Delocalisation softens polaron electronic transitions and vibrational modes in conjugated polymers

Kahmann, Simon; Loi, Maria Antonietta; Brabec, Christoph J.

doi:10.1039/c8tc00909k

Cited by 28 publications

(41 citation statements)

References 26 publications

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“…Similarly intensied IR bands have been observed in the polarons of doped conjugated polymers, 7,8 and mixed-valence coordination complexes, 9 where they are known as 'infrared active vibrations' (IRAVs). This terminology arose because they were attributed to infrared-activation of Raman modes by electric elds.…”

Section: Introductionmentioning

confidence: 86%

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Mechanisms of IR amplification in radical cation polarons

et al. 2020

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Break down of the Born-Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs). Here, we investigate the mechanism of this amplification, and show that it provides insights into intramolecular charge migration.Spectroelectrochemical time-resolved infrared (TRIR) and two-dimensional infrared (2D-IR) spectroscopies were used to investigate the radical cations of two butadiyne-linked conjugated porphyrin oligomers, a linear dimer and a cyclic hexamer. The 2D-IR spectra reveal strong coupling between all the IRAVs and the electronic p-p* polaron band. Intramolecular vibrational energy redistribution (IVR) and vibrational relaxation occur within $0.1-7 ps. TRIR spectra show that the transient ground state bleach (GSB) and excited state absorption (ESA) signals have anisotropies of 0.31 AE 0.07 and 0.08 AE 0.04 for the linear dimer and cyclic hexamer cations, respectively. The small TRIR anisotropy for the cyclic hexamer radical cation indicates that the vibrationally excited polaron migrates round the nanoring on a time scale faster than the measurement, i.e. within 0.5 ps, at 298 K. Density functional theory (DFT) calculations qualitatively reproduce the emergence of the IRAVs. The first singlet (S 1 ) excited states of the neutral porphyrin oligomers exhibit similar IRAVs to the radical cations, implying that the excitons have similar electronic structures to polarons. Our results show that IRAVs originate from the strong coupling of charge redistribution to nuclear motion, and from the similar energies of electronic and vibrational transitions.

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Section: Introductionmentioning

confidence: 86%

“…This terminology arose because they were attributed to infrared-activation of Raman modes by electric elds. 4,7,10 Their origin is attributed to two processes: coupling between charge redistribution and vibrational motion ( Fig. 2a), or mixing between low-lying electronic and vibrational transitions ( Fig.…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of IR amplification in radical cation polarons

et al. 2020

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“…As different processing conditions are known to induce different degrees of order, these polaron studies reveal trends in the nanoscale order that allow the local molecular order in the polymer to be directly related to its electronic properties. Previous work on polarons in semicrystalline polymers has mainly focused on classifying polarons as either “1D,” localized on a single chain, or “2D,” delocalized across multiple chains . These assignments are important because there are several regimes of localization and transport in semicrystalline macromolecular materials.…”

Section: Introductionmentioning

confidence: 99%

Unraveling the Effect of Conformational and Electronic Disorder in the Charge Transport Processes of Semiconducting Polymers

Chew

Ghosh

Pakhnyuk

et al. 2018

Adv Funct Materials

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Charge transport in semiconducting polymers is inextricably linked to their microstructure, making the characterization of polymer morphology at all length-scales essential for understanding the factors that limit mobility in these materials. Indeed, charge transport depends both on the ability of polarons to delocalize at the approximately nanometer length-scale and navigate a complex energetic and morphological mesoscale landscape. While characterization of the mesoscale morphology of polymers is wellestablished, studies of the local chain packing and nanoscale disorder, which affect delocalization, can be significantly more difficult to carry out. Through infrared charge modulation spectroscopy and theoretical modeling, the effect of the local chain environment on polaron delocalization is directly measured and quantified. Using a series of polymers based on the model system, poly(3-hexylthiophene), the link between disorder and polaron localization is systematically explored. Polaron delocalization is correlated with known trends in mobility, revealing that while charge delocalization is always beneficial, the formation of tie-chains is necessary to reach the highest mobilities in semicrystalline polymers. The results provide direct evidence for the importance of both nanoscale (charge carrier delocalization) and mesoscale (tie-chains) orders, demonstrating the need to distinguish the key length-scale limiting charge transport in the design of new, high mobility polymers.

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“…We first studied the mid‐infrared (MIR) spectral region using a modified FTIR spectrometer . We performed measurements at energies as low as 0.08 eV, i.e., the region characteristic of low energy polaron transitions as well as signatures due to molecular vibrations of the polymer backbone (infrared active vibrations; IRAVs) that may be used as material specific fingerprints . PTB7‐Th:PC 70 BM, exhibits a broad absorption feature between ≈0.1–0.4 eV, which is superimposed with IRAVs and two broader dips around 0.2 eV (Figure S5a, green, Supporting Information), which we attribute to the P1 transition of the polymer polaron.…”

Section: Resultsmentioning

confidence: 99%

Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

Gasparini

Kahmann

Salvador

et al. 2019

Advanced Energy Materials

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Ternary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.

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Delocalisation softens polaron electronic transitions and vibrational modes in conjugated polymers

Abstract: The delocalisation of polarons in conjugated polymers strongly impacts on optical transition energies in the infrared spectral region.

Cited by 28 publications

References 26 publications

Mechanisms of IR amplification in radical cation polarons

Mechanisms of IR amplification in radical cation polarons

Unraveling the Effect of Conformational and Electronic Disorder in the Charge Transport Processes of Semiconducting Polymers

Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

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