Slow Hopping and Spin Dephasing of Coulombically Bound Polaron Pairs in an Organic Semiconductor at Room Temperature

Polaron pairs are intermediate electronic states that are integral to the optoelectronic conversion process in organic semiconductors. Here, we report on electrically detected spin echoes arising from direct quantum control of polaron pair spins in an organic light-emitting diode at room temperature. This approach reveals phase coherence on a microsecond time scale, and offers a direct way to probe charge recombination and dissociation processes in organic devices, revealing temperature-independent intermolecular carrier hopping on slow time scales. In addition, the long spin phase coherence time at room temperature is of potential interest for developing quantum-enhanced sensors and information processing systems which operate at room temperature.

Figure 1

(a) The important step in both the light-to-charge (solar cell) and charge-to-light conversion (OLED) is the polaron pair, a state that is accessible by the spectroscopic method discussed. Polaron pairs exist both in the singlet ($S$) and triplet ($T$) configuration and can couple to excitons at energies $E_S,E_T$. (b) An electron hopping through sites 1-2-3 (blue) and hole (red) form a carrier pair. As charges hop from site to site within the Onsager radius of the Coulombically bound pair, they experience an effective temporal fluctuation in the local magnetic field, even when the nuclear spin ensemble is quasistatic. The decoherence time therefore places an upper limit on the intersite hopping rate of charges within a pair. (c) The measured loss of phase coherence within a pair can be seen as a hopping event to a new Overhauser site during the spin-echo sequence, where the current change $\Delta I$ is proportional to the singlet content of the wave function.

Figure 2

Experimentally observed magnetic resonance spectrum and spin echoes. (a) The spectrum is described by two Gaussians [orange (wide) and purple (narrow) lines] representing the hyperfine field-broadened resonance of electron and hole [sum of Gaussians: green line (fit to data)]. (b) Using a Carr-Purcell (CP) spin-echo pulse sequence as described in Fig. 1c, the effect of spin dephasing can be removed, providing a measure of the intrinsic phase coherence time $T_2$. The three black curves show CP echoes scaled to the time axis. The echo intensity follows an exponential decay with time and depends only weakly on temperature. All measurements were performed at 295 K unless otherwise marked. The inset shows $T_2$ values at different device currents to exclude the possibility of current-induced dephasing.

Figure 3

Computational modeling of the expected echo decay time $T_2^{\mathrm{sim}}$ as a function of polaron hopping time for an ensemble of polaron pairs. The inset shows the computed decay for a single hopping time, simulated for a number of different echo wait times $2\tau$. The echo is described by an exponential decay. (a) The simulated decoherence time $T_2^{\mathrm{sim}}$ is plotted in the main panel as a function of hopping time. For very short hopping times, $T_2^{\mathrm{sim}}$ increases with decreasing hopping time due to motional narrowing. The simulated data are well described by the relation $T_2^{\mathrm{sim}}=t_{\mathrm{hop}}+t_d^{\prime }$, where $t_d^{\prime }$ describes on-site dephasing due to the local Overhauser fields estimated from the resonance line width. The modeled decay time coincides with the experimentally measured time of 320 ns for two hopping times (red arrows). (b) The expected resonance line width depends on hopping time due to motional narrowing, providing a measure to differentiate between the two possible hopping times.