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 (MFEs) ensue in the conductivity and luminescence. Sources of variable spin dynamics generate much larger MFEs if their spatial structure is correlated on the nanoscale with the energetic sites governing conductivity or luminescence such as in coevaporated 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 produces MFEs 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. Both of these immense MFEs are 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 MFEs in materials emitting light from excitons by affecting the precursor pairs. Demonstration of immense MFEs 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 thermally activated delayed fluorescence process.
4 More- Received 8 September 2015
DOI:https://doi.org/10.1103/PhysRevX.6.011011
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The generation and control of light for both electronic displays and general lighting has recently been dominated by solid-state devices called light-emitting diodes (LEDs). Most electronic displays use plastic LEDs to convert an electron’s energy into light, but the magnetic nature of the electron prevents some of the electronic excitations from efficiently producing light. Light will only emerge if the spin of the excited electron and the spin of the empty state (the hole) that the electron falls into are antiparallel (spin singlet configuration; total spin 0); light will not be produced in the threefold-more-common parallel configuration (spin triplet; total spin 1). The efficiency of plastic LEDs has recently improved with special blends of plastic with similar singlet and triplet excitation energies, which allow useless triplets to be converted into useful singlets via thermally delayed activated fluorescence. Here, we show that a small magnetic field helps this conversion in these special blends of plastic by scrambling the magnetic character of the electronic excitation, thereby converting triplets to singlets.
We focus on organic semiconductor blends with a layer thickness of 180 nm at room temperature (about 300 K). The size of the magnetoelectroluminescence (the change in light emission with magnetic field) exhibits thermal activation with an energy similar to thermally delayed activated fluorescence. In ordinary devices, the effect exceeds a factor of 2 at room temperature, and the effect increases even more with device manipulation known as “conditioning,” which refers to operating the device over a period of time at a relatively high current density. We find that this conditioning procedure is stable on time scales longer than half a day.
We anticipate that the use of magnetic fields to control room-temperature light emission in materials with unusual singlet-triplet spin-energy landscapes may provide a new way to integrate magnetic memory and storage with electronic displays and to improve the efficiency of organic LEDs.