  As carriers slowly move through a disordered energy landscape in organic
semiconductors, tiny spatial variations in spin dynamics relieve spin blocking
at transport bottlenecks or in the electron-hole recombination process that
produces light. Large room-temperature magnetic-field effects (MFE) ensue in
the conductivity and luminescence. Sources of variable spin dynamics generate
much larger MFE if their spatial structure is correlated on the nanoscale with
the energetic sites governing conductivity or luminescence such as in
co-evaporated organic blends within which the electron resides on one molecule
and the hole on the other (an exciplex). Here we show that exciplex
recombination in blends exhibiting thermally-activated delayed fluorescence
(TADF) produces MFE in excess of 60% at room temperature. In addition, effects
greater than 4000% can be achieved by tuning the device's current-voltage
response curve by device conditioning. These immense MFEs are both the largest
reported values for their device type at room temperature. Our theory traces
this MFE and its unusual temperature dependence to changes in spin mixing
between triplet exciplexes and light-emitting singlet exciplexes. In contrast,
spin mixing of excitons is energetically suppressed, and thus spin mixing
produces comparatively weaker MFE in materials emitting light from excitons by
affecting the precursor pairs. Demonstration of immense MFE in common organic
blends provides a flexible and inexpensive pathway towards magnetic
functionality and field sensitivity in current organic devices without
patterning the constituent materials on the nanoscale. Magnetic fields increase
the power efficiency of unconditioned devices by 30% at room temperature, also
showing that magnetic fields may increase the efficiency of the TADF process.
