Model calculation of the optical properties of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films

Abstract

The marked difference between the optical absorption of dissolved PTCDA monomers and crystalline films is analysed in terms of the deformation of an effective internal vibrational mode of the molecule and transfer of Frenkel excitons between different molecular sites in the crystal. It is shown in detail that the coupling between equivalent molecules in different crystal unit cells, especially among stack neighbours, dominates over exciton transfer between the two different molecules in the crystal basis. The first type of interaction leads to a pronounced deformation of the expected Poisson progression over subsequent vibrational levels while the latter is responsible for a small Davydov splitting and, consequently, different absorption lineshapes along different crystal axes.

Anisotropic optical properties of single crystalline PTCDA studied by spectroscopic ellipsometry

“…However, for our measured lowest-energy exciton data we obtain / 0 ¼ 122:5 AE 3:5°, that would indicate a large admixture of CT character. This result, derived using [21], is in striking contrast to the approach of Vragovi c c et al [22] since they obtain an anisotropy by considering only FE character. Both calculations give the polarization ratios or ratio of oscillator strength of differently polarized transitions.…”

“…A Wannier-CT combination giving a rather extended and nearly spherical exciton [23] cannot explain the observed anisotropy. On the contrary, CT-FE mixed states [21] as well as pure FE excitons [22] could produce anisotropic in-plane optical spectra. Both these approaches [21,22] contain several adjustable parameters whose values must be obtained from fits to experimental data.…”

“…On the contrary, CT-FE mixed states [21] as well as pure FE excitons [22] could produce anisotropic in-plane optical spectra. Both these approaches [21,22] contain several adjustable parameters whose values must be obtained from fits to experimental data. Regarding anisotropy no experimental values were available, however, before our work [4].…”

“…The character of the lowest-energy exciton is not yet well established. Different possibilities from Wannier-like, across CT to FE, and their combinations, have been considered [21][22][23]. A Wannier-CT combination giving a rather extended and nearly spherical exciton [23] cannot explain the observed anisotropy.…”

Exciton emission in PTCDA films andPTCDA∕Alq3multilayers

“…In addition, the absorption spectrum of a 70 nm PTCDA film on Pyrex™ is displayed revealing a narrow absorption band at 2.22 eV that is attributed to a zero-vibron -* Frenkel exciton transition. 36 This 0-0 absorption line is followed by a broad and slightly structured band centered at 2.53 eV, which is attributed to Frenkel exciton transitions into higher vibronic subbands. A more detailed investigation on temperature-dependent absorption in PTCDA films 37 as well as a discussion about other absorption models will be given elsewhere.…”