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We show that it is possible to produce an efficient solution-processable phosphorescent poly(dendrimer) OLED with a 32 lm/W power efficiency at 100 cd/m2 without using a charge transporting host or any improvements in light extraction. This is achieved by using the dendrimer architecture to control inter-chromophore interactions. The effects of using 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) as a charge transporting host and using a double dendron structure to further reduce inter-chromophore interactions are also reported.
©2012 Optical Society of America
1. Introduction
OLED displays give excellent colour, wide viewing angle, and good contrast ratio. In addition they have the potential for high power efficiency as they do not suffer the losses of polarisers and colour filters that limit the efficiency of liquid crystal displays. OLEDs are now deployed commercially in significant numbers of devices including many brands of mobile telephone. They are also promising for lighting applications and are compatible with a wide variety of substrates, allowing flexible devices to be produced.
High power efficiency at the desired operating brightness is a crucial parameter for OLEDs. The increasing importance of mobile displays means that for long battery life OLEDs must be efficient. In lighting high efficiency is not just desirable, but in many nations lower efficiency lighting technologies (such as incandescent bulbs) have been banned. If this trend continues any new commercial lighting technology must have high lm/W power efficiency.
In order to achieve high efficiencies OLEDs must be able to harvest both singlet and triplet excitons that form within them [1]. By including heavy metal complexes it is possible to induce spin-orbit coupling and allow all of the excited states to emit radiatively, giving almost 100% internal quantum efficiency [2]. Iridium(III) complexes, such as fac-tris(2-phenylpyridyl)iridium(III) [3, 4], are commonly used phosphorescent emitters.
Solution-processable OLED materials offer significant advantages in cost, throughput and ease of processing [5]. By making solution-processable materials that can be inkjet printed, patterning and deposition can be combined into a single step that is compatible with roll-to-roll processing [6–8]. Unfortunately small molecules tend not to be particularly soluble in organic solvents and can suffer from aggregation and concentration quenching in the solid state. Semiconducting polymers have excellent solution-processability, but the vast majority are fluorescent, so that the triplets formed in a device are wasted.
As a solution to both of these problems, light-emitting dendrimers were developed [9]. In these materials the luminescent core is protected sterically by dendrons (branched units) that also serve to enhance solubility. As phosphorescent cores can be functionalized with dendrons it has been possible to use these materials to make highly efficient solution-processed OLEDs [10, 11]. Unfortunately inkjet printing requires higher viscosities than most organic solvents [6, 12], and dendrimers, unlike polymers, do not significantly increase the viscosity of their solutions with concentration [13, 14].
Another approach has been to try and make phosphorescent polymer systems by directly grafting the heavy metal complexes onto a polymer [15–20]. We have been developing poly(dendrimers) in order to combine the effective control of inter-chromophore interactions of dendrimers with the desirable viscosity of polymers. We have shown that dendrons can help to prevent concentration quenching by inter-chromophore interactions, and that by making poly(dendrimers) the viscosity of solutions becomes suitable for inkjet printing applications [13, 14]. In this paper we show that by using dendrimers attached to a norbornene polymer backbone we can achieve a highly efficient polymer OLED without the need for a host material. The effects of diluting this material in a 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) [21] host and adding extra dendrons to produce a double dendron material [22] are compared.
2. Experimental methods
All films were spin-coated from dicholoromethane at a concentration of 10 mg/ml. For photoluminescence quantum yield (PLQY) measurements films on 12 mm quartz substrates were prepared. The PLQY was then measured using a Hamamatsu C9920-02 integrating sphere and measurement system [23] in a nitrogen atmosphere. Neat films were excited at 325 nm and blended films at 350 nm.
OLEDs were fabricated on 12 mm x 12 mm indium tin oxide (ITO) substrates that had been etched with 37% HCl and zinc powder to give an active area of 6 mm2. The substrates were cleaned by ultrasound in acetone and 2-propanol for 15 minutes each and then treated with oxygen plasma for 5 minutes prior to spin-coating of the emissive layer.
The samples were transferred to a vacuum evaporation system where a 60 nm layer of 1,3,5,-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) was deposited followed by a 0.7 nm layer of LiF and >100 nm of aluminum as the cathode at a pressure of ~10−6 mbar. The OLEDs were characterised in vacuum using a photodiode of known responsivity and assuming lambertian emission. The electroluminescence spectra were measured using an Andor DV420-BV CCD spectrometer. Emissive layer thicknesses were measured using a Veeco DekTak 150 surface profilometer.
3. Results and discussion
The chemical structures of phosphorescent polymers 1-3 are shown in Fig. 1 . Polymer 1 has no dendrons, polymer 2 has a single dendron attached to each of its two phenylpyridine ligands and polymer 3 has two dendrons on each phenylpyridine ligand. By measuring the film photoluminescence quantum yield (PLQY) we can determine the degree to which these dendrons help to protect the iridium complexes from aggregation.
Fig. 1 The materials structures of phosphorescent polymer 1, poly(dendrimer) 2 and poly(dendrimer) 3.
For polymer 1 we obtain a neat film PLQY of 2%. This increases to 44% for single dendron (one dendron per ligand) material 2 and to 58% for double dendron (two dendrons per ligand) polymer 3, clearly showing that the increasing number of dendrons results in a significant improvement in the emissive properties. Another strategy for reducing aggregation and improving PLQY is to blend the emissive material with a host that separates the emissive cores. The host can also help to improve charge transport. By blending 20 wt% of 2 in TCTA the film PLQY was found to rise significantly to 80%, showing that even the double dendrimer does not completely prevent aggregation. The absorption, photoluminescence (PL) and electroluminescence (EL) spectra of films of 2 and 3 are shown in Fig. 2 . The PL and EL spectra are nearly identical. The emission of 3 is redder than that of the other materials because of the increased electron delocalization on the ligand due to the additional dendron attached to the pyridyl ring.
Fig. 2 Absorption (left panel), photoluminescence (center panel) and electroluminescence (right panel) of a neat film of 2 (thin black line, device A for EL), a blended film of 20 wt% 2 in TCTA (dashed thin blue line, device B for EL) and a neat film of 3 (thick red line, device C for EL).
The improved PLQY should directly correspond to improved electroluminescence efficiency in OLED devices provided that charge balance between electrons and holes can be achieved. In order to ensure this bilayer OLEDs of neat polymer 2 (device A) and 20 wt% 2 blended in TCTA (device B) and neat polymer 3 (device C) were made using TPBI as an electron-transport hole-blocking layer. OLEDs were not made using polymer 1 due to its low solid state PLQY.
The electroluminescence spectra of the OLEDs are shown in Fig. 2. The spectra of polymer 2 in neat and blended devices remained the same indicating that there was no strong aggregation affecting the neat polymer. The emission of polymer 3 is red-shifted compared to 2, and this is due to the extension of the conjugation caused by the additional dendron on the 2-phenylpyridyl ligands [22].
The current-voltage and brightness-voltage characteristics of these devices are shown in Fig. 3 . Devices B and C show almost identical currents while device A gave a higher current density. These results cannot be explained by variations in the film thicknesses as the thickness of the emissive layer was 115 nm in device A, 70 nm in device B was 70 nm and 100 nm in device C, whilst the TPBI layer was 60 nm thick in all devices. This means that the current density is not correlated with layer thickness alone.
Fig. 3 Current density (A) and brightness (B) of the devices versus voltage. The emissive layer was a neat film of 2 (device A, black line), a blended film of 2, 20 wt% in TCTA (device B, dashed blue line) and a neat film of 3 (device C, red line).
The devices achieve a brightness of 100 cd/m2 at 4.1 V, 5.6 V and 5.5 V for A, B and C, respectively. The turn-on voltage of all devices (1 cd/m2), was similar and low: 2.8 V, 3.1 V and 3.0 V for A, B and C, respectively. This suggests there was no significant barrier to charge injection in any of the devices. The fact that the current in device A is higher than in device B, suggests that the charge mobility in the neat film is higher than in the blend with TCTA, even though TCTA is a good hole transporting material [21]. The higher mobility in the neat film could arise if charge traps on the iridium complex, so that charge transport is by hopping between iridium complexes. The separation of the iridium complexes would be larger in the blend (and also in the neat film of 3), than in the neat film of 2, leading to lower mobility and the lower current observed. Hence the closer spacing of the iridium complexes in a neat film of 2 is helpful to charge transport, but leads to lower PLQY.
The external quantum efficiency (EQE) of the devices is shown in Fig. 4A and the power efficiency as a function of brightness is shown in Fig. 4B. The external quantum efficiencies were 11.9%, 12.7% and 11.7% at 100 cd/m2 for devices A, B and C, respectively. This corresponds to current efficiencies of 42 cd/A, 46 cd/A and 38 cd/A, respectively. For the same devices the efficiency remains high and at 1000 cd/m2 EQEs of 10.1%, 11.4% and 10.4% were measured for device A, B and C, respectively.
Fig. 4 External quantum efficiency (A) and power efficiency (B) of the devices versus brightness. The emissive layer was a neat film of 2 (device A, black line), a blended film of 20 wt% of 2 in TCTA (device B, dashed blue line) and a neat film of 3 (device C, red line).
The quantum efficiency of blended device B is slightly higher than that of the other devices, as we might expect from its higher film PLQY. However the difference is smaller than the difference in PLQY, suggesting that there may be scope to improve this device by optimizing the charge balance and/or the location of the recombination zone.
These high values at relatively low drive voltages result in good power efficiencies of 32 lm/W, 26 lm/W and 22 lm/W at 100 cd/m2 for A, B and C, respectively. The lower driving voltage of A gives it the advantage over the blended device B and device C based on neat double dendron polymer 3, despite their higher film PLQY values. Potentially, with an optimized charge transporting host, material 3 could be used to produce an even more efficient OLED device.
4. Conclusion
In this work we have investigated the potential of phosphorescent poly(dendrimers) for organic light-emitting diodes. These materials are attractive because of their solution processability, in particular because their higher viscosity in solution makes inkjet printing possible.
We have demonstrated that by using a dendritic architecture it is possible to control interchromophore interactions and improve the efficiency of luminescence in phosphorescent polymer films. By making use of these materials it is possible to fabricate high quantum efficiency organic light emitting diodes with low driving voltages. This results in a high power efficiency that is required for applications in displays and solid-state lighting without using a charge transporting host or using strategies for enhancing light out coupling efficiency.
Acknowledgments
J. W. Levell, S. Zhang and I. D. W. Samuel are grateful to the Engineering and Physical Sciences Research Council and the Scottish Universities Physics Alliance for financial support. P. L. Burn is the recipient of an Australian Research Council Federation Fellowship (Project FF0668728).
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