In typical operation of organic light emitting diodes (OLEDs), excitons are assumed to generate with a ratio of 1:3 for singlet and triplet excitons, respectively, based on a simple spin statistics model. This assumption has been used in designing efficient OLEDs. Despite the larger generation ratio of triplet excitons, physical properties of fluorescent OLEDs are usually evaluated only through the electroluminescence (EL) intensity from singlets and the behaviors of triplets during the LED operation are virtually black-boxed, because the triplets are mostly non-emissive. Here, we employ transient spectroscopy combined with LED-operation for directly monitoring the non-emissive triplets of working OLEDs. The spectroscopic techniques are performed simultaneously with EL- and current measurements under various operation biases. The simultaneous measurements reveal that the relative formation ratio of singlet-to-triplet excitons dramatically changes with the magnitude of bias. The measurements also show that the generation efficiency of singlets scales with the bias, whereas that of triplets is nearly bias-independent. These features of the formation ratio and efficiency are compatibly explained by considering the yield of intersystem crossing and the energy separation of excitons from electron-hole pairs. The obtained findings via the spectroscopic measurements enable prediction of the formation pathways in OLEDs.

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Silicon Carbide (SiC) power semiconductor devices achieve low switching losses with fast switching operation and miniaturized power conversion circuit. SiC power module for high voltage and large current is subject to large di/dt and dv/dt in switching transient. Large di/dt induces surge voltage and ringing oscillation by interacting with circuit parasitic inductance and capacitance. These are fatal factors for electromagnetic interference (EMI) of the circuit. This report studies modeling of parasitic inductance in SiC power module based on threedimensional (3-D) electromagnetic analysis to realize low EMI design for power conversion circuit. The developed Partial Equivalent Element Circuit (PEEC) model of SiC power module is applied to circuit analysis. The simulated time response of voltage/current in switching operation and frequency spectrum of the conducted EMI for the tested converter agree with the measurement result well.

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