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Correlation between emission and structural properties of poly(p-phenylene vinylene) thin films
Synthetic Metals 170 (2013) 25–30
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Correlation between emission and structural properties of poly(p-phenylene vinylene) thin films R.V. Fernandes a , M.A.T. da Silva a , I.F.L. Dias a , J.L. Duarte a , H. de Santana b , E. Laureto a,∗ a b
Departamento de Física, Centro de Ciências Exatas, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil Departamento de Química, Centro de Ciências Exatas, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 18 August 2012 Accepted 4 March 2013 Keywords: Conjugated polymers Thin films Photoluminescence spectroscopy Optical properties Energy transfer
a b s t r a c t In this work we used different methods and parameters of deposition in order to obtain thin films of poly(p-phenylene vinylene) (PPV) with different structural properties. These properties were characterized by measurements of absorbance, Raman scattering, and photoluminescence (PL) as a function of temperature. The analysis of experimental data allowed us to observe the dependence of optical spectra with the effective conjugation length and the interaction between the polymer chains. In particular, we were able to correlate the dependence of the PL line shape with temperature and the values of emission anisotropy factor of the films with the intra and interchains energy migration processes. These results led us to conclude that such processes are essential ingredients for a proper analysis of the optical response of thin films of conjugated polymers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Understanding and control of migration processes of photogenerated carriers are crucial for the application of conjugated polymers in devices such as light emitting diodes (LEDs) and photovoltaic cells based on organic materials. Through these processes the charge carriers are driven to the interfaces with the electrodes after the excitonic dissociation in the case of photovoltaics [1], or are injected into the region of the active layer for subsequent radiative recombination in organic LEDs [2]. In any case, the path since the photogeneration until the capture (or recombination) of carriers involves a complex scenario of events, which are usually very dependent on the chemical and morphological characteristics of the medium in which they occur [3]. In thin films of conjugated polymers, these processes are closely related to the conformational properties of the polymer chains, which are influenced by the deposition parameters such as the method of deposition, the nature of the solvent, and also the concentration of defects and/or traps [4–6]. In addition, the temperature exerts a fundamental role in the migration processes. There is a consensus that decreasing temperature causes a planarization of the chains, which leads to a decrease in the intrinsic disorder of the material and consequently a greater delocalization of the excited state [7]. Still, with increasing temperature there is an increase in overall electron vibrational interaction couplings due to the localization of the electronic states. Such
∗ Corresponding author. Tel.: +55 43 3371 4266; fax: +55 43 3371 4166. E-mail addresses: [email protected], [email protected] (E. Laureto). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.03.003
effects have been evaluated by changes in Huang-Rhys factor extracted of the photoluminescence (PL) spectra [8], since the increased coupling should lead to an increase in the PL intensity of vibronic bands with respect to purely electronic band, as observed in oligomers [9]. However, Mulazzi et al. [10] attributed the change in the ratio between the PL bands intensity with temperature to energy migration processes from shorter to longer segments contained in the chains ensemble of poly(p-phenylene vinylene) (PPV) thin films. The evaluation of the fluorescence anisotropy is a valuable tool for studying the mechanisms of excitation migration in conjugated polymers [11]. When the sample is excited with polarized light, only the chromophores with transition moment parallel to the electric field vector of the incident beam are excited. Depolarization by rotation is hindered in solid state thin films [12], so that the change of the dipole moment orientation of the emission is related to the intrachain and interchain energy migration processes [13]. Fluorescence anisotropy measurements show the average angular displacement between the chromophores absorbing excitation and the chromophores emitting light during the lifetime of the excited state [14]. In this study, we have correlated the emission properties and the emission anisotropy factor with the structural characteristics of PPV thin films. Different deposition methods and parameters related to the production of the samples resulted in films with different structural properties, which were characterized by combining optical spectroscopy measurements as absorption, PL, and Raman scattering. This correlation has led to an analysis of the results presented in this paper taking into account the processes of
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Table 1 Molar concentration of solutions, method of deposition and volume/number of bilayers for the samples investigated in this work. Sample
PTHT solution (M)
DBS solution (M)
Method of deposition
Volume or number of bilayers
P0 P8 P14
1.1 × 10−2 1.4 × 10−3 7.9 × 10−4
– 10−2 10−2
Casting LbL LbL
50 l 20 20
excitation migration within these structurally different films. Thus, this paper aims to contribute to the discussion and elucidation of effects of the energy migration processes on the optical properties of conjugated polymers. In particular, the dependence of the intensity ratio between the PL bands and the emission anisotropy factor with the structural properties of the films allowed us to infer that the energy migration processes are crucial ingredients to analyzing the optical response of thin films of conjugated polymers.
2. Experimental The precursor polymer poly(xylylidene tetrahydrothiophenium chloride) (PTHT) solution (Aldrich, 0.25% (w/w), ref. 540765) and sodium dodecylbenzenesulfonate (DBS) salt (Aldrich, ref. 289957) were used as received. BK7 glass substrates were cleaned and hydrophylized before the film deposition. In sample P0, 50 l of the PTHT solution were deposited by drop casting. Samples P8 and P14 were produced by the layer-by-layer (LbL) method, using the precursor mass ratio of 1:8 and 1:14, respectively, diluted in MilliQ ultra-pure water (resistivity of 18.2 M cm−1 ). A DBS solution at 10−2 M was used in both depositions. All of these data are summarized in Table 1 for each sample investigated in this work. The LbL deposition cycle started by immersing the substrate in the solution of precursor for 30 s. Then the substrate was washed with ultrapure water and dried by spinning for 5 min [15]. It was subsequently immersed in the DBS solution for 30 s, and then washed and dried again. The cycle was repeated twenty times so that samples P8 and P14 were composed by 20 layers of PTHT-DBS. Thermal treatment of the films was performed in air for 30 min in a home-made oven maintained at 110 ◦ C, at ambient pressure. As indicated in Fig. 1, the result of this process is the thermal conversion of the PTHT or PTHTDBS layer in a partially conjugated PPV layer, through the elimination of the side group [16]. However, for sample P0 it is expected a less efficient conversion at this temperature, since bonding with Cl− counter-ion is more stable than with the sulfonate group [17].
The optical absorption measurements were made with a USB 4000 minispectrometer (Ocean Optics) using a model DT-Mini2-GS UV-VIS lamp. The same spectrometer was used in the PL measurements. As an excitation source we used a solid state laser emitting at 405 nm with very low power intensity. The samples were accommodated in a Janis He closed cycle cryostat held at a pressure below 10−4 Torr. The temperature was monitored by a LakeShore 330E temperature controller. The Raman scattering measurements were performed by a portable Raman equipment (DeltaNu), using a laser at 532 nm as excitation beam. For the polarized measurements, an LED emitting at 400 nm was used as excitation, and linear polarizers (Edmund Optics) with high extinction ratio were employed. The emission anisotropy factor (r) is obtained by Eq. (1): r=
(Ipara,para − GIpara,perp )
Ipara,para + 2GIpara,perp
(1)
where Ix,x refers to the PL intensity and the scripts indicate the polarization direction of linear polarizers (parallel or perpendicular to the direction of the film deposition) [18]. G is a spectroscopic correction factor determined by: G=
Iperp,para . Iperp,perp
(2)
3. Results Fig. 2 shows the absorbance curves and the normalized PL spectra at room temperature for the films analyzed in this work. The PL spectra are already corrected by the self-absorption effects [19]. The absorbance intensity of sample P0 was multiplied by 0.1 for easier comparison with the curves of other samples. It can be seen that both the absorbance curve as the PL spectrum of the sample P8 are shifted to lower energy, which indicates that the polymer chains have a longer effective conjugation length in this film than in the others. Another important feature is that the onset of absorption
Fig. 1. Thermal conversion process from PTHT to PPV (1), as used in sample P0, and from PTHT + DBS to PPV (2), as used in samples P8 and P14.
R.V. Fernandes et al. / Synthetic Metals 170 (2013) 25–30
27
Table 2 Assignment of the Raman active vibrational modes shown in Fig. 3.
Fig. 2. Absorbance (right) and PL (left) spectra at room temperature of the PPV films analyzed in this work. The absorbance curve of sample P0 was multiplied by 0.1 to facilitate comparison with other curves.
for the sample P14 is slightly extended to lower energies relative to the sample P0. The energy peaks of the PL spectra are very similar in these samples, with a small red-shift in the film P14; however, there is a considerable difference in intensity of the band centered at 2.35 eV (I1 ) with respect to the higher energy band (with a maximum around 2.5 eV, I0 ), when comparing the spectra of these two samples. Fig. 3 shows the Raman spectra of these films. The region analyzed refers to the most intense Raman peaks for these samples, and are normalized with respect to the most intense band centered around 1580 cm−1 . The assignment of these vibrational modes is indicated in Table 2 [20]. We observe two main aspects when the curves are compared: firstly, we noted that the band at 1625 cm−1 is more intense than the band at 1550 cm−1 for sample P0, and the I1550 /I1625 ratio is inverted in sample P8. Also we observed that the band at 1580 cm−1 is redshifted in sample P0 when compared with sample P8. The relationship between the intensity of Raman
Fig. 3. Raman scattering spectra of the PPV films analyzed in this work.
Frequency (cm−1 )
Assignment
1550 1586 1625
C C stretching of phenyl ring C C stretching of phenyl ring C C stretching of vinyl ring
bands centered at 1550 cm−1 and 1625 cm−1 was investigated in oligomers [21] and PPV films produced under different conditions [20]. These studies indicated that the I1550 /I1625 ratio increases with the increasing of the conjugation length of the chains. Furthermore, Nguyen et al. found that the band centered on 1580 cm−1 was redshifted with the conjugation increase in oligomers [21]. Based on the above informations, the data of Fig. 3 are indicating that in sample P0 there is predominantly shorter conjugated segments, while in the sample P8 there is predominance of longer segments. The configuration of polymeric segments in the film P14 seems to be an intermediate case. These Raman data are consistent with information obtained from the absorbance curves and PL spectra. As a result, we can then consider the sample P0 consisting of PPV chains with a small effective conjugation length, whereas in the sample P8 the chains have a higher effective conjugation length. Besides the Raman data, by observing the peak energies of absorbance and PL curves, we can deduce that there is also a predominance of shorter segments in film P14 when compared with film P8. To evaluate the influence of temperature on the PL spectra, the emission curves at 15 K, 80 K and 300 K for the samples P0 and P14 are shown in Fig. 4. A comparison of the curves shows that at 15 K the PL bands of sample P14 are wider and slightly redshifted with respect to the PL bands of the sample P0. The relative intensities of these bands are almost equal in both samples. This situation is maintained for the spectra taken at 80 K, but is considerably changed when the temperature rises to 300 K. In this case, the PL bands of the different samples have almost same width, but the intensity of the band centered around 2.35 eV (I1 ) is reinforced in the sample P0 with respect to the sample P14. The ratio I1 /I0 as a function of temperature is plotted in Fig. 5, for the films P0 (triangle symbol) and P14 (circle symbol). We can observe that this ratio increases with increasing temperature for sample P0, but it does not vary much for sample P14. It should be noted here that the spectra at low temperatures were not corrected for the self-absorption effects [19]. However, because of the very low absorbance of the
Fig. 4. PL spectra at 15, 80 and 300 K for the samples P0 (continuous line) and P14 (circle symbols).
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this film. Note also that, for all samples analyzed in this paper, the values of r were practically independent of energy.
4. Discussion
Fig. 5. Plot of the ratio I1 /I0 of the PL bands (shown in Fig. 3) as a function of temperature for the samples P0 (triangle symbols) and P14 (circle symbols). The lines are just a guide for the eyes.
film P14, we expect that self-absorption correction, if necessary, will change only the band intensities in the spectrum of the film P0. As the self-absorption is more important in the region of overlap of the absorbance and PL curves, we expected some distortion in the intensity of the band at 2.5 eV (I0 ). However, taking into account the correction of this effect only would become even clearer the difference in behavior of the ratio I1 /I0 as a function of temperature for the two films under consideration (since that the spectra at 300 K has already corrected by the self-absorption effects). The emission anisotropy factor (r) at room temperature for the films P0, P8 and P14 was plotted as a function of energy in Fig. 6. These curves were obtained through the four PL measurements, one for each setting of the linear polarizers according to Eqs. (1) and (2). There is a clear distinction between the r values for different films, which is related to the depolarization degree of the excitation in each case. For the film P0, the anisotropy factor is practically zero, indicating a nearly complete depolarization of the excitation. The film P8 had an anisotropy factor of almost 0.15, in agreement with values reported previously for similar films [22]. As for the film P14, the anisotropy is high (around 0.25), indicating that mechanisms leading to depolarization of the excitation were not as effective for
Fig. 6. Emission anisotropy factor of samples P0, P8 and P14 taken at room temperature and calculated from Eqs. (1) and (2) (see text for further details).
The films analyzed in this paper have distinct structural characteristics, as can be deduced from a combination of the absorbance, PL, and Raman scattering data shown in Figs. 2 and 3. Indeed, it is known that the thermal conversion efficiency of precursor polymer to PPV occurs at high temperatures (300 ◦ C) for relatively long periods of thermal treatment time (2–3 h). The conditions employed in the production and conversion of the sample P0 provided a PPV film composed mainly by chains with small conjugation length. The high absorbance value, combined with the high blueshift of the absorption band, the relative intensity of Raman lines centered at 1550 and 1625 cm−1 , and the blue-shift of the band around 1580 cm−1 (see Fig. 3), show that this film is characterized by a high density of highly segmented PPV chains. The use of DBS as a substituent counterion in the polymer precursor was a resource proposed by Marletta et al. [23] to obtain PPV films of high quality and high conjugation degree, using lower temperatures and shorter time intervals to conversion, conditions more suitable for the use of the polymer in devices [24]. Such advantages occur because of the less stable bonding between the phenyl ring and the RSO3 − group [25]. The LbL deposition proved to be a non-self-limiting process [26], and precise control of film thickness can be obtained from the number of repetitions of the process (i.e. the number of deposited layers). The properties of the sample P8 are typical of films obtained by LbL method using solutions with standard concentrations [16,22]. The red shifts of the absorbance and PL, as well as the Raman data shown in Fig. 3, demonstrate that the film P8 is composed predominantly by segments with a high conjugation degree. As for the sample P14, a blue shift of the maximum absorption and PL is observed as compared with sample P8, as well as an absorbance with very low intensity. Therezio et al. [27] and Massuyeau et al. [17] have observed the same shift of the spectra for PPV films produced with fewer layers or by using dilute solutions of the precursor, respectively. Both of the authors have credited these spectral features to the low effective conjugation length of the film. The low intensity of absorbance was correlated with the low thickness of the film in ref. [27]. In our case we believe, due to the high dilution of the precursor in sample P14, the resulting film has a low density of chains and therefore it is composed by polymer chains widely spaced from each other, resulting in a low absorbance intensity. For the energy positions of maximum absorbance and PL, and the Raman spectrum analysis, we inferred that these chains are composed predominantly by segments with an effective conjugation length slightly larger than in sample P0, but smaller than the conjugation length predominantly found in the film P8. These conclusions are supported by comparative analysis of the Raman scattering curves contained in Fig. 3, based on the results reported in previous studies [21]. In short, the thin films analyzed in this paper have remarkable structural differences: in the sample P0, there is a high volumetric density of polymeric chains mainly composed by small conjugated segments. For the sample P8, the segments have a larger effective conjugation length, and the film has optical properties typical of standard PPV films [28]. In sample P14, in addition to the prevalence of low conjugated segments, the chains are spaced apart, because of low concentration of the polymer precursor used for the production of this film. The structural differences of the films can be used to explain the behavior as a function of temperature of the ratio I1 /I0 of the most intense bands in the PL spectra as shown in Fig. 4. Following
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the model proposed by Mulazzi et al. [10,28], the I1 increment with the increasing temperature is related to the migration process involving the interchain energy homotransfer from shorter segments (with emission centered around 2.5 eV, I0 band) to longer segments (emission around 2.35 eV, I1 band) within the film. The broadening of PL bands with increasing temperature can also be an indication of the energy redistribution between the conjugated segments participating in the emission process. However, the thermally activated migration process is also dependent on the distance between the chains, then being hampered in film P14, which results in almost constant I1 /I0 ratio for this sample as shown in Fig. 5 (circle symbols). This analysis is corroborated by data from emission anisotropy: because migration is hampered in the sample P14, the excitation slightly depolarizes, resulting in a high value of anisotropy of the emitting state. However for sample P0, depolarization of the excitation is almost complete due to the efficient interchain migration process, and the anisotropy factor results very close to zero. Defects [17] and/or film-substrate interaction [29] are aspects that cannot be discarded in the analysis of the results presented in this paper. Even though the film P14 contains 20 layers of PTHT-DBS converted to PPV, the high dilution of the precursor may result in a higher incidence of defects in the bulk film. Furthermore, the interaction with the substrate may extend beyond the limit proposed by Therezio et al. [29], since the solution of precursor used in their work is different from that employed here. A more detailed study of the influence of the concentration of the precursor is necessary to elucidate this question. Either way, both defects and film-substrate interaction would contribute to hamper the interchain migration of the charge carriers, which does not invalidate the analysis done here. Another aspect that could be contributing to the increase in the anisotropy factor in the LbL films is related to the molecular ordering. It is known that self-assembled deposition can lead to a preferential direction of alignment of the polymer chains [25–27]. A measure of the molecular ordering degree in polymers is provided by the dichroic ratio (ı), defined as the ratio of the intensities of the absorbance with light linearly polarized parallel and perpendicularly to the direction of molecular ordering [22]. However, measurements of dichroic ratio at 405 nm (not shown here) resulted in very similar values (ı∼1.1) for all samples analyzed in this work. We suggested that the drying method by spinning used in the production of our LbL films may be contributing to the directional randomness in the adsorption of the polymeric chains. The dependence of the Huang-Rhys factor with temperature, as discussed in previous works [30,31], applies very well to sample P8, which represents the “standard” material. However, for films that differ from this standard condition in terms of structural properties, as is the case obtained by the procedures adopted in the production of films P0 and P14 shown in this paper, it should be noted that the efficiency or the difficulty of the migration process of the photogenerated carriers by the interaction between the chains must be taken into account in the analysis of optical spectra of conjugated polymers.
5. Conclusion There is currently an amount of papers investigating the effects of migration and energy transfer in organic semiconductors. The goal is to understand the broad scenario of events that may occur during the relaxation process of photogenerated carriers within their lifetime. The importance of the elucidation and control of these events is crucial to improving the performance of semiconductor devices based on organic materials. Regarding this matter, the contribution of this work was to demonstrate that the emission
29
properties of the films, such as PL line shape and anisotropy factor, have a connection with their structural parameters. Thin films of PPV were produced from different deposition conditions and served as appropriate templates to investigate the dependence of emission properties with the structure of the film in terms of effective conjugation and interaction between polymer chains. The high value of emission anisotropy factor of the PPV film produced by the thermal conversion of the highly diluted solution of precursor indicated that the processes of energy migration were hindered in this film, which affected significantly the behavior of PL line shape as a function of temperature. As for another sample with lower effective conjugation length but with higher volumetric density of polymeric chains, migration between segments is facilitated and the steady state emission anisotropy is practically zero, resulting in changes in the PL line shape. Finally, the results presented in this work demonstrate that the events related to the excitation migration are essential ingredients for a proper analysis of the optical properties of conjugated polymers. Acknowledgments The authors would like to acknowledge the financial support granted by the following Brazilian agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior (CAPES), and Fundac¸ão Araucária. References [1] J.L. Bredas, J.E. Norton, J. Cornil, V. Coropceanu, Accounts of Chemical Research 42 (2009) 1691. [2] L. Akcelrud, Progress in Polymer Science 28 (2003) 875. [3] T.E. Dykstra, E. Hennebicq, D. Beljonne, J. Gierschner, G. Claudio, E.R. Bittner, J. Knoester, G.D. Scholes, Journal of Physical Chemistry B 113 (2009) 656. [4] R.F. Cossielo, M.D. Susman, P.F. Aramendía, T.D.Z. Atvars, Journal of Luminescence 130 (2010) 415. [5] S. Saini, B. Bagchi, Physical Chemistry Chemical Physics 12 (2010) 7427. [6] I.A. Levitsky, J. Kim, T.M. Swager, Journal of the American Chemical Society 121 (1999) 1466. [7] A.J. Heeger, S. Kivelson, J.R. Schieffer, W.P. Su, Reviews of Modern Physics 60 (1988) 781. [8] O. Narwark, A. Gerhard, S.C.J. Meskers, S. Brocke, E. Thorn-Csányi, H. Bässler, Chemical Physics 294 (2003) 17. [9] J. Cornil, D. Beljonne, Z. Shuai, T.W. Hagler, I. Campbell, D.D.C. Bradley, J.L. Brédas, C.W. Spangler, K. Müllen, Chemical Physics Letters 247 (1995) 425. [10] E. Mulazzi, R. Perego, J. Wéry, L. Mihut, S. Lefrant, E. Faulques, Journal of Chemical Physics 125 (2006) 014703. [11] H.L. Vaughan, A.P. Monkman, L.O. Pälsson, B.S. Nehls, T. Farrel, U. Scherf, Journal of Chemical Physics 128 (2008) 044709. [12] F. Montilla, L.M. Frutos, C.R. Mateo, R. Mallavia, Journal of Physical Chemistry C 111 (2007) 18405. [13] A. Rose, J.D. Tovar, S. Yamaguchi, E.E. Nesterov, Z. Zhu, T.M. Swager, Philosophical Transactions of the Royal Society A 365 (2007) 1589. [14] B. Valeur, Molecular Fluorescence – Principles and Applications, Wiley-VCH Verlag, Germany, 2001. [15] H.R. Favarim, D. Spadacio, A.D. Faceto, V. Zucolotto, O.N. Oliveira Jr., F.E.G. Guimaraes, Advanced Materials 17 (2007) 2862. [16] A. Marletta, R.A. Silva, P.A. Ribeiro, M. Raposo, D. Gonc¸alves, Journal of NonCrystalline Solids 354 (2008) 4856. [17] F. Massuyeau, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. Wéry, E. Mulazzi, R. Perego, Journal of Physical Chemistry C 111 (2007) 15111. [18] J.R. Lackowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 1999. [19] S.M. Cassemiro, F. Thomazi, L.S. Roman, A. Marletta, L. Akcelrud, Synthetic Metals 159 (2009) 1975. [20] E. Mulazzi, A. Ripamonti, J. Wéry, B. Dulieu, S. Lefrant, Physical Review B 60 (1999) 16519. [21] T.P. Nguyen, V.H. Tran, P. Destrue, D. Oelkrug, Synthetic Metals 101 (1999) 633. [22] E. Laureto, M.A.T. da Silva, R.V. Fernandes, J.L. Duarte, I.F.L. Dias, A. Marletta, Current Applied Physics 12 (2012) 870. [23] A. Marletta, D. Gonc¸alves, O.N. Oliveira Jr., R.M. Faria, F.E.G. Guimaraes, Macromolecules 33 (2000) 5886. [24] L.C. Pocas, S.L. Nogueira, R.S. Nobuyasu, G.G. Dalkiranis, M.J.M. Pires, J.R. Tozoni, R.A. Silva, A. Marletta, Journal of Materials Science 46 (2011) 2644. [25] A. Marletta., D. Gonc¸alves, O.N. Oliveira Jr., R.M. Faria, F.E.G. Guimaraes, Advanced Materials 12 (2000) 69. [26] M. Ferreira, M.F. Rubner, Macromolecules 28 (1995) 7107.
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[27] E.M. Therezio, E. Piovesan, M.L. Vega, R.A. Silva, O.N. Oliveira Jr., A. Marletta, Journal of Polymer Science Part B: Polymer Physics 49 (2011) 206. [28] E. Mulazzi, C. Botta, D. Facchinetti, A. Bolognesi, Synthetic Metals 142 (2004) 85. [29] E.M. Therezio, E. Piovesan, M. Anni, R.A. Silva, O.N. Oliveira Jr., A. Marletta, Journal of Applied Physics 110 (2011) 044504.
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Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Correlation between emission and structural properties of poly(p-phenylene vinylene) thin films R.V. Fernandes a , M.A.T. da Silva a , I.F.L. Dias a , J.L. Duarte a , H. de Santana b , E. Laureto a,∗ a b
Departamento de Física, Centro de Ciências Exatas, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil Departamento de Química, Centro de Ciências Exatas, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 18 August 2012 Accepted 4 March 2013 Keywords: Conjugated polymers Thin films Photoluminescence spectroscopy Optical properties Energy transfer
a b s t r a c t In this work we used different methods and parameters of deposition in order to obtain thin films of poly(p-phenylene vinylene) (PPV) with different structural properties. These properties were characterized by measurements of absorbance, Raman scattering, and photoluminescence (PL) as a function of temperature. The analysis of experimental data allowed us to observe the dependence of optical spectra with the effective conjugation length and the interaction between the polymer chains. In particular, we were able to correlate the dependence of the PL line shape with temperature and the values of emission anisotropy factor of the films with the intra and interchains energy migration processes. These results led us to conclude that such processes are essential ingredients for a proper analysis of the optical response of thin films of conjugated polymers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Understanding and control of migration processes of photogenerated carriers are crucial for the application of conjugated polymers in devices such as light emitting diodes (LEDs) and photovoltaic cells based on organic materials. Through these processes the charge carriers are driven to the interfaces with the electrodes after the excitonic dissociation in the case of photovoltaics [1], or are injected into the region of the active layer for subsequent radiative recombination in organic LEDs [2]. In any case, the path since the photogeneration until the capture (or recombination) of carriers involves a complex scenario of events, which are usually very dependent on the chemical and morphological characteristics of the medium in which they occur [3]. In thin films of conjugated polymers, these processes are closely related to the conformational properties of the polymer chains, which are influenced by the deposition parameters such as the method of deposition, the nature of the solvent, and also the concentration of defects and/or traps [4–6]. In addition, the temperature exerts a fundamental role in the migration processes. There is a consensus that decreasing temperature causes a planarization of the chains, which leads to a decrease in the intrinsic disorder of the material and consequently a greater delocalization of the excited state [7]. Still, with increasing temperature there is an increase in overall electron vibrational interaction couplings due to the localization of the electronic states. Such
∗ Corresponding author. Tel.: +55 43 3371 4266; fax: +55 43 3371 4166. E-mail addresses: [email protected], [email protected] (E. Laureto). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.03.003
effects have been evaluated by changes in Huang-Rhys factor extracted of the photoluminescence (PL) spectra [8], since the increased coupling should lead to an increase in the PL intensity of vibronic bands with respect to purely electronic band, as observed in oligomers [9]. However, Mulazzi et al. [10] attributed the change in the ratio between the PL bands intensity with temperature to energy migration processes from shorter to longer segments contained in the chains ensemble of poly(p-phenylene vinylene) (PPV) thin films. The evaluation of the fluorescence anisotropy is a valuable tool for studying the mechanisms of excitation migration in conjugated polymers [11]. When the sample is excited with polarized light, only the chromophores with transition moment parallel to the electric field vector of the incident beam are excited. Depolarization by rotation is hindered in solid state thin films [12], so that the change of the dipole moment orientation of the emission is related to the intrachain and interchain energy migration processes [13]. Fluorescence anisotropy measurements show the average angular displacement between the chromophores absorbing excitation and the chromophores emitting light during the lifetime of the excited state [14]. In this study, we have correlated the emission properties and the emission anisotropy factor with the structural characteristics of PPV thin films. Different deposition methods and parameters related to the production of the samples resulted in films with different structural properties, which were characterized by combining optical spectroscopy measurements as absorption, PL, and Raman scattering. This correlation has led to an analysis of the results presented in this paper taking into account the processes of
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Table 1 Molar concentration of solutions, method of deposition and volume/number of bilayers for the samples investigated in this work. Sample
PTHT solution (M)
DBS solution (M)
Method of deposition
Volume or number of bilayers
P0 P8 P14
1.1 × 10−2 1.4 × 10−3 7.9 × 10−4
– 10−2 10−2
Casting LbL LbL
50 l 20 20
excitation migration within these structurally different films. Thus, this paper aims to contribute to the discussion and elucidation of effects of the energy migration processes on the optical properties of conjugated polymers. In particular, the dependence of the intensity ratio between the PL bands and the emission anisotropy factor with the structural properties of the films allowed us to infer that the energy migration processes are crucial ingredients to analyzing the optical response of thin films of conjugated polymers.
2. Experimental The precursor polymer poly(xylylidene tetrahydrothiophenium chloride) (PTHT) solution (Aldrich, 0.25% (w/w), ref. 540765) and sodium dodecylbenzenesulfonate (DBS) salt (Aldrich, ref. 289957) were used as received. BK7 glass substrates were cleaned and hydrophylized before the film deposition. In sample P0, 50 l of the PTHT solution were deposited by drop casting. Samples P8 and P14 were produced by the layer-by-layer (LbL) method, using the precursor mass ratio of 1:8 and 1:14, respectively, diluted in MilliQ ultra-pure water (resistivity of 18.2 M cm−1 ). A DBS solution at 10−2 M was used in both depositions. All of these data are summarized in Table 1 for each sample investigated in this work. The LbL deposition cycle started by immersing the substrate in the solution of precursor for 30 s. Then the substrate was washed with ultrapure water and dried by spinning for 5 min [15]. It was subsequently immersed in the DBS solution for 30 s, and then washed and dried again. The cycle was repeated twenty times so that samples P8 and P14 were composed by 20 layers of PTHT-DBS. Thermal treatment of the films was performed in air for 30 min in a home-made oven maintained at 110 ◦ C, at ambient pressure. As indicated in Fig. 1, the result of this process is the thermal conversion of the PTHT or PTHTDBS layer in a partially conjugated PPV layer, through the elimination of the side group [16]. However, for sample P0 it is expected a less efficient conversion at this temperature, since bonding with Cl− counter-ion is more stable than with the sulfonate group [17].
The optical absorption measurements were made with a USB 4000 minispectrometer (Ocean Optics) using a model DT-Mini2-GS UV-VIS lamp. The same spectrometer was used in the PL measurements. As an excitation source we used a solid state laser emitting at 405 nm with very low power intensity. The samples were accommodated in a Janis He closed cycle cryostat held at a pressure below 10−4 Torr. The temperature was monitored by a LakeShore 330E temperature controller. The Raman scattering measurements were performed by a portable Raman equipment (DeltaNu), using a laser at 532 nm as excitation beam. For the polarized measurements, an LED emitting at 400 nm was used as excitation, and linear polarizers (Edmund Optics) with high extinction ratio were employed. The emission anisotropy factor (r) is obtained by Eq. (1): r=
(Ipara,para − GIpara,perp )
Ipara,para + 2GIpara,perp
(1)
where Ix,x refers to the PL intensity and the scripts indicate the polarization direction of linear polarizers (parallel or perpendicular to the direction of the film deposition) [18]. G is a spectroscopic correction factor determined by: G=
Iperp,para . Iperp,perp
(2)
3. Results Fig. 2 shows the absorbance curves and the normalized PL spectra at room temperature for the films analyzed in this work. The PL spectra are already corrected by the self-absorption effects [19]. The absorbance intensity of sample P0 was multiplied by 0.1 for easier comparison with the curves of other samples. It can be seen that both the absorbance curve as the PL spectrum of the sample P8 are shifted to lower energy, which indicates that the polymer chains have a longer effective conjugation length in this film than in the others. Another important feature is that the onset of absorption
Fig. 1. Thermal conversion process from PTHT to PPV (1), as used in sample P0, and from PTHT + DBS to PPV (2), as used in samples P8 and P14.
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Table 2 Assignment of the Raman active vibrational modes shown in Fig. 3.
Fig. 2. Absorbance (right) and PL (left) spectra at room temperature of the PPV films analyzed in this work. The absorbance curve of sample P0 was multiplied by 0.1 to facilitate comparison with other curves.
for the sample P14 is slightly extended to lower energies relative to the sample P0. The energy peaks of the PL spectra are very similar in these samples, with a small red-shift in the film P14; however, there is a considerable difference in intensity of the band centered at 2.35 eV (I1 ) with respect to the higher energy band (with a maximum around 2.5 eV, I0 ), when comparing the spectra of these two samples. Fig. 3 shows the Raman spectra of these films. The region analyzed refers to the most intense Raman peaks for these samples, and are normalized with respect to the most intense band centered around 1580 cm−1 . The assignment of these vibrational modes is indicated in Table 2 [20]. We observe two main aspects when the curves are compared: firstly, we noted that the band at 1625 cm−1 is more intense than the band at 1550 cm−1 for sample P0, and the I1550 /I1625 ratio is inverted in sample P8. Also we observed that the band at 1580 cm−1 is redshifted in sample P0 when compared with sample P8. The relationship between the intensity of Raman
Fig. 3. Raman scattering spectra of the PPV films analyzed in this work.
Frequency (cm−1 )
Assignment
1550 1586 1625
C C stretching of phenyl ring C C stretching of phenyl ring C C stretching of vinyl ring
bands centered at 1550 cm−1 and 1625 cm−1 was investigated in oligomers [21] and PPV films produced under different conditions [20]. These studies indicated that the I1550 /I1625 ratio increases with the increasing of the conjugation length of the chains. Furthermore, Nguyen et al. found that the band centered on 1580 cm−1 was redshifted with the conjugation increase in oligomers [21]. Based on the above informations, the data of Fig. 3 are indicating that in sample P0 there is predominantly shorter conjugated segments, while in the sample P8 there is predominance of longer segments. The configuration of polymeric segments in the film P14 seems to be an intermediate case. These Raman data are consistent with information obtained from the absorbance curves and PL spectra. As a result, we can then consider the sample P0 consisting of PPV chains with a small effective conjugation length, whereas in the sample P8 the chains have a higher effective conjugation length. Besides the Raman data, by observing the peak energies of absorbance and PL curves, we can deduce that there is also a predominance of shorter segments in film P14 when compared with film P8. To evaluate the influence of temperature on the PL spectra, the emission curves at 15 K, 80 K and 300 K for the samples P0 and P14 are shown in Fig. 4. A comparison of the curves shows that at 15 K the PL bands of sample P14 are wider and slightly redshifted with respect to the PL bands of the sample P0. The relative intensities of these bands are almost equal in both samples. This situation is maintained for the spectra taken at 80 K, but is considerably changed when the temperature rises to 300 K. In this case, the PL bands of the different samples have almost same width, but the intensity of the band centered around 2.35 eV (I1 ) is reinforced in the sample P0 with respect to the sample P14. The ratio I1 /I0 as a function of temperature is plotted in Fig. 5, for the films P0 (triangle symbol) and P14 (circle symbol). We can observe that this ratio increases with increasing temperature for sample P0, but it does not vary much for sample P14. It should be noted here that the spectra at low temperatures were not corrected for the self-absorption effects [19]. However, because of the very low absorbance of the
Fig. 4. PL spectra at 15, 80 and 300 K for the samples P0 (continuous line) and P14 (circle symbols).
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this film. Note also that, for all samples analyzed in this paper, the values of r were practically independent of energy.
4. Discussion
Fig. 5. Plot of the ratio I1 /I0 of the PL bands (shown in Fig. 3) as a function of temperature for the samples P0 (triangle symbols) and P14 (circle symbols). The lines are just a guide for the eyes.
film P14, we expect that self-absorption correction, if necessary, will change only the band intensities in the spectrum of the film P0. As the self-absorption is more important in the region of overlap of the absorbance and PL curves, we expected some distortion in the intensity of the band at 2.5 eV (I0 ). However, taking into account the correction of this effect only would become even clearer the difference in behavior of the ratio I1 /I0 as a function of temperature for the two films under consideration (since that the spectra at 300 K has already corrected by the self-absorption effects). The emission anisotropy factor (r) at room temperature for the films P0, P8 and P14 was plotted as a function of energy in Fig. 6. These curves were obtained through the four PL measurements, one for each setting of the linear polarizers according to Eqs. (1) and (2). There is a clear distinction between the r values for different films, which is related to the depolarization degree of the excitation in each case. For the film P0, the anisotropy factor is practically zero, indicating a nearly complete depolarization of the excitation. The film P8 had an anisotropy factor of almost 0.15, in agreement with values reported previously for similar films [22]. As for the film P14, the anisotropy is high (around 0.25), indicating that mechanisms leading to depolarization of the excitation were not as effective for
Fig. 6. Emission anisotropy factor of samples P0, P8 and P14 taken at room temperature and calculated from Eqs. (1) and (2) (see text for further details).
The films analyzed in this paper have distinct structural characteristics, as can be deduced from a combination of the absorbance, PL, and Raman scattering data shown in Figs. 2 and 3. Indeed, it is known that the thermal conversion efficiency of precursor polymer to PPV occurs at high temperatures (300 ◦ C) for relatively long periods of thermal treatment time (2–3 h). The conditions employed in the production and conversion of the sample P0 provided a PPV film composed mainly by chains with small conjugation length. The high absorbance value, combined with the high blueshift of the absorption band, the relative intensity of Raman lines centered at 1550 and 1625 cm−1 , and the blue-shift of the band around 1580 cm−1 (see Fig. 3), show that this film is characterized by a high density of highly segmented PPV chains. The use of DBS as a substituent counterion in the polymer precursor was a resource proposed by Marletta et al. [23] to obtain PPV films of high quality and high conjugation degree, using lower temperatures and shorter time intervals to conversion, conditions more suitable for the use of the polymer in devices [24]. Such advantages occur because of the less stable bonding between the phenyl ring and the RSO3 − group [25]. The LbL deposition proved to be a non-self-limiting process [26], and precise control of film thickness can be obtained from the number of repetitions of the process (i.e. the number of deposited layers). The properties of the sample P8 are typical of films obtained by LbL method using solutions with standard concentrations [16,22]. The red shifts of the absorbance and PL, as well as the Raman data shown in Fig. 3, demonstrate that the film P8 is composed predominantly by segments with a high conjugation degree. As for the sample P14, a blue shift of the maximum absorption and PL is observed as compared with sample P8, as well as an absorbance with very low intensity. Therezio et al. [27] and Massuyeau et al. [17] have observed the same shift of the spectra for PPV films produced with fewer layers or by using dilute solutions of the precursor, respectively. Both of the authors have credited these spectral features to the low effective conjugation length of the film. The low intensity of absorbance was correlated with the low thickness of the film in ref. [27]. In our case we believe, due to the high dilution of the precursor in sample P14, the resulting film has a low density of chains and therefore it is composed by polymer chains widely spaced from each other, resulting in a low absorbance intensity. For the energy positions of maximum absorbance and PL, and the Raman spectrum analysis, we inferred that these chains are composed predominantly by segments with an effective conjugation length slightly larger than in sample P0, but smaller than the conjugation length predominantly found in the film P8. These conclusions are supported by comparative analysis of the Raman scattering curves contained in Fig. 3, based on the results reported in previous studies [21]. In short, the thin films analyzed in this paper have remarkable structural differences: in the sample P0, there is a high volumetric density of polymeric chains mainly composed by small conjugated segments. For the sample P8, the segments have a larger effective conjugation length, and the film has optical properties typical of standard PPV films [28]. In sample P14, in addition to the prevalence of low conjugated segments, the chains are spaced apart, because of low concentration of the polymer precursor used for the production of this film. The structural differences of the films can be used to explain the behavior as a function of temperature of the ratio I1 /I0 of the most intense bands in the PL spectra as shown in Fig. 4. Following
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the model proposed by Mulazzi et al. [10,28], the I1 increment with the increasing temperature is related to the migration process involving the interchain energy homotransfer from shorter segments (with emission centered around 2.5 eV, I0 band) to longer segments (emission around 2.35 eV, I1 band) within the film. The broadening of PL bands with increasing temperature can also be an indication of the energy redistribution between the conjugated segments participating in the emission process. However, the thermally activated migration process is also dependent on the distance between the chains, then being hampered in film P14, which results in almost constant I1 /I0 ratio for this sample as shown in Fig. 5 (circle symbols). This analysis is corroborated by data from emission anisotropy: because migration is hampered in the sample P14, the excitation slightly depolarizes, resulting in a high value of anisotropy of the emitting state. However for sample P0, depolarization of the excitation is almost complete due to the efficient interchain migration process, and the anisotropy factor results very close to zero. Defects [17] and/or film-substrate interaction [29] are aspects that cannot be discarded in the analysis of the results presented in this paper. Even though the film P14 contains 20 layers of PTHT-DBS converted to PPV, the high dilution of the precursor may result in a higher incidence of defects in the bulk film. Furthermore, the interaction with the substrate may extend beyond the limit proposed by Therezio et al. [29], since the solution of precursor used in their work is different from that employed here. A more detailed study of the influence of the concentration of the precursor is necessary to elucidate this question. Either way, both defects and film-substrate interaction would contribute to hamper the interchain migration of the charge carriers, which does not invalidate the analysis done here. Another aspect that could be contributing to the increase in the anisotropy factor in the LbL films is related to the molecular ordering. It is known that self-assembled deposition can lead to a preferential direction of alignment of the polymer chains [25–27]. A measure of the molecular ordering degree in polymers is provided by the dichroic ratio (ı), defined as the ratio of the intensities of the absorbance with light linearly polarized parallel and perpendicularly to the direction of molecular ordering [22]. However, measurements of dichroic ratio at 405 nm (not shown here) resulted in very similar values (ı∼1.1) for all samples analyzed in this work. We suggested that the drying method by spinning used in the production of our LbL films may be contributing to the directional randomness in the adsorption of the polymeric chains. The dependence of the Huang-Rhys factor with temperature, as discussed in previous works [30,31], applies very well to sample P8, which represents the “standard” material. However, for films that differ from this standard condition in terms of structural properties, as is the case obtained by the procedures adopted in the production of films P0 and P14 shown in this paper, it should be noted that the efficiency or the difficulty of the migration process of the photogenerated carriers by the interaction between the chains must be taken into account in the analysis of optical spectra of conjugated polymers.
5. Conclusion There is currently an amount of papers investigating the effects of migration and energy transfer in organic semiconductors. The goal is to understand the broad scenario of events that may occur during the relaxation process of photogenerated carriers within their lifetime. The importance of the elucidation and control of these events is crucial to improving the performance of semiconductor devices based on organic materials. Regarding this matter, the contribution of this work was to demonstrate that the emission
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properties of the films, such as PL line shape and anisotropy factor, have a connection with their structural parameters. Thin films of PPV were produced from different deposition conditions and served as appropriate templates to investigate the dependence of emission properties with the structure of the film in terms of effective conjugation and interaction between polymer chains. The high value of emission anisotropy factor of the PPV film produced by the thermal conversion of the highly diluted solution of precursor indicated that the processes of energy migration were hindered in this film, which affected significantly the behavior of PL line shape as a function of temperature. As for another sample with lower effective conjugation length but with higher volumetric density of polymeric chains, migration between segments is facilitated and the steady state emission anisotropy is practically zero, resulting in changes in the PL line shape. Finally, the results presented in this work demonstrate that the events related to the excitation migration are essential ingredients for a proper analysis of the optical properties of conjugated polymers. Acknowledgments The authors would like to acknowledge the financial support granted by the following Brazilian agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior (CAPES), and Fundac¸ão Araucária. References [1] J.L. Bredas, J.E. Norton, J. Cornil, V. Coropceanu, Accounts of Chemical Research 42 (2009) 1691. [2] L. Akcelrud, Progress in Polymer Science 28 (2003) 875. [3] T.E. Dykstra, E. Hennebicq, D. Beljonne, J. Gierschner, G. Claudio, E.R. Bittner, J. Knoester, G.D. Scholes, Journal of Physical Chemistry B 113 (2009) 656. [4] R.F. Cossielo, M.D. Susman, P.F. Aramendía, T.D.Z. Atvars, Journal of Luminescence 130 (2010) 415. [5] S. Saini, B. Bagchi, Physical Chemistry Chemical Physics 12 (2010) 7427. [6] I.A. Levitsky, J. Kim, T.M. Swager, Journal of the American Chemical Society 121 (1999) 1466. [7] A.J. Heeger, S. Kivelson, J.R. Schieffer, W.P. Su, Reviews of Modern Physics 60 (1988) 781. [8] O. Narwark, A. Gerhard, S.C.J. Meskers, S. Brocke, E. Thorn-Csányi, H. Bässler, Chemical Physics 294 (2003) 17. [9] J. Cornil, D. Beljonne, Z. Shuai, T.W. Hagler, I. Campbell, D.D.C. Bradley, J.L. Brédas, C.W. Spangler, K. Müllen, Chemical Physics Letters 247 (1995) 425. [10] E. Mulazzi, R. Perego, J. Wéry, L. Mihut, S. Lefrant, E. Faulques, Journal of Chemical Physics 125 (2006) 014703. [11] H.L. Vaughan, A.P. Monkman, L.O. Pälsson, B.S. Nehls, T. Farrel, U. Scherf, Journal of Chemical Physics 128 (2008) 044709. [12] F. Montilla, L.M. Frutos, C.R. Mateo, R. Mallavia, Journal of Physical Chemistry C 111 (2007) 18405. [13] A. Rose, J.D. Tovar, S. Yamaguchi, E.E. Nesterov, Z. Zhu, T.M. Swager, Philosophical Transactions of the Royal Society A 365 (2007) 1589. [14] B. Valeur, Molecular Fluorescence – Principles and Applications, Wiley-VCH Verlag, Germany, 2001. [15] H.R. Favarim, D. Spadacio, A.D. Faceto, V. Zucolotto, O.N. Oliveira Jr., F.E.G. Guimaraes, Advanced Materials 17 (2007) 2862. [16] A. Marletta, R.A. Silva, P.A. Ribeiro, M. Raposo, D. Gonc¸alves, Journal of NonCrystalline Solids 354 (2008) 4856. [17] F. Massuyeau, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. Wéry, E. Mulazzi, R. Perego, Journal of Physical Chemistry C 111 (2007) 15111. [18] J.R. Lackowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 1999. [19] S.M. Cassemiro, F. Thomazi, L.S. Roman, A. Marletta, L. Akcelrud, Synthetic Metals 159 (2009) 1975. [20] E. Mulazzi, A. Ripamonti, J. Wéry, B. Dulieu, S. Lefrant, Physical Review B 60 (1999) 16519. [21] T.P. Nguyen, V.H. Tran, P. Destrue, D. Oelkrug, Synthetic Metals 101 (1999) 633. [22] E. Laureto, M.A.T. da Silva, R.V. Fernandes, J.L. Duarte, I.F.L. Dias, A. Marletta, Current Applied Physics 12 (2012) 870. [23] A. Marletta, D. Gonc¸alves, O.N. Oliveira Jr., R.M. Faria, F.E.G. Guimaraes, Macromolecules 33 (2000) 5886. [24] L.C. Pocas, S.L. Nogueira, R.S. Nobuyasu, G.G. Dalkiranis, M.J.M. Pires, J.R. Tozoni, R.A. Silva, A. Marletta, Journal of Materials Science 46 (2011) 2644. [25] A. Marletta., D. Gonc¸alves, O.N. Oliveira Jr., R.M. Faria, F.E.G. Guimaraes, Advanced Materials 12 (2000) 69. [26] M. Ferreira, M.F. Rubner, Macromolecules 28 (1995) 7107.
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