremely low because of the negligible 

intersystem crossing ISC due to the poor heavy-atom 

effect for a light metal such as aluminum. 3,4 Recently, Burrows 

et al. 5 observed clear phosphorescence from Alq 3 in an 

ethyl iodide glass matrix at 77 K by promoting the ISC due 

to the presence of the heavy-atom iodine, and estimated the 

triplet energy to be 2.17±0.10 eV. Cölle and co-workers observed 

the phosphorescence from polycrystalline Alq 3 by 

measuring time-resolved photoluminescence PL spectra, 6 

and determined the triplet energy to be 2.05±0.1 eV from 

the delayed EL spectra of Alq 3-based OLEDs. 7 More recently, 

they determined the triplet energies of the meridional 

and facial isomers in - and -Alq 3 to be 2.11±0.1 and 

2.16±0.1 eV, respectively. 8 

Baldo et al. reported the concept of “phosphorescent 

sensitization” for improvement of the emission efficiency of 

fluorescent dyes in small-molecule-based OLEDs. 9 Recently, 

phosphorescent sensitization was applied for the fluorescent 

dyes in polymer-based OLEDs 10 and white OLEDs. 11,12 

Goushi et al. 13 reported the phosphorescence enhancement 

of the fluorescent molecule, 4,4’-bisN-1-naphthyl- 

N-phenyl-aminobiphenyl -NPD by using the greenemitting 

phosphorescent sensitizer, fac tris2-phenylpyridine 

a 

Author to whom correspondence should be addressed; electronic-mail: 

tanaka.i-eo@nhk.or.jp 

JOURNAL OF APPLIED PHYSICS 97, 113532 2005 

iridiumIII Irppy 3, whose triplet energy is higher than 

that of -NPD. Similarly, we have succeeded in detecting the 

phosphorescence from 4,4’-N,N’-dicarbazole-biphenyl 

CBP by doping blue-emitting bis4,6-difluorophenylpyridinato-N,C 

2’ picolinate iridiumIII Flrpic, whose 

triplet energy is higher than that of CBP. 14 Very recently, an 

efficient phosphorescent-sensitized photocyclization has 

been reported for photochromic dithienylethene derivatives 

containing ruthenium metal units. 15 

In this work, we prepared an amorphous Alq 3 thin film 

heavily doped with Irppy 3, and studied the energy-transfer 

mechanism and the phosphorescence behavior using PL 

spectroscopy. We have succeeded in observing the 

phosphorescent-sensitized annihilation of two triplet excitons, 

triplet-triplet T-T annihilation, in Alq 3 as the deviation 

from the linearity of the phosphorescence intensity to 

the excitation power above 0.01 W/cm 2 . This excitationpower 

dependent phosphorescence behavior was analyzed on 

the basis of the simple T-T annihilation model proposed by 

Baldo et al. 16 

II. EXPERIMENT 

An amorphous Alq 3 thin film heavily doped with 50 

wt % Irppy 3 Alq 3–Irppy 3 was deposited using highvacuum 

thermal evaporation through a metal mask with a 

1010-mm 2 opening onto a precleaned quartz substrate that 

was 2020 mm 2 in size and 0.5 mm thick. The chemical 

structures of Alq 3 and Irppy 3 are illustrated in Fig. 1. The 

organic thin film was 50 nm thick. Since it is well known 

that phosphorescence is affected by oxygen, the thin film was 

encapsulated with a glass cap in a nitrogen atmosphere using 

UV-epoxy adhesive. 

The sample was set in a contact-type cryostat Daikin 

Industries, UV202CLS and cooled at 8 K. The emission 

light was dispersed by a 10-cm single monochromator Koken 

Kogyo, SG-100 with 4-nm resolution, and detected by a 

0021-8979/2005/9711/113532/4/$22.50 97, 113532-1 

© 2005 American Institute of Physics 

Downloaded 16 Sep 2005 to 210.224.157.20. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

113532-2 I. Tanaka and S. Tokito J. Appl. Phys. 97, 113532 2005 

FIG. 1. Chemical structures of Alq 3 and Irppy 3. 

photomultiplier tube Hamamatsu Photonics, R955. For the 

photoexcitation, a diode-pumped passively Q-switched 

Nd:YAG yttrium aluminum garnet laser Crystal GmbH, 

FTSS355-Q with a wavelength of 355 nm was applied. The 

excitation power density at the sample surface was varied in 

a wide range from 0.003 to 5 W/cm 2 by using neutraldensity 

ND filters. 

III. RESULTS AND DISCUSSION 

The PL spectra of the Alq 3 neat thin film and the 

Alq 3–Irppy 3 thin film at 8 K are shown in Fig. 2a. The 

Alq 3 neat thin film exhibited strong green fluorescence emission 

at around 520 nm. For the Alq 3–Irppy 3 thin film, the 

Alq 3 fluorescence intensity drastically decreased, and a vibronic 

structured emission in the wavelength range from 600 

to 800 nm was dominant. Figure 2b shows a photograph of 

the PL emission from the Alq 3–Irppy 3 thin film mounted in 

the cryostat sample holder cooled at 8 K, which was taken by 

FIG. 2. Color a Photoluminescence spectra of the Alq 3 neat thin film and 

the 50 wt % Irppy 3-doped Alq 3 thin film at 8 K. The arrow indicates the 

wavelength corresponding to the triplet energy of Alq 3. b Photograph of 

the PL emission from 50 wt % Irppy 3-doped Alq 3 thin film at 8 K. The 

excitation wavelength was 355 nm and the power was measured to be about 

0.1 W/cm 2 . 

FIG. 3. Schematic energy level alignment of singlet-excited state S 1, 

triplet-excited states T 1, and singlet-ground states S 0 in Alq 3 and Irppy 3 

and the energy-transfer and light-emission processes. 

a conventional charge-coupled device CCD camera. Interestingly, 

a red emission could be obtained by heavily doping 

the green phosphorescent Irppy 3 into the green fluorescent 

Alq 3. This red emission showed a relatively long PL lifetime, 

estimated to be 5.3 ms at 8 K. 17 The highest energy peak of 

the PL spectrum of the Alq 3–Irppy 3 thin film, indicated by 

the arrow in Fig. 2a, was at 2.03 eV. This energy is close to 

the theoretical triplet energy 2.13 eV of Alq 3 reported by 

Martin et al. 18 from time-dependent density-functional calculations, 

and comparable to the experimental values reported 

by Barrows et al. 5 and Cölle and co-workers. 6–8 The lifetime 

and energy obtained from our PL measurements clearly indicate 

that the red emission from the Alq 3–Irppy 3 thin film 

is due to the phosphorescence from Alq 3. 

Here, we discuss the energy-transfer and light-emission 

mechanisms in the Alq 3–Irppy 3 thin film. Figure 3 shows 

the schematic energy level alignment of the singlet-excited 

states S 1, triplet-excited states T 1, and singlet-ground 

states S 0 in Alq 3 and Irppy 3. After the photoexcitation, 

both the S 1 in Alq 3 and the S 1 in Irppy 3 are generated, and 

the prompt fluorescence of the order of nanoseconds from 

Alq 3 consequently occurs. Also, the delayed fluorescence 

with a longer lifetime should be considered. We compared 

the PL spectrum of the Alq 3 neat thin film with the absorption 

spectrum of the Irppy 3 neat thin film, and found a 

significant spectral overlap between the fluorescence band of 

the S 1→S 0 transition in Alq 3 and the absorption band of the 

S 0→T 1 transition in Irppy 3. 17 This suggests that the S 1 in 

Alq 3 can transfer to the T 1 in Irppy 3 through the long-range 

process of Förster energy transfer by dipole-dipole coupling 

shown in Fig. 3. For Irppy 3, the nearly 100% conversion of 

the rapid ISC from the S 1 to the T 1 might occur because of 

the strong spin-orbit coupling. 19 The phosphorescence from 

Irppy 3 was not observed in the Alq 3–Irppy 3 thin film, 

which indicates the extremely efficient energy transfer from 

the T 1 in Irppy 3 to the T 1 in Alq 3. This triplet energy transfer 

is ascribed to the short-range process of Dexter energy 

transfer, which requires an overlap of the molecular orbital 

of adjacent molecules. From these results, it is concluded 

that the Alq 3 phosphorescence in the Alq 3–Irppy 3 thin 

film occurs via the two possible processes as shown 

in Fig. 3: S 1Alq 3→T 1Irppy 3→T 1Alq 3→S 0Alq 3 

and S 1Irppy 3→T 1Irppy 3→T 1Alq 3→S 0Alq 3. The 



FIG. 4. Photoluminescence spectra of the Alq 3–Irppy 3 thin film at 8 K 

under several excitation-power I exc conditions. 

more detailed PL properties, including the temperaturedependent 

phosphorescence decay dynamics, are discussed 

in a separate article. 17 

Figure 4 shows the PL spectra of the Alq 3–Irppy 3 thin 

film at 8 K under several excitation-power I exc conditions. 

For the lower excitation power of 0.05 W/cm 2 in Fig. 4 

bottom, the red Alq 3 phosphorescence in the wavelength 

range of 600 to 800 nm was dominant. However, with increasing 

excitation power, the green Alq 3 fluorescence 

tended to be relatively strong. It is clear from Fig. 4 that 

these two emissions showed a different excitation-power dependence 

of the intensity. 

The excitation-power dependence of the PL intensities 

for the fluorescence and phosphorescence is shown in Fig. 5. 

The plotted PL intensities were obtained by integrating over 

the wavelength. The fluorescence intensity I F was obviously 

proportional to the excitation power: I FI exc. On the 

other hand, the phosphorescence intensity was not proportional 

to the excitation power, especially above 

0.01 W/cm 2 . We discuss the difference in the excitationpower 

dependent fluorescence and phosphorescence behaviors 

as follows. 

FIG. 5. Excitation-power I exc dependence of the integrated PL intensities 

for the fluorescence and phosphorescence from Alq 3 in the 50 wt % 

Irppy 3-doped Alq 3 thin film at 8 K. The broken line indicates the result of 

the least squares fit to the Alq 3 fluorescence intensities. The solid curve is a 

fit to the Alq 3 phosphorescence intensities using Eq. 2 derived from the 

simple T-T annihilation theory. 

If the energy of the S 1 is less than the sum of the energies 

of the colliding T 1, the so-called P-type of the delayed 

fluorescence should occur. 20 Since the delayed fluorescence 

originates from the S 1→S 0 transition, its spectrum line shape 

is the same as observed in conventional cw-PL measurements. 

This process, in which the S 1 is populated by the 

T-T annihilation, produces one molecule in the S 1 whose 

lifetime is much longer than that of the spontaneous fluorescence, 

and can be written as 

T 1 + T 1 → S 0 + S 1 → S 0 + S 0. 1 

In this bimolecular process, the lifetime of the delayed fluorescence 

should be half of the value of the concomitant phosphorescence, 

and the S 1Alq 3 formation through T-T annihilation 

is expected to be proportional to the square of the 

excitation power. 20 However, it was found from our timeresolved 

PL measurements that the observed lifetime of the 

delayed fluorescence about 200 s was much shorter than 

half of the phosphorescence lifetime 5.3 ms, and that the 

delayed fluorescence was extremely weak. 17 The shorter life 

time and the weaker intensity of the delayed fluorescence 

than the expected might be due to Förster transfer of the 

S 1Alq 3→T 1Irppy 3 transition. Therefore, the weak Alq 3 

fluorescence observed at around 520 nm in the cw-PL spectra 

of the Alq 3–Irppy 3 thin film shown in Fig. 4 is mainly due 

to the prompt fluorescence. This results in the linearity of the 

fluorescence intensity to the excitation power shown in 

Fig. 5. 

Finally, we discuss the nonlinear phosphorescence behavior 

as a function of the excitation power. Baldo et al. 

demonstrated excitation-pulse-intensity dependent phosphorescence 

quenching due to T-T annihilation from the 

phosphorescence-transient-decay deviation from a single exponential 

profile with increasing pulse intensity. 16 According 

to their proposed simple T-T model, 16 the phosphorescence 

intensity I P can be derived as 

IP IexcP = 0I01+8 4 Iexc −1, 2 

I0 where P is the phosphorescence quantum yield, 0 the 

phosphorescence quantum yield in the absence of T-T annihilation, 

and I 0 the excitation power at P= 0/2. The solid 

curve shown in Fig. 5 is a fit to the phosphorescence intensities 

indicated by the solid circles using Eq. 2, and demonstrates 

good agreement with the behavior expected for bimolecular 

quenching. From the above fitting, the I 0 was 

determined to be 0.1 W/cm 2 , which corresponds to a limiting 

excitation power above which T-T annihilation dominates. 

In spite of the complicated energy transfer between 

Alq 3 and Irppy 3 in the Alq 3–Irppy 3 thin film discussed 

above, the phosphorescence behavior is well explained by 

the simple T-T annihilation theory. The long phosphorescence 

lifetime of the order of milliseconds of Alq 3 enables us 

to observe T-T annihilation under relatively low excitationpower 

conditions. 



IV. SUMMARY 

We characterized the PL properties of amorphous Alq 3 

thin film heavily doped with phosphorescent molecules, 

Irppy 3, at 8 K. Not only green fluorescence but also the 

phosphorescence from Alq 3 was clearly observed. The triplet 

energy of Alq 3 was estimated to be 2.03 eV from the highest 

energy peak of the phosphorescence spectrum. It was found 

that Irppy 3 plays an important role as a phosphorescent 

sensitizer for Alq 3. The Alq 3 fluorescence intensity was proportional 

to the excitation power. On the other hand, the 

deviation from the linearity of the phosphorescence intensity 

to the excitation power was observed above 0.01 W/cm 2 . 

In spite of the complicated energy transfer between Alq 3 and 

Irppy 3 in the Alq 3–Irppy 3 thin film, the phosphorescence 

behavior is well explained by the simple T-T annihilation 

theory. From this study, it was revealed that the efficient 

triplet energy transfer from Irppy 3 enables us to observe 

T-T annihilation in addition to the phosphorescence in Alq 3. 

Doping the phosphorescent sensitizer with higher triplet energy 

into the fluorescent molecule is a useful method for 

characterizing the various phosphorescence properties, including 

T-T annihilation, of fluorescent molecules. This 

work will be helpful in fundamental studies on the photophysics 

and photochemistry of organic materials and the application 

to organic devices such as OLEDs. 

ACKNOWLEDGMENTS 

We would like to thank Nippon Steel Chemical Co., Ltd. 

for providing the high-purity Alq 3 and Takasago Interna- 

tional Corporation for providing the high-purity Irppy 3.We 

wish to thank Yuichiro Tabata of Tokyo University of Science 

for preparing the thin-film samples. We also greatly 

appreciate the advice of Emeritus Professor Katsumi Tokumaru 

of University of Tsukuba. 

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