Charge transfer complexes in silicon-organic photoconductors with admixtures of acceptor molecules

Synthetic Metals 129 (2002) 19±24

Charge transfer complexes in silicon-organic photoconductors with admixtures of acceptor molecules N.I. Ostapenkoa,*, I.V. Sekirina, D.N. Tulchynskayaa, S. Sutob, A. Watanabec a

b

Institute of Physics, NASU, Prospekt Nauki 46, 03650 Kiev, Ukraine Department of Physics, Faculty of Science, Tohoku University, Sendai 980-77, Japan c Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan

Received 5 September 2001; received in revised form 11 November 2001; accepted 11 December 2001

Abstract The absorption spectra of the solutions of polysilanes (poly(methylphenylsilane) (PMPS) and poly(dihexylsilane) (PDHS)) with various acceptors (tetracyanoquinodimetane, tetracyano-naphtoquinodimetane and trinitro¯uorenone) were investigated. It is shown that several new broad bands in visible region arise in the absorption spectra of polysilanes with addition of acceptors, which is accompanied by changes in the color of the solutions. These new bands are attributed to formation of charge transfer complexes (CT-complexes) between s-donor (polysilane) and p-acceptor, i.e. polysilanes with admixture of acceptors can be used both as transport layers and photogeneration ones. The possible nature of the structure of the absorption bands of these CT-complexes is discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Photoconductors; Poly(methylphenylsilane); Poly(dihexylsilane)

1. Introduction The quantum ef®ciency of photogeneration of charge carriers and their mobility are main parameters which determine the photoconductive properties of polymers. Siliconorganic polymers are good photoconductors, because they have high ef®ciency of photogeneration of charge carriers and high drift mobility of charge carriers (10 4 cm2/V s) [1±3]. These features determine their use as transport layers in electroluminescent devices [4,5]. Typical representatives of silicon-organic polymers are poly(methylphenylsilane) (PMPS) and poly(dihexylsilane) (PDHS). Their main polymer chains consist of silicon atom and side groups are organic fragments (Fig. 1). The processes of photogeneration of charge carriers in polymers usually proceed in two stages, namely, via formation of geminal electron±hole pairs, with consequent dissociation of these pairs which is concurrent with the processes of their recombination [6]. The ef®ciency of photogeneration of charge carriers in polymers increases under introduction of acceptor molecules into polymers [7±9], which is probably connected with formation of charge transfer complexes (CT-complexes) in the polymer. For this reason, silicon-organic polymers with *

Corresponding author.

admixture of acceptors can be used not only as transport layers, but also as photogeneration layers. In addition, the formation of CT-complexes increases the sensitivity of polymers to the visible light. One should note that, up to the present time, the number of reliable works which prove the existence of CT-complexes in PMPS and PDHS with admixture of acceptor molecules is very small. In fact, there exist one system for which there is reliably determined the existence of CT-complexes, namely, ®lms of PMPS with admixture of iodine [10,11]. The degree of charge transfer in the ground state of the polymer is not signi®cant, but becomes signi®cant under excitation of the polymer. The formation of CT-complexes and the distance between the donor (D) and acceptor (A) molecules which form the CT-complex are mainly determined by the overlapping of their boundary orbitals, namely, HOMO of donor and LUMO of acceptor, but are also in¯uenced by van der Waals forces and steric hindrances. For planar molecules, this distance corresponds to face-to-face packing and is Ê . In our case, we expect expected to be equal to 3.5±4 A formation of not usual p±p, but s±p complex, where the stype HOMO is localized at silicon chain. In contrast with planar molecules of p-donors, here the tetrahedral surrounding of silicon atoms does not allow the acceptor molecules to lie so closely to the atoms on which the HOMO is localized as it lies in p-complexes. This is of special importance for the polymers under study (bulky, chaotically curled chains).

0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 0 4 1 - 3

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Fig. 1. The molecular structures of PMPS, poly(di-n-hexylsilane), TNF, TCNQ and TNAP.

In this work, we studied the CT-complexes formed in the solutions of polysilanes under the introduction of acceptor molecules (tetracyanoquinodimetane, TCNQ; tetracyanonaphtoquinodimetane, TNAP; trinitro¯uorenone, TNF) on the basis of their absorption spectra (T ˆ 300 K) as dependent on the ionization potential of the donor polymer and on the absence or presence of aryl substituent in the polymer (PMPS and PDHS), the electron af®nity of the acceptor, the concentration of the solution and the temperature of the solution. The questions concerning the participation of the aromatic substituents in transfer of electron from the Si±Si chain to acceptor [12,13] and the in¯uence of acceptor on the degradation processes in polymer [10,11,13] are rather interesting and debatable.

prepared at room temperature and kept in dark place. The spectra of the mixtures periodically monitored until it became evident that consequent keeping would not lead to additional changes (typically from several days to 2 months). 3. Results In Fig. 2 (curves 2 and 3), we present the absorption spectra of the toluene solutions of PMPS with TCNQ admixture

2. Experiment The studied polymers, PMPS and PDHS, were the separated fractions with molecular weights Mw of 11,160 and 219,000 and with distributions (Mw/Mn) equal to 1.75 and 1.18, respectively. As acceptors, we used TCNQ, TNF and TNAP. The structures of all these compounds are presented in Fig. 1. The UV spectra were measured with the use of the spectral complex KSVU-12. We used toluene solutions (concentration of PMPS and PDHS 0.1%, the amount of acceptors equal to 5±20 wt.% of the amount of the polymers, quartz cuvettes with length 5 and 10 mm). The solutions were

Fig. 2. Absorption spectra of the toluene solutions of TCNQ and PMPS± TCNQ mixtures as dependent on the concentration of TCNQ and the time of storage: (1) pure TCNQ; (2 and 3) 5 and 10% TCNQ (with respect to polymer) after 1 day storage; (4 and 5) 10% TCNQ, 7 and 10 days storage. Concentration of polymer is 0.1 mass%, d ˆ 10 mm; all spectra are normalized to the intensity of the wing of the band of TCNQ.

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Fig. 3. Decomposition of the absorption spectra of the PMPS±TCNQ mixture (10% TCNQ, time of storage is 10 days) in the region of the CT-bands.

(concentration of polymer 0.1 mass%, d ˆ 10 mm) as dependent on the concentration of TCNQ with respect to polymer (curve 2: 5%; curve 3: 10%), as well as on the time of storage of the solution of PMPS with 10% TCNQ (3±5: 1, 7 and 10 days). The curve 1 corresponds to the absorption spectrum of pure TCNQ in toluene. The most red band in the absorption spectrum of the polymer and the acceptor in toluene lie near 350 nm (s±s transition) and 400 nm, respectively. One can see that in the absorption spectra of the solutions of PMPS with TCNQ admixture there arise a new broad weak band in the visible region (520±690 nm, see curve 2) whose intensity increases with the increase of the concentration of the acceptor up to 10% (curve 3), as well as with the time of storage of the solution of PMPS with 10% TCNQ (curves 3±5) and it mainly achieves saturation after 10 days. One can see that this band has structure. The appearance of the new band in the absorption spectrum is accompanied by the change in the color of the solution, which becomes violet.

The decomposition of the curve 5 (Fig. 2) into two Gaussian bands is presented in Fig. 3 and the numerical data are given in Table 1. On can see from this table that the energy distance between the Gaussian bands is equal to 0.17 eV. The changes observed in the absorption spectra of the toluene solutions of PDHS under the introduction of TCNQ are similar to the changes observed in the absorption spectra of the PMPS±TCNQ mixture, in visible region, a new broad band arises (460±730 nm) whose intensity increases with time of storage of this solution (Fig. 4, curves 2±4). It is clearly seen that this band has structure. The color of the solution is changed to violet. The saturation of the intensity of this band, however, takes place at greater times than for the PMPS±TCNQ mixture. Heating of the solution up to 50 8C during 1 h slightly decreases the intensity of this band. The decomposition of the curve 4 (Fig. 4) into three Gaussian bands is presented in Fig. 5 and the numerical

Table 1 Fitted parameters for the decomposition of the absorption spectra of PMPS±TCNQ, PDHS±TCNQ and PMPS±TNF complexes into Gaussian bands. PMPS solution with 10% TCNQ

D (eV)

PDHS solution with 10% TCNQ

D (eV)

PMPS solution with 7% TNF

D (eV)

Band 1 Peak position (eV) Width (eV) Amplitude (a.u.)

1.97 0.18 0.26

0.17

2.03 0.3 0.4

0.17

2.6 0.2 0.15

0.19

Band 2 Peak position (eV) Width (eV) Amplitude (a.u.)

2.14 0.2 0.32

2.20 0.19 0.3

0.17

2.79 0.2 0.15

Band 3 Peak position (eV) Width (eV) Amplitude (a.u.)

2.37 0.23 0.5

Note: D corresponds to the energy differences between the highest occupied molecular orbitals in the Si chromophore.

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Fig. 4. The absorption spectrum of the toluene solution of PDHS±TCNQ (10% TCNQ) as dependent on the time of storage: (1) pure TCNQ; (2±4) 1, 15 and 50 days.

Fig. 6. The absorption spectrum of the toluene solution of PMPS±TNF (7% TNF) as dependent on the time of storage: (1) pure TNF; (2±4) 1, 2 and 7 days.

data are given in Table 1. One can see from this table that the new bands in the PDHS±TCNQ system arise in the region with smaller wave lengths than in the case of the PMPS± TCNQ system and the distances between the neighboring maxima (1±2 and 2±3) are equal to 0.17 eV. In Fig. 6, we present the absorption spectra of the toluene solutions of PMPS with 7% TNF admixture as dependent on the time of storage of the solution. The toluene solutions of the polymer and the acceptor are almost colorless. The edge of absorption of TNF lies near 420 nm. One can see that in the absorption spectra of PMPS with 7% TNF there arises a weak band of irregular shape in visible region (420±620 nm) whose intensity increases with time and mainly achieves saturation after several days. The formation of the new absorption band in the PMPS±TNF system is accompanied by the change in the color of the solution, namely, it becomes yellow. One should note that in the PMPS±TNF system the new band arises at shorter wavelengths that in the PMPS± TCNQ or PDHS±TCNQ systems. The increase of the con-

centration of TNF to 20% leads to disappearance of the effect, i.e. the spectra of the solution with 20% TNF practically do not change with time. The decomposition of the curve 4 (Fig. 6) into two Gaussian bands was performed and the numerical data are given in Table 1. One can see from the table that the energy distance between these bands is equal to 0.19 eV. The introduction of 10% of TNF into the toluene solution of PDHS did not lead to any changes in the absorption spectra additional to trivial superposition of the spectra of the components. No new bands in the 450±600 nm region arose even at long-time storage of this mixture. Heating of the solution up to 50±100 8C, as well as the changes of the molar ratio of the components (5 and 20%), also did not lead to any changes in the absorption spectra. We also studied the absorption spectra of the mixture PMPS±TNAP (5 and 10% TNAP). Even at long-time (60 days) standing of the solution, no new bands arose in visible region.

Fig. 5. Decomposition of the absorption spectra of the PDHS±TCNQ mixture (10% TCNQ, time of storage is 50 days) in the region of the CT-bands.

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4. Discussion The appearance of new wide absorption bands in the visible region of the spectra of the silicon-organic polymers (PMPS and PDHS) under introduction of acceptors (TNF and TCNQ) can be attributed to formation of CT-complexes (with silicon backbone as donor). CT-complexes are characterized by electron transfer from donor to acceptor. Since the electronic interaction in the ground state is weak, the individual absorption bands of the molecules which form the complex are also preserved in the spectrum of the complex; in our experiments, we have exactly this situation. The energy of transition in CT-complexes can be roughly estimated as ECT  Ig

Ea

where Ig is the potential of ionization of the donor and Ea the electron affinity of the acceptor. When the Ig value of the donor increases (for the same acceptor), as in the case of PDHS as compared with PMPS (in condensed state I c ˆ 5:78 [14] and 5.3 eV [15], respectively; the values for free molecules I g  I c ‡ 1:5 eV), the band of CT-complex should be shifted toward shorter wavelengths, which agrees with our experiments with TCNQ as acceptor (Figs. 2 and 4, Table 1). On the other hand, when decreasing the value of Ea (for the same donor), as in the case of TNF (Ea ˆ 2:05 eV) [16] as compared with TCNQ (Ea ˆ 2:8 eV) [17], the band of CT-complex also should be shifted toward shorter wavelengths, which agrees with our experiments with PMPS as donor (Figs. 2 and 6, Table 1). An additional evidence for formation of CT-complexes is the increase of the quantum yield of photogeneration of charge carriers which achieves, for example, ®ve times in PMPS with TCNQ as compared with pure PMPS [9]; the increase of the quantum yield of photogeneration of charge carriers is also observed in PMPS±TNF system in the 400± 500 nm region [18]. One more evidence for formation of CT-complexes, for example, in the PMPS±TNF system, is great Stokes shift between the absorption and luminescence bands (5000 cm 1). It is known [19] that in the luminescence spectra of PMPS ®lms with 1% TNF (T ˆ 5 K, lexc ˆ 313 nm) one observes weak long-wave band in the 500± 700 nm region. This broad band with maximum at 630 nm is more pronounced for wavelength of excitation, lexc ˆ 405 nm [19]. In the luminescence spectra of PDHS ®lms with 1% TNF (T ˆ 5 K), a broad weak band arises with maximum at 610 nm. The band of CT-complexes is complex and it can be decomposed into several Gaussian peaks (Table 1); the number of peaks for PMPS±TCNQ system is two; for PDHS±TCNQ: 3; and for PMPS±TNF: 2. In all cases, the distances between the Gaussian components of the CT-bands are the same (Table 1). The fact that for various acceptors we have in fact the same energy distances between the bands suggests that this structure is connected with the

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silicon backbone. The orbitals of the polymer molecules form series of similar orbitals, so there are series of the orbitals similar to HOMO, which are also of s-type and are localized in the silicon backbone. In this case, we assume that the structure of the CT-bands is connected with transitions from several highest occupied orbitals of silicon backbone to the lowest unoccupied orbitals of acceptor. The distances between these occupied orbitals connected with silicon backbone should correspond to the distances between the bands in the photoelectron spectra of these polymers. Unfortunately, the bands in the 5±6 eV region of the photoelectron spectra which mainly correspond to the delocolized sigma orbitals of the silicon backbone are extremely weak [20] and are hardly observed due to overlapping by strong bands of side groups. From the fact that the structure of the absorption bands in the systems PMPS±TCNQ and PDHS±TCNQ are similar, one can conclude that the presence of aromatic groups in the polymer has not essential in¯uence on the formation and character of the CT-complexes. We tried to estimate possible distances between the components of the CT-complexes with taking into account stereoizomerism and possible conformations of the polymers under study. As far as the HOMO are localized on the Si±Si chain and the LUMO of acceptor are localized on heavy (nonhydrogen) atoms of acceptor, the subject of interest was the distance between the Si atoms of the polymer and the closest heavy atom of acceptor. For PMPS, isotactic or syndiotactic structure is possible, as well as mixed structures. Syndiotactic structure is more favorable for close contact with acceptor molecules and leads to Ê ; in this case, acceptor molecule has distances of ca. 4.5 A contact only with methyl groups. For isotactic structure, the distances are greater, but phenyl groups can in principle, due to mixing of s-orbitals of Si-backbone and p-orbitals of phenyl substituents, be mediators for electron transfer in the CT, notwithstanding the greater distances from the Si atoms. For PDHS, one can build two conformations, with hexyl groups aligned parallel and perpendicular to the zigzag formed by Si atoms. Despite of the tendency of PDHS to crystallization, there are no data in literature concerning really present conformations of PDHS. Again the distances Ê . This is greater than usually, but these estiare ca. 4.5 A mations do not allow us to exclude the formation of CT-complexes between the starting components. The absence of the CT-band in the PMPS±TNAP system can be correlated with the fact that no increase of the quantum yield of photogeneration is observed in the systems formed by the polymers under study with the acceptor with great Ea [9]. A problem which arises when trying to interpret the colored products as the CT-complexes between the starting compounds is to explain unusually great times of their formation. This can be explained, e.g. by the presence of van der Waals associates in the freshly prepared solution of the polymer which slowly dissociate, so that the formation of

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the complexes is also slow. The absence of CT-complexes in the absorption spectra of the system PDHS±TNF can be attributed to greater steric hindrances in PDHS as compared with PMPS. The difference in times of development of the bands of CT-complexes of PMPS and PDHS with TCNQ can be attributed to greater tendency of PDHS to form ordered supramolecular structures in solution with greater times of dissociation. This can also be accompanied by the processes of degradation of the polymers. An alternative to formation of CT-complexes between the unchanged components is slow chemical reaction between them. Indeed, the introduction of acceptor can lead to degradation of the polymer [10,11,13]. One can assume addition of two halves of the polymer molecule (with scission of Si±Si bond) to the conjugated system of double bonds of the acceptor molecule. The fragment of the acceptor molecule with the rest of this conjugated system can have yet enough pronounced acceptor character and the remaining Si±Si chains can have pronounced donor character, so the product can be intramolecular CT-complex (the problem of interatomic distances is canceled, as far as the distance between the donor and acceptor fragments is the length of normal chemical bond). We performed semiempirical quantum mechanical calculations of the UV spectra of such possible products. The calculations show that such products really can be colored. The assumption that the obtained complexes are such products explains the fact of their slow formation. Unfortunately, the uncertainty of the calculations does not allow us to verify this assumption. However, our experiments with UV irradiation of the mixture PMPS±TNF at wavelength 365 nm during 1 min led to appearance of a new absorption band, but its position (3.38 eV) is not identical with the positions of the bands observed upon staying of the solution (2.6 and 2.79 eV). This allows us to assume that the CT-complex formed upon staying is the intermolecular complex between unchanged components. 5. Conclusions The formation of CT-complexes between s-donor polymers and p-acceptor is shown on the basis of appearance of

new bands in visible region in the absorption spectra of the solutions of polymers when introducing various acceptors. It is shown that the character of the observed spectral changes does not depend on the absence or presence of aromatic substituents in the polymers under study (in particular, for all pairs polymer-acceptor we have several bands of CT-complexes with in fact the same energy distances between these bands). References [1] R.G. Kepler, J.M. Zeigler, L.A. Harrah, J.R. Kurtz, Phys. Rev. B 35 (1987) 2818. [2] M. Abkowitz, F.E. Knier, H.-J. Yuh, R.J. Weagley, M. Stolka, Solid State Commun. 62 (1987) 547. [3] T. Dohmaru, K. Oka, T. Yajima, M. Miyamoto, Y. Nakayama, T. Kawamura, R. West, Philos. Mag. B 71 (1995) 1069. [4] J. Kido, K. Nagai, Y. Okamoto, T. Skotheim, Appl. Phys. Lett. 59 (1991) 2760. [5] H. Suzuki, H. Meyer, J. Simmerer, J. Yang, D. Haarer, Adv. Mater. 5 (1993) 743. [6] S. NesÏpurek, V. Cimrova, J. Pfleger, J. Kminek, Polym. Adv. Technol. 7 (1996) 459±470. [7] R.G. Kepler, P.A. Cahill, Appl. Phys. Lett. 63 (1993) 1552. [8] Y. Nakayama, A. Saito, T. Fujii, S. Akita, J. Imag. Sci. Tech. 43 (1999) 261. [9] S. NesÏpurek, private communication. [10] A. Watanabe, Y. Tsutsumi, M. Matsuda, Synth. Met. 74 (1995) 191±196. [11] M. Kakimoto, H. Ueno, H. Kojima, Y. Yamaguchi, A. Nishimura, Mat. Res. Symp. Proc. 328 (1994) 267. [12] K. Yoshino, K. Yoshimoto, M. Haneguchi, T. Kawai, A. Zazhidov, H. Ueno, M. Kakimoto, H. Kojima, Jpn. J. Appl. Phys. 34 (2, no. 1B) (1995) L141±L144. [13] S. Ninomiya, Y. Ashihara, Y. Nakayama, K. Oka, R. West, J. Appl. Phys. 83 (7) (1998) 3652±3655. [14] N. Matsumoto, in: J.F. Horrod, R.M. Laine (Eds.), Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Dordrecht, 1991, pp. 97±113. [15] K. Takeda, J. Phys. Soc. Jpn. 63 (1994) 1±29. [16] V.EÂ. Kampar, O.Ya. Nejland, Uspekhi Khimii 46 (6) (1977) 945±966. [17] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals, Oxford University Press, New York, 1982. [18] I. Glowacki, J. Jung, J. Ulanski, Synth. Met. 103 (2000) 143±146. [19] V.N. Zaika, A.K. Kadashchuk, N.I. Ostapenko, Yu.A. Skryshevskii, S. NesÏpurek, Ukrainskii Fiz. Zhurn. 45 (10) (2000) 1246±1249. [20] K. Seni, A. Yuyama, S. Narioka, Polymeric materials for microelectronic applications, ACS Symposium Series no. 579, Sci. Technol. 32 (1994) 200±209.