An assessment of the role of dipoles on the density-of-states function of disordered molecular solids

Dieckmann, Anbangkai; Bäßler, H.; Borsenberger, P. M.

doi:10.1063/1.465640

Cited by 244 publications

(186 citation statements)

References 26 publications

Supporting

Mentioning

180

Contrasting

Order By: Relevance

“…͑5͒ for a doped polymer containing a strong dipolar additive in addition to a polar charge transport agent. The data are more consistent with the linear dependence on concentration, n ϭ1, proposed by Young 35 and Dunlap, Parris, and Kenkre, 38 while the superlinear dependence, 2nϭ4/3, of Dieckman, Bässler, and Borsenberger 36 and Hirao and Nishizawa 37 would be too strong. The NAS chromophores have a larger effect on the energy width despite having a smaller dipole moment ͑6.7 D͒ than EHDNPB ͑7.6 D͒, but EHDNPB is much larger because of its alkyl tail ͓see Fig.…”

Section: Discussionsupporting

confidence: 84%

“…35,36,38 These are the first results showing the dipolar disorder effects due to independent dopants ͑NAS and EHD-NPB͒ and the transport agent study reveals that the proper choice of transport agents is important in achieving high mobilities in MDPs that contain other strong polar species.…”

Section: Discussionmentioning

confidence: 74%

“…[28][29][30][31][32][33][34] The dipolar effect on charge transport in doped polymers has been described in a qualitative manner by a model based on dipolar disorder. 28,30 The more quantitative descriptions can be found in later work, where the random potentials due to the electric fields of the dipoles are incorporated into the Gaussian disorder model, [35][36][37] or derived from a one-dimensional random potential model. 38 The main argument of the dipolar disorder model is that the local variation of the electrostatic potential resulting from randomly distributed and oriented dipoles increases the width of the Gaussian energy distribution.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Effect of dipolar molecules on carrier mobilities in photorefractive polymers

Goonesekera

Ducharme

1999

Journal of Applied Physics

View full text Add to dashboard Cite

The grating formation speed in photorefractive polymers is greatly reduced by highly polar molecules incorporated by necessity in large concentrations to produce large diffraction efficiency and two-beam energy coupling gain. The random electric fields generated by these dipoles interfere with charge transport by increasing the width of the hopping site energy distribution and thus greatly reducing the carrier mobility and the photorefractive speed. We conducted transport studies of several model systems consisting of combinations of two polymer binders, six charge transport agents ͑four for holes and two for electrons͒, and varying concentrations of two highly polar electro-optic chromophores. The results confirm that carrier mobility is greatly reduced in the presence of polar molecules in accordance with the predictions of models of hopping transport in the presence of dipolar disorder. The randomly positioned and oriented dipoles increase the width of the hopping site energy distribution by an amount proportional to the square root of the dipole concentration and to the strength of the dipole moment. The results also show that transport agents with smaller dipole moments reduce the sensitivity to the dipolar effect. The photorefractive speed may therefore be increased by using transport agents with small dipole moments.

show abstract

Section: Discussionsupporting

confidence: 84%

Section: Discussionmentioning

confidence: 74%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Effect of dipolar molecules on carrier mobilities in photorefractive polymers

Goonesekera

Ducharme

1999

Journal of Applied Physics

View full text Add to dashboard Cite

show abstract

“…14,15 Here we show that the presence of large molecular dipole moments can be beneficial rather than disadvantageous for promoting charge hopping. Electrostatic attraction between the large fixed internal dipoles in these molecules leads to formation of centrosymmetric dimers with cofacial π-π antiparallel alignment along the long molecular axis.…”

mentioning

confidence: 70%

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

et al. 2015

View full text Add to dashboard Cite

We investigate dipolar donor materials mixed with a C 70 acceptor in an organic photovoltaic (OPV) cell. Dipolar donors that have donor-acceptor-acceptor (d-a-a') structure result in high conductivity pathways due to close coupling between neighboring molecules in the mixed films.We analyze the charge transfer properties of the dipolar donor:C 70 mixtures and corresponding neat donors using a combination of time-resolved electroluminescence from intermolecular polaron pair states and conductive tip atomic force microscopy, from which we infer that dimers of the d-a-a' donors tend to form a continuous network of nanocrystalline clusters within the blends. Additional insights are provided by quantum mechanical calculations of hole transfer coupling and hopping rates between donor molecules using nearest neighbor donor packing motifs taken from crystal structural data. The approximation using only nearest neighbor interactions leads to good agreement between donor hole hopping rates and the conductive properties of the donor:C 70 blends. This represents a significant simplification from requiring details of the nano-and mesoscale morphologies of thin films to estimate their electronic 2 characteristics. Using these dipolar donors, we obtain a maximum power conversion efficiency of 9.6 ± 0.5 % under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with C 70 .3

show abstract

“…[5,6] Via the calculation of local electric fields for a number of different hopping sites, it could be shown that the disorder parameter σ depends on the polarity and the polarizability of the transport material. [7] This result is supported by experiments demonstrating that polar side chains in polymeric semiconductors have a detrimental effect on charge carrier mobilities by increasing the disorder in the DOS. [2] A more profound understanding of these and other parameters influencing the disorder is needed to rationally design materials with favorable transport properties because small energetic disorder is decisive for efficient exciton and charge transport in molecular semiconducting thin films of optoelectronic devices.…”

mentioning

confidence: 81%

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

View full text Add to dashboard Cite

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...

show abstract

An assessment of the role of dipoles on the density-of-states function of disordered molecular solids

Cited by 244 publications

References 26 publications

Effect of dipolar molecules on carrier mobilities in photorefractive polymers

Effect of dipolar molecules on carrier mobilities in photorefractive polymers

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

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

Contact Info

Product

Resources

About