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Reactions of singlet excitons in tris-(8-hydroxyquinoline) aluminum
ELSEVIER
S~iTlthrtiCMetalS84(1997)921-922
Reactions
of singlet excitons in tris-(&hydroxyquinoline)
aluminum
I. Sokolik”, A. D. Walser”, R. Priestley’, C. W. Tangb, and R. Dorsinville” “Dept. of Electrical Engr., The City College of CUNY, 138th St. at Convent Ave. New York, NY 10031, USA bEastman Kodak Company, Rochester, NY 14650, USA Abstract
Singlet excited state reactions in thin films of fris(&hydroxyquinoline) aluminum (Alqs), a well known emitter for organic electroluminescent devices, are described. Bimolecular recombination dominates the sin let exdton decay in pristine films at . high intensities, thus decreasing the photoluminescence quantum ew$cienyy-yd singlet hf ebme. The measured rate constant of the singlet-singlet annihilation in Alq, films is yss = (3.5ti.5) X 10 cm s . The value of the diffusion coefficient of singlet excitons estimated from yss is Ds = (l.m.8) X low5(1~1’s~*. Keywords: Photolumlnescence, Time-resolved fast spectroscopy, Organic semiconductors based on conjugated molecules. Tris-(&hydroxyquinoline) aluminum (Alq,) has been widely used as an emitter for organic light emitting diodes. However, very little information on the photo hysics of this material, in general, and the reactions of singPet excited states, in particular, has been ained since the first publications by the Kodak group 9 1, 21. Because singlet excltons are created in high concentrations in the process of electroluminescence (EL), knowledge of their reactions is important for the design of EL devices and handling of the EL materials.
0.8 0.6 E
F G 72
2 0.4
52
j
0.2
0.0 300
350
400
Wavelen&
Alqs is a chelate compound with an Al ion surrounded ligands. The ligands are absorption of li ht and a photon by a moHecule in the ground state, SD creates a singlet excited state, Si So+ hv, ---> Si which can deactivate nonradiatively producing a luminescence photon -->
Sl
Soc hv.
or
(1) radiatively (2)
Absorption and photoluminescence (PL) spectra of Alqs are shown in Fi . 1. The singlet lifetime has been measured for both thin cfns and toluene solutions [2] at low intensity laser excitation where the singlet state population decays exponentially with a decay time zs = 14-16 ns. PL quantum efficiency $Iin the solid state is 8 % according to C. W. Tang ef al. [2] and (32&2)% according to the more recent work of D. Z. Garbuzov et al. [4]. We have shown elsewhere [5, 61 that photooxidation of Alg decreases $ because of the diffusion-controlled quenching of singlet excitations S1by the products of photooxidation. In turn photooxidation 03796779/971$17.00
Q 1997 Elsevier
PII SO379-6779(96)04211-7
Science S.A. All rights reserved
450
500
550
600
(run)
Fig. 1. Absorption (ABS) and PL spectra of pristine Alq3 film. Excitation spe&um of fluorescence (PLE) was measured for emission at 500 nm. hap ens as a result of the interaction of a singlet exciton witi oxygen [5], possibly including formation of a singlet oxygen and Alq, triplet, as an intermediate step. It is important to note that this reaction requires W light, contrary to the dark oxidation of Alg by water [7]. Bimolecular recombination of singlet excitors is another recess effecting the quantum efflclency and PL lifetime. Jihis process can be represented by the following scheme: s, + s1 ---> s,” + s,
(3)
where S,* is the vibronically excited singlet state and r&l. Autoionization of S,* leads to the generation of charge carriers, i.e. photoconductivity [8]. Quantum efficiency of photogeneration of charge carriers in Alq, is about (6-10) X 10-6 electrons/photon as measured by the transient photoconductivity [9]. Like in molecular crystals, S,* deactivates primarily into the lowest singlet state S1,thus contributing to the PL: s,* ---> Sl. (4)
922
I. Sokolik et al. /Synthetic Metals 84 (1997) 921-922
Quantitatively, of the kinetics of PL decay and quantum ef described on the basis of the kinetic equation for e concentration of singlet molecular exdtons [S,] in the absenceof quenching centers:
1 0.7 3
0.5
zi.*
0.3
dhl /dt = aI - (kr + km)[SJ - fuss[S~l*~ (5) where a is the abso tion coefficient, I is the intensity of the excitation, kr an‘;pknr are radiative and nonradiative rates of the monomolecular decay of singlet excitons, yss is the rate constant for bimolecular recombination, and f = l/2 is aparameter which takes into account losses due to autoiomzation and direct nonradiative relaxation to S, from states above the singlet exclton state [8]. For the case when the o&e duration is much shorter than the observation time and l/o < d, where d is the film thickness, an expression for $ as a function of excitation intensity had been obtained in Ref. 11, where 4. is the PL quantum efficiency at low intensities, k = k + k, r = c1yss, and z is variable of integration:
We measured this dependence using 355 ran excitation (a = 4.14 X lo4 cm-l) with 25 ps laser pulses with a repetition rate of 20 Hz.(Fig.2). Pulse energies ranged from 1 PJ to 220 PJcorresponding to an incident flux of 4.2 X 1O23 - 9.4 X 1O25 photons/cm2s. For that wavelength, excitation occurred in the bulk of the samole and almost all exciting photons were absorbed in a 2 pm thick sam le. Using Eq. g to fit the experimental data presented in Plg. 2, we were able to determine the value of yss = (5.0F0.5)X10-11 an3s-
1. Independent estimates of yss = (2+1) X 1011 cm3 s-l were obtained b fitting the several experimental PL decay curves usinn J a. 5 (see Fie. 3). Based on these two methods welave’found that”the~value of the rate constant of singlet-singlet annihilation yss in Alq3 is between 1.0 X lo-l1 cm3 s-l and 5.5 X lo-l1 cm3 s-l.
Incident 1010 1 0.7-r -8
0.5 I
2
0.3 0.2 0.15
0.1/z
4
intensity
1011 1012 *--. 7
1013
(photons / cm2) 1014
1015
1016
\;’ \0
cl k
0.2 0.15 0.1
10
20
40
Time (ns) Fi% 3. Fluorescence decay curves measured for low (3.5 X 10 4 photons/an;! s) and high (3.5 X 1oz5 photons/cm2 s) intensities of exdtation at 355 run. The solid lines through the experimental points were calculated using Eq. 5. Using these numbers for yss, the value of the diffusion coefficient of singlet excitons D, was estimated from the equation yss = SnD,R + 8nR2(Dsk)li2, (7) where R is the spherical reaction radius [8,11]. Taking R = 10s7 cm we obtained values for Ds within the range (1.2 M.8) X low5cn?s-‘. This work was s onsored b NSF under grants Nos. HRD-8802964, ECS-6212310, E8 S-9520561, ECS-9311608 New York State Technology Foundation. R. P. is to the NSF Center for Analysis of Structures and terfaces (CASI) for fellowship. References
PI cw. Tan and S. A. VanSlyke, Appl. Phys. Left. 51 1987) 912
El i:: W. Tang, S. A. VanSlyke and C. H. Chen, 1. Appl.
Phys., 65 (1989) 3610. [31 C A. Parke?r, Photoluminescence qf solutions, Elsevier. Amsterdam, 19t [41 D. Z. Garbuzov ~“%?l??ovic, S. R. Forrest, Chem. Phvs: J Left., 249 (1996) 433. [51 R. Priestley, I. Sokolik, A. D. Walser, C. W. Tang, and R. Dorsinville, these Proceedings, p ....... [61 I. Sokolik, A. Walser, R. Priestley, R. Dorsinville, and C W. Tan in: Elecfr., 0 f. ,and Magn. Pro of Org. Sol. StareA?afer. III (MRS symp. Proc. Ser., 4.01. 413, MRS, [71
‘:-(\ . + . .\
Bl N \
\
Fig 2. Dependence of the relative quantum efficiency of PL @loo on the intensity of the exciting light I. Squares experimental points, the broken line was calculated using Eq.6 for yss = 5.0 X lo-l1 cm3 s-l.
PI -,’ Interscience, London, i973), @.495. [lo] A. Walser, R Priestley, I. Sokolik, and R. Dorsinvllle, Ap 1.Ph s. Lett., to be published. [ll] I. &koli& R. Prlestley, A. Walser, and R. Dorsinville, to be published. [12]. A. J, Campillo, R. C H er, S. L. Sha iro, and C E. Swenberg, Chem. Phys. I!eff. 48 1977) B95.
S~iTlthrtiCMetalS84(1997)921-922
Reactions
of singlet excitons in tris-(&hydroxyquinoline)
aluminum
I. Sokolik”, A. D. Walser”, R. Priestley’, C. W. Tangb, and R. Dorsinville” “Dept. of Electrical Engr., The City College of CUNY, 138th St. at Convent Ave. New York, NY 10031, USA bEastman Kodak Company, Rochester, NY 14650, USA Abstract
Singlet excited state reactions in thin films of fris(&hydroxyquinoline) aluminum (Alqs), a well known emitter for organic electroluminescent devices, are described. Bimolecular recombination dominates the sin let exdton decay in pristine films at . high intensities, thus decreasing the photoluminescence quantum ew$cienyy-yd singlet hf ebme. The measured rate constant of the singlet-singlet annihilation in Alq, films is yss = (3.5ti.5) X 10 cm s . The value of the diffusion coefficient of singlet excitons estimated from yss is Ds = (l.m.8) X low5(1~1’s~*. Keywords: Photolumlnescence, Time-resolved fast spectroscopy, Organic semiconductors based on conjugated molecules. Tris-(&hydroxyquinoline) aluminum (Alq,) has been widely used as an emitter for organic light emitting diodes. However, very little information on the photo hysics of this material, in general, and the reactions of singPet excited states, in particular, has been ained since the first publications by the Kodak group 9 1, 21. Because singlet excltons are created in high concentrations in the process of electroluminescence (EL), knowledge of their reactions is important for the design of EL devices and handling of the EL materials.
0.8 0.6 E
F G 72
2 0.4
52
j
0.2
0.0 300
350
400
Wavelen&
Alqs is a chelate compound with an Al ion surrounded ligands. The ligands are absorption of li ht and a photon by a moHecule in the ground state, SD creates a singlet excited state, Si So+ hv, ---> Si which can deactivate nonradiatively producing a luminescence photon -->
Sl
Soc hv.
or
(1) radiatively (2)
Absorption and photoluminescence (PL) spectra of Alqs are shown in Fi . 1. The singlet lifetime has been measured for both thin cfns and toluene solutions [2] at low intensity laser excitation where the singlet state population decays exponentially with a decay time zs = 14-16 ns. PL quantum efficiency $Iin the solid state is 8 % according to C. W. Tang ef al. [2] and (32&2)% according to the more recent work of D. Z. Garbuzov et al. [4]. We have shown elsewhere [5, 61 that photooxidation of Alg decreases $ because of the diffusion-controlled quenching of singlet excitations S1by the products of photooxidation. In turn photooxidation 03796779/971$17.00
Q 1997 Elsevier
PII SO379-6779(96)04211-7
Science S.A. All rights reserved
450
500
550
600
(run)
Fig. 1. Absorption (ABS) and PL spectra of pristine Alq3 film. Excitation spe&um of fluorescence (PLE) was measured for emission at 500 nm. hap ens as a result of the interaction of a singlet exciton witi oxygen [5], possibly including formation of a singlet oxygen and Alq, triplet, as an intermediate step. It is important to note that this reaction requires W light, contrary to the dark oxidation of Alg by water [7]. Bimolecular recombination of singlet excitors is another recess effecting the quantum efflclency and PL lifetime. Jihis process can be represented by the following scheme: s, + s1 ---> s,” + s,
(3)
where S,* is the vibronically excited singlet state and r&l. Autoionization of S,* leads to the generation of charge carriers, i.e. photoconductivity [8]. Quantum efficiency of photogeneration of charge carriers in Alq, is about (6-10) X 10-6 electrons/photon as measured by the transient photoconductivity [9]. Like in molecular crystals, S,* deactivates primarily into the lowest singlet state S1,thus contributing to the PL: s,* ---> Sl. (4)
922
I. Sokolik et al. /Synthetic Metals 84 (1997) 921-922
Quantitatively, of the kinetics of PL decay and quantum ef described on the basis of the kinetic equation for e concentration of singlet molecular exdtons [S,] in the absenceof quenching centers:
1 0.7 3
0.5
zi.*
0.3
dhl /dt = aI - (kr + km)[SJ - fuss[S~l*~ (5) where a is the abso tion coefficient, I is the intensity of the excitation, kr an‘;pknr are radiative and nonradiative rates of the monomolecular decay of singlet excitons, yss is the rate constant for bimolecular recombination, and f = l/2 is aparameter which takes into account losses due to autoiomzation and direct nonradiative relaxation to S, from states above the singlet exclton state [8]. For the case when the o&e duration is much shorter than the observation time and l/o < d, where d is the film thickness, an expression for $ as a function of excitation intensity had been obtained in Ref. 11, where 4. is the PL quantum efficiency at low intensities, k = k + k, r = c1yss, and z is variable of integration:
We measured this dependence using 355 ran excitation (a = 4.14 X lo4 cm-l) with 25 ps laser pulses with a repetition rate of 20 Hz.(Fig.2). Pulse energies ranged from 1 PJ to 220 PJcorresponding to an incident flux of 4.2 X 1O23 - 9.4 X 1O25 photons/cm2s. For that wavelength, excitation occurred in the bulk of the samole and almost all exciting photons were absorbed in a 2 pm thick sam le. Using Eq. g to fit the experimental data presented in Plg. 2, we were able to determine the value of yss = (5.0F0.5)X10-11 an3s-
1. Independent estimates of yss = (2+1) X 1011 cm3 s-l were obtained b fitting the several experimental PL decay curves usinn J a. 5 (see Fie. 3). Based on these two methods welave’found that”the~value of the rate constant of singlet-singlet annihilation yss in Alq3 is between 1.0 X lo-l1 cm3 s-l and 5.5 X lo-l1 cm3 s-l.
Incident 1010 1 0.7-r -8
0.5 I
2
0.3 0.2 0.15
0.1/z
4
intensity
1011 1012 *--. 7
1013
(photons / cm2) 1014
1015
1016
\;’ \0
cl k
0.2 0.15 0.1
10
20
40
Time (ns) Fi% 3. Fluorescence decay curves measured for low (3.5 X 10 4 photons/an;! s) and high (3.5 X 1oz5 photons/cm2 s) intensities of exdtation at 355 run. The solid lines through the experimental points were calculated using Eq. 5. Using these numbers for yss, the value of the diffusion coefficient of singlet excitons D, was estimated from the equation yss = SnD,R + 8nR2(Dsk)li2, (7) where R is the spherical reaction radius [8,11]. Taking R = 10s7 cm we obtained values for Ds within the range (1.2 M.8) X low5cn?s-‘. This work was s onsored b NSF under grants Nos. HRD-8802964, ECS-6212310, E8 S-9520561, ECS-9311608 New York State Technology Foundation. R. P. is to the NSF Center for Analysis of Structures and terfaces (CASI) for fellowship. References
PI cw. Tan and S. A. VanSlyke, Appl. Phys. Left. 51 1987) 912
El i:: W. Tang, S. A. VanSlyke and C. H. Chen, 1. Appl.
Phys., 65 (1989) 3610. [31 C A. Parke?r, Photoluminescence qf solutions, Elsevier. Amsterdam, 19t [41 D. Z. Garbuzov ~“%?l??ovic, S. R. Forrest, Chem. Phvs: J Left., 249 (1996) 433. [51 R. Priestley, I. Sokolik, A. D. Walser, C. W. Tang, and R. Dorsinville, these Proceedings, p ....... [61 I. Sokolik, A. Walser, R. Priestley, R. Dorsinville, and C W. Tan in: Elecfr., 0 f. ,and Magn. Pro of Org. Sol. StareA?afer. III (MRS symp. Proc. Ser., 4.01. 413, MRS, [71
‘:-(\ . + . .\
Bl N \
\
Fig 2. Dependence of the relative quantum efficiency of PL @loo on the intensity of the exciting light I. Squares experimental points, the broken line was calculated using Eq.6 for yss = 5.0 X lo-l1 cm3 s-l.
PI -,’ Interscience, London, i973), @.495. [lo] A. Walser, R Priestley, I. Sokolik, and R. Dorsinvllle, Ap 1.Ph s. Lett., to be published. [ll] I. &koli& R. Prlestley, A. Walser, and R. Dorsinville, to be published. [12]. A. J, Campillo, R. C H er, S. L. Sha iro, and C E. Swenberg, Chem. Phys. I!eff. 48 1977) B95.