Fate of Excitations in Conjugated Polymers: Single-Molecule Spectroscopy Reveals Nonemissive “Dark” Regions in MEH-PPV Individual Chains

Lin, Hongzhen; Tian, Yuxi; Zapadka, Karolina; Persson, Gustav; Thomsson, Daniel; Mirzov, Oleg; Larsson, Per‐Olof; Widengren, Jerker; Scheblykin, Ivan G.

doi:10.1021/nl9027473

Cited by 67 publications

(129 citation statements)

References 39 publications

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“…67,68 Single-molecule spectroscopy has correlated the ultrafast relaxation to aggregated regions of the polymer. 69 To further substantiate the conclusion that the polarons are formed from the excitons, we have fitted the decay of the transient data. Already a visual interpretation of the kinetic traces shows that there is the ultrafast signal change as discussed above and an additional slower component.…”

Section: Temporal Evolution Of the Charge Generation Processmentioning

confidence: 95%

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

et al. 2011

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Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organicÀinorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UVÀVisÀNIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions, and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

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Section: Temporal Evolution Of the Charge Generation Processmentioning

confidence: 95%

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

et al. 2011

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“…Very recently, it has been reported [43] that in long MEH-PPV chains, part of the chain may stay in a permanently quenched state. For some chains, this quenched state was suggested to account for more than 90% of the chain length, and the quenched parts would remain invisible to single-molecule spectroscopy in fl uorescence mode.…”

Section: Review Articlementioning

confidence: 99%

Conformation and physics of polymer chains: a single-molecule perspective

Vácha

Habuchi

2010

NPG Asia Mater

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N anotechnology has been one of the driving forces of science and technology over the past 20 years. Scaling down the size of an object to nanometer dimensions can result in dramatic changes to its physical properties. With recent progress in the nanoscience and nanotechnology of polymeric materials, new methods are now required for the characterization of polymer structures and properties. Despite the great success of electron microscopy and scanning probe microscope techniques, there remains a need for a non-invasive method that could provide information not only on the structure but also on the physical phenomena occurring on the nanoscale level. One method that has recently emerged as an excellent tool for nanoscale characterization is single-molecule optical detection and spectroscopy. Single-molecule spectroscopy relies on the detection and measurement of the fl uorescence signal and spectra from single isolated molecules or polymer chains (Figure 1). Th ese can either be immobilized in a solid matrix or be diff using in a solution or melt. In all cases, the concentration of the emitting molecules must be suffi ciently low to ensure that the spatial separation between individual molecules is larger than the optical resolution of the setup, typically on the order of micrometers. As the fl uorescence signal from a molecule is strongly infl uenced by the nanoscale environment, measuring many molecules one by one provides the distribution of physical properties of the respective nano-environments. Moreover, monitoring a molecule over a period of time reveals dynamic changes in its environment. Th ese static and dynamic heterogeneities of nanoscale properties are averaged and hidden in the usual ensemble experiments. Th e strength of fl uorescence detection is in its sensitivity and low background level -emissions of just a few hundred photons per second can be readily detected. Th e technique also off ers remarkable dynamic range, from sub-nanosecond timescales to seconds and longer, and is non-invasive, making it possible to detect emissions from molecules buried deep in a sample.Single-molecule spectroscopy has evolved since the beginning of the 1990s from low-temperature, high-resolution optical spectroscopic

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“…[6][7][8] Moreover, the ability to correlate absorption and fluorescence spectra of the same sample offers the fascinating possibility to unravel the mechanisms underlying photoblinking and photobleaching of single-molecules, 5 or the nature of absorptive vs. emissive centers in conjugated polymers. 9,10 Although absorption-based imaging has recently been demonstrated. [11][12][13][14][15][16][17][18] no absorption spectroscopy is available for nanostructures of arbitrary size, geometry, and material.…”

mentioning

confidence: 99%

Single-Particle Absorption Spectroscopy by Photothermal Contrast

et al. 2015

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Removing effects of sample heterogeneity through single-molecule and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here we present an approach capable of recording pure absorption spectra of individual nanostructures. We demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, we are furthermore able to record absorption spectra of single gold nanorods with different aspect ratios. We find that the spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.

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Fate of Excitations in Conjugated Polymers: Single-Molecule Spectroscopy Reveals Nonemissive “Dark” Regions in MEH-PPV Individual Chains

Cited by 67 publications

References 39 publications

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

Conformation and physics of polymer chains: a single-molecule perspective

Single-Particle Absorption Spectroscopy by Photothermal Contrast

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