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Dynamic Behavior of Electroluminescence from Phosphor-Sensitized Red Fluorescent Organic Light-Emitting Diodes† Musubu Ichikawa,* Tomoya Aoyama, Junko Amagai, Toshiki Koyama, and Yoshio Taniguchi Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokita, Ueda City, Nagano 386-8567, Japan ReceiVed: NoVember 27, 2008; ReVised Manuscript ReceiVed: January 20, 2009
We study electronic excitation processes in the organometallic phosphor of fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) and fluorescent dye of 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1ij]quinolin-9-yl)-vinyl]-pyran-4-ylidene}-malononitrile (DCM2) double-doped organic light-emitting devices by using the transient electroluminescent technique. Excitation transfer from Ir(ppy)3 to DCM2 occurs in the device. However, excitons of DCM2 are directly created on DCM2 by charge carrier recombination, and the exciton formation process on DCM2 is preferred over that on Ir(ppy)3. This probably results from charge carrier trapping on DCM2 in the system. In addition, decay of DCM2 excitons in EL devices was faster than that in the case of photoluminescence. This means that the decay process is influenced by specific processes that occur under electric pumping conditions, such as the exciton-charge carrier annihilation. I. INTRODUCTION Impressive scientific and technological advances have been achieved in the field of organic light-emitting diodes (OLEDs) in the past decade1-3 due to potential applications for devices in a large variety of display technologies. To obtain the maximum luminous efficiency from OLEDs, it is necessary to harness both the spin symmetric and antisymmetric molecular excitations that result from electrical pumping. This is possible if the material is phosphorescent, and high efficiencies have been observed in phosphorescent OLEDs.3,4 An internal electronphoton conversion efficiency of nearly 100% has already been attained with fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3).5 In this context, many kinds of organometallic phosphorescent molecules utilizing heavy atoms have been developed.6-8 However, at present, phosphorescence in organic molecules is still rare at room temperature. The alternative radiative process of fluorescence is more common, and an enormous number of fluorescent dyes, which exhibit a wide variety of luminescent colors, have been synthesized, but fluorescence is approximately 75% less efficient due to the requirement of spin-symmetry conservation. If exciting a fluorescent dye using a phosphorescent sensitizer is possible, the internal efficiency of fluorescence can be as high as 100%, even with electrical pumping. This high efficiency can be maintained at dense current excitations because radiative transition probabilities of fluorophores higher than those of phosphors might reduce so-called roll-off due to triplet-triplet quenching.9,10 Phosphorescent-sensitized fluorescent OLEDs have been demonstrated by Forrest et al.11,12 Following these previous works, the radiative singlet excited state of the doped fluorescent dye is produced via the dipole-dipole Fo¨rster process from the triplet excited state of the phosphorescent sensitizer. The efficiency of Fo¨rster transfer is approximately independent of the radiative rates of the donor if the donor is efficiently phosphorescent.11 The Fo¨rster transfer radius of an appropriate combination of a phosphorescent donor and a †
Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. E-mail:
[email protected].
fluorescent acceptor is as long as 40 Å;12 this is comparable to that of a conventional fluorescent donor/acceptor system. Thus, a Fo¨rster transfer of high efficiency to produce the radiative singlet state of fluorophore probably occurs in phosphorsensitized fluorescent OLEDs. However, if the distance between the donor and acceptor becomes close, then the Dexter transfer occurs. In addition, direct recombination of electron and hole on a doped fluorophore is preferred over that on other sites because the fluorophore has the smallest band gap among all materials in OLEDs to ensure light emission from the fluorophore. These processes produce the nonradiative triplet excited-state of fluorophore, which leads to reducing the internal efficiency of electroluminescence (EL). These processes are crucial, but they are not fully understood at present. In particular, to the best of our knowledge, there is no report on cross-over behavior of Fo¨rster and Dexter transfers in phosphor-sensitized fluorescent OLEDs. We study these processes in phosphor and fluorophore double-doped OLEDs by using the transient EL technique. II. Experimental Sample devices were prepared by the following procedures. An indium-tin-oxide (ITO)-coated glass substrate was treated by O2 plasma for 5 min in advance. An organic layer, which consists of 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)-vinyl]-pyran-4-ylidene}-malononitrile (DCM2), Ir(ppy)3, 2-biphenyl-5-(4-tert-butylphenyl)oxadiazole (PBD), and polyvinyl carbazole (PVK) as a fluorescent dopant, phosphorescent material, electron transporting material, and hole-transporting matrix polymer, respectively, were prepared on the ITO substrate by using a spin coater under the condition of 3000 rpm for 60 s. Concentrations of these compounds were 74 wt % for PVK, 18 wt % for PBD, and 8 wt % for Ir(ppy)3, which was determined after optimizing to attain a high quantum efficiency of EL of about 6% without DCM2. The concentration of DCM2 was 0.13 wt %. If we added more DCM2, we obtained a clearer red EL that originated from DCM2, but the quantum efficiency of EL decreased much less
10.1021/jp810425u CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
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Figure 3. Two-dimensional (wavelength and time) EL intensity map from device under pumping with electric pulse of 23 V for 200 ns. The back line on the right represents the excitation electric pulse.
III. Results and Discussion Figure 1. Chemical structures used and energy diagram of used chemicals.
Figure 2. Schematic of transient EL measurement system. MC, monochromator; PM, photomultiplier; PA, preamplifier; and OSC, oscilloscope.
than 1%. At this concentration, we observed a red EL from DCM2, and the efficiency remained at almost 1%, at least. Then, Mg-Ag alloy (9:1 in volume) was deposited in a vacuum (3 × 10-4 Pa) as a cathode metal. The size of devices that corresponds to the overlap area of ITO and MgAg electrodes was set to be 1 mm2. Device structures and chemicals used in this study are shown in Figure 1. The transient EL waveforms were measured using a nanosecond apparatus,13 whose schematic details are shown in Figure 2. A fast electric pulse generator (model 2000D, Picosecond Pulse Laboratory.) that generates a rectangular voltage pulse of 200 ns width with a 500 ps risetime at a 10 Hz repetition rate was used to drive the device through an electric power attenuator (TRA-10B, Tamagawa Electronics). Light emitted from the sample OLED was detected by a photomultiplier (R7400U, Hamamatsu Photonics) through an optical fiber bundle and a monochromator. The signal from the photomultiplier was recorded via an ultrafast preamplifier with a fixed gain of 36 dB (C5594, Hamamatsu Photonics) using an oscilloscope (TDS-680C, Sony Tektronics). The time delay of the apparatus, including the electric delays in the apparatus itself and in the cables, for example, was experimentally determined for each measurement so that the time axis could be corrected for the delay. A N2-gas laser that generated a light pulse of 500-ps duration at 337 nm at a repetition rate of 10 Hz was used as an excitation light source to analyze photoluminescence (PL) behavior of the phosphor-sensitized fluorescent doped films. Transient PL waveforms were also recorded using the same nanosecond apparatus mentioned above.
A time-wavelength two-dimensional map of transient EL, which was constructed from some transient EL waveforms at different observed wavelengths, of the Ir(ppy)3 and DCM2 double-doped device is shown in Figure 3. The 23-V electric pulse was applied to the device for 200 ns, as depicted by the white line in the figure. As we can see from the figure, EL lights were delayed against the applied electric pulse, and the delays seemed to become bigger at shorter wavelength regions around 520 nm. On the other hand, EL intensities at various monitored wavelengths decreased almost all at once when the electric pulse turned off, and EL at about 500-550 nm remained for a longer time than that at other wavelengths. The behavior of EL means that we will be able to discuss formations and behavior of excitons in phosphor-sensitized fluorescent OLEDs on the basis of the transient EL map depicted in the figure. Time-resolved transient EL spectra when an electric pulse is applied obtained from a part of Figure 3 are shown in Figure 4a. As shown in the figure, only luminescence from DCM2 peaking at about 600 nm was observed. In addition, EL intensity rise profiles for both DCM2 at 590 nm and Ir(ppy)3 at 520 nm are shown in Figure 4b. The rise times for both are the same, and the time of 70 ns approximately agrees with the instrumental response time (75 ns) of our electrical pumping system, including the OLED sample that has a large capacitance. As also shown in Figure 4b, however, turn-on times of EL were different from each other, and the delay time for Ir(ppy)3 after DCM2 was 40 ns. These findings strongly indicate that excitons of DCM2 were directly formed on DCM2, and the exciton formation process on DCM2 took preference over that on Ir(ppy)3. This preference probably results from charge carrier (probably electron) trapping on DCM2 in the DCM2, Ir(ppy)3, PBD, and PVK mixed system. The energy diagram shown in Figure 1 also confirms this consideration. Transient EL spectra obtained just before and after turning off an electric pulse are shown in Figure 5. The EL intensity of DCM2 rapidly decayed by ending the electric excitation, as shown in the figure. This result suggests that the preferred direct exciton formation on DCM2 continuously occurred during the pulsed electric excitation for 200 ns. The decay rate of DCM2 at turn-off of the electric excitation was about 6 ns, which almost corresponds to 5 ns of fluorescence lifetime of DCM,14 a material very similar to DCM2. On the other hand, the DCM2 component of EL was observed after the pulse was turned off,
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Figure 6. Transient EL intensity decay profiles observed at 520 and 590 nm. Solid lines are fitting results with single exponential decay.
Figure 4. (a) Transient EL spectra from device during electric pulse is applied, (b) EL intensity rise profiles for both DCM2 at 590 nm and Ir(ppy)3 at 520 nm. The solid lines are a guide for the eyes, and the arrows indicate the turn-on time.
Figure 5. Transient EL spectra obtained just before (at 187 ns) and after (at 228 ns) turning off electric pulse. Solid lines are guides for the eyes.
and this strongly suggests that the DCM2 excitons were continuously created after the pulse was turned off. Next, we would like to discuss the second creation process of DCM2 excitons. Transient EL intensity profiles observed at 520 and 590 nm are shown in Figure 6. As shown in the figure, both decay profiles are single exponential, excluding the above-mentioned fast component of DCM2 observed at about the time of 200 ns. The lifetimes of DCM2 and Ir(ppy)3 are 90 and 120 ns, respectively. This DCM2 decay time is longer than the fluorescence lifetime of DCM2 of several ns. The longer lifetime of DCM2 in the doped system indicates that the exciton energy transfer produces the singlet exciton on DCM2 from Ir(ppy)3 in the doped system under the electric pulse excitation. Since the concentration of DCM2 was much smaller than that of Ir(ppy)3, the average distance from a DCM2 to the nearest adjacent energy host molecule Ir(ppy)3 is approximately the same as half of the average distance among Ir(ppy)3 (24 Å). The distance of 12 Å could not cause the Dexter type energy transfer, which is a common mechanism for triplet energy
transfer, because the distance is longer than the closest metalto-metal distance of 7.7 Å in the Ir(ppy)3 crystal.15 Therefore, other mechanisms of energy transfer, such as the Fo¨rster type transfer, which is labeled as a cause of luminescent exciton energy transfer from phosphorescent to fluorescent molecules, or the trivial mechanism (radiative emission-absorption mechanism) in this doped system were pointed out by Baldo et al.11 The energy transfer from Ir(ppy)3 to DCM2 was also observed in the same doped film under optical excitations. The PL decay rates of DCM2 and Ir(ppy)3 in the same doped system were both 120 ns, which is equal to the EL decay rate of Ir(ppy)3. In addition, the PL decay rates depend on the concentration of DCM2; that is, lower concentrations of DCM2 show slower decay times, for example, 180 ns at DCM2 concentrations of 0.07 wt %. However, molecules in solid systems are not mobile, and there is only 0.24 molecule of DCM2 in a cubic cell containing a molecule of Ir(ppy)3, for a 0.13 wt %-doped system, whose volume is 13 600 (∼243) Å3). Therefore, single exponential decays of excitonic energy shown in Figure 6 strongly suggest that excitons migrated among Ir(ppy)3 sites and transferred to an adjacent DCM2 molecule, on the basis of the above-mentioned mechanism. This occurred if the excitons were immobile on an Ir(ppy)3 molecule that has an excitonic energy first. The decay profiles become nonsingle exponential. However, each distance from each DCM2 to its nearest adjacent energy donor molecule Ir(ppy)3 in disperse systems widely varied. If a DCM2 molecule is close to an Ir(ppy)3 molecule, Dexter type transfer that produces the nonluminescent triplet excited state of DCM2 dominantly occurred. As shown in Figure 7, steady-state PL intensity significantly decreased by about 1/10 by doping DCM2 (1.3 wt %). Assuming uniform distribution of dopants (Ir(ppy)3 and DCM2), the ratio (R) of total PL intensities with and without DCM2 can be expressed from populations of DCM2 in the Dexter type transfer region (inner shell region, see the inset of Figure 7) and the non-Dexter type region (outer shell region):
R)
rC3 - rIr3 φf(r03 - rIr3)
(1)
where rC, r0, and rIr are the critical distances for the Dexter type transfer (at the critical distance the non-Dexter type transfer rate crosses over the Dexter type transfer rate), the half-distance adjacent Ir(ppy)3 (24 ÷ 2 ) 12 Å), and the radius of Ir(ppy)3 considered as a hard sphere (7.7 ÷ 2 ) 3.85 Å), respectively (see the inset of Figure 7), and DCM2 excitons transferred by non-Dexter type transfer emit fluorescence with the quantum
Electroluminescence from OLEDs
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11523 on Ir(ppy)3. This preference probably results from charge carrier trapping on DCM2 in the investigated system. In addition, the decay process of DCM2 excitons in EL devices is affected by specific processes under electric pumping, such as the excitoncharge carrier annihilation. Acknowledgment. This work was supported by the Cooperative Link of Unique Science and Technology for Economy Revitalization (CLUSTER) and CLUSTER (the second stage) of Japan’s Ministry of Education, Culture, Sports, Science, and Technology and by the Ministry’s Grand-in-Aid (No. 16760009) to M.I.
Figure 7. Steady-state PL spectra of films composed of PVK (74 wt %), PBD (18 wt %), and Ir(ppy)3 (8 wt %) with or without DCM2 (1.3 wt %). The inset is the schematic for the energy transfer scheme depending on distance in this system. The non-Dexter type transfer rate crosses over the Dexter type transfer rate at the critical distance, rC, and DCM2 excitons transferred by the non-Dexter type transfer emit fluorescence.
efficiency of φf. Supposing φf ) 0.53,12,16 an rC of 10 Å was obtained due to R ) 0.1, as seen in Figure 7. Assuming the same exciton transfer scheme in the case of the electric excitation, we believe that highly efficient phosphor-sensitized fluorescent OLED was achieved by reducing the distribution of the distance between a phosphorescent sensitizer and fluorescent dopant and maintaining the effective distance (∼10-12 Å). In addition, note here that since the EL decay times of DCM2 and Ir(ppy)3 shown in Figure 6 are different from each other, the difference strongly indicates that excitons on DCM2 probably decayed by specific processes under electric pumping; for example, the exciton-charge carrier annihilation.17 Therefore, such specific decay processes should also be suppressed to attain a highly efficient phosphor-sensitized fluorescent OLED. IV. Conclusion We investigated electronic excitation processes in phosphor and fluorescent chromophore double-doped organic lightemitting devices by using the transient EL technique. Excitation transfer from Ir(ppy)3 to DCM2 occurs in the device. However, excitons of DCM2 were directly created on DCM2, and the exciton formation process on DCM2 gets preference over that
References and Notes (1) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610–3616. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. (3) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4–6. (4) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151–154. (5) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048–5051. (6) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377–7387. (7) Li, J.; Djurovich, P. I.; Alleyn, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713–1727. (8) Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.H.; Hsu, S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285–289. (9) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967–10977. (10) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904–906. (11) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750–753. (12) D’Andrade, B. W.; Baldo, M. A.; Adachi, C.; Brooks, J.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 1045–1047. (13) Ichikawa, M.; Amagai, J.; Horiba, Y.; Koyama, T.; Taniguchi, Y. J. Appl. Phys. 2003, 94, 7796–7800. (14) Bulovic, V.; Kozlov, V. G.; Khalfin, V. B.; Forrest, S. R. Science 1998, 279, 553–555. (15) Breu, J.; Stoesel, P.; Schrader, S.; Starukhin, A.; Finkenzeller, W. J.; Yersin, H. Chem. Mater. 2005, 17, 1745–1752. (16) Cheon, K. O.; Shinar, J. Appl. Phys. Lett. 2004, 84, 1201–1203. (17) Ichikawa, M.; Naitou, R.; Koyama, T.; Taniguchi, Y. Jpn. J. Appl. Phys 2001, 40, L1068–L1070.
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