Relating transient electroluminescence lifetime and bulk transit time in OLED during switch-off

We present a study on the dependence of the transient electroluminescence lifetimes in polyfluorene-blend based light emitting diodes on the injection efficiency during the turn-off cycle. Using a specially designed driver that minimizes the interaction of the outer switch-off circuit with the diode, we were able to keep the light intensity continuous during the switching and allow the study of the dynamic mechanisms of intrinsic charge. 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 found that the slow lifetime increases, by more than 25 percent, with the increase in the injection efficiency, controlled by varying the thickness of a LiF injection layer. We attribute the slow decay to the transit time via the drift of the mobile charges between the space charge regions during discharge. We tested and verified this hypothesis using two methods that affect the electric field in the bulk region. In the first we controlled the thickness of the active layer and in the second we varied the applied operation voltage. Based on this model, we showed that the variation in the thickness of the injection layer 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. We found that under a constant applied voltage, devices with higher injection efficiency have a smaller electric field in the bulk, and therefore, a longer transit time, which correlates with the EL decay lifetime. These observations are supported by simulation based on the drift-diffusion model.