Influence of injection layer on transient electroluminescence lifetime in OLED during switch-off

We present a study of the transient electroluminescence (TREL) of a polyfluorenes-blend based light emitting diodes during the turn-off cycle. We observed a bi-exponential decay of the transient electroluminescence, a fast decay with a lifetime of tens of nano-seconds, followed by a slow decay with a lifetime of hundreds of nano-seconds. We attribute the slow decay to the transit time via drift of the mobile charges between the space charge regions during discharge. We tested this hypothesis using various methods that affect the electric field in the bulk region. In particular, we modified the injection efficiency by varying the thickness of a LiF electron injection layer. The variation of the injection layer thickness influences the width of the space-charge region and hence influences the division of the voltage drop between the space charge regions and the bulk. In all cases, the electric field inversely correlated with the measured decay time, supporting our assumptionThese observations are supported by simulation based on the drift-diffusion model.