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...