Tb-containing electroluminescent polymer with both electron- and hole-transporting side groups for single layer light emitting diodes

Synthetic Metals 144 (2004) 259–263

Tb-containing electroluminescent polymer with both electron- and hole-transporting side groups for single layer light emitting diodes Lichang Zeng a , Mujie Yang a,∗ , Peng Wu b , Hui Ye b , Xu Liu b b

a Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China The State-Key Laboratory of Modern Optical Instruments, Zhejiang University, Hangzhou 310027, China

Received 23 December 2003; received in revised form 29 March 2004; accepted 30 March 2004 Available online 18 May 2004

Abstract Electroluminescent Tb-complex containing non-conjugated polymer (PKMOTb) with dual transporting properties has been studied. The carbazole side groups serve as hole transporter and diphenyloxadiazole (DPO) groups serve as electron transporter. The photoluminescence study on the polymer film reveals an energy transfer from both carbazole and DPO groups to Tb complex moieties, enhancing Tb3+ ion’s characteristic sharp emission at 545 nm. A single layer light emitting diodes with the structure ITO/PKMOTb/Al was fabricated. Typical semiconductor current (I) voltage (V) property was observed in this device. Emissive spectrum with sharp peaks was recorded when this device was driven under a positive bias above the turn on voltage of 6.2 V. Mechanism of the electroluminescence was also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Dual transport; Tb; Electroluminescence

1. Introduction Since Burroughes et al. [1] reported the first light emitting diodes (LED) based on polymers, electroluminescent polymers have been studied extensively due to their potential application in large screen emissive flat-panel color displays or as white back-lights for liquid crystal displays [2–9]. To achieve high luminescence efficiency, the electron and hole currents must be balanced, implying similar injection and transport behavior for the two types of carriers. However, typical polymers are not simultaneously good conductors for both electrons and holes. One of the strategies to improve device efficiency is to employ multilayer structure, where additional charge transport layers are inserted between emissive layer and the electrodes to facilitate charge injection and transport [4,10]. However, polymers are generally unsuitable for constructing multilayer devices, since only in unusual cases is it possible to choose a solvent that will dissolve the material being deposited and yet will not dissolve the previously deposited layers during spin coating [11,12]. A second strategy is to blend hole- and electron-transporting materials together, or to co-polymerize monomers contain∗ Corresponding author. Tel.: +86-571-8795-2444; fax: +86-571-8795-2444. E-mail address: [email protected] (M. Yang).

0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.03.011

ing hole- and electron-transporting groups together, and retains single layer devices structure [8,13,14]. Carbazole and diphenyloxadiazole (DPO) are two of the most studied functional groups that have been extensively employed in co-polymers as hole- and electron-transporting moieties, respectively [8,9,13]. Unfortunately, intimate contact between the electron-donating groups of the hole-transporting material and the electron-withdrawing groups of the electron-transporting material would generally bring to the formation of excited-state complexes, which alter the spectrum of the emitted light [15]. One approach to obtain pure light emission is to employ energy transfer, where excited energy formed on the donor by recombination of holes and electrons was transferred to an acceptor (e.g. Forster energy transfer), and from the latter was emitted the light with sufficient purity. Lanthanide complexes have been proved to be good candidates as acceptors for this purpose [16]. They excel other luminescent organic materials in that their emission bands are usually extremely narrow, which makes them especially attractive for use in white light emitters [17–20]. It is reported that LEDs fabricated from lanthanide complexes doped into transporting hosts might give only lanthanide ion’s characteristic emission with enhanced performance [21,22]. However, this doping system with small molecules as dopants risks the disadvantage of unstable performance resulting from phase

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separation during operation [23]. One strategy to circumvent this phase separation is to attach emissive moieties directly to polymer chains so that they were dispersed uniformly in the bulk, avoiding the unfavorable aggregation. Previously, we have reported polymers containing lanthanide complexes where excited energy was transferred from carbazole groups to lanthanide complex moieties under electrical or light excitation [17,24,25]. In this article, we will report a novel polymer PKMOTb containing electron-transporting carbazole and hole-transporting diphenyloxadiazole groups as well as luminescent Tb complexes and its use in single layer LED.

the corresponding polymer’s solution. Their UV-Vis spectra were recorded on Cary100 Bio UV-Vis Spectroscopy; solutions with concentration of 1 × 10−5 g/ml in tetrahedron furan (THF) were used for this purpose. 2.3. Device fabrication and testing Etched patterned glass-substrate ITO cathode (∼100 /䊐) were cleansed by ultrasonication in de-ionized water, ethanol, acetone and THF successively. Polymer was deposited by spin coating at 3000 rpm from a 5 mg/ml solution with mixed chloroform and THF (2:3 (v/v)), producing films of 60–70 nm thickness as measured by surface profilometer. An Al cathode was then deposited onto the polymer film by vacuum evaporation, forming an active area of 5 mm×5 mm. The devices’ EL spectra were recorded with a photoelectricity amplifier equipped with a monochromatic meter. Tuning the monochromatic meter gave brightness at different emission wavelength indicated as photocurrent on the photoelectricity amplifier. Their brightness-voltage characteristics were recorded with the same photoelectricity amplifier. Tuning the driving voltage gave different brightness indicated as photocurrent on the photoelectricity amplifier. The devices’ current-voltage characteristics were studied with a Keithley 4200 Semiconductor Characterization System.

2. Experimental 2.1. Materials The chemical structure of polymers used in this study are illustrated in Scheme 1: NVK-co-MMA-co-DPO-co-Tb (MA)(acac)2 Phen co-polymer (PKMOTb), MMA-co-DPOco-Tb(MA)(acac)2 Phen co-polymer (PMOTb), and poly(Nvinylcarbazole) (PVK). Here, NVK: N-vinylcarbazole; MMA: methylmethacrylate; DPO: 2-phenyl-5-[(methacrylate amino) phenyl]-1,3,4-oxadiazole; MA: methacrylate; acac: acetylacetone; and Phen: 1,10-phenanthroline. The details on synthesis and characterization of PKMOTb and PMOTb will be reported later. The PVK is a commercial product from Aldrich. The molecular weight (MW ) of PKMOTb is 22,935 (by GPC). The polymers have decomposition temperature (Td ) above 325 ◦ C recorded as 5% weight loss by thermal gravity analysis.

3. Results and discussion The PKMOTb’s film photoluminescence (PL) spectrum was given in Fig. 1. The broad band starting at 350 nm is generated from both carbazole and DPO [15], where the peak at 405 nm should be attributed to excimer emission from total overlap carbazole groups as is explained in our previous report [24]. However, no obvious evidence of exciplex emission at 420 nm [15] from an interaction of carbazole and DPO moieties is observed in this polymer film. This could be explained that the introduction of inert segment

2.2. Measurement The materials’ photoluminescent properties were studied with a Hitachi-850 Photoluminescence Spectroscopy. The samples for PL and EX study were prepared by spin coating a polymer film onto a thoroughly cleansed glass substrate from

CH2

CH N

x

CH2

CH3 C

y

COOCH3

CH2

CH

HN

z

CH3 C

CH3

O

N

O

Tb

O O

N N

k

O

O

2

N

PKMOTb: x=0.99, y=1.01, z=0.35, k=0.05 PMOTb: x=0, y=1.01, z=0.25 PVK: x=1, y=z=k=0 Scheme 1. Chemical structure of PKMOTb, PMOTb, and PVK.

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1.2

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PKMOTb 40

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Fig. 1. PL spectrum of PKMOTb film (excited wavelength: 298 nm).

Fig. 2. Excitation (EX) spectra of PKMOTb (—) and PMOTb (- - -). Absorbance (Abs) spectra of PVK (· · · · · · ) and PMOTb (–·–·–).

methylmethacrylate (MMA) makes the distance between the carbazole and oxadiazole groups too large for formation of such an exciplex. The sharp peaks at 490 and 545 nm in Fig. 1 should be ascribed to 5 D4 –7 F6 and 5 D4 –7 F6 transitions of 4f orbital in Tb3+ ion, respectively [26]. These Tb3+ ion’s orbital transitions are excited by energy transferred from ligand triplet, and the latter is produced by energy intersystem cross (ISC) from ligand singlet. Our previous work showed that the Tb3+ ion’s emission would be enhanced by Forster energy transferred from carbazole groups in a co-polymer containing both N-vinylcarbazole and Tb(MA)(acac)2 Phen [17,25]. The Forster energy transfer rate (r) is decided by spectra overlap integral (J) and donor–acceptor distance (R): r ∝ J/R6 . Since DPO group has similar emission spectra as that of carbazole; one could expect the possibility of observing energy transfer from the former to Tb complex moieties as well. To clarify which group, carbazole or DPO, dominates the strong emission from Tb3+ ions, the excitation (EX) spectrum of PKMOTb was recorded by monitoring the emission peak at 545 nm (Fig. 2). For comparison, the EX spectrum of PMOTb (without the active NVK group) concentrated on 545 nm was also presented in Fig. 2. It is found that PMOTb’s EX spectrum is similar to that of its UV-Vis absorbance curve, which is also presented in Fig. 2. This result indicates the existence of energy transfer from DPO groups to lanthanide complexes. However, the EX spectra of PKMOTb is an analogy of PVK’s UV-Vis absorbance curve (Fig. 2), indicating that the excitation of Tb complex is, in most part, via energy transfer from carbazole groups. Therefore, we can come to the conclusion that though energy was transferred from both groups to Tb complex moieties, the energy from carbazole might greatly excel that from DPO group and thus dominates the enhancement of emission at 545 nm. It should be noted that the emission spectra of carbazole and DPO are very similar [27], and hence their overlap area

with absorption spectra of the Tb complex moieties. Therefore, their Forster transfer rates should be practically the same. As a consequence, there is no correlation between the rate sand the observed efficiency, suggesting that other mechanisms might take place. Further work needs to be done to clarify these mechanisms, which will be our future investigation. A LED was fabricated with the structure ITO/PKMOTb/ Al. Green light is observed when a proper positive bias was applied. Fig. 3 shows the current (I), voltage (V), and brightness (B) curve of this device. It is found that when the bias is above 6.2 V (turn on voltage Vto ), a green light was observed. This Vto is much lower than that of our previously reported device (24 V) based on a similar polymer without DPO groups [24]. Though a thinner active layer might partly contribute to the decrease of Vto , it is un-proportional to the nearly four-time-lower dropdown. We suppose that the main factor affecting Vto is the introduction of the electron-transporting DPO group. The DPO group not only facilitates the electron transporting through the bulk of the active polymer layer, but also makes the electron injection from the electrode much more easier because of a better matching between the polymer’s lowest unoccupied molecular orbital (LUMO) and the work function of Al [9]. The electroluminescent (EL) spectrum of this device was given in Fig. 4. At a positive voltage of 8 V, a sharp peak at 545 nm is observed, which is believed to be the characteristic emission from Tb3+ ion. Interestingly, the broad band at 400 nm is almost completely quenched under electrical field, suggesting that this polymer PKMOTb is suitable for narrow-band electroluminescence. The EL process of this single layer device is supposed as follows. Under the driving of applied voltage, holes are injected from ITO anode into the polymer layer and transported through it in a hopping mechanism between the carbazole groups, while electrons are injected from Al cathode into the polymer layer and transported through it

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50

40

8

-4

30 6 20 4 10

Relative Brightness (a.u.)

Current(I) (x10 A)

10

2

0

0 2

4

6

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Voltage (V) (V) Fig. 3. V–I–brightness characteristics of device ITO/PKMOTb/Al.

to Tb complex moieties, with the latter dominating the enhancement of Tb3+ ion’s characteristic sharp emission. A single layer LED was fabricated based on this polymer. Sharp-band green light at 545 nm with half-height width of 10 nm is emitted from this device under the positive bias above 6.2 V. Though the brightness of our device is still unsatisfactory, we have demonstrated a convenient way to design and synthesize polymer suitable for single layer pure-color light emitting diodes.

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EL Intensity(a.u.)

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Acknowledgements 0 400

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Fig. 4. EL spectra of device ITO/PKMOTb/Al at 8 V.

in a similar manner between DPO groups. The holes and electrons recombine at the interface between carbazole and DPO groups to produce excitons, which are transferred to lanthanide complex ligands in Forster mechanism, and the singlets thus obtained are transferred to triplets by intersystem crossing (ISC). The lanthanide ions are excited by intromolecular energy transfer from ligand triplets to central metal ions. The f–f transition in the Tb3+ ions produces visible green light with high purity.

4. Conclusions The film photoluminescence study of PKMOTb reveals that there is no obvious excimer formation between carbazole and oxadiazole groups. There is evident energy transfers from carbazole groups and diphenyloxadiazole groups

This work was financially supported by National Science Foundation of China (No. 29974025). The authors are grateful for the experimental assistance from Mr. Fan Ruixin (State-Key Lab. Silicon Materials, ZJU) in I–V characterization.

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