Synthesis and characterization of white light-emitting polyfluorene-based copolymers containing new red iridium complex in the main chain

Synthetic Metals 175 (2013) 68–74

Contents lists available at SciVerse ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Synthesis and characterization of white light-emitting polyfluorene-based copolymers containing new red iridium complex in the main chain Woosum Cho a , N.S. Karthikeyan a , Siwon Kim b , Suhkmann Kim b , Yeong-Soon Gal c , Jae Wook Lee d,∗ , Sung-Ho Jin a,∗ a Department of Chemistry Education, Graduate Department of 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., Kyungil University, Hayang 712-701, Republic of Korea d Department of Chemistry, Dong-A University, Busan 604-714, Republic of Korea

a r t i c l e

i n f o

Article history: Received 20 February 2013 Received in revised form 24 April 2013 Accepted 29 April 2013 Available online 5 June 2013 Keywords: Polymer light-emitting diodes Phosphorescence Iridium complex White emission Polyfluorenes

a b s t r a c t We report the synthesis of single chain white light-emitting polyfluorene (PF)-based copolymers containing a novel red iridium complex ((CVz-PhQ)2 IrdbmBr), benzothiadiazole (BT), and 2,5-bisphenyl1,3,4-oxadiazole (OXD) segments. The resulting copolymers were highly soluble in common organic solvents and could be easily spin-coated onto an indium-tin oxide (ITO)-coated glass substrate to obtain high quality optical thin films. The weight-average molecular weight (Mw ) and polydispersity of the copolymers were (17.5–36.3) × 104 and 1.54–1.65, respectively. We fabricated white polymer light-emitting diodes (WPLEDs) in ITO/PEDOT:PSS/PVK/copolymer/CsF/Ca/Al, both with and without the poly(9-vinylcarbazole) (PVK) interlayer. By carefully controlling the concentrations of the low-energyemitting species in the resulting copolymers, white light-emission with contributions from each of the three primary colors was achieved. By incorporating 2 mol% of an electron transporting material, OXD, as a comonomer into the copolymer backbone and inserting a PVK interlayer, the external quantum efficiency (EQE) and luminance efficiency (LE) of the WPLEDs were enhanced. The turn-on voltages of the copolymers were in the range of 5.5–7.5 V, and the maximum EQE and LE were 1.79% and 1.89 cd/A with CIE coordinates of (0.32, 0.34), which are close to the standard for white light emission. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Since their initial discovery by Kido and coworkers in 1994, white organic light-emitting diodes (WOLEDs) and white polymer light-emitting diodes (WPLEDs) have attracted promising attention and interest both academically and industrially [1–4]. Particularly, WPLEDs have been the subject of intensive investigation in recent years because of their unique advantages such as backlighting for liquid crystal displays and next generation lighting sources [5–7]. Despite the high performance of small molecule WOLEDs, WPLEDs are superior for the fabrication of large area displays using inexpensive solution processing techniques [8–10]. Various methods have been adopted to realize white light-emission. The general approach to obtain WPLEDs is to blend fluorescent or phosphorescent dyes into a polymer matrix [11–13] or to use polymer blending systems, such as blending red, green, and blue light-emitting polymers or

∗ Corresponding authors. Tel.: +82 51 510 2727; fax: +82 51 581 2348. E-mail addresses: [email protected] (J.W. Lee), [email protected] (S.-H. Jin). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.04.029

blue and orange light-emitting polymers [14,15]. However, these blending systems may suffer intrinsic limitations of efficiency and stability because of possible phase separation and color shift with the applied voltage. To address these issues, WPLEDs using a single polymer with broad emission covering the whole visible region from 400 nm to 700 nm as emitting layer have recently been developed [16–20]. Liu et al. reported the first single polymer with simultaneous blue, green, and red emission for white emission with a luminance efficiency (LE) of 1.59 cd/A and Commission Internationale de L’Eclairage (CIE) coordinates of (0.34, 0.34) [21]. In the past few years, various efforts have been made to improve WPLEDs with single emitting polymer materials [22–24]. Phosphorescence imparted by heavy metal complexes has played a vital role in raising the efficiency of OLEDs [25–28]. Phosphorescent emitters of heavy metal complexes, which can harvest both singlet and triplet excitation emissions and elevate the internal quantum efficiency of the devices up to 100% theoretically, have been used to produce high performance WPLEDs [29–31]. Among the transition metal complexes, iridium (Ir) complex yields very high efficiency when doped into the polymer matrix. The high ligand field strength of the carbon atom bonding to the Ir atom

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

provides high emission energy, which potentially covers the entire visible light range from blue to red. Polyfluorenes (PFs) are very promising materials for lightemitting materials because of their high photoluminescence (PL) efficiency and high thermal stability. Consequently, PFs can be used as both the host and the blue emitter in WPLEDs [32,33]. The emission spectra of PF-based polymers have been tuned to cover the entire visible region by incorporating electron deficient comonomers [34]. Hence, we incorporated the benzothiadiazole (BT) unit into the PF backbone as a green emitter. On the other hand, we have previously reported the synthesis and characterization of a series of carbazole–phenylquinoline (Cz–PhQ) main ligand-based red Ir(III) complexes with high quantum yields and appropriate lifetimes. Among them, (EtCz–PhQ)2 Ir(acac) gives an external quantum efficiency (EQE) of 4.08% and an LE of 4.31 cd/A with CIE coordinates of (0.65, 0.34) [35]. These results encouraged us to further link (Et-CVz-PhQ)2 Ir(acac) to the PF main chain via chemical bonds, with the aim of designing white light-emitting phosphorescent polymers. The occurrence of charge trapping on Ir(III) complexes and of unmatched highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) to the electrodes among the polymers has often resulted in unbalanced holes and electrons in the emitting layer, with low resulting efficiencies. To solve these problems, carrier transporting moieties have been chemically incorporated into the main chain of PFs. Many research groups have introduced electron transport moieties into the main or side chains of ␲-conjugated polymers. Among the most widely used electron transport moieties are aromatic 1,3,4-oxadiazole (OXD) derivatives, which have high electron affinities that facilitate electron transport and injection. We have incorporated the OXD unit into the PF backbone to increase the electron injection and transportation into the emitting layer. The poly(3,4-ethylenedioxythiophene):poly(4stylenesurfonate) (PEDOT:PSS) layer has already been proven to be a successful and efficient hole injection layer [36]. However, it still does not provide a fully ohmic contact to a number of OLEDs. For enhancing the hole injection, several conjugated and non-conjugated polymeric materials have been tried as an interlayer [37]. In general, conjugated polymers are difficult to use in printing process due to their solubility issues in several organic solvents [38]. However, non-conjugated polymers such as poly(9-vinylcarbazole) (PVK) have found favor due to their hole transport chemical units that have been attributed as improving hole injection [39–41]. Therefore, the use of high molecular weight PVK interlayer is advantageous, as it forms a dense film that remains stable and well preserved when the emissive layer is spin-coated on top of it [38]. Herein, we report a new series of white emission PFs both with and without the electron transport molecule, 2,5-bisphenyl1,3,4-oxadiazole (OXD) unit, in the main chain. Their photophysical properties and resulting device performance are discussed in detail. By incorporating 2 mol% of an OXD segment into the PF backbone and inserting a PVK interlayer, we obtained a maximum EQE of 1.79%, and maximum LE of 1.89 cd/A with the CIE coordinates of (0.32, 0.34).

2. Experimental 2.1. Materials and characterization 9-Ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazole (1), 2,7-bis(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2,yl)-9,9dioctylfluorene (M2) and 2,5-bis(4-bromophenyl)-1,3,4oxadiazole (M4) were synthesized according to the literature

69

procedures [35,42,43]. 4,7-Dibromobenzo[c]-1,2,5-thiadiazole (M3) and PVK (Mw : 1,100,000) were obtained from Aldrich Chemicals. All the manipulations involving air sensitive reagents were performed under nitrogen atmosphere. The solvents were of analytical grade and were purified by routine procedures before being used. All column chromatography was performed using silica gel (250–430 mesh, Merck) as the stationary phase in a column. 1 H NMR spectra were recorded on a 600 MHz Agilent NMR spectrometer and CDCl3 was used as the solvent at 298 K. The chemical shifts were reported in ppm. Electronic absorption spectra were obtained on a JASCO V-570 UV spectrophotometer with baseline correction and normalization carried out using Origin 7.0 software. Emission spectra were recorded for dilute solutions at 298 K by a Hitachi F-4500 fluorescence spectrophotometer. The solid-state emission measurements were carried out by supporting each film on a quartz substrate that was mounted to receive front-face excitation at an angle of less than 45◦ . Each polymer film was excited with several portions of visible light from a xenon lamp. The molecular weight and polydispersity of the polymer were determined by gel permeation chromatography (GPC) using a PL gel 5 ␮m MIXED-C column on an Agilent 1100 series liquid chromatography system with THF as an eluent and then calibrated with polystyrene standards. Thermal analyses were carried out on a Mettler Toledo TGA/SDTA 851, DSC 822 analyzer under nitrogen atmosphere at a heating rate of 10 ◦ C/min. Cyclic voltammetry (CV) experiments were performed with a CHI-600C electrochemical analyser at a potential scan rate of 100–150 mV/s at room temperature with a conventional three-electrode configuration consisting of a platinum working electrode, an auxiliary electrode and a nonaqueous Ag/AgNO3 reference electrode. The solvent used in all CV experiments was anhydrous acetonitrile and the supporting electrolyte was nitrogen-saturated 0.1 M solution of tetrabutylammoniumtetrafluoroborate (Bu4 NBF4 ). Each polymer film was coated on a Pt disk electrode (0.2 cm2 ) by spinning the electrode from the polymer solution (10 mg/mL). 2.2. Synthesis of 1,3-bis(p-bromophenyl)propane-1,3-dione (dbmBr) (3) [44] A mixture of ethyl p-bromobenzoate (2.3 g, 10 mmol) and sodium hydride in oil dispersion (60%, 0.53 g, 22 mmol) in dry THF (30 mL) was heated to 60 ◦ C. p-Bromoacetophenone in dry THF (20 mL) was added dropwise to the reaction mixture and stirred for 24 h. The reaction mixture was poured into water and then neutralized with hydrochloric acid. The resulting precipitate was recrystallized from ethanol to give pale yellow needle like crystals (3.1 g, 81%). 1 H NMR (600 MHz, CDCl3 ): ı (ppm) 6.7 (s, 1H), 7.6 (d, 4H), 7.8 (d, 4H); 13 C NMR (600 MHz, CDCl3 ): ı (ppm) 194.23, 136.37, 131.52, 130.18, 128.34, 52.33. 2.3. Synthesis of (CVz-PhQ)2 Ir(dbmBr) (M5) The cyclometalated Ir(III) chloride bridged dimer (2) was synthesized by the method reported by Nonoyama [45]. 9-Ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazole (Et-Cz–PhQ) (1) (4 g, 10.36 mmol) and IrCl3 H2 O (1.6 g, 45.6 mmol) were added to a mixture of 2-ethoxyethanol and water (130 mL, 3:1, v/v). The reaction mixture was stirred at 140 ◦ C for 20 h and a brown precipitate was obtained after cooling to room temperature. The precipitate was collected and washed with deionized water and methanol. Subsequently, the cyclometalated Ir(III) chloride bridged dimer (2) was dried under vacuum to give a brown solid. The cyclometalated Ir(III) chloride bridged dimer (2) (2 g, 9 mmol) and 1,3-bis(pbromophenyl)propane-1,3-dione (dbmBr) (3) (0.93 g, 24 mmol) were mixed with sodium carbonate (1.036 g, 97 mmol) in 1,2dicholoroethane (100 mL). The mixture was refluxed at 100 ◦ C for

70

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

Scheme 1. Synthetic scheme of M-5 and copolymers (P1 and P2).

12 h under a nitrogen atmosphere. After cooling to room temperature, the crude solution was poured into water, extracted with ethyl acetate, dried over anhydrous MgSO4 and evaporated in a vacuum. The residue was purified by silica gel chromatography using hexane and methylene chloride (4:1, vol%) as an eluent and further purified by recrystallization twice from methylene chloride and methanol mixture to give a red solid complex (1.1 g, 83%). 1 H NMR(600 MHz, CDCl3 ): ı (ppm) 8.70 (s, 2H), 8.24 (s, 2H), 8.15 (d, 2H) 7.78–7.62 (m, 14H) 7.48–7.42 (m, 6H), 7.37–7.31 (m, 6H) 7.25–7.18 (m, 8H) 7.15 (t, 2H), 6.67 (1H), 5.96 (s, 2H), 3.91 (m, 4H), 0.89 (t, 6H); 13 C NMR (600 MHz, CDCl3 ): ı (ppm) 180.27, 158.06, 150.64, 149.47, 141.61, 137.9, 136.55, 133.10, 131.72, 130.37, 128.7, 128.06, 126.41, 125.18, 122.24, 120.16, 11.930, 118.92, 112.05, 111.12, 102.44, 91.28, 50.13, 48.29, 15.79. 2.4. Synthesis of PFIrPhQ-CzBT (P1) Copolymers were prepared by Suzuki coupling reaction. The synthetic route for the copolymers and the feed ratio of the monomers are shown in Scheme 1 and Table 1. The following is an example for the preparation of PFIrPhQ-CzBT (P1). To a round bottom flask (100 mL), 2,7-dibromo-9,9-dioctylfluorene (M1) (162.7 mg, 0.3 mmol), 2,7-bis(4,4,5,5,-tetramethyl(M2) (163.0 mg, 1,3,2-dioxaborolan-2,yl)-9,9-dioctylfluorene

0.297 mmol), 4,7-dibromobenzo[c]-1,2,5thiadiazole (M3) (0.5 mL, 7.2 × 10−4 M), and (CVz-PhQ)2 Ir(dbmBr) (M5) (0.5 mL, 4.8 × 10−3 M) were dissolved in freshly distilled toluene (2 mL) and purged with nitrogen for 30 min. Subsequently, Pd(PPh3 )4 was added to the flask and the reaction mixture was stirred at 90 ◦ C for 5 min, after which several drops of tetraethylammonium hydroxide was slowly injected. The reaction mixture was stirred at 90 ◦ C for 48 h and exhibited gradually increasing viscosity. The resulting mixture was cooled to room temperature and poured into methanol solution. The precipitated polymer was recovered by filtration and washed with methanol. The solid was further purified by stirring for 24 h using acetone to remove oligomers and catalyst residues. The resulting copolymer was dried under reduced pressure at room temperature to give a bright green polymer powder, PFIrPhQ-CzBT (P1) (112 mg, 47%). 1 H NMR (600 MHz, CDCl3 ) ı (ppm): 7.85 (d, 2H), 7.68 (m, 4H), 2.10 (m, 4H), 1.14 (s, 24H), 0.84 (t, 6H). 13 C NMR (600 MHz, CDCl3 ) ı (ppm): 152.0, 140.7, 140.2, 126.4, 121.7, 120.2, 55.6, 40.6, 32.0, 30.3, 29.5, 24.1, 22.8, 14.3. PFIrPhQ-CzBTOXD (P2) was synthesized with the various feed ratios of monomers especially including the 2,5-bis(4bromophenyl)-1,3,4-oxadiazole (M4) using a method similar to that of PFIrPhQ-CzBT (P1) to give a polymer powder (73 mg, 31%). 1 H NMR (600 MHz, CDCl ) ı (ppm): 7.85 (d, 2H), 7.68 (m, 4H), 2.10 3

Table 1 Polymerization results, thermal and electro-optical properties of P1 and P2. Polymer

Feed ratio of OXD (mol%)

P1 P2

0 2

a b c d e f

Mw a 36,300 17,515

PDIa

DSCb (Tg )

Abs (nm)

1.54 1.65

156 154

391 380

Mw and PDI of the copolymers were determined by GPC using polystyrene standards. Determined by DSC at a heating rate of 10 ◦ C/min under N2 atmosphere. Determined from UV–visible absorption spectra. HOMO = −4.8 − [Eox onset – (E1/2 (ferrocene) = 0.35)] eV. Estimated from HOMO levels and the optical band gaps. Measured in CH2 Cl2 solution using Curmarin 1 as reference.

PL (nm) 424, 442, 514 425, 444, 507

Eg c (eV)

HOMOd (eV)

LUMOe (eV)

˚PL f (%)

3.01 3.02

−5.43 −5.43

−2.42 −2.41

91 82

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

(m, 4H), 1.14 (s, 24H), 0.84 (t, 6H). 13 C NMR (600 MHz, CDCl3 ) ı (ppm): 152.0, 140.7, 140.2, 129.0, 127.4, 126.4, 121.7, 120.2, 55.6, 40.6, 32.0, 30.3, 29.5, 24.1, 22.8, 14.3.

71

100

P1 P2

90

Each glass substrate was coated with a transparent indium-tin oxide (ITO) electrode (110 nm thick, 10–20 / sheet resistance). Patterned ITO slides were cleaned with detergent, deionized water, acetone, and isopropyl alcohol. The ITO slides were dried on a hot plate followed by UV-ozone treatment for 10 min before use. A hole injection layer of PEDOT:PSS (H.C. Starck, Clevios P VP AI 4083) was spin-coated on the ITO, which was then dried on the hot plate for 30 min at 150 ◦ C. A 10-nm-thick high molecular weight PVK was subsequently spin-coated on the PEDOT:PSS layer, as an interlayer. For complete dissolution of PVK, 0.5 wt% optimized value of PVK was dissolved in chlorobenzene. After spin coating, it was then baked at 120 ◦ C for 20 min. A thin film of emitting layer was then spin-coated onto the PEDOT:PSS (or PVK)-coated substrate using trichloroethylene solution and then annealed at 80 ◦ C for 30 min. Finally, a thin layer of CsF (1 nm) was deposited by vapor evaporation under a vacuum of 5 × 10−6 Torr, followed by a layer of Ca (20 nm) and Al (100 nm), yielding an effective area of 9 mm2 . The film thickness was measured using ␣-Step IQ surface profiler (KLA Tencor, San Jose, CA). The electroluminescence (EL) spectra and current density–voltage–luminance (J–V–L) characteristics of the WPLEDs were measured using a programmable Keithley model 236 power source and spectra scan CS-1000 photometer, respectively. 3. Results and discussion The new cyclometalated Ir(III) complex (CVz-PhQ)2 Ir(dbmBr) (M5) was synthesized successfully. 9-Ethyl-3-(4-phenylquinolin2-yl)-9H-carbazole (1) was prepared by an acid-catalyzed Friedlander condensation reaction [46]. It was further used as the main ligand for red Ir(III) complexes (M5). Iridium trichloride hydrate was reacted with the main ligand (1) to give the ␮-chloride bridged dimeric complex (2). Final Ir(III) complex (M5) was achieved by simply reacting the ancillary ligand (3) with the bridged dimer (2) under reflux condition. For the preparation of conjugated copolymer systems containing phosphorescent Ir(III) complex, the Suzuki cross-coupling reaction has found widespread application. Generally, conjugated copolymers are expected to provide better charge transport to the emitter, but the performance can suffer from the usually rather low triplet energy level of the copolymer backbone, which increases the probability of back energy transfer from the emitter to the copolymers. PFs are promising materials for PLEDs because of their thermal and chemical stability and exceptionally high fluorescence quantum yields (0.6–0.8) [32,33]. In order to improve the electron transporting capability of copolymers, OXD units were incorporated into the fluorene-based building blocks. White light-emission can be obtained by an appropriate combination of the three primary colors (red, blue, and green). In the same principle, we developed white light-emitting copolymers through the incorporation of low-band gap green light-emitting BT segments and red light-emitting new Ir(III) complex derivative as comonomers into blue light-emitting PF main chain. The design and synthesis of the two kinds of copolymers (P1 and P2) are shown in Scheme 1. In order to adjust the color purity of white emission, copolymers were synthesized from the combination of various monomers (M1–M5) both without (P1) and with (P2) the OXD units by Suzuki cross-coupling reaction with two different feed ratios. On the basis of GPC, the weight average molecular weight (Mw ) of the P1 and P2 were determined as 36,300 and 17,515 with polydispersity index (PDI) of 1.54 and 1.65, respectively.

Wight(%)

2.5. Fabrication and measurement of WPLEDs 80 70 60 50 40

0

100

200

300

400

500

600

700

800

o

Temperature( C) Fig. 1. TGA curves of the P1 and P2.

To confirm the introduction of OXD segments in P2 compared to P1, we measured the 1 H- and 13 C NMR spectra using 600 MHz Agilent NMR spectrometer system. The 1 H NMR spectrum of P2 was almost identical to that of P1, due to the lower content of OXD segment. As the polymerization proceeded, the aromatic carbon peaks of 2,5-diphenyl-1,3,4-oxadiazole segments from P2 appeared at 129.0 and 127.4 ppm. The obtained copolymers were successively Soxhlet extracted with methanol, isopropyl alcohol, and acetone to remove the unreacted monomers, catalyst residue, and oligomers and the resulting copolymers were all readily soluble in common organic solvents, such as toluene, THF, chloroform, trichloroethylene, and chlorobenzene. The advantage of our approach is that the ratio of the colored chromophores was fixed during the synthesis of copolymers, which allowed tight control of the feed ratio of chromophores and thereby facilitated the good color reproducibility of the white emission polymers. The thermal properties of P1 and P2 were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Thermal analysis data for the copolymers are illustrated in Fig. 1 and these results are summarized in Table 1. The incorporation of Ir(III) complex into copolymers potentially gave the resulting copolymer better thermal stability. Inspection of the TGA results showed that P1 and P2 had 5% weight loss temperatures (T5% ) under a nitrogen atmosphere of 362 and 408 ◦ C, respectively. The glass transition temperatures (Tg ) of P1 and P2 were 156 and 154 ◦ C, respectively. Fig. 2(a) illustrates the UV–visible absorption spectra of the copolymers in dilute chloroform solutions and thin films on quartz plates. The two copolymers exhibited absorption in dilute solutions with peaks at 391 nm and 380 nm for P1 and P2, respectively, which were ascribed to the –∗ transition of the copolymer backbone. Blue shift was observed in the absorption spectrum of P2, due to the less coplanarity between the adjacent fluorene and OXD units in P2 than that in the two fluorene units in P1. The absorption contributions from the BT and Ir(III) complex moieties were not observable in the absorption spectra, due to their very low concentrations in the copolymers. Fig. 2(b) shows the PL spectra of the copolymers. The PL spectra of the copolymers in the film state were measured in order to elucidate the origins of their EL spectra. Unlike the absorption spectra resulting from only the PF backbone, the PL spectra exhibited three characteristic peaks at 424, 442, and 514 nm for P1 and 425, 444, and 507 nm for P2. The emission range of 424–444 nm bands was attributed to the fluorene segments and the range of 507–514 nm to the BT units, which revealed the action

72

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

P1 (sol) P2 (sol) P1 (film) P2 (film)

0.8

PEDOT:PSS/P1 PEDOT:PSS/P2

1.0 Normalized intensity [a.u.]

Nomalized Absorption

1.0

0.6 (a)

0.4 0.2

0.8 (a) 0.6

0.4

0.2

0.0 300

400

500

600

700

0.0

800

400

500

Wavelength(nm)

800

0.6 (b)

0.4 0.2

PEDOT:PSS/PVK/P1 PEDOT:PSSPVK/P2

1.0

Normalized intensity [a.u.]

Nomalized PL intensity

0.8

700

Wavelength [nm]

P1 (sol) P1 (film) P2 (sol) P2 (film)

1.0

600

0.8 (b) 0.6

0.4

0.2

0.0 400

500

600

700

Wavelength(nm)

0.0 400

500

600 700 Wavelength [nm]

800

Fig. 2. UV–visible absorption (a) and PL (b) spectra of the P1 and P2.

Fig. 3. EL spectra of the P1 and P2: (a) ITO/PEDOT:PSS/P1 or P2/CsF/Ca/Al; (b) ITO/PEDOT:PSS/PVK/P1 or P2/CsF/Ca/Al.

of the partial energy transfer from the fluorene segment to the BT unit. Even though the concentration of BT (M3) was much lower than that of Ir(III) complex (M5), the energy transfer was more efficient form fluorene to the BT unit than to M5, due to the shorter distance. Incorporation of 2 mol% of the OXD unit slightly changed the peak intensities due to the less coplanarity between the adjacent fluorene and OXD units in P2, but did not influence the peak positions. The PL quantum efficiencies (˚PL ) of P1 and P2 were measured as CH2 Cl2 solutions by absolute method using Curmarin 1 (˚PL = 99% in ethyl acetate) as a reference. Compared to Curmarin 1, P1 and P2 show quantum efficiencies between 82 and 91% (Table 1). Generally, the ˚PL decreased as the content of the Ir(III) complex units increased. It can be rationalized by the greater tendency of triplet–triplet annihilation at higher Ir(III) concentrations [47]. The electrochemical properties of the copolymers were investigated by CV and the results are presented in Table 1. No reduction wave for copolymers was observed in the range of 0 to −2.5 eV vs. Ag/Ag+ . On the basis of the onset oxidation potentials, we estimated the HOMO and LUMO energy levels of P1 and P2 with regard to the energy level of ferrocene (E1/2 = 0.35 eV). As determined by CV, the onset oxidation (Eox onset ) of the copolymers was 0.98 V. The HOMO and LUMO energy levels of P1 and P2 were −5.43/−2.42 eV and −5.43/−2.41 eV, respectively. Because of the similar absorption spectra, the optical band gaps (Eg opt ) of the copolymers were very close to −3.01 eV for P1 and −3.02 eV for P2. In addition, the introduction of a small amount of OXD moiety did not affect the

electrochemical properties of the copolymers, because of their low content in the copolymer backbones. Several factors need to be considered in the fabrication of highly efficient WPLEDs. For the investigation of copolymers as an emitting layer, two types of WPLEDs were fabricated using P1 and P2 with a configuration of ITO (110 nm)/PEDOT:PSS (40 nm)/PVK/copolymer (50 nm)/CsF (1 nm)/Ca (10 nm)/Al (100 nm), both with and without the PVK interlayer. Fig. 3 shows the EL spectra of the WPLEDs covering the visible range from 400 and 760 nm. The EL spectra of WPLEDs showed similar emission peaks, both with and without the PVK interlayer. The EL emission of red unit was triggered along with the blue and green emission, which was different from the corresponding PL spectra. This discrepancy was attributed to the charge trapping effects of the Ir(III) complexes. This mechanism is consistent with their HOMO/LUMO energy levels of (CVz-PhQ)2 Ir(dbmBr) (M5) and PFO, and further proved by the difference between the PL and EL spectra [35,48]. The copolymers that contained 0.4 mol% of Ir(III) complex, P1 and P2, exhibited strong emission bands in the range from 400 and 760 nm, indicating that P1 and P2 contained well-balanced partial intramolecular and/or intermolecular energy transfer pathways in the copolymer main chain. The peaks in the blue region arose from the PF segments, and the peaks in the green and red regions originated from the BT segments and Ir(III) complex. The CIE coordinates of the copolymers (0.32, 0.34) with an operating current density of 50 mA/cm2 were close to that of the pure white light (0.33, 0.33).

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

73

Table 2 WPLED performance of P1 and P2. Device

Turn-on (V)

Lmax (cd/m2 )

EQE (%)

CE (cd/A)

PE (lm/W)

CIE (x, y)

PEDOT:PSS/P1 PEDOT:PSS/P2 PVK/P1 PVK/P2 PEDOT:PSS/PVK/P2:OXD-7

5.5 5.8 7.0 7.5 6.7

443 653 362 498 63

0.28 0.41 1.68 1.79 0.15

0.32 0.48 1.76 1.89 0.30

0.14 0.21 0.32 0.46 0.08

(0.32, 0.38) (0.29, 0.33) (0.31, 0.37) (0.33, 0.34) (0.33, 0.38)

1.0 , ,

Luminance efficiency [cd/A]

(a)

PEDOT:PSS/P1 PEDOT:PSS/P2

0.8

Power efficiency [lm/W]

0.1 0.6

0.4

0.01

0.2

0.0 0

50

100

150

200

1E-3 250

2

Current density [mA/cm ]

5

, ,

(b)

PEDOT:PSS/PVK/P1 PEDOT:PSS/PVK/P2

4

3 0.1 2

1

0.01 60

0 0

20

Power efficiency [lm/W]

Luminance efficiency [cd/A]

The current density–voltage–luminance (J–V–L) and current density–efficiency (J–) characteristics of WPLEDs, both with and without the PVK interlayer, are shown in Figs. 4 and 5, respectively, and their device performances are summarized in Table 2. The high molecular weight PVK interlayer deposited by spin coating induced effective hole carrier injection and transportation into the emitting layer. The HOMO energy level of PVK (5.6 eV), as reported in the literature [41], is close to the HOMO energy levels of the copolymers (5.43 eV), which facilitated hole injection and transportation effectively. On the other hand, the PVK interlayer produced a high electrical resistance within the device, which increased the turn-on voltages of the WPLEDs from 5.5 V to 7.5 V. The maximum brightness, EQE and LE of the WPLEDs without the PVK interlayer were 653 cd/m2 , 0.41% and 0.48 cd/A, respectively. As demonstrated in Table 2, the overall device performance of WPLEDs with PVK interlayer was superior to that of the WPLED without PVK interlayer. Furthermore, PVK as an interlayer proved advantageous in improving the device performance due to its high triplet energy and good electron blocking characteristics because of the high LUMO energy

40 2

Current density [mA/cm ]

Fig. 5. Current density–efficiency characteristics of the P1 and P2: (a) ITO/PEDOT:PSS/P1 or P2/CsF/Ca/Al; (b) ITO/PEDOT:PSS/PVK/P1 or P2/CsF/Ca/Al.

level. WPLEDs with PVK interlayer showed good performance with a maximum EQE of 1.79%, and a maximum LE of 1.89 cd/A.

4. Conclusion

Fig. 4. Current density–voltage–luminance characteristics of the P1 and P2: (a) ITO/PEDOT:PSS/P1 or P2/CsF/Ca/Al; (b) ITO/PEDOT:PSS/PVK/P1 or P2/CsF/Ca/Al.

We have successfully synthesized PF-based copolymers consisting of BT and the new iridium (III) complex [(CVz-PhQ)2 Ir(dbmBr)]. The resulting copolymers emitted blue, green, and red light and were applied to the development of WPLEDs with excellent CIE coordinates of (0.32, 0.34). The WPLEDs were fabricated in ITO/PEDOT:PSS/PVK/copolymer/CsF/Ca/Al, both with and without the PVK interlayer. The performance of the WPLEDs with PVK interlayer was superior to that of without PVK interlayer. The turn-on voltages were in the range of 5.5–7.5 V and the maximum EQE and LE were 1.79% and 1.89 cd/A at 10 mA/cm2 , respectively. These study results demonstrated the suitability of these copolymers for display applications. Nevertheless, we are planning further improvements to these copolymers in future work.

74

W. Cho et al. / Synthetic Metals 175 (2013) 68–74

Acknowledgments This work was supported by a Grant funds from the National Research Foundation of Korea (NRF) of the Ministry of Education, Science and Technology (MEST) of Korea (no. 2011-0028320). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet.2013. 04.029. References [1] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Applied Physics Letters 64 (1994) 815–817. [2] M.C. Gather, H. Kc¸hnen, K. Meerholz, Advanced Materials 23 (2011) 233–248. [3] Q. Wang, J.Q. Ding, D.G. Ma, Y.X. Cheng, L.X. Wang, X.B. Jing, F.S. Wang, Advanced Functional Materials 19 (2009) 84–95. [4] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (2009) 234–238. [5] M. Strukelj, R.H. Jordan, A. Dodabalapur, Journal of the American Chemical Society 118 (1996) 1213–1214. [6] Z. Xie, J.S. Huang, C.N. Li, Y. Wang, Y.Q. Li, J.C. Shen, Applied Physics Letters 74 (1999) 641–643. [7] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Advanced Materials 14 (2002) 147–151. [8] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angewandte Chemie International Edition 37 (1998) 402–428. [9] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121–128. [10] M.T. Bernius, M. Inbasekaran, J. O’Brien, W. Wu, Advanced Materials 12 (2000) 1737–1750. [11] J. Liu, G. Tu, Q. Zhou, Y. Cheng, Y. Geng, L. Wang, D. Ma, X. Jing, F.J. Wang, Journal of Materials Chemistry 16 (2006) 1431–1438. [12] K. Tada, M. Onodo, Japanese Journal of Applied Physics 44 (2005) 4167–4170. [13] L. Wang, G. Lei, Y. Qiu, Applied Physics Letters 97 (2005) 114501–114503. [14] H.J. Su, F.I. Wu, C.F. Shu, Macromolecules 37 (2004) 7197–7202. [15] J. Huang, G. Li, E. Wu, Q. Xu, Y. Yang, Advanced Materials 18 (2006) 114–117. [16] J.Y. Deng, Y. Liu, Y.F. Wang, H. Tan, Z.Y. Zhang, G.T. Lei, J.T. Yu, M.X. Zhu, W.G. Zhu, Y. Cao, European Polymer Journal 47 (2011) 1836–1841. [17] Y. Wang, Y. Liu, Z. Zhang, J. Luo, D. Shi, H. Tan, G. Lei, M. Zhu, W. Zhu, Y. Cao, Dyes and Pigments 91 (2011) 495–500. [18] Y. Xu, R. Guan, J. Jiang, W. Yang, H. Zhen, J. Peng, Y. Cao, Journal of Polymer Science Part A: Polymer Chemistry 46 (2008) 453–463. [19] H.M. Shih, R.C. Wu, P.I. Shih, C.L. Wang, C.S. Hsu, Journal of Polymer Science Part A: Polymer Chemistry 50 (2012) 696–710. [20] C.N. Lo, C.S. Hsu, Journal of Polymer Science Part A: Polymer Chemistry 49 (2011) 3355–3365.

[21] J. Liu, Q. Zhou, Y. Cheng, Y.H. Geng, L. Wang, D. Ma, X. Jing, F.S. Wang, Advanced Materials 17 (2005) 2974–2978. [22] J. Liu, Q.G. Zhou, Y.X. Cheng, Y.H. Geng, L.X. Wang, D.G. Ma, X.B. Jing, F.S. Wang, Advanced Functional Materials 16 (2006) 975–979. [23] M.J. Park, J. Lee, J. Kwak, I.H. Jung, J.H. Park, H. Kong, C. Lee, D.H. Hwang, H.K. Shim, Macromolecules 42 (2009) 5551–5557. [24] J.X. Jiang, Y.H. Xu, W. Yang, R. Guan, Z.Q. Liu, H.Y. Zhen, Y. Cao, Advanced Materials 18 (2006) 1769–1773. [25] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [26] Z. Minrong, L. Yanhu, Z. Cheng, Y. Chuluo, W. Hongbin, Q. Jingui, C.J. Yong, Journal of Materials Chemistry 22 (2012) 11128–11133. [27] S.L. Lai, S.L. Tao, M.Y. Chan, M.F. Lo, T.W. Ng, S.T. Lee, W.M. Zhao, C.S. Lee, Journal of Materials Chemistry 21 (2011) 4983–4988. [28] D. Shi, Y. Wang, Y. Liu, Z. Zhang, J. Luo, J. He, Q. Chen, G. Lei, W. Zhu, Chemistry: An Asian Journal 7 (2012) 2096–2101. [29] J. Zou, H. Wu, C.S. Lam, C. Wang, J. Zhu, C. Zhong, S. Hu, C.L. Ho, G.J. Zhou, H. Wu, W.C. Choy, J. Peng, Y. Cao, W.Y. Wong, Advanced Materials 23 (2011) 2976–2980. [30] M. Zhu, J. Zou, S. Hu, C. Li, C. Yang, H. Wu, J. Qin, Y. Cao, Journal of Materials Chemistry 22 (2012) 361–366. [31] H. Wu, G. Zhou, J. Zou, C.L. Ho, W.Y. Wong, W. Yang, J. Peng, Y. Cao, Advanced Materials 21 (2009) 4181–4184. [32] G. Klaerner, R.D. Miller, Macromolecules 31 (1998) 2007–2009. [33] M.T. Bernius, M. Inbasekaran, J.O. Brien, W. Wu, Advanced Materials 13 (2000) 1737–1750. [34] P. Herguth, X. Jiang, M.S. Liu, A.K.Y. Jen, Macromolecules 35 (2002) 6094–6100. [35] S.J. Lee, J.S. Park, M. Song, I.A. Shin, Y.I. Kim, J.W. Lee, J.W. Kang, Y.S. Gal, S. Kang, J.Y. Lee, S.H. Jung, H.S. Kim, M.Y. Chae, S.H. Jin, Advanced Functional Materials 19 (2009) 2205–2212. [36] Y. Yang, A.J. Heeger, Applied Physics Letters 64 (1994) 1245–1247. [37] M.J. Harding, D. Poplavskyy, V.E. Choong, A.J. Campbell, F. So, Organic Electronics 9 (2008) 183–190. [38] J.S. Park, T.J. Park, W.S. Jeon, R. Pode, J. Jang, J.H. Kwon, E.S. Yu, M.Y. Chae, Organic Electronics 10 (2009) 189–193. [39] H. Yan, P. Lee, N.R. Armstrong, A. Graham, G.A. Evonenenkes, P. Dutta, T.J. Marks, Journal of the American Chemical Society 127 (2005) 3172–3183. [40] M.C. Gather, A. Kohnen, A. Falcou, H. Hecker, K. Meerholz, Advanced Functional Materials 17 (2007) 191–200. [41] T.W. Lee, M.G. Kim, S.Y. Kim, S.H. Park, O. Kwon, T. Noh, T.S. Oh, Applied Physics Letters 89 (2006), 123505-1-3. [42] D.C. Shin, J.H. Ahn, Y.H. Kim, S.K. Kwon, Journal of Polymer Science Part A: Polymer Chemistry 38 (2000) 3086–3091. [43] X.W. Zhan, Y.Q. Liu, X. Wu, S.A. Wang, D.B. Zhu, Macromolecules 35 (2002) 2529–2537. [44] K. Zhang, Z. Chen, Y. Zou, C. Yang, J. Qin, Y. Cao, Organometallics 26 (2007) 3699–3707. [45] M. Nonoyama, Bulletin of the Chemical Society of Japan 47 (1974) 767–768. [46] A.K. Agrawal, S.A. Jenekhe, Chemistry of Materials 4 (1992) 95–104. [47] W.S. Huang, Y.H. Wu, H.C. Lin, J.T. Lin, Polymer Chemistry 1 (2010) 494–505. [48] S.A. Choulis, V.E. Choong, A. Patwardhan, M.K. Mathai, F. So, Advanced Functional Materials 16 (2006) 1075–1080.