  As for its inorganic counterparts the future developments in organic
electronics are driven by an advanced device miniaturization. Therefore, the
opto-electronic behavior of up-to-date devices is progressively governed by the
local structural environment. However, there is a lack of organic semiconductor
materials providing access to the fundamental structure-functionality relation,
either due to limitations by their inherent growth or their optical
characteristics. In this work we present a systematic investigation of the
optical states, so-called excitons, and their temporal evolution in the
prototypical organic semiconductor rubrene by means of time and temperature
dependent photoluminescence studies. This material offers the unique
possibility of preparing well-defined morphologies with adjustable degree of
confinement. By this approach we are able to confirm the direct influence on
the temperature dependent optical processes with picosecond resolution already
for a spatial localization of excitation on the \mu m length scale. While in
bulk single crystals the exciton decay dynamics are governed by thermally
activated singlet fission, excitons created in microcrystals are trapped by
dark states localized on the surface and leading to a pronounced enhancement of
their average lifetime. Our results highlight the impact of the local
environment on the excitonic states and their dynamics in organic
semiconductors. With respect to the spatial dimensions of organic thin film
devices, this correlation and the reported effects emerging by the confinement
have to be considered upon further miniaturization and in the development of
innovative device concepts, such as photovoltaic cells based on
triplet-harvesting.
