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Avoiding trap states in poly(n-vinylcarbazole) thin films
Organic Electronics 13 (2012) 2843–2849
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Avoiding trap states in poly(n-vinylcarbazole) thin films K.A.S. Araujo a, P.S.S. Guimarães a, L.A. Cury a,⇑, L. Akcelrud b, D. Sanvitto c, M. De Giorgi c, M. Valadares d, H.D.R. Calado e a
Departamento de Física, Instituto de Ciências, Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil Laboratório de Polímeros Paulo Scarpa (LaPPS), Departamento de Química, Universidade Federal do Paraná, 81531-990 Curitiba, Paraná, Brazil c National Nanotechnology Laboratory, Istituto Nanoscienze – CNR, Via Arnesano, 73100 Lecce, Italy d Departamento de Física, Centro de Ciências, Exatas e Tecnológicas, Universidade Federal de Viçosa, 36570-000 Viçosa, Minas Gerais, Brazil e Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil b
a r t i c l e
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Article history: Received 1 June 2012 Received in revised form 3 August 2012 Accepted 16 August 2012 Available online 8 September 2012 Keywords: Triplet trap states PVK Aggregates Exciton energy transfer PVK/PODT adjacent bilayer
a b s t r a c t Optical properties of poly(n-vinylcarbazole) (PVK) thin films are revisited. Steady-state emission spectra put in evidence a strong red band whose intensity increases with decreasing temperature when the solid state PVK film is excited by a continuous 375 nm laser line. This red band is assigned to the emission from PVK aggregate states which act as trap states for the monomeric PVK triplet high energy (blue) excitons. At the same low temperatures, these trap states can be avoided when the excitation of the PVK film is made by a 355 nm pulsed laser line with 10 Hz repetition rate. The red band was also observed to compete with the emission of guest poly(3-octadecylthiophene) (PODT) molecules in a PVK/PODT sequential bilayer structure. Different optical geometries enabled us to show that the exciton energy transfer effect from PVK donor to PODT acceptor states dominates the scenario in the bilayer structure, suppressing almost completely the trap state emissions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the first polymer light emitting devices (PLEDs) reported in the literature [1], white PLEDs [2–4] have been investigated in order to consecrate conjugated polymers as a prominent, low cost and efficient new class of organic electronic materials. Many strategies have been planed and executed to obtain polymer-based white light-emitting diodes [5–10]. These devices were mainly based on polymer-blend or small-molecule-doped polymer systems [11,12]. However, disadvantages of this approach, such as difficulties in finding polymer components with suitable deep blue or red wavelengths, can happen in the quest to obtain a balanced white-light emission. In this context, poly(n-vinylcarbazole) (PVK), a non-conjugated polymer with a luminescent carbazole side group, has been extensively used as a blue phosphorescent host material ⇑ Corresponding author. Fax: +55 031 3409 5600. E-mail address: cury@fisica.ufmg.br (L.A. Cury). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.034
[13–16]. However, the use of PVK as a host for the blue phosphorescent component in PLEDs has been put in doubt by Jankus and Monkman [17]. These authors demonstrated the existence of triplet trap species, which were assigned as ground state triplet dimers in PVK with an energy around 2.5 eV. These trap states would likely compete with monomeric triplet states during recombination in the PVK host, thus reducing its deep blue emission. In this work, steady state emissions from PVK films, a green band (507 nm) and a red shoulder (665 nm), were observed at room temperature when the sample was pumped by a continuous 375 nm laser line. With decreasing temperature these two emissions presented an intensity enhancement with a significant reduction of the PVK blue emission. These emissions are assigned, respectively, to recombinations of the ground state triplet dimers [17] and recombinations of aggregate species in the PVK film. However, the enhancement effect of these two species was not observed when excitation of the PVK film was done by a pulsed laser at a low repetition rate. In this case, even at
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low temperatures the PVK film presents only its well known phosphorescent blue emission. In addition, by using a PVK/PODT sequential bilayer at the same pulsed laser excitation and low temperature conditions it is shown that the PVK blue emission survives and contributes effectively to a more balanced white emission.
by a Spectra Physics Tsunami Ti:Sapphire laser coupled to a Spectra Physics GWU-23PS second harmonic generator, were used to produce a vertically polarised excitation source at 360 nm. A time resolution around 7 ps and a spectral resolution of 4 nm can be achieved with this setup, with fluorescence decays obtained by averaging over a 5 nm wide boxed section of the camera image.
2. Samples and experimental details 3. Results and discussions The PVK (5.0 mg/mL in chlorophorm) and PODT (5.0 mg/mL in chlorophorm) solutions were produced and left to stir for more than 24 hs. Additional information about the synthesis of the PODT polymer used in this work can be found elsewhere [18]. Thin films of PVK and PODT were fabricated by spin-coating on glass substrates at 1000 rpm. The PODT film was heated at 60 °C in a hot-plate for 30 min to further remove the solvent. The PVK film was left drying inside the glove-box to further remove its solvent. In order to fabricate the PVK/PODT sequential bilayer structure we first spread the PODT solution at 1000 rpm on top of a glass substrate and then heated it at 60 °C for 30 min, as for the pure film. A hexamethyldisilane (HMDS) solution was deposited by spin-coating at 1500 rpm on top of the PODT film prior to the PVK deposition in order to promote its better spreading. The HMDS coated surface becomes neutral, hydrophobic and non-oily. In addition, it offers increased resistivity and is not affected by solvents that are not readily hydrolyzed. The bilayer structure was completed by spreading the PVK solution at 1000 rpm. The bilayer structure was then left drying inside the glove-box to further remove its solvent. The respective thicknesses and refractive indexes of the samples were measured by ellipsometry using a J.A. Woollan Co., Inc. ellipsometer, model M2000. The absorption spectra were obtained using a 3600 Shimadzu spectrophotometer. The steady-state emission measurements were performed at different temperatures using a liquid He immersion cryostat and a temperature controller. All films were always in contact with the cold He gas at atmospheric pressure or under vacuum at room temperature to avoid any photo-oxidation effect on them. The detection of the emitted light was made by an Ocean Optics USB2000 mini-spectrometer. For the excitation of the films we have used a CW 375 nm line from a diode laser and a CW 457.9 nm line from an Ar-laser. In this work the detection of the emission signal at an angle of 45° to the normal of the film surface is called 45° geometry. This is the more standard photoluminescence collection geometry. The detection from the edge of the film, with the laser incidence parallel to the normal of the film surface is called 90° geometry. A special experimental set-up was used in order to shine each laser line at the same position on the films, enabling us to compare the respective emission intensities. A pulsed Nd:YAG laser line at 355 nm, with 10 Hz of repetition rate and pulse duration of 4 ns was also used to study the samples. Picosecond time resolved fluorescence measurements were made using a Hamamatsu C5680 streak camera coupled to a monochromator (Acton SpectraPro-2300i). Femtosecond pulses with 80 MHz repetition rate, generated
) for the PVK film as a funcThe average decay time (s tion of the temperature is shown in Fig. 1. A decay curve obtained at 120 K is shown in the inset of Fig. 1 as an example. All decay curves have been well fitted with a triP ple exponential function IðtÞ ¼ 3i¼1 Ii eðt=ti Þ . The average decay time decreases with increasing temperature. At room temperature our PVK thin film has the main peak emission at 420 nm, recognized in the literature as a triplet state [17,19]. The molecule conformation of PVK films spin cast from chloroform solution, which is our case, corresponds to a fully eclipsed face to face flat carbazole chromophore stacking [20]. The PVK molecules with more face to face stacked carbazole units have a higher probability of forming triplet excitons. This statement is consistent with previous works [21,22] where the authors reported that the probability of triplet exciton formation depends on the molecular conformation, the film morphology, and increases with decreasing effective conjugation length. The face to face conformation of two carbazole unities, highly ordered with short intermolecular contact, characterizes a polymeric system with short conjugation length. Thus, such shorter conjugation length would favor a higher intersystem crossing rate of singlet excitons to form triplet excitons. A quantum-chemical calculation [23] has confirmed the statements above. In addition, the relatively
Fig. 1. Average decay time for the PVK film as a function of the temperature. The dashed line is a guide for the eyes. The excitation of the PVK film was made at kExc = 360 nm. All decays have been fitted by a triple exponential function. In the inset it is shown a decay curve obtained at 120 K for a collect wavelength kCollect = 440 nm, with the corresponding fitting (Ii, ti) parameters shown inside the figure. The values of the average P P ¼ ðIi t 2i = Ii ti Þ. decay times were calculated through the expression s
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in Fig. 1, of the order of nanosechigh values obtained for s onds (ns), confirm the phosphorescent nature of the PVK emission. The same order of our PVK lifetime was also observed [24] for a PVK film made from a chloroform solution. The phosphorescent emissions of our PVK film as a function of temperature with the excitation made by a CW 375 nm laser line are shown in Fig. 2a. At room temperature a green band appears centered at 507 nm, which has been well characterized and assigned as the emission from ground state triplet dimers in PVK [17]. However, the red band, which intensity is enhanced significantly with decreasing temperature, has not been previously observed so clearly in the literature. As it happens for the triplet dimer emission, the red band also keeps a predominant excitonic character, corresponding to a relatively well structured spectrum. It is worth noticing that the enhancement of this red band occurs in detriment of the PVK electronic peak intensity when excitation is done by the CW 375 nm laser line. This indicates that exciton migration from the triplet mono-
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meric blue states to those lower energy red states is occurring. The red band could be interpreted as the emission of excimers. However, by definition [25], excimer is an electrically excited physical dimer consisting of the same species ground state molecule and excited state molecule. Most importantly, it is dissociative and non-bonding in a ground state, not unlike a dimer; hence, normally no new absorption band is observed with an increase of excimer concentration. Thus, absorption from excimers is not possible and, therefore, energy transfer from triplet monomeric to excimer states is not possible to occur. Emission of excimers is normally characterized by redshifted broad unstructured spectra, which it is not happen in our case, where as mentioned before, the red emission possess a relatively thin Gaussian shape well structured. The red emission is interpreted here as coming from aggregate species, where the average conjugation length is observed to increase with decreasing temperature, which is confirmed by the redshift of the red peak emission (Fig. 2a). When, in turn, the excitation of the PVK film is made by a pulsed 355 nm laser line (repetition rate of 10 Hz), at the
Fig. 2. (a) Steady-state emission spectra for the PVK film at different temperatures when excited by the CW 375 nm laser line (45° geometry shown by the scheme in the inset of the Fig. 4). (b) Normalized steady-state emission spectra at 45 K for the PVK film, at 45° geometry, using the pulsed 355 nm (full line) and the CW 375 nm (dashed line) laser lines. (c) Steady-state emission spectra for the PVK film at different temperatures after excitation by a CW 476 nm laser line.
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same low temperature conditions (see Fig. 2b), the emission spectrum presents a more standard profile, with the higher intensity corresponding to the PVK electronic peak at 400 nm. Qian et al. [14,20] have observed a red band in PVK PLED emissions, where the corresponding intensity increased with increasing applied voltage. The origin of this red band was claimed by the authors to be due to the exposition of the PVK solution to different UV irradiation doses prior to spin-coating the PVK PLED active layer. By exciting the PVK film with a CW Ar-laser line at 476 nm we were able to observe a steady-state green band (Fig. 2c), which is also assigned to the ground state triplet dimers emission in PVK. The corresponding intensity of this band increases significantly with decreasing temperature, giving rise to a featureless shoulder structure around 610 nm, which probably is coming from the contribution of the aggregate states at the lowest temperatures. As shown by the results in Fig. 2a–c, the green and red bands are well formed, becoming more intense and more structured at lower temperatures, without any further broadening. Thus, they are very unlike to arise from the presence of impurities or molecular defects. They were not either created by the incidence of UV light. If so, the pulsed 355 nm UV laser line (Fig. 2b) would also induce a reduction of the PVK electronic peak intensity due to migration effects to the triplet dimer or to the aggregates, which act as trap states for the PVK deep blue states. Contrarily, at excitation with the 355 nm UV laser line the deep blue emission of PVK is always present at different experimental conditions, as shown in Figs. 2, 5 and 6b, without any further intensity reduction. The absorption tail observed for the PVK film in Fig. 3a and b at room temperature, which optical density increases monotonically with decreasing temperature, is probably related to the aggregate states. However, at excitation by the CW 476 nm laser line, only the triplet dimer emission is observed in Fig. 2c at higher temperatures. The featureless shoulder around 610 nm, which we related to the contribution of the aggregate states, appears only at lower temperatures. The photon energy at 476 nm is thus more effectively absorbed by the triplet dimer states. This result is consistent with our statement that the enhancement of the aggregate emission peak (Fig. 2a) occurs due to the more effective exciton energy transfer from the monomeric triplet to the aggregate states. A PVK:PODT blend structure, with 1.0 wt.% of PODT in the PVK matrix and a PVK/PODT bilayer were fabricated. In these structures, we expected that the PODT acceptor would have a role of competing with the triplet dimer and aggregate trap states for the quench of the PVK monomeric triplet blue emission. The optical properties of this 1% blend, excited by the 375 nm CW laser line, however, have not put in evidence any kind of host–guest interaction, presenting the emission spectra with the same fea525 tures as the pure PVK film. The average decay times s 630 , respectively obtained at collect wavelengths kColand s lect = 525 nm (around the triplet dimer green band) and kCollect = 630 nm (around the aggregate red band), for the 1% blend sample varied from 5.6 ns to 3.3 ns and from 1.4 ns to 0.7 ns with increasing temperature (the same
Fig. 3. (a) Absorption spectra at room temperature for the PVK and PODT films and for the PVK/PODT bilayer; (b) absorption spectra for the PVK film at different temperatures.
qualitative behavior as observed for the pure PVK film at kCollect = 440 nm in Fig. 1). We interpret this common trend in temperature as being due to the fact that the PVK monomeric triplet states are the initial source, which is followed by the exciton migration (probably by Förster and/or Dexter transfer) to triplet dimer states and more effectively to aggregate trap states, corroborating the ideas of the previous paragraph. The PVK/PODT bilayer have helped us to obtain more information on how the triplet dimer and the aggregate trap states affect the emission of the triplet monomeric (fundamental) electronic states of the PVK layer. Different optical geometries have been applied, the more standard 45° geometry (scheme at the inset of Fig. 4) and the 90° geometry (scheme at the inset of Fig. 5a). The latter enabled us to explore the waveguide properties of the bilayer and the consequences on the optical properties at different CW or pulsed laser line excitations. At 45° geometry, with excitation by the 375 nm CW laser line, the PVK/PODT bilayer exhibited spectra with a major contribution of the PODT layer emission (Fig. 4), which we have assigned to exciton energy transfer (EET) [26]
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Fig. 4. Emission spectra for the PVK/PODT bilayer at T = 285 K, excited respectively by the CW 375 nm (full curve) and by the CW 457.9 nm (dashed curve) laser lines in the 45° geometry (scheme shown in the inset of the figure). For both excitations the laser power was 21 lW. The dashed curve shows specifically the emission of the PODT layer component for 457.9 nm laser line excitation, which is the wavelength corresponding to a relatively high absorption of PODT. Its emission intensity must be multiplayed by 7 to be comparable to the emission (full curve) coming from the exciton energy transfer between the PVK donor to the PODT acceptor.
from the PVK donor to the PODT acceptor. The EET in the bilayer remained effective even at low temperatures (figure not shown), without any traceable signal from the red aggregate band. We have just observed a relatively small shoulder which is due to the contribution of the triplet dimer emission at around 525 nm, as shown in the spectra at T = 285 K in Fig. 4. The edge emission spectra for the PVK/PODT bilayer using the 90° geometry, with excitation by the CW laser line at 375 nm are shown in Fig. 5a. An important quench of the triplet PVK monomeric blue emission occurs in favor of the intensity enhancement of the triplet dimer and the aggregate trap state emissions, and also to the PODT emission enhancement via EET. Initially, at 290 K (Fig. 5a), the emission spectrum shows just a bump for the triplet dimer emission at around 525 nm and a well formed and more intense band due to EET to the PODT layer around 615 nm. With decreasing temperature the triplet dimer emission intensity is enhanced and the aggregate red band starts the competition with the PODT emission band. The aggregate red band becomes the dominant emission at the lowest temperatures. It is worth to note that these are guided emissions travelling from the middle point (excitation point) up to the edge of the sample. The refractive index for PVK (1.64 ± 0.03) and for PODT (1.63 ± 0.02), for a wavelength around 650 nm, and the respective layer thicknesses LPVK = (50 ± 2) nm and LPODT = (72 ± 8) nm, were measured by ellipsometry. Although these values do not allow a trully confined waveguide mode, the emission light can be guided by means of a substrate radiation mode [27] or simply by multiple internal reflexions, with losses in both cases. At the lowest temperatures the dominant aggregate and the triplet dimer emissions travel
Fig. 5. Emission spectra for the PVK/PODT bilayer at different temperatures, excited by the CW 375 nm laser line (a); and by the pulsed 355 nm laser line (b). In both cases we have used the 90° geometry with the laser directed to the middle of the sample as indicated by the inset in part (a).
through the PVK layer, overcoming the losses during their passage towards the sample edge. Different results (Fig. 5b) were observed at the same 90° geometry but using the pulsed laser line at 355 nm (10 Hz of repetition rate). Firstly, no quenching of the triplet PVK monomeric blue emission occurs, this band, around 425 nm, still increases with decreasing temperature. The monomeric blue exciton migration to the aggregate states, or to the acceptor PODT molecules via EET, reduces considerably. Both processes still compete between them with the final dominance of the aggregate emission at the lowest temperatures, as observed in Fig. 5a. The triplet dimer band presents a higher intensity, spreading the overall emission of the PVK/PODT bilayer to a larger range of wavelength. At 45° geometry (Fig. 6a) this also occurs, with the advantage that we can tune the color components of the emission by the applied laser power in order to reach a color balance shifted more towards the white, as shown in Fig. 6b. Thus, the results in Figs. 2a, 4 and 5a show a significant quenching of the PVK monomeric triplet blue emission,
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peratures any excess of temperature will also affect its emission efficiency). The emission bands from these two trap states were also observed when a 325 nm CW laser line from a He– Cd laser was used for excitation (results not shown). At that wavelength the absorption of the PVK layer is relatively higher (Fig. 3) and the exciton migration from the monomeric triplet blue emission to the trap states would be likely expected. In addition, at this 325 nm excitation the corresponding PVK aggregate band (red band) intensity presents a linear dependence on the laser power, which is another indication that the red band has not its origin due to impurities or defects. At excitation by a 355 nm pulsed laser line, on the other hand, the red band is not observed, even at low temperatures (Fig. 2b), and no effective quenching of the PVK monomeric triplet blue emission occurs (Figs. 5b and 6a).
(b)
Fig. 6. (a) Steady-state emission spectra for the PVK/PODT bilayer excited by the pulsed 355 nm laser line (45° geometry) at T = 29 K and different average laser powers. The arrows indicate, respectively, where the PODT and the aggregate of PVK emissions are contributing to the spectra. (b) Diagram of chromaticity for the respective spectra in part a. The letters a, b, c, d, e, f, and g correspond respectively to the 0.10, 0.26, 0.52, 1.05, 1.65, 2.30, and 3.00 mW average laser powers.
due to exciton migration (probably Förster and/or Dexter transfer) to triplet dimer and aggregate trap states. This has been clearly observed with decreasing temperature and with the PVK film being excited by the 375 nm CW laser line. The intensity enhancement with decreasing temperature of these trap state emissions is in part explained because internal conversion process (the excess of energy lost by vibrational relaxation) or other radiationless transfers of energy compete so successfully with phosphorescence from trap states that the latter is usually seen only at low temperatures (We cannot confirm the phosphorescent character of the aggregate emission but considering the fact that aggregate emission only occurs at lower tem-
4. Conclusions In summary, trap states corresponding to triplet dimer and aggregate states were investigated in PVK films. The presence of these trap states can represent quenching sites for the PVK own monomeric triplet blue excitons, and also for other guest materials. Indeed, a significant quenching of the PVK monomeric triplet blue emission was observed mainly in favor of the aggregate states, which emission intensity increases with decreasing temperature. We have attributed this to exciton migration via Förster and/or Dexter transfer. The latter is a very likely transfer mechanism in films where emissive sites can be within angstrons of each other. The quenching of monomeric triplet blue excitons occurred effectively with the excitation of the PVK film made by a 375 nm CW laser line. On the other hand, with the excitation made by a pulsed 355 nm laser line the PVK monomeric triplet blue emission was always active and its intensity increased with decreasing temperature. The pulse duration of the 355 nm pulsed laser we used is 4 ns, with 10 Hz repetition rate. It is worth to remember that the lifetime for the triplet monomeric blue excited states is larger than the pulse duration, for the full range of temperature studied (see Fig. 1). This means that during the pulse incidence PVK monomeric triplet excitons are being created continuously or are being kept frozen at the excited state. Since we do not observe the formation of the red band under 355 nm pulsed excitation, we conclude that the overall conditions for this excitation do not favor the corresponding exciton migration to aggregate states. In addition, simple calculations show us that the number of exciting photons per second created by the pulsed laser is of the order of 108 less than those created by the CW laser at 375 nm. Thus, in the CW case we are in a steadystate regime, always over-populating the monomeric triplet blue states, enabling a much more efficient exciton migration to the aggregate or to the PVK dimer triplet trap states. The PVK:PODT blend and the PVK/PODT bilayer structures enabled us to investigate the competition processes between the PVK trap states with the emissive states of the guest PODT molecules. This competition was clearly
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observed in the PVK/PODT bilayer at the 90° geometry, through the guided emission from the sample edge. At the 45° geometry the exciton energy transfer from the PVK states to the PODT guest states becomes the dominant transfer process. In that case no transfer to the PVK trap states was observed, characterising a way to avoid these trap states, a feature which could be helpful for specific device applications. Acknowledgments We kindly acknowledge G. Gigli from National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, Italy for the helpful discussions. K.A.S. Araujo, P.S.S. Guimarães, L.A. Cury thank FAPEMIG, CAPES, CNPq, the Instituto Nacional de Eletrônica Orgânica (INCT-INEO) and the Instituto Nacional de Ciência e Tecnologia em Dispositivos Semicondutores (INCT-DISSE) from Brazil for the financial support. References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Light-emitting diodes based on conjugated polymers, Nature 347 (1990) 539–541. [2] J. Kido, H. Shionoya, K. Nagai, Single-layer white light-emitting organic electroluminescent devices based on dye-dispersed poly(Nvinylcarbazole), Appl. Phys. Lett. 67 (1995) 2281–2283. [3] M. Granström, O. Inganäs, White light emission from a polymer blend light emitting diode, Appl. Phys. Lett. 68 (1996) 147–149. [4] J. Huang, G. Li, E. Wu, Q. Xu, Y. Yang, Achieving high-efficiency polymer white-light-emitting devices, Adv. Mater. 18 (2006) 114– 117. [5] M. Strukelj, R.H. Jordan, A. Dodabalapur, Organic multilayer white light emitting diodes, J. Am. Chem. Soc. 118 (1996) 1213. [6] S. Tasch, E.J.W. List, O. Ekström, W. Graupner, G. Leising, P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Möllen, Efficient white light-emitting diodes realized with new processable blends of conjugated polymers, Appl. Phys. Lett. 71 (1997) 2883–2885. [7] Z. Xie, J.S. Huang, C.N. Li, Y. Wang, Y.Q. Li, J. Shen, White light emission induced by confinement in organic multiheterostructures, Appl. Phys. Lett. 74 (1999) 641–643. [8] C.W. Ko, Y.T. Tao, Bright white organic light-emitting diode, Appl. Phys. Lett. 79 (2001) 4234–4236. [9] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Controlling exciton diffusion in multilayer white phosphorescent organic light emitting devices, Adv. Mater. 14 (2002) 147–151. [10] X. Gong, S. Wang, D. Moses, G.C. Bazan, A.J. Heeger, Multilayer polymer light-emitting diodes: white-light emission with high efficiency, Adv. Mater. 17 (2005) 2053–2058.
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Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Avoiding trap states in poly(n-vinylcarbazole) thin films K.A.S. Araujo a, P.S.S. Guimarães a, L.A. Cury a,⇑, L. Akcelrud b, D. Sanvitto c, M. De Giorgi c, M. Valadares d, H.D.R. Calado e a
Departamento de Física, Instituto de Ciências, Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil Laboratório de Polímeros Paulo Scarpa (LaPPS), Departamento de Química, Universidade Federal do Paraná, 81531-990 Curitiba, Paraná, Brazil c National Nanotechnology Laboratory, Istituto Nanoscienze – CNR, Via Arnesano, 73100 Lecce, Italy d Departamento de Física, Centro de Ciências, Exatas e Tecnológicas, Universidade Federal de Viçosa, 36570-000 Viçosa, Minas Gerais, Brazil e Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil b
a r t i c l e
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Article history: Received 1 June 2012 Received in revised form 3 August 2012 Accepted 16 August 2012 Available online 8 September 2012 Keywords: Triplet trap states PVK Aggregates Exciton energy transfer PVK/PODT adjacent bilayer
a b s t r a c t Optical properties of poly(n-vinylcarbazole) (PVK) thin films are revisited. Steady-state emission spectra put in evidence a strong red band whose intensity increases with decreasing temperature when the solid state PVK film is excited by a continuous 375 nm laser line. This red band is assigned to the emission from PVK aggregate states which act as trap states for the monomeric PVK triplet high energy (blue) excitons. At the same low temperatures, these trap states can be avoided when the excitation of the PVK film is made by a 355 nm pulsed laser line with 10 Hz repetition rate. The red band was also observed to compete with the emission of guest poly(3-octadecylthiophene) (PODT) molecules in a PVK/PODT sequential bilayer structure. Different optical geometries enabled us to show that the exciton energy transfer effect from PVK donor to PODT acceptor states dominates the scenario in the bilayer structure, suppressing almost completely the trap state emissions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the first polymer light emitting devices (PLEDs) reported in the literature [1], white PLEDs [2–4] have been investigated in order to consecrate conjugated polymers as a prominent, low cost and efficient new class of organic electronic materials. Many strategies have been planed and executed to obtain polymer-based white light-emitting diodes [5–10]. These devices were mainly based on polymer-blend or small-molecule-doped polymer systems [11,12]. However, disadvantages of this approach, such as difficulties in finding polymer components with suitable deep blue or red wavelengths, can happen in the quest to obtain a balanced white-light emission. In this context, poly(n-vinylcarbazole) (PVK), a non-conjugated polymer with a luminescent carbazole side group, has been extensively used as a blue phosphorescent host material ⇑ Corresponding author. Fax: +55 031 3409 5600. E-mail address: cury@fisica.ufmg.br (L.A. Cury). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.034
[13–16]. However, the use of PVK as a host for the blue phosphorescent component in PLEDs has been put in doubt by Jankus and Monkman [17]. These authors demonstrated the existence of triplet trap species, which were assigned as ground state triplet dimers in PVK with an energy around 2.5 eV. These trap states would likely compete with monomeric triplet states during recombination in the PVK host, thus reducing its deep blue emission. In this work, steady state emissions from PVK films, a green band (507 nm) and a red shoulder (665 nm), were observed at room temperature when the sample was pumped by a continuous 375 nm laser line. With decreasing temperature these two emissions presented an intensity enhancement with a significant reduction of the PVK blue emission. These emissions are assigned, respectively, to recombinations of the ground state triplet dimers [17] and recombinations of aggregate species in the PVK film. However, the enhancement effect of these two species was not observed when excitation of the PVK film was done by a pulsed laser at a low repetition rate. In this case, even at
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low temperatures the PVK film presents only its well known phosphorescent blue emission. In addition, by using a PVK/PODT sequential bilayer at the same pulsed laser excitation and low temperature conditions it is shown that the PVK blue emission survives and contributes effectively to a more balanced white emission.
by a Spectra Physics Tsunami Ti:Sapphire laser coupled to a Spectra Physics GWU-23PS second harmonic generator, were used to produce a vertically polarised excitation source at 360 nm. A time resolution around 7 ps and a spectral resolution of 4 nm can be achieved with this setup, with fluorescence decays obtained by averaging over a 5 nm wide boxed section of the camera image.
2. Samples and experimental details 3. Results and discussions The PVK (5.0 mg/mL in chlorophorm) and PODT (5.0 mg/mL in chlorophorm) solutions were produced and left to stir for more than 24 hs. Additional information about the synthesis of the PODT polymer used in this work can be found elsewhere [18]. Thin films of PVK and PODT were fabricated by spin-coating on glass substrates at 1000 rpm. The PODT film was heated at 60 °C in a hot-plate for 30 min to further remove the solvent. The PVK film was left drying inside the glove-box to further remove its solvent. In order to fabricate the PVK/PODT sequential bilayer structure we first spread the PODT solution at 1000 rpm on top of a glass substrate and then heated it at 60 °C for 30 min, as for the pure film. A hexamethyldisilane (HMDS) solution was deposited by spin-coating at 1500 rpm on top of the PODT film prior to the PVK deposition in order to promote its better spreading. The HMDS coated surface becomes neutral, hydrophobic and non-oily. In addition, it offers increased resistivity and is not affected by solvents that are not readily hydrolyzed. The bilayer structure was completed by spreading the PVK solution at 1000 rpm. The bilayer structure was then left drying inside the glove-box to further remove its solvent. The respective thicknesses and refractive indexes of the samples were measured by ellipsometry using a J.A. Woollan Co., Inc. ellipsometer, model M2000. The absorption spectra were obtained using a 3600 Shimadzu spectrophotometer. The steady-state emission measurements were performed at different temperatures using a liquid He immersion cryostat and a temperature controller. All films were always in contact with the cold He gas at atmospheric pressure or under vacuum at room temperature to avoid any photo-oxidation effect on them. The detection of the emitted light was made by an Ocean Optics USB2000 mini-spectrometer. For the excitation of the films we have used a CW 375 nm line from a diode laser and a CW 457.9 nm line from an Ar-laser. In this work the detection of the emission signal at an angle of 45° to the normal of the film surface is called 45° geometry. This is the more standard photoluminescence collection geometry. The detection from the edge of the film, with the laser incidence parallel to the normal of the film surface is called 90° geometry. A special experimental set-up was used in order to shine each laser line at the same position on the films, enabling us to compare the respective emission intensities. A pulsed Nd:YAG laser line at 355 nm, with 10 Hz of repetition rate and pulse duration of 4 ns was also used to study the samples. Picosecond time resolved fluorescence measurements were made using a Hamamatsu C5680 streak camera coupled to a monochromator (Acton SpectraPro-2300i). Femtosecond pulses with 80 MHz repetition rate, generated
) for the PVK film as a funcThe average decay time (s tion of the temperature is shown in Fig. 1. A decay curve obtained at 120 K is shown in the inset of Fig. 1 as an example. All decay curves have been well fitted with a triP ple exponential function IðtÞ ¼ 3i¼1 Ii eðt=ti Þ . The average decay time decreases with increasing temperature. At room temperature our PVK thin film has the main peak emission at 420 nm, recognized in the literature as a triplet state [17,19]. The molecule conformation of PVK films spin cast from chloroform solution, which is our case, corresponds to a fully eclipsed face to face flat carbazole chromophore stacking [20]. The PVK molecules with more face to face stacked carbazole units have a higher probability of forming triplet excitons. This statement is consistent with previous works [21,22] where the authors reported that the probability of triplet exciton formation depends on the molecular conformation, the film morphology, and increases with decreasing effective conjugation length. The face to face conformation of two carbazole unities, highly ordered with short intermolecular contact, characterizes a polymeric system with short conjugation length. Thus, such shorter conjugation length would favor a higher intersystem crossing rate of singlet excitons to form triplet excitons. A quantum-chemical calculation [23] has confirmed the statements above. In addition, the relatively
Fig. 1. Average decay time for the PVK film as a function of the temperature. The dashed line is a guide for the eyes. The excitation of the PVK film was made at kExc = 360 nm. All decays have been fitted by a triple exponential function. In the inset it is shown a decay curve obtained at 120 K for a collect wavelength kCollect = 440 nm, with the corresponding fitting (Ii, ti) parameters shown inside the figure. The values of the average P P ¼ ðIi t 2i = Ii ti Þ. decay times were calculated through the expression s
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in Fig. 1, of the order of nanosechigh values obtained for s onds (ns), confirm the phosphorescent nature of the PVK emission. The same order of our PVK lifetime was also observed [24] for a PVK film made from a chloroform solution. The phosphorescent emissions of our PVK film as a function of temperature with the excitation made by a CW 375 nm laser line are shown in Fig. 2a. At room temperature a green band appears centered at 507 nm, which has been well characterized and assigned as the emission from ground state triplet dimers in PVK [17]. However, the red band, which intensity is enhanced significantly with decreasing temperature, has not been previously observed so clearly in the literature. As it happens for the triplet dimer emission, the red band also keeps a predominant excitonic character, corresponding to a relatively well structured spectrum. It is worth noticing that the enhancement of this red band occurs in detriment of the PVK electronic peak intensity when excitation is done by the CW 375 nm laser line. This indicates that exciton migration from the triplet mono-
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meric blue states to those lower energy red states is occurring. The red band could be interpreted as the emission of excimers. However, by definition [25], excimer is an electrically excited physical dimer consisting of the same species ground state molecule and excited state molecule. Most importantly, it is dissociative and non-bonding in a ground state, not unlike a dimer; hence, normally no new absorption band is observed with an increase of excimer concentration. Thus, absorption from excimers is not possible and, therefore, energy transfer from triplet monomeric to excimer states is not possible to occur. Emission of excimers is normally characterized by redshifted broad unstructured spectra, which it is not happen in our case, where as mentioned before, the red emission possess a relatively thin Gaussian shape well structured. The red emission is interpreted here as coming from aggregate species, where the average conjugation length is observed to increase with decreasing temperature, which is confirmed by the redshift of the red peak emission (Fig. 2a). When, in turn, the excitation of the PVK film is made by a pulsed 355 nm laser line (repetition rate of 10 Hz), at the
Fig. 2. (a) Steady-state emission spectra for the PVK film at different temperatures when excited by the CW 375 nm laser line (45° geometry shown by the scheme in the inset of the Fig. 4). (b) Normalized steady-state emission spectra at 45 K for the PVK film, at 45° geometry, using the pulsed 355 nm (full line) and the CW 375 nm (dashed line) laser lines. (c) Steady-state emission spectra for the PVK film at different temperatures after excitation by a CW 476 nm laser line.
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same low temperature conditions (see Fig. 2b), the emission spectrum presents a more standard profile, with the higher intensity corresponding to the PVK electronic peak at 400 nm. Qian et al. [14,20] have observed a red band in PVK PLED emissions, where the corresponding intensity increased with increasing applied voltage. The origin of this red band was claimed by the authors to be due to the exposition of the PVK solution to different UV irradiation doses prior to spin-coating the PVK PLED active layer. By exciting the PVK film with a CW Ar-laser line at 476 nm we were able to observe a steady-state green band (Fig. 2c), which is also assigned to the ground state triplet dimers emission in PVK. The corresponding intensity of this band increases significantly with decreasing temperature, giving rise to a featureless shoulder structure around 610 nm, which probably is coming from the contribution of the aggregate states at the lowest temperatures. As shown by the results in Fig. 2a–c, the green and red bands are well formed, becoming more intense and more structured at lower temperatures, without any further broadening. Thus, they are very unlike to arise from the presence of impurities or molecular defects. They were not either created by the incidence of UV light. If so, the pulsed 355 nm UV laser line (Fig. 2b) would also induce a reduction of the PVK electronic peak intensity due to migration effects to the triplet dimer or to the aggregates, which act as trap states for the PVK deep blue states. Contrarily, at excitation with the 355 nm UV laser line the deep blue emission of PVK is always present at different experimental conditions, as shown in Figs. 2, 5 and 6b, without any further intensity reduction. The absorption tail observed for the PVK film in Fig. 3a and b at room temperature, which optical density increases monotonically with decreasing temperature, is probably related to the aggregate states. However, at excitation by the CW 476 nm laser line, only the triplet dimer emission is observed in Fig. 2c at higher temperatures. The featureless shoulder around 610 nm, which we related to the contribution of the aggregate states, appears only at lower temperatures. The photon energy at 476 nm is thus more effectively absorbed by the triplet dimer states. This result is consistent with our statement that the enhancement of the aggregate emission peak (Fig. 2a) occurs due to the more effective exciton energy transfer from the monomeric triplet to the aggregate states. A PVK:PODT blend structure, with 1.0 wt.% of PODT in the PVK matrix and a PVK/PODT bilayer were fabricated. In these structures, we expected that the PODT acceptor would have a role of competing with the triplet dimer and aggregate trap states for the quench of the PVK monomeric triplet blue emission. The optical properties of this 1% blend, excited by the 375 nm CW laser line, however, have not put in evidence any kind of host–guest interaction, presenting the emission spectra with the same fea525 tures as the pure PVK film. The average decay times s 630 , respectively obtained at collect wavelengths kColand s lect = 525 nm (around the triplet dimer green band) and kCollect = 630 nm (around the aggregate red band), for the 1% blend sample varied from 5.6 ns to 3.3 ns and from 1.4 ns to 0.7 ns with increasing temperature (the same
Fig. 3. (a) Absorption spectra at room temperature for the PVK and PODT films and for the PVK/PODT bilayer; (b) absorption spectra for the PVK film at different temperatures.
qualitative behavior as observed for the pure PVK film at kCollect = 440 nm in Fig. 1). We interpret this common trend in temperature as being due to the fact that the PVK monomeric triplet states are the initial source, which is followed by the exciton migration (probably by Förster and/or Dexter transfer) to triplet dimer states and more effectively to aggregate trap states, corroborating the ideas of the previous paragraph. The PVK/PODT bilayer have helped us to obtain more information on how the triplet dimer and the aggregate trap states affect the emission of the triplet monomeric (fundamental) electronic states of the PVK layer. Different optical geometries have been applied, the more standard 45° geometry (scheme at the inset of Fig. 4) and the 90° geometry (scheme at the inset of Fig. 5a). The latter enabled us to explore the waveguide properties of the bilayer and the consequences on the optical properties at different CW or pulsed laser line excitations. At 45° geometry, with excitation by the 375 nm CW laser line, the PVK/PODT bilayer exhibited spectra with a major contribution of the PODT layer emission (Fig. 4), which we have assigned to exciton energy transfer (EET) [26]
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Fig. 4. Emission spectra for the PVK/PODT bilayer at T = 285 K, excited respectively by the CW 375 nm (full curve) and by the CW 457.9 nm (dashed curve) laser lines in the 45° geometry (scheme shown in the inset of the figure). For both excitations the laser power was 21 lW. The dashed curve shows specifically the emission of the PODT layer component for 457.9 nm laser line excitation, which is the wavelength corresponding to a relatively high absorption of PODT. Its emission intensity must be multiplayed by 7 to be comparable to the emission (full curve) coming from the exciton energy transfer between the PVK donor to the PODT acceptor.
from the PVK donor to the PODT acceptor. The EET in the bilayer remained effective even at low temperatures (figure not shown), without any traceable signal from the red aggregate band. We have just observed a relatively small shoulder which is due to the contribution of the triplet dimer emission at around 525 nm, as shown in the spectra at T = 285 K in Fig. 4. The edge emission spectra for the PVK/PODT bilayer using the 90° geometry, with excitation by the CW laser line at 375 nm are shown in Fig. 5a. An important quench of the triplet PVK monomeric blue emission occurs in favor of the intensity enhancement of the triplet dimer and the aggregate trap state emissions, and also to the PODT emission enhancement via EET. Initially, at 290 K (Fig. 5a), the emission spectrum shows just a bump for the triplet dimer emission at around 525 nm and a well formed and more intense band due to EET to the PODT layer around 615 nm. With decreasing temperature the triplet dimer emission intensity is enhanced and the aggregate red band starts the competition with the PODT emission band. The aggregate red band becomes the dominant emission at the lowest temperatures. It is worth to note that these are guided emissions travelling from the middle point (excitation point) up to the edge of the sample. The refractive index for PVK (1.64 ± 0.03) and for PODT (1.63 ± 0.02), for a wavelength around 650 nm, and the respective layer thicknesses LPVK = (50 ± 2) nm and LPODT = (72 ± 8) nm, were measured by ellipsometry. Although these values do not allow a trully confined waveguide mode, the emission light can be guided by means of a substrate radiation mode [27] or simply by multiple internal reflexions, with losses in both cases. At the lowest temperatures the dominant aggregate and the triplet dimer emissions travel
Fig. 5. Emission spectra for the PVK/PODT bilayer at different temperatures, excited by the CW 375 nm laser line (a); and by the pulsed 355 nm laser line (b). In both cases we have used the 90° geometry with the laser directed to the middle of the sample as indicated by the inset in part (a).
through the PVK layer, overcoming the losses during their passage towards the sample edge. Different results (Fig. 5b) were observed at the same 90° geometry but using the pulsed laser line at 355 nm (10 Hz of repetition rate). Firstly, no quenching of the triplet PVK monomeric blue emission occurs, this band, around 425 nm, still increases with decreasing temperature. The monomeric blue exciton migration to the aggregate states, or to the acceptor PODT molecules via EET, reduces considerably. Both processes still compete between them with the final dominance of the aggregate emission at the lowest temperatures, as observed in Fig. 5a. The triplet dimer band presents a higher intensity, spreading the overall emission of the PVK/PODT bilayer to a larger range of wavelength. At 45° geometry (Fig. 6a) this also occurs, with the advantage that we can tune the color components of the emission by the applied laser power in order to reach a color balance shifted more towards the white, as shown in Fig. 6b. Thus, the results in Figs. 2a, 4 and 5a show a significant quenching of the PVK monomeric triplet blue emission,
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peratures any excess of temperature will also affect its emission efficiency). The emission bands from these two trap states were also observed when a 325 nm CW laser line from a He– Cd laser was used for excitation (results not shown). At that wavelength the absorption of the PVK layer is relatively higher (Fig. 3) and the exciton migration from the monomeric triplet blue emission to the trap states would be likely expected. In addition, at this 325 nm excitation the corresponding PVK aggregate band (red band) intensity presents a linear dependence on the laser power, which is another indication that the red band has not its origin due to impurities or defects. At excitation by a 355 nm pulsed laser line, on the other hand, the red band is not observed, even at low temperatures (Fig. 2b), and no effective quenching of the PVK monomeric triplet blue emission occurs (Figs. 5b and 6a).
(b)
Fig. 6. (a) Steady-state emission spectra for the PVK/PODT bilayer excited by the pulsed 355 nm laser line (45° geometry) at T = 29 K and different average laser powers. The arrows indicate, respectively, where the PODT and the aggregate of PVK emissions are contributing to the spectra. (b) Diagram of chromaticity for the respective spectra in part a. The letters a, b, c, d, e, f, and g correspond respectively to the 0.10, 0.26, 0.52, 1.05, 1.65, 2.30, and 3.00 mW average laser powers.
due to exciton migration (probably Förster and/or Dexter transfer) to triplet dimer and aggregate trap states. This has been clearly observed with decreasing temperature and with the PVK film being excited by the 375 nm CW laser line. The intensity enhancement with decreasing temperature of these trap state emissions is in part explained because internal conversion process (the excess of energy lost by vibrational relaxation) or other radiationless transfers of energy compete so successfully with phosphorescence from trap states that the latter is usually seen only at low temperatures (We cannot confirm the phosphorescent character of the aggregate emission but considering the fact that aggregate emission only occurs at lower tem-
4. Conclusions In summary, trap states corresponding to triplet dimer and aggregate states were investigated in PVK films. The presence of these trap states can represent quenching sites for the PVK own monomeric triplet blue excitons, and also for other guest materials. Indeed, a significant quenching of the PVK monomeric triplet blue emission was observed mainly in favor of the aggregate states, which emission intensity increases with decreasing temperature. We have attributed this to exciton migration via Förster and/or Dexter transfer. The latter is a very likely transfer mechanism in films where emissive sites can be within angstrons of each other. The quenching of monomeric triplet blue excitons occurred effectively with the excitation of the PVK film made by a 375 nm CW laser line. On the other hand, with the excitation made by a pulsed 355 nm laser line the PVK monomeric triplet blue emission was always active and its intensity increased with decreasing temperature. The pulse duration of the 355 nm pulsed laser we used is 4 ns, with 10 Hz repetition rate. It is worth to remember that the lifetime for the triplet monomeric blue excited states is larger than the pulse duration, for the full range of temperature studied (see Fig. 1). This means that during the pulse incidence PVK monomeric triplet excitons are being created continuously or are being kept frozen at the excited state. Since we do not observe the formation of the red band under 355 nm pulsed excitation, we conclude that the overall conditions for this excitation do not favor the corresponding exciton migration to aggregate states. In addition, simple calculations show us that the number of exciting photons per second created by the pulsed laser is of the order of 108 less than those created by the CW laser at 375 nm. Thus, in the CW case we are in a steadystate regime, always over-populating the monomeric triplet blue states, enabling a much more efficient exciton migration to the aggregate or to the PVK dimer triplet trap states. The PVK:PODT blend and the PVK/PODT bilayer structures enabled us to investigate the competition processes between the PVK trap states with the emissive states of the guest PODT molecules. This competition was clearly
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observed in the PVK/PODT bilayer at the 90° geometry, through the guided emission from the sample edge. At the 45° geometry the exciton energy transfer from the PVK states to the PODT guest states becomes the dominant transfer process. In that case no transfer to the PVK trap states was observed, characterising a way to avoid these trap states, a feature which could be helpful for specific device applications. Acknowledgments We kindly acknowledge G. Gigli from National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, Italy for the helpful discussions. K.A.S. Araujo, P.S.S. Guimarães, L.A. Cury thank FAPEMIG, CAPES, CNPq, the Instituto Nacional de Eletrônica Orgânica (INCT-INEO) and the Instituto Nacional de Ciência e Tecnologia em Dispositivos Semicondutores (INCT-DISSE) from Brazil for the financial support. References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Light-emitting diodes based on conjugated polymers, Nature 347 (1990) 539–541. [2] J. Kido, H. Shionoya, K. Nagai, Single-layer white light-emitting organic electroluminescent devices based on dye-dispersed poly(Nvinylcarbazole), Appl. Phys. Lett. 67 (1995) 2281–2283. [3] M. Granström, O. Inganäs, White light emission from a polymer blend light emitting diode, Appl. Phys. Lett. 68 (1996) 147–149. [4] J. Huang, G. Li, E. Wu, Q. Xu, Y. Yang, Achieving high-efficiency polymer white-light-emitting devices, Adv. Mater. 18 (2006) 114– 117. [5] M. Strukelj, R.H. Jordan, A. Dodabalapur, Organic multilayer white light emitting diodes, J. Am. Chem. Soc. 118 (1996) 1213. [6] S. Tasch, E.J.W. List, O. Ekström, W. Graupner, G. Leising, P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Möllen, Efficient white light-emitting diodes realized with new processable blends of conjugated polymers, Appl. Phys. Lett. 71 (1997) 2883–2885. [7] Z. Xie, J.S. Huang, C.N. Li, Y. Wang, Y.Q. Li, J. Shen, White light emission induced by confinement in organic multiheterostructures, Appl. Phys. Lett. 74 (1999) 641–643. [8] C.W. Ko, Y.T. Tao, Bright white organic light-emitting diode, Appl. Phys. Lett. 79 (2001) 4234–4236. [9] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Controlling exciton diffusion in multilayer white phosphorescent organic light emitting devices, Adv. Mater. 14 (2002) 147–151. [10] X. Gong, S. Wang, D. Moses, G.C. Bazan, A.J. Heeger, Multilayer polymer light-emitting diodes: white-light emission with high efficiency, Adv. Mater. 17 (2005) 2053–2058.
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