Immense Magnetic Response of Exciplex Light Emission due to Correlated Spin-Charge Dynamics

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.

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.

Figure 1

Exciplexes, $\mathrm{\Delta }g$ mechanism, and hyperfine mechanism for magnetic field effects. (a) Schematic of the exciplex with m-MTDATA as the donor and 3TPYMB as the acceptor. The electron (hole) wave function is schematically represented by the orange (gray) oval and is overlaid upon the respective chemical structures. (b) $\mathrm{\Delta }g$ spin mixing between the electron and hole spins (shown to be orthogonal to the applied field $\mathit{B}$) which are initiated at time $t_0$ in the $|T_0⟩$ spin configuration. In the top panel, where there is no difference in $g$ factors, the spins remain in their initial configuration. In the lower panel, with nonzero $\mathrm{\Delta }g$, the spinor picks up singlet ($|S⟩$) character over time $t_1$. (c) As in (b), but hyperfine spin mixing between electron and hole spins. In this case, spin mixing occurs among all four spin states. (d) Energy diagram of exciplex showing the exchange splitting ${\mathrm{\Delta }}_{\mathrm{ST}}$ between the singlet and triplet levels. For efficient TADF, ${\mathrm{\Delta }}_{\mathrm{ST}}$ should be similar in magnitude to the thermal energy $k_{B}T$. The much quicker singlet recombination (with rate $k_S$) is depicted by the thicker arrow compared to triplet recombination (with rate $k_T$) and luminescence is assumed only for the singlet recombination. Spin mixing occurs due to the $\mathrm{\Delta }g$ mechanism (i.e., transitions between $S$ and the activated $T_0$ level of $T^{*}$) and the hyperfine mechanism (i.e., transitions between $S$ and all activated triplet levels of $T^{*}$). (e) Theoretical calculations for the $\mathrm{MEL}=\mathrm{\Delta }\mathrm{EL}/\mathrm{EL}$ at large field with the $\mathrm{\Delta }g$ mechanism ($B_{0\mathrm{hf}}=0$). (f) Theoretical calculations for the $\mathrm{MEL}=\mathrm{\Delta }\mathrm{EL}/\mathrm{EL}$ from hyperfine spin mixing at large field ($\mathrm{\Delta }g=0$). Increasing either $G_T/G_S$ or $B_{0\mathrm{hf}}$ causes the MEL to be more negative for exciplex recombination values. Orange lines are $G_T/G_S=0.25$ and black lines are $G_T/G_S=1$. For panels (e) and (f), $k_D$ is the exciplex dissociation rate and $G_S$ is the singlet exciplex generation rate, whereas $G_T$ is the generation rate for thermally activated triplet exciplexes (see text for additional details).

Figure 2

Exciplex device structure and emission spectrum. (a) Schematic device structure showing the two measurement modes we use. The solid circuit is for a constant-voltage measurement, whereas the dashed circuit is for a constant current. (b) Electroluminescence spectrum for our device (red dashed line), compared with exciplex emission (solid black line) and exciton emission spectra (solid blue and green lines) taken from the literature [3]. This shows that exciplexes are responsible for EL in our devices.

Figure 3

Molecular structures of (a) MEH-PPV and (b) ${\mathrm{Alq}}_3$ that have been used in control experiments. Typical magnetconductance (red line) and magnetoelectroluminescence (blue line) in (c) MEH-PPV and (d) ${\mathrm{Alq}}_3$ control devices that are dominated by exciton emission. The dashed black lines represent fits to the non-Lorentzian line shape (see text).

Figure 4

(a) Magnetoconductance (MC) and (b) magnetoelectroluminescence (MEL) before device conditioning for three representative constant voltages. (c) Magnetovoltage and (d) magnetoelectroluminescence before device conditioning for three representative constant currents. The same color is used throughout the figure to designate measurements with a comparable current density. Thick dashed lines are non-Lorentzian (as defined in text) fits to the largest displayed curve in each panel.

Figure 5

Magnetic-field effect on electroluminescence efficiency $\eta$ in the pristine device for three representative constant voltages. The curves shown here are for the same device and experimental data set as in Figs. 4 and 4.

Figure 6

(a) Magnetoconductance and (b) magnetoelectroluminescence after device conditioning for three representative constant voltages. (c) Magnetovoltage and (d) magnetoelectroluminescence after device conditioning for three representative constant currents. The same color is used throughout the figure to designate measurements with a comparable current density. Thick dashed lines are non-Lorentzian (as defined in text) fits to the largest displayed curve in each panel.

Figure 7

(a),(b) Temperature dependence of magnetic-field effects in optimally conditioned device. Dashed lines are non-Lorentzian (as defined in text) fits. (c),(d) Arrhenius plot of the saturation magnetic-field effect extracted from the data in (a) and (b), respectively. Solid lines are fits to an activated Boltzmann law indicating the range of data points used in the fit, whereas the dashed lines are extrapolations to lower temperatures. (e) Dependence of the saturation magnetoconductance ($\mathrm{\Delta }I/I$) and magnetoluminescence ($\mathrm{\Delta }\mathrm{EL}/\mathrm{EL}$) versus the thickness of the coevaporated layer, whereas (f) plots the same quantities versus the composition of the coevaporated layer.

Figure 8

(a) Current-voltage and (b) electroluminescence-voltage curves for different stages of device conditioning [protocols of the conditioning are specified as labels to the curves of (a); other panels use same color coding]. Dashed (solid) lines are measurements at $B=0$ (0.3) T. (c),(d) Constant-voltage and (e),(f) constant-current magnetic-field effect measurements for varying device conditioning.

Figure 9

Magnetoconductance and magnetoelectroluminescence in TADF-exciplex-based devices that differ in the fabrication procedure used to deposit the top metal electrode. (a),(b) Devices whose top electrode is fabricated by thermal evaporation. These devices have been maximally conditioned. (c),(d) Devices whose top electrode is fabricated by $e$-beam evaporation. These devices are not sensitive to device conditioning.

Figure 10

(a),(b) Conditioning dependence of the device studied in Fig. 6. The blue line is for the optimally conditioned device after a rest period of 12 h and shows that the effect does not return to the pristine value.

Figure 11

Magnetotransport and magnetoluminescence in MEH-PPV and ${\mathrm{Alq}}_3$ devices. Magnetotransport and magnetoluminescence at (a),(b) constant voltage and (c),(d) constant current before and after device conditioning in MEH-PPV and ${\mathrm{Alq}}_3$ devices.