Letter pubs.acs.org/JPCL
Probing Inter- and Intrachain Exciton Coupling in Isolated Poly(3hexylthiophene) Nanofibers: Effect of Solvation and Regioregularity Mina Baghgar,† Joelle Labastide,‡ Felicia Bokel,§ Irene Dujovne,‡ Aidan McKenna,∥ Austin M. Barnes,† Emily Pentzer,§ Todd Emrick,§ Ryan Hayward,§ and Michael D. Barnes*,†,‡ †
Department of Physics, ‡Department of Chemistry, §Polymer Science and Engineering, and ∥Department of Chemical Engineering, University of Massachusetts-Amherst, Amherst, MA 01003, United States S Supporting Information *
ABSTRACT: We report wavelength and time-resolved photoluminescence studies of isolated extended (1−10 μm length) poly(3-hexylthiophene) (P3HT) nanofibers (xNFs) cast on glass from suspension. The PL spectra of xNFs show multiple vibronic replicas that appear to be associated with the existence of both H- and J-type aggregates. The PL spectra of xNFs made from regioregular (rr)- (93%) and highly regioregular (hrr)-P3HT (98%) both show similarities in PL spectra suggestive of common chain packing features, as well as subtle differences that can be attributed to higher long-range order in the hrr-xNFs. Specifically, PL spectral measurements on isolated xNFs made from highly regioregular (>98%) P3HT showed a red-shifted electronic origin (≈30 meV) and increased 0−0/0−1 PL intensity ratio for the J-type species, suggestive of enhanced structural coherence length and intrachain order. SECTION: Spectroscopy, Photochemistry, and Excited States
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proposed by Spano.15 In this model, electronic excitation is accompanied by a molecular elongation, which gives rise to relatively simple vibronic progressions associated with 0, 1, or 2 (or higher) quanta of excitation in a symmetric stretching mode (e.g., CC stretch, ω0 ≈ 170 meV) coupled with the electronic excitation.14,16 Since H- and J-type excitonic coupling gives rise to different selection rules for absorption and emission, the nature of the coupling can be readily determined from qualititative differences in the emission spectra. In weakly coupled H-aggregate systems, typical of most thin films,17,18 the 0−0 transition in emission is only weakly allowed due to structural or thermal disorder, and most of the PL intensity is carried in the 0−1 vibronic transition. This gives rise to 0−0/ 0−1 intensity ratios in PL < 1 for H-aggregate systems. Conversely, a rigid linear (and coplanar) arrangement of thiophene units gives rise to dominant intrachain coupling manifested as J-aggregate behavior, where the 0−0 transition in emission is strongly allowed, thus giving rise to 0−0/0−1 intensity ratios in PL > 1.14,19 Motivated by intriguing observations by Moule and co-workers correlating solutionphase absorption features of NFs and structural properties obtained from X-ray diffraction (XRD),11 we explore the question of whether different nanoscopic structural “grains” (pointed to in XRD measurements) manifest in distinct
rystalline nanostuctures made from organic semiconductors are attracting enormous interest as a route to highefficiency organic photovoltaics.1−6 In particular, there has been a surge of interest in extended crystalline nanowires or nanofibers (NFs) as a means of enabling long-range chargetransport.7−11 In crystalline poly(3-hexylthiophene) (P3HT) NFs, the polymer chains align mostly prependicular to the NF growth axis via π−π interactions to form extended ribbon-like structures.11,12 Despite the significant research interest in these species, only very limited literature exists on properties of isolated NFs. Shimomura and co-workers examined effects of structural coherence on carrier transport properties in singlenanowire field-effect transistor (FET) devices.13 More recently, PL spectra on isolated NFs from purified suspensions showed unambiguous spectral signatures of exclusively J-aggregate behavior in single NFs, where a high degree of planarization of the P3HT chains results in a dominant intrachain coupling.14 Of particular interest is the question of competition between interchain and intrachain coupling in these species, which may be tunable via different solution processessing conditions. In this Letter, we present results of photoluminescence (PL) measurements on isolated extended (1−10 μm) NFs that show spectral signatures of both J- (intrachain) and H-type (interchain) excitonic coupling, suggestive of the existence of both aggregation types within crystalline domains in extended P3HT nanostructures. Spectroscopic manifestations of inter- and intrachain
Received: May 10, 2012 Accepted: May 31, 2012 Published: May 31, 2012
coupling in P3HT are usually discussed in the language of polaronic Frenkel excitons in H- and J-type aggregates, as © 2012 American Chemical Society
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440-nm excitation (PicoQuant PDL LDH−P-C-440B), and ≈250 ps for 532 nm (PicoQuant PDL 532) excitation. Figure 1 (a−c) shows PL images of three different extended NFs illustrating some of the different gross NF morphologies observed in our PL measurements. Most of the NFs probed appeared linear and uniform in PL intensity, while others show
spectroscopic signatures that can be detected using single xNF spectroscopy. We used a combination of wide-field single-molecule spectroscopy and scanning probe techniques to investigate the PL properties from individual P3HT xNFs. Spectrally resolved measurements revealed three different vibronic progressions with different electronic origins: one J-type progression similar in electronic origin, intensity ratio, and line width to that observed in ref 14, along with two H-type species with electronic origins offset by ∼75 meV. The linewidths for these H-aggregate components are quite narrow (≈65 meV), suggesting a high degree of structural order. Different NF macroscopic morphologies were observed (straight, curly, or zigzag) with only small changes in the PL spectra but with rather large changes in the time-resolved PL decay dynamics. Comparison with PL spectra obtained from xNFs prepared from highly regioregular (>98%) P3HT shows similar spectral features but with important quantitative differences that derive from enhanced long-range structural order. Also, the contrast between the spectra of xNFs in suspension and in the solid phase indicates significant changes in the relative contribution of each vibronic species to the spectrum owing to the formation of further crystalline structure during the drying process. These results provide additional insight into the connection between NF structure and associated PL properties, and their incorporation into the design and processing of organic photovoltaic systems. Crystalline P3HT NFs were assembled in solution by addition of a poor solvent (dichloromethane (DCM)) to a chloroform-solvated P3HT solution to a final v/v ratio of 1:7 CHCl3:CH2Cl2. We investigated PL properties of xNFs made from both commercial regioregular (rr)-P3HT (Reike Metals, 15 000 MW, 93% regioregularity) and P3HT of high regioregularity (hrr) (98%) prepared by the Grignard metathesis polymerization (GRIM) of 2,5-dibromo-3-hexylthiophene, as reported by McCullough and co-workers.20 A critical experimental issue in single-NF PL measurements is to reach suitably dilute concentrations without degrading the NF structure. To accomplish this, P3HT NF suspensions were diluted in heptane (a marginal solvent for P3HT with high vapor pressure) in order to minimize fiber bunching (typically 10 μL of NFs in 1:7 chloroform:DCM to ≈0.75 mL of heptane). To prevent precipitation, 0.25 mL of the chloroform:DCM mixture was added to the heptane suspension for a final volume of about 1 mL. A small amount of the suspension of NFs in heptane was drop-cast onto a plasma cleaned glass coverslip, which gave a NF density of about 5 well-separated xNFs in an area of ≈100 μm2. All xNFs were probed directly on glass with no host polymer (e.g., poly(methyl methacrylate) (PMMA)) matrix. Wavelengthresolved PL images were recorded by positioning a single NF near the center of a weakly focused laser spot (≈10 μm diam.) registered with the spectrometer slit (Acton 2150i, 300 grooves/in grating, blazed at 500 nm) positioned at a confocal plane on the side port of our microscope (Nikon TE 300 configured in an epi-illumination geometry with fluorescence collected through a 1.4 N.A./100× oil immersion objective). Time-resolved PL measurements were made on a separate setup using a precision timing avalanche photodiode (APD, ID Quantique 400) registered with the microscope focal spot. The illuminated area sampled by the APD was ≈1.5 μm2 with an overall instrument response function approximately 70 ps for
Figure 1. (a−c) PL images of three different xNFs (straight, bent, and zigzag) on glass. The vertical line indicates approximate spectrograph slit location for spectral measurements. (d) Fluorescence decay curves with the best triple exponential fits, and (e) PL emission spectra for the same xNFs structures displayed in panels a−c using 532 nm excitation. (f) PL emission spectra of xNF using 405 nm (purple line) and 532 nm excitation (green, filled). The gray trace in panels e and f shows solution-phase absorption spectra from rr-P3HT (>93% rr) NFs, with electronic (aggregate) origin at 2.049 eV (605 nm) to which all PL spectra were referenced. 1675
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curly, branched (bifurcated), and zigzag morphologies with significant variation in PL intensity along the NF axis. Atomic force microscopy (AFM) measuremnets of the NFs (Supporting Information, Figure S1) show a variety of xNF structures ranging in height from as small as 3 nm to ≈50 nm, and up to several micrometers in length. The P3HT NF absorption spectrum (typical of most of our NF suspensions) in the chloroform/DCM mixed solvent is shown by the solid gray curve in both Figure 1e and f with electronic origin at 605 nm (2.049 eV); all NF spectra were referenced to this value. In contrast with the NF absorption spectrum in ref 1 (showing an A(0−0)/A(0−1) intensity ratio slightly greater than 1), our typical measured A(0−0)/A(0−1) absorption intensity ratios of ≈0.75 correspond to a value of J0 = +20 meV (see eq 5 in ref 15), suggesting primarily H-type (interchain) electronic coupling in the (ensemble) aggregate. Figure 1e shows the PL spectra associated with the same xNFs in Figure 1a−c using 532 nm excitation. Aside from small differences in integrated PL intensity, sideband intensities, and line width, the general features of the PL spectrum appeared to be independent of NF morphology and position within the NF. However, comparing the PL spectra of xNFs obtained with 405 and 532 nm excitation (Figure 1f) indicates that the PL spectrum is quite sensitive to excitation energy; for 405 nm excitation, the vibronic structure in the PL spectrum appeared mostly J-like (with variable higher energy contributions), while multiple vibronic replicas appeared for 532 nm excitation. In constrast with the steady-state spectral properties, the PL decay dynamics shows significant variation with NF shape (Figure 1e). Analysis of PL decays from different NFs using a threeexponential fit showed that all three time constants are affected by NF shape. The fastest decay dynamics were observed for “bent” or sickle-shaped xNFs, with two short time constants (τ1 ∼ 220 ps and τ2 ∼ 500 ps), and a third (long) time constant of about τ3 ∼ 1.7 ns. Figure 1(e) shows that the NF with the zigzag morphology has a slightly faster overall decay than the straight xNF owing to the difference in their second time constants (τ2 = 1.8 ns for zigzag vs τ2 = 2.1 ns for straight), although they have similar prompt components (≈320 ps), and long-time components of 2.4 ns. The most striking feature of the xNF PL is in the low energy region of the PL spectra. Figure 2 shows an experimental PL spectrum from a single xNF (red crosses) that appears as a set of vibronic replicas offset by ≈75 meV (all transition energies were referenced to the solution-phase origin, 605 nm). The highest energy feature near the solution-phase origin carries most of the intensity as one might expect from a J-aggregate spectrum.19 However, the middle features of the experimental spectrum (−219 and −294 meV with respect to solution-phase origin) are much sharper and cannot be connected to a broad origin peak by addition of (nominal) 170 meV quanta. Since origin (Stokes) shifts for J-aggregates are expected to be small, we assigned the central peaks (−219 and −294 meV) in NF PL spectra as (0−1) and (0−1)′ transitions of two different H-type species, labeled H1 and H2. Figure 2 (top) shows a comparison of typical experimental NF PL spectra (red crosses) with a model spectrum composed of two H-type aggregate species with approximately the same vibrational frequency and 0−0 origins at −48 and −134 meV, respectively, (solid black line). Since structural disorder for Htype structures gives rise to nonzero intensity in the 0−0 transition in luminescence, our model spectrum includes 0−0 contributions to the PL spectrum, with relative intensities as
Figure 2. Experimental PL spectrum from an isolated extended NF (red crosses) and model spectra (solid black line). (Top) Fit to lowenergy spectral region using a sum of two H-type vibronic progressions (H1 and H2; filled gray curves) separated by ≈75 meV. The central peaks (−219 and −298 meV) were assigned as (0− 1) and (0−1)′ peaks for H1 and H2, respectively. (Bottom) Sum of H1, H2, and a J-type progression (solid red line), with origin at −105 meV (full width at half-maximum (fwhm) = 150 meV).
adjustable parameters. The model spectrum (H1 + H2) reproduces the central and low energy features of the spectrum, but does not account for the high PL intensity near the solution-phase origin. Figure 2 (bottom) shows a model spectrum including the addition of a J-type species (solid red line) with 0−0 origin at −105 meV (comparable to the ca. −90 meV origin reported in ref 14) with a much larger line width (≈150 meV) than the H-type components (≈65 meV). From our fitting of the experimental PL spectrum, the contribution from J-type aggregates accounts for ≈60% of the spectrally integrated PL intensity. (For details of all PL spectra fitting parameters, see Supporting Information, S2.) Although PL spectra of all the xNFs surveyed show similar features, we observed slight differences in sideband intensities and linewidths between the PL spectra of individual xNFs. Comparison of fitting parameters of PL spectra from different xNFs revealed that (0−0)/(0−1) intensity ratios changed mildly for different xNFs, which could be due to small differences in packing structure affecting H- and J-type coupling. To test this, we measured the PL spectra of xNFs made from hrr-P3HT to examine the impact of intensity ratios and electronic orgins (spectral properties related to structral order) on the different vibronic species. Figure 3 (top) shows a representative PL spectrum of a hrrP3HT NF excited with 532 nm radiation. In comparing with spectra from NFs made from commercial P3HT, we observe a red-shift in the origin of the J-type species of 27 meV, consistent with longer range intrachain order enforced by the 1676
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The observation of multiple aggregate types in the PL spectrum begs the question as to whether the solution phase (suspended) absorption spectrum accurately represents species in the dry state. Such questions were originally posed by Moule and co-workers in reconciliation of aggregate absorption features with structural features from dry NFs using XRD. Here, we examined this question by studying the transition in PL spectra (and associated differences in excitonic coupling) that takes place in solvated (suspended) versus solid state single NFs. To do this, we took a small droplet of suspended rr-NFs in the dilute chloroform/DCM mixed solvent, and trapped the NFs with a secondary coverslip to partially immobilize the NFs in solution and reduce the evaportation rate. In this way, we were able to make successive PL spectral measurements on the same NF over a time period of several minutes during solvent evaporation. We did not observe any significant changes in NF shape during the solvent evaporation process. Figure 4 shows the PL spectra of an individual suspended rrxNF and the same NF after solvent evaporation. While both
Figure 3. Effect of regioregularity on NF PL spectra. (Top) hrr-P3HT NF PL spectrum (red crosses) along with model H1 (dark filled), H2 (light filled), and J1 (solid red) spectral components. For comparison, the PL spectrum of the rr-NF in Figure 3 is shown in light gray (see Supporting Information for complete fitting details). There is a redshift in the origin of the J1 spectral component of 27 meV, while the H-type aggregates are weakly affected by the regioregularity. (Bottom) Summary of two-PL component decay analysis showing decreased spread of τ1 and τ2 values for the hrr-NFs, consistent with higher longrange order.
higher regioregularity. In addition, we find that the 0−0/0−1 intensity ratio for J1 increases by ≈40% for the hrr-NF, again consistent with higher intrachain structural order. The H-type spectral components appeared to be only weakly affected by the regioregularity (with respect to NFs made from commercial P3HT). The picture of higher long-range order afforded by higher regioregularity is also supported by analysis of the PL decay, where the hrr-NFs show signifantly more uniform PL decay dynamics.21 Figure 3 (bottom) shows a scatter plot of amplitude (relative contribution) versus decay constant (τ1 and τ2) for the two prompt decay components for NFs made from commercial rr-P3HT fibers (red markers) and hrr-P3HT fibers (blue markers), indicating tight clustering of the PL decay components as a result of a higher degree of structural order. It is perhaps surprising that the linewidths for the H- and J-type species are so markedly different. We speculate that this is a result of additional inhomogeneous broadening of the J-type species; we observe both an origin (red) shift relative to the lower regioregular polymer, as well as an increase in the 0−0/ 0−1 PL intensity ratio, both of which are indicative of a higher structural coherence length, yet the linewidths of the J-species for both NF types are nearly identical. This seems to support the idea that the additional broadening for the J-species is inhomogeneous in nature.
Figure 4. PL spectra of an isolated rr-xNF in solution phase (blue line with crosses) and of the same NF after solvent evaporation (red line with crosses). (Top) The model PL spectrum (black solid line) reveals that in solution, the NF has an aggregate spectrum (red solid line) that appears mostly H-like with a significant contribution from free P3HT (see ref 22) represented by the filled blue region. (Bottom) The model PL spectrum of the same rr-xNF after solvent evaporation (black solid line) acquires a dominant J-like character (dashed red line); virtually all the “free P3HT” luminescence vanished after drying.
suspended and dry NFs share common spectral components in the low energy region (region II), the suspended NF has a prominent high energy feature centered at ca. +0.087 eV (with respect to solution phase origin), that disappears almost completely upon drying (Region I). Recently, Bielawski and Van den Bout reported PL spectra of free rr-P3HT in choloroform with origin at 577 nm, close to the peak of 1677
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the case of a “mixed crystal”, the emission might be expected to look like “a mixture of ideal H- and J-aggregates” (ref 15). Very recently, the possibility of HJ hybrid coupling was proposed by Yamagata and Spano,26 who considered the competition between H and J coupling in P3HT nanostructures where mixed aggregate PL properties emerged from a charge-transferbased interaction between polymer chains. They found that the sign of the interaction (hence H- or J-like coupling) depends sensitively on highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) overlap between adjacent chains. Thus, the appearance of both Hand J-excitonic coupling could be due to multiple crystalline polymorphs in the NF. This is particularly intriguing from the point of view of enhancing charge transport along the NF backbone, and optoelectronic applications of these species.
Region I (+0.087 eV),22 thus we conclude that the high energy component of the suspended NF is uncoupled free P3HT within the aggregate that has a 0.015 eV red-shift in the electronic origin with respect to that of free P3HT chains in chloroform, and acounts for about 50% of the spectrally integrated PL intensity from the suspended NF. After drying, the total contribution of J1 and H2 increase by factors of 2.5 and 0.6, respectively, while the contribution from H1 is essentially unchanged (Table 2S in Supporting Information). This suggests a physical picture in which a significant amount of mobile free P3HT is weakly bound to the NF core in suspension where the aggregate component of the spectrum appears mostly H-like (consistent with solution phase absorption spectrum); as the solvent evaporates, the free P3HT becomes integrated into the NF assembly primarily by side-chain interactions that fit the new chains onto the surface of the NF core, and the PL spectrum acquires a dominant J-like character. We have shown that isolated extended xNFs have PL properties associated with both H- and J- type aggregated P3HT, whose spectral details (primarily the J-type) are sensitive to polymer regioregularity. This is in marked contrast to the NF PL spectra reported in ref 14, where exclusively Jtype coupling was observed. This could be a result of the smaller molecular weight (15 kDa) of the P3HT used in our study (compared to ≈50 kDa in ref 1), but also presumably points to a sensitivity to processing conditions. Similar PL spectra were observed for a variety of different NF lengths and morphologies, with only small differences in sideband intensities and linewidths, but nearly identical in spectral (0− 0) origins for the different vibronic series. In comparing PL spectra from xNFs made from commercial and hrr-P3HT, we observe a small but significant red-shift in electronic origin (≈27 meV) and a decrease in sideband intensity for the J-type species; both phenomena are indicative of higher intrachain order for the hrr-xNFs. The picture of enhanced structural order in the hrr-xNFs was supported by analysis of PL timetransients, which were observed to be highly variable for xNFs made from commercial P3HT, but fairly reproducible for the hrr-xNFs. There are two different NF structures that could account for the observation of multiple vibronic replicas associated with different excitonic coupling:1 Distinct structural domains (grains) with dimensions that are small compared to optical wavelength, whose PL signatures add incoherently and/or2 mixed H/J aggregate results from a “mixed crystal” (polymorphs) where both coupling types are present at the unit cell level. Moule and co-workers made the observation that, for P3HT NF solutions in p-xylene and toluene, a decrease in the 0−0/0−1 intensity ratio was accompanied by an increase in structural order as probed by XRD,11 suggesting either the existence of multiple domains in the NF solvent suspension (but difficult to observe in (ensemble) absorption), or the formation of additional crystalline grains during the drying process. Our measurements of the PL spectra of individual NFs during the solvent evaporation process indicate that much of the aggregate structure is maintained from suspended to dry state, but with significant changes in each vibronic species’ relative contribution to the spectrum. Alternatively, polymorphism at the unit-cell level could give rise to a Davydov splitting observable in polarization-resolved absorption, with an origin shift in absorption for polarization components parallel and perpdendicular to the NF axis.23−25 In
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ASSOCIATED CONTENT
S Supporting Information *
Transmission electron microscopy (TEM) and AFM images of xNFs, PL spectral fitting details, including fit parameters. This material is available free of charge via the Internet http://pubs. acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.B. (PL spectral measurements) acknowledges support for the U.S. Department of Energy (DE-FG02-05ER15695), J.L. (time-resolved measurements) acknowledges support from the NSF MRSEC (DMR-0820506). I.D. and E.P. acknowledge support from the Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001087. R.H. and F.B. acknowledge support from U.S. Department of Energy, Basic Energy Sciences (DE-SC0006639), and M.D.B. gratefully acknowledges support from the U.S. Department of Energy (Program Manager: Larry Rahn, DE-FG0205ER15695). The authors thank Prof. F. C. Spano for bringing to our attention ref 26.
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REFERENCES
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