Efficiency improvement of solution processed red phosphorescent organic light-emitting diodes using optimized hole transport material

Optical Materials 35 (2013) 685–689

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Short Communication

Efficiency improvement of solution processed red phosphorescent organic light-emitting diodes using optimized hole transport material Juhyeon Park a, Myungkwan Song a, Siwon Kim b, Suhkmann Kim b, Yeong-Soon Gal c, Sung-Ho Jin a,⇑ a Department of Chemistry Education, Graduate Department of Frontier Materials Chemistry and Institute for Plastic Information and Energy Materials, Pusan National University, Busan 609-735, Republic of Korea b Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea c Polymer Chemistry Lab, College of General Education, Kyungil University, Hayang 712-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 18 July 2012 Received in revised form 10 October 2012 Accepted 10 October 2012 Available online 11 November 2012 Keywords: PhOLEDs Solution processed Phosphorescent Hole transport material

a b s t r a c t We fabricated highly efficient solution processed red phosphorescent organic light-emitting diodes (PhOLEDs) composed of bis[9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C20 ]iridium picolinate N-oxide (Et-CVz-PhQ)2Ir(pic-N-O) and bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9Hcarbazolato-N,C20 ]iridium picolinate N-oxide (EO-CVz-PhQ)2Ir(pic-N-O) as an emitter, which were doped into poly(N-vinylcarbazole) (PVK), electron transport material, 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) and optimized concentration of hole transport material, N,N0 -diphenylN,N0 -(bis(3-methylphenyl)-[1,1-biphenyl]-4,40 -diamine (TPD). The optimized PhOLEDs doped with 16 wt% of TPD shows a maximum external quantum efficiency of 9.21% and a luminance efficiency of 15.24 cd/A with a CIE coordinate of (0.62, 0.38). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Phosphorescent organic light-emitting diodes (PhOLEDs) have been actively investigated for the past decade because they can, in theory, approach 100% internal quantum efficiency by harvesting both singlet and triplet excited states [1]. One of the major advantages of PhOLEDs is their full-color capability with various emission wavelengths which are easily gained by changing the molecular structure of the organic materials [2–4]. Another advantage is the solution processability for the fabrication of PhOLEDs using the conjugated organic material. Generally, the fabrication of the PhOLEDs are divided in two approaches, those that are thermal vapor deposition for small molecules [5] and solution processing for polymers [6]. Small molecules have many advantages such as easy synthesis and purification. However, thermal evaporation technology under high vacuum does increase fabrication complexity and may be a relatively costly process. On the other hand, solution processing method is easy to fabricate and cost effective process. Numerous efforts have been carried out to improve the performance of the solution processed PhOLEDs. Nevertheless, it is more challenging to achieve good charge balance with these devices [7–12]. For instance, incorporation of small molecules (molecular doping) into the fluorescent and phosphorescent emitting layers or synthesis of novel polymers has been used to ⇑ Corresponding author. E-mail address: [email protected] (S.-H. Jin). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.009

improve the charge injection and transport [13–16]. In order to maximize the performance of PhOLEDs, it is necessary to choose both a suitable host for efficient energy transfer from the host to the phosphorescent dopant, and a hole and exciton blocking layer to confine both holes and electrons within the emitting layer. The most commonly used attempt is to blend with a low molecular weight phosphorescent emitter into the proper polymer matrix [17–19]. In these systems, it is required that polymer host should possess triplet energies higher than those of the dopant molecules, better charge balance properties, and good charge carrier transport properties. In this regard, poly(N-vinylcarbazole) (PVK) is generally selected as the polymer host in PhOLEDs for its relatively high triplet energy, better film formation and moderate hole transport ability. However, the electron transporting property of PVK is poor [20]. To achieve better balance of charge transport, PVK based on PhOLEDs introduced the electron transporting material to the emitting layer. One of the most common used electron transporting material for this purpose is 1,3-bis[2-(4-tert-butylphenyl)1,3,4-oxadiazole-5-yl]benzene (OXD-7), which has a high triplet energy (2.7 eV) and a high electron mobility (2.1  105 cm2/V s at 105 V cm1) [21,22]. We have recently reported a novel red emitting Ir(III) complexes based on carbazole containing phenylquinoline (CVz-PhQ) derivatives to fabricate solution processed PhOLEDs [23]. Carbazole molecules exhibit inherent electron-donating nature, excellent photoconductivity, and relatively intense luminescence. The thermal stability or the glass-state durability of organic

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compounds can be greatly enhanced by the introduction of a carbazole group in the core structure. Moreover, the carbazole group can be easily functionalized at the 3-, 6-, or 9-position and covalently linked to other molecular groups. The N-aryl carbazoles, in which a phenyl or a naphthyl group is attached on the 9-position of carbazole, have shown excellent thermal stability and good electro-optical properties [24–30]. Quinolines, along with its significant biological activities, are well known for their formation of conjugated molecules and polymers that combine enhanced electronic, optoelectronic, or nonlinear optical properties with excellent mechanical properties [31]. However, performance and configuration of solution processed PhOLEDs are still needed for further investigation and development. In recent pure red-emitting bifunctional Ir(III) complexes with triphenylamine dendron gave a peak external quantum efficiency of 11.65% at 5 V and a luminance of 7451 cd/m2 [32]. For this purpose, we have substantially improved the efficiency of PhOLEDs by doping hole transporting material into the emitting layer, which facilitate the transport of carriers into emitting layer for increasing the probability of carrier recombination. In this work, we report on highly efficient solution processed red PhOLEDs using a novel (Et-CVz-PhQ)2Ir(pic-N-O) or (EO-CVzPhQ)2Ir(pic-N-O) as phosphorescent dopants into PVK or CBP (4,40 -bis(9-cabazolyl)biphenyl) as a host materials blended with hole transport material of N,N0 -diphenyl-N,N0 -(bis(3-methylphenyl)-[1,1-biphenyl]-4,40 -diamine (TPD) and electron transport material of OXD-7. TPD is particularly suitable host because of the HOMO level of TPD at 5.5 eV leaves ca. 0.3 eV barrier for hole injection to PVK [33]. It is expected that adding TPD at a sufficiently concentration will allow hole to penetrate deeper into emitting layer and reduces the barrier for hole injection. The PhOLEDs in this work were prepared by the following procedures. The patterned indium tin oxide (ITO, thickness 110 nm) glass substrates with a sheet resistance of 20 X were ultrasonically cleaned with detergent, deionized water, acetone, and isopropyl alcohol, and finally treated in UV–ozone chamber for 15 min. The layer of 40 nm thick PEDOT:PSS (P VP AI 4083) was spin-coated onto the pre-cleaned and UV–ozone treated ITO substrates. And then, spin-coated film baked in air at 150 °C for 20 min. The PVK (or CBP), OXD-7, TPD, and (Et-Cvz-PhQ)Ir2(pic-N-O) or (EO-CvzPhQ)Ir2(pic-N-O) were dissolved separately in chlorobenzene. The

PVK (or CBP) and OXD-7 blend solutions (weight ratio, 2:1) and TPD solution were mixed various ratios to give appropriate weight percentage. The 70–80 nm thick emitting layer was spin-coated on the PEDOT:PSS layer for 40 s to minimize the residual solvent. The emitting layer was then annealed at 80 °C for 30 min in glove box. Finally, as typical cathode, consisting of OXD-7 (20 nm)/Ba (3 nm)/ Al (100 nm) was thermal vapor deposited with effective area of 4 mm2 at a pressure 5  106 Torr. The film thickness was measured by using a-Step IQ surface profiler (KLA Tencor, San Jose, CA). All solutions used in the PhOLEDs fabrication were filtered with 0.45 lm PTFE syringe filter. In all cases, doping concentrations of the (Et-Cvz-PhQ)Ir2(pic-N-O) or (EO-Cvz-PhQ)Ir2(pic-N-O) were fixed at 6 wt%. EL spectra and current density–voltage–luminance (J–V–L) characteristics of PhOLEDs were measured with a programmable Keithley model 236 power source and spectrascan CS-1000 photometer, respectively. All measurements were carried out at room temperature under an ambient atmosphere. The absorption and photoluminescent (PL) spectra were recorded using JASCO V570 UV–vis spectrophotometer and Hitachi F-4500 fluorescence spectrophotometer, respectively. The molecular structures of (Et-Cvz-PhQ)2Ir(pic-N-O) and (EOCvz-PhQ)2Ir(pic-N-O) along with the PhOLEDs configuration and HOMO and LUMO energy levels of the materials used in this study are illustrated in Fig. 1. The thermal stability of the Ir(III) complexes was evaluated using TGA and DSC under a nitrogen atmosphere. The temperature of 5% weight loss and the glass transition temperatures were 381 °C, 287 °C for (Et-CVz-PhQ)2Ir(pic-N-O) and 358 °C, 182 °C for (EO-CVz-PhQ)2Ir(pic-N-O), respectively. The phosphorescent quantum yields (Upl) of Ir(III) complexes in chloroform solution were measured with (piq)2Ir(acac) as a standard (0.20) [34]. The Upl for (Et-CVz-PhQ)2Ir(picN-O) and (EO-CVz-PhQ)2Ir(pic-N-O) were in the range of 0.20– 0.30. The HOMO and LUMO energy (5.14/2.62 eV and 5.14/ 2.64 eV) levels of (Et-Cvz-PhQ)2Ir(pic-N-O) and (EO-Cvz-PhQ)2Ir(pic-N-O) were determined by using cyclic voltammerty. The energy level diagrams show that the HOMO and LUMO energy levels of Ir(III) complexes lie above and below those of PVK, respectively. Therefore, it is possible that Ir(III) complexes will trap both electrons and holes in the emitting layer. In addition, the hole blocking and electron transport (OXD-7) layer was introduced for effective electron transport and charge carrier balance within emitting

Fig. 1. Molecular structures of (Et-Cvz-PhQ)2Ir(pic-N-O) and (EO-Cvz-PhQ)2Ir(pic-N-O) and the energy band diagram of PhOLEDs.

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layer. This good alignment of energy levels in PhOLEDs is important to facilitate carrier injection into both transport molecules and Ir(III) complexes. To investigate the energy transfer between the (Et-Cvz-PhQ)2Ir(pic-N-O) or (EO-Cvz-PhQ)2Ir(pic-N-O) and PVK (or CBP) doped with OXD-7 in film state, absorption and emission spectra have

Fig. 4. Current density–voltage–luminance (J–V–L) characteristics of (a) (Et-CvzPhQ)2Ir(pic-N-O) and (b) (EO-Cvz-PhQ)2Ir(pic-N-O) with different TPD concentration using PVK.

been measured as shown in Fig. 2. These spectra show a large spectral overlap between the PL emission spectrum of PVK doped with OXD-7 and the absorption spectrum of the two Ir(III) complexes. Such a good spectral overlaps enable efficient Förster energy transfer from host to guest and/or direct charge trapping of the Ir(III) complexes. Fig. 3 shows electroluminescent (EL) spectra of the (Et-CvzPhQ)2Ir(pic-N-O) (a) and (EO-Cvz-PhQ)2Ir(pic-N-O) (b) at a current density of 30 mA/cm2 and their thin film PL spectrum using PVK host. The EL spectra exhibited red emission with peak maxima at 605 nm and an excellent color purity at a CIE coordinate of (0.62, 0.37). PL spectra of (Et-Cvz-PhQ)2Ir(pic-N-O) and (EO-Cvz-PhQ)2Ir(pic-N-O) showed a very similar to their EL spectra. From these results, EL emission originated from the triplet excited state of the Ir(III) complex from the direct recombination of holes and electrons located within emitting layer. In order to investigate the effect of TPD concentration on device performance, PhOLEDs were fabricated with variable concentrations (0, 4, 8, 12, and 16 wt%) of TPD using PVK or CBP host. Fig. 4 shows the current density–voltage–luminance (J–V–L) characteristics of (Et-Cvz-PhQ)2Ir(pic-N-O) (a) and (EO-Cvz-PhQ)2Ir(pic-N-O) (b) with a different concentrations of TPD in PhOLEDs using PVK host. The maximum luminance and turn-on voltage without the TPD concentration in the emitting layer were 2439 cd/m2 and 10 V for (Et-Cvz-PhQ)2Ir(pic-N-O) and 2214 cd/ m2 and 9.5 V for (EO-Cvz-PhQ)2Ir(pic-N-O). However, by adding and increasing the TPD concentrations led to dramatic changes on the performances of PhOLEDs. When the concentration of TPD

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was 16 wt%, the maximum luminance and turn-on voltage of the PhOLEDs were obtained 7860 cd/m2 and 6 V for (Et-Cvz-PhQ)2Ir(pic-N-O) and 7211 cd/m2 and 6 V for (EO-Cvz-PhQ)2Ir(pic-NO), respectively. From the J–V characteristics of PhOLEDs, it is clear that the current density is directly dependent on the TPD

concentration. We found that the performance of PhOLEDs improves with increasing the concentration of TPD from 0% to 16%, but the performances gradually decreased if the concentration of TPD is above 16 wt%. Fig. 5 shows the luminance and power efficiencies versus current density of the PhOLEDs based on (Et-Cvz-PhQ)2Ir(pic-N-O) (a) and (EO-Cvz-PhQ)2Ir(pic-N-O) (b) and the performances of PhOLEDs are summarized in Table 1. Without the TPD concentration in the emitting layer, the external quantum, luminance and power efficiencies of (Et-Cvz-PhQ)2Ir(pic-N-O) were 5.46%, 7.95 cd/A and 1.57 lm/W using PVK host. Similarly, in case of (EO-Cvz-PhQ)2Ir(pic-N-O), the external quantum, luminance and power efficiencies were 4.80%, 6.65 cd/A and 1.04 lm/W, respectively. The external quantum, luminance and power efficiencies of PhOLEDs based on (Et-Cvz-PhQ)2Ir(pic-N-O) with a 16 wt% of TPD were significantly increased up to 9.21%, 15.24 cd/A and 4.53 lm/W, respectively. The slightly lower device performance of (EO-Cvz-PhQ)2Ir(pic-N-O) based on PhOLEDs are due to the lower thermal stability than that of (Et-Cvz-PhQ)2Ir(pic-N-O). The performances of the PhOLEDs using the (EO-Cvz-PhQ)2Ir(pic-N-O) showed the similar effect with same TPD concentration as the (Et-Cvz-PhQ)2Ir(pic-N-O). Consequently, addition of TPD in emitting layer acts as a bridge for efficient hole injection into the PVK and the reduced the turn-on voltages and thus improved the PhOLEDs performances. To investigate the PhOLEDs performance of (Et-Cvz-PhQ)2Ir(picN-O) and (EO-Cvz-PhQ)2Ir(pic-N-O) using different host material, PhOLEDs were fabricated using CBP as a host instead of PVK. As shown in Table 1, device performances of PhOLEDs using CBP host are lower than those of PVK based PhOLEDs. As shown in Fig. 1, HOMO energy level of Ir(III) complexes lie above, but LUMO energy level of Ir(III) complexes do not lie blow those of CBP. Therefore, it is possible that Ir(III) complexes will not perfectly trap both electrons and holes in the emitting layer. In addition, CBP is a small molecule host so that it is difficult to make uniform thin film via solution processable EML. As a result, the overall device performances are lower than PVK based PhOLEDs. In summary, we fabricated highly efficient solution processed red PhOLEDs with a configuration of ITO/PEDOT:PSS/PVK:OXD7:TPD:Ir complex/cathode and characterized the performance of PhOLEDs. We investigated the effect of TPD concentration in PhOLEDs based on (Et-Cvz-PhQ)2Ir(pic-N-O) and (EO-Cvz-PhQ)2Ir(picN-O), respectively. The increasing TPD concentration up to

Table 1 Performances parameters of PhOLEDs based on different TPD concentrations. Dopant

HOST

TPD (%)

Turn-on (V)

Lmax (cd/m2)

EQEmax (%)

LEc (cd/A)

PEmax (lm/W)

LE (cd/A) at 100 cd/cm2

(Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (Et-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O) (EO-Cvz-PhQ)2Ir(pic-N-O)

PVK PVK PVK PVK PVK PVK PVK PVK PVK PVK CBP CBP CBP CBP CBP CBP CBP CBP CBP CBP

0 4 8 12 16 0 4 8 12 16 0 4 8 12 16 0 4 8 12 16

10 8.5 7.5 6.5 6 9.5 8.5 7.5 7 6 8 7.5 7.2 6.8 6.8 7.3 7 6.7 6.7 6.5

2439 3250 4321 6580 7860 2214 3082 4215 6056 7211 1732 2055 2307 2463 3726 503 1503 2305 2707 2824

5.46 6.54 7.16 7.40 9.21 4.80 5.78 6.02 6.76 7.40 3.41 5.92 7.76 8.21 9.10 5.17 6.01 6.81 6.94 7.64

7.95 10.50 10.86 12.46 15.24 6.65 7.25 9.08 9.81 11.72 5.24 8.97 11.85 13.32 14.80 7.46 9.09 10.1 10.8 12.0

1.57 2.00 2.56 3.55 4.53 1.04 1.65 2.56 3.09 3.55 2.1 1.8 2.5 4.2 4.9 2.3 2.9 2.8 3.4 3.3

7.89 9.74 4.99 12.14 7.72 3.28 3.12 2.78 4.36 7.92 5.2 6.6 10.1 9.8 14.2 3.9 9.0 10.1 10.5 11.2

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