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Disorder‐induced inhomogeneity in blue‐fluorescent‐based organic light‐emitting diodes (OLEDs) based on mixtures of host and guest molecules is studied using magneto‐electroluminescence, MEL(B), response based on the so called “Δgmechanism”, where Δgis the difference in the Landég‐factor of electrons and holes. The disorder in the organic active layer is manifested by a unique non‐Lorentzian MEL(B) response that is analyzed using a distribution of spin lifetimes for the injected electron–hole pairs that is determined by a dispersive parameter, α (<1). The carriers’ inhomogeneous response also influences the free carrier absorption spectrum, which shows characteristic properties described by a dispersive parameter β (<1). From the measured MEL(B) response at various injection conditions it is found that α is robust at increasing current density showing that the inhomogeneity is governed by intrinsic disorder in the device active layer. Also the obtained increase in α at low temperature indicates that the organic layer becomes more ordered, where longer‐lived electron–hole spin pairs are formed.

Organic‐based magnetic materials have been used for spintronic device applications as electrodes of spin aligned carriers and spin‐pumping substrates. Their advantages over more traditional inorganic magnets include reduced magnetic damping and lower fabrication costs. Vanadium tetracyanoethylene, V[TCNE]_x(x ≈ 2), is an organic‐based ferrimagnet with an above room‐temperature magnetic order temperature (T_c ≈ 400 K). V[TCNE]_xhas deposition flexibility and can be grown on a variety of substrates via low‐temperature chemical vapor deposition (CVD). A systematic study of V[TCNE]_xthin‐film CVD parameters to achieve optimal film quality, reproducibility, and excellent magnetic properties is reported. This is assessed by broadband ferromagnetic resonance (FMR) that shows most narrow linewidth of ≈1.5 Gauss and an extremely low Gilbert damping coefficient. The neat V[TCNE]_xfilms are shown to be efficient spin injectors via spin pumping into an adjacent platinum layer. Also, under an optimized FMR linewidth, the V[TCNE]_xfilms exhibit Fano‐type resonance with a continuum broadband absorption in the microwave range, which can be readily tuned by the microwave frequency.

The isotope effect is studied in the magneto‐electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light‐emitting diodes based on thermally activated delayed fluorescence (TADF) from donor–acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin‐dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin‐mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials.