Dr. Klaus Eckstein

• [ 2023 ]
• [ 2022 ]
• [ 2021 ]
• [ 2019 ]
• [ 2017 ]

• Trions, Exciton Dynamics, and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism. Eckstein, Klaus H.; Kunkel, Pascal; Voelckel, Markus; Schöppler, Friedrich; Hertel, Tobias. In J. Phys. Chem. C, 127(39), pp. 19659–19667. American Chemical Society, 2023.
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  @article{eckstein2023trions, author = {Eckstein, Klaus H. and Kunkel, Pascal and Voelckel, Markus and Schöppler, Friedrich and Hertel, Tobias}, booktitle = {The Journal of Physical Chemistry C}, journal = {J. Phys. Chem. C}, keywords = {}, month = 10, number = 39, pages = {19659–19667}, publisher = {American Chemical Society}, title = {Trions, Exciton Dynamics, and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism}, volume = 127, year = 2023 }

• Single-Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics. Abudulimu, Abasi; Eckstein, Klaus; Gemayel, Mirella El; Namal, Imge; Phillips, Adam B.; Ellingson, Randy J.; Heben, Michael J.; Hertel, Tobias; Meier, Sebastian B.; Lüer, Larry. In Solar RRL, 6(4), p. 2101010. 2022.
  • [ Abstract ]
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  Employing single-walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap-free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond-microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high-performance organic solar cells.

  @article{https://doi.org/10.1002/solr.202101010, abstract = {Employing single-walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap-free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond-microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high-performance organic solar cells.}, author = {Abudulimu, Abasi and Eckstein, Klaus and Gemayel, Mirella El and Namal, Imge and Phillips, Adam B. and Ellingson, Randy J. and Heben, Michael J. and Hertel, Tobias and Meier, Sebastian B. and Lüer, Larry}, journal = {Solar RRL}, keywords = {}, number = 4, pages = 2101010, title = {Single-Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics}, volume = 6, year = 2022 }

• Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes. Eckstein, Klaus H.; Hirsch, Florian; Martel, Richard; Hertel, Tobias. In The Journal of Physical Chemistry C. American Chemical Society (ACS), 2021.
  • [ Abstract ]
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  Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.

  @article{Eckstein_2021, abstract = {Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.}, author = {Eckstein, Klaus H. and Hirsch, Florian and Martel, Richard and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {}, month = {03}, publisher = {American Chemical Society ({ACS})}, title = {Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes}, year = 2021 }
• Coherent two-dimensional electronic spectroelectrochemistry. Heitmüller, Julia; Eckstein, Klaus; Renner, Rebecca; Stolte, Matthias; Hertel, Tobias; Würthner, Frank; Brixner, Tobias. In Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 253, p. 119567. 2021.
  • [ Abstract ]
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  We report the development of a new spectroscopic scheme, coherent two-dimensional (2D) electronic spectroelectrochemistry. Conventional 2D electronic spectroscopy has become well established to investigate molecular energy transfer, charge transfer, or structural dynamics with femtosecond time resolution following electronic excitation, providing frequency resolution for both the excitation and the detection step. Here we combine this method with electrochemistry in a flow cell. Thus we have established access to the dynamics of various oxidized and reduced molecular species in solution. We investigate the photophysics of a tetraphenoxy-substituted perylene bisimide dye and its reduced species as a proof of principle and find substantially different dynamics for the neutral and the twofold reduced compound. The electrochemical flow cell is furthermore applied in conventional transient absorption spectroscopy and photoluminescence spectroscopies as an application in different setups.

  @article{HEITMULLER2021119567, abstract = {We report the development of a new spectroscopic scheme, coherent two-dimensional (2D) electronic spectroelectrochemistry. Conventional 2D electronic spectroscopy has become well established to investigate molecular energy transfer, charge transfer, or structural dynamics with femtosecond time resolution following electronic excitation, providing frequency resolution for both the excitation and the detection step. Here we combine this method with electrochemistry in a flow cell. Thus we have established access to the dynamics of various oxidized and reduced molecular species in solution. We investigate the photophysics of a tetraphenoxy-substituted perylene bisimide dye and its reduced species as a proof of principle and find substantially different dynamics for the neutral and the twofold reduced compound. The electrochemical flow cell is furthermore applied in conventional transient absorption spectroscopy and photoluminescence spectroscopies as an application in different setups.}, author = {Heitmüller, Julia and Eckstein, Klaus and Renner, Rebecca and Stolte, Matthias and Hertel, Tobias and Würthner, Frank and Brixner, Tobias}, journal = {Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy}, keywords = {}, pages = 119567, title = {Coherent two-dimensional electronic spectroelectrochemistry}, volume = 253, year = 2021 }
• Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes. Eckstein, Klaus H.; Kunkel, Pascal; Voelckel, Markus; Schöppler, Friedrich; Abudulimu, Abasi; Lüer, Larry; Hertel, Tobias. 2021.
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  Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of \($2.0\,\rm ps^-1$\). Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.

  @preprint{eckstein2021diffusive, abstract = {Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of $2.0\,\rm ps^{-1}$. Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.}, author = {Eckstein, Klaus H. and Kunkel, Pascal and Voelckel, Markus and Schöppler, Friedrich and Abudulimu, Abasi and Lüer, Larry and Hertel, Tobias}, keywords = {}, note = {cite arxiv:2103.15477}, title = {Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes}, year = 2021 }

• Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy. Eckstein, Klaus H.; Oberndorfer, Florian; Achsnich, Melanie M.; Schöppler, Friedrich; Hertel, Tobias. In The Journal of Physical Chemistry C, 123(49), pp. 30001–30006. 2019.
  • [ Abstract ]
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  Controlling doping is essential for the successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being “high” or “low” or entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting single-wall carbon nanotubes (s-SWNTs) using vis–NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 nm–1 can be quantified with a sensitivity of roughly 1 charge per 104 carbon atoms assuming inhomogeneous or homogeneous carrier distributions.

  @article{doi:10.1021/acs.jpcc.9b08663, abstract = {Controlling doping is essential for the successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being “high” or “low” or entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting single-wall carbon nanotubes (s-SWNTs) using vis–NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 nm–1 can be quantified with a sensitivity of roughly 1 charge per 104 carbon atoms assuming inhomogeneous or homogeneous carrier distributions.}, author = {Eckstein, Klaus H. and Oberndorfer, Florian and Achsnich, Melanie M. and Schöppler, Friedrich and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {}, number = 49, pages = {30001-30006}, title = {Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy}, volume = 123, year = 2019 }

• Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes. Eckstein, Klaus H.; Hartleb, Holger; Achsnich, Melanie M.; Schöppler, Friedrich; Hertel, Tobias. In ACS Nano, 11(10), pp. 10401–10408. American Chemical Society, 2017.
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  Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive excitors transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.

  @article{Eckstein_2017, abstract = {Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive excitors transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.}, author = {Eckstein, Klaus H. and Hartleb, Holger and Achsnich, Melanie M. and Schöppler, Friedrich and Hertel, Tobias}, journal = {ACS Nano}, keywords = {}, month = {09}, number = 10, pages = {10401-10408}, publisher = {American Chemical Society}, title = {Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes}, volume = 11, year = 2017 }