Rare-earth metal complexes as emitter materials in organic electroluminescent devices

Transcription

1 64 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig Rare-earth metal complexes as emitter materials in organic electroluminescent devices Siegfried Dirr, Hans-Hermann Johannes, Wolfgang Kowalsky Emitter materials for electroluminescent devices using rare-earth metal complexes have been investigated. Narrow bandwidths of about 5 nm in the visible spectral region have been observed. First realized organic light emitting diodes (OLEDs) with an europium complex as emitter exhibit highly monochromatic red light with a luminance of 2 cd/m 2 and low drive voltages of about 5 V. 1 Introduction Electroluminescent devices using organic fluorescent dyes show emission with broad spectral bands of nearly 80 nm. The luminescent colors appear dull and were not well suited for display applications [1,2]. Filters or Fabry-Perot microcavities may reduce this problem, however, they are not practicable for lowcost fabrication. Rare-earth metal complexes with luminescent bandwidths of about 5 nm are a potential molecular class to overcome these disadvantages. The narrow emission is due to radiative transitions of the inner 4f-orbitals of the trivalent rare-earth ion. In addition, the lifetime of these materials is improved, because the emission does not originate from the organic dye, but from the inorganic rare-earth element. Possible candidates for green, red and blue light emitting devices are the fluorescent rare-earth elements terbium, europium and thulium, respectively [3,4]. A wide range of possible molecules with different rare-earth elements may open other spectral regions for new applications of organic electroluminescent devices. The molecular structure of the complexes, the relevant energy transfer mechanisms, and the spectral characteristics are discussed with emphasis on an europium and a terbium complex. First experiments of realized red light emitting diodes are presented. 2 Rare-earth complexes From the variety of possible molecular complexes containing rare-earth elements (RE) as the central ion, -diketonate chelates are shown in Fig. 1. Organic ligands surround the rare-earth ion and thus 4f-electrons are shielded both from the 5s and 5p electrons and the organic environment. Therefore the atomic properties of the rare-earth ions are retained after formation of complexes with different ligands. The ligand s substituents R 1 and R 2 are for example acetyl groups (CH 3 ), phenyl groups (C 6 H 5 ) or thenoyl groups (C 4 SH 3 ). They are responsible for the energy levels of the organic ligand.

2 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig 65 Fig. 1: Structure of the -diketonate chelate with the substituents R 1 and R 2. The organic ligand absorb visible or UV radiation. Reemission of the absorption energy originates from the rareearth ion via an internal molecular energy transfer [5]. For the investigations, an europium complex europiumtris(1-thenoyl-3-trifluoracetonate)-1,10- phenanthroline and a terbium complex, terbium-tris(1-benzoyl-3-trifluoracetonate)- 1,10-phenanthroline have been synthesized. Fig. 2 and 3 show the molecular structure, the energy level diagram, and the relevant energy transitions of the two materials. Fig. 2: Structure and energy level diagram of Eu(TTFA) 3 Phen. Fig. 3: Structure and energy level diagram of Tb(BTFA) 3 Phen. The ligands act as antennas. The absorbed energy excites the first singlet state S 1. Energy migration takes place in a non-radiative intersystem crossing from the S 1 state to the first triplet state T 1. The energy of the T 1 state is now transfered nonradiatively to the 5 D ground multiplets of the europium ion and the 7 F ground multiplets of the terbium ion, respectively. Emission from excited levels leads to the characteristic fluorescence lines, especially in the red and green spectral region. No emission would take place if the energy transfering T 1 state lies below the rare-earth energy level [6].

3 66 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig 3 Spectral characteristics For photoluminescence measurements the complexes are sublimed onto a copper plate mounted in a cryogenic system and excited by the UV-radiation of an Ar -laser. The spectra were recorded in the temperature region from 10 K to 300 K with optical powers of about 6 to 20 mw. Fig. 4 and 5 show the PL-spectra of the europium and the terbium complexes at 10 K and 300 K. The Eu 3 emission lines originates both from the 5 D 1 and the 5 D 0 states to the 7 F-levels. The most efficient fluorescent transition is from 5 D 0 to 7 F 2 at 613 nm with a bandwidth of about 5 nm. Quenching processes are very weak because the intensity at 10 K has only twice the value of the maximum at 300 K. On the other hand the green and red emission line intensities of the terbium complex decrease nearly by a factor of 50. This indicates that the transitions are thermally quenched, due to the T 1 -level of the ligand which is not optimally chosen for this element. The emission peak at 543 nm originates from the 5 D 4 7 F 5 transition and has a bandwidth of nearly 10 nm. Fig. 4: PL-spectrum of a Eu(TTFA) 3 Phen-thin film at 10 K and 300 K. Fig. 5: PL-spectrum of a Tb(BTFA) 3 Phen-thin film at 10 K and 300 K.

4 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig 67 4 Red light emitting diodes: first experiments Electroluminescent devices using a multilayer structure are investigated. For red-light-emitting diodes, the europium complex Eu(TTFA) 3 Phen was used as emitter material. The cells were fabricated by means of vacuum deposition in an OMBD system [7]. The organic layers were subsequently deposited onto a transparent ITO-coated glass substrate 120 kept at room temperature. The top electrode consists of a Mg layer followed by an Ag layer, both with thicknesses of 120 nm. First results were achieved with two different cell structures. Fig. 6 shows the cell structure of a single heterostructure (SH) device with the current-voltage and the optical power-voltage characteristics. 15 nm of a new modified starburst molecule m-mtdata and 5 nm of 4,4 -bis(3-methylphenylphenylamino)biphenyl (TAD) form a sufficient electron barrier between the emitter and the hole transport layers. The total thickness of the organic layer is about 28 nm. In addition, the results of a double heterostructure (DH) device extended by an electron transport layer of 15 nm aluminium-tris-(8-hydroxyquinoline) (Alq 3 ) are shown in Fig. 7. In this structure the thicknesses of the m-mtdata layer and the emitter layer are 20 nm and 25 nm, respectively. Fig. 6: Cell structure, current-voltage and optical power-voltage characteristics of a SH device. Fig. 7: Cell structure, current-voltage and optical power-voltage characteristics of a DH device. Red emission was observed from both device structures, when a DC positive voltage was applied to the ITO electrode. The luminance was measured with a Minolta LS110 luminance meter. For the SH device the luminance started at 4 V due to the relatively thin organic layer. A luminance of 2 cd/m 2 and a current density of 7 ma/cm 2 was achieved at 6 V. The values of the DH device are 17 V for the threshold voltage and 0.3 cd/m 2 at 18 V and 3 ma/cm 2.

5 68 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig Fig. 8: CIE coordinates of the red light emitting diode. Further improvements are expected from opimization of the layer sequence and the layer thicknesses. Due to the rather low carrier transport ability of the europium complex thin film, coevaporation of a host material with a higher carrier transport ability like Alq 3 would improve the luminance. The luminescence could also be enhanced by mixed complexes with different inorganic fluorescent materials [8]. Finally the CIE coordinates of the red OLED have been measured (Fig. 8). The electroluminescent cell exhibits highly monochromatic red light at a wavelength of 613 nm. 5 Summary In conclusion, it has been shown that thin films of metal complexes with the rare-earth elements europium and terbium exhibits very narrow emission lines. Red light emitting diodes with an europium complex as emitter have sucessfully been developed. The devices have to be optimized due to the rather low luminances. Further metal complexes with the rare-earth elements terbium, thulium, erbium and praseodymium are currently under investigation. References [1] C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51 (1987) [2] C. Adachi, S. Tokito, T. Tsutsui and S. Saito, Jpn. J. Appl. Phys., 27 (1988) L269-L271. [3] J. Kido, K. Nagai and Y. Ohashi, Chem. Lett., (1990) [4] J. Kido, K. Nagai, Y. Okamoto and T. Skotheim, Chem. Lett., (1991) [5] S. P. Sinha, Complexes of the Rare Earth, Pergamon, London (1966). [6] G. E. Bouno-Core, H. Li and B. Marciniak, Coord. Chem. Rev., 99 (1990) [7] C. Rompf, D. Ammermann and W. Kowalsky, Mater. Sci. Technol., 11 (1995) [8] W. Li, W. Li, G. Yu, Q. Wang and R. Lin, J. All. Comp. 192 (1993)