  The orientation of luminescent molecules in organic light-emitting diodes
(OLEDs) strongly influences device performance. However, our understanding of
the factors controlling emitter orientation is limited as current measurements
only provide ensemble-averaged orientation values. Here, we use single-molecule
imaging to measure the transition dipole orientation of individual molecules in
a state-of-the-art thermally evaporated host and thereby obtain complete
orientation distributions of the hyperfluorescence-terminal emitter C545T. We
achieve this by realizing ultra-low doping concentrations (10-6 wt%) of C545T
and minimising background levels to reliably measure the photoluminescence of
the emitter. This approach yields the orientation distributions of >1000
individual emitter molecules in a system relevant to vacuum-processed OLEDs.
Analysis of solution- and vacuum-processed systems reveals that the orientation
distributions strongly depend on the nanoscale environment of the emitter. This
work opens the door to attaining unprecedented information on the factors that
determine emitter orientation in current and future material systems for OLEDs.
