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Two-Dimensional Electronic Excitations in Self-Assembled Conjugated Polymer Nanocrystals

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Science  04 Feb 2000:
Vol. 287, Issue 5454, pp. 839-842
DOI: 10.1126/science.287.5454.839

Abstract

Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.

Self-assembled organic semiconductor polymers with supramolecular two-dimensional (2D) structures are of interest (1–11) because the traditional one-dimensional (1D) electronic properties of the π-conjugated polymer chains can be modified by the increased interchain coupling. The regioregular (RR)-substituted polythiophene polymers (7) such as poly(3-alkylthiophene) (P3AT), in which the alkyl side groups are attached to the third position of the thiophene rings in a head-to-tail stereoregular order (Fig. 1A), form thin films with nanocrystalline lamellae (6), resulting in relatively high hole mobilities of 0.1 cm2 V−1 s−1(8–11). Thin-film transistors of RR P3AT have already been used to fabricate integrated circuits (8), which, in conjunction with highly luminescent polymers, may also lead to all-organic light-emitting diodes and smart pixels (9). On the contrary, P3AT films of regiorandom (RRa) stereo order (Fig. 1B) do not show supramolecular structures, and the hole mobility is very small, on the order of 10−5 cm2V−1 s−1 (8–11).

Figure 1

The (A) regioregular and (B) regiorandom stereo orders of P3AT polymer chains. R, alkyl group; S, sulfur. (C) The lamellae geometry of RR P3AT films. The substrate is perpendicular to the polymer planes. The in-plane interchain distance a and the interplane interchain distance b are assigned. (D) Schematic representation of a positive polaron on the P3AT backbone chain structure; Δr is the dimerization amplitude. The localized energy levels and allowed optical transitions of a positively charged (E) 1D polaron, P+, and (F) 2D delocalized polaron, DP+, where 2Δ is the level splitting, are shown. HOMO is blue-shifted and LUMO is red-shifted for the coupled chain (F) in relation to the uncoupled chain (E).

In this work, we applied linear and nonlinear optical spectroscopies to show that the charge carriers (or polarons) in the RR P3AT lamellae become delocalized over several polymer chains, consistent with the high hole mobility. These spectroscopies include photoinduced absorption (PIA), PIA detected magnetic resonance (PIA DMR), doping-induced absorption (DIA), and electroabsorption (EA). In all of these techniques, the changes ΔT in the films' optical transmission T that are induced by the external perturbation were measured versus the photon frequency ω, using phase-sensitive methods, and the absorption modulation spectra ΔT(ω)/T(ω) were obtained. In our measurements, we used visible and near-infrared (NIR) as well as Fourier transform infrared spectrometers. In PIA (12), ΔT is photoinduced by a continuous-wave Ar+laser, which generates long-lived photoexcitations with densityN. In PIA DMR (12), we modulated the densityN by microwave absorption in a magnetic field to obtain the spin state of the photoexcitations that are associated with specific bands in the PIA spectrum. In the DIA measurements (13), the films were doped by exposure to I2 vapor for a few seconds. The electric field in EA spectroscopy was on the order of 105 V cm−1, and we found a quadratic field-dependent EA in all cases (14).

We studied thin films of RR and RRa P3AT (Fig. 1, A and B), where the alkyl side group (CH2)nCH3 varied fromn = 4 to 12. We purchased the P3AT powders from Aldrich, and the films were evaporated from xylene solutions. The RR polymers (>99% head to tail) self-organized to form lamellae perpendicular to the substrate (11), with grain size of ∼150 Å (6) and high hole mobility (11). The interplane distance b (Fig. 1C) was measured by x-ray diffraction to be ∼3.8Å, whereas the in-plane, interchain distance a (Fig. 1C) varied from 13 to 27 Å forn = 4 to 12, respectively (6).

In a strictly 1D chain model (15), a single- charge carrier added onto the polymer chain forms a spin −1/2 polaron (Fig. 1D), with two localized states in the gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) (Fig. 1E). The two localized states are shifted inside the gap caused by the polaronic relaxation of the local electronic, bond dimerization (Δr in Fig. 1D), and lattice structure, and thus, polarons have two allowed optical transitions, P1 and P2, (Fig. 1E) below the HOMO-LUMO gap (15). Because of the relatively strong electron-phonon coupling in π-conjugated polymers, the polaron excitation renormalizes the frequencies of the Raman-active amplitude modes (16), which consequently borrow IR intensity from the electronic transitions. The small polaronic mass causes these IR-active vibrations (IRAVs) to have very large oscillator strengths that are comparable to that of the electronic transitions (17). As seen in Fig. 2A, two DIA bands, P1 at 0.45 eV and P2 at 1.3 eV, and associated IRAVs are indeed generated in RR poly(3-hexylthiophene) (P3HT) films upon I2doping. The large interstitial iodine ions may actually isolate the polymer chains and cause quasi-1D electronic properties. In addition, the dopant counterion may further localize the induced polaron excitation.

Figure 2

(A) The DIA bands of RR P3HT film doped with I2. Long-lived photoexcitations in films of (B) RR P3HT, where the broken line is for isolated chains in polystyrene, and (C) RRa P3HT, measured by PIA at 80 K, are shown. There is a change in scale at 0.6 eV (B) and 0.85 eV (C). The PIA bands (P1 and P2 and DP1 and DP2) are associated with 1D polarons (Fig. 1E) and 2D delocalized polarons (Fig. 1F), respectively. IEX and T1are related to interchain singlet exciton and intrachain triplet exciton, respectively, whereas IRAVs are IR-active vibrations associated with the 1D polarons. ℏ︀, Planck's constant divided by 2π.

Similar results were obtained upon photogeneration of isolated RR P3HT chains in a polystyrene matrix (Fig. 2B). We observed two correlated PIA bands, P1 at 0.4 eV and P2 at 1.35 eV, with associated IRAVs. PIA DMR studies show a spin −1/2 resonance with Landé g factor of 2.012 associated with these PIA bands. We therefore conclude that, similar to I2doping, the long-lived photoexcitations in isolated RR P3HT chains are localized polarons, with polaronic relaxation energy given by P1 ≃ 0.35 eV, and 1D electronic properties.

The photoinduced charged excitations in RRa P3HT films are also 1D polarons with P1 ≃ 0.5 eV and strong IRAVs (Fig. 2C). However, these films also show an improved fluorescence efficiency with a singlet-exciton lifetime on the order of 0.5 ns, which allows efficient intersystem crossing. Thus, the dominant long-lived photoexcitations are triplet excitons with a PIA band at 1.5 eV (Fig. 2C); this excitation was identified in the PIA DMR spectrum by the associated triplet powder pattern at g ≃ 4 (12). We therefore conclude that the interchain coupling in RRa P3AT films is also weak and results in 1D electronic properties.

However, the PIA spectrum of an undoped RR P3HT film (Fig. 2B), where lamellae are formed (11), substantially differs from those of the other films discussed above. The PIA spectrum shows multiple PIA bands: In addition to the P1 band at 0.37 eV and the P2 band at 1.3 eV as in isolated chains in polystyrene, the PIA spectrum in the RR P3HT film also contains three additional PIA bands, denoted DP1 (for delocalized polarons) with a peak at 0.06 eV (Fig. 2B), interchain excitons (IEXs) at 1.05 eV, and DP2 at 1.7 eV. The DP1 and DP2 PIA bands are correlated: They have the same dependencies on the laser excitation intensityI L, laser modulation frequency, and temperature. Moreover, we found from PIA DMR measurements that the two DPi bands (DP1 and DP2) are also associated with spin −1/2 excitations, but with g ≃ 2.008, compared to g ≃ 2.012 for the Pi bands, P1 and P2. The IEX band, however, does not show any PIA DMR and therefore is due to spin singlet excitations. In addition, it is not correlated with the DPi and Pi bands and is not generated by charge injection (11). We conjecture, therefore, that the IEX band is due to a neutral, spin singlet excitation. Because intrachain excitons are short-lived in π-conjugated polymers with a lifetime on the order of 1 ns, we think that IEX is caused by interchain excitons (18). We note that the Pi bands are not correlated with the DPi bands; in addition to a differentg value and microwave modulation frequency dependence in PIA DMR, the Pi bands saturate more easily than the DPi bands with I L. We therefore conclude that the DPi bands are due to delocalized polarons in the RR P3HT ordered phase (in the lamellae), whereas the Pi bands are due to localized polarons in disordered P3HT chains surrounding the lamellae.

The influence of increasing the interchain couplingt on the stability, delocalization, and other electronic properties of polaron excitations in π-conjugated polymer chains has been investigated theoretically (19–24). It has been predicted that, for t on the order of 0.15 eV, the polaron excitation substantially delocalizes over adjacent chains. Similar to the case of π dimers (25,26), we assumed that neighboring chains are strongly coupled, which results in an energy splitting 2Δ (≃ 2t ) for each of the two polaron levels in the gap, whereas the HOMO-LUMO gap shrinks by the same amount (Fig. 1F). For the two allowed transitions, DP1 and DP2, we then find DP1 = P1 − 2Δ and DP2 = P2 + 2Δ, and thus, the sum of the transition energies is conserved; that is, DP1 + DP2 = P1 + P2. Moreover, the intensity ratio of the DPi transitions is very different from that of isolated polarons, with the lowest energy transition (DP1) becoming dominant (24). The four new gap states and associated allowed optical transitions related to a DP+ excitation (Fig. 1F) can be compared with the DPi bands of the RR P3HT film (Fig. 2B). We identify two PA bands related to the delocalized polaron excitation, DP1 ≃ 0.06 eV and DP2 ≃ 1.8 eV, where the DP1 band is the strongest. We also note that P1 + P2 ≃ 1.75 eV, whereas DP1 + DP2 ≃ 1.85 eV, in agreement with the conservation of the transition energies concluded from the above model.

The DP1 band (Fig. 2B) is superimposed by narrow bands with some resemblance to the IRAV bands in I2-doping films (Fig. 2A) or in PIA of RRa films (Fig. 2C). However, a more detailed examination of the DP1 band (Fig. 3) indicates that this is not true. First, the narrow superimposed features appear to have negative absorption contributions with antiresonances (ARs) rather than peaks (or resonances), as for regular IRAVs (16, 17). Second, there are many more AR bands than IRAV bands. Third, the AR bands diminish at low frequency, on the DP1 absorption tail. Finally, the AR frequencies are very near the frequencies of the Raman-active modes and the IR-active modes of the RR P3HT film (Fig. 3). It seems, therefore, that the AR bands are due to Fano-type interferences (27) between the discrete vibrational lines (both Raman and IR active) and the DP1 electronic band. Such AR bands cannot be described as a set of independent Fano interferences, but the complete phonon propagator has to be considered. In this case, AR bands appear in between each adjacent pair of Raman-active modes (27). Our results differ substantially from this theoretical prediction: First, we observe AR bands with Raman-active mode as well as with IR-active modes. Second, the AR frequencies are very close to the Raman and IR frequencies, indicating a reduced electron-phonon coupling. We speculate that both of these findings may be related to the acquired 2D properties of the delocalized polaron excitation caused by the increased interchain coupling.

Figure 3

The DP1 PIA band in RR P3AT films for alkyl side chains (CH2)nCH3 of various length n. Arrows mark the DP1 peak energy. The resonant Raman scattering and IR absorption spectra of a RR P3HT film are shown for comparison with the AR bands. P3DDT, poly(3-dodecylthiophene); P3DT, poly(3-decylthiophene); and P3OT, poly(3-octylthiophene).

Increasing the side-chain length n of the polymers may decrease t (28) because the interplane interchain separation b (Fig. 1C) increases (6). The change in t may then be reflected in the delocalized polaron properties: The DP1 peak energy will increase with decreasingt (24). The DP1 band in various RR P3AT films blue-shifts fromn > 6 (Fig. 3), indicating that, indeed, the delocalized polaron excitation becomes more localized when bincreases. However, the DP1 peak increases again forn = 4 [RR poly(3-butylthiophene) (P3BT)]. The reason for this increase is not exactly understood at present; however, it may be related to the difficulties that we encountered in dissolving the RR P3BT powder.

We studied the influence of the increased structural order in RR P3HT films on the intrachain excitons by EA spectroscopy. We found that, in addition to spectral features at 2 eV (derivative-like) and 2.7 eV (absorption band), which are similar to those in disordered films (14), the RR P3HT EA spectrum also contains strong oscillation with zero crossing at 2.75 eV (Fig. 4). Such oscillation was previously seen in α-sexithienyl microcrystallites and analyzed in terms of a charge-transfer exciton near the continuum level (29). This may be a prerequisite for the formation of Franz-Keldysh band-edge oscillation, such as in EA of single-crystal polydiacetylene (30). We also found that this oscillation gradually disappears upon aging of the film, due to reduced electron coherence caused by the oxygen attack on the polymer chains (Fig. 4).

Figure 4

(A) The EA spectrum of a freshly prepared RR P3HT film. The energies of the intrachain excitons 1Bu, mAg, and charge transfer (CT) excitons are assigned. The inset shows the four essential states for explaining the EA spectrum. (B) The EA spectrum of an aged RR P3HT film.

In conclusion, we studied and compared long-lived photoexcitations in RR and RRa P3AT films. In RR P3AT films, where lamellae are formed, the increased interplane interchain interaction,t delocalizes the traditional 1D charged polarons onto adjacent chains, acquiring 2D electronic properties. The most prominent optical signature of the delocalized polaron excitation is a strong red-shifted absorption band in the mid-IR range (DP1) that may be used to directly measuret in polymer films by optical means. In addition, the relatively weak transitions of the delocalized polaron excitation in the visible and NIR spectral range may help to achieve laser action in nanocrystalline polymer devices using current injection, by reducing the optical loss at wavelengths near the exciton-stimulated emission band; so far, this has been an acute problem in the field of plastic lasers. Moreover, although the absorption spectrum of delocalized polaron excitation approaches that of free carriers in more conventional 3D semiconductors, it still contains a low-frequency spectral gap of ∼60 meV, which is due to the polaronic relaxation and which may be reflected in the relatively low carrier mobility in these films. A further increase oft will close the gap, destabilize the delocalized polaron excitation, and enhance carrier mobility by additional orders of magnitudes. All organic electronic and optoelectronic applications of nanocrystalline π-conjugated polymers may then become a reality.

  • * On leave from the Department of Physics and the Graduate School of Materials Research, Åbo Akademi University, Porthansgatan 3, FIN-20500 Turku, Finland.

  • To whom correspondence should be addressed. E-mail: val{at}physics.utah.edu

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