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

Abstract

Pulsed electrically detected magnetic resonance offers a unique avenue to distinguish between polaronpair (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.

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

“…[14,50]. In Figure 1(a) the sketch of the experimental setup for carrying out simultaneous measurements of pEDMR and pODMR on OLED structures is shown.…”

“…In this case, device current changes will, of course, also induce recombination rate changes. An example of a spin-dependent transport mechanism which has previously been resolved directly in MEH-PPV devices is the quenching interaction between triplet excitons and charge carriers, the socalled triplet exciton-polaron mechanism [14,[43][44][45][46][47][48][49]. This process depends on triplet density and triplet lifetime, and therefore on temperature and exhibits clear signatures of s=1 triplet species in the half-field magnetic resonance.…”

“…For typical room temperature device operating conditions, it has been established in recent years that weakly spin-exchange and spin-dipolar coupled pairs of charge carriers with spin s=1/2 are responsible on their own for spin-related material effects such as DC magnetoresistance [9][10][11] or conductivity changes due to magnetic resonant excitation by a radiofrequency field [12]. One of the crucial experiments in this context was the demonstration that spin-dependent electric currents under strong coherent magnetic resonant excitation of the charge carrier spins reveal characteristic beat oscillations due to Rabi-nutation of weakly coupled pairs of s=1/2 spin states [13][14][15].…”

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

“…Here we perform pulsed electrically detected magnetic resonance (EDMR) [2,3] on devices consisting of a an organic -conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), similar to devices previously studied [1,4]. The polymer was incorporated into an OLED with indium tin oxide (ITO) and calcium electrodes, as well as a hole injection layer of poly(3,4-ethylenedioxythiophene) (PEDOT) for balanced hole injection [5].…”

“…The correspondence between electrically and optically detected pulsed electron spin resonance experiments [7] on similar structures indicates that the polaron pairs seen in this work are oppositely charged. However, this method may also be of use for materials which contain bipolaron pairs [8] (pairs of likecharge polarons) or other more complex spin pairings [4].…”