SPECTROSCOPIC PROPERTIES OF A PROTOTYPIC ORGANIC
SEMICONDUCTOR: THE CASE OF PTCDA
In the present work, we derive a quantitative interpretation of the
large Stokes shift observed in crystalline PTCDA (3,4,9,10-perylene
tetra\-carboxylic dianhydride). This issue requires both a
microscopic analysis of the isolated molecule and the interactions
between the molecules in the crystalline phase. The starting point
is the computation of the deformation of the isolated molecule in
the relaxed excited state, resulting in elongations of internal
vibrational modes. On this basis, we can model the linear absorption
of dissolved PTCDA monomers. As the elongations of the internal
vibrations can also be observed in resonant Raman spectra obtained
on epitaxial films, we conclude that the internal geometric changes
in the relaxed excited state are only weakly influenced by the
surroundings. However, the optical properties in crystalline films
are strongly altered by exciton transfer between neighbouring
molecules. From microscopic calculations of the Frenkel exciton
transfer, we deduce a consistent set of model parameters for a
quantitative interpretation of recent experimental data concerning
the linear optical properties of bulk PTCDA. On this basis, we can
compute the ${\bf k}$-space dispersion of the excitonic transitions,
resulting in a minimum at the boundary of the Brillouin zone. The
origin of the low-temperature photoluminescence can be assigned to
this low-lying excited state, while the room-temperature
photoluminescence is attributed to charge transfer recombination
between positively and negatively charged stack neighbours. First
generalizations of the Frenkel exciton transfer model to ultrathin
films are discussed.
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