Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3326−3332
pubs.acs.org/JPCL
Coherent Charge Transfer Exciton Formation in Regioregular P3HT: A Quantum Dynamical Study Wjatscheslaw Popp, Matthias Polkehn, Robert Binder, and Irene Burghardt* Institute of Physical and Theoretical Chemistry, Goethe University Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
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S Supporting Information *
ABSTRACT: The ultrafast formation of charge transfer excitons (CTXs) in regioregular poly(3-hexyl thiophene) (rrP3HT) domains is elucidated by electronic structure and quantum dynamical studies of an aggregate model system comprising five stacked quaterthiophene units. Using a multistate vibronic coupling Hamiltonian parametrized by TDDFT calculations for 13 electronic states of Frenkel and CTX type, along with 78 vibrational modes, quantum dynamical simulations are carried out using the Multi-Layer MultiConfiguration Time-Dependent Hartree (ML-MCTDH) method. In line with time-resolved spectroscopic results [De Sio, A.; et al. Nat. Commun. 2016, 7, 13742], it is found that CTX formation occurs immediately upon photoexcitation, accompanied by sustained regular oscillations with a ∼22 fs periodicity. These coherent features, whose presence may seem surprising in a high-dimensional aggregate or thin film material, can be traced back to a dominant vibronic signature of CC stretch-type high-frequency modes. These vibrational signatures are found to be enhanced due to a collective vibronic response that is prompted by the initial generation of a delocalized bright exciton and its subsequent relaxation, by internal conversion, to a polaronic local exciton ground state. vibrational coherence, with a dephasing time of ∼250 fs, was inferred from 2D electronic spectroscopy of P3HT films. However, these observations leave open the rationale for the special role of a single dominant high-frequency mode in a molecular thin film material comprising many electronic states and a large number of vibrational degrees of freedom. Furthermore, one cannot rule out the possibility that the ultrafast oscillations are excitonic beatings, given the observed CTX generation time of 20 fs or less, or else combined vibrational−excitonic (vibronic) beatings as attributed to biological exciton systems.14,15 Against this background, the present study seeks to develop a comprehensive quantum dynamical picture of the ultrafast vibronic dynamics in a representative fragment of rrP3HT, composed of five stacked quaterthiophene units, denoted (OT4)5 in the following; see Figure 1. The (OT4)5 pentamer represents a minimal system size permitting a realistic description of spatially extended excitons in a typical oligothiophene H-aggregate.16 For this model system, an electronic structure analysis relying on time-dependent density functional theory (TDDFT) in conjunction with a suitable diabatization scheme13,17 is used to generate a multistate linear vibronic coupling (LVC) Hamiltonian, similar to the Hamiltonians discussed in refs 13 and 18. Vibronic couplings are obtained from excited-state Franck−Condon (FC)
P
oly-3-hexylthiophene (P3HT) plays a key role as a donor material in organic photovoltaics,1−5 which has been employed in prototypical donor−acceptor blends with the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and also in combination with alternative acceptor species like perylene diimide (PDI)6 or in hybrid organic− inorganic heterojunctions exemplified by P3HT/ZnO.7 A noteworthy fact is that in lamellar, regioregular (rrP3HT) domains4,5 charge transfer excitons (CTXs) or polarons are observed,8−10 likely due to photoinduced interchain polaron pair formation that occurs even in the absence of an acceptor species. This raises the question whether the presence of CTX polarons affects the yield of charge-separated species between donor and acceptor domains and, hence, the power conversion efficiency. Recent spectroscopic8−12 and theoretical13 work contributes to answering these questions from a molecularlevel perspective. The present Letter focuses specifically on the ultrafast formation step of the CTX species in rrP3HT-type materials. In the time-resolved spectroscopic observations of rrP3HT thin films reported in ref 10, an extremely short time scale of 30 fs) are less pronounced, and the system remains in a coherent superposition state. This is likely due to the fact that energy dissipation is not complete within the observation interval. The regular oscillations observed in the integrated diabatic populations of panel (a) are also visible in the adiabatic populations, but to a lesser extent. In a complementary representation, Figure 3 shows the time dependence of the XT and CTX populations with the spatial 3330
DOI: 10.1021/acs.jpclett.9b01105 J. Phys. Chem. Lett. 2019, 10, 3326−3332
Letter
The Journal of Physical Chemistry Letters
ena that attract increasing attention in atomic and molecular quantum nanotechnology.35
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01105.
Figure 4. (a) Fourier transform of the integrated CTX state populations of Figure 2a (blue) and Fourier transform of the corresponding purely electronic dynamics (gray). (b) SD of Figure 1d, with a scaled representation of the lower-frequency part. The correspondence between the spectral features of the upper and lower panels is emphasized by red dashed lines. Both the dominant highfrequency spectral features and the lower-amplitude features are in good correspondence with the observations of ref 10; see especially Figure 2b,c of this reference.
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Details regarding the parametrization of the model Hamiltonian, the setup of the multiconfigurational (MLMCTDH) calculations, supplementary dynamical simulation results, and detailed considerations regarding the size dependence of the exciton dynamics (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Irene Burghardt: 0000-0002-9727-9049 Notes
The authors declare no competing financial interest.
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Additional, more approximate simulations for larger (OT4)n aggregates, i.e., (OT4)7 (19 states, 114 effective modes), (OT4)9 (25 states, 150 effective modes), and (OT4)11 (31 states, 186 effective modes), are reported in the Supporting Information (see section S4) to ascertain that the dynamics that we analyzed for (OT4)5 is representative of larger aggregates. We found that both the bright-state excitation pattern and the relaxation toward the lowest-lying exciton are similar (see Figure S17). The extremely rapid internal conversion dynamics within 20−30 fs (see Figures 2 and S18) is shared by all systems, even though the large aggregates tend to exhibit a longer period of S1/S2 mixing before the exciton fully relaxes toward the center of the lattice (see Figures S19−S21). Presumably for the same reasons, the asymptotic exciton delocalization ranges from 3 to 5 units (see Table S8). These findings are in line with combined experimental and theoretical analyses that report on delocalization lengths of around 3 units in rrP3HT.33,34 To summarize, our analysis is in excellent agreement with the experimental observations of refs 10 and 11, both regarding the time scale of CTX formation and the presence of sustained high-frequency oscillations. Our first-principles analysis shows that the photogenerated state as such corresponds to a XT/ CTX superposition, representing the high-lying bright state of the modified H-aggregate with a strong CTX admixture. The subsequent vibronic dynamics triggers an ultrafast internal conversion process via a cascade of nonadiabatic curve crossings, leading to the local exciton ground state of the aggregate. The present analysis paves the way toward a better understanding of the role of polaronic CTX species in the charge separation at donor−acceptor interfaces and establishing whether CTX polarons appear as a loss channel or promote interfacial charge separation.13 As highlighted above, the CTX formation dynamics is mainly driven by a small subset of high-frequency CC stretch modes, which play a prominent role in the SD and whose vibronic coupling effects are synchronized and enhanced by excitonic delocalization. The emergence of collective electron− phonon coupling in extended systems and the resulting dissipation and dephasing effects are indeed general phenom-
ACKNOWLEDGMENTS We thank Hiroyuki Tamura, Rocco Martinazzo, and Keith Hughes for valuable discussions. Funding by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Project BU-1032-2 is gratefully acknowledged.
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REFERENCES
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DOI: 10.1021/acs.jpclett.9b01105 J. Phys. Chem. Lett. 2019, 10, 3326−3332
