Enhanced triplet exciton generation in polyfluorene blends

Ford, T.A.; Avilov, I. V.; Beljonne, David; Greenham, Neil C.

doi:10.1103/physrevb.71.125212

Cited by 107 publications

(124 citation statements)

References 29 publications

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“…This geminate recombination, which should be distinguished from the subsequent bimolecular recombination of the dissociated free charges, has been suggested to be a key factor in limiting device performance for PPV-based devices. Similar observations have been made for polyfluorene-based polymer/polymer blend films 35 and for polyfluorene/fullerene blends. 36 In particular, in a study employing three different PPV-based polymers, an inverse correlation was observed between this exciplex-like emission strength and PV device performance.…”

Section: Introductionsupporting

confidence: 85%

“…29,35 Such 'intermediate' charge transfer (CT) states have also recently been reported in a photophysical study of small bandgap co-polymer blended with PCBM. 68 As we have discussed previously, 37 our observation of triplet exciton formation in the presence of quenching of singlet exciton, provides strong evidence for the relevance of this model to polythiophene/PCBM blend films.…”

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 84%

“…Previous studies of PPV and polyfluorene-based donor/acceptor blends 29,35,36,86,87 have reported significant red-shifted exciplex-like emission and CT absorption, suggesting that the BRP states observed in these systems are radiatively coupled to the ground state. A sub-band gap absorption has also been reported for P3HT/PCBM blend films by Fourier-transform photocurrent spectroscopy.…”

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 92%

“…29,35 In photosynthetic reaction centers, the back electron transfer produces a triplet state with an unusual spin polarization showing the charge recombination within a weakly coupled, spin-correlated radical ion pair. 78,82 In D-A molecules, a triplet formation is observed following the back electron transfer.…”

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 99%

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Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

et al. 2008

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Abstract:We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt% 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C 61 (PCBM). Three polymers are observed to give amorphous films, attributed to a non-planar geometry of their backbone whilst the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (~ 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt% PCBM resulted in 70 -90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer + and PCBM − polarons, assayed by the appearance of a long-lived, powerlaw decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over two orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find a excellent correlation between the free energy difference for charge separation (ΔG CS rel ) and yield of the long-lived charge generation yield, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL 3 quenching. We suggest this observation is consistent with a model where the excess thermal energy of the initially formed polarons pairs is necessary to overcome their coulomb binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.4

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Section: Introductionsupporting

confidence: 85%

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 84%

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 92%

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 99%

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Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

et al. 2008

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“…[21,22] It can be used to identify charged states and to further give information about state origin and lifetimes in the μs to ms regime. The use of PIA spectroscopy to measure the lifetime of photo-excited states in semiconductors has a well established theory for both mono-and bi-molecular relaxation mechanisms, such as those we expect for charges in photovoltaic-and solar-cells, where the excitation intensities are such that charge-charge interaction (e.g.…”

Section: Experimental Procedures and Background Theorymentioning

confidence: 99%

Photophysical studies of poly-isocyanopeptide based photovoltaic blends

Finlayson¹,

Whitney²

2010

J. Phys. D: Appl. Phys.

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We report a photophysical study of photovoltaic blends containing perylene-substituted polyisocyanide (pery-PIC) materials, using time-correlated single photon counting (TCSPC) and photo-induced absorption spectroscopy, and compare the key characteristics to analogous perylene-diimide (PDI) monomer blends with polythiophene-and polyfluorene-based conjugated polymers. Pery-PIC consists of semiconducting perylene units, which self-stack in a regular fashion around a rigid helical poly-isocyanopeptide backbone. In particular, the charged state lifetimes in pery-PIC blends are found to be of order 10s of μs; this being typically less than half those of the perylene-anion in the corresponding PDI blends. We consider the influence of photophysical factors on the superior photovoltaic device performance of the pery-PIC blends, relative to the corresponding PDI-based devices, in addition to the morphological effects described in earlier studies.

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Concept of Excitons

Singh

Ruda

2006

Optical Properties of Condensed Matter and Applications

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Enhanced triplet exciton generation in polyfluorene blends

Cited by 107 publications

References 29 publications

Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

Photophysical studies of poly-isocyanopeptide based photovoltaic blends

Concept of Excitons

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