Mat. Res. Soc. Symp. Proc. Vol. 708 © 2002 Materials Research Society

Electrophosphorescent Light Emitting Devices Using Mixed Ligand Ir(III) Complexes Hae Won Lee, R. R. Das, Chang-Lyoul Lee, Yong-Young Noh and Jang-Joo Kim Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 1 Oryong-dong, Buk-gu, Kwangju 500-712, Korea ABSTRACT We have fabricated phosphorescent light emitting devices using mixed ligand bisorthometalated Ir(III) complexes, chlorobis-(2-phenylpyridinato-N,C2’)pyridineiridium(III) [Ir(Cl)(ppy)2(py)] and chlorobis-(2-phenylpyridinato-N,C2’)triphenylphosphine iridium(III) [Ir(Cl)(ppy)2P(Ph) 3] chlorobis-(2-phenylpyridinato-N,C2’)tri-n-butylphosphine iridium(III) [Ir(Cl)(ppy)2P(n-Bu)3], where ppy is the orthometalating ligand. These complexes vary in their HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) and emissive states in accordance with the π-accepting abilities of the non-orthometalating ligands. The lifetimes and EL spectrum of the devices were studied and compared. INTRODUCTION At present there is a great effort to synthesize transition metal complexes, that might effectively emit from their triplet metal-to-ligand charge-transfer (MLCT) states by an energy transfer from a fluorescent host, thus harvesting triplet as well as singlet excitons in organic light emitting devices (OLEDs). The coincidence of short triplet lifetime and reasonable photoluminescence efficiency of fac-[Ir(ppy)3] help [Ir(ppy)3]-based OLEDs to achieve a high performance [1]. Recently high efficiency light emitting devices have been reported using cyclometalated iridium complexes with 2-(p-tolyl)pyridine, benzoquinoline, 2phenylbenxothiazole, and 2-phenylquinoline ligands [2]. Although this technique has proved to be effective, many structural features of the complex control the emissive state, their lifetime, emission quantum yield and subsequent performance of the device. The mode of ligand coordination, presence of π-accepting delocalized ligand orbitals and the probability of back bonding from metal to ligand define the metal centered (MC), ligand centered (LC) and MLCT states. The MLCT character is highest in complexes, which have a maximum number of Ir-C σ-bonds and a good delocalized π-acceptor ligand with relatively high energy LC excited states. These complexes also produce in-accessible MC excited states of higher energy. In this presentation, we will report the effect of the strength of the π-accepting and electron delocalizing abilities of non-orthometalating ligands coordinated to Ir(III) in the bisorthometalated complexes on the device properties. We have fabricated and studied the device function by using three bis-orthometalated mixed ligand complexes (Figure 1), [Ir(Cl)(ppy)2(py)], [Ir(Cl)(ppy)2P(Ph)3], and[ Ir(Cl)(ppy)2P(n-Bu)3].

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N

N Cl

Cl

Ir N

N

(a) Figure 1. Molecular structure of [Ir(Cl)(ppy)2P(n-Bu)3].

N Ir

P

Ir

Cl N

P

(b)

N

(c)

(a) [Ir(Cl)(ppy)2Py], (b) [Ir(Cl)(ppy)2P(Ph)3], and (c)

EXPERIMENTAL DETAILS

The mixed ligand complexes, [Ir(Cl)(ppy)2Py], and [Ir(Cl)(ppy)2P(Ph)3] were synthesized according to the literature [4] from [Ir(ppy)2Cl]2 [3]. [Ir(Cl)(ppy)2P(n-Bu)3] was synthesized by stirring of [Ir(ppy)2Cl]2 and of tri-n-butylphosphine in dichloromethane for 24 h. The fabricated double layer organic light emitting devices have the structure of ITO/PEDOT (40 nm)/PVKIr(III)complex (80 nm)/Mg:Ag (100 nm)/Ag (20 nm). Absorption spectra were recorded with a Hewlett-packard HP8452A UV-visible diode array spectrophotometer. Photoluminescence and electroluminescence spectra were detected by a spectrometer connected to a photomultiplier tube with a xenon lamp as excitation source. The spot size is 2 mm in our measurement. Emission lifetimes were measured with the storage-sampling oscilloscope by exciting the samples at 355 nm with a Nd-YAG pulsed laser. For the laser measurements, the temporal pulse duration was 5 ns and the pulse repetition rate was 10 Hz. The current-voltage characteristics of the devices were measured by using a Keithley 237 Source Measurement Unit. Cyclic voltametry was performed with an Autolab potentiostat. RESULTS AND DISCUSSION

Molecular structure of [Ir(Cl)(ppy)2(py)], [Ir(Cl)(ppy)2(PPh3)] and [Ir(Cl)(ppy)2P(n-Bu3)] are shown in figure 1. Table 1 summarizes the experimental HOMO and LUMO levels found from the cyclovoltametric and absorption data. All the complexes show only one oxidation process within the investigated redox window ( tri-n-butylphosphine > pyridine. Since chloro and ppy ligands are not changed in these complexes, the observed increase in their oxidation potentials indicate the varying ligand strengths of these nonorthometalating compounds which follows the reverse order i.e., Pyridine > tri-nbutylphosphine > triphenyphosphine. The metal dπ-orbitals are raised in energy in the sequence, Pyridine > tri-n-butylphosphine > triphenyphosphine.

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Table 1. Energy levels, PL and EL emission, and lifetime of orthometalated mixed ligand complexes.

Ir(Cl)(ppy)2Py 0.87 5.54 360, 400, 460, 480 505 510, 540 0.625

Compound E1/2(oxdn)(V) HOMO (eV) λabs (nm) λmax, PL (nm) λmax, EL (nm) Lifetime (µs)

Ir(Cl)(ppy)2P(n-Bu)3 0.89 5.55 360, 400, 430 490 495 1.19

Ir(Cl)(ppy)2P(Ph)3 0.96 5.62 350, 400 485 490 1.24

0.8 1.5 0.6 1.0 0.4 0.5

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0.2 0.0 400

500

600

700

Wavelength (nm)

PL Intensity (a. u.)

1.0

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Normalized EL Intensity (a. u.)

Extinction Coefficient x 10

-4

The HOMO states in the phosphine complexes can be ascribed to Ir(III) dπ-orbital as it has been assigned in several instances of orthometalated Pt(II) phosphine complexes [5]. Figure 2 shows the solution absorbance and PL of [Ir(Cl)(ppy)2Py], [Ir(Cl)(ppy)2P(Ph)3] and [Ir(Cl)(ppy)2P(n-Bu)3]. Intense absorption bands below 300nm can be assigned to the π-π* transition of the pyridyl and phenyl ring of the ligand(s) [6. 7]. Because of the strong mixing of metal and ligand orbitals occurring in orthometalated complexes, these bands in the range of 350 - 510 nm can be ascribed to an admixture of MLCT and ligand centered π-π* bands. Again the

0

1

2

3

Time (µs)

4

Figure 3. Transient PL spectra of Figure 2. Absorption and PL spectra of 10-4 M (solid line), [Ir(Cl)(ppy)2Py] [Ir(Cl)(ppy)2Py] (solid line), [Ir(Cl)(ppy)2P(Bu)3] [Ir(Cl)(ppy)2P(n-Bu)3] (dashed line) and (dashed line) and [Ir(Cl)(ppy)2P(Ph)3] (dotted [Ir(Cl)(ppy)2P(Ph)3] (dotted line) doped in line). PMMA film with the concentration of 4%.

long absorption tail toward lower energies is attributed to triplet MLCT transitions. These are enhanced in magnitude due to the large spin-orbit coupling coefficient (of the order of 103 cm-1) of iridium [8]. The MLCT bands in the phosphine complexes involve Ir(dπ)-ppy(π*) transitions as observed in similar Pt(II) phosphine complexes [5]. The MLCT peaks are better resolved in BB3.38.3

pyridyl complex and have different peak maxima than the phosphine complexes. The pyridyl complex shows a long tail toward the visible regions assigned to singlet-to-triplet charge transfer transitions because of enhanced strong spin-orbit coupling due to the heavy Ir(III) ion [6]. The three complexes vary in emission energy at room temperature. CH2Cl2 solutions of [Ir(Cl)(ppy)2(py)], [Ir(Cl)(ppy)2P(Ph)3], and [Ir(Cl)(ppy)2P(Bu)3] emit at 500 and 485, 490 nm respectively. Distortion of the geometry of the complexes, because of the π-backbonding from Ir(III) dπ orbital to the phosphine ligands moves up the energy of the MLCT states [9. 10]. Emissions from ppy centered 3MLCT states are evident from the observed lifetimes of less than 1.5 µs in these complexes. Figure 3 shows the time resolved PL response in PMMA film that provides evidence for phosphorescence of these complexes. The life time of [Ir(Cl)(ppy)2Py] is 625 ns, [Ir(Cl)(ppy)2P(n-Bu)3] is 1.19 , and [Ir(Cl)(ppy)2P(Ph)3] is 1.24 . These complexes demonstrate shorter lifetime than Ir(ppy)3. EL spectra of a 4% Ir complex doped PVK film is displayed in figure 4. Pyridine, butylphosphine and phenylphosphine complexes emit at 510, 495, 485 nm respectively. The phosphine complexes exhibit the same emission in PL, the similar excited states in both PL and EL. The EL peak maximum is red shifted in the pyridine complex compared to its PL. [Ir(Cl) (ppy)2P(n-Bu)3] complex doped device does not show any host PVK emission, indicating complete energy transfer by Forster and/or Dexter mechanisms from the host.

Normalized Intensity (a. u.)







1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm) Figure 4. Electroluminescence spectra of Ir(Cl)(ppy)2Py (solid line), Ir(Cl)(ppy)2P(n-Bu)3 (dashed line) and Ir(Cl)(ppy)2P(Ph)3 (dotted line) doped in PVK film with the concentration of 4%.

On the other hand a little host emission is seen in the devices containing other two complexes. A short lifetime of 625 ns in the case of the pyridine complex and incomplete energy transfer from the host indicates possible pathways for nonradiative loss. 545 nm emission of [Ir(Cl)(ppy)2Py] OLED might come from another excited state. We expect that charge confinement occur in lower excited energy states or energy is transferred effectively to lower energy states. BB3.38.4

CONCLUSION

We synthesized mixed ligand bis-orthometalated Iridium complexes with different nonorthometalating and π-accepting ligands and fabricated electrophosphorescent double-layered polymer light emitting devices. Through PL and EL spectra we can demonstrate that energy transfer takes place from PVK excimer to mixed ligand Iridium complexes. These materials can be complied to the device, demanding energy transfer, since the lifetimes of the luminescent excited states of the complexes [Ir(Cl)(ppy)2(Py)], [Ir(Cl)(ppy)2P(n-Bu)3] and [Ir(Cl) (ppy)2P(Ph)3] are assigned as MLCT involving ppy ligand [10]. The MC excited states in these complexes are moved to very high energies by the strong σ-bonding effect of ppy negating the possibility of emission from these states. ACKNOWLEDGMENTS

This work was supported by the Ministry of Education (BK21) and KOSEF through the Center for Electro- and Photo-responsive Molecules. REFERENCE

1. 2.

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