Density of States and the Role of Energetic Disorder in Charge Transport in an Organic Radical Polymer in the Solid State

Kemper, Travis; Larsen, Ross E.; Gennett, Thomas

doi:10.1021/acs.jpcc.5b06368

Cited by 26 publications

(50 citation statements)

References 34 publications

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“…These maximal values compare well with calculated widths for the hole state DOS in polar polymers of 660 meV. [12] Experimentally characterized disorder parameters for bulk semiconducting phases are usually within an energy range of 50 to 150 meV. [2] While the calculated disorder of excitation energies in the vicinity of the interfaces lies within this range, the disorder of ionization potentials is considerably larger.…”

Section: Resultssupporting

confidence: 71%

“…[12] Since an excitation (ionization) accompanied by the polarization and deformation of the surrounding lattice corresponds to an exciton (a polaron), [1] it seems reasonable to associate the disorder of excitation energies (ionization potentials) with the disorder of excitonic (polaronic) transport levels. The distribution of these transport levels determines the efficiency of the corresponding transport processes.…”

Section: Resultsmentioning

confidence: 99%

“…Hence, theoretical studies are needed because insight into the formation of the energy landscapes of the transport states in the vicinity of these interfaces is particularly essential for a rationally guided design of efficient organic solar cells (OSCs) and organic light-emitting diodes. [11] They are of special interest because the shapes of the interfacial DOS might deviate from a Gaussian one, and the parameters of interest might differ from the ones found for the corresponding bulk phases.Several computational studies have addressed the influence of disorder on charge transport in the bulk of organic semiconductors, for instance, by Gennett and coworkers [12] and by Brédas and coworkers. [13,14] Van Voorhis et al outlined that also disorder at organic-organic interfaces is decisive and can significantly influence the character of the chargegeneration process.…”

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confidence: 99%

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QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

Brückner

Stolte²,

Würthner³

et al. 2017

J of Physical Organic Chem

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A QM/MM approach using ωB97X-D combined with AMOEBA calculations was used to analyze the energetic disorder in the vicinity of interfaces in amorphous organic heterostructures. Distributions of ground-, excited-, and cationic-state energies as well as of ionization potentials and excitation energies, all being relevant quantities for the transport properties of thin films, were calculated. As already found for bulk amorphous organic semiconductors, local densities of states at molecular interfaces possess Gaussian-shaped profiles with a significant amount of disorder. Assuming a disorder-limited activation of exciton and charge transport, a decrease in the amount of disorder could improve the transport properties. Relating calculated disorders to molecular parameters revealed that in line with the Bässler model, especially the molecular polarity, its change upon electronic excitation, and the molecular polarizability are relevant quantities leading to energetically largely disordered films. Moreover, because of the different mechanisms of exciton and polaron delocalization, a given morphology with disordered charge-transport levels gives not necessarily rise to disordered exciton-transport levels and vice versa. | INTRODUCTIONDisorder in molecular organic semiconductors has been recognized as a key parameter limiting the semiconductor's transport properties for both charges and excitons. [1] A random distribution of the molecules in a disordered amorphous film results in considerable variations in local intermolecular interactions. This gives rise to a distribution of ionization and excitation energies, the relevant quantities for charge and exciton transport, which depend among others on these intermolecular interaction energies. [2] According to the central limit theorem, resulting densities of states (DOSs) for excitons and charges have a Gaussian shape because intermolecular interaction energies are composed of many independently varying contributions. [3] The width of the Gaussian-shaped DOSs, ie, the standard deviation of the Gaussian normal distribution σ, is referred to as the disorder parameter and characterizes the amount of site energy disorder, ie, of static disorder, in the semiconducting solid-state system. [2] Based on this Gaussian DOS, the Bässler model describes charge transport in organic semiconductors as an incoherent hopping process between the normally distributed sites. [4] The expression "disorder" can be connected with geometrical and energetical disorder. In line with the Bässler nomenclature, in the following the expression, "disorder" is always used to characterize the energetic disorder unless otherwise noted. Although experimentally detected field-and temperature-dependent charge carrier mobilities confirm the predictions of the Bässler model in principle, no direct experimental proof for the Gaussian-shaped DOS, particularly of charge carriers, in disordered molecular semiconductors † This article is published as part of a special issue to celebrate the 80 th birthday of P...

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Section: Resultssupporting

confidence: 71%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

Brückner

Stolte²,

Würthner³

et al. 2017

J of Physical Organic Chem

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“…The most widely studied family of stable radical polymers is based on 2,2,6,6‐tetramethyl‐piperidin‐1‐yl (TEMPO, 1 ) radicals, while examples based on other families of radicals, including nitronyl nitroxide ( 2 ), 2,2,5,5‐tetramethyl‐1‐pyrrolidinylloxy (PROXYL, 3 ), spirobisnitroxide ( 4 ), aminoxy ( 5 ), galvinoxyl ( 6 ), and 6‐oxoverdazyl ( 7 ) radicals have received considerably less attention. Further expansion of the stable radical polymer field to include examples based on these and other stable radicals will allow for the realization of materials with targeted properties that are suitable for the applications described above.…”

Section: Introductionmentioning

confidence: 99%

Synthesis, characterization, and thin‐film properties of 6‐oxoverdazyl polymers prepared by ring‐opening metathesis polymerization

Paquette

Ezugwu

Yadav

et al. 2016

J. Polym. Sci. Part A: Polym. Chem.

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Redox-active 6-oxoverdazyl polymers were synthesized via ring-opening metathesis polymerization (ROMP) and their solution, bulk, and thin-film properties investigated. Detailed studies of the ROMP method employed confirmed that stable radical polymers with controlled molecular weights and narrow molecular weight distributions (Ð < 1.2) were produced. Thermal gravimetric analysis of a representative example of the title polymers demonstrated stability up to 190 °C, while differential scanning calorimetry studies revealed a glass transition temperature of 152 C. Comparison of the spectra of 6-oxoverdazyl monomer 12 and polymer 13, including FT-IR, UV-vis absorption, and electron paramagnetic resonance spectroscopy, was used to confirm the tolerance of the ROMP mechanism for the 6-oxoverdazyl radical both qualitatively and quantitatively.Cyclic voltammetry studies demonstrated the ambipolar redox properties of polymer 13 (E 1/2,ox = 0.25 and E 1/2,red = −1.35 V relative to ferrocene/ferrocenium), which were consistent with those of monomer 12. The charge transport properties of thin films of polymer 13 were studied before and after a potential of 5 V was applied, revealing a drastic drop in the resistivity from 10 6 10 10 m or more to 1.7  10 4 m and suggesting the potential usefulness of polymer 13 in bistable electronics.

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“…In addition to battery applications, radical polymers have been shown to be electrically conductive in the solid state (i.e., in the absence of a supporting electrolyte). It is postulated that, in the presence of an electric field, adjacent radical sites will undergo electron transfer with one another if there is a sufficiently high density of radical sites present within the film to allow for electrochemical reaction to propagate from one electrode to the other . This mechanism has allowed radical polymers to be implemented in non‐volatile memory devices, diodes, and as an interfacial modifying layer in solar cells .…”

Section: Radical Polymersmentioning

confidence: 99%

Recent advances in the syntheses of radical-containing macromolecules

Wingate

Boudouris

2016

J. Polym. Sci. Part A: Polym. Chem.

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Tailor-made polymers containing specific chemical functionalities have ushered in a number of emerging fields in polymer science. In most of these next-generation applications the focus of the community has centered upon closed-shell macromolecules. Conversely, macromolecules containing stable radical sites have been less studied despite the promise of this evolving class of polymers. In particular, radical-containing macromolecules have shown great potential in magnetic, energy storage, and biomedical applications. Here, the progress regarding the syntheses of open-shell containing polymers are reviewed in two distinct subclasses. In the first, the syntheses of radical polymers (i.e., materials composed of nonconjugated macromolecular backbones and with open-shell units present on the polymer pendant sites) are described. In the second, polyradical (i.e., macromolecules containing stabi-lized radical sites either within the macromolecular backbone or those containing radical sites that are stabilized through a large degree of conjugation) synthetic schemes are presented. Thus, the state-of-the-art in open-shell macromolecular syntheses will be reported and future means by which to advance the current archetype will be discussed. By detailing the synthetic pathways possible for, and the inherent synthetic limitations of, the creation of these functional polymers, the community will be able to extend the bounds of the radical-containing macromolecular paradigm.

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Density of States and the Role of Energetic Disorder in Charge Transport in an Organic Radical Polymer in the Solid State

Cited by 26 publications

References 34 publications

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

Synthesis, characterization, and thin‐film properties of 6‐oxoverdazyl polymers prepared by ring‐opening metathesis polymerization

Recent advances in the syntheses of radical-containing macromolecules

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