Heavy-Metal Organometallic Complexes as Yellow and Orange Triplet Emitters for Organic Light-Emitting Diodes

CHEUK-LAM HO AND WAI-YEUNG WONG

Institute of Molecular Functional Materials and Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China

1.1 Introduction

While lighting applications account for about 19% of the electricity consumption of the world, the need to reduce energy consumption associated with the low efficiency of conventional lighting systems (e.g. the incandescent bulbs) has prompted researchers to pay considerable research attention to developing new energy-saving technologies such as organic light-emitting devices (OLEDs). Actually, incandescent bulbs that have long been the most common lighting sources are very inefficient (converting only 5–10% of this energy into light) and dissipate the main part of the electrical energy absorbed as heat. Even the energy-saving compact fluorescent lamps are only about 20% energy efficient with typical power efficiency of 40–70 lm W-1. Moreover, fluorescent lamps contain a small but significant amount of toxic mercury in the tube, which complicates their disposal and causes an important environmental impact. Recently, the efficiencies of white organic light-emitting devices (WOLEDs) have been shown to approach or surpass those of the fluorescent lamps due to recent advances in novel material synthesis and optimisation of device structures in the past few years. The key advantages of OLEDs for flat-panel display applications are their self-emitting property, high luminous efficiency, full colour capability, wide viewing angle, high contrast, low power consumption, low weight, potentially large-area colour displays and flexibility. In particular, recent developments in using phosphorescent materials have led to significant improvements in OLED performance up to 100 lm W-1, thus providing organic semiconducting lighting with a very bright future and allowing WOLEDs to become the next generation of light illumination systems.

White-light emission can be obtained based on the principle of additive colour mixing. In practice, this is mostly done by mixing the three primary colours (red, green and blue, RGB). Besides R-G-B phosphorescent emitters, phosphors showing complementary colours, such as blue (B) and orange or yellow (O or Y), can also be utilised to produce white-light emission in the devices. This approach can eliminate the necessity for excessive emissive dopants in a device, hence reducing structural heterogeneities and the device fabrication process can generally be simplified. Building on these attractive properties, research on two-colour WOLEDs still remains scarce and is driving many researchers to investigate high-efficiency yellow or orange triplet emitters.

Of the various heavy-metal ions that could be envisaged for promoting radiative emission of triplet states in OLEDs, iridum(III), platinum(II) and other transition metals have attracted most attention to date. Flourishing studies over the past decade have revealed how the excited-state energies and hence emission colours in several classes of their complexes can be controlled through rational ligand design. Here, we summarise a number of triplet emitters that show yellow or orange electroluminescence (EL) that may serve as good candidates for WOLED applications.

1.2 Iridium(III) Complexes

Iridium(III) complexes are considered to be the seminal generation of phosphorescent emitters. As a general approach, the emission peak wavelength was found to be greatly dependent on the molecular design of the cyclometallating ligand chelates.

1.2.1 Homoleptic and Heteroleptic Iridium(III) Complexes with Modified 2-Phenylpyridyl Moieties

Recently, for OLEDs based on heavy-metal Ir(III) complexes, the benchmark green emitters fac-[Ir(ppy)3] and [Ir(ppy)2(acac)](Hppy = 2-phenylpyridine, Hacac = acetylacetone) have drawn great attention due to their ease of synthesis and high efficiency. To generate yellow or orange colour, a very versatile avenue can be adopted towards emission colour tuning of Ir(III) complexes viathe facile derivatisation of the phenyl or pyridyl moiety of ppy with various substituents or functionalities. Attachment of the main-group moieties SO2 and PO to ppy in Ir-1 and Ir-2 was reported by Wong et al. in which the approach successfully shifts the charge-transfer character from the pyridyl group in some ppy-type complexes to the electron-withdrawing main-group moieties. This kind of complex improves their electron-injection (EI) and electron-transporting (ET) features. The strongly electron-withdrawing inductive influence of the polar PO group in Ir-2 was shown to lower significantly the lowest unoccupied molecular orbital (LUMO) energy, thus redshifting the emission peak as compared to [Ir(ppy)2(acac)]. Ir-2 shows a shorter emission wavelength at 541 nm than Ir-1 (550 nm). The vacuum-deposited yellow-emitting device based on Ir-1 turned on at 3.7 V, with a maximum luminance (Lmax) of 48 567 cd m-2 at 19.9 V and maximum external quantum efficiency (ηext) of 10.7%, luminance efficiency (ηL) of 35.1 cd A-1, and power efficiency (ηP) of 23.1 lm W-1, while a higher turn-on voltage (12.1 V) was found in the solution-processed yellow-orange device based on Ir-2 with Lmax of 7103 cd m-2 at 27.9 V and maximum efficiencies of 3.49%, 11.89 cd A-1 and 2.40 lm W-1.

By extending the p-electron delocalisation of the aromatic ligand, the energy gap between the ground and lowest excited states can be effectively reduced to provide a redshifted emission. Complexes Ir-3 to Ir-7 consist of additional fused aromatic rings either on the phenyl or pyridyl ring of the ppy ligand. Yellow phosphorescence from Ir-3 exhibits a very broad and featureless peak at ca. 550 nm with a wide full spectral width at half-maximum (FWHM) of 87 nm without the formation of excimer emission. It gives high peak forward-viewing ηL, ηP and ηext of 33.6 cd A-1, 21.8 lm W-1 and 10.5% along with Commission international de l'Eclairage (CIE) coordinates of (0.45, 0.54). This broad emission and nonexcimer formation effectively overcome the problem of inevitably low colour rendering indexes (CRI) that are common in many WOLEDs. Highly efficient WOLEDs with maximum efficiencies as high as 46 cd A-1 and 41 lm W-1 under forward voltage bias can be achieved.

3-Phenylisoquinolinyl (Ir-4 to Ir-6) and phenylbenzoquinoline-based (Ir-7) Ir(III) complexes also achieve yellow or orange electrophosphorescence. Their corresponding CIE coordinates are (0.49, 0.51), (0.46, 0.53), (0.41, 0.54) and (0.55, 0.44) for Ir-4 to Ir-7, respectively. The better EL performance observed in Ir-4 can be ascribed to its shorter phosphorescence lifetime. The triplet-triplet (T–T) annihilation becomes more serious for the fluoro derivatives Ir-5 and Ir-6.

Substitution of benzothiazole for pyridine leads to a redshift in the phosphorescence emission. The EL peaks of benzothiazole-based complexes Ir-8 and Ir-9 [7e,13,14] appear at around 560 nm in the yellow region and the replacement of S by O in the chromophore causes a blueshift in the emission wavelength to the green region for Ir-10 due to the lower polarisability and basicity of oxygen relative to sulfur. However, by introducing a CF3 substituent to Ir-10, the phosphor dye emits yellow light at 544 nm with luminance up to 9200 cd m-2 at 100 mA cm-2. Orange-emitting OLEDs were also fabricated by using crosslinkable compound Ir-11, which can be crosslinked with a hole-conducting matrix via cationic ring-opening polymerisation to yield an insoluble emitting layer by using the spin-coating method. The optimal device has a maximum ηL of 18.4 cd A-1 at 100 cd m-2 and ηP of 11.7 lm W-1. At a luminous density of 1000 cd m-2, the efficiency was still 15 cd A-1 and 11.5 lm W-1, respectively. The results gave a performance that compares favourably with other multilayered devices fabricated by thermal evaporation.

Kwon and coworkers designed a Ir(III) complex Ir-12 with high device efficiency owing to its regulated energy levels and high stability stemming from the introduction of the imide group as well as the suppressed T–T annihilation in the EL devices in the presence of a bulky isopropyl unit. All the devices with different doping concentrations emit intense orange EL at 566-570 nm with the CIE chromaticity coordinates of (0.51, 0.49) to (0.52, 0.47). The orange-emitting device (λ = 570 nm) with 4 wt.% dopant level exhibited a low operation voltage of 5.3 V at 1000 cd m-2, a maximum ηext of 13.8%, and a maximum ηP of 32.7 lm W-1. Adoption of a double layer-structure enhanced the ηext further to 14.4%.

The use of styrylbenzoimidazole derivatives instead of the traditional pyridine derivatives enhances the ET ability and OLED efficiency at high brightness levels. Devices based on Ir-13 and Ir-14 gave yellow EL at 570 and 585 nm, respectively. The performances of the devices based on Ir-14 are inferior to those of the corresponding devices based on Ir-13. The device based on Ir-13 exhibited a Lmax of 56 162 cd m-2, accompanied by high ηL of 25.7 cd A-1 at high brightness of 1000 cd m-2 and 20.7 cd A-1 at 10 000 cd m-2.

To investigate the structure-property relationship and develop durable, high-efficiency devices, the groups of Guo and Zou synthesised a series of Ir(III) complexes Ir-15 to Ir-17 bearing the pyrazine ligands. High yellow colour purity with a featureless emission peak at 580 nm was obtained using Ir-15. A systematic study on colour tuning via variation of the cyclometallating and ancillary ligands was then carried out. Their phosphorescence peak wavelength can be fine tuned in the yellow range. The emission shifts to the blue in Ir-16 (λEL = 561 nm; CIE 5 (0.45, 0.54)) by adding some fluoro groups to the ligand. The use of picolinic acid instead of acetylacetonate as the ancillary ligand in Ir-17 also causes a hypsochromic shift in the EL wavelength to 569 nm with CIE of (0.48, 0.50). This result suggests that the strategies for tuning the emission colour and colour purity by changing the ligand substituents can also be applied to the pyrazine system.

The use of a sterically hindered amidinate as the chelating ancillary ligand in Ir-18 effectively relieves the self-quenching problem in the EL emission and such a group also acts as a good ambipolar charge-transporting material with high hole and electron mobilities. The highest occupied molecular orbital (HOMO) level of this complex is higher than that with acac ligand and therefore the energy gap of Ir-18 is significantly smaller than that with acac and this can be ascribed in part to the higher π-bonding ability of the amidinate ligand. The better hole-transporting ability of Ir-18 resulted in excellent EL performance for the orange-emitting device with peak ηext of 18.4%, ηL of 21.4 cd A-1 and ηP of 18.7 lm W-1. A WOLED with the structure of ITO/NPB/Ir-18:Bepp2/Bepp2/LiF/Al was fabricated (Bepp2 = bis(2-(2-hydroxyphenyl)-pyridine)beryllium, a deep blue-emitting fluorescent complex). The CIE coordinates remain almost unchanged (0.35 [+ or -] 0.02, 0.33 [+ or -] 0.02) upon varying the luminance and so the device possesses a high colour reproducibility. A reasonably low turn-on voltage (2.7 V) was recorded with remarkable EL efficiency at a peak ηext of 27.8% (ηL of 60.8 cd A-1) at a luminance of 300 cd m-2 (4.4 V) and the highest ηP of 48.8 lm W-1 at 80 cd m-2 (3.8 V). Such high performance of the WOLED should be attributed to the carrier direct-injection mechanism as well as efficient energy transfer from Bepp2 to Ir-18 in the emitting layer.

Devices based on the heteroleptic Ir(III) cyclometallates Ir-19 and Ir-20 containing both azolate and diphenylphosphinoaryl chelates displayed bright orange emission. Their photophysical properties are almost identical, regardless of the identity of X and their phosphorescence quantum efficiency can reach near unity for good device performance. The device structure employed was ITO/PEDOT:PSS/NPD/TCTA/host:dopant(10%)/TPBI/LiF/Al (PEDOT:PSS = poly(ethylenedioxythiophene)-poly(styrenesulfonic acid); NPD = N,N'-di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl -4,4'diamine; TCTA = 4,4',4"-tris(carbazol-9-yl)-triphenylamine; TPBI = 1,3,5-tris[N-(phenyl)ben-zimidazole]benzene). Two different host materials, namely, 4,5-diaza-2',7'-bis(carbazol-9-yl) -9,9'-spirobifluorene and 4,4'-N,N'-dicarbazolebiphenyl (CBP), were used for comparison. The results identified that effective energy confinement and appropriate guest-host combinations were observed for the devices using the spirobifluorene-based host that showed higher EL efficiencies with Lmax of 19 300 cd m-2, ηext = 17.1%, ηP = 49.3 lm W-1, CIE at (0.51, 0.48) for Ir-19, and Lmax of 21 100 cd m-2, ηext = 15%, ηp = 37 lm W-1, CIE at (0.51, 0.49) for Ir-20.

Besides modification of the ligand structures, colour tuning was also realised by incorporation of ligands with different electrochemical properties in some heteroleptic coumarin-based structures. Complexes Ir-21 to Ir-24 were reported by Ren and coworkers. Due to the fact that coumarin is strongly electron withdrawing and very electron deficient, any replacement of ppy ligands in Ir(ppy)3 with coumarin derivatives can significantly decrease the HOMO energy but it has a less pronounced effect on the LUMO energy, consequently leading to an increased HOMO-LUMO gap and a blueshift in the emission energy. Therefore, the λPL of Ir-21 shifts from 550 to 536 nm in Ir-22, whereas Ir-23 shifts from 570 to 544 nm in Ir-24. The OLEDs fabricated using these coumarin-based Ir(III) complexes as emissive dopants are highly efficient and stable with ηext as high as 20–21% and ηp above 45 lm W-1 at 1 mA cm-2. Improved charge balance in the emissive layer was observed in Ir-21; thanks to the fact that Ir-21 is already a good charge-transporting material. Its ηext showed a steady increase as the doping concentration was increased.