Pure red electroluminescence from novel heteroleptic cyclometalated platinum(II) emitters embedded in polyvinylcarbazole

Synthetic Metals 160 (2010) 615–620

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Pure red electroluminescence from novel heteroleptic cyclometalated platinum(II) emitters embedded in polyvinylcarbazole Hidetaka Tsujimoto a , Yoshiaki Sakurai b,∗ , Shigeyuki Yagi a , Yuichiro Honda a , Hotaka Asuka a , Hiroto Terao a , Takeshi Maeda a , Hiroyuki Nakazumi a,∗∗ a b

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Department of Environment and Chemistry, Technology Research Institute of Osaka Prefecture, Ayumino 2-7-1, Izumi, Osaka 594-1157, Japan

a r t i c l e

i n f o

Article history: Received 26 October 2009 Received in revised form 17 December 2009 Accepted 19 December 2009 Available online 22 January 2010 Keywords: Polymer light-emitting diodes Heteroleptic cyclometalated platinum(II) complex Photoluminescence Pure red electrophosphorescence Polyvinylcarbazole

a b s t r a c t We fabricated molecularly doped, polymer-based light-emitting diodes possessing a single emitting layer containing a hole-transporting host polymer poly(N-vinylcarbazole) and an electron-transporting auxiliary, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, doped with novel phosphorescent cyclometalated Pt(II) complexes bearing arylpyridine and 1,3-diketone ligands. These novel cyclometalated Pt(II) complexes emit pure red color both in steady-state emissions (poly(methyl methacrylate) films)) and electrophosphorescence. They exhibited pure red emissions with the Commission Internationale de l’Eclairage coordinates (X = ∼0.67, Y = ∼0.33), which is almost identical to the coordinates of standard red (0.66, 0.34) demanded by the National Television System Committee. The color coordinates remained unchanged over a range of operating voltages, even at luminances greater than 1 × 104 cd/m2 . The maximum external quantum efficiency of these devices exceeded 3.6% and the maximum brightness was greater than 1 × 104 cd/m2 . © 2009 Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) fabricated by a dry method have attracted much attention during past decades for their applications in flat panel displays and solid-state lighting. With the incorporation of phosphorescent heavy metal complexes, both singlet and triplet excitons could be harvested for light emission and therefore, in principal, 100% internal quantum efficiency could be achieved in the phosphorescent OLEDs [1–3]. Very high efficiency has been achieved from small molecular-based OLEDs, via multilayer structures, by using iridium complexes [4–7]. Prompted by success in this area, research activities were then directed towards polymeric electrophosphorescent light-emitting diodes (PLEDs) [8–10]. The PLEDs are attractive because of their low cost, and potential roll-to-roll manufacturing of large areas, based on various printing techniques. For PLEDs, poly(N-vinylcarbazole) (PVK) is widely used as a host polymer because of its excellent film-forming properties, high glass transition temperature, high triplet energy level, and good hole mobility [11–23]. In the field of phosphorescent materials, OLEDs and PLEDs prepared using phosphorescent mate-

∗ Corresponding author. Tel.: +81 725 51 2674; fax: +81 725 51 2699. ∗∗ Corresponding author. Tel.: +81 72 254 9320; fax: +81 72 254 9320. E-mail addresses: [email protected] (Y. Sakurai), [email protected] (H. Nakazumi). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.12.017

rials other than iridium complexes have received attention. There have been an increasing number of interesting reports on OLEDs and PLEDs prepared using platinum complexes such as porphyrin derivatives and cyclometalated derivatives [16,24–32]. However, there are only a few phosphorescent cyclometalated platinum complexes applicable to PLEDs that have good solubility and exhibit pure red-light emission [31,32]. Therefore, we systematically prepared various novel phosphorescent cyclometalated Pt(II) complexes, applicable to PLEDs, bearing arylpyridine and 1,3-diketone ligands, with the aim of achieving red-light emission [33]. In particular, the 1,3-diketone ligands based on 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione were chosen as they were expected to play an important role in enhancing the solubility in the organic and polymer media. Unlike conventional phosphorescent materials, as a result, these novel Pt(II) complexes exhibit high solubility in organic solvents. These Pt(II) complexes also have strong spin–orbit coupling of the 5d orbit, resulting in efficient intersystem crossing from the singlet excited state to the triplet excited state. The strong ligand field of the arylpyridine ligand, together with the added stabilization of donation from the platinum to the aromatic ligand, contributes to making these types of complexes very stable, and enables them to exhibit significant optoelectronics properties in PLEDs [34–36]. In this paper, we report on the preparation of efficient PVK-based single-layer PLEDs incorporating a novel phosphorescent cyclometalated Pt(II) complex (an emitting center) and

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2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, an electron-transport material). We also report that the novel cyclometalated Pt(II) complexes emit pure red color both in steadystate emissions (poly(methyl methacrylate), PMMA, films) and electrophosphorescence. This PLED exhibited a pure red emission having the Commission Internationale de l’Eclairage coordinates (X = ∼0.67, Y = ∼0.33), which are almost identical to those of standard red (0.66, 0.34) demanded by the National Television System Committee (NTSC). The color coordinates remained unchanged over a range of operating voltages, even when the luminance was greater than 1 × 104 cd/m2 . The maximum external quantum efficiency of these devices exceeded 3.6% and a maximum brightness greater than 1 × 104 cd/m2 . 2. Experimental

Chemical Industries. PVK was purified by reprecipitation from tetrahydrofuran and methanol. All other materials were used without further purification. The prepatterned indium–tin oxide (ITO) glass substrate with a sheet resistance of ∼10 /sq was purchased from Sanyo Vacuum Industries. 2.2. Films used for optical measurements The optical properties of Pt(II) complexes in solid state were studied in PMMA films, spin-coated from toluene onto quartz plates, in appropriate thickness (to obey the Lambert–Beer law). PMMA was chosen as a matrix for optical measurements of Pt(II) complexes, because PMMA is optically and electronically inert, and has good film-forming properties and a high glass transition temperature of 105 ◦ C.

2.1. Materials 2.3. PLEDs The molecular structures used here are shown in Fig. 1. Details on the synthesis and characterization of the phosphorescent cyclometalated Pt(II) complexes (Pt1–Pt3) are described in Ref. [33]. Poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT-PSS, Clevious P VP AI 4083), PVK (Mw = 25,000–50,000), PBD, and Al were purchased from H.C. Starck Corporation, Sigma–Aldrich Co., Tokyo Chemical Industry Co. Ltd., and Nilaco Corporation, respectively. LiF and CsF, to be used as electron injecting materials (EIMs), were purchased from Wako Pure

The PLEDs were prepared according to a standard procedure [11,12]. The prepatterned ITO glass substrate was routinely cleaned by ultrasonic treatment in detergent solution, distilled water, methanol, acetone, chloroform, hexane, and boiling isopropanol. The ITO substrate was then air-dried. A hole injection layer of PEDOT-PSS (40 nm) was first spin-coated on top of the ITO glass after UV–O3 treatment, and then dried at 120 ◦ C for 1 h under an argon atmosphere. A mixture of the Pt(II) complex with PVK:PBD (15%) was filtered through a 0.2-␮m Millex-FG filter (Millipore)

Fig. 1. Chemical structures of (a) PVK, (b) PBD, (c) Pt 1, (d) Pt 2, (e) Pt 3 and general structure for the single-layered PLED devices (f).

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and then spin-coated onto a PEDOT-PSS layer under an argon atmosphere and adapted rotate speed. A profilometer (KLA Tencor P-15) was used to measure the thickness of the films. Thereafter, 0.3 nm LiF and 1.0 nm CsF, as EIM, and aluminum (250 nm), as a cathode, were deposited onto the organic layers by a vacuum deposition method, through a shadow mask, at a base pressure of ∼10−4 Pa. The thicknesses of the EIMs and the Al layer were monitored after deposition using a crystal thickness deposition monitor (XTM-2, Inficon). Finally, the device was covered with a glass cap and encapsulated with a UV-curing epoxy resin in an argon atmosphere to prevent oxidation of the cathode and the organic layer. The area of the emitting part was 10 (2 × 5) mm2 . Fabrication of the device was carried out in a controlled atmosphere glove box (Seinan Industries) under an argon atmosphere. The final configuration of the PLED was ITO/PEDOT-PSS (40 nm)/PVK:PBD:Pt(II) complex (100 nm)/EIM (0.3 nm for LiF and 1.0 nm for CsF)/Al (250 nm) (see Fig. 1). The PLEDs were operated and, using an organic electroluminescence (EL) device evaluating system (Hamamatsu Photonics C-9920-11), the current density, luminescence, emitting spectrum, and luminous efficiency were measured at room temperature. 3. Results and discussion 3.1. Optical properties of Pt complexes in PMMA films Table 1 summarizes the absorption and emission characteristics of Pt(II) complexes in PMMA films. The table shows that the red shift of max from 520 nm of Pt1 to 610 nm of Pt3 was produced when the electron-rich group (benzothiophene group) was attached to the electron-deficient part (pyridine ring). The absorption and emission spectra of Pt(II) complexes in PMMA films are shown in Fig. 2. In addition to intense ␲–␲ absorption bands at approximately 300 nm, the Pt(II) complexes display low energy absorption bands at 380–420 nm, attributed to metal-toligand charge transfer (MLCT) transitions. When excited at their absorption maxima, Pt(II) complexes exhibit efficient room temperature phosphorescence in doped PMMA thin films. Although excitation of either ␲–␲* or MLCT absorption bands can be used to promote phosphorescence in these complexes, corresponding excitation spectra show maxima that are centered close to the MLCT bands. An increase in the photoluminescence (PL) quantum

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yields in PMMA:Pt(II) complexes indicates the likelihood of higher emission efficiency in PMMA because of the stabilization of the phosphorescent molecules in a rigid environment and a reduction of intermolecular interactions that lead to nonradiative decay processes. 3.2. Single-layer PLEDs Single-layer PLEDs are one of the architecturally simplest lightemitting structures. A PLED comprises a single-layer polymer PVK:PBD:Pt(II) complex, sandwiched between an ITO/PEDOT-PSS anode and an EIM/Al cathode, to give a general device structure of ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/EIM/Al. Table 2 summarizes the characteristics of ITO/PEDOTPSS/PVK:PBD:Pt(II) complex/EIM/Al devices. Figs. 3 and 4 show the current density–voltage and light output–voltage characteristics of ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/EIM/Al PLEDs with Pt1, Pt2, and Pt3 as the Pt(II) complex, and with LiF and CsF as EIMs. To enhance the electron injection from the Al cathode, EIM was used between the polymer layer and the Al electrode. The film (PVK:PBD:Pt(II) complex layer) thickness of all the devices was 1000 Å. Table 2 also shows that in all Pt(II) complexes the external quantum efficiency, ext (%), and power efficiency, p (lm/W), recorded when using the CsF layer were higher than when using the LiF layer because of the improved electron injection. We consider that in our devices the CsF generates interfacial dipoles more easily than LiF, resulting in an improved alignment of the Fermi levels of the cathode and the lowest unoccupied molecular orbital energy levels of the electron-transport layers. As the number of Cs+ ions generated from CsF is more than the number of Li+ ions from LiF (because CsF is more dissociable than LiF), the Cs+ might migrate more easily into the polymer layer than the Li+ [37]. As a result, when CsF is used, the barrier against electron injection is reduced more than when LiF is used. Under the chosen deposition conditions, the devices show high turn-on voltages (Vturn-on , see Table 2) of about 8–10 V for all Pt(II) complexes. We define the turn-on voltage as the voltage at which the light level measured on the optical power meter rises above 1 cd/m2 (for these dopants). The turn-on voltage decreased when the CsF layer was inserted, and the maximum luminance was higher, compared with when inserting the LiF layer. Despite having a high turn-on voltage,

Table 1 Absorption and emission characteristics of Pt(II) complexes in PMMA film. Pt(II) complex

Absorption, max /nm

Emission, max /nm (ex /nm)

Emission quantum yield, PL

Pt1 Pt2 Pt3

297, 366 368 354

516, 549 (379) 559, 605 (370) 613, 665 (381)

0.13 0.28 0.08

Table 2 Characteristics of ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/EIM/Al PLEDs. Pt(II) complex

EIM

Vturn-on (V)

L (cd/m2 )

ext (%)

L (cd/A)

Pt1

LiF

8.5

7736 (18)

2.5 (12)a 2.3b

7.9 (12)a

Pt2

Pt3

b

CsF

5

17,500 (15.5)

3.6 (11)

LiF

8

2315 (18)

0.87 (12)a 0.81b a

3.5

b

2.6

7864 (14.5)

2.8 (9)

LiF

9.5

319 (17.5)

0.3 (11.5)a 0.28b

5.5

1845 (16)

1.8 (9.5)

a

b

1.7

b

EL max (nm)

CIE

2.1 (12)a

1.8b

520, 557

(0.39, 0.58)

a

b

11.4 (11) 11.2

3.6 (10)

3.3

520, 556

(0.39, 0.58)

2.3 (12)a

0.6 (12)a

0.53b

560, 607

(0.53, 0.47)

561, 607

(0.54, 0.46)

2.1b b

7.0 (8)

5

a

7.5b

a

CsF

CsF a

a

p (lm/W)

a

6.4

b

2.5 (8)

2.1

0.28 (11.5)a 0.26b

0.076 (11.5)a 0.067b

611, 669

(0.66, 0.33)

a

a

615, 670

(0.67, 0.33)

1.5 (9.5)

b

1.5

0.53 (8.5)

b

0.41

Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. b At ∼20 mA cm−2 . Device structures are ITO(150 nm)/PEDOT:PSS(40 nm)/PVK:PBD:Pt(II) complex (140 nm)/LiF(0.3 nm)/Al(250 nm) and ITO(150 nm)/PEDOT:PSS(40 nm)/ PVK:PBD:Pt(II) complex (100 nm)/CsF(1.0 nm)/Al(250 nm).

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Fig. 2. Absorption and emission spectra of Pt(II) complex in PMMA film.

the ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/CsF/Al devices reach quantum efficiency comparable with those of PVK:PBD devices with other Pt(II) complexes [16]. Current density values corresponding to peak quantum efficiency are in the range typical for molecular OLEDs (5–20 mA/cm2 ). The ext (%) of the ITO/PEDOTPSS/PVK:PBD:Pt(II) complex/CsF/Al devices was 3.6% for Pt1, 2.8% for Pt2, and 1.8% for Pt3 (Table 2). This depression of the exter-

nal quantum efficiency is based on the depression of the emission quantum yield of the Pt(II) complex in PMMA. High driving voltages contribute to the low power efficiency, p (lm/W), of these devices (less than 10 lm/W), as has been reported for other PLEDs [16]. Brightness of the devices is in the peak efficiency range from 100 to 1000 cd/m2 , whereas maximum brightness of more than 10,000 cd/m2 is typically achieved at higher voltages.

Fig. 3. Luminance and current density characteristics as functions of applied voltages of ITO/PEDOT-PSS/PVK:PBD:Pt1/EIM/Al PLEDs with CsF and LiF as EIM.

Fig. 4. Luminance and current density characteristics as functions of applied voltages of ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/CsF/Al PLEDs with Pt2 and Pt3 as Pt(II) complex.

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Fig. 5. Electroluminescence spectra of ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/CsF/Al PLEDs with Pt1, Pt2, and Pt3 as Pt(II) complex.

The ITO/PEDOT-PSS/PVK:PBD:Pt(II) complex/EIM/Al devices exhibit EL with peaks, EL max (nm), corresponding to the dopant emission maxima at 520 and 556 for Pt1, 561 and 607 for Pt2, and 615 and 670 nm for Pt3 (Fig. 5). The wavelength of emission from PLED when using CsF or LiF as an EIM was almost the same. In the case of Pt3, the EL emission maximum is red-shifted to a greater extent, because of the ␲-extension of the benzothiophene unit. External quantum efficiency of Pt3 devices as a function of current density is shown in Fig. 6. A decrease in the efficiency was efficiently suppressed by increasing the current density in the device. Furthermore, decay of the efficiency of the devices as a function of current density appears to be independent of the dopant concentration in the PVK–PBD–Pt3 devices. At the doping concentrations used here (5–20%), we expect that the electrons and holes will be trapped at the Pt(II) complex. The concentrations in this regime are low enough that carrier hopping between Pt(II) complexes will most likely not be facile. At higher concentrations, interdopant hopping may be possible, and may be an important electron–hole conduction pathway in these dopants in PVK films. Considering that charges are likely to be trapped at Pt(II) complexes,

619

carrier recombination at a Pt(II) complex may be a significant pathway to exciton formation at a Pt(II) complex. In this recombination process, a phosphor-bound carrier (hole or electron) would recombine with an opposite carrier on an adjacent site (PVK segment or PBD) to give the phosphor-bound exciton. Similar charge-trapping behavior, proposed for some phosphorescence-based PLEDs, can proceed via either an energy transfer from PVK to a Pt(II) complex or via hole and electron trapping by a Pt(II) complex, followed by Pt(II) complex/PVK carrier recombination. The Commission Internationale de l’Eclairage (CIE) coordinates of Pt2 and Pt3 are (X = 0.54, Y = 0.46) and (X = 0.67, Y = 0.33), respectively. The CIE coordinates of Pt3 are almost identical to those of standard red (0.66, 0.34) demanded by the NTSC [31,32,38]. We are currently investigating the lifetime and efficiency of the devices. For electroluminescence, electrons and holes from PBD and PVK, respectively, are sequentially trapped by a Pt(II) complex, resulting in exciton formation on the Pt(II) complex and then photon emission due to exciton decay. Further studies of the mechanism of exciton formation in these phosphorescent PLEDs by evaluation of the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the Pt(II) complexes PBD and PVK are necessary to improve the lifetime and efficiency of these devices. In summary, a new type of Pt(II) complex was demonstrated to be promising as pure red phosphorescent emitters in PLED. The new triplet emitters reported here can be further improved by structural modifications of the ligand substituents to fine-tune both the device color and its efficiency. 4. Conclusion We demonstrated for the first time that novel cyclometalated Pt(II) complexes bearing arylpyridine and 1,3-diketone ligands emit pure red color in electrophosphoresence. The PL and EL maxima of the Pt(II) complexes were red-shifted when changing the aromatic groups from phenyl to thiophene. The Pt3 devices exhibited a maximum external quantum efficiency of 1.8% at 9.5 V, with a brightness of 1845 cd/m2 . Significantly, both the Pt2 and Pt3 devices exhibited pure red emission; their chromaticity values were (X = 0.54, Y = 0.46) and (X = 0.67, Y = 0.33), respectively, which are almost identical to those of standard red (0.66, 0.34) demanded by the NTSC. Moreover, our findings can be considered valuable for solidstate emitting applications because of their simple device architecture and low cost of fabrication. References

Fig. 6. External quantum efficiency–current density characteristics of ITO/PEDOTPSS/PVK:PBD:Pt(II) complex/CsF/Al PLEDs at different concentrations of Pt3.

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