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Electronic and Vibrational Spectra of Positive Polarons and Bipolarons in Regioregular Poly(3-hexylthiophene) Doped with Ferric Chloride Jun Yamamoto and Yukio Furukawa* Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan ABSTRACT: We studied the carriers generated in regioregular poly(3hexylthiophene) (P3HT) upon FeCl3 vapor and solution doping using visible/near-infrared (VIS/NIR) absorption, infrared (IR), and Raman spectroscopy. Upon doping with an FeCl3 solution in air, the main carriers that were generated were positive polarons. Upon doping with FeCl3 vapor, positive polarons also formed initially, but at higher doping levels, positive bipolarons formed with the concomitant disappearance of the positive polarons. The Raman and IR spectra of the positive bipolarons and the positive polarons were obtained. Raman spectroscopy is very useful for characterizing positive polarons and bipolarons. The Raman results indicated that the positive bipolarons were converted to polarons upon heating to 85 °C, indicating that the positive bipolarons formed a metastable state. The temporal changes in the electrical conductivity of a P3HT film upon doping with FeCl3 vapor were measured. The conductivity reached a maximum and then decreased by 2 orders of magnitude. This result suggests that the mobility of the polarons is approximately 100 times as high as that of the bipolarons.

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INTRODUCTION The 2000 Nobel Prize in Chemistry was awarded to Drs. A. J. Heeger, A. J. MacDiarmid, and H. Shirakawa for the discovery and development of electrically conductive polymers, such as polyacetylene, polythiophene, and polypyrrole.1 These polymers are used as active materials for organic polymer electronic devices, such as light-emitting diodes, thin-film transistors, and solar cells.2−4 Regioregular poly(3-hexylthiophene) (P3HT) is one such conjugated polymer that demonstrates high performance in polymer transistors and solar cells; indeed, a variety of polymers with complex chemical structures demonstrate high performance in polymer electronic devices. When a P3HT film is doped with an electron acceptor such as iodine or FeCl3, it exhibits high electrical conductivity. Polarons and bipolarons serve as carriers in polymers with a nondegenerate ground state, such as P3HT.5−8 Upon acceptor doping, the polymer chains become positively charged and form complexes with dopant anions such as I3− or FeCl4−. When an electron is removed from a polymer chain during this process, the positive charge +e becomes localized over several thiophene rings, and structural changes occur in this region. This charge carrier is called a positive polaron; it has a charge +e and spin 1/2. When an electron is subsequently removed from a positive polaron, a positive bipolaron, with charge +2e and no spin, is formed. The bipolaron is a spinless carrier. The chemical structures of positive polarons and positive bipolarons in P3HT are schematically illustrated in Figure 1. In the case of donor doping, negative polarons (charge −e, spin 1/2) and negative bipolarons (charge −2e, no spin) are formed. © 2015 American Chemical Society

Figure 1. Chemical structures of (a) a positive polaron and (b) a positive bipolaron.

When a polaron or a bipolaron forms in a polymer chain such as a polythiophene chain, localized electronic states form within the bandgap of the polymer chain. One-electron theory predicts that a polaron should exhibit two strong absorption bands from the VIS region to the IR region, whereas a bipolaron should exhibit one strong band.9,10 A radical cation (anion) and a dication (dianion) of short oligomers correspond to a positive (negative) polaron and a positive (negative) bipolaron, respectively. Optical absorption studies of short oligomers have provided support for this paradigm, revealing Received: December 18, 2014 Revised: February 7, 2015 Published: March 13, 2015 4788

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approximately 100 nm using a KLA-Tencor Alpha-Step 500 surface profiler. FeCl3 solution doping was performed as follows. A P3HT film formed on a glass or BaF2 substrate was immersed in an acetonitrile solution of anhydrous FeCl3. The concentration of FeCl3 was 10 × 10−3 mol/L. The immersion time was varied to control the doping level. Then, the doped film was washed with fresh acetonitrile and dried. The film was subjected to spectroscopic measurements. FeCl3 vapor doping and spectroscopic measurements were performed using a cell such as that depicted in Figure 2. A pair of glass windows was used for VIS/

two strong bands, P1 and P2, for polarons and a single strong band, BP1, for bipolarons between the positions of the P1 and P2 bands.11−21 For doubly charged long oligomers, a twopolaron state that exhibits the P1 and P2 bands forms;20,21 this two-polaron state is more stable than the one-bipolaron state. On the basis of the theoretical consideration and experimental data of oligomers, the two-band feature is assigned to polarons and the one-band feature is assigned to bipolarons in conjugated in conjugated polymers.22 Shimoi et al.23 theoretically showed the two-band feature of the polaron and the oneband feature of the bipolaron in the presence of electron− electron interactions. A polaron can be detected via electron spin resonance (ESR) spectroscopy because the polaron has spin 1/2. By contrast, a bipolaron cannot be detected via ESR because a bipolaron has no spin. However, a singlet polaron pair also has no spin. It is difficult to distinguish a bipolaron from a singlet polaron pair using ESR alone. The charge on a polaron or a bipolaron is localized within several thiophene rings due to the structural changes that occur from the benzenoid to the quinoid structure. These structural changes can be detected via IR and Raman (vibrational) spectroscopy. Doping-induced infrared-active vibrational (IRAV) bands have been observed and explained using the effective conjugation coordinate (ECC) theory proposed by Zerbi and colleagues.24 Although charged species give rise to strong IRAV bands, polarons and bipolarons have not been identified using infrared spectroscopy. The Raman spectra of polyaniline,25 poly(p-phenylenevinylene),26−28 poly(p-phenylene),18,29 and polythiophene30 have been analyzed within the context of the polaron and the bipolaron, which have been identified based on the Raman spectra of the radical ions and divalent ions of short oligomers. The performance of polymer electronic devices is associated with charge carriers, i.e., polarons and bipolarons. Polarons are considered to be the charge carriers in polymer photovoltaic solar cells.31−33 A relation between the open-circuit voltages of solar cells and polaron energy levels has been proposed.34 Using Raman spectroscopy, McCreery et al.35,36 reported that polarons are generated in P3HT-based memory devices. Raman and IR spectroscopy are very useful for in situ investigations of charge carriers in polymer electronic devices.37 Indeed, it is important to study polarons and bipolarons in detail to improve the performance of polymer electronic devices. However, the complete Raman and IR characterization of polarons and bipolarons in P3HT has yet to be achieved. In this paper, we present a study of the positive polarons and the positive bipolarons that form in P3HT upon FeCl3 vapor or solution doping using VIS/NIR absorption, IR, and Raman spectroscopy. The key Raman bands that are characteristic of positive polarons and positive bipolarons were obtained in this study. We investigated the stability of two polarons and a single bipolaron upon heating as well as the electrical conductivities of films in which polarons or bipolarons are formed.

Figure 2. Cell for in situ absorption and Raman measurements of a P3HT film upon FeCl3 vapor doping.

NIR and Raman measurements. A pair of BaF2 windows was used for IR measurements. Solid powders of FeCl3 were placed into the cell, and the cell was evacuated to a pressure of 0.1 Pa. Electrical conductivity measurements were performed using a P3HT film formed on a glass substrate with two linear indium tin oxide (ITO) electrodes. The distance L between the ITO electrodes was 3.0 mm. The width W and the thickness t of the P3HT film were 6.0 mm and 130 nm, respectively. The resistance R was measured using a Solartron 1260 impedance analyzer with a frequency of 0.1 Hz and an amplitude of 0.1 V. The conductivity σ of the film was calculated using the following equation: L σ= (1) RWt Absorption spectra in the VIS/NIR range were measured using a JASCO V-570 UV/vis/NIR or V-650 UV/vis spectrophotometer. Raman spectra were measured using a Renishaw InVia Raman microscope at an excitation wavelength of 830 nm. The power density of the excitation source was typically 125 W/cm2. IR spectra were measured using a Digilab FTS-7000 FT-IR spectrometer equipped with a liquid-nitrogencooled linearized HgCdTe detector.

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RESULTS AND DISCUSSION VIS/NIR Absorption Spectra. Figure 3 shows the changes in the VIS/NIR absorption spectrum of a P3HT film that occurred upon FeCl3 solution doping. The spectrum of the neutral P3HT film is presented as spectrum (a). The absorption at 520 nm is assigned to the π−π* transition of a conjugated polymer chain. The number-average molecular weight of P3HT is between 54 000 and 75 000. The content of the head-to-tail coupling is more than 98%. Thus, the calculated average numbers of the head-to-tail coupling thiophene rings are between 43 and 45. Absorption spectra (b)−(e) represent the spectra of the P3HT film at various doping levels. The FeCl3 concentrations of the doping solutions and the immersion times were 10 mmol/L and 2 s, 10 mmol/L and

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EXPERIMENTAL METHODS Chlorobenzene, acetonitrile, and anhydrous FeCl3 were purchased from Kanto Chemical, and regioregular P3HT (head to tail >98%; Mn, 54 000−75 000) was purchased from Sigma-Aldrich. A thin film of P3HT was prepared on a glass or BaF2 substrate by spin-coating from a chlorobenzene solution of P3HT formed by dissolving 24 mg of P3HT in 1 mL of chlorobenzene. The thickness of the film was measured to be 4789

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electronic configurations22 of a neutral P3HT chain, a positive polaron, and a positive bipolaron in the one-electron picture are illustrated in Figure 5. The 520 nm absorption band observed

Figure 3. Changes in the VIS/NIR absorption spectrum of a P3HT film doped with an acetonitrile solution of FeCl3.

Figure 5. Schematic energy-level diagrams of (a) neutral P3HT, (b) a positive polaron, and (c) a positive bipolaron.

4 s, 10 mmol/L and 304 s, and an additional 50 mmol/L and 2 min for (b), (c), (d), and (e), respectively. The doping level increased with each subsequent preparation. As the doping level was increased (from (b) to (e)), initially, the 520 nm absorption band of the P3HT decreased and new absorption bands appeared at 788 nm (1.57 eV) and in the NIR range; subsequently, the 788 nm absorption band decreased slightly, and the NIR absorption band increased slightly. Upon additional doping, no further spectral changes were observed. Figure 4 shows the changes in the VIS/NIR absorption spectrum of a P3HT film that occurred upon FeCl3 vapor

for P3HT originates from the transition from the valence band (VB) to the conduction band (CB), as shown in Figure 5a. When a positive polaron forms, two localized electronic states, the +ω0 and −ω0 levels indicated in Figure 5b, are generated. The 788 nm absorption band is assigned to the transition P2 from the −ω0 level to the ω0 level. This transition is observed only for the polaron. The observed NIR absorption band is assigned to the transition P1. When a positive bipolaron forms, two localized electronic states, the +ω0′ and −ω0′ levels indicated in Figure 5c, are generated. The ω0′ level of the bipolaron is deeper than the corresponding ω0 level of the polaron. The NIR absorption band observed at high doping levels can be assigned to the transition BP1 of the bipolaron. Based on the foregoing considerations, the observed spectral changes can be explained as follows. In the case of solution doping, positive polarons form, as indicated by the observation of the 788 nm P2 transition. A comparison of Figures 3d and 3e reveals a decrease in the 788 nm absorption band and a simultaneous increase in the NIR absorption band. This spectral change originates from a decrease in the number of positive polarons and an increase in the number of positive bipolarons. In the case of vapor doping, positive polarons form during the initial stage of doping, as evidenced by the increasing strength of the 790 nm P2 transition. However, during the latter stage of doping, positive bipolarons form; at the maximum doping level, only positive bipolarons exist, as indicated by the absence of the 790 nm P2 absorption band and the increase in the NIR absorption band. As the FeCl3 doping proceeded, the intensity of the 520 nm absorption decreased. This originates from the decreases in the energy level densities of the VB and the CB. It is expected that the width of the P2 absorption between the localized energy levels for a positive polaron (Figure 5b) is narrow. However, the observed width of the P2 absorption is not narrow. The observed absorption band is associated with the Franck−Condon type vibrational transitions. Thus, the P2 absorption has a considerable width. Raman Spectra. Plot “a” in Figure 6 represents the Raman spectrum of a neutral P3HT film excited at 830 nm. The strong band observed at 1452 cm−1 is assigned to the CαCβ stretching vibration.38,39 The 1386 cm−1 band is assigned to the Cβ−Cβ stretching vibration.38,39 Figures 6b−d are the resonance Raman spectra of an FeCl3-doped P3HT film (excited at 830 nm) that was prepared at various doping levels

Figure 4. Changes in the VIS/NIR absorption spectrum of a P3HT film doped with FeCl3 vapor: (a) neutral P3HT film; (b) after 200 min; (c) after 20 h.

doping. As the doping proceeded, the absorption band at 520 nm associated with the π−π* transition decreased. Meanwhile, a new band appeared at 790 nm and then subsequently disappeared. The strength of the absorption in the NIR range, however, increased monotonically. It should be noted that the 790 nm band disappeared in the case of vapor doping, whereas this band persisted during solution doping. This result suggests that the maximum doping level that can be achieved through vapor doping is higher than that achievable through solution doping. The spectral changes observed upon solution and vapor doping can be explained as follows. When P3HT is doped with FeCl3, positive polarons or positive bipolarons form. The 4790

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Figure 7. Raman spectrum of a P3HT film doped heavily with FeCl3 vapor for longer than 20 h.

spectrum indicates the formation of positive bipolarons, as previously described. The 830 nm laser line lies within the NIR absorption region. Therefore, the observed Raman spectrum originates from positive bipolarons. No Raman spectra of positive bipolarons have been reported so far. The Raman spectrum of the positive bipolarons is different from that of the positive polarons; its most prominent features lie within the frequency range between 1470 and 1400 cm−1. The band at 1470 cm−1 is attributed to positive bipolarons, whereas the band at 1425−1412 cm−1 is attributed to positive polarons. A band at 1452 cm−1 is observed for neutral P3HT. The frequency of the bipolaron band is higher than that of the band for neutral P3HT, whereas the frequency of the polaron band is lower than that of the band for neutral P3HT. The high frequency of the bipolaron band (1470 cm−1) is quite unusual and may be explained as follows. The Raman spectrum of the positive bipolarons (excited at 830 nm) is resonant with the BP1 bipolaron transition, whereas the corresponding spectrum of the positive polarons is resonant with the P2 transition. These spectra reflect different resonance effects. Thus, the vibrational mode of the 1470 cm−1 band is likely different from the modes of the 1452 cm−1 band of neutral P3HT and the 1425−1412 cm−1 band of the positive polarons. Different vibrational modes are most likely observed under different resonant conditions. Positive polarons and positive bipolarons could be identified via Raman spectroscopy. The intense 1412 cm−1 band of the positive polarons and the 1470 cm−1 band of the bipolarons can be used as markers. The 1382 and 727 cm−1 bands can also be used as markers of positive polarons, and the 1234 cm−1 band can be used as a marker of positive bipolarons. This latter band corresponds to the 1211 cm−1 band of polarons and the 1213 cm−1 band of neutral P3HT. Infrared Spectra. Figure 8 shows the changes in the infrared spectrum of a P3HT film that occurred upon FeCl3 solution doping. Spectrum (a) represents the infrared spectrum of the neutral P3HT film. The infrared spectra (b)−(e) are attributed to IRAV modes. No bands associated with neutral P3HT are observed because the absorption coefficients of these modes are large. The infrared spectra (b), (c), (d), and (e) were obtained from the sample films that produced the VIS/ NIR spectra (b), (c), (d), and (e), respectively, in Figure 3. These VIS/NIR spectra indicate that the main carriers that were generated were positive polarons. Thus, the observed infrared spectra (b)−(e) are attributed to positive polarons. The bands observed at 1312, 1151, and 1081 cm−1 in Figure 8b exhibited significant downward shifts to 1284, 1129, and 1063

Figure 6. Raman spectra of (a) a neutral P3HT film and (b−d) a P3HT film doped with an acetonitrile solution of FeCl3. The doping level increases in the following order: (b) < (c) < (d). The doping level of (d) is the maximum level.

through FeCl3 solution doping at an FeCl3 concentration of 10 mmol/L. The doping levels of these films were similar to those of the films represented by Figures 3b−d. The 830 nm laser line lies within the doping-induced absorption band at 788 nm. Raman spectroscopy using 830 nm light should thus reveal doping-induced spectral changes. As previously described, the VIS/NIR absorption spectra indicate that predominantly positive polarons form upon solution doping. Accordingly, Raman spectra in Figures 6b−d are quite similar to each other and can be attributed to positive polarons. The band observed at 1425 cm−1 shifted downward to 1419 and 1412 cm−1 as the doping level was increased. The frequency of this mode may depend on the delocalization length of charge or interactions between polarons, among other factors. The Raman spectra of the positive polarons are consistent with the results published by McCreery et al.35 The observed Raman spectra are quite similar to the 676.4 nm excited Raman spectrum of electrochemically oxidized regioregular poly(3-dodecylthiophene);38 the spectrum showed the bands at 1410, 1378, 1210, 1186, and 726 cm−1. The observed spectrum was significantly contributed by a quinoid structure on the basis of a normal coordinate analysis.38 The bands between 1425 and 1412 cm−1 are assigned to the CαCβ stretching vibration of polarons, corresponding to the 1452 cm−1 CαCβ stretching mode of neutral P3HT. The lower frequencies in the positive polaron bands are consistent with the extraction of an electron from the bonding orbital of the conjugated thiophene rings. The 1382 and 1381 cm−1 bands of the positive polarons are assigned to the CβCβ stretching mode, corresponding to the 1386 cm−1 CβCβ stretching mode of neutral P3HT. These bands are strong for positive polarons in P3HT, whereas the corresponding bands are weak for positive polarons in polythiophene.30 Figure 7 shows the Raman spectrum of a FeCl3-doped P3HT film (excited at 830 nm) that was prepared via FeCl3 vapor doping for a doping time exceeding 20 h. The same film produced the VIS/NIR absorption spectrum in Figure 4c; this 4791

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correspond to the polaron bands observed at 1390, 1284, 1129, 1063, 976, 848, and 835 cm−1, respectively, in Figure 8e. However, the relative intensities of the 1383 and 1264 cm−1 bands with respect to intensity of the 1129 cm−1 band are low, which is a unique feature of the bipolaron spectrum. The observed 1383, 1264, 1129, and 1068 cm−1 bands are consistent with those predicted theoretically.45 It is difficult to distinguish polarons from bipolarons in infrared spectra because of the presence of many overlapping bands. Effect of Heating on the Raman Spectra. We investigated the stability of two polarons and that of a single bipolaron by heating a sample. A P3HT film containing bipolarons was prepared via doping with FeCl3 vapor for 20 h. Figure 10a presents the Raman spectrum of the film after

Figure 8. Changes in the IR absorption spectrum of a P3HT film upon FeCl3 solution doping.

cm−1 in Figure 8e, respectively, as doping proceeded. These shifts are characteristic of modes with large contributions from the effective conjugation coordinate according to the ECC theory.24 The doping-induced IRAV spectra of regiorandom poly(3-methylthiophene)40,41 and poly(3-hexylthiophene)42 were reported. The spectra observed in the present study are similar to them. The sharp weak bands observed at 1396−1388 cm−1 are not observed for polythiophene41,43,44 and are characteristic for poly(3-alkylthiophene)s. Figure 9 presents the infrared spectrum of a P3HT film doped with FeCl3 vapor to the maximum doping level. The

Figure 10. Changes in the Raman spectrum of a P3HT film doped heavily with FeCl3 vapor upon 1 h of thermal treatment at (a) 40, (b) 60, and (c) 85 °C.

heating to 40 °C for 1 h; Figure 10b presents the Raman spectrum after further heating to 60 °C for 1 h. The frequency of the strong band shifted downward to 1455 and then 1450 cm−1. The frequency of this mode is 1425−1412 cm−1 for polarons and 1470 cm−1 for bipolarons. Thus, these downward shifts can be explained in terms of the conversion of a number of bipolarons into polarons. Figure 10c presents the Raman spectrum observed after additional heating to 85 °C for 1 h. In this spectrum, large spectral changes are observed. The intense band is located at 1431 cm−1, a 1378 cm−1 band has appeared, and the intensity at approximately 1229 cm−1 has decreased. These results indicate that bipolarons are converted to polarons upon heating. Accordingly, the bipolarons form a metastable state. Electrical Conductivity. Figure 11 shows the change in the electrical conductivity of a P3HT film that occurred over time upon doping with FeCl3 vapor. At the beginning of the doping process, the electrical conductivity increased rapidly. After approximately 3.5 h, the electrical conductivity reached a maximum value of 6.3 × 10 S/cm. The VIS/NIR absorption and Raman spectra of a similar film indicated the formation of polarons at this doping level, after which the electrical conductivity began to gradually decrease. After 24 h, the

Figure 9. IR absorption spectrum of a P3HT film doped heavily with FeCl3 vapor for longer than 20 h.

same film produced the VIS/NIR absorption spectrum in Figure 4c. This spectrum indicates that the carriers that were generated were positive bipolarons. Thus, the observed infrared bands are associated with positive bipolarons. The bands observed at 1383, 1264, 1129, 1068, 974, 853, and 813 cm−1 4792

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ACKNOWLEDGMENTS This work was financially supported in part by the Grants for Excellent Graduate Schools (Practical Chemical Wisdom), MEXT, Japan.

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Figure 11. Change in the electrical conductivity of a P3HT film as a function of FeCl3 vapor doping time.

electrical conductivity reached a nearly constant value. The electrical conductivity after 46 h was 5.7 × 10−1 S/cm. VIS/ NIR and Raman measurements indicated that only positive bipolarons exist at the maximum doping level. Thus, the electrical conductivity of a film that contains only polarons, 6.3 × 10 S/cm, is higher than that of a film that contains only bipolarons, 5.7 × 10−1 S/cm. The electrical conductivity σ can be expressed in terms of the charge of a carrier q, the density of carriers n, and the carrier mobility μ as σ = qnμ. It is reasonable to assume that the electrical conductivity of a film that contains polarons at high concentrations would be saturated at 6.3 × 10 S/cm. The charges of a polaron and a bipolaron are e and 2e, respectively. The polaron density is 2 times the bipolaron density. Thus, the mobility of polarons is approximately 100 times that of bipolarons.

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CONCLUSIONS Changes in the VIS/NIR absorption, IR, and Raman spectra of a P3HT film upon FeCl3 vapor and solution doping were studied. Upon doping with an FeCl3 solution in air, the main carriers that are generated are positive polarons. Upon doping with FeCl3 vapor, positive polarons also form initially, followed by the formation of positive bipolarons with the concomitant disappearance of the positive polarons. The Raman and IR spectra of these positive polarons and bipolarons were obtained. These spectra can be used to identify positive polarons and bipolarons. The Raman results indicate that positive bipolarons are converted to positive polarons upon heating. Accordingly, the bipolarons form a metastable state. As FeCl3 vapor doping progressed, the electrical conductivity of a P3HT film first reached a maximum and then decreased by 2 orders of magnitude; this decrease is associated with the conversion of positive polarons to bipolarons. Thus, the mobility of positive polarons was estimated to be 100 times that of positive bipolarons. Raman spectroscopy will be used in further investigations of these carriers in organic electronic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.F.). Notes

The authors declare no competing financial interest. 4793

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The Journal of Physical Chemistry B

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DOI: 10.1021/jp512654b J. Phys. Chem. B 2015, 119, 4788−4794