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Time-dependence of erbium(III) tris(8-hydroxyquinolate) near-infrared photoluminescence: implications for organic light-emitting diode efficiency
Synthetic Metals 138 (2003) 463–469
Time-dependence of erbium(III) tris(8-hydroxyquinolate) near-infrared photoluminescence: implications for organic light-emitting diode efficiency S.W. Magennisa, A.J. Fergusonb, T. Brydenb, T.S. Jonesb, A. Beebyc,*, I.D.W. Samuela,* a
b
Ultrafast Photonics Collaboration, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK Ultrafast Photonics Collaboration, Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College, London SW7 2AY, UK c Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK Received 8 July 2002; accepted 17 October 2002
Abstract We report the first time-resolved photoluminescence studies of erbium tris(8-hydroxyquinolate), [ErQ3], as a powder, a thin film blend with polycarbonate, an evaporated film and in DMSO-d6 solution. The 4 I13=2 ! 4 I15=2 transition at 1.5 mm displays biexponential kinetics with photoluminescence lifetimes in the region of 0.2 ms for the powder, 0.4 ms for the thin film blend, 0.9 ms for the evaporated film, and 2 ms in solution. These observed lifetimes are much shorter than the natural radiative lifetimes of this Er3þ transition, implying photoluminescence quantum yields that range from 0.002 to 0.03%. This has significant consequences for the device efficiencies achievable by near IR-emitting organic light-emitting diodes (OLEDs) containing [ErQ3] as the emissive layer. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Erbium; Photoluminescence; 8-Hydroxyquinolate; OLED
1. Introduction The 4 I13=2 ! 4 I15=2 optical transition of the trivalent erbium ion, Er3þ, occurs at 1.5 mm, which falls in one of the standard telecommunications windows, and erbiumdoped silica has been a tremendously successful material for optical amplifiers [1,2]. In recent years, however, there has been a growing interest in using organic materials to replace traditional inorganic optoelectronic components because of advantages such as solution processing, flexibility and low cost [3]. A disadvantage of erbium-doped inorganic matrices is the low concentration of erbium ions attainable, but this could be overcome by incorporating the erbium in a polymer matrix [4]. This can be accomplished by surrounding the erbium ion with organic ligands; the resultant complex will provide the erbium ion with the increased solubility necessary for dispersal at high concentration in a polymer matrix [5,6]. In addition, the organic ligands may serve as sensitisers of Er3þ luminescence, whereby the * Corresponding authors. Tel.: þ44-1334-463114; fax: þ44-1334-463104, þ44-191-384-4737. E-mail addresses: [email protected], [email protected] (A. Beeby).
ligand absorbs strongly and then transfers this energy to the metal ion, circumventing the problem of the erbium ion’s inherently low extinction coefficient [4]. It has been shown that erbium tris(8-hydroxyquinolate), [ErQ3] (see inset of Fig. 1), can be used in organic lightemitting diodes (OLEDs) operating at 1.5 mm [7,8]. The complexes erbium tris(acetylacetonate)(1,10-phenanthroline) [9] and erbium tris(dibenzoylmethanate)(bathophenanthroline) [10] have also been used to make OLEDs. In addition, Curry and Gillin [11] have made a [ErQ3]-based OLED on a silicon substrate and have estimated the internal efficiency to be 0.01%. In the present work, we use timeresolved studies of photoluminescence of [ErQ3] to give a better understanding of the emission process in this material. Due to the sensitivity of Er3þ to its local environment, we have studied the photoluminescence in both solution and the solid state. We report the first example of [ErQ3] incorporated in a polymer host: a thin-film blend of [ErQ3] in polycarbonate. In addition, we have studied films of [ErQ3] produced by vacuum deposition, the fabrication technique utilised by Curry and Gillin [7,8,11] in their work on [ErQ3]. The time-resolved photoluminescence measurements are particularly significant in the context of [ErQ3]containing OLED technologies.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 5 0 1 - 5
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Fig. 1. PL spectrum of spin-coated [ErQ3] in polycarbonate (15%, w/w), following excitation at 355 nm; inset shows the structure of [ErQ3].
2. Experimental 2.1. Materials [ErQ3] was prepared by the addition of a methanolic solution of erbium(III) chloride (Aldrich) to a methanolic solution of 8-hydroxyquinoline (Aldrich). The yellow precipitate that immediately formed was filtered, washed repeatedly with methanol (HPLC grade, Fisher) and dried under vacuum to give the desired product. The FT-IR spectrum of a KBr disc of this product matched perfectly with a previously reported spectrum [12]; the FAB (þ) mass spectrum showed peaks centred at m/z 454, corresponding to [ErQ2]þ; and only one component was evident by TLC on silica. (Found: C, 49.59; H, 3.22; N, 6.38%. C27H18ErN3O33H2O requires C, 49.60; H, 3.70; N, 6.43%.) For the preparation of the thin film blend, solutions of [ErQ3] in methanol-d4 (44 mg/ml) and polycarbonate (Acros, approx. MW 64,000) in dry THF (85 mg/ml) were mixed together in a ratio of 1:3 v/v to give a final concentration of ca. 15% (w/w), and the resultant mixture was spin coated at 1500 rpm for 1 min to give ca. 300 nm thick films. Film thickness was measured using a Dektak stylus profiler. Solutions and films were prepared in a glove box and sealed under nitrogen until used. For solution studies, [ErQ3] was dissolved in DMSO-d6 and used immediately. Deuterated solvents were obtained from Aldrich and used as received. Evaporated [ErQ3] films were grown in an organic molecular beam deposition (OMBD) chamber with a base pressure of 5 109 Torr [13,14]. The [ErQ3] powder was outgassed for 72 h at 200 8C prior to being sublimed from an effusion cell onto quartz substrates. The substrates were sonicated in methanol for 10 min and dried in a stream of
nitrogen prior to being transferred into the vacuum chamber. The [ErQ3] was sublimed at a temperature of 330 8C, which ˚ s1. The growth corresponds to a growth rate of 13 A rate was determined using a quartz crystal microbalance positioned close to the substrate. The temperature of the substrates during deposition was 25 8C. The structure and morphology of the evaporated [ErQ3] films were analysed using atomic force microscopy (AFM) (Burleigh Instruments, Metris-2000) and powder X-ray diffraction (XRD) (Phillips, PW1700). The AFM images were acquired in non-contact mode and were filtered using line and plane subtraction routines. The XRD data was acquired using Cu Ka radiation and a step size of 0.048 for 2y scans. 2.2. Photophysical measurements Samples were illuminated over an area of 1 cm2 with the 355 nm Q-switched output of a Nd:YAG laser (Spectra Physics) operating at 10 Hz. Typical pulse energies were 3–4 mJ with a pulse FWHM of 8 ns. The IR photoluminescence was collected at right angles and focussed onto the entrance slit of a Jobin-Yvon Triax 320 monochromator with a grating blazed at 1 mm and was detected with a nitrogen-cooled germanium photodiode/amplifier (North Coast EO-817P) operating in high sensitivity mode. The signal was captured and averaged by a digital storage oscilloscope (Tektronix TDS320) and transferred to a PC for data analysis. Decays were analysed by iterative reconvolution and non-linear least-squares analysis of the instrument response profile with single, biexponential or triexponential functions. The quality of the fits was assessed by the randomness of the residuals and a satisfactory reduced chi-squared. Further details of the
S.W. Magennis et al. / Synthetic Metals 138 (2003) 463–469
experimental technique are published elsewhere [15]. The fractional intensities (fi) were calculated from the values of lifetimes (ti) and pre-exponential factors (Ai) as follows: f1 ¼ A1 t1 =ðA1 t1 þ A2 t2 Þ and f2 ¼ A2 t2 =ðA1 t1 þ A2 t2 Þ. Photoluminescence spectra were generated using the time-resolved setup described above by measuring the decay of the luminescence intensity at 2.5 nm wavelength intervals followed by integration of the area under the decay to give the total emission intensity at each wavelength.
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Table 1 Time-resolved PL data for [ErQ3] in the solid state and solution; the excitation wavelength was 355 nm and the detection wavelength was 1500 nm (the fractional intensity of each decay component is given in parentheses) [ErQ3] sample
Lifetimes (ms) t1
t2
3. Results and discussion
Powder Polycarbonate blend film 20 nm evaporated film 100 nm evaporated film 200 nm evaporated film DMSO-d6
0.13 (71%) 0.21 (37%) 0.18 (10%) 0.29 (11%) 0.24 (8%) 1.2 (14%)
0.42 (29%) 0.75 (63%) 1.5 (90%) 1.2 (89%) 1.1 (92%) 2.9 (86%)
Photoluminescence (PL) measurements were made for [ErQ3] as a powder, as evaporated thin films (with 20, 100 and 200 nm thickness), in a DMSO-d6 solution and as a spincoated thin film, in which the erbium complex is dispersed in a polycarbonate polymer matrix ([ErQ3]:polycarbonate 15% (w/w)). For each sample, excitation into ligand-centred bands at 355 nm leads to the characteristic emission at 1.5 mm due to the intraconfigurational 4 I13=2 ! 4 I15=2 transition. A representative PL spectrum, in this case for the spin-coated film, is shown in Fig. 1. The time-dependence of this emission was examined, following excitation at 355 nm, and the results are summarised in Table 1. For the powder sample of [ErQ3], the decay was best represented by a biexponential process with lifetimes of 0.13 ms and 0.42 ms and fractional intensities of 71% and 29%, respectively. Similarly, two components were required to adequately describe the decay kinetics of the thin film of [ErQ3] blended with polycarbonate. In this case, the measured lifetimes were 0.21 and 0.75 ms, with the longer lifetime now the dominant component (63% fractional intensity). The decay of the emission from the evaporated
films was also biexponential, and was independent of film thickness (within experimental error); for example, the data for the 200 nm thick film was fitted with lifetimes of 1.1 ms (92%) and 0.24 ms (8%). For the solution of [ErQ3] in DMSO-d6, three components were used to fit the observed data: two exponential decay components with lifetimes of 2.9 ms (86%) and 1.2 ms (14%) and a rise time component of ca. 30 ns. Rise times are sometimes observed in time-resolved measurements of lanthanide ion luminescence and arise from non-radiative relaxation of the initially generated higher excited states of the metal ion to the emissive levels. Although we are not aware of any similar observations for other Er3þ complexes, this may reflect the experimental difficulties in working with this ion (e.g. the rise time in Eu3þ luminescence due to the 5 D1 ! 5 D0 transition is on the microsecond timescale). The decay curve, instrument response function and fit obtained for the powder, thin film blend, evaporated film (200 nm thick) and solution samples are shown in Figs. 2–5,
Fig. 2. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of powdered [ErQ3], following excitation at 355 nm.
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Fig. 3. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of [ErQ3] in a thin-film blend with PC, following excitation at 355 nm.
respectively. Excellent fits are obtained; the measured decay and fit are indistinguishable in the figures. The biexponential nature of the decays for all of these samples reflects the presence of different local environments for Er3þ ions and is most probably due to the presence of [ErQ3] with different numbers of coordinated solvent molecules. The coordination number of lanthanide ions is typically 8 or 9, depending on the nature of the ligand(s), with any vacant coordination sites filled by solvent molecules. It is well established that the luminescence of lanthanide ions can be efficiently quenched via vibronic coupling with ligands and solvent molecules. For the sample used in this study, it is likely that there will be solvent molecules
coordinated directly to the Er3þ because the 8-hydroxyquinolate ligands will not provide sufficient steric bulk to completely surround the metal ion. It has recently been demonstrated that the rate of water exchange for lanthanide complexes can be sufficiently slow that biexponential decays are observed, corresponding to complexes with different numbers of coordinated solvent molecules [16]. This would also explain the behaviour observed for the [ErQ3] samples. There is a significant variation in observed [ErQ3] PL lifetimes between the different samples (Table 1). The lifetime is much longer in solution than in the solid state and we believe that this is due to replacement of coordinated water or
Fig. 4. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of a 200 nm thick evaporated film of [ErQ3], following excitation at 355 nm.
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Fig. 5. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of [ErQ3] in DMSO-d6, following excitation at 355 nm.
methanol molecules by DMSO-d6 molecules; such preferential binding of DMSO is well known [15]. Since the efficiency of the vibronic deactivation process is related to the number of vibrational quanta that are required to bridge the energy gap between the lowest emissive state and the highest non-emissive state of the lanthanide ion [17], the removal of the high energy O–H oscillators (nOH 3400 cm1) leads to an increase in lifetime. As the 4 I13=2 ! 4 I15=2 energy gap for Er3þ is only ca. 6500 cm1, it can also be efficiently bridged by other lower energy oscillators such as C–H (nCH
2950 cm1). Thus, the use of deuterated DMSO as solvent (nCD 21002200 cm1) will also have contributed to the increase in lifetime observed in solution. For the preparation of the spin-coated sample, the polycarbonate was dissolved in dry THF, and the [ErQ3] dissolved in dry deuterated methanol. The longer lifetime observed for the spin-coated sample relative to the powder may be due to replacement of innersphere water or methanol molecules with deuterated methanol or by replacement of outer-sphere solvent molecules, which can also provide an important non-radiative pathway for lanthanide ions [18]. The evaporated films displayed lifetimes in between those for the spin coated thin film and DMSO-d6 solution, and this can be explained by solvent removal during purification of the material in vacuum and the subsequent deposition process. The longest PL lifetime observed for [ErQ3] in this study was ca. 3 ms for the DMSO-d6 solution. In contrast, typical radiative lifetimes for the 4 I13=2 ! 4 I15=2 transition of Er3þ are on the millisecond timescale, and this is the case whether the Er3þ ion is in an inorganic matrix [2] or is complexed with organic ligands [19]. The difference between these inorganic and organic hosts is that in the former case the observed lifetimes are often comparable to the radiative lifetime. When the metal ion is bound to organic ligands,
however, the luminescence is significantly quenched due to the deactivation mechanisms already discussed. For example, reported natural radiative lifetimes for erbium complexes are in the range 4–14 ms, whilst observed lifetimes for the same complexes are in the range 0.5–3 ms [6,19,20]. We can estimate the PL quantum yield (PLQY) for the erbium-centred 4 I13=2 ! 4 I15=2 transition at 1.5 mm in [ErQ3] from our lifetime measurements. We take a radiative lifetime, tr, of 8 ms, which is an average of lifetimes reported for erbium complexes. We assume this lifetime applies to both components of our measured decays. The PLQY is then estimated using ðA1 t1 þ A2 t2 Þ=tr , where A1 and A2 are the pre-exponential factors, normalised such that A1 þ A2 ¼ 1, and t1 and t2 are the decay times for the two components. Using this approach, we estimate the PLQYof [ErQ3] to be ca. 0.002% as a powder, 0.005% as a thin-film blend with polycarbonate, 0.01% as an evaporated film, and 0.03% as a DMSO-d6 solution. Whilst these are the first reports of the time-dependence of the PL of [ErQ3], the time-dependence of the electroluminescence (EL) of [ErQ3] has been briefly mentioned previously [21]. The reference describes erbium-related EL
Fig. 6. AFM image of a 100 nm evaporated [ErQ3] film.
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Fig. 7. Powder XRD pattern of a 100 nm evaporated [ErQ3] film.
with two distinct lifetimes: 0.23 and 2.2 ms. These are much longer than the PL lifetimes we have measured and, as discussed, are inconsistent with values reported for other erbium complexes [6,19,20,22,23]. In addition, the PL lifetime of the related complex [NdQ3] in DMSO-d6 is reported to be 2 ms [24], which is also very short compared to the natural radiative lifetime of Nd3þ. The energy gap for Nd3þ (ca. 5300 cm1) is only slightly smaller than that of Er3þ, so the lifetimes in isostructural erbium(III) and neodymium(III) complexes are usually similar, in agreement with our measured [ErQ3] lifetime in DMSO-d6. We speculate that the EL lifetimes previously reported may be either due to [ErQ3] that has decomposed or to a delayed emission resulting from the EL process. Our time-dependent PL studies suggest that the low efficiency of 0.01% that was estimated for a [ErQ3] OLED device [11] may actually be close to the maximum device efficiency possible when [ErQ3] is the emissive material. It matches the PLQY that we have estimated for films produced by evaporation, which is the same technique that was used to prepare the [ErQ3] OLEDs. The structure and morphology of the evaporated [ErQ3] films used in this study were analysed using AFM and powder XRD. The AFM image of a 100 nm film (Fig. 6) shows elongated crystallites with a lateral size of ca. 40 nm. The peak observed in XRD at 2y ¼ 8:24 (Fig. 7) corre˚ . This is consistent with a sponds to a cell spacing of 10.7 A previous report of the spacing of the (1 0 0) plane of monoclinic AlQ3 [25]. Fitting the XRD peak with a single Gaussian yields an average crystalline domain size of ca. 6 nm. The size of the features observed in AFM compared to those found by XRD implies the film is mostly amorphous, consisting of small crystalline domains. The AFM and XRD dimensions are unchanged by post-growth annealing or by varying the growth rate (data not shown). The low signal to
noise observed in the XRD scans is also consistent with low crystallinity.
4. Conclusions We have examined the time-dependence of the nearinfrared emission of [ErQ3] as a powder, spin-coated thin film in a polymer matrix, evaporated film and as a DMSO-d6 solution; the PL lifetimes range from 0.13 to 2.9 ms. The observed rate of decay of the Er3þ excited state is explained by efficient non-radiative decay via vibronic coupling with ligand and solvent oscillators. The dramatic reduction in lifetime for [ErQ3] compared with the millisecond radiative lifetime of the Er3þ ion means that the efficiency of [ErQ3] luminescence is in the range 0.002–0.03%, which explains the very low efficiencies that have been reported for [ErQ3]based OLEDs. These results demonstrate that solvent and ligand deactivation processes will need to be addressed in order to make efficient erbium-based organic thin film optoelectronic devices.
Acknowledgements We are grateful to the EPSRC Ultrafast Photonics Collaboration, and SHEFC for financial support. IDWS is a Royal Society University Research Fellow.
References [1] R.J. Mears, L. Reekie, I.M. Jauncey, D.N. Payne, Electron. Lett. 23 (1987) 1026.
S.W. Magennis et al. / Synthetic Metals 138 (2003) 463–469 [2] E. Desurvire, Phys. Today 97 (1994) 20. [3] A.G. MacDiarmid, Angew. Chem. Int. Ed. 40 (2001) 2581. [4] M.P. Oude Wolbers, F.C.J.M. van Veggel, B.H.M. Snellink-Rue¨ l, J.W. Hofstraat, F.A.J. Guerts, D.N. Reinhoudt, J. Chem. Soc., Perkin Trans. 2 (1998) 2141. [5] S. Lin, R.J. Feuerstein, A.R. Mickelson, J. Appl. Phys. 79 (1996) 2868. [6] S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, D.N. Reinhoudt, L.H. Slooff, A. Polman, J.W. Hofstraat, J. Appl. Phys. 86 (1999) 1181. [7] W.P. Gillin, R.J. Curry, Appl. Phys. Lett. 74 (1999) 798. [8] R.J. Curry, W.P. Gillin, Appl. Phys. Lett. 75 (1999) 1380. [9] R.G. Sun, Y.Z. Wang, Q.B. Zheng, H.J. Zhang, A.J. Epstein, J. Appl. Phys. 87 (2000) 7589. [10] Y. Kawamura, Y. Wada, S. Yanagida, Jpn. J. Appl. Phys. 1 40 (2001) 350. [11] R.J. Curry, W.P. Gillin, Appl. Phys. Lett. 77 (2000) 2271. [12] H.F. Aly, F.M. Abdel Kerim, A.T. Kandil, J. Inorg. Nucl. Chem. 33 (1971) 4340. [13] S. Heutz, A.J. Ferguson, G. Rumbles, T.S. Jones, Org. Electron. 3 (2002) 119. [14] S. Heutz, T.S. Jones, J. Appl. Phys. 92 (2002) 3039.
469
[15] A. Beeby, S. Faulkner, Chem. Phys. Lett. 266 (1997) 116. [16] A.S. Batsanov, A. Beeby, J.I. Bruce, J.A.K. Howard, A.M. Kenwright, D. Parker, Chem. Commun. (1999) 1011. [17] G. Stein, E. Wu¨ rzberg, J. Chem. Phys. 62 (1975) 208. [18] A. Beeby, I.M. Clarkson, R.S. Dickins, S. Faulkner, D. Parker, L. Royle, A.S. de Sousa, J.A.G. Williams, M. Woods, J. Chem. Soc., Perkin Trans. 2 (1999) 493. [19] L.H. Sloof, A. Polman, M.P. Oude Wolbers, F.C.J.M. van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 83 (1998) 497. [20] M.H.V. Werts, J.W. Verhoeven, J.W. Hofstraat, J. Chem. Soc., Perkin Trans. 2 (2000) 433. [21] R.J. Curry, W.P. Gillin, Synth. Met. 111–112 (2000) 35. [22] G.A. Hebbink, S.I. Klink, P.G.B. Oude Alink, F.C.J.M. van Veggel, Inorg. Chim. Acta 317 (2001) 114. [23] S.I. Klink, L. Grave, D.N. Reinhoudt, F.C.J.M. van Veggel, M.H.V. Werts, F.A.J.G. Geurts, J.W. Hofstraat, J. Phys. Chem. A 104 (2000) 5457. [24] M. Iwamuro, T. Adachi, Y. Wada, T. Kitamura, N. Nakashima, S. Yanagida, Bull. Chem. Soc. Jpn. 73 (2000) 1359. [25] M. Brinkman, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi, J. Am. Chem. Soc. 122 (2000) 5147.
Time-dependence of erbium(III) tris(8-hydroxyquinolate) near-infrared photoluminescence: implications for organic light-emitting diode efficiency S.W. Magennisa, A.J. Fergusonb, T. Brydenb, T.S. Jonesb, A. Beebyc,*, I.D.W. Samuela,* a
b
Ultrafast Photonics Collaboration, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK Ultrafast Photonics Collaboration, Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College, London SW7 2AY, UK c Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK Received 8 July 2002; accepted 17 October 2002
Abstract We report the first time-resolved photoluminescence studies of erbium tris(8-hydroxyquinolate), [ErQ3], as a powder, a thin film blend with polycarbonate, an evaporated film and in DMSO-d6 solution. The 4 I13=2 ! 4 I15=2 transition at 1.5 mm displays biexponential kinetics with photoluminescence lifetimes in the region of 0.2 ms for the powder, 0.4 ms for the thin film blend, 0.9 ms for the evaporated film, and 2 ms in solution. These observed lifetimes are much shorter than the natural radiative lifetimes of this Er3þ transition, implying photoluminescence quantum yields that range from 0.002 to 0.03%. This has significant consequences for the device efficiencies achievable by near IR-emitting organic light-emitting diodes (OLEDs) containing [ErQ3] as the emissive layer. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Erbium; Photoluminescence; 8-Hydroxyquinolate; OLED
1. Introduction The 4 I13=2 ! 4 I15=2 optical transition of the trivalent erbium ion, Er3þ, occurs at 1.5 mm, which falls in one of the standard telecommunications windows, and erbiumdoped silica has been a tremendously successful material for optical amplifiers [1,2]. In recent years, however, there has been a growing interest in using organic materials to replace traditional inorganic optoelectronic components because of advantages such as solution processing, flexibility and low cost [3]. A disadvantage of erbium-doped inorganic matrices is the low concentration of erbium ions attainable, but this could be overcome by incorporating the erbium in a polymer matrix [4]. This can be accomplished by surrounding the erbium ion with organic ligands; the resultant complex will provide the erbium ion with the increased solubility necessary for dispersal at high concentration in a polymer matrix [5,6]. In addition, the organic ligands may serve as sensitisers of Er3þ luminescence, whereby the * Corresponding authors. Tel.: þ44-1334-463114; fax: þ44-1334-463104, þ44-191-384-4737. E-mail addresses: [email protected], [email protected] (A. Beeby).
ligand absorbs strongly and then transfers this energy to the metal ion, circumventing the problem of the erbium ion’s inherently low extinction coefficient [4]. It has been shown that erbium tris(8-hydroxyquinolate), [ErQ3] (see inset of Fig. 1), can be used in organic lightemitting diodes (OLEDs) operating at 1.5 mm [7,8]. The complexes erbium tris(acetylacetonate)(1,10-phenanthroline) [9] and erbium tris(dibenzoylmethanate)(bathophenanthroline) [10] have also been used to make OLEDs. In addition, Curry and Gillin [11] have made a [ErQ3]-based OLED on a silicon substrate and have estimated the internal efficiency to be 0.01%. In the present work, we use timeresolved studies of photoluminescence of [ErQ3] to give a better understanding of the emission process in this material. Due to the sensitivity of Er3þ to its local environment, we have studied the photoluminescence in both solution and the solid state. We report the first example of [ErQ3] incorporated in a polymer host: a thin-film blend of [ErQ3] in polycarbonate. In addition, we have studied films of [ErQ3] produced by vacuum deposition, the fabrication technique utilised by Curry and Gillin [7,8,11] in their work on [ErQ3]. The time-resolved photoluminescence measurements are particularly significant in the context of [ErQ3]containing OLED technologies.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 5 0 1 - 5
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Fig. 1. PL spectrum of spin-coated [ErQ3] in polycarbonate (15%, w/w), following excitation at 355 nm; inset shows the structure of [ErQ3].
2. Experimental 2.1. Materials [ErQ3] was prepared by the addition of a methanolic solution of erbium(III) chloride (Aldrich) to a methanolic solution of 8-hydroxyquinoline (Aldrich). The yellow precipitate that immediately formed was filtered, washed repeatedly with methanol (HPLC grade, Fisher) and dried under vacuum to give the desired product. The FT-IR spectrum of a KBr disc of this product matched perfectly with a previously reported spectrum [12]; the FAB (þ) mass spectrum showed peaks centred at m/z 454, corresponding to [ErQ2]þ; and only one component was evident by TLC on silica. (Found: C, 49.59; H, 3.22; N, 6.38%. C27H18ErN3O33H2O requires C, 49.60; H, 3.70; N, 6.43%.) For the preparation of the thin film blend, solutions of [ErQ3] in methanol-d4 (44 mg/ml) and polycarbonate (Acros, approx. MW 64,000) in dry THF (85 mg/ml) were mixed together in a ratio of 1:3 v/v to give a final concentration of ca. 15% (w/w), and the resultant mixture was spin coated at 1500 rpm for 1 min to give ca. 300 nm thick films. Film thickness was measured using a Dektak stylus profiler. Solutions and films were prepared in a glove box and sealed under nitrogen until used. For solution studies, [ErQ3] was dissolved in DMSO-d6 and used immediately. Deuterated solvents were obtained from Aldrich and used as received. Evaporated [ErQ3] films were grown in an organic molecular beam deposition (OMBD) chamber with a base pressure of 5 109 Torr [13,14]. The [ErQ3] powder was outgassed for 72 h at 200 8C prior to being sublimed from an effusion cell onto quartz substrates. The substrates were sonicated in methanol for 10 min and dried in a stream of
nitrogen prior to being transferred into the vacuum chamber. The [ErQ3] was sublimed at a temperature of 330 8C, which ˚ s1. The growth corresponds to a growth rate of 13 A rate was determined using a quartz crystal microbalance positioned close to the substrate. The temperature of the substrates during deposition was 25 8C. The structure and morphology of the evaporated [ErQ3] films were analysed using atomic force microscopy (AFM) (Burleigh Instruments, Metris-2000) and powder X-ray diffraction (XRD) (Phillips, PW1700). The AFM images were acquired in non-contact mode and were filtered using line and plane subtraction routines. The XRD data was acquired using Cu Ka radiation and a step size of 0.048 for 2y scans. 2.2. Photophysical measurements Samples were illuminated over an area of 1 cm2 with the 355 nm Q-switched output of a Nd:YAG laser (Spectra Physics) operating at 10 Hz. Typical pulse energies were 3–4 mJ with a pulse FWHM of 8 ns. The IR photoluminescence was collected at right angles and focussed onto the entrance slit of a Jobin-Yvon Triax 320 monochromator with a grating blazed at 1 mm and was detected with a nitrogen-cooled germanium photodiode/amplifier (North Coast EO-817P) operating in high sensitivity mode. The signal was captured and averaged by a digital storage oscilloscope (Tektronix TDS320) and transferred to a PC for data analysis. Decays were analysed by iterative reconvolution and non-linear least-squares analysis of the instrument response profile with single, biexponential or triexponential functions. The quality of the fits was assessed by the randomness of the residuals and a satisfactory reduced chi-squared. Further details of the
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experimental technique are published elsewhere [15]. The fractional intensities (fi) were calculated from the values of lifetimes (ti) and pre-exponential factors (Ai) as follows: f1 ¼ A1 t1 =ðA1 t1 þ A2 t2 Þ and f2 ¼ A2 t2 =ðA1 t1 þ A2 t2 Þ. Photoluminescence spectra were generated using the time-resolved setup described above by measuring the decay of the luminescence intensity at 2.5 nm wavelength intervals followed by integration of the area under the decay to give the total emission intensity at each wavelength.
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Table 1 Time-resolved PL data for [ErQ3] in the solid state and solution; the excitation wavelength was 355 nm and the detection wavelength was 1500 nm (the fractional intensity of each decay component is given in parentheses) [ErQ3] sample
Lifetimes (ms) t1
t2
3. Results and discussion
Powder Polycarbonate blend film 20 nm evaporated film 100 nm evaporated film 200 nm evaporated film DMSO-d6
0.13 (71%) 0.21 (37%) 0.18 (10%) 0.29 (11%) 0.24 (8%) 1.2 (14%)
0.42 (29%) 0.75 (63%) 1.5 (90%) 1.2 (89%) 1.1 (92%) 2.9 (86%)
Photoluminescence (PL) measurements were made for [ErQ3] as a powder, as evaporated thin films (with 20, 100 and 200 nm thickness), in a DMSO-d6 solution and as a spincoated thin film, in which the erbium complex is dispersed in a polycarbonate polymer matrix ([ErQ3]:polycarbonate 15% (w/w)). For each sample, excitation into ligand-centred bands at 355 nm leads to the characteristic emission at 1.5 mm due to the intraconfigurational 4 I13=2 ! 4 I15=2 transition. A representative PL spectrum, in this case for the spin-coated film, is shown in Fig. 1. The time-dependence of this emission was examined, following excitation at 355 nm, and the results are summarised in Table 1. For the powder sample of [ErQ3], the decay was best represented by a biexponential process with lifetimes of 0.13 ms and 0.42 ms and fractional intensities of 71% and 29%, respectively. Similarly, two components were required to adequately describe the decay kinetics of the thin film of [ErQ3] blended with polycarbonate. In this case, the measured lifetimes were 0.21 and 0.75 ms, with the longer lifetime now the dominant component (63% fractional intensity). The decay of the emission from the evaporated
films was also biexponential, and was independent of film thickness (within experimental error); for example, the data for the 200 nm thick film was fitted with lifetimes of 1.1 ms (92%) and 0.24 ms (8%). For the solution of [ErQ3] in DMSO-d6, three components were used to fit the observed data: two exponential decay components with lifetimes of 2.9 ms (86%) and 1.2 ms (14%) and a rise time component of ca. 30 ns. Rise times are sometimes observed in time-resolved measurements of lanthanide ion luminescence and arise from non-radiative relaxation of the initially generated higher excited states of the metal ion to the emissive levels. Although we are not aware of any similar observations for other Er3þ complexes, this may reflect the experimental difficulties in working with this ion (e.g. the rise time in Eu3þ luminescence due to the 5 D1 ! 5 D0 transition is on the microsecond timescale). The decay curve, instrument response function and fit obtained for the powder, thin film blend, evaporated film (200 nm thick) and solution samples are shown in Figs. 2–5,
Fig. 2. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of powdered [ErQ3], following excitation at 355 nm.
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Fig. 3. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of [ErQ3] in a thin-film blend with PC, following excitation at 355 nm.
respectively. Excellent fits are obtained; the measured decay and fit are indistinguishable in the figures. The biexponential nature of the decays for all of these samples reflects the presence of different local environments for Er3þ ions and is most probably due to the presence of [ErQ3] with different numbers of coordinated solvent molecules. The coordination number of lanthanide ions is typically 8 or 9, depending on the nature of the ligand(s), with any vacant coordination sites filled by solvent molecules. It is well established that the luminescence of lanthanide ions can be efficiently quenched via vibronic coupling with ligands and solvent molecules. For the sample used in this study, it is likely that there will be solvent molecules
coordinated directly to the Er3þ because the 8-hydroxyquinolate ligands will not provide sufficient steric bulk to completely surround the metal ion. It has recently been demonstrated that the rate of water exchange for lanthanide complexes can be sufficiently slow that biexponential decays are observed, corresponding to complexes with different numbers of coordinated solvent molecules [16]. This would also explain the behaviour observed for the [ErQ3] samples. There is a significant variation in observed [ErQ3] PL lifetimes between the different samples (Table 1). The lifetime is much longer in solution than in the solid state and we believe that this is due to replacement of coordinated water or
Fig. 4. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of a 200 nm thick evaporated film of [ErQ3], following excitation at 355 nm.
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467
Fig. 5. PL decay dynamics of the 4 I13=2 ! 4 I15=2 transition of [ErQ3] in DMSO-d6, following excitation at 355 nm.
methanol molecules by DMSO-d6 molecules; such preferential binding of DMSO is well known [15]. Since the efficiency of the vibronic deactivation process is related to the number of vibrational quanta that are required to bridge the energy gap between the lowest emissive state and the highest non-emissive state of the lanthanide ion [17], the removal of the high energy O–H oscillators (nOH 3400 cm1) leads to an increase in lifetime. As the 4 I13=2 ! 4 I15=2 energy gap for Er3þ is only ca. 6500 cm1, it can also be efficiently bridged by other lower energy oscillators such as C–H (nCH
2950 cm1). Thus, the use of deuterated DMSO as solvent (nCD 21002200 cm1) will also have contributed to the increase in lifetime observed in solution. For the preparation of the spin-coated sample, the polycarbonate was dissolved in dry THF, and the [ErQ3] dissolved in dry deuterated methanol. The longer lifetime observed for the spin-coated sample relative to the powder may be due to replacement of innersphere water or methanol molecules with deuterated methanol or by replacement of outer-sphere solvent molecules, which can also provide an important non-radiative pathway for lanthanide ions [18]. The evaporated films displayed lifetimes in between those for the spin coated thin film and DMSO-d6 solution, and this can be explained by solvent removal during purification of the material in vacuum and the subsequent deposition process. The longest PL lifetime observed for [ErQ3] in this study was ca. 3 ms for the DMSO-d6 solution. In contrast, typical radiative lifetimes for the 4 I13=2 ! 4 I15=2 transition of Er3þ are on the millisecond timescale, and this is the case whether the Er3þ ion is in an inorganic matrix [2] or is complexed with organic ligands [19]. The difference between these inorganic and organic hosts is that in the former case the observed lifetimes are often comparable to the radiative lifetime. When the metal ion is bound to organic ligands,
however, the luminescence is significantly quenched due to the deactivation mechanisms already discussed. For example, reported natural radiative lifetimes for erbium complexes are in the range 4–14 ms, whilst observed lifetimes for the same complexes are in the range 0.5–3 ms [6,19,20]. We can estimate the PL quantum yield (PLQY) for the erbium-centred 4 I13=2 ! 4 I15=2 transition at 1.5 mm in [ErQ3] from our lifetime measurements. We take a radiative lifetime, tr, of 8 ms, which is an average of lifetimes reported for erbium complexes. We assume this lifetime applies to both components of our measured decays. The PLQY is then estimated using ðA1 t1 þ A2 t2 Þ=tr , where A1 and A2 are the pre-exponential factors, normalised such that A1 þ A2 ¼ 1, and t1 and t2 are the decay times for the two components. Using this approach, we estimate the PLQYof [ErQ3] to be ca. 0.002% as a powder, 0.005% as a thin-film blend with polycarbonate, 0.01% as an evaporated film, and 0.03% as a DMSO-d6 solution. Whilst these are the first reports of the time-dependence of the PL of [ErQ3], the time-dependence of the electroluminescence (EL) of [ErQ3] has been briefly mentioned previously [21]. The reference describes erbium-related EL
Fig. 6. AFM image of a 100 nm evaporated [ErQ3] film.
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Fig. 7. Powder XRD pattern of a 100 nm evaporated [ErQ3] film.
with two distinct lifetimes: 0.23 and 2.2 ms. These are much longer than the PL lifetimes we have measured and, as discussed, are inconsistent with values reported for other erbium complexes [6,19,20,22,23]. In addition, the PL lifetime of the related complex [NdQ3] in DMSO-d6 is reported to be 2 ms [24], which is also very short compared to the natural radiative lifetime of Nd3þ. The energy gap for Nd3þ (ca. 5300 cm1) is only slightly smaller than that of Er3þ, so the lifetimes in isostructural erbium(III) and neodymium(III) complexes are usually similar, in agreement with our measured [ErQ3] lifetime in DMSO-d6. We speculate that the EL lifetimes previously reported may be either due to [ErQ3] that has decomposed or to a delayed emission resulting from the EL process. Our time-dependent PL studies suggest that the low efficiency of 0.01% that was estimated for a [ErQ3] OLED device [11] may actually be close to the maximum device efficiency possible when [ErQ3] is the emissive material. It matches the PLQY that we have estimated for films produced by evaporation, which is the same technique that was used to prepare the [ErQ3] OLEDs. The structure and morphology of the evaporated [ErQ3] films used in this study were analysed using AFM and powder XRD. The AFM image of a 100 nm film (Fig. 6) shows elongated crystallites with a lateral size of ca. 40 nm. The peak observed in XRD at 2y ¼ 8:24 (Fig. 7) corre˚ . This is consistent with a sponds to a cell spacing of 10.7 A previous report of the spacing of the (1 0 0) plane of monoclinic AlQ3 [25]. Fitting the XRD peak with a single Gaussian yields an average crystalline domain size of ca. 6 nm. The size of the features observed in AFM compared to those found by XRD implies the film is mostly amorphous, consisting of small crystalline domains. The AFM and XRD dimensions are unchanged by post-growth annealing or by varying the growth rate (data not shown). The low signal to
noise observed in the XRD scans is also consistent with low crystallinity.
4. Conclusions We have examined the time-dependence of the nearinfrared emission of [ErQ3] as a powder, spin-coated thin film in a polymer matrix, evaporated film and as a DMSO-d6 solution; the PL lifetimes range from 0.13 to 2.9 ms. The observed rate of decay of the Er3þ excited state is explained by efficient non-radiative decay via vibronic coupling with ligand and solvent oscillators. The dramatic reduction in lifetime for [ErQ3] compared with the millisecond radiative lifetime of the Er3þ ion means that the efficiency of [ErQ3] luminescence is in the range 0.002–0.03%, which explains the very low efficiencies that have been reported for [ErQ3]based OLEDs. These results demonstrate that solvent and ligand deactivation processes will need to be addressed in order to make efficient erbium-based organic thin film optoelectronic devices.
Acknowledgements We are grateful to the EPSRC Ultrafast Photonics Collaboration, and SHEFC for financial support. IDWS is a Royal Society University Research Fellow.
References [1] R.J. Mears, L. Reekie, I.M. Jauncey, D.N. Payne, Electron. Lett. 23 (1987) 1026.
S.W. Magennis et al. / Synthetic Metals 138 (2003) 463–469 [2] E. Desurvire, Phys. Today 97 (1994) 20. [3] A.G. MacDiarmid, Angew. Chem. Int. Ed. 40 (2001) 2581. [4] M.P. Oude Wolbers, F.C.J.M. van Veggel, B.H.M. Snellink-Rue¨ l, J.W. Hofstraat, F.A.J. Guerts, D.N. Reinhoudt, J. Chem. Soc., Perkin Trans. 2 (1998) 2141. [5] S. Lin, R.J. Feuerstein, A.R. Mickelson, J. Appl. Phys. 79 (1996) 2868. [6] S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, D.N. Reinhoudt, L.H. Slooff, A. Polman, J.W. Hofstraat, J. Appl. Phys. 86 (1999) 1181. [7] W.P. Gillin, R.J. Curry, Appl. Phys. Lett. 74 (1999) 798. [8] R.J. Curry, W.P. Gillin, Appl. Phys. Lett. 75 (1999) 1380. [9] R.G. Sun, Y.Z. Wang, Q.B. Zheng, H.J. Zhang, A.J. Epstein, J. Appl. Phys. 87 (2000) 7589. [10] Y. Kawamura, Y. Wada, S. Yanagida, Jpn. J. Appl. Phys. 1 40 (2001) 350. [11] R.J. Curry, W.P. Gillin, Appl. Phys. Lett. 77 (2000) 2271. [12] H.F. Aly, F.M. Abdel Kerim, A.T. Kandil, J. Inorg. Nucl. Chem. 33 (1971) 4340. [13] S. Heutz, A.J. Ferguson, G. Rumbles, T.S. Jones, Org. Electron. 3 (2002) 119. [14] S. Heutz, T.S. Jones, J. Appl. Phys. 92 (2002) 3039.
469
[15] A. Beeby, S. Faulkner, Chem. Phys. Lett. 266 (1997) 116. [16] A.S. Batsanov, A. Beeby, J.I. Bruce, J.A.K. Howard, A.M. Kenwright, D. Parker, Chem. Commun. (1999) 1011. [17] G. Stein, E. Wu¨ rzberg, J. Chem. Phys. 62 (1975) 208. [18] A. Beeby, I.M. Clarkson, R.S. Dickins, S. Faulkner, D. Parker, L. Royle, A.S. de Sousa, J.A.G. Williams, M. Woods, J. Chem. Soc., Perkin Trans. 2 (1999) 493. [19] L.H. Sloof, A. Polman, M.P. Oude Wolbers, F.C.J.M. van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 83 (1998) 497. [20] M.H.V. Werts, J.W. Verhoeven, J.W. Hofstraat, J. Chem. Soc., Perkin Trans. 2 (2000) 433. [21] R.J. Curry, W.P. Gillin, Synth. Met. 111–112 (2000) 35. [22] G.A. Hebbink, S.I. Klink, P.G.B. Oude Alink, F.C.J.M. van Veggel, Inorg. Chim. Acta 317 (2001) 114. [23] S.I. Klink, L. Grave, D.N. Reinhoudt, F.C.J.M. van Veggel, M.H.V. Werts, F.A.J.G. Geurts, J.W. Hofstraat, J. Phys. Chem. A 104 (2000) 5457. [24] M. Iwamuro, T. Adachi, Y. Wada, T. Kitamura, N. Nakashima, S. Yanagida, Bull. Chem. Soc. Jpn. 73 (2000) 1359. [25] M. Brinkman, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi, J. Am. Chem. Soc. 122 (2000) 5147.