Dark Carriers, Trapping, and Activation Control of Carrier Recombination in Neat P3HT and P3HT:PCBM Blends

Ferguson, Andrew J.; Kopidakis, Nikos; Shaheen, Sean E.; Rumbles, Garry

doi:10.1021/jp208014v

Cited by 137 publications

(310 citation statements)

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“…Rumbles and coworkers have discussed such a CT or CT precursor state using the nomenclature {D + …A − }. [ 15 ] The inset to Figure 5 captures the above in a simplifi ed energy diagram with only three (labeled) transitions that matter. On the time scales of Figure 5 the charge separation is complete, i.e., the concentration of excitons has dropped to below 1% of the initial concentration.…”

Section: Resultsmentioning

confidence: 99%

“…[ 15,17 ] In the rate equation model this is readily incorporated by setting γ pre < 1. The result of fi tting this rate equation to the experimental data is shown as dashed lines on Figure 6 .…”

Section: Resultsmentioning

confidence: 99%

“…In view of the (deliberate) neglect of phase separation in the minimalistic MC model a likely explanation for a sub-unity γ pre is the presence of an aggregated PCBM phase (and/or pure polymer phases) with lower-lying energy levels that act almost as charge carrier sinks within the experimental devices. [ 15,19 ] Let us fi nally discuss the implications of the physical picture sketched in Figure 6 b for OPV operation at normal light intensity, i.e., around 1 sun steady-state illumination. The wileyonlinelibrary.com multiple dissociation and re-association of CT complexes at short time scales remains at low light levels as this is a direct consequence of the diffusive relaxation in the disordered DOS.…”

Section: Resultsmentioning

confidence: 99%

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Nonequilibrium Charge Dynamics in Organic Solar Cells

Howard

Etzold

Laquai

et al. 2014

Advanced Energy Materials

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mean that (undesired) geminate charge carrier recombination always competes with the (desired) charge carrier separation and therefore quantum effi ciencies are always moderate. The surprising experimental observations of exceptionally high quantum effi ciencies revealed that such a simplistic picture is insufficient to describe charge carrier separation in organic photovoltaics, and motivated a broad range of close physical investigations into this critical process aimed at developing an accurate mechanistic understanding of charge separation (that could be used to design high quantum effi ciency materials). In this manuscript we present a rich multidimensional set of transient absorption observations which provide detailed information (on the ps to µs timescale) about the behavior of mobile, separating charge carriers in the prototypical high quantum effi ciency poly[N-11′′-henicosanyl-2,7-carbaz olealt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): fullerene derivative blends. The measurements concurrently observe the relaxation of charge carriers within the disordered density of states (DOS), and the recombination of charge carriers at a sequence of different initial charge carrier concentrations. Understood through the lens of kinetic Monte Carlo calculations, these observations reveal how mobile charge carriers hop and relax, dissociating and re-associating potentially multiple times before extraction or recombination.The current literature regarding observations of charge carrier separation in organic bulk heterojunctions is in agreement with regards to the conclusion that charge carrier separation out of the geminate pairs occurs on the sub nanosecond timescale. A clear indication of the upper bound for the time within which charge carriers must separate was given by the precise measurement using an ultrafast pump-push-photocurrent technique of the lifetimes of geminate interfacial bound charge carrier pairs (also called interfacial charge-transfer state). [ 2 ] This work found that the lifetime for a geminate charge carrier pair which remained at the interface in relatively high quantum effi ciency blends (such as P3HT:PCBM) was in general on the order of 500 ps. Thus, charge carriers which separate must certainly achieve separation in less than 500 ps. Bakulin and coworkers in this work suggest that signifi cant charge carrier delocalization during the transient occupation of a higher lying interfacial geminate state (which would occur on the sub-picosecond timescale) is the crucial initial step towards separation. [ 2,3 ] This sub-nanosecond timescale for geminate charge pair separation is supported by a wide range The dynamics of charge carriers after their creation at, or near, an interface play a critical role in determining the effi ciency of organic solar cells as they dictate, via mechanisms that are not yet fully understood, the pathways for charge separation and recombination. Here, a combination of ultrafast transient spectroscopy and kinetic Monte Carlo simulations based on a...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Nonequilibrium Charge Dynamics in Organic Solar Cells

Howard

Etzold

Laquai

et al. 2014

Advanced Energy Materials

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“…36,37 Using a timeresolved microwave conductivity (TRMC) approach, Rumbles and coworkers studied the effect of doping of solubilized fullerene in P3HT films on the photoinduced conductivity transients as a result of trapped and mobile holes in the film. The results of this study provided a somewhat different picture of carrier dynamics in the long-time regime emerged from this work relative to that of the power law behavior described by Silva, namely, that carrier trapping (and subsequent release)-as opposed to a single recombination mechanism via tunneling.…”

Section: Introductionmentioning

confidence: 99%

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

Barnes

Baghar

2012

J Polym Sci B Polym Phys

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This mini-review on the photophysics of poly-alkyl thiophenes (e.g., P3HT) and its blends with electron-acceptor moeties such as fullerenes (e.g., PCBM) and carbon nanotubes focuses on highlights of recent literature on spectroscopic probes of exciton formation, diffusion, charge-separation, and transport in these materials. The literature in this area is vast: more than 3000 papers have been published in on P3HT (and related materials) and applications to organic solar energy harvesting devices over the last 20 years. Thus, no single review can capture the breadth and depth of this research. Here, we attempt to highlight some of the exciting new research efforts aimed at understanding photophysical processes in organic photovoltaic materials. This mini-review is organized as follows: First, a summary of the theoretical framework commonly used to describe fundamental physical processes of charge generation in organic (polymeric) semiconductor materials is presented. We then discuss recent exciting results on ultrafast spectroscopic probes of exciton dynamics in these materials. Finally, we present highlights of new research on polymer nanostructures (nanoparticles and nanofibers) and their exciting applications to organic photovoltaics.

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“…We have recently used the time-resolved microwave conductivity (TRMC) technique to probe charge generation and recombination in a number of organic donor-acceptor systems [2,[11][12][13], including systems pairing SWCNTs with either semiconducting polymer electron donors [3,14,15] or fullerene electron acceptors [7][8][9]16]. TRMC is a time-resolved pump-probe technique that measures pump-induced charge generation through the absorbance of the microwave probe beam by mobile carriers.…”

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confidence: 99%