  Organic light emitting devices and solar cells are machines that create,
manipulate and destroy excited states in organic semiconductors. It is crucial
to characterize these excited states, or excitons, to optimize device
performance in applications like displays and solar energy harvesting. This is
complicated if the excited state is a triplet because the electronic transition
is dark with a vanishing oscillator strength. As a consequence, triplet state
spectroscopy must usually be performed at cryogenic temperatures to reduce
competition from non-radiative rates. Here, we control non-radiative rates by
engineering a solid-state host matrix containing the target molecule, allowing
the observation of phosphorescence at room temperature and alleviating
constraints of cryogenic experiments. We test these techniques on a wide range
of materials with functionalities spanning multi-exciton generation (singlet
exciton fission), organic light emitting device host materials, and thermally
activated delayed fluorescence type emitters. Control of non-radiative modes in
the matrix surrounding a target molecule may also have broader applications in
light emitting and photovoltaic devices.
