Differentiation between polaron-pair and triplet-exciton polaron spin-dependent mechanisms in organic light-emitting diodes by coherent spin beating

Pulsed electrically detected magnetic resonance offers a unique avenue to distinguish between polaron-pair (PP) and triplet-exciton polaron (TEP) spin-dependent recombination, which control the conductivity and magnetoresistivity of organic semiconductors. Which of these two fundamental processes dominates depends on carrier balance: by injecting surplus electrons we show that both processes simultaneously impact the device conductivity. The two mechanisms are distinguished by the presence of a half-field resonance, indicative of TEP interactions, and transient spin beating, the signature of PPs. Coherent spin Rabi flopping in the half-field (triplet) channel is observed, demonstrating that the triplet exciton has an ensemble phase coherence time of at least 60 ns, offering insight into the effect of carrier correlations on spin dephasing.

Figure 1

(a) IV curves of both balanced (dashed line) and electron-rich devices (solid line) showing a consistently higher current injection in the balanced devices over the imbalanced resulting from more effective hole injection as a result of the PEDOT. (b) The 15 K IV curve displays Child's law V${}^2$-dependent current from single carrier dominated injection of the devices without the PEDOT hole-injection layer.

Figure 2

X-band EDMR of OLEDs with (a) carrier balance and (b) electron preference. (c), (d) At 295 K, only the full-field resonance is observed. (e), (f) At 10 K, the half-field resonance is seen, which depends on carrier balance. Microwave frequencies are 9.66 GHz at 295 K and 9.74 GHz at10 K.

Figure 3

Transient EDMR spectra at different times after spin-resonant excitation at 10 K. (a) In the balanced device the spectrum is time invariant and described by two Gaussians (see text). (b) With excess electrons the spectrum (described by the same two Gaussians) changes with time, indicating the presence of two spin-dependent mechanisms.

Figure 4

Rabi flopping in the current of an electron-dominated device at 10 K. The differential current on resonance (B${}_0$ = 345.5 mT) is plotted as a function of the microwave-pulse duration and time after the pulse. During the application of the microwave (B${}_1$) field spin precession occurs, leading to Rabi flopping in the current along the vertical graph axis. At short times after B${}_1$ application single-frequency Rabi flopping is seen—the TEP mechanism (left panel; Fourier transform inset). At longer times, spin beating arises in the PP process (right panel) with a harmonic in the Fourier transform (top right).

Figure 5

Spin-1 (triplet) half-field Rabi flopping in the electron-rich device. As the temperature increases, both the half-field resonance amplitude (hf) and the corrected amplitude of the nonbeating component of the full-field resonance (ff) disappear (upper inset).