Multi-layer organic light-emitting diodes processed from solution using phosphorescent dendrimers in a polymer host

Multi-layer organic light-emitting diodes processed from solution

using phosphorescent dendrimers in a polymer host

Li-Chun Ko

, Tsung-Yu Liu

, Chun-Yao Chen

, Chung-Ling Yeh

, Shin-Rong Tseng

,

Yu-Chiang Chao

, Hsin-Fei Meng

, Shih-Chun Lo

, Paul L. Burn

, Sheng-Fu Horng

Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC b_{Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC} c_{Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC} d

Centre for Organic Photonics and Electronics, The University of Queensland, School of Chemistry and Molecular Biosciences, Brisbane, QLD 4072, Australia

a r t i c l e

i n f o

Article history:

Received 11 November 2009

Received in revised form 12 March 2010 Accepted 16 March 2010

Available online 20 March 2010

Keywords: Dendrimer Blade coating

Organic light-emitting diodes

a b s t r a c t

A uniform dispersion of highly soluble phosphorescent dendrimer emitters is achieved by blending with a polymer host poly(9-vinylcarbazole) (PVK) containing N,N0_-diphenyl-N, N0_{-(bis(3-methylphenyl)-[1,1-biphenyl]-4,4}0_{-diamine (TPD) and 2-(4-biphen-4}0 -yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). No visible aggregation or self-quenching was observed for guest-to-host weight ratios of up to 33:67. The dendrimers contain a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3] core, first generation biphenyl-based dendrons,

and 2-ethylhexyloxy surface groups. The guest–host blend is used for all solution pro-cessed organic light-emitting diodes. A maximum external and current efficiency of 10.2% and 38 cd/A (at 5 V and a brightness of 50 cd/m2_{), and a maximum brightness of}

27,000 cd/m2_{(at 14.5 V), were obtained when a CsF/Al cathode was used. Blade coating}

was used to fabricate a multi-layer structure that also contained an electron-transport layer. The device that had a LiF/Al cathode had a maximal efficiency of 40 cd/A correspond-ing to an external quantum efficiency of 10.8% (at 5 V and a brightness of 19 cd/m2_{). The}

maximum brightness of the second device was 17,840 cd/m2_{at 14 V.}

Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction

Solution processable organic light-emitting diodes (OLEDs) have the great potential for applications in large-area lighting and displays[1]. Compared with fluorescent OLEDs, phosphorescent OLEDs have much higher efficiency because the triplet emitters used can theoretically harvest 100% of the excitons that are formed during device opera-tion[2]. A common way to achieve efficient phosphores-cent OLEDs is to blend heavy metal complexes such as iridium(III) or platinum(II) complexes into polymer hosts.

However, a major challenge for such simple blending is that small molecule phosphorescent emitters in general have poor solubility, resulting in aggregation in the poly-mer host. In fact most of the iridium(III) complexes are vacuum deposited where aggregation is avoided by careful co-evaporation together with a small molecule host[3,4]. Aggregation of the iridium(III) complexes in the polymer hosts reduces the electroluminescence efficiency due to two reasons. One is the quenching of the luminescence through triplet–triplet annihilation, and the second is that aggregation may leave some volume in the host without any emitter so the exciton will form and decay in the host. The hosts often have a low radiative decay efficiency and emit light of a different color to the guest complex. In order to raise the efficiency of solution processed polymer based light-emitting diodes to the level of vacuum deposited small molecule organic light-emitting diode, the dispersion

1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.03.014

*Corresponding author. **Corresponding author.

E-mail addresses:[email protected](H.-F. Meng),[email protected]. au(S.-C. Lo).

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Organic Electronics

of the metal complex emitter in the polymer host must be improved.

Chemical modification of the phosphorescent green iridium(III) complex fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3], which has been shown to have a high

photolu-minescence efficiency and relatively short radiative life-time, has led to some improvement in the solubility. For example, methylation of Ir(ppy)3on the ligand phenyl ring

at the position para to the ligand pyridyl ring, i.e. tris[2-(4-tolyl)pyridyl]iridium(III), [Ir(mppy)3], enhances the

solu-bility. However, although the solubility is improved, this approach still does not fully prevent the aggregation of the emissive molecules. This is due to Ir(mppy)3still

hav-ing only poor to at best moderate solubility in organic sol-vents commonly used for processing polymer hosts such as toluene and chlorobenzene. The range of concentrations that the complex is used as a guest in a polymer host in which a uniform dispersion is achieved is therefore quite limited. Moreover, the low solubility of Ir(mppy)3makes

it difficult to accurately control the doping concentrations and the long mixing time required for dissolution makes the fabrication process rather time-inefficient.

Dendrimer OLEDs offer the advantages of combining the high efficiency of small molecule OLEDs and the solu-tion processing properties of polymer OLEDs [5,6]. High efficiency dendrimer OLEDs have been achieved when the dendrimer layer has been deposited by spin-coating, and an evaporated electron-transport layer has been used

[7–9]. Lately, blade coating has shown potential as an

alternative processing method to spin-coating and ink-jet printing. It also opens up the possibility of multi-layer OLEDs, in which all the layers are deposited by solution processing[10]without the use of post UV-cured polymer-ization of soluble cross-linkable precursors[11]. In general multi-layer structures are required to balance electron and hole currents for achieving highly efficient OLEDs.

In this Letter we use highly soluble dendrimers, p-G1-Ir and m-G1-Ir, as the light-emitting guest (chemical struc-tures shown inFig. 1c). The host is a mixture comprised of poly(9-vinylcarbazole) (PVK), and charge-transport materials, N,N0_{-diphenyl-N,N}0_{-[bis(3-methylphenyl)]-[1,1}0

-biphenyl]-4,40_{-diamine (TPD) and 2-(4-biphen-4}0

-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [12]. The p-G1-Ir and m-G1-Ir dendrimers consist of a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3] core, first generation

biphenyl-based dendrons, and 2-ethylhexyloxy surface groups[13,14]. The synthesis of the dendrimers has been previously reported[15]. As shown inFig. 1c, the differ-ence between p-G1-Ir and m-G1-Ir is that p-G1-Ir has the dendron attached on the ligand phenyl ring para to the pyridyl ring whereas m-G1-Ir has the dendron attached on the ligand phenyl ring meta to the pyridyl ring

[7,15,16]. Unlike most phosphorescent emitters based on

small molecules, the phosphorescent dendrimers have su-perb solubility in common organic solvents due to the sur-face groups attached to the distal ends of the dendrons. In addition, the dendrons act as rigid spacers that reduce the intermolecular interactions of the emissive cores that can cause luminescence quenching [5,6,17]. Indeed, there is dramatic difference in solubility between the dendrimers used and Ir(mppy)3. The solubility of p-G1-Ir is more than

10 wt.% both in toluene and chlorobenzene, whereas Ir(mppy)3is less than 0.5 wt.% in chlorobenzene and it is

virtually insoluble in toluene. Bi-layer OLEDs with CsF/Al cathode and tri-layer OLEDs with an electron-transport layer (ETL) and a more stable LiF/Al cathode were fabri-cated with the dendrimers as the guest in a PVK:TPD:PBD blend host. The electron transport materials were depos-ited by a newly developed blade-coating method to avoid dissolution [10]. The ETL materials studied included 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), 3-(4-biphen-40

-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), 4,7-diphenyl-1,10-phenanthroline (Bphen), and tris-[3-(3-pyridyl)mesityl]borane (3TPYMB). High effi-ciencies were achieved for dendrimer weight concentra-tions of up to 33% with easy concentration control and instant dissolution during fabrication. An efficiency of 38 cd/A (at 5 V) and luminance of 27,000 cd/m2 _(at

14.5 V) were obtained in the bi-layer devices prepared by blade coating. Although direct comparison is not possible as different host materials have different solubility for the studies, current device efficiencies are higher than

LiF

CsF

EML

ETL

a

b

c

Fig. 1. (a) Schematic diagram of the OLED structures employed. The cathodes are CsF/Al for the devices without the ETL and LiF/Al for those with the ETL. (b) Energy diagram of the materials in this work, the numbers are in eV. (c) Structures of p-G1-Ir and m-G1-Ir.

those previously reported for similar bi-layer device fabri-cated from spin-coating[18]. In addition, the amount of dendrimer used in the emissive layer is less than that of previous reports, making the blade-coating fabrication more cost effective.

2. Experimental

Fig. 1shows the OLED device structures used in the

study. To fabricate the OLEDs, the indium-tin-oxide (ITO) substrate was first precleaned in deionized water with detergent, which was purchased from Alconox, and then UV–ozone treated. In order to use the blade-coating technique it was necessary to have large ITO substrates (7 cm  8 cm). A 50 nm poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS, CLEVIOS™ P VP AI 4083) was then spin-coated onto the ITO substrate and an-nealed at 100 °C for 40 min in vacuum. To improve hole injection in the devices [19,20], poly[(9,9-dioctylfluore-nyl-2,7-diyl)-co-(4,40_{-{N-[4-s-butylphenyl]}diphenylamine)]}

(TFB) in toluene (1 wt.%) was blade-coated on top of the PEDOT:PSS and spin-rinsed with toluene after annealing at 180 °C for 40 min in vacuum to leave a TFB layer with thickness of about 5 nm. The light-emitting solution was prepared by mixing host solution, PVK:PBD:TPD in chloro-benzene (2 wt.%), and dendrimer guest solution, dendrimer in toluene (2 wt.%), and then blade-coated onto the TFB layer to give a 70 nm thick ‘light-emitting’ film. In the blend system, dendrimer:PVK:TPD:PBD, the ratio of TPD:PBD was fixed at 9:24 and the ratio of dendrimer:PVK was tuned. The ratios of p-G1-Ir:PVK were 1:66, 2:65, 6:61, 13:54, and 33:34. The ratios of m-G1-Ir:PVK were 2:65, 6:61, 13:54, and 33:34. The emissive layer was annealed at 80 °C for 60 min in vacuum. To complete the devices without an elec-tron-transport layer, a 2 nm layer of CsF was deposited, fol-lowed by a 100 nm capping layer of Al. The latter two layers were deposited using thermal evaporation at a pressure of 106_{Torr. For the devices with electron-transport layers,}

TPBi, TAZ, Bphen, and 3TPYMB, were dissolved in n-butanol (0.5 wt.%) and blade-coated on top of the emissive layer (dendrimer:TPD:PBD:PVK with a ratio of 6:9:24:61) to give thicknesses of order 20 nm [12]. Dissolution between the ETL and emissive layer was avoided by using the newly developed blade-coating method with n-butanol as the sol-vent. A thin (1 nm) layer of LiF was evaporated under the pressure of 106_{Torr and covered with a 100 nm of Al to}

form the cathode and complete the devices. The active area of each pixel was 4 mm2_{. I–L–V characteristics were}

mea-sured with a Keithley 2400 source meter with the light out-put integrated with a PR650 photometer. PVK with a molecular weight (Mw) of 1,100,000 was purchased from

Sigma–Aldrich. TPD, PBD, TPBi, TAZ, BPhen, and 3TPYMB were obtained from Luminescence Technology Corp, and TFB was purchased from American Dye Source.

3. Results and discussion

Figs. 2 and 3show the bi-layer device characteristics

and emission spectra based on p-G1-Ir and m-G1-Ir, respectively, and using CsF/Al as cathode. In general, the

luminance and efficiency were not high at low dendrimer doping concentrations in the PVK (i.e. 1:66 and 2:65 with a fixed ratio of TPD:PBD of 9:24). This could be due to the fact that there were not enough emitters in the host to harvest the excitons, which is supported by the emission spectra where there is blue emission from the host

(Fig. 2b). The blue emission at the wavelengths near

400 nm can be attributed to the TPD or PVK host while the broad emission peaking at 470 nm may originate from the (TPD + PBD-)* exciplex [12]. Interestingly, the blue emission from m-G1-Ir based devices is not as obvious as those of p-G1-Ir at the same doping concentration (2:65)

(Fig. 3). Given that the two dendrimers have exact the

same molecular weights and the same dendrimer doping concentrations, the smaller amount of blue emission from the (2:65) device could be due to the different shape of the dendrimers. The different shape results from the dendrons

0 5000 10000 15000 20000 25000 30000 Voltage (V) p-G1-Ir:PVK 1:66 2:65 6:61 13:54 33:34 Luminance (cd/m 2 ) spin-coating without TFB 0 100 200 300 400 500 600 Current Density ( mA/cm 2) Voltage (V) spin-coating without TFB

a

b

Normalized EL Itensity (a.u.) Wavelength (nm)

4 6 8 10 12 14 4 6 8 1 0 1 2 1 4 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Voltage (V)

Current Efficiency (cd/A)

1:66 2:65 6:61 13:54 33:34 p-G1-Ir:PVK spin-coating without TFB

Fig. 2. Device performance of p-G1-Ir:PVK:TPD:PBD. (a) The luminance and the inset is the current density versus voltage. (b) The current efficiency versus voltage and the inset shows the electroluminescent spectra at 1000 cd/m2_{. The ratio of TPD:PBD was fixed at 9:24. The} performance of the device without TFB layer is indicated by dark yellow line, while the performance of the device fabricated by spin-coating instead of blade coating is indicated by pink line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

being attached to different positions on the ligand phenyl ring of the emissive core, which in turn could give rise to a different film morphology. The fact that m-G1-Ir has a larger effective hydrodynamic radii is evidence of the dif-ference in shape between the two materials[15].

Both the luminance and efficiency significantly in-creased as the dendrimer concentrations inin-creased above the ratio of 6:9:24:61 [dendrimer:TPD:PBD:PVK (TPD:PBD: PVK = host)]. The peak luminance of 22,900 cd/m2 _(at

13 V), 27,000 cd/m2 (at 14.5 V), and 22,380 cd/m2 (at 14 V) was achieved for p-G1-Ir:PVK with blending ratios of 33:34, 13:54, and 6:61, respectively. The current effi-ciency was around 22–38 cd/A for the mixing ratios of den-drimer:PVK between 33:34 and 6:61, showing that the dendrimers can be used in rather high blending concentra-tions due to the uniform distribution in the emissive layer with less aggregation and self-quenching. The highest external quantum efficiency is 10.2% at 5 V with a bright-ness of 50.1 cd/m2_{for p-G1-Ir:PVK at ratio of 6:61. Such}

excellent dispersion in the polymer host is attributed to the good solubility of the dendrimers in comparison to small molecule iridium(III) complexes.

While the efficiency of device with a p-G1-Ir:PVK ratio of 6:61 reaches up to 40 cd/A, it drops rapidly as the volt-age increases. In contrast, the devices with a p-G1-Ir:PVK ratio of 13:54 keeps a high efficiency at high luminance. In fact the ratio 13:54 of dendrimer:PVK was found to be the optimum ratio for both dendrimers in the host. For OLEDs with higher dendrimer concentrations, although the dispersion is still good, some level of self-quenching sets in causing a slight reduction in the efficiency.

The significance of the blade-coating method can be understood from the performance of the device fabricated by spin-coating. The concentration ratio of p-G1-Ir:PVK is 6:61. As shown inFig. 2, the luminance and current effi-ciency are low for the device fabricated by spin-coating. The peak luminance of 14,980 cd/m2 _{was achieved at}

13.5 V, and the maximum current efficiency of 10.7 cd/A was achieved at 9.5 V. This inferior performance may result from the layer-to-layer dissolution while using spin-coat-ing technique or due to a completely different film mor-phology being formed. The layer-to-layer dissolution can be prevent by adopting the blacoating technique as de-scribed in previous work[10]. The need for the TFB layer is

0 4000 8000 12000 16000 20000 24000 m-G1-Ir:PVK 33:34 13:54 6:61 2:65 Luminance (cd/m 2) Voltage (V) 0 50 100 150 200 250 300 350 400 Voltage (V) Current Density ( mA/cm 2)

a

b

Normalized EL Intensity (a.u.) Wavelength (nm)

4 6 8 10 12 14 4 6 8 10 12 14 400 500 600 700 4 6 8 10 12 14 0 5 10 15 20 25 30 35 40 45 50 55 m-G1-Ir:PVK 33:34 13:54 6:61 2:65

Current Efficiency (cd/A)

Voltage (V)

Fig. 3. Device performance of the m-G1-Ir:PVK:TPD:PBD. (a) The lumi-nance and the inset shows the current density versus voltage. (b) The current efficiency versus voltage and the inset is the electroluminescent spectra at 1000 cd/m2

. The ratio of TPD:PBD was fixed at 9:24.

0 5000 10000 15000 20000 25000 EML/TPBi/LiF/Al EML/TAZ/LiF/Al EML/Bphen/LiF/Al EML/3TPYMB/LiF/Al EML/CsF/Al EML/LiF/Al Luminance ( cd/m 2 ) Voltage (V) 0 5 10 15 20 25 30 35 40 45 EML/TPBi/LiF/Al EML/TAZ/LiF/Al EML/Bphen/LiF/Al EML/3TPYMB/LiF/Al EML/CsF/Al EML/LiF/Al

Current Efficiency (cd/A)

Voltage (V)

a

b

Fig. 4. The results of the multi-layer p-G1-Ir:PVK:TPD:PBD devices with ETL materials, TPBi, TAZ, Bphen, and 3TPYMB. (a) The luminance and the inset is the current density versus voltage. (b) The current efficiency versus voltage.

also shown inFig. 2. For the device without TFB hole-trans-porting and electron-blocking layer, the peak luminance of 13,730 cd/m2_{was achieved at 13 V, and the maximum}

cur-rent efficiency of 11.5 cd/A was achieved at 9.5 V. The superior performance of the device with TFB layer results from the fact that more holes can be injected.

Fig. 4 shows the performance of the devices with

TPBi, TAZ, Bphen, and 3TPYMB as the electron-transport layers and p-G1-Ir as the emitter. The ratio of dendri-mer:TPD:PBD:PVK was 6:9:24:61. The results were com-pared with those without the electron-transport layer. Of the tri-layer devices, the device with TAZ shows the high-est luminance as it and PBD have highhigh-est electron affinities [most stable Lowest Unoccupied Molecular Orbital (LUMO) energies] and this leads to more efficient electron injection into the light-emitting layer. However, the overall effi-ciency of TAZ devices is slightly lower than those contain-ing TPBi. The device that had a LiF/Al cathode had a maximal efficiency of 40 cd/A (at 5 V with external quan-tum efficiency = 10.8% and brightness of 19 cd/m2) and maximal luminance of 17,840 cd/m2_{(at 14 V). This}

differ-ence probably arises from the fact that while the TAZ has the higher electron affinity it is a poorer hole blocking material than TPBi (see energy diagramFig. 1b). In general the electron-transport layer in combination with LiF/Al cathode shows similar performance to devices with CsF/ Al cathode but the latter devices tend to be less stable to the moisture. Without the electron-transport layer, the LiF/Al cathode has poorer electron injection capability and this may be due to a poor metal/organic interface lead-ing to relatively low efficiencies (Fig. 4). The blade-coated layer of the electron transport materials therefore has a similar effect as with the equivalent vacuum deposited lay-ers. Importantly, there is no dissolution of the emissive layer observed while blade coating the electron-transport layer.

4. Conclusion

In conclusion, we demonstrate that the good solubility of phosphorescent dendrimer emitters allows the forma-tion of a dispersion of the dendrimers in a polymer host. The uniform dispersion enabled OLEDs with high efficiency and luminance to be achieved with a wide range of emitter concentrations because of low level of intermolecular quenching of the phosphorescence. The good compatibility of the phosphorescent dendrimers and the polymer host

indicates that it is crucial to add functional groups to en-hance the solubility when designing solution processable triplet emitters for use in blends. Importantly, all solution processed multi-layer OLEDs without layer-to-layer disso-lution can be fabricated using the blade-coating technique. Blade-coating phosphorescent dendrimers show promise for the development of large-area and low-cost lighting emitting applications.

Acknowledgements

This work was supported by the National Science Coun-cil of the Republic of China (NSC NO. 97-3114-M-009-002, NO. 97-2628-M-009-016) and Professor Paul L. Burn is re-cipient of an Australian Research Council Federation Fel-lowship (Project Number FF0668728).

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