Characterizing the Polymer:Fullerene Intermolecular Interactions

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Characterizing the Polymer:Fullerene Intermolecular Interactions Sean Sweetnam, Koen Vandewal, Eunkyung Cho, Chad Risko, Veaceslav Coropceanu, Alberto Salleo, Jean-Luc Bredas, and Michael D. McGehee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03378 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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Chemistry of Materials

Sean Sweetnam,1 Koen Vandewal,1,2 Eunkyung Cho,3,4 Chad Risko,5 Veaceslav Coropceanu,3 Alberto Salleo,1 Jean-Luc Brédas,3,6 Michael D. McGehee1 1. Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. 2. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01069 Dresden, Germany. 3. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, USA. 4. Materials & Devices Advanced Research Institute, LG Electronics, 38 Baumoe-ro, Seocho-gu, Seoul, 137-724, Korea 5. Department of Chemistry and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506-0055, United States. 6. Solar and Photovoltaics Engineering Research Center, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. ABSTRACT Polymer:fullerene solar cells depend heavily on the electronic coupling of the polymer and fullerene molecular species from which they are composed. The intermolecular interaction between the polymer and fullerene tends to be strong in efficient photovoltaic systems, as evidenced by efficient charge transfer processes and by large changes in the energetics of the polymer and fullerene when they are molecularly mixed. Despite the clear presence of these strong intermolecular interactions between the polymer and fullerene, there is not consensus on the nature of these interactions. In this work, we use a combination of Raman spectroscopy, charge transfer state absorption, and density functional theory simulations to show that the intermolecular interactions do not appear to be caused by ground state charge transfer between the polymer and fullerene. We conclude that these intermolecular interactions are primarily van der Waals in nature.

INTRODUCTION Molecular mixing of semiconducting polymers and fullerene derivatives results in significant changes in the position of the polymer energy levels, shifting the polymer valence band away from vacuum.1 Such an energy level shift occurs whenever molecular mixing occurs, regardless of polymer or fullerene structure. These shifts tend to be large, ranging in magnitude from 100 meV to 350 meV. Because of their ubiquity and large magnitude, it is important to understand the physical mechanism that drives these energy level shifts. At the molecular interface, two mechanisms are likely to describe the intermolecular interaction responsible for a shift in frontier energy levels upon mixing (Figure 1). One mechanism is based on partial ground-state charge transfer (GSCT) from the polymer to the fullerene via occupation of states in the electronic gaps of the organic materials.2–5 A second mechanism proposes that the energy levels shifts are caused by mutual polarization of the molecules due to electrostatic interactions between the

polymer and fullerene.6–12 A prominent example of such electrostatic interactions is the quadrupole-induced dipole interaction, such as that observed between pentacene, which possesses a permanent quadrupole moment, and fullerene molecules, which are polarized relatively easily.13–15 In this work we investigate the hypothesis that the intermolecular interaction is caused by GSCT from the polymer to the fullerene following three approaches. First, using Raman spectroscopy, a technique sensitive to molecular GSCT,16,17 we compare the vibrational spectra of semiconducting polymers in the absence and presence of fullerene derivatives to probe for the presence of GSCT between the molecular species. Second, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are performed to explore the Raman modes and quantify the degree of GSCT expected in polymer:fullerene blends. Third, we quantify the GSCT between the polymer and fullerene by measuring the optical absorption of the charge transfer complex (CTC) formed between the two molecular species. Using these three

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van der Waals Isolated molecules

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Polymer monomer Fullerene

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Figure 1: Cartoon depicting the two polymer:fullerene intermolecular interaction mechanisms discussed in this work. When ground state charge transfer dominate the intermolecular interactions, electron density is donated from the polymer to the fullerene. The reduction in polymer electron density deepens the polymer valence band and decreases the vibrational frequencies of polymer bonds. When van der Waals interactions dominate intermolecular interactions, the electron density of the polymer is redistributed over the polymer monomer in response to the change in its electrostatic environment caused by the presence of a fullerene. The redistribution of the electron density will deepen the polymer valence band, but will not strongly impact vibrational frequencies as the monomer electron density has not been reduced, only redistributed. This work finds no significant evidence of ground state charge transfer, indicating van der Waals interactions dominate the polymer:fullerene intermolecular interaction.

approaches to quantify the degree of GSCT, we find little to no GSCT occurs between polymer and fullerene. We thus conclude that electrostatic interactions between the polymer and fullerene are responsible for the energy level shifts caused by the mixing of polymers and fullerenes. RESULTS AND DISCUSSION RAMAN SPECTROSCOPY OF POLYMER:FULLERENE BLENDS Vibrational spectroscopy techniques such as Raman spectroscopy can be used to probe for the presence of GSCT in molecular blends by measuring changes in the frequency of the molecular vibrational modes between pure and blended systems. In the case where charge transfer absorption bands absorb strongly, for example small molecule charge transfer complexes like TTFTCNQ,18 resonant Raman conditions can be used to preferentially excite the CTC. In the case where molecular disorder is high and charge transfer is weak, as is typically encountered in polymer:fullerene blends, resonant Raman condition cannot be expected to isolate the weak CT transitions which lie near the band edge and may be confounded by overlapping molecular absorption form the pristine materials. In these cases, and in this work, nonresonant Raman is used instead to characterize the CTC by probing at subgap wavelengths where the pure molecular transitions are less likely to confound analysis. In both resonant and non-resonant conditions, the molecular vibrations of the polymer and fullerene can be modeled as harmonic oscillators, with the frequency of a given molecular vibrational mode varying as the square root of the bond force constant. The bond force constant of a molecular bond will depend on the electron density associated with the molecular bond. If electron density is removed from that molecular bond the bond force constant will decrease in magnitude, resulting in a reduction in the vibrational frequency of the molecular bond.16,17 In a GSCT process, where electron density is transferred from the donor species to the acceptor species, it is expected that some or all of the donor bond force constants will be reduced as the donor loses electron density, and

the frequency of the vibrational modes associated with these bonds will be reduced. In the case of polymer:fullerene blends, the polymer species is expected to donate electron density to the fullerene acceptor species, so a decrease in the frequencies of the polymer vibrational modes is expected. However, the interpretation of changes in molecular vibrational mode frequencies is not straightforward as any process that reduces the electron density of a molecular bond will also change the bond force constant. A relevant example for semiconducting polymers would be a reduction in molecular bond electron density due to increased electron wavefunction delocalization along the polymer backbone resulting from increased conjugation length of the polymer. We begin with polymer:fullerene blends comprised of the well known polymers regioregular (RR) and regiorandom (RRa) poly(3-hexylthiophene-2,5-diyl) (P3HT), and the widely used fullerene derivative [6,6]phenyl C71 butyric acid methyl ester (PC70BM). Cyclic voltammetry (CV) reveals that the addition of PC70BM to RR-P3HT and RRa-P3HT causes the polymer valence band to shift away from vacuum by ~150-350 meV.1 This shift occurred only in the amorphous polymer phase, i.e. in the phase where P3HT and PC70BM mix,19 indicating there are intermolecular interactions occurring at the polymer:fullerene molecular interface. Figure 2 shows the Raman spectrum of the main vibrational modes of films of pure RR-P3HT and a 1:1 wt:wt RRP3HT:PC70BM blend. The peak at ~1445 cm-1 is assigned to the symmetric C=C stretch mode, and the peak at ~1381 cm-1 is assigned to the C-C intraring stretch mode.20 It has been previously found that p-doping of P3HT with FeCl3 causes the peak at 1445 cm-1 to shift to lower frequencies,20 consistent with the observed behavior of other doped molecular blends.16,17 However, addition of PC70BM causes no change in the position of the Raman peak at 1445 cm-1, suggesting no GSCT occurs in the RR-P3HT:PC70BM blend. The Raman spectra of both RR-P3HT and RRP3HT:PC70BM agrees well with other measurements of this system reported in literature.21


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Given that RR-P3HT is a semicrystalline polymer with both crystalline and amorphous domains, it seems possible that the dominant Raman signal could originate in the crystalline polymer phase, where no polymer:fullerene mixing occurs. To isolate the amorphous P3HT phase, RRa-P3HT, a chemically identical isomer of RR-P3HT with randomly placed sidechains, is used. The random placement of the sidechains causes RRa-P3HT to form only an amorphous phase, making it ideal for isolating the amorphous P3HT phase. 1.0

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Figure 2: Raman spectra of the main vibrational modes of RR-P3HT (black) and a 1:1 wt:wt RR-P3HT: PC70BM blend (red, dotted).

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Figure 3 shows the Raman spectra of the main vibrational modes of a pure RRa-P3HT film and a 1:1 wt:wt RRa-P3HT:PC70BM blend film. We observe a symmetric C=C stretch peak near ~1455 cm-1, and the C-C intra-ring stretch peaks has split into a double peak with peaks near ~1375 and 1390 cm-1. The RRa-P3HT spectra observed here again agree well with the literature.21 As observed in the case of RR-P3HT, addition of PC70BM does not cause any substantial change in the position of Raman peaks of RRa-

P3HT, again suggesting that no GSCT occurs, even when the mixed polymer:fullerene phase is isolated. Next, blends containing the polymer poly-(2,5-bis(3tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT) are characterized with Raman spectroscopy. pBTTT is capable of forming a crystalline mixed phase with some fullerene derivatives (e.g. PC70BM),22 but does not mix with some other fullerene derivatives (e.g. bisPC70BM).23,24 It has also been shown that the pBTTT valence band deepens only when mixing with the fullerene occurs, i.e. when PC70BM is blended with pBTTT, but not when bisPC70BM is blended with pBTTT. If GSCT is the mechanism that drives the intermolecular interactions, then a shift of the pBTTT Raman peaks to lower frequency in a pBTTT:PC70BM blend is expected, but no change in the position of the pBTTT peaks in a pBTTT:bisPC70BM blend should occur. The Raman spectra of neat pBTTT, pBTTT:PC70BM, and pBTTT:bisPC70BM (Figure 4) are thus compared to see whether blending with fullerenes, molecular mixing with fullerenes, or both, causes a change in the position of the pBTTT Raman peaks. The peaks are assigned to the thiophene C-C stretch (~1391 cm-1), the thienothiophene C=C stretch (~1420 cm-1), the inter-ring C-C stretch (~1465 cm-1), and the thiophene C=C stretch (~1489 cm-1).25 There is no change in the position of the pBTTT Raman peaks upon addition of bisPC70BM, which is consistent with expectations: even if GSCT were to occur when polymer:fullerene mixing occurs, no change is expected in the pBTTT Raman peaks in the pBTTT:bisPC70BM blend because pBTTT and bisPC70BM do not mix. However, addition of PC70BM to pBTTT causes all pBTTT Raman peaks to shift to lower frequency, possibly indicating GSCT between pBTTT and PC70BM. The presence of GSCT in pBTTT:PC70BM would be at odds with the lack of such shifts in the Raman spectra of P3HT:PC70BM blends. Both pBTTT and P3HT display deepened valence bands upon mixing with PC70BM, implying there is an intermolecular interaction between the polymer and fullerene. On the other hand, only pBTTT displays a change in the position of its Raman peaks when mixed with PC70BM. It is unlikely that one physical mechanism would produce the same response using one technique (CV), and a different response for a different technique (Raman), suggesting that the cause of the observed shift in the Raman peaks of pBTTT upon mixing with PC70BM is not due to GSCT, but is due to some other mechanism. This discrepancy between the CV and Raman spectroscopic results can be explained by considering the absorption spectrum of pBTTT, pBTTT:PC70BM, and pBTTT:bisPC70BM (Figure 5). Both pure pBTTT and pBTTT:bisPC70BM have broad, relatively unstructured absorption spectra, suggesting the pBTTT is not highly ordered in these samples. In contrast, the pBTTT:PC70BM absorption spectrum displays several sharp vibronic absorption features that are characteristic of well ordered polymers.26,27 From these absorption measurements we


Chemistry of Materials can conclude the pBTTT in pBTTT:PC70BM is more ordered than it is in either pure pBTTT or pBTTT:bisPC70BM, which will result in a more delocalized wavefunction. Increased delocalization of the wavefunction i.e. a longer conjugation pathway along the pBTTT backbone, results in a reduction in the molecular bond force constant, causing a decrease in the frequency of the associated molecular vibrational mode. The most reasonable explanation for the change in the frequency of the pBTTT vibrational modes upon mixing with PC70BM is not GSCT, but rather increased electron wavefunction delocalization due to improved polymer ordering in the blend. This explanation is consistent with the conclusions of recent work by Gao et al,25 who conclude that the formation of the polymer:fullerene bimolecular crystal increases the polymer ordering. pBTTT

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Figure 4: Raman spectra of the main vibrational modes of films of pure pBTTT (black), pBTTT:PC70BM (red, dotted), and pBTTT:bisPC70BM (blue, dashed).

Furthermore, CTC formation in semiconducting molecules is expected to have a more pronounced impact on the Raman spectra of the molecular. For example, large changes in relative intensity of Raman peaks are often observed in conjunction with the formation of CTCs in polymer:small molecule blends.28,29 The absence of radical changes in Raman peak intensity is further evidence that no significant GSCT occurs in P3HT:PCBM or pBTTT:PCBM blends.To confirm the hypothesis that the observed Raman peak shifts in pBTTT upon addition of PCBM are due to improved polymer ordering and not GSCT, the Raman spectra of pBTTT in planar, twisted, and cation conformations, were determined through DFT calculations. Figure 6 shows the simulated Raman spectra of an isolated BTTT dimer (i.e. an oligomer consisting of two BTTT monomers) in these conformations. The planarization of the pBTTT backbone leads to a marked decrease in the frequency of several of the vibrational modes. This shift in the simulated pBTTT Raman peaks is consistent with shift in pBTTT Raman peaks observed experimentally, and supports the conclusion that the observed shift in pBTTT Raman peaks is due to a change in the delocalization of the electron wavefunction along the polymer backbone due to improved polymer ordering in the polymer:fullerene blend. In addition, the Raman spectra of the polymer cation (i.e. a polymer which has donated one electron which serves as a model of a polymer in a strongly coupled CTC) shows dramatic change, with the peak near 1500 cm-1 disappearing entirely. Such a change is consistent with our earlier comment that GSCT is often accompanied by large changes in Raman spectra. The lack of significant decrease in the 1500 cm-1 peak in the experimentally measured pBTTT:PCBM blend Raman spectra is consistent with an absence of strong GSCT.

dimer-twisted dimer-planar dimer-cation

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Figure 5: Scaled absorption spectra of thin films of pBTTT (blue, solid), pBTTT:PC70BM (red, dashed) and pBTTT:bisPC70BM (black, dotted). Linear extrapolations of absorption edges are indicated with colored lines and are used to estimate polymer bandgap in each film. Bandgap estimates are listed next to the figure legend.

Figure 6: Simulated Raman spectra of a BTTT dimer in twisted (black, solid),planar (red, dashed), and cation (blue, dotted) conformations.

Calculation of the Degree of Charge Transfer from the Charge Transfer Molar Extinction Coefficient


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It is also possible to estimate the degree of charge transfer by directly measuring the molar extinction coefficient of a CTC in a polymer:fullerene blend. CTC absorption involves direct excitation of an electron from the polymer valence band into the PC70BM conduction band. The molar extinction coefficient can be used to determine the transition dipole moment M of the CTC, and can in turn be related to the degree of GSCT by

(1) where M is the transition dipole moment of the CTC and Δμ is the change in the dipole moment of the diabatic (non-interacting) states involved in the electron transfer. However, this analysis requires a system with a known density of CTC states and a known value of Δμ. The crystalline unit cell of pBTTT:PC70BM has been determined previously,30 allowing us to calculate the CTC density and Δμ for this system. The unit cell of pBTTT:PC70BM has dimensions a = 31.2 Å, b = 9.8 Å, c = 13.5 Å, α = 108°, and β = γ = 89°, and has two polymer monomers and two PC70BM molecules per unit cell. The unit cell volume of a monoclinic unit cell is given by

(2) which yields a unit cell volume for pBTTT:PC70BM of 3.924*10-27 m3. There is one CTC for each polymerfullerene pair yielding two CTCs per unit cell, resulting in a CTC density of . 100000 10000 1000

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Figure 7: Molar extinction coefficient of pBTTT:PC70BM blend obtained by scaling subgap absorption obtained from PDS (black) to above gap absorption of pBTTT:PC70BM (blue) and applying equation (4). A Gaussian fit (dashed red) is used to model the CTC absorption band.

The molar extinction coefficient of the CTC can be determined by measuring the CTC absorption spectrum in a polymer:fullerene blend sample. Measuring CTC absorption in thin films is challenging, due to its low absorption coefficient as compared to the absorption of the neat donor and acceptor materials. We therefore use photothermal deflection spectroscopy (PDS) which is sufficiently sensitive to detect CT absorption bands. PDS, however only provides a relative measurement of absorption spectrum, and is therefore scaled to absolute absorp-

tion units by matching the PDS spectrum in the strongly absorbing region with an integrating sphere UV-VIS transmission and reflection measurement. Figure 7 shows the absorption profile of pBTTT:PC70BM created when the PDS spectrum is scaled to fit the tail of an absorption spectrum. This low energy tail cannot be due to absorption by either the pure polymer or fullerene species, which have absorption coefficients approximately 100 times weaker than that of the blend in the sub-bandgap region,31 and we therefore attribute the low energy tail to the CT complex. The molar extinction coefficient can be related to the absorption through the equation

(3) (4) where A is the sample absorbance, α is the sample absorption coefficient, ε is the molar extinction coefficient, and d is the sample thickness. Next, it is possible to relate the molar extinction coefficient ε to the transition dipole moment M through the equation32

(5) where v is the frequency of light, in cm-1, vmax the frequency at which the absorption maximum occurs, ε is in M-1 cm-1 and M in Debye (D). Integrating over the Gaussian fit of the CT band yields a value of 1.48D for M. For comparison, TD-DFT calculations were carried out to determine the transition dipole moment M and the change in the state dipole moment between the ground and charge-transfer state Δµ for a BTTT trimer and PC70BM complex. The results of natural transition obitals analysis reveal that the prominent CT states of the complex are 1st, 2nd, 4th excited states; combining the respective M’s of these transitions leads to a total CT M of 1.6D. The obtained experimental and theoretical values are in the good agreement. We turn now to the estimation of Δμ. A simple way to estimate this parameter is based on geometric considerations. By taking the center-to-center separation (d=7.2Å) of a pBTTT monomer and a PC70BM molecule in the pBTTT:PC70BM bimolecular crystal as electron transfer distance and assuming a full charge transfer the dibatic change in the dipole moment can be computed as Δμ = qd that yields of approximately 33.6 D for Δμ. A more accurate way to derive Δμ is to express it in terms of related adiabatic states, i.e. the ground state of pBTTT:PC70BM and the CT state. According to Cave and Newton, Δμ can be written as33


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Here is the difference between the adiabatic dipole moments of the system ground state and the CT state. Experimental values of are, in principle, accessible through Stark spectroscopy.34 was obtained using both the finite-field method through TD-DFT calculations in the presence of a uniform external electric field,35 and from the excited state density. The calculated ΔM values from the finite-field method and from the excited state density are 20 D and 27 D, respectively. Using these values and the experimental and TD-DFT values of M we estimate a range of 20.2 D-27.2 D for These values are comparable with the estimated value based on the geometric approach. Finally, application of (1) yields a range of possible degrees of charge transfer of 10-4-10-2 electrons. These degrees of GSCT are small, and the CTC formed by pBTTT:PCBM is too weak to be responsible for the observed shifts in polymer energy levels observed in CV measurements.1 This measurably small degree of charge transfer is, in our minds, compelling evidence that no significant GSCT occurs in the pBTTT:PCBM blend. this observation is consistent with the analysis of the Raman spectra of pBTTT:PCBM and P3HT:PCBM blends. CONCLUSION The polymer:fullerene intermolecular interactions play an important role in the function and formation of a polymer:fullerene bulk heterojunction solar cell. They drive the formation of the mixed polymer:fullerene phase by stabilizing the energy of the system, and can also impact the energetic landscape of the solar cell, influencing charge carrier transport and recombination. Understanding the nature of the polymer:fullerene interactions is thus important. The results of both experimental and computational characterizations in this work indicate that there is no significant GSCT from the polymer to the fullerene in the polymer:fullerene blends studied here. We must therefore conclude that the physical mechanism behind the intermolecular interactions is not GSCT. This conclusion agrees with the literature on the subject of intermolecular interactions at the donor:fullerene interface, which finds little evidence of charge transfer13,14,36 with few exceptions.37,38 These works instead attribute the intermolecular interactions to electrostatic interactions between the donor and fullerene,39 namely the interaction of induced and permanent dipoles, quadrupoles, and higherorder multipoles present in the donor species, and permanent and induced dipoles in fullerene derivatives. We therefore conclude that the polymer:fullerene intermolecular interactions have primarily a van der Waals nature. With an enhanced understanding of the polymer:fullerene intermolecular interaction it should be possible to utilize the intermolecular interactions to inform future molecular and device design, enabling chemists to design molecular species to leverage the polymer:fullerene interactions to tune bulk heterojunction morphologies, optimize the energetic landscape, and

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much more, taking organic semiconducting devices to the next level of performance. ACKNOWLEDGEMENTS

This work was supported by the Department of the Navy, Office of Naval Research Award No. N00014-14-10580, and by ONR Global, Grant N62909-15-1-2003. S.S. acknowledges support from the National Science Foundation through the National Science Foundation Graduate Research Fellowship under Grant No. DGE-114747, and support from Stanford University through a Benchmark Stanford Graduate Fellowship. EXPERIMENTAL RAMAN MEASUREMENTS Raman samples were fabricated on quartz slides which were sonicated in a 1:9 extran:deionized water solution, then acetone, then isopropanol. The quartz slides were exposed to a UV-ozone plasma for 10 minutes prior to sample deposition to improve wetting of the polymer:fullerene blend on the quartz. Thick films were required for the Raman measurements due to the small scattering cross section of non-resonant Raman processes, so Raman samples were fabricated by drop casting to obtain films with thicknesses on the order of um-100um. Solution concentrations for the blends were 5 mg/ml polymer concentration with a 4:6 polymer:fullerene weight:weight ratio. pBTTT was provided by the group of Martin Heeney. pBTTT, 4:6 wt:wt pBTTT:PC60BM and 4:6 wt:wt pBTTT:bisPC70BM solutions were prepared in DCB at polymer concentrations of 5 mg/ml and were heated and stirred overnight at 70 °C. For pBTTT and PC70BM/bisPC70BM 4:6 weight:weight corresponds roughly to 1:1 molar:molar, the stoichiometry for pure bimolecular crystal in the case of pBTTT:PC70BM. pBTTT/pBTTT:PC70BM/pBTTT:bisPC70BM films were annealed at 180 °C for 10 minutes to ensure complete intercalation occurred. Raman measurements were performed on a Vertex 70 FTIR with RAM II Raman accessory using 1064 nm light. In order to obtain improved signal to noise ratios, a mirror was placed behind the sample to increase the optical path length of light through the sample. In the blend samples, a strong background was often observed which was removed prior to analysis. The shape and position of this feature suggest the large background is due to emission by the charge transfer complex, whose emission energy is in the same range of energy as the 1064 nm excitation source and can thus be registered as Raman scattering. MEASUREMENT COEFFICIENT

OF

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Reprinted with permission from 40 copyright 2014 Nature Publishing Group. PDS was performed using a homebuilt set-up: chopped (3.333 Hz) monochromated light from a 100 W QTH lamp is focused onto the sample. Perfluorohexane (C6F14, 3M Fluorinert FC-72) is used as the deflection medium. The deflection of a HeNe laser (633 nm) is detected by a position-sensitive Si detector, connected to a Stanford Research Systems SR830 lock-in amplifier. PDS samples of active layers were spin-coated


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on quartz substrates, The PDS spectra were set to absolute scale by matching the spectra with integrating sphere measurements on a Varian Cary 5000 spectrophotometer. DFT SIMULATIONS The geometry optimization, Raman, and dipole moment calculations were performed with DFT using the B3LYP/6-31G** hybrid functional. All DFT calculations were performed by means of the Gaussian 09 program.41


Chemistry of Materials Case of Pentacene vs TIPS-Pentacene. J. Am. Chem. Soc. 2014, 136, 6421–6427.

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Sweetnam, S.; Graham, K. R.; Ngongang Ndjawa, G. O.; Heumueller, T.; Bartelt, J. A.; Burke, T. M.; You, W.; Amassian, A.; McGehee, M. D. Characterization of the Polymer Energy Landscape in Polymer:fullerene Bulk Heterojunctions with Pure and Mixed Phases. J. Am. Chem. Soc. 2014, 136, 14078–14088.

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polymer:fullerene Interactions

pBTTT

pBTTT:PC₇₀BM

Raman Shift (cm-1)

10


Characterizing the Polymer:Fullerene Intermolecular Interactions

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