Optical Reflection Spectroscopy of Organic Solids

Philpott, Michael R.

doi:10.1146/annurev.pc.31.100180.000525

Cited by 28 publications

(9 citation statements)

References 64 publications

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“…Typically, spectroscopic studies are performed with large-scale crystalline samples, where the large oscillator strengths (and hence strong absorption) of molecular solids prohibit the use of transmission geometry due to limitations in dynamic range. Therefore, experiments are often performed in reflection geometry and are analyzed by using the Kramers-Kronig transformation (15)(16)(17), a relation only rigorously valid if the entire spectrum is considered.…”

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confidence: 99%

Low-lying excited states in crystalline perylene

Rangel

Rinn

Sharifzadeh

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with firstprinciples calculations for the absorption based on the GW plus Bethe-Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying excitations in perylene emerges, including evidence of an exciton-polariton stopband, as well as an assessment of the commonly used Tamm-Dancoff approximation to the GW-BSE approach. Our findings on this well-controlled system can guide understanding and development of advanced molecular solids and functionalization for applications. molecular crystals | many-body perturbation theory | excited states | spectroscopy

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confidence: 99%

Low-lying excited states in crystalline perylene

Rangel

Rinn

Sharifzadeh

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…In contrast to the short-range forces, the long-range contribution is fairly well approximated by dipole-dipole interaction, and can be evaluated analytically in the framework of the continuum optical model. 10,11 For any angle ␤ between k and the transition dipole ed, the exciton frequency reads where p 2 ϭ4me 2 /m e c is the plasma frequency, c being the volume of the unit cell, and m the number of nonequivalent molecules; T is the short-range contribution to the exciton frequency, which coincides with the energy of transverse excitons (␤ϭ90°). The second term in Eq.…”

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Primary optical excitations and excited-state interaction energies in sexithiophene

et al. 2002

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Based on a unique combination of angle-resolved transmission spectroscopy and transmission data at high pressure, we identify the primary photoexcitations and the relevant excited-state interaction energies in a sexithiophene crystal. Optical excitations include charge-transfer excitons and Davydov polaritons. By extrapolation, we predict that in sexithiophene at hydrostatic pressures above 180 kbar, intermolecular excitations are lower in energy than intramolecular ones. The results are representative for a wide class of -conjugated molecular semiconductors because ͑1͒ the pertinent interaction energies and lengths scales are nearly identical and ͑2͒ published data on different molecules are consistent with our interpretation. DOI: 10.1103/PhysRevB.66.113102 PACS number͑s͒: 78.40.Me, 78.55.Kz, 78.66.Qn Linear -conjugated molecules, such as oligothiophenes, oligophenylenes, and oligophenylene-vinylenes, are attracting increasing attention for their use in electronic and optoelectronic devices.1 Despite the high technological interest, the knowledge of their fundamental electronic and optical properties remains inadequate for various aspects. Our interest is to address to the nature and interacting energies of the primary photoexcitations, for which long-lasting and unresolved controversies exist. [2][3][4][5][6] In the crystal, the lowest molecular excitation gives rise to as many intramolecular exciton bands as the number of nonequivalent molecules in the unit cell. The energy difference between the lowest and highest exciton transitions, the socalled Davydov splitting (E DS ), represents a fundamental energy scale, which provides an estimate of the intermolecular interaction strength in the excited state. Experimental optical spectra feature, however, complex patterns, which have precluded an unequivocal evaluation of E DS .In crystals, excitons with small wave vector k couple to photons with the same quasimomentum to yield a new particle called polariton. 7 The latter describes the photonexciton dynamics when the interaction energy (E EPI ) with photons exceeds any line broadening ͑strong-coupling regime͒. The polariton concept is the extension of the weakcoupling limit case, in which photons and delocalized excitations of matter are treated as independent particles. In linear -conjugated molecules, the lowest -* transition exhibits a very large oscillator strength (ӷ1). Therefore, we expect that polaritons, rather than bare excitons, are the primary photoexcitations in the crystal. The ensuing optical response can differ substantially from the one predicted in the weak-coupling regime.Besides intramolecular Frenkel excitons, crystals also support excitations for which the bound electron and hole reside on different molecules ͓charge-transfer excitons ͑CTE͔͒. The energy difference E B between these two kinds of excitations provides an estimate of the intermolecular interaction energy between electrons and holes. Positive ͑nega-tive͒ E B implies that stable exitations have an intramolecular ͑intermolecular...

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“…The polariton model has also been used to explain directional dispersion in reflectance and emission spectra from cyanine dye crystals (118) and thin GaAs layers (119). Surface exciton-polaritons (120) have been used to explain similar observations in attenuated total reflection (ATR) signals from ZnO (121,122).…”

Section: Excitons and The Exciton-photon Interactionmentioning

confidence: 99%

Nonlinear polariton effects in naphthalene

Stevenson¹

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Optical Reflection Spectroscopy of Organic Solids

Cited by 28 publications

References 64 publications

Low-lying excited states in crystalline perylene

Low-lying excited states in crystalline perylene

Primary optical excitations and excited-state interaction energies in sexithiophene

Nonlinear polariton effects in naphthalene

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