Increase in the Mobility of Photogenerated Positive Charge Carriers in

J. Phys. Chem. B 2005, 109, 10015-10019

10015

Increase in the Mobility of Photogenerated Positive Charge Carriers in Polythiophene Akinori Saeki,*,† Shu Seki,† Yoshiko Koizumi,† Takeyoshi Sunagawa,‡ Kiminori Ushida,§ and Seiichi Tagawa*,† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, Fukui UniVersity of Technology, 3-6-1 Gakuen, Fukui, Fukui 910-8505, Japan, and RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ReceiVed: December 20, 2004; In Final Form: March 8, 2005

We report the increase in the mobility of charge carriers in regioregular poly 3-hexyl thiophene (RR-P3HT) films by mixing them with tetracyanoethylene (TCNE), which is examined by in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). TCNE acts not only as an electron acceptor which increases the number of charge carriers on photoexposure but also as a functional additive which enhances the mobility of the charge carriers. This conclusion was deduced from the results of fluorescence quenching, transient optical absorption and photobleaching, and comparison of the TRMC signal with the TOS signal. The combination of the TRMC and TOS techniques represents a comprehensive and fully experimental approach to the determination of the intrinsic carrier mobility in conjugated polymers.

Introduction The σ- and π-conjugated polymers1 have been subjected to intensive investigations because of the feasibility of their optical, electrical, and optoelectrical properties for flexible and lowcost electric devices such as organic electroluminescent (EL) displays. Polythiophene is one of the most probable candidates for organic electronic polymer materials. Optimization of the charge carrier mobility in the materials plays a crucial role in their applications to devices. A systematic, rapid, and accurate technique has been in strong demand for evaluation of the intrinsic carrier mobility of conjugated polymer chains. Timeresolved microwave conductivity (TRMC)2-6 is a powerful tool for measuring the conductivity and dynamics of charge carriers in organic materials, semiconducting materials, and so on, because the conductivity can be probed by low-power microwaves without having electrodes attached. This advantage provides us with an approach to the measurement of the intrinsic mobility of the charge carriers in various media, which excludes the interference of the electrode-polymer interface, disordered structures, and so on.2 The charge carriers were generated by a pulsed laser in the present work, followed by in situ measurement of the conductivity and density of carriers with the TRMC and transient optical spectroscopy (TOS) techniques. Pulsed radiation from an accelerator such as a high-energy electron beam has been also used to produce transient charge carriers.2,3 The radiation offers the advantage that it promotes a uniform distribution of charge carriers with precise information on their density. On the other hand, the efficiency of photogeneration of the charge carriers is dependent on the optical properties of the materials and the excitation wavelength. To increase the concentration of positive charge carriers, the addition of an electron acceptor is one of the most effective choices, simul* To whom correspondence should be addressed. Phone: +81-6-68798502. Fax:+81-6-6876-3287. E-mail: [email protected] (A.S.); [email protected] (S.T.). † Osaka University. ‡ Fukui University of Technology. § RIKEN.

taneously providing the highly quantitative analysis of optical spectroscopy. A large number of investigations have been devoted to the spectroscopy of conjugated polymer solutions with electron acceptors in order to elucidate the dynamics of donor-acceptor systems such as the formation of contact ion pairs, forward and backward electron transfers, and the formation of charge transfer complexes. In these experiments, organic materials such as tetracyanobenzene (TCNB), tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), anthraquinone (AQ), methyl viologen (MV), and fullerene (C60) are often used as electron acceptors. Among them, TCNE7-10 is one of the most powerful electron acceptors, having both a high reduction potential and a high compatibility with solid polythiophenes. In the present work, TCNE is adopted as an electron acceptor, and its concentration dependence on photoinduced conductivity and fluorescence quenching is discussed. At the end of this article, the mobility of charge carriers in RRP3HT-TCNE films is shown. Experimental Section Regioregular (head-to-tail >98%) poly 3-hexyl thiophenes (RR-P3HT) and tetracyanoethylene (TCNE) were purchased from Aldrich and Wako Chemical, respectively, and used without further purification. A 5 mM (base mol unit) methyl tetrahydrofuran (MeTHF) solution of RR-P3HT and a 10 mM MeTHF solution of TCNE were prepared. A 2 mL portion of the RR-P3HT solution was mixed with various volumes of the TCNE solution. The concentration was determined from the mixing ratio of each molecule and was converted to M (mol/L) using the molecular weight of TCNE (128) and its density (1.348 g/mL) and the unit molecular weight of RR-P3HT (166) and its density (1.5). The solution was drop-cast onto a quartz substrate (1 mm thick). The sample preparation involved the following two procedures: (A) the sample was placed in a vacuum oven and dried at 50 °C under vacuum, and (B) the sample was placed in an oven and dried at 80 °C in air. The thicknesses of the films were measured using a stylus surface profiler (Dektak 3ST) to be ∼500 nm.

10.1021/jp0442145 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/03/2005

10016 J. Phys. Chem. B, Vol. 109, No. 20, 2005

Saeki et al.

Figure 1. Dependence of peak values of φΣµ on TCNE concentration. The insets show the chemical structure of materials used in the film experiments: regioregular poly 3-hexyl thiophene (RR-P3HT) and tetracyanoethylene (TCNE). They were mixed in methyl tetrahydrofuran solution and casted on a quartz substrate. The open and closed circles were obtained under 355 and 532 nm excitation, respectively. The solid line is a fitting function of 2 × 10-4 exp(8.5[TCNE]).

Transient conductivity, photoinduced absorption, and fluorescence were measured with the identical geometry using in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS) systems. A resonant cavity was used to obtain a high degree of sensitivity in the measurement of conductivity. The resonant frequency and the microwave power were set at ∼9.1 GHz and 3 mW, respectively, so that the electric field of the microwave was sufficiently small not to disturb the motion of charge carriers. The value of conductivity is converted to the product of the quantum yield, φ, and the sum of charge carrier mobilities, Σµ, by the following equation:

∑

φ

µ)

∆Pr 1 eAI0Flight Pr

Figure 2. Dependence of the normalized peak of fluorescence from RR-P3HT films on TCNE concentration. The open and closed circles were obtained under 355 and 532 nm excitation, respectively. These data were collected from the insets. The solid line is a fitting function of exp(-1.2[TCNE]).

product of the quantum efficiency and the sum of the mobilities, and therefore, such a high value of φΣµ means the increase in either φ or Σµ. To obtain information on the formation process of charge carriers, the quenching of fluorescence from the singlet (1PT*) was observed, as shown in Figure 2. With the increase in [TCNE], a rapid decrease in the fluorescence intensity was observed for both 355 and 532 nm excitations, which supported an effective fluorescence quenching through an electron transfer reaction:10

PT* + TCNE f PT•+ + TCNE•-

1

(1)

where e, A, I0, Flight, ∆Pr, and Pr are the unit charge of a single electron, a sensitivity factor (S-1 m), the incident photon density of the excitation laser (photon m-2), a correction (or filling) factor (m-1), a change of the reflected microwave power, and the reflected microwave power, respectively. The details of the in situ TRMC-TOS system will be reported elsewhere. The THG (355 nm) and SHG (532 nm) light pulses from a Nd: YAG laser (5-8 ns pulse duration) were used as an excitation source. The photon density of the laser was approximately 1.5 × 1016 photons/cm2. A white light continuum from a Xe lamp was used for a probe light source. The probe light and emission were guided into a high-dynamic-range streak camera (Hamamatsu C7700) which collects a two-dimensional image of the spectrum profiles of light intensity. All the experiments were performed at room temperature in air. Results and Discussion Figure 1 shows the dependence of φΣµ on the TCNE concentration ([TCNE]). The charge carriers were generated by 355 or 532 nm light from the Nd:YAG pulsed laser. The value of φΣµ exceeded the order of 10-2 cm2 V-1 s-1 with [TCNE] > 4.5 M. The maximum value of φΣµ was nearly 2 orders of magnitude higher than those found in the pristine polymer. The corresponding conductivity is on the order of 10-3 S cm-1 for a 1016 cm-2 photon density. The solid line in Figure 1 is an exponential function of a × eb[TCNE], where a and b are a scaling factor and a fitting parameter, respectively. The solid curve in Figure 1 was obtained using b ) 8.5 M-1. φΣµ represents the

(2)

TCNE subtracts an electron from the excited state of the polymer, yielding a radical anion of TCNE, TCNE•-, and a polythiophene radical cation, PT•+. It is well-known that the radical cation interacts with the phonon of the polymer backbone and forms a positive polaron within the femtosecond to picosecond time scale.11 The quenching of the fluorescence was fitted by an exponential function of e-b[TCNE], as shown in Figure 2. This function is explained by a result of dynamic and static quenching in terms of charge delocalization12 and/or electron tunneling13 within the time resolution of the TOS system. In the former explanation,12 the energy transfer from the excited state to the quencher occurs if the quencher is located in the area where the dispersion of energy or diffusion of excited molecule can reach within its lifetime. The static quenching plays an important role, particularly in the presence of highly concentrated quenchers, because more quenchers happen to be within the reaction radius of the energy transfer, leading to the immediate energy transfer after the excitation. On the basis of only the static quenching, the parameter b was interpreted to include a distribution of solute association/aggregation, a delocalization of charge or energy on a molecule, and a ratio of an excluded volume. The solid curve in Figure 2 was obtained with a fitting parameter, b, of 1.2 M-1. This value is close to that used in the previous study,12 suggesting the delocalization of the excited state over several thiophene units of the polythiophene chain. In the latter explanation,13 efficient electron transfer from donor to acceptor via electron tunneling occurs, which results in a large reaction radius. Application of this model to the present case (donor, RR-P3HT; acceptor, TCNE) gives a reaction radius of ∼0.8

Mobility of Photogenerated Positive Charge Carriers

Figure 3. Transient absorption spectrum at end of pulse obtained in RR-P3HT films mixed with 0.22 M TCNE on 355 nm excitation (dots). The solid line, gray line, and dotted line represent a ground-state absorption spectrum of pristine RR-P3HT, an absorption spectrum of a radical cation of pristine RR-P3HT produced by γ-rays from 60Co in a 1-butyl chloride matrix at 80 K, and an emission spectrum of pristine RR-P3HT, respectively. The inset in the upper part of the figure shows the dependence of end-of-pulse optical density at 460 nm on TCNE concentration. The graphics below the spectra are the kinetic traces of transient absorption monitored at 460 and 635 nm, respectively.

nm based on a spherical quenching volume. This radius is approximately twice that of TCNE if it is assumed as a sphere, suggesting a charge transfer via electron tunneling. However, in the present donor-acceptor system, it is difficult to separate the effects of charge delocalization and electron tunneling, because the structure of a polymer is indiscrete and seems to have a distributed reaction radius. In the case of 532 nm excitation, only polythiophene is excited and the electron transfer reaction occurs, because the π-π* transition in polythiophene is observed at around 510 nm and there is no optical absorption of TCNE at 532 nm. At 355 nm, the extinction coefficient of polythiophene is approximately 20% of that at 532 nm. However, the extinction coefficient of TCNE at this wavelength is 1 order of magnitude lower than that of polythiophene. Therefore, even by the excitation at 355 nm, the electron transfer reaction from PT to TCNE given by eq 2 is predominant in the quenching of the PT singlet. This is also supported by the fact that no significant difference between 355 and 532 nm excitation was observed in the dependences of φΣµ and fluorescence quenching yield on [TCNE]. In Figure 3, the photoinduced transient absorption of polythiophene at [TCNE] ) 0.22 M (dots) is shown, together with the normalized ground-state absorption (solid line) and emission spectra (dotted line) of the pristine polymer. As can be seen in the inset of Figure 3, the amplitude of the transient absorption at 460 nm increased to some extent by adding TCNE; however, a significant increase in the amplitude was not observed upon more addition of TCNE. The photobleaching at 635 nm also showed the same dependence. This indicates that the main path

J. Phys. Chem. B, Vol. 109, No. 20, 2005 10017 of the photo charge carrier generation is the direct exciton dissociation even in the presence of TCNE. The transient absorption at around 460 nm can be seen clearly with the bleaching of the steady-state absorption band at around 635 nm. The bleaching area lies in the edge of steady-state absorption, suggesting that the positive polarons are formed on the relatively long π-conjugated segments in the polymer chain after the intrachain charge transfer reactions. It should be noted that the intramolecular migration of polarons from the conjugated segments of distributed length to the longer one is completed within the time resolution of the experiment.1 The kinetic traces at 460 nm showed a good correspondence with that at 635 nm. This implies that both the transient absorption and bleaching originate from a single species, which is the positive polaron lying on the longer π-conjugated segment. We observed a broad absorption centered at 475 nm using low-temperature γ-radiolysis, in which the radical cations of polythiophene were formed in the glassy matrix of 1-butyl chloride (BuCl) by γ-ray irradiation. The normalized absorption spectrum is shown in Figure 3 as a gray line. The absorption maximum of TCNE•- was reported to be around 420 nm,7,8 and its extinction coefficient at this wavelength is relatively small, being around 7 × 103 M-1 cm-1.7 The extinction coefficient of the radical cation of polythiophene is expected to be much higher than that of TCNE•-. In addition, the decay of TCNE•- is attributable to the charge recombination given by eq 3, indicating that the kinetic trace of TCNE•- is the same as that of PT•+.

PT•+ + TCNE•- f PT + TCNE

(3)

Therefore, it is difficult to distinguish the contribution of TCNE•- from the kinetic trace at around 460 nm. The photoinduced absorption peak shown in Figure 3 seems to be blue-shifted by 15 nm compared with the peak obtained by γ-radiolysis (475 nm). This is probably due to the overlap of TCNE•- and the difficulty of spectroscopy in the region where the steady-state absorption is strong. Figure 4 demonstrates the comparison of kinetic traces between the TRMC and TOS signals which were obtained in the identical geometry of the experiment at [TCNE] ) 0.22 M upon 355 nm exposure. The samples shown in Figure 4 were prepared from the same solution with the two different procedures, resulting in a higher ratio of lamellar structures in part b than in part a. The kinetic traces of the TRMC signal coincided with that of the TOS signal for each sample over the entire time scale. The peak positions of the transient absorption and the bleaching observed in Figure 4b were the same as those of Figure 3, and the kinetic traces of the absorption and the bleaching showed good correspondence over the entire time scale. This indicates that the conductive transients of the polythiophene films originate mainly from the radical cations on the polythiophene chains, the positive polarons. The rise time of the kinetic trace in Figure 4a was approximately 200 ns, while that in Figure 4b was approximately 6 µs. Both rise times are much longer than the response time of the measurement system (3 × 104 M-1 cm-1. The uncertainty of this value comes from the experimental difficulty because of the thermochromism of RR-P3HT. An accurate value will be reported in the near future. Using this extinction coefficient, the quantum efficiency, φ, was calculated to be 0.5 cm2 V-1 s-1 at [TCNE] ) 4.9 M. The increase in mobility is considered to enhance the efficiency of charge carrier transport in organic electric devices. The mobility obtained at [TCNE] ) 0 M is the same order as that measured by pulse radiolysis TRMC2,3 in which the charge carriers are generated by a high-energy electron beam. This value is, however, lower than the maximum value of ∼0.1 cm2 V-1 s-1 that was obtained by field effect transistors (FETs).16 In general, the TRMC mobility should be higher than the FET mobility,2 because the TRMC mobility is not affected by grain boundary, electrodes, and so on, while the FET mobility is dependent on them and is based on transport properties of long channel between the electrodes, which results in the charge carriers being more trapped by trapping sites. The lower TRMC mobility than expected may be due to the difference of the sample conditions (the lamellar structure) and the overestimation of charge carrier density. In general, the predominant effect of TCNE doping into polythiophenes is expected to be a considerable increase in the yield and concentration of photogenerated charge carriers, and the effects on the mobility should be the minor path. However, the experimental results indicate that the increase in conductivity is attributed to the increase in mobility rather than the enhancement of charge carrier generation. Figure 5 shows ground-state absorption spectra of 0, 1.5, and 4.9 M TCNE-RR-P3HT mixed films. The two absorption bands at 650-1000 nm and >2600 nm increase with the contents of TCNE. These two bands can also be seen in pristine RR-P3HT film, although their amplitudes are small. The former band is close to an absorption of delocalized polarons in RR-P3HT observed by femtosecond spectroscopy,1c while the latter band is close to an absorption of intrachain polarons in regiorandom polythiophene (RRaP3HT) rather than that in RR-P3HT. The correlation in the relative amplitudes of these two bands differs from the literature,1c but it is probable that more charge carriers and/or charge transfer (CT) complexes are formed by the addition of

Mobility of Photogenerated Positive Charge Carriers TCNE. This is also suggested by the experimental fact that too large of an added amount of TCNE caused strong absorption of microwave, resulting in a failure of TRMC experiments based on a resonant cavity. It should be emphasized that 100% TCNE did not give an intense TRMC signal on exposure at 355 nm where TCNE absorbs the laser pulse. Combinations of TCNE and other polymers such as polysilane were examined, but significant increases in conductivity like the case of RR-P3HT were not observed. Combinations of RR-P3HT and other electron acceptors such as those presented in the Introduction were also examined, but the increases were not observed either. Preliminary experiments for regiorandom polythiophene (RRaP3HT) and TCNE mixed films showed an increase in the conductivity. Therefore, we predicted that the increase in TRMC mobility is due to the matching of the molecular orbital of polythiophene and TCNE rather than the enhancement in microordering of the molecules. In such a case, questions about an activation energy of charge hopping and FET or TOF mobility for RR-P3HT-TCNE mixed films would arise. Unfortunately, the activation energy cannot be measured by our present measurement system to date, because of the lack of a temperature controlling system. Measurements of FET and TOF mobility, which reflect a transport property of long distance, are expected to be hard, because the existing charge carriers narrow the depletion layer, resulting in difficulties of gate modulation, estimation of carrier density, and insulation of voltage. Further investigations on the activation energy, comparison of TRMC and FET or TOF mobility, and interaction between polythiophene and TCNE are required in the future. It should be noted that the increase in charge carrier mobility in RR-P3HT by the addition of TCNE suggests a realization of polymer-based organic electric devices of which the carrier mobility is comparable to that found in low molecular materials such as pentacene.17 The carrier mobilities in polymers are, in general, low compared to those in low molecular materials, but polymers are applicable to wet processing, which would offer low-cost organic electric devices. The strong steady-state absorption of polythiophene at ∼510 nm, which is corresponding to the peak of the solar spectrum in visible, also suggests the use of polythiophene-TCNE film for photoinduced functional materials such as organic solar cells because of the efficient current generation upon light exposure. Conclusion The dependence of φΣµ and fluorescence quenching on TCNE concentration was measured in the polythiophene-TCNE mixed films using in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). The observation revealed that the mobility of charge carriers increased by nearly 2 orders of magnitude, reaching >0.5 cm2 V-1 s-1 at [TCNE] ) 4.9 M. This findings suggests an enhancement of the electric property of polymer-based organic materials and

J. Phys. Chem. B, Vol. 109, No. 20, 2005 10019 an efficient current generation upon light exposure. The combination of the TRMC and TOS techniques demonstrated a comprehensive and fully experimental approach to determining the intrinsic mobility on conjugated materials. Acknowledgment. The authors thank Mr. K. Abe at Canon Inc. for his assistance in experiments and meaningful discussion. This work was supported in part by a grant-in-aid for scientific research from Ministry of Education, Culture, Sports, Science and Technology in Japan. References and Notes (1) (a) Handbook of Conducting Polymer; Skotheim T. A., Elsenbaumer R. L., Reynolds J. R., Eds.; Marcel Dekker: New York, 1998. (b) Seki, S.; Koizumi, Y.; Kawaguchi, T.; Habara, H.; Tagawa, S. J. Am. Chem. Soc. 2004, 126, 3521. (c) Jiang, X. M.; O ¨ sterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.; Janssen, R. A. J.; Vardeny, Z. V. AdV. Funct. Mater. 2002, 12, 587. (d) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwing, P.; de Leeuw, D. M. Nature 1999, 401, 685. (2) Warman, J. M.; Gelinck, G. H.; de Haas, M. P. J. Phys.: Condens. Matter 2002, 14, 9935. (3) (a) Grozema, F. C.; Siebbeles, L. D. A.; Warman, J. M.; Seki, S.; Tagawa, S.; Scherf, U. AdV. Mater. 2002, 14, 228. (b) Wegewijs, B.; Grozema, F. C.; Siebbeles, L. D. A.; de Hass, M. P.; de Leeuw, D. M. Synth. Met. 2001, 119, 431. (4) Kroeze, J. E.; Savenije, T. J.; Vermeulene, M. J. W.; Warman, J. M. J. Phys. Chem. B 2003, 107, 7696. (5) Dicker, G.; de Haas, M. P.; Siebbeles, L. D. A.; Warman, J. M. Phys. ReV. B 2004, 70, 045203. (6) Dicker, G.; de Hass, M. P.; de Leeuw, D. M.; Siebbeles, L. D. A. Chem. Phys. Lett. 2005, 402, 370. (7) Grellmann, K. H.; Watkins, A. R.; Weller, A. J. Phys. Chem. 1972, 76, 3132. (8) Achiba, Y.; Katsumata, S.; Kimura, K. Chem. Phys. Lett. 1972, 13, 213. (9) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (10) (a) Evans, C. H.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 2694. (b) Delabouglise, D.; Hmyene, M.; Horowitz, G.; Yassar, A.; Garnier, F. AdV. Mater. 1992, 4, 107. (11) (a) Kanner, G. S.; Wei, X.; Hess, B. C.; Chen, L. R.; Vardeny, Z. V. Phys. ReV. Lett. 1992, 69, 538. (b) Yu, G.; Phillips, S. D.; Tomozawa, H.; Heeger, A. J. Phys. ReV. B 1990, 42, 3004. (12) Saeki, A.; Kozawa, T.; Yoshida, Y.; Tagawa, S. J. Phys. Chem. A 2004, 108, 1475. (13) Miller, J. R.; Peeples, J. A.; Schmitt, M. J.; Closs, G. L. J. Am. Chem. Soc. 1982, 104, 6488. (14) (a) Magnani, L.; Rumbles, G.; Samuel, I. D. W.; Murray, K.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Synth. Met. 1997, 84, 899. (b) van der Horst, J.-W.; Bobbert, P. A.; de Jong, P. H. L.; Michels, M. A. J.; Siebbeles, L. D. A.; Warman, J. M.; Gelinck, G. H.; Brocks, G. Chem. Phys. Lett. 2001, 334, 303. (c) Rumbles, G.; Samuel, I. D. W.; Magnani, L.; Murray, K. A.; DeMello, A. J.; Crystall, B.; Moratti, S. C.; Stone, B. M.; Holmes, A. B.; Friend, R. H. Synth. Met. 1996, 76, 47. (15) (a) Burrows, H. D.; de Melo, J. S.; Serpa, C.; Arnaut, L. G.; Miguel, M. da G.; Monkman, A. P.; Hamblett, I.; Navaratnam, S. Chem. Phys. 2002, 285, 3. (b) Cadby, A. J.; Lane, P. A.; Bradley, D. D. C.; Holdcroft, S. J.; Yang, C. Synth. Met. 2001, 119, 573. (16) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (17) (a) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science 1999, 282, 822. (b) Kato, Y.; Iba, S.; Teramoto, R.; Sekitani, T.; Someya, T.; Kawaguchi, H.; Sakurai, T. Appl. Phys. Lett. 2004, 84, 3789.