Light-assisted deep-trapping of holes in conjugated polymers

Bolinger, Josh C.; Fradkin, Leonid; Lee, Kwang-Jik; Palacios, Rodrigo E.; Barbara, Paul F.

doi:10.1073/pnas.0811900106

Cited by 27 publications

(36 citation statements)

References 37 publications

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“…It has been suggested that there are at least two main types of trap states in CPs:1 )shallow trap states, [16][17][18] in which the chargec an be highly mobile and can therefore access al arge amount of material, and 2) deep trap states, in which the charge is immobile and confined within as mall region of the material. [16][17][18][19][20] Fortunately,i np rinciple, the photon statistics obtained from as ingle CP chain contain the information required to differentiate between quenching events arising from either shallow or deep traps, as is outlined in the following.Photon statisticsa re typicallyu sed to count the number of independently emitting entities-fluorophores-present within the observation volumeo faconfocal fluorescencem icroscope. [21][22][23][24] Such counting is achieved by exploitingt he fact that as inglef luorophore can only emit one photon,t wo fluorophores up to two photons and so forth per excited state lifetime, that is, the PL lifetime.…”

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Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

et al. 2015

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Blinking of the photoluminescence (PL) emitted from individual conjugated polymer chains is one of the central observations made by single-molecule spectroscopy (SMS). Important information, for example regarding excitation energy transfer, can be extracted by evaluating dynamic quenching. However, the nature of trap states, which are responsible for PL quenching, often remains obscured. We present a detailed investigation of the photon statistics of single poly(3-hexylthiophene) (P3HT) chains obtained by SMS. The photon statistics provide a measure of the number and brightness of independently emitting areas on a single chain. These observables can be followed during blinking. A decrease in PL intensity is shown to be correlated with either 1) a decrease in the average brightness of the emitting sites; or 2) a decrease in the number of emitting regions. We attribute these phenomena to the formation of 1) shallow charge traps, which can weakly affect all emitting areas of a single chain at once; and 2) deep traps, which have a strong effect on small regions within the single chains.

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Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

et al. 2015

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“…It also allows predictions of the absolute ionization potentials (IP) of both polaronic localized and free delocalized hole state in solids, revealing large errors in semilocal (sl) DFT functionals that to our knowledge have never been discussed in the literature. This successfully treatment of hole localization and delocalization properties will aid the understanding of charge trapping [8,9] and carrier mobility [10] that are important for organic electronics.A key prerequisite to such studies is the correct choice of DFT functionals. Previous theoretical studies reveal an extreme sensitivity of charge localization to theoretical approximations [6,7,[11][12][13][14].…”

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confidence: 99%

“…The gas phase ionization potentials of small molecules, arguably related to the energies of localized holes in solids, are however accurately predicted by even the semi-local functionals that we have considered. Our study may contribute to the understanding of possible mechanisms of deep-hole trap states [9] commonly observed in organic electronics, and may potentially be applicable to covalently bonded solids [6,7,[11][12][13].…”

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Hole Localization in Molecular Crystals from Hybrid Density Functional Theory

2011

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We use first-principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals form self-trapped molecular polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values.Charge localization in organic solids and interfaces has important implications for the functioning of organic devices [1, 2]. It has been long suggested that charge carriers in organic molecular crystals may self-trap to form localized small polarons [1][2][3][4] through interaction with the surrounding electron and lattice. Models based on the concept of polaron have led to theoretical understanding of charge transport of these materials (see e.g. [1,5]). On the other hand, direct evidence and atomic scale probe of localized charge states in organic crystals remain lacking. Ab initio computations have recently been summoned to provide accurate quantitative description of polaron state in covalent solids [6,7]. In this work, we apply hybrid density functional theory (DFT) with carefully calibrated admixtures of exact exchange to molecular crystals and conclusively demonstrate from first principles the existence of self-trapped hole polaron in defect-free molecular crystals. Our choice to work with small organic molecules permits systematic analysis of DFT exchange correlation (xc) functionals in both gas and solid phases. It also allows predictions of the absolute ionization potentials (IP) of both polaronic localized and free delocalized hole state in solids, revealing large errors in semilocal (sl) DFT functionals that to our knowledge have never been discussed in the literature. This successfully treatment of hole localization and delocalization properties will aid the understanding of charge trapping [8,9] and carrier mobility [10] that are important for organic electronics.A key prerequisite to such studies is the correct choice of DFT functionals. Previous theoretical studies reveal an extreme sensitivity of charge localization to theoretical approximations [6,7,[11][12][13][14]. Standard semilocal functionals for the xc energy, such as the local density approximation (LDA) and generalized gradient approximation (GGA), have been successful in predicting atomic structures and electronic properties of many molecular complexes and solid state materials. But they tend to fail in systems involving fractional charges, e.g. they dissociate molecular ion H + 2 into 2 H 0.5+ rather than into H + and H. The problem stems from self-interaction error (SIE) in the standard density functionals [15]. For a one electron system SIE is conveniently described as the incomplete cancellation of spurious sel...

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“…The amount of fluorescence quenching observed upon the injection of a single hole polaron into a polymer chain also varied with the molecular weight: 60% quenching for 150 kDa 8 , 40% quenching for 1000 kDa. 4,12 Point-spread function fitting techniques used to track nanometer-scale position changes of fluorophores 13 were combined with an electro-optical technique developed in the Barbara group 14 to produce bias-modulated intensity-centroid spectroscopy (BIC), used to monitor exciton migration in MEH-PPV as holes were reversibly injected into single polymer chains. 8 This study revealed remarkably long-range energy transfer in the chain, but also showed a curious change in direction of the fluorescence centroid shift when more than one hole was injected into the polymer.…”

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confidence: 99%

Direct measurement of energy transport in organic nanosystems

2014

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Conjugated polymers are appealing materials for cost-effective production of flexible electronic devices, but the efficiency of these devices may be compromised by exciton quenching by hole polarons. Exciton migration was monitored directly in single poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) chains embedded in a hole-injection device by applying super-resolution point-spread function fitting techniques to the fluorescence image of each chain as holes were reversibly injected into the polymer. Correlated excitation polarization spectroscopy techniques reported the orientation of the longitudinal axis of the rod-like polymers. The wide-field microscopy images are diffraction-limited in one dimension but slightly elongated in the direction of the longitudinal axis of the rod-like polymer in most cases and at every depth of fluorescence quenching explored.

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Light-assisted deep-trapping of holes in conjugated polymers

Cited by 27 publications

References 37 publications

Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

Hole Localization in Molecular Crystals from Hybrid Density Functional Theory

Direct measurement of energy transport in organic nanosystems

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