  Triplet excited states in organic semiconductor materials and devices are
notoriously difficult to detect and study with established spectroscopic
methods. Yet, they are a crucial intermediate step in next-generation organic
light emitting diodes (OLED) that employ thermally activated delayed
fluorescence (TADF) to upconvert non-emissive triplets to emissive singlet
states. In organic photovoltaic (OPV) devices, however, triplets are an
efficiency-limiting exciton loss channel and are also involved in device
degradation. Here, we introduce an innovative spin-sensitive method to study
triplet states in both, optically excited organic semiconductor films, as well
as in electrically driven devices. The method of transient optically detected
magnetic resonance (trODMR) can be applied to all light-emitting materials
whose luminescence depends on paramagnetic spin states. It is thus an ideal
spectroscopic tool to distinguish different states involved and determine their
corresponding time scales. We unravel the role of intermediate excited spin
states in opto-electronic and photovoltaic materials and devices and reveal
fundamental differences in electrically and optically induced triplet states.

Applied Physics;Materials Science