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Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved – Electrochemical Impedance Spectroscopy
Polymer Degradation and Stability 126 (2016) 204e208
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Short communication
Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved e Electrochemical Impedance Spectroscopy c, V. Nada d, M. O b, L. Tka F. Schauer a, b, *, M. Tka cova zdy d, K. Gmucova zvoldova c b, J. Chlpík c a
Faculty of Applied Informatics, Tomas Bata University in Zlín, 760 05 Zlín, Czech Republic Faculty of Education, Trnava University in Trnava, 918 43 Trnava, Slovak Republic Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, 812 19 Bratislava, Slovak Republic d cesta 9, 845 11 Bratislava, Slovak Republic Institute of Physics SAS, Dúbravska b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2015 Received in revised form 15 January 2016 Accepted 24 February 2016 Available online 26 February 2016
The white photo luminescence after UV degradation in long wavelength range 400e600 nm was examined on the prototypical polysilane, poly[methyl(phenyl)silane], using both photoluminescence spectroscopy and a new method of Energy Resolved - Electrochemical Impedance Spectroscopy (ER-EIS). Two groups of defect states, situated at approximately 440 nm (DE1 ¼ 2.8 eV with respect to electron transport energy) and 520 nm (DE2 ¼ 2.4 eV with respect to electron transport energy) were found by both spectroscopic methods. The white radiative recombination is ascribed to the recombination from trapping sites following the extreme energy migration. The forming of the crosslinking and bridging defects after photochemical scission of SieSi via the series of various kinds of intermediates is feasible (esilyl R3Si 380 nm, silylene Si2H4 480 nm, silene and silylsilylene 550 nm emissions). On the grounds of the IR absorption spectroscopy results we suppose the presence of the bonding by methylene bridging and carbosilane unit SieCH2eSi creation after SieSi Si s sp3 bond scission. The ER-EIS method turned out to be extremely suitable for elucidation of the electronic structure and its changes in organic semiconductors due to its great resolving power and wide range both in the energy and the density of electronic states. © 2016 Published by Elsevier Ltd.
Keywords: Polysilanes Poly[methyl(phenyl)silylene UV degradation Photochemical scission Crosslinking defects Bridging defects Electrochemical Impedance Spectroscopy
1. Introduction The high molecular weight polysilanes are interesting materials, predominantly studied as photoconductors, emitting materials and/or carrier transporting materials for light emitting diodes (LEDs), photoresists, and photo-induced radical initiators [1,2]. They belong to the class of s-conjugated polymers with a silicon polymer backbone, whose electronic properties are attributed to the s-conjugation originating from the overlap of Si s sp3 orbitals. The optical and electrical properties of these polymers differ significantly from those of structurally analogous carbon-based sbound systems such as polystyrene and polyethylene, resembling
* Corresponding author. Faculty of Applied Informatics, Tomas Bata University in Zlín, 760 05 Zlín, Czech Republic. E-mail addresses: [email protected] (F. Schauer), [email protected] ), [email protected] (V. Nad (M. Tk a cova a zdy), [email protected] (K. Gmucov a), [email protected] (L. Tk a c), [email protected] (J. Chlpík). http://dx.doi.org/10.1016/j.polymdegradstab.2016.02.016 0141-3910/© 2016 Published by Elsevier Ltd.
rather fully p-conjugated systems, for example, polyacetylenes [3]. The lowest energy optically allowed excited state of polysilanes was found to be of the excitonic nature [4]. Polysilanes are typically wide band gap semiconducting polymers, which makes them promising for UV- and white-light OLED applications [5,6]. In addition to a strong excitonic UV photoluminescence (PL), some polysilanes, e.g. poly[methyl(phenyl)silane] (PMPSi), show a broad emission peak in the visible region. The origin of this peak has been claimed to be controversial and unclarified, resulting in a long lasting debate, acute till now [7]. Several investigators considered the artificial or natural branching as the major cause for the white PL, examined by various experimental methods [8e12]. Degradation by UV radiation showed similar effects like the flash photolysis absorption on UV degraded polysilanes, i.e. creation of various kinds of intermediates e silylR3Si (with absorption at 380 nm) silylene Si2H4 (480 nm) and silene. On top of this, another emission occurred (550 nm) ascribed to silylsilylene [13e16]. Next cause of the white PL turned out to be conformation defects [17] artificially
F. Schauer et al. / Polymer Degradation and Stability 126 (2016) 204e208
introduced by the size effect. Watanabe et al. [12] introduced and studied profound conformation changes by interaction of hostguest type in poly(di-n-hexylsilane)/TiO2 system. The detailed quantum-mechanical calculation of the weak bond (WB) and the dangling bond (DB) positions were presented by Takeda et al. [18]. The major problem and the obstacle for the study of organic materials is the lack of the spectroscopic methods for the measurements of the electronic structure expressed by the density of electronic states (DOS) that would enable a faster progress in the material optimization [19,20]. This paper aims both to contribute to the white PL issue in polysilanes and to introduce the new spectroscopic method - Energy Resolved-Electrochemical Impedance Spectroscopy (ER-EIS) [21] - for its elucidation. Using the ER-EIS method, we intend to show the principal role of dangling bonds (DB) on the branching of polysilanes in question and to show the energy and the DOS distribution with a high precision that coincides with the broad PL emission originating from the trapping sites following the extreme energy migration. 2. Experiment and method PMPSi films were deposited by spin-coating (Spin Coater, K.L.M.), for the PL on the quartz substrates and for the EIS measurements on the ITO covered float glass substrates. Thin films were prepared from 5 wt % solution of PMPSi (Gelest, Inc., USA) in tetrahydrofuran spin-coated at 900 rpm for 60 s. The samples were subsequently annealed in N2 atmosphere in a glove box (Jacomex) at 60 C for 4 h.The thickness of the samples was approximately 100 nm and was measured by the profilometer (Veeco DEKTAK 150). For the PL spectroscopy measurements the fluorimeter JobinYvon Fluorolog-3 spectrophotometer, model FL3-22 was used. The PMPSi degradation was carried out in the glow box at 345 nm (6.2 mW cm2) and the degradation time varied from 0 to 1900 s. For ER e EIS measurements the impedance/gain-phase analyser (Solartron Analytical 1260) was used in the frequency range 0.1e1.0 Hz with the amplitude of AC signal 100 mV and the sweep rate for the superimposed voltage ramp of 10 mV/s. The electrochemical measurements were performed in the arrangement with the electrochemical microcell (volume about 200 ml) filled with 0.1 M electrolyte solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. The electrochemical microcell was built on the ITO coated substrate, the typical threeelectrode compartment was used and all measurements were carried out in the glovebox [21]. The ER-EIS method [21] for DOS elucidation is based on the measurement of the charge transfer resistance Rct of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using the external voltage. The method allows the mapping over unprecedentedly wide energy and DOS ranges of both the principal HOMO and LUMO bands, and defect states [22]. ER-EIS, by virtue of its redox basis, is exceptionally advantageous for the spectroscopic purposes of energy distribution of DOS, because the redox processes are neither temperature activated, nor rely on the temperature and concentration dependent transport tin the bulk of the material examined. All the quantities e the electrolyte concentration and transfer coefficient, entering the final redox process, are weakly temperature dependent and more or less known. The chargetransfer current density j between the semiconductor surface with the electron concentration ns and the electrolyte with the
205
concentration [A] of the redox (donor/acceptor) is
j ¼ eket ns ½A;
(1)
where, e is the elemental charge and ket is the charge-transfer coefficient in the interval 1017e1016 cm4 s1 [23e25]. The electron concentration ns at the semiconductor surface is a variable that can be measured and controlled experimentally by changing the semiconductor surface potential. Using eq. (1), we can express the DOS function in the semiconductor at the Fermi energy, g(EF), in terms of the charge transfer resistance Rct(U) measured at the applied voltage U as
gðEF ¼ eUÞ ¼
dns 1 dðjSÞ 1 ¼ ¼ : dðeUÞ eket ½AS dðUÞ eket ½ASRct
(2)
We can access experimentally the charge transfer resistance Rct ¼ dU/d(jS) using the superimposed harmonic perturbation U with a suitable frequency u at the applied voltage U, where Sis the sample area. The sought DOS function g(E) may be then derived directly from the measured charge-transfer resistance Rct (U) at the instantaneous position of the Fermi energy given by the applied potential EF ¼ eU in eq. (2). 3. Experimental results Fig. 1 shows the normalized singlet excitonic s* - s PL emission spectra of PMPSi peaked at (356 ± 5) nm for excitation at the wavelengths of 345 nm for increasing irradiation time (from 0 to 1900s). The excitonic PL emission band decreases with the photodegradation time, exhibiting blue shift to the shorter wavelengths. The degradation kinetics of the PL intensity decrease I, shows the following dependence: I~tn for t < tT and I~tm for t > tT, where n ¼ 0.19, m ¼ 0.43 and tT ¼ 12 s (see the inset of Fig. 1). This behaviour is in accordance with the observation of Nakayama et al. [26]. The s*-s PL at 356 nm returns to its original state after the annealing at 140 C if not degraded more than to 50% of its original value. Besides, the PL bands were observed in PL, situated at approximately 440 nm (2.8 eV) and 520 nm (2.4 eV). They are depicted in Fig. 2a, normalized to the excitonic s* - s 356 nm PL band, for three irradiation times 600, 1200, and 1800s in accordance with [16]. The
Fig. 1. PL PMPSi emission excitonic emission s* - s spectra for the excitation wavelength of 345 nm situated at ( 356 ± 5) nm for increasing irradiation time (from 0 to 1900 s). Subtle PL bands at about 440 nm (2.8 eV with respect to electron transport energy) and about and z520 nm (2.4 eV) are also visible. The inset gives the degradation kinetics of the 356 nm excitonic band.
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F. Schauer et al. / Polymer Degradation and Stability 126 (2016) 204e208
E1 = 2.8 eV E2 = 2.4 eV
Fig. 2. PMPSi PL spectra situated at about 440 nm (2.8 eV) and about 520 nm (2.4 eV) normalized to 356 nm PL band for three irradiation times 600, 1200 and 1800 s a), Integrated number of PL counts b).
integrated number of PL bands after irradiation (Fig. 2b) shows the linear growth of the white PL bands with degradation time. The resulting electronic structure DOS of PMPSi, measured by ER-EIS is shown in Fig. 3a for both the virgin sample (t ¼ 0 s) and that irradiated by the 345 nm UV radiation (up to the degradation time 1900 s). The HOMO and LUMO polaronic transport bandgap DE HOMO-LUMO ¼ 4.09 eV, and the DOS band g (E) vs (E-Eo)2 plots reveal that DOS extremities can be fitted with the Gaussian distributions. The standard deviations for the HOMO and LUMO are sHOMO ¼ 0.48 eV and sLUMO ¼ 0.29 eV, respectively. On the progressive degradation with the UV radiation at 345 nm the defect bands near HOMO occur, possible to approximate by two Gaussian distributions at about 4.5 eV and 4.2 eV (with respect to vacuum, only the corresponding energy positions of the maxima are denoted in Fig. 3a with respect to the transport level of PMPSi, DE1 ¼ 2.8 eV and DE2 ¼ 2.4 eV). The integral of the defect states in the energy interval from 3.0 eV to 5.5 eV (Fig. 3b) gives the generation rate of the defect states 6 1014 s1 cm3 for the defect states created by the 345 nm UV radiation. The resulting quantum efficiency for the crosslinking defects creation F(x) was determined to be F(x) ¼ 0.0059. A coincidence of two wide energy distributions of the white PL, situated approximately at 440 nm (or at DE1 ¼ 2.8 eV) and 520 nm
Fig. 3. PMPSi DOS spectra measured by Energy e Resolved Electrochemical Impedance Spectroscopy (ER-EIS) for virgin and irradiated sample for increasing irradiation time (from 0 to 1900 s). a), defect DOS integrated in the energy interval (from 3.0 eV to 5.5 eV with respect to vacuum) b).
(or at DE2 ¼ 2.4 eV), corresponds to the transitions from the distributions determined by the ER - EIS method to the LUMO of PMPSi. As to experimental errors of our ER-EIS method, we may give the absolute energy error as ±50 meV and the relative error of the DOS function d g(E)/g(E)$100 ¼ 5% [21]. 4. Discussion and conclusions In spite of the excessive efforts, the origin of white PL in polysilanes in general and PMPSi in particular is still controversial. The white PL origin rests presumably in the manifold reasons because of the softness of their backbones and susceptibility to conformation defects, branching and networking and conforming energy HOMO-LUMO energy difference for white PL. WB electrons and holes maintain their original orbital symmetries and the optical transitions between them is possible, so a WB acts as a radiative centre with the PL. On the other hand the DB is a non-radiative centre, suppressing the PL. The chemical modification during the PMPSi polymerization results in the variable degree of branching and occurrence of the white PL, together with predicted
F. Schauer et al. / Polymer Degradation and Stability 126 (2016) 204e208
disappearance of PL excitonic emission s* - s spectra for a defect density of less than 1%. Wilson and Weidman [8], using time resolved PL spectroscopy, were the first, who suggested that the branching may be responsible for the presence of the bands often observed in the emission spectra of samples assumed to be completely linear. Higher degrees of the branch incorporation result in the loss of the excitonic emission altogether with a further red shift of the visible band with the increased branching ratio. The visible band appeared as a “traplike” relaxation phenomenon and may be successfully described in terms of an electron-hole pair recombination model often applied to band tail emission in amorphous semiconductors. Kishida et al. [9] have shown that the introduction of branching points (one silicon atom chemically bonded to three other Si atoms instead of two, as is usual in linear chains) enhances the white PL, whereas the sharp UV luminescence band is suppressed. In Si NMR studies on branching Fujiki et al. [9,10] claimed the origin of the defects were due to the unit, consisting of an organosilane unit and about three PMPSi monomer units near the branch, resulting in a fairly lengthened SieSi WB bonding state. The empirical linear relationship between the relative intensity of the broad PL and the defect density predicted the disappearance of the broad PL for a defect density of less than 1%.Toyoda and Fujiki [11] have compared the luminescent properties materials with different branching levels. The data suggested that branching might be responsible for the presence of the white PL. Similar results were presented by PL study in Ref. [9]. The next step in revealing the origin of the white PL were ingenious experiments by Ostapenko et al. [17], who decisively showed the role of the conformation defects and resulting WB. Building in the poly(din-hexylsilane) (PDHS) into 2.8 nm silica pores led to a strong PL at 410 nm, whereas for the pores of 5.8 nm no change in the PL spectra was observed. The authors formulated their result as the proof of defect nature of the visible fluorescence of polysilanes occurring due to the orientation of the polymer chain along the surface of the pore. The interaction of the polymer with the surface of the pore can lead to the substantial changes in the conformation of the polymer chain owing to the change in the angle between SieSi bonds [27]. A structure change of the polymer found in a restricted volume is confirmed by the Kumar et al. data [28]. The theoretical calculations [29] also indicate a change in the angle between SieSi bonds near defects of the cross-link type in the case of specially synthesized polysilanes with branched structures. Degradation by UV radiation showed similar effects. Watanabe et al. [13] demonstrated with flash photolysis absorption on UV degraded polysilanes various kinds of intermediates e silylR3Si (with absorption at 380 nm)silylene Si2H4 (480 nm) and silene. On top of this, another emission occurred (550 nm), which was ascribed to silylsilylene. The detailed quantum-mechanical calculation of WB and DB electron distributions and energies were presented by Takeda et al. [18]. The detailed quantum-mechanical calculation of WB and DB electron distributions and energies were presented by Takeda et al. [18]. They have found that WB, by the local Si skeleton-bond stretching, weakens the ps bonding character of the highest occupied valence band and destabilizes it, forming energetically shifted states upward in the energy. Conversely, a reduction in the sp s* antibonding character stabilizes the lowest unoccupied conduction band state downwards. The 35% reduction of bandgap energy Eg may result. The results of PMPSi degradation influence on the excitonic s* s PL situated at 356 nm and its degradation kinetics in Fig. 1 are in accordance with the results of [26]. These results also correlate with our previous work [14], when we supposed an energy excitonic transfer to the longest PMPSi segments and that photochemical events occurring in these segments. The scission of SieSi bonds and the transfer of the excitation to the longest segments are the
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competitive processes cutting the longest segments (and those with defects). The shifting of the exciton absorption to shorter wavelengths therefore testifies the remaining shorter segments. The kinetics of the degradation process clearly shows both mechanisms one for the time t < tT ¼ 12 s and the other starting at the times t > tT ¼ 12 s. The white PL linear growth with prolonged degradation is starting approximately at the degradation time tT ¼ 12 s (see in Fig. 2) and exhibits two peaks at 440 nm and 520 nm, whereas the peak at 440 nm shifts both with the intensity of degradation radiation and its wavelength shortening to longer wavelengths. The data of the ER e EIS method [21] in Fig. 3 give, beside the exhaustive characterisation of the HOMO-LUMO DOS, the detailed energy spectroscopy of the wide distributed defect states due to the crosslinking peaked at 4.6 eV and e4.2 eV, with respect to the vacuum energy. These defect states result in radiative recombination PL with energy 2.8 eV and 2.4 eV, respectively. The quantum efficiency for the crosslinking defects formation was found to be F(x) ¼ 0.0059, which compares favourably with 0.0036 found by Trefonas et al. for PMPSi [30]. They claim the phenyl groups on the silicon backbone of the polymer are photochemically destabilizing the backbone, possibly due to a combination of steric and electronic effects. These process leads to the high quantum efficiency of scission F(s) ¼ 0.017, typical of the aryl polysilanes, which is much higher compared to the alkyl polysilanes. Since the defect states are not amenable to anneal (by 140 C), the creation siloxane of SieOeSi units may be excluded, because all experiments including chemicals preparation, purification and films deposition are performed in protective, highly purified, N2 atmosphere of the glovebox. The possible candidate is the carbosilane unit, which we found after electron induced degradations of PMPSi under vacuum, where changes in IR spectra lead to both the decrease of SieSi vibrations at 463 cm1 and increase of SieCH2eSi at the wide band 1020e1070 cm1. We tentatively explain that this process starts by the SieSi bond scission with subsequent bonding by methylene bridging and carbosilane unit SieCH2eSi creation. This supposition is supported by the increasing of the SieH stretching mode at 2100 cm1 in IR vibrations. 5. Conclusions - The concentration and energy distribution of the DOS of crosslinking and bridging defects in PMPSi during UV degradation were characterized - The white PL both in intensity and the energy position of white PL correlated well with ER-EIS - ER - EIS is the suitable method for organic materials elucidation for its resolving power and wide range both in energy and density of electronic states. Acknowledgments The authors acknowledge the financial support of the Slovak Research and Development Agency under the Project No. APVV0096-11, the Scientific Grant Agency VEGA under Project Nos. 1/ 0501/15 and 2/0165/13. References [1] K. Oka, Polysilanes: novel sigma-conjugated polymers, in: M. Anpo, K. Mizuno (Eds.), Environmentally Harmonious Chemistry for the 21st Century, Nova Science Publishers, , New York, 2010, pp. 181e203. [2] K. Mizuno, H. Ikeda, H. Maeda, in: M. Anpo, K. Mizuno (Eds.), Environmentally Harmonious Chemistry for the 21st Century Environmentally Harmonious Organic Photochemical Reactions, Nova Science Publishers, New York, 2010, pp. 89e122.
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[3] R.D. Miller, J. Michl, Polysilane high polymers, Chem. Rev. 89 (6) (1989) 1359e1410. [4] L.A. Harrah, J.M. Zeigler, Electronic spectra of polysilanes, Macromolecules 20 (3) (1987) 601e608. [5] A. Fujii, K. Yoshimoto, M. Yoshida, Y. Ohmori, K. Yoshino, H. Ueno, M. Kakimoto, H. Kojima, Electroluminescent diodes utilizing polysilanes, Jpn. J. Appl. Phys. 35 (7R) (1996) 3914e3917. [6] K. Ebihara, S. Koshihara, T. Miyazawa, M. Kira, Polysilane-based polymer diodes emitting ultraviolet light, Jpn. J. Appl. Phys. 35 (10A) (1996) L1278eL1280. [7] A. Kadashchuk, Y. Skryshevski, A. Vakhnin, S. Toliautas, J. Sulskus, R. Augulis, V. Gulbinas, S. Nespurek, J. Genoe, L. Valkunas, Highly efficient intrinsic phosphorescence from a s conjugated poly(silylene) polymer, J. Phys. Chem. C 118 (40) (2014) 22923e22934. [8] W.L. Wilson, T.W. Weidman, Excited-state dynamics of one-dimensional and 2-dimensional sigma-conjugated silicon polymers-dramatic effects of branching in a series of hexylsilylene-branched poly(hexylmethylsilylene) copolymers, J. Phys. Chem. 95 (11) (1991) 4568e4572. [9] H. Kishida, H. Tachibana, M. Matsumoto, Y. Tokura, Visible luminescence from branched silicon polymers, J. Appl. Phys. 78 (5) (1995) 3362e3366. [10] M. Fujiki, Structural defects in poly(methylphenylsilylene), Chem. Phys. Lett. 198 (1) (1992) 177e182. [11] S. Toyoda, M. Fujiki, Photoluminescence and absorption spectra of poly(pentylphenylsilylene). Absence of broad photoluminescence of poly(alkylarylsilylene) around 2.7 eV, Chem. Phys. Lett. 293 (1) (1998) 38e42. [12] A. Watanabe, T. Miyashita, A. Kasuya, M. Takahashi, Y. Kawazoe, Fluorescence spectra of poly(di-n-hexylsilane)/TiO2 nanoparticle hybrid film, Polymer 49 (2) (2008) 554e560. [13] A. Watanabe, H. Miike, T. Tsutsumi, M. Matsuda, Photochemical properties of network and branched polysilanes, Macromolecules 26 (8) (1993) 2111e2116. [14] F. Schauer, P. Schauer, I. Kuritka, H. Bao, Conjugated siliconebased polymer resists for nanotechnologies: EB and UV meditated degradation processes in polysilanes, Mater. Trans. 51 (2) (2010) 197e201. Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis. [15] I. Kuritka, F. Schauer, P. Saha, J. Zemek, P. Jiricek, S. Nespurek, UV degradability of polysilanes for nanoresists examined by electron spectroscopies and photoluminescence, Czech. J. Phys. 56 (1) (2006) 41e50. [16] S. Nespurek, F. Schauer, A. Kadashchuk, Visible photoluminescence in
polysilanes, Chem. Mon. 132 (1) (2001) 159e168. [17] N. Ostapenko, G. Telbiz, V. Ilyin, S. Suto, A. Watanabe, Criticality of fluorescence spectra of the nanosized polysilanes oriented in the mesoporous silica, Chem. Phys. Lett. 383 (5) (2004) 456e461. [18] K. Takeda, K. Shiraishi, M. Fujiki, M. Kondo, K. Morigaki, Photocreated metastable states in polysilanes, Phys. Rev. B 50 (8) (1994) 5171e5179. [19] R.A. Street, S.A. Hawks, P.P. Khlyabich, G. Li, B.J. Schwartz, B.C. Thompson, Y.J. Yang, Electronic structure and transition energies in polymerfullerene bulk heterojunctions, Phys. Chem. C 118 (38) (2014) 21873e21883. [20] C. Deibel, D. Mack, J. Gorenflot, A. Scholl, S. Krause, F. Reinert, D. Rauh, V. Dyakonov, Energetics of excited states in the conjugated polymer poly(3hexylthiophene), Phys. Rev. B 81 (8) (2010) 085202. [21] V. N ada zdy, F. Schauer, K. Gmucov a, Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors, Appl. Phys. Lett. 105 (14) (2014) 42109. da , Electrochemical [22] K. Gmucov a, V. Na zdy, F. Schauer, M. Kaiser, E. Majkova spectroscopic methods for the fine band gap electronic structure mapping in organic semiconductors, J. Phys. Chem. C 119 (2015) 15926e15934. [23] R.A. Marcus, On the theory of oxidation-reduction reactions involving electron transfer 1, J. Chem. Phys. 24 (5) (1956) 966e978. [24] N.S. Lewis, An analysis of charge-transfer rate constants for semiconductor liquid interfaces, Rev. Phys. Chem. 42 (1991) 543e580. [25] A.M. Fajardo, N.S. Lewis, The Spectrum 10, A Quarterly Publ vol. 1, Bowling Green State Uni, Ohio 43403, Winter, 1997. [26] Y. Nakayama, H. Inagi, M. Zhang, Photobleaching and photodegradation of photoluminescence in thin films of poly(methylphenylsilane), J. Appl. Phys. 86 (2) (1999) 768e773. [27] P. Toman, S. Nesp urek, W. Bartkowiak, J. Sworakowski, Excitations in oligosilanes with photochromic side groups, J. Luminescence 112 (1) (2005) 386e390. [28] S.K. Kumar, M. Vacatello, D.Y. Yoon, Off-lattice Monte-Carlo simulations of polymer melts confined between 2 plates, J. Chem. Phys. 89 (8) (1988) 5206e5215. [29] A. Watanabe, M. Nanjo, T. Sunaga, A. Sekiguchi, Dynamics of the excited state of polysilanedendrimers: origin of the broad visible emission of branched silicon chains, J. Phys. Chem. A 105 (26) (2001) 6436e6442. [30] P. Trefonas, R. West, R.D. Miller, D. Hofer, Organosilane high polymers: electronic spectra and photodegradation, J. Polym. Sci. Polym. Lett. Ed. 21 (10) (1983) 823e829.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Short communication
Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved e Electrochemical Impedance Spectroscopy c, V. Nada d, M. O b, L. Tka F. Schauer a, b, *, M. Tka cova zdy d, K. Gmucova zvoldova c b, J. Chlpík c a
Faculty of Applied Informatics, Tomas Bata University in Zlín, 760 05 Zlín, Czech Republic Faculty of Education, Trnava University in Trnava, 918 43 Trnava, Slovak Republic Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, 812 19 Bratislava, Slovak Republic d cesta 9, 845 11 Bratislava, Slovak Republic Institute of Physics SAS, Dúbravska b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2015 Received in revised form 15 January 2016 Accepted 24 February 2016 Available online 26 February 2016
The white photo luminescence after UV degradation in long wavelength range 400e600 nm was examined on the prototypical polysilane, poly[methyl(phenyl)silane], using both photoluminescence spectroscopy and a new method of Energy Resolved - Electrochemical Impedance Spectroscopy (ER-EIS). Two groups of defect states, situated at approximately 440 nm (DE1 ¼ 2.8 eV with respect to electron transport energy) and 520 nm (DE2 ¼ 2.4 eV with respect to electron transport energy) were found by both spectroscopic methods. The white radiative recombination is ascribed to the recombination from trapping sites following the extreme energy migration. The forming of the crosslinking and bridging defects after photochemical scission of SieSi via the series of various kinds of intermediates is feasible (esilyl R3Si 380 nm, silylene Si2H4 480 nm, silene and silylsilylene 550 nm emissions). On the grounds of the IR absorption spectroscopy results we suppose the presence of the bonding by methylene bridging and carbosilane unit SieCH2eSi creation after SieSi Si s sp3 bond scission. The ER-EIS method turned out to be extremely suitable for elucidation of the electronic structure and its changes in organic semiconductors due to its great resolving power and wide range both in the energy and the density of electronic states. © 2016 Published by Elsevier Ltd.
Keywords: Polysilanes Poly[methyl(phenyl)silylene UV degradation Photochemical scission Crosslinking defects Bridging defects Electrochemical Impedance Spectroscopy
1. Introduction The high molecular weight polysilanes are interesting materials, predominantly studied as photoconductors, emitting materials and/or carrier transporting materials for light emitting diodes (LEDs), photoresists, and photo-induced radical initiators [1,2]. They belong to the class of s-conjugated polymers with a silicon polymer backbone, whose electronic properties are attributed to the s-conjugation originating from the overlap of Si s sp3 orbitals. The optical and electrical properties of these polymers differ significantly from those of structurally analogous carbon-based sbound systems such as polystyrene and polyethylene, resembling
* Corresponding author. Faculty of Applied Informatics, Tomas Bata University in Zlín, 760 05 Zlín, Czech Republic. E-mail addresses: [email protected] (F. Schauer), [email protected] ), [email protected] (V. Nad (M. Tk a cova a zdy), [email protected] (K. Gmucov a), [email protected] (L. Tk a c), [email protected] (J. Chlpík). http://dx.doi.org/10.1016/j.polymdegradstab.2016.02.016 0141-3910/© 2016 Published by Elsevier Ltd.
rather fully p-conjugated systems, for example, polyacetylenes [3]. The lowest energy optically allowed excited state of polysilanes was found to be of the excitonic nature [4]. Polysilanes are typically wide band gap semiconducting polymers, which makes them promising for UV- and white-light OLED applications [5,6]. In addition to a strong excitonic UV photoluminescence (PL), some polysilanes, e.g. poly[methyl(phenyl)silane] (PMPSi), show a broad emission peak in the visible region. The origin of this peak has been claimed to be controversial and unclarified, resulting in a long lasting debate, acute till now [7]. Several investigators considered the artificial or natural branching as the major cause for the white PL, examined by various experimental methods [8e12]. Degradation by UV radiation showed similar effects like the flash photolysis absorption on UV degraded polysilanes, i.e. creation of various kinds of intermediates e silylR3Si (with absorption at 380 nm) silylene Si2H4 (480 nm) and silene. On top of this, another emission occurred (550 nm) ascribed to silylsilylene [13e16]. Next cause of the white PL turned out to be conformation defects [17] artificially
F. Schauer et al. / Polymer Degradation and Stability 126 (2016) 204e208
introduced by the size effect. Watanabe et al. [12] introduced and studied profound conformation changes by interaction of hostguest type in poly(di-n-hexylsilane)/TiO2 system. The detailed quantum-mechanical calculation of the weak bond (WB) and the dangling bond (DB) positions were presented by Takeda et al. [18]. The major problem and the obstacle for the study of organic materials is the lack of the spectroscopic methods for the measurements of the electronic structure expressed by the density of electronic states (DOS) that would enable a faster progress in the material optimization [19,20]. This paper aims both to contribute to the white PL issue in polysilanes and to introduce the new spectroscopic method - Energy Resolved-Electrochemical Impedance Spectroscopy (ER-EIS) [21] - for its elucidation. Using the ER-EIS method, we intend to show the principal role of dangling bonds (DB) on the branching of polysilanes in question and to show the energy and the DOS distribution with a high precision that coincides with the broad PL emission originating from the trapping sites following the extreme energy migration. 2. Experiment and method PMPSi films were deposited by spin-coating (Spin Coater, K.L.M.), for the PL on the quartz substrates and for the EIS measurements on the ITO covered float glass substrates. Thin films were prepared from 5 wt % solution of PMPSi (Gelest, Inc., USA) in tetrahydrofuran spin-coated at 900 rpm for 60 s. The samples were subsequently annealed in N2 atmosphere in a glove box (Jacomex) at 60 C for 4 h.The thickness of the samples was approximately 100 nm and was measured by the profilometer (Veeco DEKTAK 150). For the PL spectroscopy measurements the fluorimeter JobinYvon Fluorolog-3 spectrophotometer, model FL3-22 was used. The PMPSi degradation was carried out in the glow box at 345 nm (6.2 mW cm2) and the degradation time varied from 0 to 1900 s. For ER e EIS measurements the impedance/gain-phase analyser (Solartron Analytical 1260) was used in the frequency range 0.1e1.0 Hz with the amplitude of AC signal 100 mV and the sweep rate for the superimposed voltage ramp of 10 mV/s. The electrochemical measurements were performed in the arrangement with the electrochemical microcell (volume about 200 ml) filled with 0.1 M electrolyte solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. The electrochemical microcell was built on the ITO coated substrate, the typical threeelectrode compartment was used and all measurements were carried out in the glovebox [21]. The ER-EIS method [21] for DOS elucidation is based on the measurement of the charge transfer resistance Rct of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using the external voltage. The method allows the mapping over unprecedentedly wide energy and DOS ranges of both the principal HOMO and LUMO bands, and defect states [22]. ER-EIS, by virtue of its redox basis, is exceptionally advantageous for the spectroscopic purposes of energy distribution of DOS, because the redox processes are neither temperature activated, nor rely on the temperature and concentration dependent transport tin the bulk of the material examined. All the quantities e the electrolyte concentration and transfer coefficient, entering the final redox process, are weakly temperature dependent and more or less known. The chargetransfer current density j between the semiconductor surface with the electron concentration ns and the electrolyte with the
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concentration [A] of the redox (donor/acceptor) is
j ¼ eket ns ½A;
(1)
where, e is the elemental charge and ket is the charge-transfer coefficient in the interval 1017e1016 cm4 s1 [23e25]. The electron concentration ns at the semiconductor surface is a variable that can be measured and controlled experimentally by changing the semiconductor surface potential. Using eq. (1), we can express the DOS function in the semiconductor at the Fermi energy, g(EF), in terms of the charge transfer resistance Rct(U) measured at the applied voltage U as
gðEF ¼ eUÞ ¼
dns 1 dðjSÞ 1 ¼ ¼ : dðeUÞ eket ½AS dðUÞ eket ½ASRct
(2)
We can access experimentally the charge transfer resistance Rct ¼ dU/d(jS) using the superimposed harmonic perturbation U with a suitable frequency u at the applied voltage U, where Sis the sample area. The sought DOS function g(E) may be then derived directly from the measured charge-transfer resistance Rct (U) at the instantaneous position of the Fermi energy given by the applied potential EF ¼ eU in eq. (2). 3. Experimental results Fig. 1 shows the normalized singlet excitonic s* - s PL emission spectra of PMPSi peaked at (356 ± 5) nm for excitation at the wavelengths of 345 nm for increasing irradiation time (from 0 to 1900s). The excitonic PL emission band decreases with the photodegradation time, exhibiting blue shift to the shorter wavelengths. The degradation kinetics of the PL intensity decrease I, shows the following dependence: I~tn for t < tT and I~tm for t > tT, where n ¼ 0.19, m ¼ 0.43 and tT ¼ 12 s (see the inset of Fig. 1). This behaviour is in accordance with the observation of Nakayama et al. [26]. The s*-s PL at 356 nm returns to its original state after the annealing at 140 C if not degraded more than to 50% of its original value. Besides, the PL bands were observed in PL, situated at approximately 440 nm (2.8 eV) and 520 nm (2.4 eV). They are depicted in Fig. 2a, normalized to the excitonic s* - s 356 nm PL band, for three irradiation times 600, 1200, and 1800s in accordance with [16]. The
Fig. 1. PL PMPSi emission excitonic emission s* - s spectra for the excitation wavelength of 345 nm situated at ( 356 ± 5) nm for increasing irradiation time (from 0 to 1900 s). Subtle PL bands at about 440 nm (2.8 eV with respect to electron transport energy) and about and z520 nm (2.4 eV) are also visible. The inset gives the degradation kinetics of the 356 nm excitonic band.
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E1 = 2.8 eV E2 = 2.4 eV
Fig. 2. PMPSi PL spectra situated at about 440 nm (2.8 eV) and about 520 nm (2.4 eV) normalized to 356 nm PL band for three irradiation times 600, 1200 and 1800 s a), Integrated number of PL counts b).
integrated number of PL bands after irradiation (Fig. 2b) shows the linear growth of the white PL bands with degradation time. The resulting electronic structure DOS of PMPSi, measured by ER-EIS is shown in Fig. 3a for both the virgin sample (t ¼ 0 s) and that irradiated by the 345 nm UV radiation (up to the degradation time 1900 s). The HOMO and LUMO polaronic transport bandgap DE HOMO-LUMO ¼ 4.09 eV, and the DOS band g (E) vs (E-Eo)2 plots reveal that DOS extremities can be fitted with the Gaussian distributions. The standard deviations for the HOMO and LUMO are sHOMO ¼ 0.48 eV and sLUMO ¼ 0.29 eV, respectively. On the progressive degradation with the UV radiation at 345 nm the defect bands near HOMO occur, possible to approximate by two Gaussian distributions at about 4.5 eV and 4.2 eV (with respect to vacuum, only the corresponding energy positions of the maxima are denoted in Fig. 3a with respect to the transport level of PMPSi, DE1 ¼ 2.8 eV and DE2 ¼ 2.4 eV). The integral of the defect states in the energy interval from 3.0 eV to 5.5 eV (Fig. 3b) gives the generation rate of the defect states 6 1014 s1 cm3 for the defect states created by the 345 nm UV radiation. The resulting quantum efficiency for the crosslinking defects creation F(x) was determined to be F(x) ¼ 0.0059. A coincidence of two wide energy distributions of the white PL, situated approximately at 440 nm (or at DE1 ¼ 2.8 eV) and 520 nm
Fig. 3. PMPSi DOS spectra measured by Energy e Resolved Electrochemical Impedance Spectroscopy (ER-EIS) for virgin and irradiated sample for increasing irradiation time (from 0 to 1900 s). a), defect DOS integrated in the energy interval (from 3.0 eV to 5.5 eV with respect to vacuum) b).
(or at DE2 ¼ 2.4 eV), corresponds to the transitions from the distributions determined by the ER - EIS method to the LUMO of PMPSi. As to experimental errors of our ER-EIS method, we may give the absolute energy error as ±50 meV and the relative error of the DOS function d g(E)/g(E)$100 ¼ 5% [21]. 4. Discussion and conclusions In spite of the excessive efforts, the origin of white PL in polysilanes in general and PMPSi in particular is still controversial. The white PL origin rests presumably in the manifold reasons because of the softness of their backbones and susceptibility to conformation defects, branching and networking and conforming energy HOMO-LUMO energy difference for white PL. WB electrons and holes maintain their original orbital symmetries and the optical transitions between them is possible, so a WB acts as a radiative centre with the PL. On the other hand the DB is a non-radiative centre, suppressing the PL. The chemical modification during the PMPSi polymerization results in the variable degree of branching and occurrence of the white PL, together with predicted
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disappearance of PL excitonic emission s* - s spectra for a defect density of less than 1%. Wilson and Weidman [8], using time resolved PL spectroscopy, were the first, who suggested that the branching may be responsible for the presence of the bands often observed in the emission spectra of samples assumed to be completely linear. Higher degrees of the branch incorporation result in the loss of the excitonic emission altogether with a further red shift of the visible band with the increased branching ratio. The visible band appeared as a “traplike” relaxation phenomenon and may be successfully described in terms of an electron-hole pair recombination model often applied to band tail emission in amorphous semiconductors. Kishida et al. [9] have shown that the introduction of branching points (one silicon atom chemically bonded to three other Si atoms instead of two, as is usual in linear chains) enhances the white PL, whereas the sharp UV luminescence band is suppressed. In Si NMR studies on branching Fujiki et al. [9,10] claimed the origin of the defects were due to the unit, consisting of an organosilane unit and about three PMPSi monomer units near the branch, resulting in a fairly lengthened SieSi WB bonding state. The empirical linear relationship between the relative intensity of the broad PL and the defect density predicted the disappearance of the broad PL for a defect density of less than 1%.Toyoda and Fujiki [11] have compared the luminescent properties materials with different branching levels. The data suggested that branching might be responsible for the presence of the white PL. Similar results were presented by PL study in Ref. [9]. The next step in revealing the origin of the white PL were ingenious experiments by Ostapenko et al. [17], who decisively showed the role of the conformation defects and resulting WB. Building in the poly(din-hexylsilane) (PDHS) into 2.8 nm silica pores led to a strong PL at 410 nm, whereas for the pores of 5.8 nm no change in the PL spectra was observed. The authors formulated their result as the proof of defect nature of the visible fluorescence of polysilanes occurring due to the orientation of the polymer chain along the surface of the pore. The interaction of the polymer with the surface of the pore can lead to the substantial changes in the conformation of the polymer chain owing to the change in the angle between SieSi bonds [27]. A structure change of the polymer found in a restricted volume is confirmed by the Kumar et al. data [28]. The theoretical calculations [29] also indicate a change in the angle between SieSi bonds near defects of the cross-link type in the case of specially synthesized polysilanes with branched structures. Degradation by UV radiation showed similar effects. Watanabe et al. [13] demonstrated with flash photolysis absorption on UV degraded polysilanes various kinds of intermediates e silylR3Si (with absorption at 380 nm)silylene Si2H4 (480 nm) and silene. On top of this, another emission occurred (550 nm), which was ascribed to silylsilylene. The detailed quantum-mechanical calculation of WB and DB electron distributions and energies were presented by Takeda et al. [18]. The detailed quantum-mechanical calculation of WB and DB electron distributions and energies were presented by Takeda et al. [18]. They have found that WB, by the local Si skeleton-bond stretching, weakens the ps bonding character of the highest occupied valence band and destabilizes it, forming energetically shifted states upward in the energy. Conversely, a reduction in the sp s* antibonding character stabilizes the lowest unoccupied conduction band state downwards. The 35% reduction of bandgap energy Eg may result. The results of PMPSi degradation influence on the excitonic s* s PL situated at 356 nm and its degradation kinetics in Fig. 1 are in accordance with the results of [26]. These results also correlate with our previous work [14], when we supposed an energy excitonic transfer to the longest PMPSi segments and that photochemical events occurring in these segments. The scission of SieSi bonds and the transfer of the excitation to the longest segments are the
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competitive processes cutting the longest segments (and those with defects). The shifting of the exciton absorption to shorter wavelengths therefore testifies the remaining shorter segments. The kinetics of the degradation process clearly shows both mechanisms one for the time t < tT ¼ 12 s and the other starting at the times t > tT ¼ 12 s. The white PL linear growth with prolonged degradation is starting approximately at the degradation time tT ¼ 12 s (see in Fig. 2) and exhibits two peaks at 440 nm and 520 nm, whereas the peak at 440 nm shifts both with the intensity of degradation radiation and its wavelength shortening to longer wavelengths. The data of the ER e EIS method [21] in Fig. 3 give, beside the exhaustive characterisation of the HOMO-LUMO DOS, the detailed energy spectroscopy of the wide distributed defect states due to the crosslinking peaked at 4.6 eV and e4.2 eV, with respect to the vacuum energy. These defect states result in radiative recombination PL with energy 2.8 eV and 2.4 eV, respectively. The quantum efficiency for the crosslinking defects formation was found to be F(x) ¼ 0.0059, which compares favourably with 0.0036 found by Trefonas et al. for PMPSi [30]. They claim the phenyl groups on the silicon backbone of the polymer are photochemically destabilizing the backbone, possibly due to a combination of steric and electronic effects. These process leads to the high quantum efficiency of scission F(s) ¼ 0.017, typical of the aryl polysilanes, which is much higher compared to the alkyl polysilanes. Since the defect states are not amenable to anneal (by 140 C), the creation siloxane of SieOeSi units may be excluded, because all experiments including chemicals preparation, purification and films deposition are performed in protective, highly purified, N2 atmosphere of the glovebox. The possible candidate is the carbosilane unit, which we found after electron induced degradations of PMPSi under vacuum, where changes in IR spectra lead to both the decrease of SieSi vibrations at 463 cm1 and increase of SieCH2eSi at the wide band 1020e1070 cm1. We tentatively explain that this process starts by the SieSi bond scission with subsequent bonding by methylene bridging and carbosilane unit SieCH2eSi creation. This supposition is supported by the increasing of the SieH stretching mode at 2100 cm1 in IR vibrations. 5. Conclusions - The concentration and energy distribution of the DOS of crosslinking and bridging defects in PMPSi during UV degradation were characterized - The white PL both in intensity and the energy position of white PL correlated well with ER-EIS - ER - EIS is the suitable method for organic materials elucidation for its resolving power and wide range both in energy and density of electronic states. Acknowledgments The authors acknowledge the financial support of the Slovak Research and Development Agency under the Project No. APVV0096-11, the Scientific Grant Agency VEGA under Project Nos. 1/ 0501/15 and 2/0165/13. References [1] K. Oka, Polysilanes: novel sigma-conjugated polymers, in: M. Anpo, K. Mizuno (Eds.), Environmentally Harmonious Chemistry for the 21st Century, Nova Science Publishers, , New York, 2010, pp. 181e203. [2] K. Mizuno, H. Ikeda, H. Maeda, in: M. Anpo, K. Mizuno (Eds.), Environmentally Harmonious Chemistry for the 21st Century Environmentally Harmonious Organic Photochemical Reactions, Nova Science Publishers, New York, 2010, pp. 89e122.
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