Electronic Polarization in Organic Crystals: A Comparative Study of Induced Dipoles and Intramolecular Charge Redistribution Schemes

D’Avino, Gabriele; Muccioli, Luca; Zannoni, Claudio; Beljonne, David; Soos, Z. G.

doi:10.1021/ct500618w

Cited by 94 publications

(141 citation statements)

References 84 publications

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“…The electrostatic interaction V within a DA pair and the Madelung energy M are computed by adopting the point-charge approximation of the molecular charge density based on ESP atomic charges computed for neutral/charged molecules and lattices. Crystal electrostatic sums have been obtained for finite clusters of increasing size using the MESCAL code [26].…”

Section: Methodsmentioning

confidence: 99%

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

Castagnetti

Masino

Rizzoli

et al. 2018

Phys. Rev. Materials

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We report extensive structural and spectroscopic characterization of four mixed stack chargetransfer (ms-CT) crystals formed by the electron donor 3,3',5,5'-tetramethylbenzidine (TMB) with Chloranil (CA), Bromanil (BA), 2,5-difluoro-tetracyanoquinodimethane (TCNQF2) and tetrafluorotetracyanoquinodimethane (TCNQF4). Together with the separately studied TMB-TCNQ [Phys. Rev. B 95, 024101 (2017)] the TMB-acceptor series span a wide range of degree of CT, from about 0.14 to 0.91, crossing the neutral-ionic interface, yet retaining similar packing and donor-acceptor CT integrals. First principle calculations of key phenomenological parameters allows us to get insight into the factors determining the degree of CT and other relevant physical properties.

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Section: Methodsmentioning

confidence: 99%

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

Castagnetti

Masino

Rizzoli

et al. 2018

Phys. Rev. Materials

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“…where ρ is the in-plane radius and (z) represents the distance between the hole (electron) and the interface in the surface-normal direction; 2/ 1 , 1 / 1 , and ε is the dielectric constant (5.84 for hexacene [38]). Since the effective mass of the hole in an organic semiconductor is expected to be much larger than that of a free electron, we approximate the reduced mass of the excitonic quasiparticle with the free electron mass and set the fixed hole position (z h ) so the binding energy of the lowest CTE matches the experimental value of -0.84 eV (see below).…”

Section: Introductionmentioning

confidence: 99%

“…The hole is treated as a point charge fixed in space within the hexacene layer and the electron is confined to the positive half-space (vacuum) by a potential at the dielectric slab/vacuum interface. The total interaction between the electron and hole is a sum of their screened Coulomb interaction (V h ) and the image potential (V i ) due to polarization of the surface [9]:where ρ is the in-plane radius and (z) represents the distance between the hole (electron) and the interface in the surface-normal direction; 2/ 1 , 1 / 1 , and ε is the dielectric constant (5.84 for hexacene [38]). Since the effective mass of the hole in an organic semiconductor is expected to be much larger than that of a free electron, we approximate the reduced mass of the excitonic quasiparticle with the free electron mass and set the fixed hole position (z h ) so the binding energy of the lowest CTE matches the experimental value of -0.84 eV (see below).…”

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confidence: 99%

Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface

et al. 2015

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How an electron-hole pair escapes the Coulomb potential at a donor/acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast timescales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor/vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.Charge generation in organic photovoltaic (OPV) devices is contingent upon the dissociation of excitons into charge-separated states across donor/acceptor (D/A) interfaces, a process that can occur on femtosecond timescales with near unity quantum efficiency [1][2][3]. However, such rapid formation of charge-separated states appears contrary to the excitonic nature of the materials that comprise organic solar cells [4]. Given their low dielectric constants, it is not immediately obvious how the electron-hole pair is able to escape the poorly screened Coulomb potential that can give rise to charge transfer excitons (CTEs) with binding energies an order of magnitude higher than thermal energy at room temperature [5][6][7][8][9]. Recent experimental [10][11][12][13][14] and theoretical [15][16][17] studies suggest that electronic delocalization enables the electron-hole pair to escape the CTE trap and promotes long-range charge separation, in agreement with the Onsager model for ionization in solution [18]. Excess energy from the offset in energy levels at the donor/acceptor interface [10][11][12][13][14][15][16][17] or from the initial excitation photon [19,20] is believed to assist long-range charge separation. The electron-hole pair in the CTE trap can also dissociate with the help of an additional photoexcitation step [ 21 ]. Despite this progress, the exact mechanism of long-range electron-hole pair formation at donor/acceptor interfaces remains Phys. Rev. Lett. submitted. 2 poorly defined and there are also seemingly contradicting findings of photo-carrier generation from low energy CTEs that are supposedly trapped [22][23][24]. In the latter case, there is likely a potential energy gradient that counters the Coulomb potential [25][26][27][28]. One universally present driving force for photo-carrier generation, which may be partially responsible for the initial longrange charge separation or the subsequent escape from the CTE trap, may be the entropic gain with increasing electron-hole separation [1,29 ]. However, there has been little experiment evidence for this proposal.Here we provide the first direct time domain view of entropy-driven charge separation using the model system of CTEs at a molecular semiconductor/vacuum interface. In this system, a molecular semiconductor is the donor and the free-electron...

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“…Given the Π matrix, iterative evaluation of the electric field parameters

boldx

and molecular parameters

boldy

is a straightforward and rapid process. This procedure will be hereafter referred to as Multipole Redistribution (MR) methodology, in analogy to the earlier CR approach …”

Section: Methodsmentioning

confidence: 99%

“…This extension is provided by molecular electric susceptibility, which constitutes the link between quantum chemistry of a single molecule and solid‐state calculations. As a result, electrostatic field theory combined with molecular response theory offers a complete calculation framework that in various implementations is used to predict diverse electrostatic properties such as material permittivity tensor or charge carriers polarization energy …”

Section: Introductionmentioning

confidence: 99%

Partial atomic multipoles for internally consistent microelectrostatic calculations

2017

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An extension of the extant microelectrostatic methodologies, based on the concept of distributed generalized polarizability matrix derived from the Coupled Perturbed Hartree-Fock (CPHF) equations, is proposed for self-consistent calculation of charge carrier and charge-transfer (CT) state electrostatic energies in molecular solids, including the doped, defected and disordered ones. The CPHF equations are solved only once and the generalized molecular polarizability they yield enables low cost iterations that mutually adjust the molecular electronic distributions and the local electric field in which the molecules are immersed. The approach offers a precise picture of molecular charge densities, accounting for atomic partial multipoles up to order 2, which allows one to reproduce the recently reported large charge-quadrupole contributions to CT state energies in low-symmetry local environments. It is particularly well suited for repetitive calculations for large clusters (up to 300,000 atoms), and may potentially be useful for describing electrostatic solvent effects. © 2017 Wiley Periodicals, Inc.

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Electronic Polarization in Organic Crystals: A Comparative Study of Induced Dipoles and Intramolecular Charge Redistribution Schemes

Cited by 94 publications

References 84 publications

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface

Partial atomic multipoles for internally consistent microelectrostatic calculations

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