Formation of a Ground‐State Charge‐Transfer Complex in Polyfluorene//[6,6]‐Phenyl‐C61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells

Benson-Smith, Jessica J.; Goris, Ludwig; Vandewal, Koen; Haenen, Ken; Manca, Jean; Vanderzande, Dirk; Bradley, Donal D. C.; Nelson, Jenny

doi:10.1002/adfm.200600484

Cited by 267 publications

(278 citation statements)

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“…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%

“…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. 32 However, a clear understanding of the parameters determining the efficiency of charge dissociation versus geminate recombination in organic blend films is yet to be established.…”

Section: Introductionmentioning

confidence: 97%

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

confidence: 92%

Section: Introductionmentioning

confidence: 97%

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

et al. 2008

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“…Recently, the existence of an intermediate charge transfer (CT) state between the exciton and the free charges in some blend systems based on polyfluorene has been reported. [62][63][64] Understanding the nature of such an intermediate state and its dependence on blend composition is important in understanding the effect of microstructure on charge generation. As an example, Veldman et al confirmed the existence of a charge transfer (CT) state in the blend of a fluorene copolymer (PF10TBT) ''poly[2,7-(9,9-dialkylfluorene)-alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)] with PCBM.…”

Section: Blend Compositionmentioning

confidence: 99%

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

et al. 2011

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This tutorial review discusses the factors influencing blend microstructure and performance in bulk heterojunction organic photovoltaic cells.Please check this proof carefully. Our staff will not read it in detail after you have returned it. Translation errors between word-processor files and typesetting systems can occur so the whole proof needs to be read. Please pay particular attention to: tabulated material; equations; numerical data; figures and graphics; and references. If you have not already indicated the corresponding author(s) please mark their name(s) with an asterisk. Please e-mail a list of corrections or the PDF with electronic notes attached --do not change the text within the PDF file or send a revised manuscript.Please bear in mind that minor layout improvements, e.g. in line breaking, table widths and graphic placement, are routinely applied to the final version.Please note that, in the typefaces we use, an italic vee looks like this: n, and a Greek nu looks like this: n.We will publish articles on the web as soon as possible after receiving your corrections; no late corrections will be made.Please return your final corrections, where possible within 48 hours of receipt, by e-mail to: chemsocrev@rsc.org Reprints-Electronic (PDF) reprints will be provided free of charge to the corresponding author. Enquiries about purchasing paper reprints should be addressed via: http://www.rsc.org/Publishing/ReSourCe/PaperReprints/. Costs for reprints are below: Q3The sentence beginning ''For example it has been shown. . .'' has been altered for clarity, please check that the meaning is correct. Q4The citation to ' Fig. 8(a)' in the sentence beginning 'Grazing-incidence X-ray diffraction (XRD) measurements. . .' has been changed to ' Fig. 10(a)' as the text appears to discuss ' Fig. 10(a)'. Please check that this is correct. Q5The citation to ' Fig. 8(b)' in the sentence beginning 'Upon annealing, P3HT chains begin to crystallize. . .' has been changed to ' Fig The performance of organic photovoltaic devices based upon bulk heterojunction blends of donor and acceptor materials has been shown to be highly dependent on the thin film microstructure. In this tutorial review, we discuss the factors responsible for influencing blend microstructure and how these affect device performance. In particular we discuss how various molecular design approaches can affect the thin film morphology of both the donor and acceptor components, as well as their blend microstructure. We further examine the influence of polymer molecular weight and blend composition upon device performance, and discuss how a variety of processing techniques can be used to control the blend microstructure, leading to improvements in solar cell efficiencies.

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“…By comparing PDS signal of the polymer:fullerene blend with those of the individual components, weak absorption from the CT state was observed (Figure 2a), indicating interaction between polymer and fullerene in the ground state. [35][36][37] In addition to PDS, we can also detect the CT state absorption by measuring the external quantum efficiency (EQE) of the solar cell in the CT state absorption regime, where neither polymer nor fullerene absorbs. The EQE in the CT state absorption regime results from the photocurrent generated by absorption of the CT state.…”

Section: Absorptionmentioning

confidence: 99%

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Gao

Inganäs

2014

Phys. Chem. Chem. Phys.

204

252

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Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer-fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.2

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Formation of a Ground‐State Charge‐Transfer Complex in Polyfluorene//[6,6]‐Phenyl‐C61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells

Cited by 267 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

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

Charge generation in polymer–fullerene bulk-heterojunction solar cells

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