A detailed study of the photophysics of organic semiconducting nanospheres

Synthetic Metals 139 (2003) 609–612

A detailed study of the photophysics of organic semiconducting nanospheres T. Piok a,b,∗ , L. Romaner a,b , C. Gadermaier a,b , F.P. Wenzl a,b , S. Patil c , R. Montenegro d , K. Landfester d , G. Lanzani e , G. Cerullo e , U. Scherf c , E.J.W. List a,b a

c

Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria b Institute of Nanostructured Materials and Photonics, A-8160 Weiz, Austria Makromolekulare Chemie, Fachbereich Chemie, Bergische Universität Wuppertal, Gauß-Str. 20, D-42097 Wuppertal, Germany d Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany e Dipartimento di Fisica, Politecnico di Milano, National Laboratory for Ultrafast and Ultraintense Optical Science—INFM, Piazza L. da Vinci 32, I-20133 Milano, Italy

Abstract Conjugated polymers are an intriguing class of organic semiconductors which can be utilized as active medium in a wide range of electronic applications. Semiconducting polymer nanospheres fabricated from a conjugated polymer dispersed in aqueous phase have been realized successfully utilizing the miniemulsion process. This class of material combines the properties of conjugated polymers and nanostructured matter and can overcome certain limitations in the film formation of defined multi-layer structures and self assemblies and can, therefore, enable new device design concepts. We present first spectroscopic investigations to characterize nanospheres of methyl-substituted ladder-type poly(para-phenylene) in film and compare them to conventional bulk films of the same polymer. The dynamics of the various photo-excited states is probed via both steady state and transient differential transmission spectroscopy. © 2003 Elsevier B.V. All rights reserved. Keywords: Conjugated polymer; Miniemulsion; Time resolved spectroscopy; Nanostructured materials

1. Introduction The controlled deposition of functional materials on the nano- to mesoscopic scale is one of the key challenges on the way to nano-scale organic electronics. Controlling the material at such a scale one will be able to fabricate improved as well as novel devices from organic semiconductors such as conjugated polymers in a cost-effective way. As demonstrated over the last decade, conjugated polymers can be utilized as the active medium in various optoelectronic applications [1–4]. By adopting the so-called miniemulsion principle one can combine the properties of semiconducting polymers with those of nanostructured matter, as recently shown [5]. Miniemulsions are stable emulsions of droplets with a distinct size of 50–500 nm, achieved by shearing a system containing water, a solution of a highly water insoluble com-

pound (the so-called hydrophobe, e.g. a polymer) and a small amount of a surfactant [6]. Semiconducting conjugated polymer nanospheres (SPNs) have been produced successfully from such a water-based miniemulsion. This novel approach enables the application of environmentally safe water-based deposition techniques such as ink-jet printing and opens new ways for controlled multilayer formation, simply by using one of the materials as an aqueous SPN dispersion [7,8]. A first basic study of the optoelectronic properties of SPNs fabricated from methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) is given [9]. In the present work, we show a more comprehensive study including a comparison between the regular m-LPPP and m-LPPP SPNs via photomodulation spectroscopy and femtosecond pump-probe spectroscopy.

2. Experimental ∗ Corresponding author. E-mail address: [email protected] (T. Piok).

0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00251-0

The preparation of the SPNs is described in detail in [5] while the synthesis of the utilized conjugated polymer

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m-LPPP is described in [10]. All films were spin cast on cleaned glass substrates. UV-Vis transmission spectra were measured using a Perkin-Elmer ␭9 spectrophotometer. Photoluminescence (PL) emission spectra were recorded using a Shimadzu RF5301 spectrofluorometer. The photoinduced absorption (PA) was measured at temperature of 80 K at a pressure of 10−5 mbar. A standard halogen lamp operated at 160 W was used as the light for the transmission measurement and an Ar+ laser operated in multiline UV mode (3.53 eV and 3.41 eV, respectively) as the excitation source. A frequency-doubled Ti–sapphire laser system with a chirped pulse amplification stage provided the pump pulses with a duration of approximately 150 fs. The probe pulses, covering the spectral region of 1.2–3 eV, were obtained by super-continuum generation in a thin sapphire plate and focused onto the photo-excited region of the sample. The pump pulse was focused onto a spot of 60 ␮m in diameter. The signals are picked up by a an Oriel MS125 spectrograph coupled to a CCD spectrometer Instaspec4 for recording spectra at fixed pump probe delay, or via a photodiode preceded by an interference filter (bandwidth: 10 nm) for measuring the temporal dynamics at selected wavelengths. A detailed description of the set-up can be found in [11].

3. Results and discussion In Fig. 1, the emission and absorption spectra of a spin-coated m-LPPP SPN film are compared to those of a spin cast film of regular m-LPPP; all prepared from a same batch of m-LPPP. Note that the emission and absorption spectra of SPN dispersions of different particle sizes did not reveal significant differences, i.e. the optical properties of the particles are not size-dependent, at least for diameters down to 70 nm. SPN dispersion, SPN film, and “common” m-LPPP display no significant differences in all recorded types of spectra. All

Fig. 1. (Right) Normalized photoluminescence and absorbance spectra of a m-LPPP film (dashed lines), and of a SPN film (solid lines). (Left) Photoinduced absorption of a m-LPPP film (solid line) and of a m-LPPP colloid film (dashed line). The triplet exciton absorption in both the cases reveals for both at an energy of 1.3 eV.

emission spectra are characterized by a steep onset at 2.7 eV accompanied by well resolved vibronic side bands; they are a mirror image of the absorption spectrum as has been discussed in detail for regular m-LPPP [12]. The tailing of the thin film absorption spectra originates from scattering effects. Moreover, a low energy tail due to an enhanced energy transfer to defect sites (as also known for thin films of the polymer) [13] can be observed in the PL spectra of the SPN dispersion [14]. Spin or drop casting of the aqueous SPN dispersions yields thin layers with the same emission properties. Please note that no significant differences in the solid-state quantum yields have been observed for bulk and SPN layers. Defect stabilized excited states such as polarons can be revealed from PA measurements [15,16]. The PA spectra depicted in Fig. 1 show very similar shapes and relative intensities for bulk and SPN-based films. This gives evidence that no additional electronic defects are incorporated upon processing of m-LPPP into SPNs. In both spectra, the triplet absorption is observed at 1.3 eV. The dependence of in-phase and out-of-phase components of the observed signal discloses the lifetime of the electronic state assigned to a spectral feature. For low excitation densities the decay is essentially monomolecular. The monomolecular lifetime can be calculated from the maximum in the frequency dependence of the out of phase component. For the dominantly bimolecular behavior observed for high excitation densities this maximum yields an equivalent bimolecular lifetime [17]. The lifetime for the monomolecular process for the m-LPPP film is τmono = 9 ms and for the colloid film τmono = 15 ms. For high enough excitation densities a bimolecular annihilation parameters of γ = 6E − 16 cm3 /s for m-LPPP films and of γ = 2E − 16 cm3 /s for SPN films is found. Since annihilation involves migration of triplets, this finding can be rationalized by a reduced migration due to the peculiar film morphology. Although the SPN come into contact at some regions of their surface, on average the SPN boundaries have a mutual distance which is much larger than the average distance between polymer chains and hence the SPN surfaces constitute barriers for exciton migration. Therefore, the longer triplet lifetimes can be interpreted in the following way: the monomolecular lifetime of triplets in the colloid system is higher due to less migration to quenching centers. The equivalent bimolecular recombination, which is related to triplet–triplet annihilation, is longer again due to lower probability of the annihilation event. Note that the different excitation densities and the different film thickness of the colloid film (300 nm) and the m-LPPP film (125 nm) have been taken into account. In order to evaluate the influence of the surfactant introduced in the miniemulsion process we carried out the same measurements with carefully dialyzed m-LPPP SPNs, i.e. the surfactant has been removed. No difference was observable either in the absorption and photoluminescence spectra or in the photoinduced absorption measurements. It can, therefore, be concluded that the surfactant does not alter the spectroscopic properties of the SPN films.

T. Piok et al. / Synthetic Metals 139 (2003) 609–612

Fig. 2. Transmission difference spectra of m-LPPP colloid film at room temperature excited at 3.2 eV for various pump-probe delays.

In order to study the early electronic processes immediately following the optical excitation we performed transient differential transmission measurements with a time resolution of 150 fs. The T/T spectrum of a m-LPPP SPN film for various delays after excitation is shown in Fig. 2. Below 2.4 eV the signal is negative (photoinduced absorption (PA)) with a broad peak at 1.5 eV (subsequently referred to as PA1 ). On the high energy tail of this feature are two peaks at 1.9 and 2.1 eV (PA2 ). In previous works, PA1 has been assigned to an absorptive transition S1 –Sn from singlet excitons and PA2 to an overlap of PA1 with a transition D0 –Dn of charged polaron states (doublets) [18]. The absorption edge of m-LPPP is 2.7 eV, hence the positive feature at 2.7 eV is assigned to photobleaching (PB). In regular m-LPPP films, stimulated emission (SE) from singlet states S1 has been reported [18] for the spectral ranges from 2.2 to 2.6 eV, with two peaks at 2.3 and 2.5 eV. Remnants of these peaks can be intuited for the SPN films as well, but the SE obviously overlaps with an absorption which nearly cancels out the emission. An absorption feature spanning major fractions of the SE spectrum has been found in m-LPPP [19]. From anisotropy measurements it has been concluded that the transition dipole of the absorbers is tilted with respect to the chain axis. On the other hand, in an oligomer of a chemical structure analogous to m-LPPP the high energy absorption has been found to originate from polarons [20]. The conjecture of an enhanced polaron absorption compared to regular m-LPPP is supported by the stronger PA2 band in the SPN film, however, a definitive assignment needs further investigation. The temporal evolution of the individual spectral signatures is displayed in Fig. 3. Initially, the dynamics is congruent for all three features and is thus dominated by the same state of origin, specifically S1 . For longer delays after excitation, PA2 is slower than the other features, which

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Fig. 3. Temporal evolution of the differential transmission spectra of m-LPPP colloid film at room temperature excited at 3.2 eV at different spectral positions.

reflects the longer lifetime of polarons compared to singlet states. At an energy of 2.53 eV, where stimulated emission overlaps with photoinduced absorption the signal is initially negative, thus PA prevails. The temporal evolution of the signal closely matches the decay of singlet excitons (1.48 eV) during the first 20 ps. However, the signal changes sign at 70 ps and a weak SE is observed. Hence, the absorption must be partially due to a species which is shorter lived than the emitting singlet excitons. This issue is the subject of further investigations.

4. Conclusion A detailed spectroscopic investigation revealed that the processing of m-LPPP into SPNs does not significantly alter its spectroscopic properties. Nevertheless, some differences in the photo-excitation kinetics have been found. The bimolecular annihilation of triplets is reduced since the SPN surfaces constitute migration barriers. Stimulated emission is mostly canceled out by an overlapped absorption.

References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [2] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [3] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (1994) 1684. [4] P.K.H. Ho, D.S. Thomas, R.H. Friend, N. Tessler, Science 285 (1999) 233. [5] K. Landfester, R. Montenegro, U. Scherf, R. Güntner, U. Asawapirom, S. Patil, D. Neher, T. Kietzke, Adv. Mater. 14 (2002) 651.

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[6] K. Landfester, Macromol. Rapid Comm. 22 (2001) 896. [7] K. Landfester, Adv. Mater. 13 (2001) 765. [8] J.-A. He, K. Yang, J. Kumar, S. Tripathy, L.A. Samuelson, T. Oshikiri, H. Patagi, H. Kasai, S. Okada, H. Oikawa, H. Nakanashi, J. Phys. Chem. B 103 (1999) 11050. [9] T. Piok, S. Gamerith, C. Gadermaier, H. Plank, F.P. Wenzl, S. Patil, R. Montenegro, T. Kietzke, U. Scherf, K. Landfester, D. Neher, E.J.W. List, Adv. Mater. 15 (2003) 800. [10] U. Scherf, K. Müllen, Makromol. Chem. Rapid Commun. 12 (1991) 489. [11] C. Gadermaier, G. Lanzani, J. Phys., Condens. Matter 14 (2002) 9785. [12] S. Tasch, G. Kranzelbinder, G. Leising, U. Scherf, Phys. Rev. B. 55 (1997) 5079. [13] E.J.W. List, R. Guentner, P. Scandiucci de Freitas, U. Scherf, Adv. Mater. 14 (2002) 374.

[14] T. Piok, F.P. Wenzl, C. Gadermaier, U. Scherf, K. Landfester, E.J.W. List, unpublished results. [15] E.J.W. List, C.H. Kim, J. Shinar, A. Pogantsch, G. Leising, W. Graupner, Appl. Phys. Lett. 76 (2000) 2083. [16] J.M. Lupton, A. Pogantsch, T. Piok, E.J.W. List, S. Patil, U. Scherf, Phys. Rev. Lett. 89 (2002) 167401. [17] E.J.W. List, C.-H. Kim, A.K. Naik, U. Scherf, G. Leising, W. Graupner, J. Shinar, Phys. Rev. B 64 (2001) 155204. [18] W. Graupner, G. Leising, G. Lanzani, M. Nisoli, S. De Silvestri, U. Scherf, Phys. Rev. Lett. 76 (1996) 847. [19] G. Cerullo, S. Stagira, M. Nisoli, S. De Silvestri, G. Lanzani, G. Kranzelbinder, W. Graupner, G. Leising, Phys. Rev. B 57 (1998) 12806. [20] C. Gadermaier, G. Cerullo, M. Zavelani-Rossi, G. Snsone, G. Lanrani, E. Zojer, A. Pongantsch, D. Beljonne, Z. Shuai, J.L. Bredas, U. Scherf, G. Leising, Phys. Rev. B 66 (2002) 125203.