Seoyoung Paik

Spin Rabi flopping in the photocurrent of a polymer light-emitting diode

Electron spin is fundamental in electrical and optical properties of organic electronic devices. Despite recent interest in spin mixing and spin transport in organic semiconductors, the actual spin coherence times in these materials have remained elusive. Measurements of spin coherence provide impartial insight into spin relaxation mechanisms, which is significant in view of recent models of spin-dependent transport and recombination involving high levels of spin mixing. We demonstrate coherent manipulation of spins in an organic light-emitting diode (OLED), using nanosecond pulsed electrically detected electron spin resonance to drive singlet-triplet spin Rabi oscillations. By measuring the change in photovoltaic response due to spin-dependent recombination, we demonstrate spin control of electronic transport and thus directly observe spin coherence over 0.5 s. This surprisingly slow spin dephasing underlines that spin mixing is not responsible for magnetoresistance in OLEDs. The long coherence times and the spin manipulation demonstrated are crucially important for expanding the impact of organic spintronics.

Modulation frequency dependence of continuous-wave optically/electrically detected magnetic resonance

Continuous wave optically and electrically detected magnetic resonance spectroscopy (cwODMR/cwEDMR) allow the investigation of paramagnetic states involved in spin-dependent transitions, like recombination and transport. Although experimentally similar to conventional electron spin resonance (ESR), there exist limitations when applying models originally developed for ESR to observables (luminescence and electric current) of cwODMR and cwEDMR. Here we present closed-form solutions for the modulation frequency dependence of cwODMR and cwEDMR based on an intermediate pair recombination model and discuss ambiguities which arise when attempting to distinguish the dominant spin-dependent processes underlying experimental data. These include: 1) a large number of quantitatively different models cannot be differentiated, 2) signs of signals are determined not only by recombination, but also by other processes like dissociation, intersystem-crossing, pair generation, and even experimental parameter such as, modulation frequency, microwave power, and temperature, 3) radiative and non-radiative recombination cannot be distinguished due to the observed signs of cwODMR and cwEDMR experiments.

T1 and T2 spin relaxation time limitations of phosphorous donor electrons near crystalline silicon to silicon dioxide interface defects

31 P =1 0 15 cm �3 and 31 P =1 0 16 cm �3 at about liquid- 4 He temperatures T =5–1 5 K. Using pulsed electrically detected magnetic resonance pEDMR, spin-dependent transitions between the 31 P donor state and two distinguishable interface states are observed, namely, i Pb centers, which can be identified by their characteristic anisotropy, and ii a more isotropic center which is attributed to E defects of the SiO2 bulk close to the interface. Correlation measurements of the dynamics of spin-dependent recombination confirm that previously proposed transitions between 31 P and the interface defects take place. The influence of these electronic near-interface transitions on the 31 P donor spin-coherence time T2 as well as the donor spin-lattice relaxation time T1 is then investigated by comparison of spin Hahn-echo decay measurements obtained from conventional bulk sensitive pulsed electron paramagnetic resonance and surface sensitive pEDMR, as well as surface sensitive electrically detected inversion recovery experiments. The measurements reveal that the T2 times of both interface states and 31 P donor electron spins in proximity to them are consistently shorter than the T1 times, and both T2 and T1 times of the near-interface donors are reduced by several orders of magnitude from those in the bulk, at T 13 K. The T2 times of the 31 P donor electrons are in agreement with the prediction by de Sousa that they are limited by interface-defect induced field noise.

Spin-dependent processes in amorphous silicon-rich silicon-nitride

A study of spin-dependent charge carrier transitions in silicon-rich hydrogenated amorphous silicon-nitride (a-SiNx:H) p-i-n devices is presented. Pulsed electrically detected magnetic resonance allows us to determine the paramagnetic states that influence the photocurrent and provides insights into the nature of spin-coupling between charge carriers. We show that, in contrast to hydrogenated amorphous silicon, a-SiNx:H allows strongly spin-coupled, correlated (geminate) pairs of charge carriers to dissociate into nongeminate pairs that contribute to the photocurrent. This is discussed with regard to the application of a-SiNx:H as a photoelectrochemical electrode material.