K. W. Lee

Impedance spectroscopy of a poly(p-phenylenevinylene)-based light-emitting diode device

A poly-p-phenylenevinylene (PPV)-based light-emitting diode (LED) device was studied by means of comprehensive ac impedance measurements. Separate regions of frequency-dependent responses, the bulk and depleted (interface) regions, with distinct electrical properties, were identified and characterized. The ac J–E characteristics revealed the space-charge limited conduction mechanism, which was specifically assigned to the depleted region. Besides, a direct estimate of the trap density in the bulk region was obtained from the applied voltage dependence of the charge concentration.

Phase transitions in layer structured (C10H21NH3)2SnCl6

Phase transitions in the quasi-two-dimensional layer structured (C10H21NH3)2SnCl6, an order–disorder transition and a conformational transition of the hydrocarbon chains, were studied by means of 1H NMR (nuclear magnetic resonance). A newly defined order parameter in the low-temperature phase, the dipolar splitting of the rigid hydrocarbon lineshape component, was shown to represent the order–disorder motion of the hydrocarbon chains, explicitly revealing its three-dimensionality. The conformational transition was marked by a discontinuity in the spin-lattice relaxation rate (1/T1). On the other hand, no obvious anomaly was observed in the spin-lattice relaxation rate at the order–disorder transition at 200 MHz, whereas a weak anomaly was observed at 45 MHz, indicating that the critical dynamics associated with the order–disorder transition is manifest in the low-frequency region of the spectral density.