Transient Simulation of Current and Electrophosphorescence in Organic Light-Emitting Diodes Using the Master Equation

Lack of consistent and efficient analyses on electrical excitation, charge carrier, and exciton dynamics has inspired us to perform transient simulations on current and electroluminescence characteristics of a phosphorescent organic light-emitting diode (OLED). The backward Euler scheme is utilized to discretize the time-dependent master equation (ME) for charge carriers and excitons, and a Newton-Raphson approach ensures quadratic convergence in both potential and triplet occupancy. The OLEDs doped with various emitter profiles of Ir(ppy)₃ in the emission layer (EML) are investigated under step and pulse voltage stimuli. Results suggest that, under a high step voltage, a broadening recombination zone in sufficiently doped devices contributes to enhancement in turn-on electrophosphorescence. On the other hand, after a pulse stimulus is switched off, the decay in turn-off phosphorescence slows down with a longer pulse duration resulting from the temporary recombination of residual charge carriers within the EML. Nevertheless, provided that further spreading of charge carriers in the EML is inhibited, strengthening in the triplet quenching effects under highly growing triplet accumulation will lead to a faster phosphorescence decay.