Immense magnetic response of exciplex light emission due to correlated spin-charge dynamics
| Date/Time: | Wednesday, 25 May 2016 from 11:00 am to 11:50 am |
|---|---|
| Location: | Physics Hall Room 18/19 |
| Phone: | 515-294-7377 |
| Channel: | College of Liberal Arts and Sciences |
| Actions: | Download iCal/vCal | Email Reminder |
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). I will describe our recent discovery 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. Our
theoretical description of this process traces this MFE and its unusual
temperature dependence to changes in spin mixing between triplet
exciplexes and light-emitting singlet exciplexes. 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.