Electrodeless Determination of Charge Carrier Mobility in Poly(3

J. Phys. Chem. C 2008, 112, 16643–16650

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Electrodeless Determination of Charge Carrier Mobility in Poly(3-hexylthiophene) Films Incorporating Perylenediimide as Photoconductivity Sensitizer and Spectroscopic Probe Akinori Saeki,*,† Shin-ichi Ohsaki,† Shu Seki,‡ and Seiichi Tagawa† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, and DiVision of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: May 4, 2008; ReVised Manuscript ReceiVed: July 4, 2008

The charge carrier mobility and its generation efficiency in regioregular and regiorandom poly(3-hexylthiophene) (P3HT) films upon exposure to light pulses are studied by flash-photolysis time-resolved microwave conductivity and transient absorption spectroscopy. For the full experimental determination of alternating current mobility without electrodes, we incorporated a perylenecarboxydiimide (PDI) derivative into the films not only to increase the photoconductivity but also estimate the quantum efficiency of charge generation by kinetic tracing of PDI radical anions. We discuss the mobility, quantum efficiency, decay of charge carriers, and film morphology observed by an X-ray diffraction and an atomic force microscope with various PDI concentrations. It is found that the three-dimensional mobility mainly attributed to the positive charges on regioregular P3HT is decreased from 0.12 to 0.070 cm2/Vs by addition of PDI which disturbs the intermolecular π-stacking, while that on regiorandom P3HT shows an almost constant value of 0.006 cm2/Vs. The quantum efficiency in regioregular P3HT reveals a peak at 6 mol % PDI for 355-nm excitation. The reason for the appearance of the peak is examined by changing excitation wavelength and inspecting the steady-state absorption and fluorescence spectra. The methodology and findings obtained by utilizing the additive as both an acceptor and a probe is proved to be useful for the investigations on optoelectronic properties of wide varieties of organic semiconductors. 1. Introduction The successful development of organic semiconductor devices based on conjugated polymers is a pervasive theme due to their adaptability, flexibility, simplicity, and low cost of manufacture. The fundamental research on photochemical and photophysical properties, dynamics of charge carriers, and discovery of novel functionalities has been required for further extension of the field of practical applications. Among wide varieties of conjugated polymer family, regioregular polythiophenes bearing headto-tail repeating unit have emerged as a viable candidate for the purpose abovementioned, because the regioregulaity induces the intermolecular π stacking, leading to highly ordered lamellar structures partially formed in a film1 (Figure 1), and thereby demonstrating high charge carrier mobility as much as 0.1 cm2/ Vs and more.2 An organic photovoltatic device, which harvests and converts sunlight to electricity, has substantial promise for the realization of sustainable society, where precise knowledge of charge separation between donor and acceptor and resultant charge carrier dynamics would facilitate a better understanding of the organic semiconductive materials. Bulk heterojunction has attracted much attention for the use of conjugated polymer as a solar cell, from the viewpoints of simple process of preparation and high charge separation probability due to the large crosssection between donors and acceptors.3 A fullerene derivative (e.g. 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61, PCBM) is one of the most common electron acceptors * To whom all correspondence should be addressed. Phone: +81-6-68798502. Fax: +81-6-6876-3287. E-mail: [email protected]. † The Institute of Scientific and Industrial Research, Osaka University. ‡ Graduate School of Engineering, Osaka University.

Figure 1. Illustration of π-stacking structure of Reg-PT. The chemical structure at the bottom is perylenecarboxydiimide (PDI) used in the present study. They were mixed and drop-cast on a quartz substrate.

which are often mixed into conjugated polymers such as poly(thiophene)4 and poly(p-phenylenevinylene).3,5 Mobility of charge carrier plays a key role in the organic electric devices, where high mobility enables a fast response to external signal, increase of current, and efficient charge collection upon photoirradiation. Generally, transport property of charge carrier is measured by a direct-current technique such as field-effect transistors (FET); however, it is reflected significantly from undesired material and device factors like impurities, grain boundary, and charge injection barrier at the metal-semiconductor interface. On the other hand, timeresolved microwave conductivity (TRMC) provides a pathway to inspect nanometer-scale charge vibration under an alternating electric field, where short-pulsed laser/radiation is used to produce transient charge carriers without electrodes.6,7 Therefore,

10.1021/jp8039252 CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

16644 J. Phys. Chem. C, Vol. 112, No. 42, 2008 abovementioned unfavorable factors are eliminated totally or as small as possible. Flash-photolysis (FP) TRMC gives a photoconductivity that is defined as a product of mobility and concentration of charge carriers, and thus the former can not be derived without estimation of the latter. The quantum efficiency of charge carrier generation is, however, varied by materials, wavelength of light, incident photon density, and measurement time scale. To circumvent this drawback and gain access to information on concentration and dynamics of shortlived charged species, we have utilized transient absorption spectroscopy (TAS) for pristine organic semiconductors.6 However, this methodology is not applicable if charged species are not detected by TAS. In recent years, PDI derivatives have evolved as a major class of acene compounds which serve as not only an electron acceptor but also a n-type semiconductor.8 Functionalized PDI derivatives have exhibited unique properties, where supramolecular architectures are formed via utilizing noncovalent intermolecular interaction between the extended π conjugations.9 Novel PDI-bound donor systems synthesized to be aimed for efficient energy and charge transfer have been reported for oligo(p-phenylenevinylene),10 oligo(p-phenylene)s,11 oligothiophenes,12 oligopyrroles,13 oligofluorene,14 porphyrin,15 and phthalocyanine.16 Although examples of the combination of oligo- and polythiophene with PDI are not so common,12,17 fabrication of ambipolar transistor and improvement of photoconductivity in the long-wavelength region were demonstrated exploiting the capability of negative charge transport.18 In this article, we demonstrate electrodeless determination of charge carrier mobility in regioregular and regirandom poly(3-n-hexylthiophene) (P3HT) films mixed with PDI. The incorporation of PDI enhances the photoconductivity utilizing donor-acceptor system, and moreover one electron reduced state of PDI has a characteristic absorption with high extinction coefficient in visible to near-infrared region, which enables an assessment of the quantum efficiency of charge carrier generation. We examine the effect of PDI concentration on photoconductivity and charge carrier mobility accompanied by X-ray diffraction (XRD) and atomic force microscopy (AFM) observation. It is expected that the methodology reported here would become a prototype of electrodeless determination of alternating current mobility of charge carrier in organic semiconductors upon exposure to light based on the combination of TRMC and TAS.

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Figure 2. TRMC transients converted to φΣµ observed for Reg-PT films with each molar concentration of PDI. The excitation wavelength was 355 nm.

respectively, so that the electric field of the microwave was sufficiently small not to disturb the thermal 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

φΣµ )

∆Pr 1 eAI0Flight Pr

(1)

where e, A, I0, Flight, ∆Pr, and Pr are the unit charge of a single electron, a sensitivity factor [(S/m)-1], incident photon density of excitation laser (photons/m2), a correction (or filling) factor (m), a change of reflected microwave power, and a power of reflected microwave, respectively. The details of the in situ TRMC-TAS system was reported elsewhere.6 Third harmonic generation (THG, 355 nm) and second harmonic generation (SHG, 532 nm) of a Nd:YAG laser (5-8 ns pulse duration) with incident photon density into samples of 5.9 × 1015 photons/ cm2 was used as excitation source. A white light continuum from a Xe lamp was used as a probe light for TAS. The probe light was guided into a wide-dynamic-range streak camera (Hamamatsu C7700) that collects two-dimensional image of the spectrum and time profiles of light intensity. The surface morphology of the polymer film was observed by tapping mode AFM (SPA-400, SEIKO Instruments Inc.). X-ray diffraction pattern was measured using Rigaku RU-200 (Cu KR: 1.5418 Å, 50 kV, 150 mA). The steady-state absorption was measured by a Shimadzu UV-3100PC spectrometer. All the experiments were performed at room temperature in an air.

2. Experimental Section

3. Results and Discussion

Regioregular P3HT, regiorandom P3HT, and N,N′-bis(2,5di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide were purchased from Aldrich and used without further purification. They are denoted in this article as Reg-PT, Rand-PT, and PDI, respectively (Figure 1). Polythiophene (100 mol %) and PDI (x mol%) relative to the base (one thiophene ring) mole unit of polythiophene were dissolved in o-dichlorobenzene and dropcast onto a quartz substrate in a vacuum oven at 60 °C. The conversion of PDI concentration from mol % to weight % is approximated by multiplying mol % by 4.6 (the ratio of molecular weight), for example, 10 mol % PDI and 100 mol % polythiophene are equivalent to 46 wt % PDI and 100 wt % polythiophene (32 wt % PDI and 68 wt % polythiophene). Transient photoconductivity and photoabsorption spectroscopies were performed by the TRMC and TAS system. 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,

Figure 2 shows the TRMC signal converted to φΣµ: a product of quantum efficiency of charge carrier generation (φ) and sum of mobilities of positive and negative charges (Σµ ) µ+ + µ-) observed for Reg-PT film with various molar concentration of PDI relative to base mole unit of Reg-PT. The φΣµ induced by photoirradiation at 355 nm was dramatically increased with PDI, reaching the maximum at around 6 mol % PDI, and then it turned to a decrease. While the amplitude increases, the decay rates of the kinetic traces do not indicate a distinct change. The increase of φΣµ is a result of increase in φ arisen from the enhancement of charge carrier generation efficiency via electron transfer to PDI. Noteworthy is that decays of φΣµ were accelerated in single-crystal rubrene6a and supra-molecular nanotubes6b by the effect of second-order bimolecular charge recombination through the increase of carrier concentration. In disordered semiconductors such as conjugated polymers, such dependence is not always observed, because the decay is dominated mainly by charge trap at physical and chemical

Charge Carrier Mobility in Poly(3-hexylthiophene) Films

Figure 3. (a) Transient absorption spectra of Reg-PT film with 6 mol % PDI induced by 355-nm laser. The inset is the magnification. (b) Kinetic decays of Reg-PT films in the presence of PDI monitored at ca. 720 nm. The decays are attributed to the PDI radical anion.

defects which intrinsically exist in these materials as, e.g., a metal catalyst used during polymerization and kinks of a polymer chain.19 The experimental fact that the decay kinetics showed no significant dependence on excitation density is consistent with the previous reports,7d-g where charge trap was proved to be a predominant factor for the decay of conductivity transients. The decrease in the amplitude of φΣµ after passing the maximum obtained for ca. 6 mol% PDI is referred to the decrease of φ and/or Σµ. To elucidate the PDI effects on the charge carrier generation, transient photoabsorption spectroscopy was performed using the same laser as FP-TRMC. Figure 3a represents the transient absorption spectrum observed for Reg-PT with 6 mol % PDI. The strong bleach at around 630 nm and weak absorption at ca. 720 nm are recognized. The latter absorption arises from the PDI radical anion, which is in good agreement with the photoabsorption spectrum of PDI radical anion in the literature.20 The spectrum shape found in the mixed films, however, looks distorted to some extent in comparison with that reported in a solution where an additional peak at around 800 nm was also observable. This difference would be due to the intermolecular interaction and change of molecular conformation in the films in addition to the low signal-to-noise ratio to distinguish such a small structure of the spectrum feature. The peak of triplet-triplet absorption is located at around 500 nm, 21 so that the contribution from PDI triplet is ruled out from the spectrum in Figure 3a. The bleach is mainly attributed to radical cation of polythiophene and was observed in spite of the presence of PDI. The intensity of photoabsorption assigned to the PDI radical anion increases with PDI concentration and then decreases after around 6 mol % as shown in Figure 3b. The decay rates also do not differ considerably by the PDI concentration, which are analogous to those found for TRMC. However, at low concentration of PDI, the radial anion was not observed or hindered by noises from fluorescence due to the small intensity of transient absorption. The φΣµ values at the end-of-pulse are plotted as a function of PDI concentration and shown in Figure 4a. Reg-PT films indicate a maximum of about 2.5 × 10-3 cm2/Vs at 6 mol %

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Figure 4. (a) Peak values of TRMC transients (φΣµ) found for RegPT (open circles) and Rand-PT (closed triangles) films in the presence of PDI. The excitation wavelength was 355 nm. (b) Quantum efficiencies of charge carrier generation (φ) estimated from photoabsorption of PDI radical anion on the basis of eq 2.

PDI and a decrease down to 1.2 × 10-3 cm2/Vs at 15 mol % PDI. The φΣµ values of mixed films of Rand-PT and PDI, however, increase and saturate after around 3-6 mol %. It should be noted that φΣµ of Rand-PT is smaller than that of Reg-PT by a factor of approximately 20, indicating the generation efficiency and mobility of charge carriers are much less for those in Rand-PT. On the basis of photoabsorption of PDI radical anions at 720 nm of which extinction coefficient had been known (7.42 × 104 mol-1 dm3 cm-1),20 we estimated the quantum efficiency of the PDI radical anion generation (φPDI). In addition to φPDI, charge carrier generation yield without PDI (φPT) must be taken into account, since pristine polythiophene also exhibits photoconductivity which is not negligible compared with the increase by addition of PDI. By incorporation of R, the ratio of φΣµ observed for polythiophene film with PDI to that without PDI, the total charge carrier generation efficiency is formulated as follows

φ ) φPDI + φPT ) (1 + 1/(R - 1))φPDI

(2)

The relationship between φPT and φPDI through R expressed in eq 2 is valid under the condition that Σµ is not changed by addition to PDI. We examined this relationship in the PDI concentration from 0.5 to 15 mol % and found that φPT estimated using eq 2 is almost a constant value of 4.6 × 10-3 for 2-10 mol % of PDI. The lower (10 mol %) concentrations seem inappropriate for the estimation of φPT using eq 2, due to the low signal-to-noise ratio and possibility of the change in mobility, respectively. It should be noted that once φPT is determined, eq 2 is applicable to the estimation of total charge carrier generation efficiency φ for every PDI concentration using each φPDI assessed by TAS. Figure 4b indicates φ (φPT + φPDI) at each PDI concentration. The φ for Reg-PT increases with addition of PDI and reaches the maximum of 2.4 × 10-2 at around 6 mol % PDI. Regarding Rand-PT, the φ rapidly saturates at around 2-3 mol % PDI. We expected that φ of Reg-PT also saturates after the maximum; however, it in turn decreased by further introduction of PDI. The reason is discussed later by comparing with a different excitation wavelength and steady-state photoabsorption.

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Figure 5. Sum of charge carrier mobility (Σµ) in Reg-PT (open circles) and Rand-PT (closed triangles) films at each PDI concentration. This was calculated from Figure 4.

From the combination of TRMC (φΣµ) and TAS (φ) results shown in Figure 4, we successfully obtained mobility of charge carrier (Σµ) as a function of PDI contents and present it in Figure 5. The Σµ of Reg-PT shows an almost constant value of 0.12 cm2/Vs up to 10 mol % and drops to 0.070 cm2/Vs at 15 mol %. The former value coincides with the high FET mobility reported in Reg-PT,2 demonstrating the substantial plausibility of Reg-PT to the use of organic electric devices. We next discuss the deterioration of Σµ that was revealed at high PDI concentration. A functionalized PDI has facilitated intermolecular π-stacking capable of efficient negative charge transport as much as 0.1 to 1.7 cm2/Vs;8 however, the PDI used in the present study has a bulky side-chain and is expected not to form such a defined intermolecular stacking. Therefore, the dominant charge carrier in the polythiophene-PDI mixed film would be still a positive charge on polythiophene. This presumption is strongly corroborated by the decrease of Σµ in the presence of large amount of PDI, where the Σµ should increase if PDI molecules form extended π-stacking leading to effective electron transport along the intermolecular column. Consequently, the decrease of Σµ is due to the decrease in the mobility of positive charge carrier on Reg-PT of which intermolecular π-stacking was disturbed by PDI at the higher concentration. On the other hand, Σµ of Rand-PT remains constant (averaged Σµ ) 0.006 cm2/Vs) for all of the PDI concentrations, although the data are scattered because of the low signal intensity. The one-dimensional mobility (Σµ1D) calculated by three multiple of this value yields 0.018 cm2/Vs, which is close to Σµ1D of pulse-radiolysis (PR) TRMC (0.014-0.020 cm2/Vs) ascribed to an intramolecular charge migration along a polythiophene backbone isolated in a benzene solution.7b,f This is suggestive of the Σµ obtained in Rand-PT films originating from intramolecular charge migration along a polymer main chain rather than intermolecular charge migration. The latter has been reported to be a few to several orders of magnitude smaller than the present results. The high mobility found for Reg-PT arises from the intermolecular π-stacking; however, it is difficult to distinguish so far whether intra- or intermolecular charge migration is responsible for the mobility obtained in the present study, since π-stacking induces both ordered intermolecular packing and extension of conjugation length of polymer backbone. A specimen that eliminates the intermolecular interaction without changing conformation of polymer chain might be necessary to clarify this issue. The intermolecular ordering and surface morphology were examined by XRD and AFM, respectively, to support the mobility derived from TRMC and TAS experiments. The XRD spectra shown in Figure 6 indicates the intensity of the diffraction peak at 2θ ) ca. 5.3° corresponding to intermolecular spacing of the lamellar in Reg-PT1 is decreased by PDI addition,

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Figure 6. XRD of Reg-PT films with 0, 3, 7, 10, and 15 mol % PDI.

Figure 7. Phase images of AFM micrographs (2 × 2 µm) of Reg-PT and Rand-PT films with 0, 7, and 15 mol % PDI.

which is in good accordance with the decrease of mobility. This supports our expectation that some portion of PDI molecules penetrates into intermolecular spacing of stacked Reg-PT chains. The same trend is seen in the phase images of Reg-PT with PDI measured by tapping mode AFM (parts a-c of Figure 7), where the surface images are disturbed by incorporation of PDI. The micrographs of Rand-PT (parts d-f of Figure 7) also show morphological change in the presence of PDI. These results by XRD and AFM support the mobility dependence on PDI concentration as shown in Figure 5. Fluorescence measurement gives additional information on the quenching via electron transfer to PDI. The normalized fluorescence intensities of Reg-PT and Rand-PT films obtained by photoexcitation with the same 355-nm laser are indicated in Figure 8. The fluorescence from Reg-PT decreases with an increase in PDI concentration and reaches the minimum at around PDI ) 5 mol %; however, the intensity is about 30% of the initial one and likely to increase to some extent at PDI ) 15 mol %. The fluorescence from Rand-PT was quenched as well but showed more rapid and larger decrease than Reg-PT. These dependences of fluorescence quenching are complementary with the φ in Figure 4b and support the quenching via electron transfer to PDI. To examine the contribution of energy transfer to the fluorescence quenching, we performed fluorescence studies in chloroform solutions of polythiophene and PDI. As shown in Figure 9, the intensities are, in contrast to film studies, increased with PDI concentration for both Reg-PT and Rand-PT, The intensity and feature of the spectra observed in the mixed solution of Reg-PT and PDI are in good coincidence with those calculated by simple summation of independent

Charge Carrier Mobility in Poly(3-hexylthiophene) Films

Figure 8. (a) Normalized intensities of fluorescence from Reg-PT (open circles) and Rand-PT (closed triangles) films in the presence of PDI excited at 355 nm. (b) Fluorescence spectra from mixed-films of Rand-PT (left side) and Reg-PT (right side). The black dots are PDI without polythiophene.

Figure 9. Emission spectra in chloroform solutions of 50 µM (a) RegPT and (b) Rand-PT with 0, 1, 2, 5, 10, 15, and 30 mol % PDI relative to 100 mol % polythiophene. The excitation wavelength is 355 nm. The arrow represents the increase of PDI concentration. The inset is the summation of independent emissions from polythiophene and PDI using their relative concentrations. (c) Normalized intensity of the emission.

emissions from Reg-PT and PDI (Figure 9a). This is suggestive of the small effect of energy transfer from Reg-PT to PDI. The increase of emission is mainly due to the increase of direct excitation of PDI and resultant emission from PDI excited state. On the contrary, the spectrum shape and intensity from RandPT and PDI solution are different from those calculated in the same manner (Figure 9b). This indicates the involvement of energy transfer from Rand-PT to PDI in addition to the direct excitation of PDI. The emission spectrum of Rand-PT is welloverlapped with the absorption spectrum of PDI, while the

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Figure 10. (a) Peak values of TRMC transients (φΣµ) found for RegPT (open circles) and Rand-PT (closed triangles) films in the presence of PDI. The excitation wavelength was 532 nm. (b) Quantum efficiencies of charge carrier generation (φ) estimated by using mobility (Σµ) of Figure 5.

emission from Reg-PT is red-shifted by a few tens of nanometer, leading to the decrease of the overlap. Figure 9c is the normalized emission intensity from polythiophene-PDI solution as a function of PDI concentration. A sublinear relation was observed for Reg-PT, while Rand-PT shows a linear correlation. This implies that some amount of Reg-PT excited states are quenched via electron transfer to PDI even in a solution. Although the increase of emission intensity was observed in solutions due to the energy transfer and direct excitation of PDI, the emission was quenched in the mixed films. This indicates the electron transfer rather than energy transfer is responsible for the quenching observed in the films. To examine the reason for the decrease of φ found in RegPT (Figure 4 b), the films were exposed to another wavelength. Figure 10a represents φΣµ of Reg-PT and Rand-PT with various PDI concentrations under an excitation at 532 nm. The incident photon density was tuned to the same as that of 355-nm excitation. The amplitude of φΣµ in Reg-PT film without PDI was reduced to less than the half of the 355-nm excitation as shown in Figure 4a. The PDI effect on the increase of φΣµ was suppressed to approximately a factor of 3, while that was about 5 for 355 nm. It is of interest to find that the PDI concentration giving the maximum φΣµ of Reg-PT films was shifted from 6 mol % for 355 nm to 10 mol % for 532 nm. A similar-type dependence of φΣµ on the contents of acceptor was found also for supramolecular nanotube having acceptor chromophore,6b where the decrease of φΣµ was speculated to be due to the decrease of Σµ. This was supported by the scanning electron microscope observation, i.e., too much amount of acceptor layers was proved to distort the formation of nanotube architecture and shorten the length of the nanotubes. The φ for 532 nm was reconstructed on the basis of the mobility obtained for the 355nm excitation (Figure 5) and is shown in Figure 10. In the case of 532-nm excitation, decrease of φ was not observed up to 15 mol % PDI. The following two candidates are plausible to account for the decrease of φ observed for 355 nm excitation. One is a lack of homogeneity in the films of polythiophene with concentrated

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Figure 12. Suggested pathway of charge carrier generation from (a) the polythiophene excited-state and (b) the PDI excited state. Figure 11. (a) Steady-state absorption spectra of Reg-PT (red line), Rand-PT (blue line), and PDI (black line) films normalized by their maximums. (b) Spectra obtained by dividing the absorption spectrum of PDI by that of Reg-PT (red line) or Rand-PT (blue line).

PDI. Since the molecular weight of PDI is large (767) in comparison with the monomer unit of poly(3-hexylthiophene) (166), 15 mol % PDI means approximately 41 wt % PDI and 59 wt % polythiophene. This is the case giving considerable contribution from the homogeneity of film. Actually, AFM images at high PDI concentration indicate nonhomogeneous pattern (morphology) of the film. The other candidate is the change of photoabsorption property at an excitation wavelength, where a large amount of PDI may cause the decrease in the yield of polythiophene excited-state and increase of PDI excited state, leading to the restriction in the pathway of charge carrier generation. The experimental fact that the PDI concentration which gives the maximum φΣµ depends on the excitation wavelength prefers the second candidate, since the homogeneity is not affected by the excitation wavelength. To examine the photoabsorption properties of the films, steady-state absorption spectra of Reg-PT, Rand-PT, and PDI films were measured. As shown in Figure 11a, the films of RegPT and Rand-PT indicate the absorption peaks at ca. 430 and 510 nm, respectively. Although the absorption peak of RandPT does not show significant difference with that in chloroform solution, Reg-PT film exhibits distinct shoulders in the long wavelength region and ca. 60-nm red shift of the peak relative to the solution, which is indicative of intermolecular stacking. To facilitate the comparison of photoabsorption at the excitation wavelengths (355 and 532 nm), the normalized absorption spectrum of PDI film (AbsPDI) was divided by those of Reg-PT and Rand-PT (AbsPT). It should be noted that no distinct photoabsorption ascribed to the formation of charge transfer complex was observed in the mixture films. In the case of RegPT shown in Figure 11b, the (AbsPDI/AbsPT) value at 355 nm is about 2.1 times higher than that at 532 nm, indicating the larger contribution of PDI to the steady-state photoabsorption of the mixed film at 355 nm than that at 532 nm. On the contrary, the (AbsPDI/AbsPT) of Rand-PT films at 355 nm is 0.08 in comparison to that at 532 nm, showing the absorption of PDI at 532 nm is more dominant than at 355 nm. These results are consistent with the fluorescence quenching shown in Figure 8, i.e., the fluorescence intensity from Reg-PT film excited at 355 nm was quenched via electron transfer to PDI, but it

increased to some extent at high PDI concentration. The increase of fluorescence intensity is due to that the PDI molecules begin to absorb more photons in the presence of concentrated PDI and they convert to excited states, leading to the appearance of fluorescence from PDI singlet. The fluorescence from the mixed films of Reg-PT and PDI is a result of contributions from both the quenching of Reg-PT fluorescence and increase of fluorescence from PDI. This can be confirmed as well by the fact that the peak was blue-shifted toward the peak of PDI fluorescence. As for Rand-PT, fluorescence seems quenched almost completely, because Rand-PT has higher intensity of fluorescence than PDI and the excited-state of PDI is formed with lesser extent than Reg-PT under the 355 nm excitation. On the basis of the above discussion on the steady-state absorption and fluorescence quenching, the following two pathways are provided for the charge separation which PDI molecule participates in

PT* + PDI f PT•++PDI••+

•-

PT + PDI* f PT +PDI PT•+,

(3) (4)

PDI•-

where PT*, PDI*, and denote excited states of polythiophene and PDI, radical cation of PT, and radical anion of PDI, respectively. The schemes of eqs 3 and 4 are illustrated in Figure 12. The electron transfer occurs from PT* to PDI, giving rise to PT•+ and PDI•-, while both electron transfer and energy transfer are involved in the formation of radical ion pair from PDI*. As is expected, the concentration of PT* decreases and that of PDI* increases with the content of PDI. Since the fraction of PDI absorption in the total absorption of the mixed film is larger at 355 nm than at 532 nm, the charge carrier generation scheme expressed by eq 4 becomes effective with lesser amount of PDI at 355 nm. The existence of maximum in the φ dependence on PDI content found in Reg-PT films (Figure 4b) is understood as the result of increased concentration of PDI*, on the assumption that the efficiency of charge carrier generation via eq 4 is lower than that via eq 3. This assumption is supported by the fact that the PDI concentration at the maximum of φ moved to higher region from 355 to 532 nm excitation where the PDI* contribution in the former is larger than the latter. 4. Conclusions The mobility and generation efficiency (φ) of charge carrier in Reg-PT and Rand-PT films were investigated for various

Charge Carrier Mobility in Poly(3-hexylthiophene) Films concentrations of PDI by an electrode less measurement of TRMC and TAS. The φ estimated from transient photoabsorption of PDI radical anion was increased with PDI via an electron transfer from Reg-PT* to PDI, and then it decreased after passing the maximum. This decrease is due to the increased contribution from PDI* of which charge separation pathway is expected less sufficient than that of Reg-PT*. This was corroborated from the steady-state absorption and fluorescence spectra. The mobility of positive charge carrier in Reg-PT obtained in the PDI concentration range from 0 to 10 mol % exhibited as high as 0.12 cm2/Vs comparable to the high mobility assessed by FET study; however, it turned to drop to 0.070 cm2/Vs at 15 mol %. The decrease of mobility is ascribed to the disturbance of π-stacking of Reg-PT by incorporating PDI into the films, which was supported by XRD and AFM experiments. On the other hand, Rand-PT resulted in much smaller mobility (0.006 cm2/Vs), of which three multiplied value is in good coincidence with the one-dimensional mobility along polythiophene chain measured by PR-TRMC, suggesting the mobility obtained in Rand-PT films reflects from intramolecular charge migration. The mobility found in Reg-PT films is referred to as the result of enhanced intermolecular charge migration and/or extended polymer backbone capable of efficient intramolecular charge transport. We demonstrated that the use of PDI as not only a photoconductivity sensitizer but also spectroscopic probe to estimate φ is an effective methodology for the electrodeless determination of alternating current mobility of charge carrier in organic semiconductors upon exposure to light based on the combination of TRMC and TAS. Acknowledgment. We greatly appreciate the experimental assistance of Dr. Yoshiko Koizumi at Osaka University. 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 (MEXT) and from Research Foundation for Opto-Science and Technology. References and Notes (1) (a) McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910. (b) Chen, T.-A.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114, 10087. (c) Yamamoto, T.; Arai, M.; Kokubo, H.; Sasaki, S. Macromolecules 2003, 36, 7986. (e) Jiang, ¨ sterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.; Janssen, X. M.; O R. A. J.; Vardeny, Z. V. AdV. Funct. Mater. 2002, 12, 587. (2) (a) 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. (b) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (c) Kline, R. J.; MucGehee, M. D.; Kadnikova, E. N; Liu, J.; Fre´chet, J. M. J. AdV. Mater. 2003, 15, 1519. (d) Wang, G.; Swensen, J.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2003, 93, 6137. (e) Salleo, A.; Chen, T. W.; Volkel, A. R.; Wu, Y.; Liu, P.; Ong, B. S.; Street, R. A. Phys. ReV. B 2004, 70, 115311. (f) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378. (g) Kim, D. H.; Park, Y. D.; Jang, Y.; Yang, H.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S.; Chang, T.; Chang, C.; Joo, M.; Ryu, C. Y.; Cho, K. AdV. Funct. Mater. 2005, 15, 77. (3) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (b) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (c) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (4) (a) Gadisa, A.; Svensson, M.; Andersson, M. R.; Inganas, O. Appl. Phys. Lett. 2004, 84, 1609. (b) Popescu, L. M.; van’t Hof, P.; Sieval, A. B.; Jonkman, H. T.; Hummelen, J. C. Appl. Phys. Lett. 2006, 89, 213507. (c) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (d) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 3030. (5) (a) Brabec, C. J.; Cravino, A.; Zerza, G.; Sariciftci, N. S.; Kiebooms, R.; Vanderzande, D.; Hummelen, J. C. J. Phys. Chem. B. 2001, 105, 1528. (b) Tuladhar, S. M.; Poplavskyy, D.; Choulis, S. A.; Durrant, J. R.; Bradley, D. D. C.; Nelson, J. AdV. Funct. Mater. 2005, 15, 1171. (c) Riedel, I.; von

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