Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection

Nelson, Jenny

doi:10.1103/physrevb.67.155209

Cited by 358 publications

(342 citation statements)

References 29 publications

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“…55,56 The exponent in the power equation was α = 0.3, which is similar to that reported for the poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylenevinylene) (MDMO-PPV) and PCBM blend system. 57 The sub-unity value for α (α would be expected to equal 1 for ideal bimolecular recombination) has been discussed previously in terms of polaron trapping in a distribution of energetic traps in the polymer.…”

Section: Characterization Of Polythiophene Pristine Filmssupporting

confidence: 82%

“…57 The sub-unity value for α (α would be expected to equal 1 for ideal bimolecular recombination) has been discussed previously in terms of polaron trapping in a distribution of energetic traps in the polymer. [55][56][57] Indistinguishable power-law decay dynamics were observed under O 2 atmosphere, confirming that this power-law decay cannot be assigned to triplet excitons. We note however that the amplitude of this power-law transient decreased by ~ 30% under the O 2 atmosphere.…”

Section: Characterization Of Polythiophene Pristine Filmsmentioning

confidence: 70%

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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: Characterization Of Polythiophene Pristine Filmssupporting

confidence: 82%

Section: Characterization Of Polythiophene Pristine Filmsmentioning

confidence: 70%

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

et al. 2008

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“…It has been suggested that the parameter α can be related to the density of states of the blend component with the slower charge diffusion. [ 41 ] Figure 2 shows the normalized polaron absorbance decay in blends of P3HT, SiPCPDTBT and PCDTBT combined with four different fullerene adducts: PCBM, bis -PCBM, ICBA and ICTA. In blends with P3HT (Figure 2 a), the acceptors PCBM, bis -PCBM and ICBA show a similar charge generation yield (signal amplitude), while the polaron yield of the blend with ICTA is reduced.…”

Section: Studies Of Charge Generation Effi Ciency Of the Blend Filmsmentioning

confidence: 99%

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Faist

Shoaee

Tuladhar

et al. 2013

Advanced Energy Materials

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The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.

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“…Multiparticle MC simulation methods have also been used to study diffusion-limited recombination in polymer-fullerene blends. 24 Another reason to study the effects of Coulomb interactions explicitly is a double-counting problem that occurs when both the space charge and the image-charge potential are taken into account either in a 3D master-equation approach or in a 1D continuum drift-diffusion model. The problem is that, in taking into account the space charge and the correct boundary conditions for the electrostatic potential at the electrodes, the image-charge potential related to the space charge has already been taken into account.…”

Section: Introductionmentioning

confidence: 99%

Monte Carlo study of charge transport in organic sandwich-type single-carrier devices: Effects of Coulomb interactions

et al. 2011

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Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection

Cited by 358 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

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Monte Carlo study of charge transport in organic sandwich-type single-carrier devices: Effects of Coulomb interactions

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