21 research outputs found
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Morphology controls the thermoelectric power factor of a doped semiconducting polymer.
The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F n TCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics
Recommended from our members
Morphology controls the thermoelectric power factor of a doped semiconducting polymer
The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (s) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (m). The Seebeck coefficient (a) is largely independent of OCL. This reveals that, unlike s, leveraging strategies to improve m have a smaller impact on a. Our best-performing sample with the largest OCL, vapor-doped PBTTT: F4TCNQ thin film, has a s of 670 S/cm and an a of 42 μV/K, which translates to a large PF of 120 mW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics
Tethered tertiary amines as solid-state n-type dopants for solution-processable organic semiconductors
A scarcity of stable n-type doping strategies compatible with facile processing has been a major impediment to the advancement of organic electronic devices. Localizing dopants near the cores of conductive molecules can lead to improved efficacy of doping. We and others recently showed the effectiveness of tethering dopants covalently to an electron-deficient aromatic molecule using trimethylammonium functionalization with hydroxide counterions linked to a perylene diimide core by alkyl spacers. In this work, we demonstrate that, contrary to previous hypotheses, the main driver responsible for the highly effective doping observed in thin films is the formation of tethered tertiary amine moieties during thin film processing. Furthermore, we demonstrate that tethered tertiary amine groups are powerful and general n-doping motifs for the successful generation of free electron carriers in the solid-state, not only when coupled to the perylene diimide molecular core, but also when linked with other small molecule systems including naphthalene diimide, diketopyrrolopyrrole, and fullerene derivatives. Our findings help expand a promising molecular design strategy for future enhancements of n-type organic electronic materials
Power Factor Enhancement in Solution‐Processed Organic n‐Type Thermoelectrics Through Molecular Design
A new class of high‐performance n‐type organic thermoelectric materials, self‐doping perylene diimide derivatives with modified side chains, is reported. These materials achieve the highest n‐type thermoelectric performance of solution‐processed organic materials reported to date, with power factors as high as 1.4 μW/mK^2. These results demonstrate that molecular design is a promising strategy for enhancing organic thermoelectric performance
Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase
We demonstrate how processing methods
affect the thermoelectric
properties of thin films of a high mobility semiconducting polymer,
PBTTT. Two doping methods were compared: vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane
(FTS) or immersion in a solvent containing 4-ethylbenzenesulfonic
acid (EBSA). Thermally annealed, thin films doped by FTS deposited
from vapor yield a high Seebeck coefficient (α) at high electronic
conductivity (σ) and, in turn, a large power factor (PF = α<sup>2</sup>σ) of ∼100 μW m<sup>–1</sup> K<sup>–2</sup>. The FTS-doped films yield α values that are
a factor of 2 higher than the EBSA-doped films at comparable high
value of σ. A detailed analysis of X-ray scattering experiments
indicates that perturbations in the local structure from either dopant
are not significant enough to account for the difference in α.
Therefore, we postulate that an increase in α arises from the
entropic vibrational component of α or changes in scattering
of carriers in disordered regions in the film
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Relationship between Mobility and Lattice Strain in Electrochemically Doped Poly(3-hexylthiophene).
Conjugated semiconducting polymers, such as poly(3-hexylthiophene) (P3HT), are poised to play an integral role in the development of organic electronic devices; however, their performance is governed by factors that are intrinsically coupled: dopant concentration, carrier mobility, crystal structure, and mesoscale morphology. We utilize synchrotron X-ray scattering and electrochemical impedance spectroscopy to probe the crystal structure and electronic properties of P3HT in situ during electrochemical doping. We show that doping strains the crystalline domains, coincident with an exponential increase in hole mobility. We believe these observations provide guidance for the development of improved theoretical models for charge transport in semiconducting polymers
First-Principles Predictions of Near-Edge X‑ray Absorption Fine Structure Spectra of Semiconducting Polymers
The electronic structure and molecular
orientation of semiconducting
polymers in thin films determine their ability to transport charge.
Methods based on near-edge X-ray absorption fine structure (NEXAFS)
spectroscopy can be used to probe both the electronic structure and
microstructure of semiconducting polymers in both crystalline and
amorphous films. However, it can be challenging to interpret NEXAFS
spectra on the basis of experimental data alone, and accurate, predictive
calculations are needed to complement experiments. Here, we show that
first-principles density functional theory (DFT) can be used to model
NEXAFS spectra of semiconducting polymers and to identify the nature
of transitions in complicated NEXAFS spectra. Core-level X-ray absorption
spectra of a set of semiconducting polymers were calculated using
the excited electron and core-hole (XCH) approach based on constrained-occupancy
DFT. A comparison of calculations on model oligomers and periodic
structures with experimental data revealed the requirements for accurate
prediction of NEXAFS spectra of both conjugated homopolymers and donor–acceptor
polymers. The NEXAFS spectra predicted by the XCH approach were applied
to study molecular orientation in donor–acceptor polymers using
experimental spectra and revealed the complexity of using carbon edge
spectra in systems with large monomeric units. The XCH approach has
sufficient accuracy in predicting experimental NEXAFS spectra of polymers
that it should be considered for design and analysis of measurements
using soft X-ray techniques, such as resonant soft X-ray scattering
and scanning transmission X-ray microscopy
Ionic Conductivity in the Metal-Organic Framework UiO-66 by Dehydration and Insertion of Lithium tert-Butoxide
Shields up! Post-synthetic modification of the secondary building units in the metal-organic framework UiO-66 (Zr6O4(OH)4(O2CR)12) by dehydration and subsequent grafting of LiOtBu yields a solid Li(+) electrolyte with a conductivity of 1.8×10(-5) S cm(-1) at 293 K. As the grafting leads to screening of the anionic charge, the activation energy for ionic conduction is significantly lower than when Li(+) is introduced through deprotonation.status: publishe
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Relationship between Mobility and Lattice Strain in Electrochemically Doped Poly(3-hexylthiophene)
Conjugated semiconducting polymers,
such as poly(3-hexylthiophene)
(P3HT), are poised to play an integral role in the development of
organic electronic devices; however, their performance is governed
by factors that are intrinsically coupled: dopant concentration, carrier
mobility, crystal structure, and mesoscale morphology. We utilize
synchrotron X-ray scattering and electrochemical impedance spectroscopy
to probe the crystal structure and electronic properties of P3HT <i>in situ</i> during electrochemical doping. We show that doping
strains the crystalline domains, coincident with an exponential increase
in hole mobility. We believe these observations provide guidance for
the development of improved theoretical models for charge transport
in semiconducting polymers