Application of alternating fluorene and thiophene copolymers in polymer light-emitting diodes

Synthetic Metals 129 (2002) 129–134

Application of alternating fluorene and thiophene copolymers in polymer light-emitting diodes Bin Liua, Yu-Hua Niub, Wang-Lin Yua, Yong Caob, Wei Huanga,* a

Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore College of Materials Science, South China University of Technology, Wushan Road, Guangzhou 510640, PR China

b

Received 9 May 2001; received in revised form 28 December 2001; accepted 31 December 2001

Abstract Four polyfluorene copolymers comprised of fluorene and thiophene or bithiophene moieties were investigated and applied for device fabrication. Relatively high photoluminescence quantum yields (23–40%), good optical and electrochemical properties have been demonstrated for this series of polymers. Single and double layer devices fabricated with the polymers in a conventional configuration appeared to have electrons as the majority carrier, and their performance was remarkably improved when poly(3,4-ethylenedioxythiophene) (PEDOT) was selected as the hole-injection/transporting layer. The best performance was demonstrated for a double layer green-emitting device, which exhibited luminescence of 356 cd/m2 at a bias of 8 V with an external efficiency of 0.38%. The current–voltage and luminescence characteristics of the devices with PEDOT or poly(N-vinylcarbazole) (PVK) as the buffer layer were studied and discussed. # 2002 Published by Elsevier Science B.V. Keywords: Fluorene; Thiophene; Light-emitting diode

1. Introduction Since the first report of polymer light-emitting diodes (LEDs) in 1990, significant efforts have been devoted in realizing commercial available polymer light-emitting diodes (PLEDs) with high brightness and efficiency, good stability as well as long operation life-times [1–6]. This progress has relied heavily on a multidisciplinary approach that includes both the field of solid-physics and that of the technology of macromolecules [7]. For chemists, the accomplishment of polymeric LEDs requires efforts in the design and synthesis of electroluminescent polymers with tailored properties, such as high photoluminescent quantum efficiency, good processibility as well as thermal, optical and electrical stability. So far, electroluminescent polymers with the emission that covers the whole visible spectrum have been successfully developed [8]. It is reported that poly(p-phenylenevinylene) (PPV) and its derivatives, poly(p-phenylene)s (PPP), polythiophenes, polyfluorenes and their copolymers have been widely used as the emissive layers for PLED devices [9–14]. Thiophene-based polymers have been intensively studied in terms of their synthesis, chemical and electronic properties. * Corresponding author. Tel.: þ65-874-8592; fax: þ65-872-0785. E-mail address: [email protected] (W. Huang).

0379-6779/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 0 1 4 - 0

The most striking features of this class of polymers are their good stability, as well as their easy and wide electronic tunability by side chain modifications [15]. Moreover, different structural regioregularity arising from side chain substitutions in PTs offered additional opportunities for tuning electronic properties by controlling the coupling configurations [16,17]. However, relatively low photoluminescence (PL) quantum efficiency of PTs in the solid state, typically 1–3%, has limited their applications in PLEDs [18]. Although it was reported that for a dioctyl-phenyl substituted polythiophene derivative, the PL efficiency could be improved to 24% [19]; Gigli et al. [20,21] demonstrated that higher absolute PL efficiency of 37% could be obtained for thiophene oligomers through the modification of backbone structure with dioxide functionalization of the thiophene heteroatom. In our previous efforts, significant increases in PL efficiency for thiophene-based polymers were achieved by modifying the backbone structure with phenylene rings [22]. A more recent progress in our group has demonstrated that the copolymers of 9,9-disubstituted polyfluorene with bithiophenes, exhibited relatively high PL and electroluminescence (EL) efficiencies as well as good EL performance [23,24]. Although Morgado and coworkers have also studied the fluorene and thiophene copolymers, their molecular weights as well as the fluorescence quantum yields in the solid state are rather low [25]. In this paper, we

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present the EL properties of a new series of polyfluorene–cothiophene copolymers. Through the modification of bithiophene moieties with different configurations, as well as by introducing different number of substituents on the thiophene rings, both the HOMO and LUMO energy levels of the copolymers could be tuned, thereby facilitating charge from the electrodes.

Table 1 Number average (Mn) and weight average (Mw) molecular weight of P1–P4 Polymer

Mn

Mw

Mw/Mn

P1 P2 P3 P4

15 200 22 600 18 700 18 600

29 100 39 400 26 600 29 300

1.9 1.7 1.4 1.6

2. Materials and experimental methods The chemical structures of the polymers are shown in Fig. 1, and their molecular weights are listed in Table 1. The synthesis and characterization of the polymers could be found in our previous publication [24]. The UV–Vis spectra were recorded on a Shimadzu 3101 PC Spectrometer, and the fluorescence measurement was carried out on a Perkin-Elmer LS 50B PL spectrometer with the xenon lamp as the light source. The absolute PL efficiencies of the polymers in neat films were measured in an integrated sphere at room temperature in air following the procedure described by Greenham et al. [26] using an argon ion laser line of 358 nm as the excitation source. The HOMO and LUMO levels of the polymers were estimated from the electrochemically oxidative and reductive onset potentials. The details for the electrochemical measurements and the calculation are described in the previous publication [24]. Device fabrication was carried out in a nitrogen-purged glove box. The indium tin oxide (ITO) coated glass substrates were cleaned via repeated washing and oxygen

plasma treated steps. The poly(3,4-ethylenedioxythiophene) (PEDOT) layer was spin-coated from water and the poly(Nvinylcarbazole) (PVK) layer from the chloroform solution, with a thickness of 50 nm. This film was dried overnight at 50 8C under rotary vacuum. The EL layer was spin-coated on top of the HTL (or ITO coated glass substrates) from toluene (15 mg/ml) at a spin rate of 700 rpm at room temperature with a thickness of around 75 nm. The thickness of different layers was measured by surface-profiler TENCOR Alfa step-500. The resultant films were vacuum dried overnight at room temperature prior to deposition of the cathode metal. Calcium covered by aluminum was thermally evaporated onto the light emission polymer in a vacuum chamber (pressure <2  104 Pa), with a thickness of 200 nm. The device was biased by computer-controlled source meter Keithley 236 and the light intensity was recorded by calibrated Si-photodiode. All of the electrical characteristics, luminance and quantum efficiencies were measured in the dry box.

3. Results and discussion The absorption and PL spectra of the films for all of the four polymers are shown in Fig. 2. The onset of the absorption of P1 film occurred at 398 nm, and the peak appeared at 403 nm. Its emission peaked at 490 nm with a shoulder around 520 nm, which corresponded to a green

Fig. 1. The structures of P1–P4.

Fig. 2. The UV–Vis absorption spectra and PL spectra of P1–P4 measured from the spin-coated films on quartz plates at room temperature.

B. Liu et al. / Synthetic Metals 129 (2002) 129–134

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Table 2 Optical and electrochemical data of P1–P4 Polymer

P1 P2 P3 P4

Sol. lmax (nm)a Abs.

Em.

401 398 403 367

482 (515) 483 (520) 461 (490) 447

Ffl

0.18 0.21 0.41 0.25

Film lmax (nm)a Abs.

Em.

403 401 412 378

490 493 492 458

(520) (520) (477) (475)

Eg (eV)b

2.57 2.60 2.63 2.78

p-Doping (V)c

n-Doping (V)c

Epa

Epc

E1/2

Epc

Epa

E1/2

1.30 1.28 1.27 1.49

0.92 0.93 1.02 1.18

1.11 1.10 1.15 1.34

– 2.50 2.31 2.48

2.11 2.07 2.09 2.23

– 2.28 2.20 2.36

a

The data in the parentheses are the wavelengths of shoulders and sub-peaks. Eg stands for the bandgap energy estimated from the onset wavelength of the optical absorption. c Epa and Epc stand for anodic peak potential and cathodic peak potential, respectively. b

light emission. Both the absorption and the PL spectra of P2 were similar to those of P1, implying that the difference of coupling configurations between the thiophene rings did not obviously affect the optical properties of the resulting polymers. P3 also gave green light emission, while P4 emitted blue light upon UV excitation. The spectroscopic parameters of the polymers are summarized in Table 2. In general, the polymers copolymerized with monothiophenes (P3 and P4) have a higher fluorescence quantum yield (Ffl) than those copolymerized with bithiophenes (P1 and P2). The Ffl of P3 (0.41) was measured to be almost twice as high as those measured for P1 and P2. The fluorescence quantum yields of P1 (0.18) and P2 (0.21) were very close indicating that the variation in the configuration of the b-substituted bithiophene had little effect on the PL quantum efficiency of the polymers based on the same backbone structure. On the other hand, the introduction of the second decyl chain onto the thiophene ring in P3 reduced the Ffl from 0.41 of P3 to 0.25 of P4. This observation is consistent with the finding for the substituted polythiophenes, in which the fluorescence quantum yields of disubstituted polythiophenes (0.008–0.02) are usually much lower than those of the monosubstituted polythiophenes (0.04–0.24) [27]. The results of PL efficiency are also listed in Table 2. The cyclic voltammograms of the polymers are given in Fig. 3. All of the four polymers are reversible in both ndoping and p-doping processes. Take P3 as an example, on sweeping the polymer cathodically, the onset of the reduction occurred at 1.84 V, and a cathodic peak appeared at – 2.31 V. The corresponding reoxidation peak appeared at 2.09 V. The n-doping potential Ered 1=2 was thus calculated to be 2.20 V. In the oxidation (p-doping) process, the onset potential was determined to be 0.99 V, and an anodic peak occurred at 1.27 V with the corresponding re-reduction peak at 1.02 V, which gave a value of p-doping potential, Eox 1=2 , as 1.14 V. The HOMO and LUMO energy levels of P3 were estimated from the n-doping and p-doping onset potentials to be 5.39 and 2.56 eV, respectively, according to the formula proposed by Bre´ das et al. [28]. The electrochemical data as well as the determined HOMO and LUMO energy levels for all of the four polymers are summarized in Table 2. A schematic diagram showing the HOMO and

Fig. 3. Cyclic voltammograms of P1–P4 films coated on platinum plate electrodes in acetonitrile containing 0.1 M Bu4NClO4. Counter electrode: platinum wire; reference electrode: Ag/AgNO3 (0.10 M in acetonitrile); scan rate: 50 mV/s.

LUMO values relative to the work functions of the electrode materials used in the EL devices described below is given in Fig. 4. Single-layer devices with the configuration of ITO/EML/ Ca/Al and double layer devices with the structures of ITO/ PEDOT or PVK/EML/Ca/Al were fabricated for all the four polymers. The current (I)–voltage (V) and light intensity (L)–voltage (V) characteristics of the single-layer device of ITO/P3/Ca/Al are depicted in Fig. 5. Under a forward bias (ITO wired positively), the light turn on occurred at 11 V. The luminescence reached about 200 cd/m2 at a driving voltage of 16 V with a current of 14.3 mA. As shown in Fig. 6, the EL spectrum of P3 is identical to its PL spectrum,

Fig. 4. The schematic energy level structure for the devices.

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corresponding to a green light emission. The maximum external EL quantum efficiency of the single-layer devices was measured to be 0.064%. A critical factor in determining the EL quantum efficiency is the balanced injection and transporting between electrons and holes in a device. As sketched in Fig. 4, for P3 the energy barrier for electroninjection is 0.34 eV, whereas at the anode interface, the energy barrier for hole-injection is 0.69 eV. The devices with ITO anode and calcium cathode thus should undergo a facile electron-injection, but a relatively difficult hole-injection. The unbalanced charge injection should be responsible for the low EL quantum efficiency. PEDOT (doped with poly(styrenesulfonate)) is one of the most widely used buffer layer materials applied at the anode interface with the functions of enhancing hole-injection and improving device life-time [29]. By adding a PEDOT layer (50 nm) between ITO and P3 film, the turn on voltage (the voltage of giving measurable light intensity of 0.1 cd/m2) for light output was reduced to 7 V. The luminescence reached 1500 cd/m2 at a drive voltage of 10 V, and the maximum external EL quantum efficiency

Zext was increased to 0.38% at the bias of 8 V with the luminescence of 356 cd/m2. The I–V and L–V curves recorded from the devices are also given in Fig. 5. The improved device performance with the PEDOT layer is likely to be due to the work function of the PEDOT compared with that of ITO (and therefore easier holeinjection). As shown in Fig. 6, the addition of PEDOT layer does not change the emission spectrum obviously. PVK is another material that is often used in PLEDs at anode interface to improve device efficiency. Different from PEDOT, a heavily doped, high conductivity polymer, PVK is a low conductivity, undoped non-conjugated polymer. For P3, when PVK was used as the buffer layer (50 nm) instead of PEDOT, the maximum external EL quantum efficiency was further increased to 0.64% (at 25 V). However, the driving voltage was also noticeably increased with a turn on voltage of 17 V. The corresponding curves are shown in Fig. 7. As in other PLEDs [31], the function of PVK is somewhat complicated. Its deep HOMO level (6.1 eV below the vacuum) causes a large injection barrier for holes, while its high LUMO level provides an even larger barrier for electron-injection. Considering the lower electron mobility of PVK, the improvement of device performance by employing the PVK layer seems to arise from PVK serving as an electron-blocking layer. It was also noted that there is a possibility of interpenetrating between the copolymers and the PVK layer, since PVK was spin-casting from its chloroform solution and chloroform can more or less dissolve the copolymers, which will result in a close and rough interface between PVK and the copolymer layer. Although there is not a clear picture, the importance of interface contact properties in the operation of polymer LEDs has been widely recognized [30]. The PVK layer improves the balance between the electron and hole currents, which helps to increase the radiative recombination within the polymer, causing an increased EL efficiency. The increased EL efficiency with the successful blocking of electrons clearly demonstrated that electrons are the majority carriers in the device.

Fig. 6. Comparison of PL and EL spectra for P3.

Fig. 7. L–V and external QE of the device ITO/PVK/P3/Ca/Al.

Fig. 5. I–V and L–V characteristics of devices ITO/P3/Ca/Al and ITO/ PEDOT/P3/Ca/Al.

B. Liu et al. / Synthetic Metals 129 (2002) 129–134 Table 3 The device performance for P1–P4 EL polymer ˚) (thickness, A

Anode buffer

Device performance Bias (V)

Current (mA)

Light intensity (cd/m2)

QE (%)

P1 P1 P1 P2 P2 P3 P3 P3 P3 P4 P4

PEDOT PEDOT PVK PEDOT PVK – PEDOT PEDOT PVK PEDOT PVK

8.0 8.0 30.0 6.0 24.0 14.0 6.0 8.0 25.0 8.0 20.0

75.4 57.5 13.5 20.5 17.9 14.3 13.9 12.4 6.77 66.7 9.00

388 647 252 144 405 69.5 359.6 356 325 824 252

0.07 0.15 0.25 0.09 0.30 0.064 0.34 0.38 0.64 0.16 0.37

(715) (900) (715) (745) (745) (650) (650) (770) (770) (690) (735)

According to Bernius et al. [26–31] the fluorene polymers and some of its copolymers seem to have electrons as the majority carrier. While Isabelle et al. [27–32] reported that hole transporters such as thiophene or phenylene units could change the balance of the nature of the carriers. As depicted in the energy band diagram of Fig. 4, for all of the four polymers, the energy barriers for holes are more or less large than those for electrons. It is expected that the presence of a hole-transporting layer will have the same beneficial effect in the devices with the other three polymers as the emissive layer. Double layered devices with PEDOT or PVK inserted between the ITO and the emissive layer were thus fabricated. The characteristics of the devices are summarized in Table 3. When PEDOT was introduced as the hole transporting layer, in a forward bias (ITO wired as positive), double layered devices (ITO/PEDOT/EML/Ca/Al) for all of the four polymers turned on for current at about 5 V and emitted visible light above 8 V. The best external EL quantum efficiency was obtained from P3, measured to be 0.4%. The EL spectra of the four polymers very closely resemble their PL spectra, indicating that the emissions originate from the radiative recombination of singlet excitons within the polymers. It is also interesting to find that, for this series of devices, P3 gave the highest external EL quantum efficiency, which was followed by P4. The EL efficiency of P3 was three times higher than those of both P1 and P2, which exhibited efficiency of around 0.08%. This sequence is inconsistent with those observed for their respective PL efficiencies in the film states; increase of thiophene moieties or additional side chain on thiophene ring lowers EL performance. When PVK was employed as the buffer layer between ITO and the polymer films, the maximum external EL quantum efficiencies for the devices of all of the four polymers were further increased to 0.25–0.64%, but the operating voltage is also significantly increased. Again, P3 gave the highest EL efficiency among this series of polymers (depicted in Fig. 7). However, the difference of the external quantum efficiency among the four polymers is not so obvious as in the PEDOT-buffered devices. Given

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the assumptions that electrons are the majority carriers in the devices and large barrier for hole-injection is the main limiting factor for EL efficiency in the devices, the electron blocking caused by the PVK layer will lead to a better balance between electron and hole currents for each polymer, and thus improved EL efficiency. Due to the deep low HOMO of PVK, the EL efficiency of the devices becomes more dependent on the hole-injection across the ITO–PVK interface. As a result, the difference of HOMO levels in the polymers becomes less important in determining the hole-injection and EL efficiency, which explained the less molecular dependence of EL efficiency compared to the PEDOT-buffered devices. The obvious increase of the operating voltage could be understood in terms of the blocking of PVK to electrons, the majority charge carrier in the devices. Due to the poor conductivity of the PVK layer, the thickness of the PVK might be another reason that affects the turn on voltage.

4. Conclusions With the backbone structures of 9,9-dihexylfluorene-altco-thiophene, the emission colors of the resulting polymers could be turned from green to blue by changing the thiophene ring number in the thiophene moieties or by attaching different substituents on the thiophene rings. The thiophene ring number in the thiophene moieties and the attachment on the thiophene rings also play an important role in determining the PL efficiency of the resulting polymers. However, both the emission spectrum and the PL efficiency are not sensitive to the coupling configurations between the thiophene rings in the polymers comprised of bithiophene moieties. All the four polymers demonstrate EL with the external quantum efficiencies up to 0.25–0.64%. The EL performance heavily depends on the device structures. Like other fluorene-based light-emitting polymers, hole-injection is more difficult than electron-injection for all the four polymers because of the large energy barriers presented at the interface between ITO and the polymer layers. The application of PEDOT as a buffer layer between ITO and the polymer layer greatly improves the device performance (much higher EL efficiency and lower driving voltage) by enhancing the hole-injection. The utilization of PVK instead of PEDOT as the buffer layer can further increase the EL efficiency of the devices because of the electron-blocking effect of PVK, but the devices suffer from the noticeably increased driving voltages. For getting higher device performance of the polymers, hole-injection should be further improved.

Acknowledgements Y.-H. Niu and Y. Cao thank NSFC for the financial support by a research grant of NSFC 29992530-6.

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