Dendrimeric phosphorescent iridium(III) complexes for organic light-emitting diodes

Abstract

Commercial interest in utilizing organic light-emitting diodes (OLEDs) as light-emitting elements in displays and lighting applications has soared in recent years. The slowly increasing market share of products utilizing OLED displays has been driven by consumer demand for displays featuring higher image quality than has been historically possible. Concurrently; interest in OLED lighting is continuing to grow thanks to their ability to achieve high efficiencies capable of significantly reducing the current consumption of global electrical energy production for lighting purposes of approximately 20 %.The successful development of highly efficient OLEDs has largely been reliant upon employing phosphorescent iridium(III) complexes as emissive dopants. While these materials exhibit many advantageous properties such as photoluminescence quantum yields up to unitary, colour tunibility across the visible spectrum and the ability to harvest both singlet and triplet excitons; they generally exhibit poor solubility in most solvents limiting their processability to the relatively expensive, wasteful and energy inefficient vacuum thermal evaporation. For existing commercial products incorporating OLEDs, the high expense of processing is overcome by the large unit cost of the current products; smartphones and televisions. If OLEDs are to be successfully employed in lighting and other less expensive display technologies, vacuum thermal evaporation must be replaced with more cost effective processing techniques. Of the candidates for alternative processing techniques, fabrication from solution by spin-coating or ink-jet printing is particularly attractive. Not only to these solution processes have relatively low cost per unit, can be affordably scaled to large-area devices and can be implemented in roll-to-roll production.It has been reported that solubility of phosphorescent iridium(III) complexes can be enhanced through functionalization with branched moieties to produce a class of materials referred to as dendrimers. As well as enhanced solubility, the dendrimers containing iridium(III) complexes display significantly less quenching by triplet-triplet annihilation in the neat solid resulting from increased interchromophore distance from the bulky dendrons. Poly(dendrimer)s, an extension upon dendrimers where a dendronized monomer is polymerized, have been reported with the capacity to improve solution processability and device performance of iridium(III) complexesWith the aim of developing solution processable phosphorescent poly(dendrimer)s to produce higher efficiency OLEDs, the initial targets synthesized herein were designed to improve our understanding of structural variations on poly(dendrimer) properties. The two green-emitting poly(dendrimer)s, PDB and PDC, were designed to be equivalent to two previously reported poly(dendrimer)s, L1 and L2, but with an additional phenyl moiety lengthening the tether between the emissive pendant and the polymer backbone.The solution photoluminescence quantum yields of PDB and PDC were found to be 55 ± 6 % and 56 ± 6 %, respectively. Compared to the photoluminescence quantum yields of 65 ± 7 % and 72 ± 7 % observed with L1 and L2, respectively; the introduction of an additional phenyl moiety to length the tether results in an increase in non-radiative decay via inter-chain interchromophore interactions.OLEDs featuring solution processed emissive layers containing PDC were observed to have very high external quantum efficiencies relative to the film photoluminescence quantum yield. With the hero device reaching an impressive external quantum yield of 19 % at 100 cd/m2, it is believed that PDC undergoes significant preferential alignment during deposition from solution which allows for a greatly enhanced outcoupling efficiency. These results coincide well with the analogous literature poly(dendrimer) L2 which was also found to produce the same enhanced outcoupling efficiency.With the development of green-emitting poly(dendrimer)s showing interesting properties, the next targets described herein were designed to investigate whether dendrimers and poly(dendrimer)s provide equivalent benefits to red-emitting iridium(III) complexes. A series of four structurally interrelated dendrimers, as well as a poly(dendrimer), were designed and synthesized toward this goal.The homoleptic dendrimers, Hom1 and Hom2, were found to have impressive solution photoluminescence quantum yields of 74 ± 7 % and 86 ± 9 %, respectively. The heteroleptic dendrimers, Het1 and Het2, were found to have high solution photoluminescence quantum yields of 66 ± 7 % and 61 ± 6 %, respectively. The poly(dendrimer), PDCR, was designed by replacing the ancillary ligand of PDC with the red-emissive ligand of the dendrimers and was found to have a solution photoluminescence quantum yield of 72 ± 7 %.Preliminary red-emitting OLEDs have been fabricated using the four dendrimers and the poly(dendrimer) as dopants. At present the highest external quantum efficiency has been achieved by Hom2 with of 11.0 ± 0.4 % at 8.4 ± 5.5 cd/m2 with CIE 1931 coordinates of (0.66, 0.34) and a maximum luminance of 6000 cd/m2. The PDCR was observed to achieve a similar external quantum efficiency of 10.0 ± 0.2 % at 5.2 ± 2.6 cd/m2 with CIE 1931 coordinates of (0.66, 0.34). It is known that these devices are still below the values expected and significant optimization is still required.