Radical ion pair mediated triplet formation in polymer–fullerene blend films

Ohkita, Hideo; Cook, Steffan; Astuti, Yeni; Duffy, Warren; Heeney, Martin; Tierney, Steve; McCulloch, Iain; Bradley, Donal D. C.; Durrant, James R.

doi:10.1039/b608832e

Cited by 55 publications

(71 citation statements)

References 21 publications

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“…Both, the absorption and emission energies are consistent with previously reported values for TTP. [46][47] With increasing content of Pt-porhyrin in the backbone, the fluorescence intensity decreases (Figure 2), indicating an efficient singlet exciton quenching, which we tentatively assign to quenching induced by the Pt-porphyrin backbone. In order to study the singlet and triplet exciton dynamics and the mechanism of the triplet exciton formation in TTP, TTP-Pt-5 and TTP-Pt-10, we used Transient Absorption Spectroscopy (TAS) on multiple timescales.…”

Section: Resultsmentioning

confidence: 81%

Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

et al. 2015

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We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple time scales and investigated the mechanism of triplet exciton formation. During sensitization, singlet exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and found that 60% of the complex triplet excitons were transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and upconversion layers.

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Section: Resultsmentioning

confidence: 81%

Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

et al. 2015

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“…17 We have recently extended these studies to two higher ionization potential (IP) thiophene-based copolymers. 37 In contrast to P3HT, quenching of the polymer singlet exciton by the inclusion of PCBM was shown to generate a high yield of triplet excitons rather than dissociated polarons. This was attributed to efficient triplet formation through geminate charge recombination of bound triplet radical ion pairs, 37 analogous to that also reported for polymer/polymer blend films.…”

Section: Introductionmentioning

confidence: 99%

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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“…This method is widely-known and has previously been applied to numerous polymer:blend systems. [12,[17][18][19][20][21][22][23][24][25] In general, the charge carrier decay kinetics exhibit a power law decay (∆OD ∝ t -α ); this has been observed in P3HT, [26] polyselenophenes, [27] and MDMO-PPV, [28,29] all blended with various fullerene derivatives. This power law behaviour is consistent with models describing bimolecular recombination of dissociated charge carriers in the presence of an exponential distribution of localised (trapped) states.…”

Section: Steady State and Transient Absorption Spectroscopymentioning

confidence: 99%

Photodegradation in Encapsulated Silole‐Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements

Clarke

Lungenschmied²,

Peet³

et al. 2013

Advanced Energy Materials

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. (2013). Photodegradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient absorption spectroscopy and charge extraction measurements. Advanced Energy Materials, 3 (11), 1473-1483. Photodegradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient absorption spectroscopy and charge extraction measurements AbstractLight induced degradation has been observed in the performance of organic solar cells in the absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic blend devices comprised of the silole-based donor-acceptor polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power conversion efficiency, primarily as a result of a reduction in short circuit current, JSC. The initial burn-in phase of the photodegradation has been examined using a combination of transient absorption spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction by linearly increasing voltage) and timeresolved charge extraction using a nanosecond switch. These measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities at long times. At the lower charge densities operating at short circuit conditions and under continuous white light illumination, where bimolecular recombination is much less significant, the JSC decreases after light soaking due to a reduction in the efficiency of trapped charge carrier extraction. Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsClarke, T. M., Lungenschmied, C., Peet, J., Drolet, N., Sunahara, K., Furube, A. & Mozer, A. J. (2013

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Radical ion pair mediated triplet formation in polymer–fullerene blend films

Abstract: Efficient triplet formation is observed for films of high ionisation potential polythiophenes blended with a fullerene derivative, and assigned to formation via geminate charge recombination of bound radical ion pair states.

Cited by 55 publications

References 21 publications

Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

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

Photodegradation in Encapsulated Silole‐Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements

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