On Charge Carrier Photogeneration Mechanisms in Organic Molecular Crystals

Silinsh, E. A.; Kolesnikov, V. A.; Muzikante, I.; Balode, D. R.

doi:10.1002/pssb.2221130141

Cited by 71 publications

(20 citation statements)

References 25 publications

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“…Applying the polarization model to the gas-phase gap of 4.57 eV, obtained from a GW calculation based on a consistent starting point, as described in Ref. 31, we obtain a bulk gap of 2.22 eV in good agreement with experiment 32 and with explicit GW calculations for pentacene crystal. 3 We note that the optical gap is further reduced with respect to the fundamental gap due to excitonic effects, 3,13,18 which are not accounted for by the SCS model.…”

supporting

confidence: 76%

Electrodynamic response and stability of molecular crystals

Schatschneider

Liang²,

Reilly

et al. 2013

Phys. Rev. B

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We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal "atom-in-a-molecule" model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals. Polyacene molecular crystals form a fundamental class of aromatic solids, and have been extensively studied as potential materials for organic electronics. [1][2][3] It is understood that the optical properties of polyacenes are very sensitive to longrange intra-and intermolecular electrodynamic interactions. This is reflected by shifts in the optical absorption frequencies upon increasing the molecule size or upon solid formation, 4 and is further exhibited by the visible color of oligoacene crystals, which changes from transparent in naphthalene and anthracene, to bright orange in tetracene, and deep blue in pentacene. 4,5 The optical absorption spectrum is directly related to the polarizability through the Kramers-Kronig transformation.6 Therefore, the observed changes in the optical spectrum upon crystallization of polyacenes are accompanied by a change in the molecular polarizability. In addition, these changes in polarization should directly impact the crystal lattice energy. However, the effect of electrodynamic intermolecular interactions on the cohesive properties of molecular crystals remains poorly understood. In this Rapid Communication, we show that the dipolar electrodynamic coupling between polyacene molecules reduces the solid dielectric constant by 15%, and has an impact of up to 0.5 eV per molecule on the computed van der Waals (vdW) energies and sublimation enthalpies of these molecular crystals. Our results imply that electrodynamic response is crucial for describing both the cohesive energy and the optical properties of molecular crystals, also providing strong quantitative support to empirical relations between stability and refractive index of molecular crystals. 7Polyacene crystals are extended aromatic networks characterized by polarizable π clouds. Therefore, an appreciable part of the crystal lattice energy stems from ubiquitous attractive vdW dispersion interactions. When studying the cohesion of molecular systems, for example, using densityfunctional theory (DFT) 8,9 or classical potentials, 10 the vdW energy is typically computed using effective polarizabilities for hybridized "atoms" inside a molecule. It is common to approximate the frequency-dependent polarizability of every atom using a single effective excitation frequency (also called t...

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supporting

confidence: 76%

Electrodynamic response and stability of molecular crystals

Schatschneider

Liang²,

Reilly

et al. 2013

Phys. Rev. B

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“…The gratifying feature about the present results is that they confirm earlier conclusions derived from short-time transit signals [6, 71, thus proving that those initial photocurrent spikes lasting for some 10 ns are indeed due to motion of free carriers before relaxing to a trap-controlled mobility state rather than to a polarization current resulting from orientation of coulonibically bound electron-hole pairs in the applied field before they either dissociate or recombine [15]. By the same token they confirm that the dispersive transport regime in a disordered organic solid is confined to a short time interval after carrier generation [ 161.…”

Section: Discussionsupporting

confidence: 87%

Mobility and Recombination Coefficient of Charge Carriers in Disordered Pentacene

Maruyama

Bäßler

1981

Physica Status Solidi (b)

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Both transient and stationary photoconduction of pentacene layers prepared by vapor deposition a t 180 K and annealing a t 295 K is studied in a surface electrode arrangement. Except a t low light intensities photocurrents are recombination controlled. A ratio y/,u = 1.8 x V cm is obtained. The sum of electron and hole mobility monitored in a time interval 5 x s after carrier generation is cm2/Vs. It agrees with a value determined previously employing the initial photocurrent pulse method thus proving the reliability of that method to monitor transport properties of disordered solids on a short time scale.Unter Anwendung der Oberfllchen-Elektroden-Anordnung wird das zeitabhiingige und stationare Photoleitungsverhaltn von Pentazen-Schichten untersucht, die bei 180 K aus der Dampfphase abgeschieden und bei 295 K getempert werden. AuBer bei niedrigen Lichtintensitiiten sind die Photostrome rekombinationsbestimmt. Das Verhaltnis y/,u wird zu 1,8 x lo-' V cm bestimmt.Die Summe von Elektronen und Locherbeweglichkeit, verfolgt im Zeitbereich 5 X bis 5 x cm2/Vs. Sie stimmt mit einem Wert uberein, der kurzlich aus einer Analyse von Kurzzeit-Photoirnpulsen gewonnen wurde. Dies stellt die Aussagekraft jener Methoden hinsichtlich des Kurzzeit-Transportverhaltens ungeordneter Festkorper unter Beweis.to 5 x s nach Ladungstrager-Erzeugung, ergibt sich zu

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“…7 ) and is reported to be 2.5-2.7 eV for PTCDA. [10][11][12] Both interpretations lead to transport gaps that are in the range of those extracted from (photo) conductivity measurements 11,97 but are clearly based on differing and opposing physical intuition. Moreover, our GW calculations of the bulk transport gap, with differing GW start points, yield 2.2-2.4 eV for pentacene and 2.7-3.0 eV for PTCDA, in excellent agreement with both the above-mentioned transport-based data and the experimental spread of photoemission based data.…”

Section: B Bandstructure Densities Of States and Photoemissionmentioning

confidence: 99%

Quasiparticle and optical spectroscopy of the organic semiconductors pentacene and PTCDA from first principles

et al. 2012

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The broad use of organic semiconductors for optoelectronic applications relies on quantitative understanding and control of their spectroscopic properties. Of paramount importance are the transport gapthe difference between ionization potential and electron affinityand the exciton binding energyinferred from the difference between the transport and optical absorption gaps.Transport gaps are commonly established via photoemission and inverse photoemission spectroscopy (PES/IPES). However, PES and IPES are surface-sensitive, average over a dynamic lattice, and are subject to extrinsic effects, leading to significant uncertainty in gaps.Here, we use density functional theory and many-body perturbation theory to calculate the spectroscopic properties of two prototypical organic semiconductors, pentacene and 3,4,9,10perylene tetracarboxylic dianhydride (PTCDA), quantitatively comparing with measured PES, IPES, and optical absorption spectra. For bulk pentacene and PTCDA, the computed transport gaps are 2.4 and 3.0 eV, and optical gaps are 1.7 and 2.1 eV, respectively. Computed bulk quasiparticle spectra are in excellent agreement with surface-sensitive photoemission measurements over several eV only if the measured gap is reduced by 0.6 eV for pentacene, and 0.6-0.9 eV for PTCDA. We attribute this redshift to several physical effects, including incomplete charge screening at the surface, static and dynamical disorder, and experimental resolution. Optical gaps are in excellent agreement with experiment, with solid-state exciton binding energies of ~0.5 eV for both systems; for pentacene, the exciton is delocalized over several molecules and exhibits significant charge transfer character. Our parameter-free calculations provide new interpretation of spectroscopic properties of organic semiconductors critical to optoelectronics.

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On Charge Carrier Photogeneration Mechanisms in Organic Molecular Crystals

Cited by 71 publications

References 25 publications

Electrodynamic response and stability of molecular crystals

Electrodynamic response and stability of molecular crystals

Mobility and Recombination Coefficient of Charge Carriers in Disordered Pentacene

Quasiparticle and optical spectroscopy of the organic semiconductors pentacene and PTCDA from first principles

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