High-performance poly(2,3-diphenyl-1,4-phenylene vinylene)-based polymer
light-emitting diodes by blade coating method
Yung-Ming Liao
a, Hung-Min Shih
a, Kuang-Hui Hsu
a, Chain-Shu Hsu
a,*, Yu-Chiang Chao
b,
Sheng-Chia Lin
b, Chun-Yao Chen
b, Hsin-Fei Meng
baDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC bInstitute of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC
a r t i c l e i n f o
Article history: Received 30 March 2011 Received in revised form 15 June 2011
Accepted 18 June 2011 Available online 25 June 2011
Keywords: Electroluminescence Gilch polymerization Polymer light-emitting diodes
a b s t r a c t
A new series of super high brightness and luminance efficient poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV)-based electroluminescent (EL) polymers containing methoxy or long branched alkoxy chains were synthesized via Gilch polymerization. The branched alkoxy groups were introduced to enhance solubility for blade and spin-coating processes. Monomers of DMeO-PPV and m-Ph-PPV were used to increase steric hindrance and prevent close packing of the main chain. By controlling the feeding ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weight were obtained. All synthesized polymers possess high glass transition temperatures and thermal stabilities. The maximum photoluminescent emissions of the thinfilms are located between 544 and 547 nm. Cyclic voltammetry analysis reveals that the band gaps of these light-emitting materials are in the range of 2.75 e2.84 eV. Blade coating was used to fabricate multilayer polymer light-emitting diodes. A multilayer electroluminescent device with the configuration of ITO/PEDOT:PSS/TFB/P1/TPBi/LiF/Al exhibited a very high luminescence efficiency (10.96 cd A1). The maximum brightness of the multilayer EL device ITO/ PEDOT:PSS/TFB/P3/CsF/Al reached up to 78,050 cd m2with a low turn-on voltage (4.0 V). For further investigation, polymer P3 was blended with DPPFBNA to achieve white light-emitting device; the multilayer devices generated a maximum brightness of 1085 cd m2and a luminance efficiency of 0.75 cd A1, with CIE coordinates (0.28, 0.33) at 11 V.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Semiconducting polymers have been intensively investigated for their potential applications in light-emitting diodes[1,2], thin film transitors[3], and solar cells[4]. Polymer light-emitting diode (PLED) has the potential to be more competitive than OLED in many future applications due to its low cost solution process[5,6,7]. The most common fabrication process for PLED is spin coating. However, only 5% of material remains on substrate after spinning and the manufacturing throughput of spin coating is low for large areas, dramatically raising the cost of PLED. More importantly, it has been proved that it is difficult to make polymeric multilayer by spin coating because the solvent of the second layer tends to dissolve the previous one. Blade coating is a common method to form large-area polymerfilms with micrometer thickness, such as photoresists and colorfilters[8]. Recently we verified the feasibility
of blade coating for high-efficiency polymer light-emitting diodes [9]. Unlike spin coating, the area can be easily scaled up and almost all coating materials can remain on the substrate. Furthermore, it is possible to deposit not only single layer but also multilayer without a buffer liquid. The performance of a single layer PLED is as good as that of the spin coated one. The bilayer PLED is even better than the one created by the liquid buffer method. Hence, blade coating was applied to fabricate PLED devices and enhanced electroluminescent (EL) properties were obtained.
Poly(1,4-phenylene vinylene) (PPV) and its derivatives are some of the most attractive classes of conjugated polymers due to their unique structure and highly electroluminescent properties [10]. Long alkyl chains and/or bulky substituents have been incorporated onto the PPV main chain to improve its solubility in order to cast thin films by solution process. Electron donating/withdrawing groups have also been introduced to adjust the optical and electrical properties. For example, poly[2-methoxy-5-(20 -ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV) is an orange-red emissive polymer and soluble in common organic solvents[11]; thinfilms of MEH-PPV can be obtained from a spin-coating process. Cyano-substituted
* Corresponding author.
E-mail address:[email protected](C.-S. Hsu).
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Polymer
j o u rn a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / p o l y m e r
0032-3861/$e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.06.025
poly(2,5-dialkoxy-1,4-phenylene vinylene) (CN-PPV) is a red emis-sive polymer with high electron affinities[12,13]. Silyl-substituted PPV is a greenish emissive material with a tendency to be easily charged by electrons rather than holes[14,15]. Synthesis of phenyl/ alkoxy-substituted PPV copolymers wasfirst reported by Spreitzer, Becker et al.[16,17]The polymerization was performed via Gilch route using different co-monomer feeding ratios. Introducing the DMeO-PPV and m-Ph-PPV moieties into the polymer main chain resulted in high PL quantum efficiency and PLED fabricated alkoxy-substituted phenyl PPVs showed improved EL performance owing to their goodfilm-forming properties. All copolymers showed high EL efficiency above 10 cd A1and a low driving voltage (w3.5 V). In addition, very high emission brightness (10,000 cd m2) was easily achieved by applying a reasonable voltage of 6e8.
In 1997, Hsieh et al. used the DielseAlder reaction, a synthetic route, to obtain a full conjugated polymer, poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV), which exhibits high photo-luminescence efficiency in the solid state[18]. This is a versatile method for preparation of a variety of substituted monomers for PPV. Different substituents were introduced at the C-5 position of the phenylene moiety to modify its properties. For example, highly phenylated DP-PPV was synthesized to further improve PL efficiency [19]. Long alkyl chains were incorporated to improve the solubility of the polymer[20]. Liquid crystalline side chains were also incorpo-rated to achieve polarized emissions [21,22]. By following this synthetic route, monomers containing diverse functional groups are easily synthesized and therefore soluble DP-PPV derivatives with high molecular weights are also easily obtained. Recently, we have reported various types of novel DP-PPV-based copolymers [23,24,25]. Devices using DP-PPV derivatives containing long branched alkoxy orfluorenyl substituents with the configuration of ITO/PEDOT/polymer/Ca/Al exhibited a low turn-on voltage (4.0 V), a high external quantum efficiency (3.39 cd A1), and the highest brightness found in this survey (16,910 cd m2)[23].
In the present article, we synthesized three new DP-PPV derivatives (P1eP3) containing the DMeO-PPV and m-Ph-PPV moieties in the polymer main chain with different feeding ratios.
The monomers were copolymerized via the Gilch polymerization method. These copolymers show high molecular weights, narrow polydispersity, good thermal stability, and are easily purified. The electrical and spectroscopic properties of these polymers were systematically investigated. Finally, we demonstrate the feasibility of fabrication PLEDs by blade and spin-coating methods. The working principle of multilayer fabrication process by blade coating was reported previously[9]. To the best of our knowledge, so far, there are no reports focusing on improving device performance by using the blade coating method on organic soluble DP-PPV copol-ymers. The PLED performance is much better than that reported previously in the literature.
2. Experimental 2.1. Instruments
1H and13C NMR spectra were recorded on a Varian-300 MHz spectrometer. Mass spectra were obtained using a JEOL JMS-HX 110 mass spectrometer. Size exclusion chromatography was measured using a Viscotek GPC system equipped with a Viscotek T50A differential viscometer, Viscotek LR125 laser referactometer, and a Viscotek VE2001 pump. Three 10
m
m American Polymer Column were connected in series in order of decreasing pore size (105, 104, and 103 Å), Polystyrene standards were used for calibration, and THF was used as an eluent. Differential scanning calorimetry (DSC) was performed using a TA Instruments Unpacking the Q Series DSC unit operated at a heating and cooling rate of 10 C min1, respectively. Samples werefirst scanned from 30 to 300C; after cooling to 30C, they were scanned again from 30 to 300C. The glass transition temperature (Tg) was determined from the first heating scan. Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer Pyris instrument. The thermal stabilities of the samples were determined under a nitrogen atmosphere by moni-toring their weight losses while being heated at a rate of 10 C min1. UVevis spectra were measured using an HP 8453 spectrophotometer. PL spectra were obtained using an ARCO CH2Cl ClH2C M1 M2 H3COC COCH3 CH2Br BrH2C M3
+
+
t-BuOK THF O * C9H19O CH2Br BrH2C C9H19O COCH3 H3COC * X Y Z P1, x=90, y=0, z=10 P2, x=85, y=5, z=10 P3, x=70, y=20, z=10SpectraPro-150 luminescence spectrometer. Cyclic voltammetry (CV) measurements were made in acetonitrile with 0.1 M tetra-butylammonium hexafluorophosphate as the supporting electro-lyte at a scan rate of 50 mV s1. Platinum wires were used as both the counter and working electrodes, silver/silver ions (Ag in 0.1 M AgNO3solution, from Bioanalytical Systems, Inc.) were used as the reference electrode, and ferrocene was used as an internal stan-dard. The corresponding highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels were estimated from the onset redox potentials. All the devices were packaged in a glove box and measured in the ambient environment. The current-voltage-luminance characteristics were measured using an optical power meter PR-650 and a digital source meter Keithley 2400. The EL spectra were measured using a Photo Research PR-650 spectrophotometer under ambient conditions after encapsulation.
2.2. Synthesis
Materials: All reagents and chemicals were purchased from commercial sources (Aldrich, Merck, Lancaster or TCI) and used without further purification. Tetrahydrofuran (THF) and dichloro-methane were dried by distillation from sodium/benzophenone and calcium hydride, respectively. Monomers M1wM3 were synthesized as described previously in the literatures [11,16,25,26,27]. The polymer DPPFBNA used in white light devices was synthesized according to the literature[28].
2.2.1. Synthesis of polymersP1wP3
Different molar ratios of M1wM3 were used to synthesize polymers P1wP3 via the modified Gilch method[29]. An experi-mental procedure for polymer P3 is given below. To a mixture of M1 (0.3 g, 0.7 mmol), M2 (0.1 g, 0.2 mmol), and M3 (0.03 g, 0.1 mmol) in THF (20 mL), a solution of potassium tert-butoxide (1.2 g, 10.6 mmol) in THF (46 mL) was added. The resulting mixture was stirred at room temperature for 14 h under nitrogen atmo-sphere. The polymer was obtained by pouring the mixture into methanol andfiltered. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone, ethyl acetate, and THF, respectively. Afterfiltration and removal of the solvent, the polymer was re-dissolved in THF again and dropped into methanol to form precipitant polymer. The purified polymer was collected by filtration and dried under vacuum for 1 day, resulting as orange-red solid P3 (0.5 g, 51%).1H NMR (300 MHz, CDCl3, TMS):
d
¼ 0.88e1.90 (m, 32H), 3.37e4.18 (m, 10H), 6.41e7.60 (m, 20H).2.3. EL device fabrication
The ITO glasses were degreased in an ultrasonic solvent bath and then dried in a heating chamber at 120C. Before transferring the substrate into the glove box, the PEDOT:PSS layer was spin-coated on ITO glass and baked at 200C for 15 min in air. It has been reported that thermal annealing of PLEDs can result in improvement of EL performance[30,31,32,33]. Except the emission material in device F was annealed in N2, emission materials in other devices were annealed in vacuum under different conditions. P1
was annealed at 50 C overnight, P2 was annealed at 80 C for 30 min, and P3 was annealed at 80C for 30 min. For single layer devices (devices A and D), emission layers were formed on PEDOT:PSS by blade and spin coating with P1 and P2 in toluene solution. After the emission materials were bladed from a blade coater with a 60
m
m gap, the substrate was then spun immediately to form 50w60 nm film. As for bilayer devices (devices B, C, E, F, G, and H), TFB (20 nmw30 nm) was first formed on PEDOT:PSS by blade and spin coating from toluene solution. After TFB was bladed on the PEDOT:PSS, the substrate was then spun at 4000 rpm for 30 s. Immediately TFB was then baked at 180 C to remove the solvent. The 40w50 nm thick emission layer about in bilayer devices was then formed by blade and spin coating on TFB. For device C, and H, a layer of 15 nm TPBI was further formed by blade and spin coating on emission material, and LiF (1 nm)/Al (100 nm) was used as the cathode. Ca (35 nm)/Al (100 nm) was used as the cathode of device A and B, while CsF (2 nm)/Al (100 nm) was used as the cathode of device D, E, F, and G. For white light devices I and J, a blend of DPPFBNA (400 mg) and P3 (10 mg) were dissolved in toluene (32 mL) to form 1.5 wt.% polymer solution which was blade-coated onto the PEDOT and TFB layer as emitting layer. CsF (2 nm) Al (100 nm) were used as the cathode for both devices. 3. Results and discussion3.1. Synthesis and characterization of the polymers
The synthesis of the copolymers P1eP3 is depicted inScheme 1. Monomer M1 has been synthesized in our laboratory according to our previous publications [23,24,25]. We introduced the alkyl substituents into the phenyl ring of DP-PPV so as to minimize gelation and to improve the solubility of the obtained polymers. Monomer M2 containing a nonoxy substituent linked to the ortho-position of 2-phenyl-1,4-bis(bromomethyl)benzene was
Table 1
Feeding ratio and polymerization results of the polymers.
Polymer x y z Yield (%) Mn 105 Mw 105 PDI
P1 90 e 10 56 3.11 4.06 1.31 P2 85 5 10 57 5.62 7.13 1.27 P3 70 20 10 51 2.82 3.57 1.26 350 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 P1
Absorbanc
e (a
.u.)
Wavelength (nm)
P2 P3Fig. 1. Normalized absorption spectra of P1eP3 in solid state.
Table 2
Thermal, absorption, and emission data of the polymers. Polymer Tg(C) Td(C) UVevis
(nm)
PL (nm) FPL (%) THF Film THF Film THF Film P1 143 446 442 434 520 544 57 28 P2 135 451 444 440 518 547 55 23 P3 120 414 439 450 516 545 43 28
synthesized according to the literature methods[26,27]. The MEH-PPV monomer M3 was prepared as described earlier[11]. It has been reported that incorporation of M2 and M3 increases carrier mobility inside the polymer layer [34]. The polymerization was carried out via a modified Gilch route to obtain soluble DP-PPV derivatives. For a typical Gilch synthetic route,
a
,a
-dihalo-p-xylene is employed with excess amount of tert-BuOK in organic solvents. The monomer feeding molar ratio of M1/M2/M3 was 90:0:10, 85:5:10, and 70:20:10, respectively, and corresponding copolymers are denominated as P1, P2, and P3, respectively.The resultant DP-PPV copolymers are readily dissolved in common organic solvents such as tetrahydrofuran (THF), chloro-form, toluene, and chlorobenzene. Their number-average molec-ular weights (Mn), determined by size exclusion chromatography (SEC, eluent: THF) and calibrated against polystyrene standards, were determined to be 3.11 105, 5.62 105, and 2.82 105 g mol1for P1, P2, and P3, respectively, with poly-dispersity index of 1.26e1.31 (Table 1). The thermal properties of the polymers were determined via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the polymers exhibited good thermal stability with high glass transition temperatures (Tg) over 120 C and high decomposition
temperatures (Td) over 414 C. Polymers P1 and P2 show even higher Tg(>135C), which can be attributed to their high molecular weights. Moreover, such a high Tg can prevent morphological changes upon exposure to excessive heat treatment, which is desirable for polymers in the fabrication of light-emitting devices. 3.2. Optical properties
Fig. 1shows the UVevis absorption spectra of polymers P1eP3 in thinfilm.Table 2summarizes the UVevis absorption maxima of all polymers in different states. The absorption maxima of synthesized polymers in THF are located in the range of 439e444 nm, which is attributable to the
p
-p
* transition along the conjugated backbone. In comparison with P1, the absorption maximum peaks of the thinfilm absorption spectra of P2 and P3 were red-shifted about 6 and 14 nm, respectively, which is consistent with the addition of the feeding ratio of M2, where the effective conjugated length is increased because ofp
-p
stacking.Fig. 2reveals the PL emission spectra of polymers P1eP3 in thin film. The PL emission maxima in different states are also summa-rized inTable 2. The maximum emission bands are located from 516 to 520 nm in the solution state and from 544 to 547 nm in the thin film state. A tendency to red-shift from solution to thin film state was also observed. For all synthesized polymers, no significant vibronic band was observed, indicating no or weak chainechain interactions. The PL quantum efficiencies (
F
PL) of the P1, P2 and P3 in THF solution are 0.57, 0.58 and 0.43, respectively. By comparing polymers P1eP3, it was found thatF
PL values were gradually decreased with decreasing feeding ratio of M1, implying that the high feeding ratio of M1 can suppress the formation of aggregates. The results demonstrate that the incorporation of appropriate bulky moieties into polymers brought benefits of high quantum efficiency and decreased chain aggregation.3.3. Cyclic voltammetry properties
To investigate the information on the charge injection, cyclic voltammogram was used to estimate the HOMO and LUMO energy levels of synthesized polymers. The oxidation and reduction processes are clear and directly associated with the conjugation structure of the polymer.Fig. 3shows CV of polymers P1eP3 in both processes. The HOMO, LUMO, and energy gap of polymers
450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0
PL Intensity (a.u.)
Wavelength (nm)
P1 P2 P3Fig. 2. Normalized PL spectra of P1eP3 in solid state.
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
C
urrent (a.u.)
Voltage vs Fc/Fc+(V)
P1 P2 P3Fig. 3. Cyclic voltammograms of P1eP3 in thin film at a scan rate of 80 mV/s.
Table 3
Electrochemical onset potentials and electronic energy levels of P1eP3. Polymer Eoxonset(V) Eredonset(V) HOMO (eV) LUMO (eV) Egec(eV)
P1 0.52 2.22 5.32 2.57 2.75 P2 0.65 2.18 5.45 2.62 2.83 P3 0.65 2.19 5.45 2.61 2.84
P1eP3 are measured[25]and summarized inTable 3. The energy level diagram of these materials is illustrated inFig. 4. The HOMO level of the PEDOT layer is known to be w5.2 eV. Thus, hole injection and transportation from PEDOT to P1 is expected to be easier than to P2 and P3. It is known that lowering oxidation potential favors hole-injection, which shows advantages for EL applications. As for LUMO levels, P2 and P3 have the smaller energy barriers to the Ca or Al cathode than P1, implying facilitated elec-tron injection from the cathode to the polymer layer.
3.4. Electroluminescence properties
Eight devices were prepared to study the multilayer device structure, and their device conformation and performance were summarized inTable 4. Different device conformations were made for each polymer in order tofind out the best electroluminescence performance. For these multilayer devices, TFB (Mw ¼ 197,000, American Dye Source), which is poly[(9,9-dioctyl fluorenyl-2,7-diyl)-co-(4,40-(N-(4-s-butylphenyl))diphenylamine)], was used as the hole-transport layer (HTL) as well as the electron blocking layer
(EBL); TPBI, namely 1,3,5-tris(N-phenylbenzimi-dazol-2-yl) benzene, possesses both good electron transport characteristic and large IP to block holes. The single layer devices were fabricated by spin-coating method while the multilayer devices were fabricated by blade coating method.Fig. 5shows the AFM images of P1wP3 films prepared by both spin-coating and blade coating methods. The root-mean-square (RMS) roughnesses of these films were averaged from 6.3 to 1.55 nm; this means that both spin-coating and blade coating method prepared smooth film in the fabrica-tion process.
Initial investigations of the EL properties of P1 were made by fabricating a single layer device (device A) and two bilayer devices (devices B and C). The EL spectra (a), current density-voltage (b), luminescence-voltage (c), and current efficiency-voltage (d) char-acteristics of P1-based devices are shown inFig. 6. The positions of the main peak and the vibronic transition peaks under an electric field were very close to those of the corresponding peaks observed in the PL spectra, indicating that the PL and EL properties are very similar. The current density increased exponentially with increasing forward bias voltage, which is a typical diode charac-teristic (Fig. 6b). The turn-on voltages of the devices A, B, and C were approximately 3.0, 3.0 and 5.0 V, respectively. The maximum brightness of the device A was found to be 6138 cd m2at 9 V, while the corresponding current efficiency was only 0.46 cd A1at 5.5 V. Device B with a structure of TFB/P1 was fabricated by blade and spin coating on a hot plate for the second layer. The maximum luminance was increased from 6138 to 8926 cd m2and the device efficiency was enhanced from 0.46 to 1.0 cd A1. The TFB interlayer plays roles in transporting holes from the PEDOT:PSS layer and blocking electrons at the TFB/emitter interface, resulting in a more balanced electron-hole recombination and the confinement of excitation in the EML [7,35]. The device efficiency was further improved by introducing the electron-transporting interlayer (TPBI) between the EML and the LiF/Al, leading to an efficiency of
Table 4
Performance of devices AeH. Device structure (Device No.) ELlmax (nm) Von (V) Lmax (voltage/V) (cd m2) efficiency (B/V) (cd A1) ITO/PEDOT:PSS/P1/Ca/Al (A) 552 3.0 6138(9.0) 0.46 (5.5) ITO/PEDOT:PSS/TFB/P1/Ca/Al (B) 548 3.0 8926(9.0) 1.0(5.0) ITO/PEDOT:PSS/TFB/P1/TPBI/LiF/Al (C) 548 5.0 19660(12.5) 10.96(5.5) ITO/PEDOT:PSS/P2/CsF/Al (D) 548 3.0 43160(12.0) 6.58(3.0) ITO/PEDOT:PSS/TFB/P2/CsF/Al (E) 548 3.0 72170(9.5) 6.29(4.0) ITO/PEDOT:PSS/TFB/P3(N2)/CsF/Al (F) 544 4.0 34430(9.5) 3.68(5.0) ITO/PEDOT:PSS/TFB/P3/CsF/Al (G) 548 4.0 78050(10.5) 9.15(4.5) ITO/PEDOT:PSS/TFB/P3/TPBI/LiF/Al (H) 544 4.0 20870(13.5) 7.96(6.0)
10.96 cd A1at 5.5 V. The maximum efficiency of device C with LiF was 24-fold higher than that of device A with Ca. Moreover, the maximum luminance was 19660 cd m2for the C device; it was about 3.2-fold larger than that of the A device. The work function of
Li (2.5 eV) is lower than Ca (2.9 eV) and, therefore, more efficient electron injection is provided by the LiF/Al cathode. In addition, the presence of the hole/exciton-blocking (TPBI) layer also effectively confines the excitations within the emitting layer, thus preventing
c
d
a
b
Fig. 6. EL spectra (a), Current density-voltage (I-V) (b), luminance-voltage (L-V) (c), and current efficiency (B-V) (d) characteristics of the P1-based devices.
c
d
a
b
luminescence quenching.Fig. 7shows the EL spectra and current density-voltage-luminance (I-V-L) characteristics of the P2-based LED devices. Their EL spectra were also similar to their PL spectra. The current of device E was smaller than that of device D because the electron current is blocked by the TFB layer. The turn-on volt-ages for D and E devices were around 3 V; the maximum lumi-nescence of device D was 43160 cd m2at 12 V, and device E with TFB interlayer showed a maximum brightness of 72179 cd m2at 9.5 V. It can be found that by introducing M2 into the main chain of P2, the P2-based devices show a much better device performance than those of P1-based devices because of the improved electron-transporting ability of P2. Further comparing of the cathodes Ca/ Al and CsF/Al in P1-based and P2-based devices also demonstrated differences. For single layer devices, the maximum efficiency of device D with CsF was 14-fold higher than that of device A with LiF and reached about 6.58 cd A1. The efficiencies of the PLED bilayer were 1.0 cd A1for device B with LiF and 6.29 cd A1for device E with CsF; the advantages of using CsF/Al cathode is about 5-fold increase. Owing to the work function of Cs (2.1 eV) being lower than Ca (2.9 eV), more efficient electron injection is provided by the CsF/Al cathode. That is why the efficiency and luminance of devices D and E were higher than those of devices A and B.Fig. 8presents emission spectra and device characteristics based on P3, with CsF/ Al or LiF/Al as cathodes. The turn-on voltages of P3-based devices were about 4 V. Device G demonstrated a better EL performance when comparing to device F without using N2annealing to treat the emissive layer. For device E and G, the maximum efficiency increased with an increasing feeding ratio of M2 in the copolymers. Therefore, device G achieved certain improvement for the device performance; it showed a great efficiency of 9.15 cd A1at 4.5 V, and a maximum brightness reaching 78050 cd m2was observed due to improved electron-transporting ability. This might be the highest value for DP-PPV derivatives reported in the literature so far. As a result, these materials are excellent candidates for PLED and show dramatic potential in further applications.
a
b
c
d
Fig. 8. EL spectra (a), Current density-voltage (I-V) (b), luminance-voltage (L-V) (c), and current efficiency (B-V) (d) characteristics of the P3-based devices.
0
2
4
6
8
10
12
14
16
18
20
0
200
400
600
800
1000
1200
Luminance
cd
/m
2Voltage (V)
P3:DPPFBNA(1:40) TFB/P3:DPPFBNA(1:40) 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 EL Intensity (a.u.) Wavelength (nm) C6H13O OC6H13 nDPPFBNA
a
b
Fig. 9. (a) The chemical structure of DPPFBNA and (b) the brightness plots vs. voltage for white light-emitting devices. The insert shows the EL spectra at 11 V.
3.5. Electroluminescence properties for white light devices
To investigate the superior properties of P3 further, we fabri-cated two kinds of white light-emitting devices based on the blend of P3 and DPPFBNA, a blue emitting polymer synthesized in our laboratory[28]. The chemical structure of DPPFBNA was depicted inFig. 9a. The double-layered device structure was made with the configuration of ITO/PEDOT:PSS (50 nm)/EML (70e80 nm)/CsF (2 nm)/Al (100 nm), and the multilayer device structure was fabricated with the configuration of ITO/PEDOT:PSS (50 nm)/TFB (20 nm)/EML (50e60 nm)/CsF (2 nm)/Al (100 nm). The EML composite film was prepared from 1.5 wt.% of P3 and DPPFBNA blending solution (P3:DPPFBNA¼ 1:40, w/w) and blade-coated onto the PEDOT or TFB layers.Table 5summarizes the results of the device performances. The TFB/P3:DPPFBNA composite film produced a maximum brightness of 1085 cd m2and a maximum luminance efficiency of 1.18 cd A1. The brightness and luminance efficiency in these multilayer devices were higher than those in double-layered devices. As illustrated in Fig. 9b, the TFB/ P3:DPPFBNA EL spectrum exhibits dual emissions, with normal-ized blue and yellow emission intensities covering the entire visible region. The CIE coordinates of the EL emission at 11 V are (0.28, 0.33) which are very close to the desired white light coordinates (0.33, 0.33). When we increased the driving voltages, the white emission CIE coordinates remained stable.
4. Conclusions
This study focused on the design and synthesis of highly ef fi-cient electroluminescent DP-PPV derivatives through Gilch poly-merization. All synthesized EL polymers with high molecular weights exhibited good solubility in conventional organic solvents and possess high thermal stabilities. The maximum PL emission bands of thinfilms are located between 544 and 547 nm. Impor-tantly, we have developed a way to simultaneously reduce the cost of PLED and at the same time prevent the dissolution between two polymer layers by blade coating. This is a very simple method to fabricate all-solution-processed multilayer polymer devices. A maximum brightness of 78050 cd m2was achieved. This is the highest value for DP-PPV derivatives reported in the literature so far. The results reveal that these materials are well suitable for the blade coating method and provide a huge potential in large-area and low-cost light-emitting applications.
In addition, we fabricated double and multilayer white light-emitting devices based on the P3 and DPPFBNA blending film. The multilayer devices showed the maximum brightness of 1085 cd m2and current efficiency of 0.75 cd A1, with CIE coor-dinates of (0.28, 0.33) at 11 V. The white emission remained stable at higher driving voltages.
Acknowledgments
The authors thank the National Science Council (NSC) of the Republic of China forfinancially supporting this research.
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Table 5
Performance of P3 in double-layer and multilayer white light devices. Device No. Vona Bmax LEmax ELb CIE 1931b
V cd m2 cd A1 nm (x, y) Double layer device I 7.5 460 0.67 465, 558, 595 (0.37, 0.43) Multilayer device J 7.5 1085 0.75 470, 550, 595 (0.28, 0.33) Double layer device I: ITO/PEDOT:PSS/P3:DPPFBNA(1:40)/CsF/Al.
Multilayer device J: ITO/PEDOT:PSS/TFB/P3:DPPFBNA(1:40)/CsF/Al.
aRecorded at 1 cd m2. bRecorded at 11 V.
