Article pubs.acs.org/cm
Regioregularity and Intrachain Ordering: Impact on the Nanostructure and Charge Transport in Two-Dimensional Assemblies of Poly(3-hexylthiophene) Avishek R. Aiyar,† Jung-Il Hong,‡,∥ and Elsa Reichmanis*,†,‡,§ †
School of Chemical and Biomolecular Engineering, ‡School of Materials Science and Engineering, §School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: The properties of supramolecular assemblies in conjugated macromolecular systems are strongly dependent on single chain effects. We report that differences in regioregularity (RR) of side chain attachment in poly(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient to induce dramatic changes in the electronic and morphological properties of the material. Casting the electronic absorption spectra in the framework of Spano’s model reveals that the conjugation length is surprisingly sensitive to RR, with differences in free exciton bandwidth between the two P3HT samples approaching 73 meV. The enhanced main chain planarization motivates a concomitant increase in nanofibril width as well as crystallinity observed in thin films of the higher RR variant. This observation correlates well with the field effect mobilities that are attenuated by 1 to 2 orders of magnitude in the lower RR polymer film. We suggest that the increased intrachain order coupled with a reduced fraction of grain boundaries in the higher RR film is responsible for the reported differences. KEYWORDS: regioregularity, exciton bandwidth, OFET, nanofibrils, grain boundaries, mobility, conjugagtion
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transport through “controlled” supramolecular self-assembly, either through improved molecular design12,13 or by exploiting solvent−solute−substrate interactions.14−16 Poly(3-hexylthiophene) (P3HT), a widely studied solution processeable organic semiconductor with excellent hole transport properties,17,18 has served as a model system for understanding the correlation between thin film microstructure and optoelectronic properties in conjugated systems.19,20 Specifically, the molecular weight (MW)21−23 and regioregularity (RR),18,24 defined as the percentage of head−tail (H−T) attachment of side chains in regiochemical diads, have been identified as key molecular parameters that control thin film microstructure and thus charge delocalization. In essence, both parameters are intricately linked, in that they both directly influence single chain properties and hence multichain assemblies. An understanding of the effects of MW forms a useful platform to elucidate the role of RR. Although a multitude of theories, all unified by the observation of a significant increase in field effect mobility with MW, have been reported to explain the effects of increasing chain length on macroscopic charge transport in P3HT films,20−23 questions remain regarding the etiology of this phenomenon. Kline et al.22,25 proposed two plausible explanations for the observed increase in mobility:
INTRODUCTION Over the past two decades, the mobility of charge carriers in organic semiconductors has seen a steady improvement spanning several orders of magnitude ranging from 10−5 cm2 V−1 s−1 in polythiophene thin films1 to 20 cm2 V−1 s−1 in single crystals of rubrene.2 This extraordinary difference in mobilities is evidence of the dominant role played by structural order, which results in sequentially improved intermolecular charge transport arising from disordered films to perfectly ordered single crystals.3,4 Although significant strides toward development of both single crystal arrays5 as well as solution processed small molecules6 have been made, polymeric semiconductors may be more amenable for use in large-area applications such as flexible printed electronics in light of their better mechanical and rheological properties.7 However, these attractive properties are somewhat offset by relatively poor charge transport in conjugated polymers, where large free energy barriers to crystallization8 inevitably lead to a structurally inhomogeneous system. Conjugated polymer thin films are thus typically composed of ordered crystallites embedded in a largely amorphous matrix,9 and charge hopping between transport sites is significantly impeded by disorder.4,10 In such variegated systems, identifying the exact individual roles of intrachain, interchain, and intergrain charge transport in determining the macroscopic field effect mobility becomes a substantial challenge.9,11 Much of the research in the area of conjugated polymers has thus focused on understanding and suppressing the effects of structural disorder on charge © 2012 American Chemical Society
Received: September 9, 2011 Revised: July 11, 2012 Published: July 19, 2012 2845
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P3HT samples persist even when comparing the results obtained from films that are spin coated from solvents that differ by as much as ca. 120° in their boiling point (BP), suggesting that RR has an overriding influence on charge transport. These data provide clear evidence that RR must be considered with great care when optimizing charge transport in solution processed conjugated polymer films. The importance of intra- vs interchain effects are also highlighted.
longer chains in high MW P3HT (i) facilitate a net reduction in interchain hopping events as intramolecular charge transport through longer conjugated segments becomes facile26 and (ii) act as charge transport conduits connecting crystalline domains separated by low conductivity amorphous grain boundaries.9 Alternatively, Zen et al.23 and Chang et al.20 ascribed the mobility increase to improved intermolecular transport, resulting from either reduced distortion of the conjugated backbone or fewer chain defects within individual grains. The early work of Bredas et al. shows that deviations from backbone coplanarity can have detrimental consequences for the electronic properties in conjugated systems leading to the limiting case of near vanishing bandwidths (BWs) for bithiophene having a 90° dihedral angle.27 The presence of alkyl substituents, required to solubilize the intractable main chain, only serve to amplify the effect.28,29 Such effects are also manifest through the reduction of the RR of the polymer.18,24 A higher percentage of sterically demanding head−head (H−H) substituents results in significant loss of backbone planarity through the formation of conformational defects or “twistons”30 kinks in the delocalization pathway of charge carriers that lead to loss of conjugation.26 Although the effect of RR on the microstructure and charge transport in P3HT films is documented,18 the subtlety of the effect remains open to question. While Sirringhaus et al. showed that a ca. 15% difference in RR can lead to a hundred fold increase in field effect mobility,18 Kim et al. demonstrated that even a mere 5% difference is sufficient to significantly alter the performance of bulk heterojunction solar cells.24 Coincidentally, Mauer et al. demonstrated an order of magnitude improvement in the time-of-flight mobility of the same HT98 versus HT94 materials investigated here, although they did not conduct a detailed examination or discuss the root cause.31 Further, Snyder et al. demonstrated that similar subtle differences in P3HT RR can impact the crystal lamellar thickness.32 The results of both Mauer and Snyder highlight the potential significant impact of side-chain RR on macroscopic charge transport in thin conjugated polymer films, a precise evaluation of which has not been reported. Identification and understanding of the level of significance of the relationship between side-chain RR and macroscopic mobility is a critical step required for the design of effective conjugated polymers for semiconductor applications. Developing an understanding of the level of significance RR plays in allowing polymer chains to assemble into nano- and microstructures is essential for identification of design rules for high performance polymer based semiconductors. Here, we seek to understand how subtle differences in RR of the hexyl side chains in P3HT can influence the supramolecular assembly of polymer chains when deposited from the solution state and subsequent thin film properties. We demonstrate that differences in RR as small as ca. 4% can, surprisingly, result in dramatic variations in field effect mobilities by as much as 2 orders of magnitude. The differences in the electronic absorption spectra, which are analyzed in light of Spano’s model,33 are consistent with the spectral signatures of enhanced intrachain order in higher RR P3HT. The observed morphological differences are in agreement with the photophysical changes, and also point to an increase in the conjugation length of the polymer chain, resulting in nanofibrillar structures that are consistently wider for the higher RR variant. Moreover, the difference in mobility between the two
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EXPERIMENTAL PROCEDURES
Materials. Regioregular P3HT (P100 and P200 Sepiolid: hereon referred to as HT94 and HT98 P3HT, respectively) used for the study was purchased from Rieke Metals, Inc. and used without further purification. The molecular weight and regioregularity of both samples are summarized in Table 1. All the solvents used in the study were anhydrous (except for benzyl chloride) and purchased from Sigma Aldrich and used without any further purification.
Table 1. Molecular Weight and Regioregularity of the P3HT Samples Used molecular weight (kDa) P3HT sample
MN
MW
regioregularity (% HT)
HT94 HT98
19.7 12.7
41.9 23
92−94 >98
The MW data were obtained through gel permeation chromatography (GPC) of the polymer samples in tetrahydrofuran solution. A Waters 1515 Isocratic high performance liquid chromatography system with a Waters 2489 UV/vis detector, fitted with a Styragel HR 5E column was used. MW data were determined using polystyrene standards. Polymer regioregularity was estimated from the 1H NMR spectra obtained from deuterated chloroform solution at 293 K using a Bruker DSX 300 (see the Supporting Information). Organic Field-Effect Transistor (OFET) Fabrication and Characterization. The FET devices used for electrical characterization consisted of two contact devices where P3HT films were prepared via spin coating the relevant polymer solution onto a 300 nm thick SiO2 gate dielectric. The highly doped silicon wafer served as the gate electrode, while Au/Cr was used for the source and drain contacts. The source and drain contacts were fabricated using a standard photolithography based lift-off process, followed by E-beam evaporation (CVC Inc.) of 50 nm Au contacts with 5 nm of Cr as the adhesion layer. Before spin coating P3HT solutions, all devices were cleaned for 15 min in a UV−ozone cleaner (Novascan PSD-UV) to ensure complete removal of any residual photoresist and other organic contaminants. At the end of the cleaning process, the substrates were hydrophilic. The solutions used for fabricating the OFETs were prepared in a nitrogen glovebox ( μHT94 (μ = field effect mobility) in spite of the lower MW of HT98 P3HT, our observations can be isolated to the effect of RR alone. While not explicitly included in this study, experiments performed with a P3HT sample, OS1100 (Plextronics Inc., RR ∼ 92%, MN ∼ 24.1 kDa), which is very similar to HT94, yielded an average mobility of 4.6 × 10−4 cm2 V−1 s−1 (data not shown). It is also noted that OFET measurements using regiorandom (ca. 57% H−T) P3HT, obtained by spin coating from CHCl3 solutions, yielded an
2 I0 − 0 ⎛ 1 − 0.24W /E P ⎞ ≈⎜ ⎟ I0 − 1 ⎝ 1 + 0.073W /E P ⎠
(1)
I0−0 and I0−1 are the intensities of the (0−0) and (0−1) transitions, respectively. Spano’s model is insightful because it provides a quantitative estimate of the free exciton bandwidth, which in turn strongly correlates with the conjugation length of individual polymer chains (and thus intrachain ordering) through the interchain excitonic coupling J (W = 4 J).33,44 Applying Spano’s model to our experimental absorption data reveals differences in W, Figure 2c, that are in agreement with enhanced main chain planarization, given that the excitonic coupling decays significantly with extension of π conjugation.44 The W values are calculated using eq 1 and the intensities of the (0−0) and (0−1) absorption bands (I0−0 and I0−1, respectively) obtained from fits to the experimental absorption spectra (see the Supporting Information). Although differences between HT94 and HT98 dominate our observations, W is also found to vary slightly as a function 2848
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ordering between the effects of BP and RR, the latter strongly influencing the solubility of the polymer as well. Conceivably, the lower RR, and thus greater steric barrier, in HT94 might prevent the manifestation of solvent BP effects, explaining the relatively flat profile for W observed in Figure 2c. Films obtained from BCl are a clear exception, routinely exhibiting the lowest exciton BW for both HT94 and HT98, thus ostensibly pointing at a role of solvent BP. However, this anomaly is addressed by noting that films obtained by spin coating a solution of HT98 in p-xylene (BP ∼ 138 °C), a relatively poor solvent for P3HT, that has been allowed to cool to room temperature for several hours prior to spin coating exhibits an exciton BW of 32.4 meV, a value that is significantly lower than that for films obtained by spin coating the hot solution (W = 84.5 meV) (see the Supporting Information). In addition, Clark et al. reported similar values for the free exciton BW in P3HT films obtained from isodurene (BP ∼ 198 °C), also a poor solvent. The role of solubility in the formation of ordered π stacked precursors in P3HT solutions is documented,45,46 and it is plausible that the facile π−π associations are a result of enhanced main chain planarization predicted by the low exciton BW. Since BCl is a relatively poor solvent for P3HT, a similar phenomenon is expected to prevail for this solvent as well, explaining the observed minima in exciton BW. Spectroscopic evidence coupled with prior results leads to the conclusion that the increased conjugation length observed for HT98 emanates from a greater resistance to chain tortuosity. Conceivably, the morphology of the films would reflect this change in the configurational character of individual polymer chains. Figure 3 depicts AFM phase images of the
Figure 2. Normalized solid state absorption spectra of P3HT films processed on glass slides from different solvents using (a) HT94 and (b) HT98 P3HT. The inset in (b) shows the comparison between the two P3HT films obtained from MCB. (c) Free exciton bandwidth (W) calculated using Spano’s model.
Figure 3. Tapping mode AFM phase images of HT94 and HT98 P3HT films obtained by spin coating from (a) chloroform, (b) pxylene, (c) chlorobenzene, and (d) benzyl chloride. The insets in b show the respective magnified images (250 nm × 250 nm).
P3HT samples where changes associated with increased RR are clearly seen. As expected,39 films obtained via spin coating P3HT from CHCl3 solutions are featureless, irrespective of the RR (Figure 3a). Films obtained for both polymer systems from other solvents however, show mesoscale features in the form of nanofibrillar structures that are randomly distributed throughout the scan area. Examination of the images in Figure 3 reveals distinct differences in the nanofibril widths for HT94 and HT98 films obtained from xylene, MCB, and BCl solution. An analysis of the nanofibril dimensions is revealing, the results of which are
of the solvent used for a given RR. As with the mobility, the spectroscopic evidence indicates no strict dependence of W (and thus J) on solvent BP, although it assumes the lowest value for both HT94 and HT98 films obtained from BCl (BP ∼ 179 °C). This apparent enhancement of intrachain ordering is believed to be borne out of solubility effects and may not be related to solvent BP (vide inf ra). Nevertheless, given that the extent of intrachain ordering in HT98 is the least for films obtained from CHCl3 (note that HT98 mobilities were also lowest for devices prepared using CHCl3 as a solvent), it might be indirect evidence of a competitive mechanism for chain 2849
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useful for understanding the packing geometry of the P3HT chains within individual nanofibers. Barford’s model predicts that for parallel chains with lengths larger than the separation between them, the excitonic coupling, J, varies as L−1.8, where L is the chain length.47 We can thus obtain an effective conjugation length (LEFF,BARFORD), defined as the ratio of conjugation lengths in HT98 and HT94, by combining Barford’s model with Spano’s model, using the exciton coupling integrals obtained from the latter. As shown in Figure 4b, these values are in close proximity to the effective nanofiber width (LEFF,AFM), defined as the ratio of nanofiber widths in HT98 and HT94 films obtained from the AFM phase images, ascribing the increase in conjugation and nanofiber width to a common phenomenon: enhanced main chain planarization in HT98. Moreover, it conclusively demonstrates that the microstructure of the P3HT films is strongly coupled with features observed in the absorption spectra. Thus, the packing within each fiber can be envisaged as consisting of lamellar stacks of P3HT with the nanofiber axis coincident with the π−π stacking axis, as has been suggested in the characterization of P3HT whiskers.48 Such a packing motif has implications for the conformation of individual polymer chains as well. A comparison with the number average contour lengths of P3HT chains in HT94 and HT98 (ca. 450 Å and 280 Å, respectively), estimated from the MW data, hints at the presence of chain folding given that the nanofiber widths are significantly lower than the contour lengths.48 Moreover, this marks a substantially larger departure from full extension in the case of HT94 (ca. 300 Å) than for HT98 (ca. 70 Å), a result made especially intriguing by the fact that it emanates from a mere 4% difference in RR. However, caution must be exercised in such an analysis because the accuracy of the GPC data hinges on knowledge of the conformational flexibility of P3HT chains in solution, a subject of continued debate.49,50 The increased planar assembly of HT98 chains is expected to provide improved opportunities for intermolecular interactions, which, in theory, should lead to higher crystallinity of the films.12 Figure 5 shows the X-ray diffractograms obtained from GIXD measurements of the P3HT films. Although reproducibility tests conducted to confirm the GIXD results reveal that peak intensities obtained from each film differ across two sets of experiments, a result that is ascribed to the sensitivity of the microstructure to the processing conditions noted earlier, the
summarized in Figure 4a. The ca. 4% difference in RR leads to nanofibril widths that, surprisingly, vary by as much as 84 Å in
Figure 4. (a) Nanofibril widths obtained from a size analysis of the AFM phase images shown in Figure 3. (b) Comparison of the effective nanofibril width (LEFF) obtained from experiment (LEFF, AFM) and from theory (LEFF,BARFORD).
the case of films obtained from xylene, Figure 3b. While the nanofibrillar widths are sensitive to the processing conditions, particularly in the case of poor solvents such as BCl and xylene, and minor variations in both solution concentration and temperature can impact nanofibril width, differences in those widths are reproducibly observed to be dependent upon polymer RR. The values varied between a minimum of ca. 43 Å to a maximum of ca. 84 Å. As with the results of the spectroscopic and mobility studies (vide inf ra), the effect of solvent is nominal in comparison to the dominating influence of RR. AFM phase images obtained from the alternate 92% RR P3HT sample, OS1100 (described earlier), very closely resemble those obtained from HT94 P3HT (see the Supporting Information). That the increase in nanofiber width is coincident with the increase in conjugation length arising from the 4% difference in RR for HT94 to HT98 is
Figure 5. Grazing incidence X-ray diffraction profiles of HT94 (solid lines) and HT98 (dashed lines) films obtained by spin-coating from different solvents. 2850
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participation of intrachain transport cannot be entirely ruled out. In fact, the coincident observation of enhanced mobility and intrachain order justifies revisiting the role of intrachain charge transport in the determination of macroscopic mobility. This perspective is supported by studies on isolated polythiophene “molecular wires” using pulse radiolysis microwave conductivity (PRMC) measurements that have yielded mobilities as high as 0.02 cm2V−1s−1,51 similar to values obtained from “bulk” films from the current OFET experiments involving HT98 films. Grozema et al. have further reported that even higher intrachain mobilities can be observed by reducing the number of torsional defects that impede orbital overlap between neighboring monomeric units.26 Kline et al. have argued that even though interchain delocalization maybe the primary mechanism of charge transport, longer chains with lower conjugation defects allow for facile intrachain delocalization, thus providing charge carriers with ample opportunities for hopping between chains.22 This mechanism could, at least in part, account for the differences in mobility observed between HT94 and HT98 films. This observation can also explain why variations in mobility as a function of solvent are relatively minor, since the differences in exciton BW across solvents (for a given RR) is at least half that observed for differences in RR (for a given solvent), once again underscoring the dominance of RR over solvent choice. There are further, potentially profound implications to the increased conjugation. It is well-known that the process of charge hopping between adjacent polymer chains can be cast into the framework of the Marcus−Hush theory for electrontransfer reactions, with the hopping rate at a given temperature governed by a combination of the internal reorganization energy (λ) that accompanies the charge transfer and the electronic coupling matrix element.52−55 This would imply that, in systems with comparable electronic coupling between adjacent segments, a decrease in λ would lead to lower reorganization energy barriers and thus a substantial increase in the charge hopping rate (assuming self-localized polaron effects are absent53). HT98 chains, which show an extension of the conjugation relative to HT94, would represent such a scenario,53 which could, in theory, contribute to the enhanced mobilities observed in HT98. In addition, because of the exponential dependence of the hopping rate on λ, the latter would exert a dominating influence, superseding the effect of intermolecular electronic coupling.53 This expectation is verified by the results of Zhang et al. who reported an exponential dependence of the mobility on the width of nanofibers with increasing MW, suggesting that extended conjugated states indeed have a dominating influence on macroscopic charge transport.21 Interestingly, when applying the model derived by Zhang using the maximum difference in nanofiber widths observed in this study (ca. 84 Å), we obtain calculated mobilities that are very close to the ones determined experimentally for HT98 (ca. 8.2 × 10−3 cm2 V−1 s−1 compared to an average experimental value of 1.77 × 10−2 cm2 V−1 s−1). Based on the variations in nanofiber widths (vide inf ra), although the model is unlikely to be a foolproof explanation for all the mobility data presented here, it does provide useful insight into the role of conjugation. Significantly, the role of intrachain ordering is not relegated to influencing intrachain transport alone, but it clearly influences the interchain hopping rate as well. In the absence of tests that probe intrinsic charge transport along a chain (i.e., a molecular wire), it is a substantial challenge
relative intensities between HT94 and HT98 are preserved (see the Supporting Information). The (100) peak appears relatively more intense in all the HT98 films, with the exception of the one obtained from BCl, indicative of better crystal packing within the nanofibers for the higher RR polymer. Although the (100) peaks in both polymer films obtained from CHCl3 solution remain weak, a difference in peak intensities between HT94 and HT98 is apparent in spite of the kinetically limiting film formation mechanism in the volatile solvent. In addition, we note that differences in line-widths of the diffractograms for both P3HT samples are minimal, suggesting that the increase in peak intensity for the HT98 samples can be ascribed primarily to an increased number of crystallites with marginal changes to the crystallite size. Subtle differences in RR can thus overcome the barriers to crystallization imposed by the rapid quenching of polymer chains. Increases in the crystallinity of films spin coated from xylene, MCB and BCl are evident, but as noted earlier, given the sharp increase in (100) peak intensity for HT98-MCB, followed by a dramatic drop in the case in the case of HT98-BCl, the effect cannot be reduced to that of solvent BP. Specifically, the (100) peak intensity is reproducibly the highest for HT98 films obtained from MCB and significantly higher than those obtained from BCl. Surprisingly, although HT98 films appear more crystalline than HT94 films in general, this is clearly not the case for the films obtained from BCl. Based upon the AFM results, it can be expected that wider nanofibrils in HT98 would result in a greater areal fraction of ordered structures, which is mirrored in the XRD peak intensities, but there appears to be no evidence for the anomalous behavior in the case of BCl. The physical origins of this behavior are not completely understood; solubility effects may be responsible for altering the internal ordering within the nanofibers, without inhibiting their tendency to form.
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DISCUSSION We have shown that a subtle difference in RR can induce changes at the single molecule level (i.e., conjugation); as well as morphological changes associated with assemblies of conjugated polymer chains (i.e., nanofibrils) on the one hand, and in the macroscopic electrical properties on the other. The 1 to 2 orders of magnitude increase in field effect mobility associated with wider nanofibrils in the higher RR HT98 films bears a resemblance to the effects of increasing MW, which thus forms an important precedent for the present results.20−23 Interestingly, the various hypotheses that have so far been proposed to explain the MW dependence of mobility can essentially be reduced to single chain effects,20−23 which can also be expected from variations in RR. As noted, the effect on mobility is manifest as either elongated chains that bridge ordered domains,22 an extended conjugated state that may have important consequences for the hopping rate of charge carriers,21 or simply through improved crystal packing that supports enhanced polaron delocalization between chains.20 The results discussed here also imply that the origin of the improvement in charge transport properties can be assigned to an improvement in any one (or combinations) of intrachain, interchain, and intergrain charge transport. Although we concede that, given the finite molecular dimensions of the polymer chains, coupled with the fact that the conjugation is actually limited to shorter oligomeric segments (most likely because of conformational disorder), intrachain charge transport is highly unlikely to dominate the movement of carriers over tens of micrometers. Nevertheless, 2851
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to isolate the role of intrachain order. This challenge is compounded by the fact that the improvement in single chain order simultaneously results in supramolecular assemblies with improved crystal quality,20 which in turn would impact the interchain transfer integral.4 The OOP GIXD data reflect this apparent improvement in crystalline order in HT98 films, characterized by the higher degree of lamellar packing as well as a narrowing of the absorption bandwidth. However, this result alone cannot form a holistic explanation for the differences in mobility observed between HT94 and HT98 films. For example, the large difference in (100) peak intensity, along with a concomitant increase in mobility from 5.2 ± 1.79 × 10−4 cm2 V−1 s−1 to 2.3 ± 1.37 × 10−2 cm2 V−1 s−1 for HT94 and HT98 films obtained from MCB would form a strong case for the degree of crystallinity being a dominant influence on the transport of charges. However, results from films obtained from other solvents clearly argue against such an assessment, thereby justifying the use of different solvents to help clarify the microstructure−charge transport correlation. For instance, in the case of films obtained from CHCl3 (μHT94 = 3.3 ± 2.3 × 10−3 cm2 V−1 s−1 while μHT98 = 1.6 ± 0.7 × 10−2 cm2 V−1 s−1) and BCl (μHT94 = 4.9 ± 1.9 × 10−4 cm2 V−1 s−1 while μHT98 = 2.1 ± 0.4 × 10−2 cm2 V−1 s−1), the differences in mobility between HT94 and HT98 OFETs are clearly incommensurate with the changes registered by the respective diffractograms. This is especially apparent for films obtained from BCl, where the difference in (100) peak intensities between HT94 and HT98 films are almost negligible. In addition, the weak and broad peak in the case of HT94 films obtained from CHCl3 would be completely unexpected given that these films routinely exhibit the highest mobility among HT94 OFETs. This observation is consistent with results that demonstrate an intense (100) peak in low MW P3HT films compared to a relatively weak peak in the high MW variant, although the latter, remarkably, has a mobility that is 4 orders of magnitude higher.22 In a separate study, we identified two distinct regions of charge transport in P3HT films, characterized by an initial sharp, 2 orders of magnitude increase due to an increase in crystallinity up to the percolation limit, followed by a gradual saturation where the mobility becomes independent of the degree of crystallinity.16 This result is further evidence that film crystallinity is clearly not a dominant factor in dictating charge transport. Other factors, such as structural anisotropy and textural variations are likely to influence charge transport as well.20 The RR of P3HT has been purported to influence the texture of the corresponding films, with 81% and 96% RR P3HT adopting so-called “face-on” and “edge-on” orientations respectively, with the latter configuration being more favorable for in-plane charge transport.18 Although the AFM phase images do not directly reflect the crystallinity of the films, they do provide contrast between the nanofibrils, which we suggest are the ordered segments, and the amorphous grain boundaries surrounding them. The wider nanofibrils obtained from HT98 films should thus result in fewer grain boundaries that are known to impede charge transport,56 while simultaneously improving the crystallinity of the film. Macroscopic charge transport would then require charges to travel across regions of low (grain boundaries) and relatively high (nanofibers) conductivity, with the effective mobility a simple harmonic mean of the mobilities within the grains and at the grain boundaries.56
The results presented here also raise critical questions about the role of conjugation in macromolecular systems. Although Spano’s model predicts, albeit indirectly, improved packing motifs in HT98 P3HT (motivated by enhanced intrachain order) with higher interchain transfer integrals,33 recent theoretical analysis conducted by Gierschner et al. has shown that the transfer integrals in polymeric systems are actually near independent of the conjugation length,57 in apparent contradiction with the above argument. The implication, then, is that the enhanced interchain hopping is a consequence of reduced reorganization energies afforded by the extended conjugation in HT98 chains. Thus, we suggest that the dramatic difference in field effect mobility observed as a result of the minor increase in RR can be narrowed to two primary effects: (i) an increase in hopping rate due to reduced reorganization energies predicted by Marcus theory and (ii) a reduction in the number of grain boundaries as a result of wider nanofibrils. In essence, both effects are a direct consequence of enhanced conjugation and are thus strongly coupled with the RR of the polymer.
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CONCLUSION We have demonstrated that changes in H−T attachments of hexyl side chains to a polythiophene backbone by as little as 4% can have a dramatic impact on the field effect mobility in P3HT thin films. The effect of solvent, BP in particular, is found to be minimal in comparison to that of RR, which has an overriding influence on the microstructure and the field effect mobility. More importantly, RR provides an effective handle for interrogating single chain effects, which can be multifaceted, on the structural and electronic properties of supramolecular assemblies. Thus, a fundamental understanding of polymer chain conformation will not only provide avenues for maximizing charge transport in polymeric semiconductors, but must also be considered carefully in the design of the next generation of polymeric semiconductors. In addition, the correlations we have reported may be useful in advancing our still evolving understanding of the complex correlations between microstructure and macroscopic charge transport in conjugated polymer systems.
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ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Present Address
∥ Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology, Daegu, Korea
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded in part by the Center for Organic Photonics and Electronics (COPE), Georgia Tech, and by the STC Program of the National Science Foundation (DMR0120967). Also, the authors would like to thank Prof. Jean-Luc Bredas and Dr. Mariacristina Rumi for useful discussions.
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REFERENCES
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dx.doi.org/10.1021/cm202700k | Chem. Mater. 2012, 24, 2845−2853
