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* E-mail: qiushuzhang@uestc.edu.cn; Tel.: 0086-028-61830598; Fax: 0086-028-61830598 Received November 20, 2009; revised and accepted April 1, 2010.

1482 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 14821486

Polymer Light-emitting Diodes Based on End-capped Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]

Zhang, Qiushu*() School of Mechatronics Engineering, University of Electronic Science and Technology of China,

Chengdu, Sichuan 611731, China

We demonstrate polymer light-emitting diodes (LEDs) based on poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl] with end capper dimethylphenyl or N,N-bis(4-methylphenyl)-N-phenylamine. The introduction of end-capper groups increased the device luminance and efficiency, while greatly depressing the green emission. For the devices constructed of poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl] end capped with dimethylphenyl, the maximum lumi-nance reached 381 cd/m2 at 122 mA/cm2. The maximum external quantum efficiency was 0.16% at 117 mA/cm2, which is more than five times higher than that of the non-end-capped polymer LEDs. The electroluminescence (EL) maximum was at 485 nm, blue shifted by 52 nm with respect to that of the non-end-capped polyfluorene devices. It is proposed that efficient hole trapping at end capper and increased resistance of polyfluorene to oxidation are re-sponsible for the improved device performance and color stability.

Keywords polymers, light-emitting diodes, polyfluorene, thin films, end-capping

Introduction End-capping is one of the strategies in macromo-

lecular engineering which can be exploited to modify polymer properties to satisfy specific application re-quirements. For polymer light-emitting diode (LED) applications, end-capping has been directed toward im-proving device efficiency,1-4 stabilizing blue emis-sion,1,4-6 tuning the emission color,2,3,7 as well as realiz-ing white electroluminescence.8 As a means for in-creasing polymer LED device performance, end-cap-ping will not result in phase separation (a common phenomenon in polymer blend systems) with time and does not alter the electronic properties of the polymer backbone.1 Furthermore, when end-capped with some hole transport moieties, polyfluorenes can become more resistant to oxidation.9

A few mechanisms have been proposed to explain the increase in polymer LED device efficiency arising from end-capping. Miteva et al. supposed that for their devices based on end-capped polymer, most holes in the emissive layer might be pointed to the end-capper groups instead of sites with less efficient emission, which subsequently recombined with the electrons on the polyfluorene main chain.1 However, Nakazawa et al. believed that the position of the exciton recombination zone has an effect on light emission and device effi-ciency.4 They suggested that end-capping gives rise to improved hole injection so that the recombination zone is moved away from the polymer/anode interface. In addition, energy transfer from polymer backbone to

end-capper could also lead to the enhanced efficiency.3 It was reported that end-capping could significantly

suppress troublesome long wavelength EL band in polyfluorenes, which has been assigned to aggregates/ excimers,10,11 an emissive keto defect generated by vir-tue of thermo-, photo-, or electro-oxidative degrada-tion,12 or a chemical defect located close to the cath-ode.13 Many end-cappers have been found to play an important role in reducing green emission, among which are mono-functional fluorene derivatives,14 anthra-cene,

15 crosslinkable moieties,16,17 hole-trapping

groups,1 sterically hindered groups,18,19 polyhedral oli-gomeric silsesquioxanes,20 and so on. A possible reason for this improvement of polyfluorene blue emission is that the chain ends, rather than aggregates and excimer forming sites, preferentially become the places where the electron-hole recombinations occur.1 In some cases, suppression of low energy emission is ascribed to the shift of the recombination zone away from the polymer/ anode interface, since the polyfluorene molecules only have strong interchain interactions near the anode inter-face but not in the bulk.4 Kadashchuk et al. found that polyfluorenes end capped with certain hole-transporting molecules, like triphenylamine derivatives, demonstrate high stability against oxidation, which leads to consid-erably decreased concentration of keto defects.9

In this work, we fabricated and characterized poly-mer LED devices based on poly[9,9-di-(2'-ethylhexyl)-fluorenyl-2,7-diyl] (PF2/6),21 end capped with dimeth-ylphenyl (DMP) or N,N-bis(4-methylphenyl)-N-phenyl-amine (TPA).4 Compared to PF2/6 based devices,

Polymer Light-emitting Diodes Based on Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]

Chin. J. Chem. 2010, 28, 14821486 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 1483

end-capped PF2/6 devices displayed improved proper-ties in terms of luminance and efficiency. Predominant long wavelength emission observed in PF2/6 was sig-nificantly suppressed in end-capped PF2/6s. The effect of end-capping on the enhancement of device perform-ance and color stability is discussed.

Experimental The chemical structures of PF2/6 and end-capped

PF2/6s are shown in Figure 1. PF2/6 was purchased from Sigma-Aldrich Corporation, United States. It is comprised of yellow-green crystal-like particulates and soluble in common organic solvents such as toluene, xylene, CHCl3 and tetrahydrofuran. Both end-capped PF2/6s were synthesized by American Dye Source, Inc., Canada. They are light yellow powder, and highly solu-ble in toluene and tetrahydrofuran. The weight average molecular weight Mw and polydispersity of DMP- and TPA-end-capped PF2/6 are 69 000, 54 000 and 2.4, 2.5, respectively, as determined by gel permeation chroma-tography in tetrahydrofuran using polystyrene standards.

Figure 1 Chemical structures of light emitting polymers: (a) PF2/6, (b) DMP-end-capped PF2/6, (c) PF2/6 end-capped with TPA.

The devices with a configuration of ITO/PEDOT: PSS/emissive layer/aluminum were fabricated at ambi-ent conditions on glass substrates covered by patterned indium-tin-oxide (ITO) electrodes. The ITO substrates were precleaned by two successive ultrasonic rinses in acetone and isopropyl alcohol. After drying them with a nitrogen gun, a 45-nm thick film of poly(3,4-oxy-ethyleneoxythiophene) doped with poly(styrene sul-

fonate) (PEDOT:PSS) was spin coated over the sub-strate from 1.3 wt% water dispersion. The emissive lay-ers consisted of PF2/6, or end-capped PF2/6. They were formed at the top of PEDOT:PSS films by spin casting from polymer toluene solutions (10 mg/mL) at a speed of 2000 r/min. The film thicknesses are ca. 40 nm for PF2/6, ca. 70 nm for DMP-end-capped PF2/6, and ca. 80 nm for TPA-end-capped PF2/6. Devices were dried in a vacuum chamber at room temperature for a mini-mum of 24 h before the deposition of the Al film. The manufacturing was completed with the thermal evapo-ration of the aluminum cathode (ca. 200 nm) at 2.67104 Pa through a shadow mask. The overlap between the two electrodes gave device active areas of 9 mm2.

The optical absorption (UV-Vis) spectra were meas-ured with an Agilent-8453 UV-visible spectrophotome-ter from polymer films spin cast onto a quartz plate. Photoluminescence (PL) spectra were obtained with a PSI (Photon Technology International, Canada) fluo-rescence spectrometer. All device testing was imple-mented in air at room temperature. Current-voltage characteristics were measured on a Keithley 236 source-measure unit. The power of EL emission was measured through using an ILX Lightwave OMM-6810B optical multimeter equipped with a silicon power/wavehead (OMH-6722B). By assuming Lamber-tian distribution of the EL emission, luminance (cd/m2) was calculated via utilizing the forward output light power and the EL spectra of the devices.

Results and discussion The optical absorption (UV-Vis) spectra of thin

films of PF2/6 and end-capped PF2/6s are shown in Figure 2. The absorption of PF2/6 exhibited an onset at 415 nm and a peak at 362 nm. The UV-Vis spectra of DMP- and TPA-terminated PF2/6 are very similar to each other. The absorption of DMP-end-capped PF2/6 had an onset at 425 nm, the same as that of TPA-end-capped PF2/6. And both spectra exhibited a peak at nearly the same wavelength. However, the opti-

Figure 2 Optical absorption (UV-Vis) spectra of thin films of PF2/6, DMP-end-capped PF2/6 and TPA-end-capped PF2/6.

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cal absorption spectra of PF2/6 with end-capper groups were clearly red-shifted compared to that of PF2/6, which indicates that the conjugation length in end- capped PF2/6s might be longer than that in PF2/6. From the UV-Vis spectra, the band-gap energies were deter-mined to be 2.99 eV for PF2/6 and 2.92 eV for end-capped PF2/6s, respectively.

PL spectra of thin films of PF2/6, DMP-end-capped PF2/6 and TPA-end-capped PF2/6 are presented in Fig-ure 3. The PL emissions from PF2/6 and end-capped PF2/6 thin films showed well-defined vibronic features. The PL of PF2/6 had a maximum at ca. 440 nm. A weaker long-wavelength emission (ca. 530 nm) is pro-nounced in the spectrum. The two end-capped PF2/6s displayed very similar PL spectra with the maximum at ca. 410 nm. The low energy emission bands were com-pletely suppressed in the case of end-capped PF2/6s. End-capping brings about red-shift in absorption and blue-shift in PL emission. As a consequence, the Stokes shift was reduced, which might be due to the increased stiffness of the polymer upon end-capping.

Figure 3 Photoluminescence spectra of thin films of PF2/6, DMP-end-capped PF2/6 and TPA-end-capped PF2/6.

Figure 4 shows the current density-electric field characteristics of polymer LEDs based on PF2/6 and end-capped PF2/6s. The turn-on voltage was around 2 V (5105 V/cm) for PF2/6 based devices, around 5 V (7.14105 V/cm) for DMP-end-capped PF2/6 devices, and around 5 V (6.25105 V/cm) for TPA-end-capped PF2/6 devices, respectively. A crossover is observed between the current density-electric field characteristics of the devices based on PF2/6 and end-capped PF2/6s, which arises at ca. 2.5106 V/cm for DMP-end-capped polymer, and at ca. 2.1106 V/cm for TPA-end-capped polyfluorene. As the charge carrier mobility in conju-gated polymers is field-dependent,22 it is reasonable to assume that the charge carrier mobility in end-capped PF2/6s is more sensitive to the electric field than that in PF2/6 as evidenced by the greater currents in the end-capped PF2/6 devices than PF2/6 devices at the higher fields.

The current dependence of light power for the poly-

Figure 4 Current density-electric field characteristics of the LEDs based on PF2/6 (), DMP-end-capped PF2/6 (), and TPA-end-capped PF2/6 ().

mer LED devices was recorded to calculate device effi-ciencies and brightness. Figure 5 displays luminance as a function of the current density for the three types of devices under study. End-capping decreases the thresh-old current for light emission from 96.9 mA/cm2 to 2.4 (DMP-end-capped PF2/6) and 1.2 mA/cm2 (TPA-end- capped PF2/6), which suggests that there is better bal-ance between holes and electrons within the end-capped PF2/6 emission layer. The charge balance between holes and electrons significantly affects the device efficiency because a surplus of either of the charge carriers results in a current increase that does not enhance the emission, but raises Joule heating which causes more rapid poly-mer degradation.23,24 For PF2/6 active layer, the maxi-mum brightness was 179 cd/m2 at 214 mA/cm2. The maximum luminance efficiency and maximum external quantum efficiency were calculated to be 0.084 cd/A and 0.03% at 214 mA/cm2, respectively. In comparison with the devices based on non-end-capped polymer, end-capping improves EL properties in light of lumi-nance and efficiency. In the case of TPA-terminated PF2/6, the luminance reached 327 cd/m2 at 211 mA/cm2. The maximum luminance efficiency and maximum ex-

Figure 5 Luminance-current density characteristics of the LEDs based on PF2/6 (), DMP-end-capped PF2/6 (), and TPA-end-capped PF2/6 ().

Polymer Light-emitting Diodes Based on Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]

Chin. J. Chem. 2010, 28, 14821486 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 1485

ternal quantum efficiency were 0.156 cd/A and 0.069% at 205 mA/cm2. The best performance was observed for DMP-end-capped PF2/6 devices with a maximum lu-minance of 381 cd/m2 at 122 mA/cm2, a maximum lu-minance efficiency of 0.319 cd/A at 117 mA/cm2, and a maximum external quantum efficiency of 0.16% at 117 mA/cm2, which are higher than the PF2/6 devices by a factor of 2.1, 3.8 and 5.3, respectively.

As shown in Figure 6, end-capping significantly suppresses the long wavelength emission that is over-whelming in the PF2/6 EL spectrum. The blue excitonic emission bands that display the intrinsic characteristics of polyfluorenes become predominant in EL emission from end-capped PF2/6s. It is especially pronounced for DMP-end-capped PF2/6. Its EL peaks at 420, 445 and 485 nm, which all fall into violet-blue region. The maximum intensity is at 485 nm, blue shifted by 52 nm relative to that of PF2/6.

Figure 6 Electroluminescence spectra of the LEDs based on PF2/6, DMP-end-capped PF2/6, and TPA-end-capped PF2/6.

For polyfluorene devices having a calcium cathode, electron-hole recombination is expected to happen close to the anode since electron injection from calcium to polyfluorene is considered to be easier than hole injec-tion from PEDOT:PSS to polyfluorene.25,26 With cal-cium cathode in the device structure, the enhancement of device performance originating from end-capping is therefore believed to correlate with the improved hole injection caused by end-capping, which moves the ex-citon recombination zone from the polymer/anode in-terface into the bulk of the light emitting polymer.4 However, in all of our devices, electron-hole recombi-nation could occur near the cathode because of the poor electron injection from aluminium cathode to polymer. This indicates that the end-capped PF2/6 device proper-ties could have nothing to do with the position of the exciton recombination zone. The increased efficiency and color stability achieved with the end-capped PF2/6 devices might be due to efficient hole trapping at the end-capper groups.1 In the PF2/6 emissive layer, a cer-tain portion of the electron-hole recombinations might occur at sites with less efficient emission, like aggre-

gates or excimer forming sites. However, for the active layer of end-capped PF2/6, the end-cappers may have preference over aggregates or excimer forming sites when they compete with each other with respect to hole trapping. Consequently, most holes could be directed to the end-capper groups, subsequently recombining with the electrons on the polymer main chain. Additionally, it is also likely that end-capping enhances the resistance of polyfluorene to oxidation, hence considerably decreas-ing concentration of keto defects.9 DMP-end-capping could give rise to more efficient hole trapping at the end-capper groups and/or higher resistance of polyfluo-rene to oxidation than TPA-end-capping, which is per-haps responsible for the better device performance ob-served from DMP-end-capped PF2/6 based LEDs.

Besides the end-capper moieties, the end-capper concentration can also affect device properties. End- capping could have an influence upon charge transport and injection. At low end-capper concentration, the current could drop sharply compared to non-end-capped polymer because of severe charge trapping on end- capper group. However, current could increase gradu-ally at higher concentrations since charge transport could be enabled by hopping via end-capper. Moreover, charge injection could be facilitated at higher end-capper concentrations. As shown in Figure 4, the devices based on end-capped PF2/6 exhibited the cur-rent comparable to that of the non-end-capped poly-fluorene devices, which suggests the high end-capper concentration in the end-capped PF2/6s. The same con-clusion can be reached from the fact that the long wavelength emission is nearly completely depressed in the case of end-capped PF2/6 (Figures 3 and 6) as the green emission bands observed from PF2/6 will de-crease continuously with increasing end-capper concen-tration.1 On the other hand, exciton quenching at charged traps might occur.27 Energy of excitons could be transferred to the trapped charges via Forster mecha-nism, which is very efficient at a sufficiently large charge concentration and eventually results in the re-lease of charges from the traps. With the high end-capper concentration, exciton quenching might harm EL emission from the end-capped PF2/6 devices under study. However, the process of carrier recombina-tion and light emission is still dominated by the EL en-hancement resulting from efficient hole trapping at the end-capper groups rather than sites with less efficient emission. Consequently, end-capped PF2/6 devices demonstrated better device properties (Figure 5). Fur-ther investigation is needed concerning the influence of end-capper concentration on polymer LED device per-formance.

Conclusion End-capped PF2/6 was used as active layer to fabri-

cate polymer LED devices. Investigation results demon-strate that end-capping PF2/6 with DMP or TPA clearly

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improves device properties, while significantly sup-pressing green emission bands. The best performance is obtained from devices with a DMP-end-capped PF2/6 emissive layer that exhibited a maximum luminance of 381 cd/m2, a maximum luminance efficiency of 0.319 cd/A, and a maximum external quantum efficiency of 0.16 %, which are 25 times higher than those of the PF2/6 LEDs. All three EL peaks belong to the vio-let-blue zone with a maximum emission at 485 nm, which has a blue shift of 52 nm with respect to that of the PF2/6 based devices. Efficient hole trapping at the end-capper groups as well as increased resistance of polymer to oxidation might lead to the enhancement of device properties and blue emission.

Acknowledgment The author would like to acknowledge David Keith

Chambers and Joseph Cannon at Louisiana Tech Uni-versity for help in device testing.

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