Immense magnetic response of exciplex light emission due to correlated spin-charge dynamics

TL;DR

This paper demonstrates exceptionally large magnetic-field effects in organic exciplex light-emitting devices at room temperature, driven by correlated spin-charge dynamics, with potential applications in magnetic sensing and device efficiency enhancement.

Contribution

It reveals that exciplex recombination in TADF-organic blends produces record-high magnetic-field effects, linked to spin mixing between triplet and singlet states, and shows how device conditioning amplifies these effects.

Findings

Magnetic-field effects exceeding 60% in organic blends at room temperature.

Effects greater than 4000% achieved through device conditioning.

Magnetic fields increase device power efficiency by 30%.

Abstract

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.