Study On Transient Electroluminescence Decay Behaviors Of Phosphorluminescence OLEDs

The storage of charges both accumulated and trapped play an important role in charge transporting and reducing the performance of PhOLEDs, but the role of stored charges in the recombination process after the pulse voltage has not been addressed. Understanding the physical processes PhOLEDs is essential for structure designing and material chosing. The transient electrophosphorescence is of great importance to understand the dynamics of phosphorescent emission in organic devices. We present a comprehensive study on charge accumulation which elucidates carrier transport and exciton dynamics in such devices.1、Investigated the transient electrophosphorescence with different hole transport material and hole blocking material respectively. The transient EL is strongly dependent on device materials and structure.Only those devices with higher electron accumulation exhibit the spikes at～1-3μs. We monitored their dependence on the pulsed voltage polarity and amplitude. The spikes became stronger and appear earlier under reverse turn-off voltage, while the spikes delayed under the forward turn-off voltage untill it end.Our results shows that upon the removal of the voltage, the accumulated charges at the interface recombined, yielding the spike.The long tails was attributed to the charges trapped at the interface defects caused by spin-coating method.2、To further elucidate the origin of the spikes, we used Ir(ppy)3 as doping molecules with different host materials in PhOLEDs. The charge recombination, accumulation and light emission mechanisms of the PhOLEDs with different host materials were analyzed. These results showed the transient EL was strongly dependent on host materials, we found out that only the devices with the electro-active host material exhibited strong electroluminescence overshoots after turning off the voltage pulse at room temperature. To further elucidate the generality of the overshoots, we monitored their dependence on the dopant concentration. The transient EL results in host-guest-system devices demonstrated a direct link between the strong overshoot effect and charge trapping in the emitting guest molecules. The excessive electrons in the guest sites could be a major factor inducing significant strong overshoot phenomenon in the TAZ:Ir(ppy)3 layer. We attributed these overshoot effect to the electrons accumulated on Ir(ppy)3 sites and accumulated holes in the vicinity of the HBL/EML interface. The characteristics of overshoot effect were also investigated through the effect of the turn-off voltage. The overshoot effect becomes more significant with increasing the reverse applied electric field. As a result, we obtained a better understanding of carrier’s dynamics and recombination process of PhOLEDs after turning off the voltage pules.3、Transient current characterics from devices having different carrier-transporting layers were investigated. The transient current can be divided into three parts:positive current spike (Ip), negative current spike (In) and steady-state current (Is), we found out steady-state current (Is) is the operating current of PhOLEDs. Further study shows that the charge accumulation can significantly affect transient current. Charge carrier accumulation on the carrier transport layer (HTL/ETL) side of electrode interface are shown to play a significant role in establishing space charge limited current of PhOLEDs. Results show that negative current spike (In) correspond to the discharging processes of space charge. The current spikes were found only affected by the injection barrier and independent with internal device structure.As a result, our experimental results from transient electroluminescence measurements provide a new understanding of overshoot effect based on charge trapping on the emitting guest molecules in a guest-host device. Hence the transient EL can serve as an important tool to identify the dominant emitting mechanisms in the guest-host systems. We believe that transient current analysis will be useful for assessing space charge and capacitance of organic light-emitting devices.