Studies On Performances And Its Emission Process Of Non-doped CBP/Ir（Ppy）₃ Phosphorescent Organic Light-emitting Devices

Comparing to inorganic light-emitting, organic light-emitting has a wide range of material selection, full-color display, low driving voltage, high brightness and efficiency; wide viewing angle, fast response, relatively simple manufacturing process, low cost, flexible displays and so on. Therefore, organic electroluminescences have been rapidly developed in recent years. It is believed to be the mainstream of the next generation of display devices.This paper investigate the performance of the non-doped PhOLEDs based on 4,40-bis(9-carbazolyl)-1,10-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)3) with the emitting layer(EML) of [CBP/Ir(ppy)3]n/CBP. In order to understand the emission processes (the exciton formation, the carrier ransport, the energy transfer, etc.) of the PhOLEDs with non-doped EML, the delayed electroluminescence (EL) technique is introduced into non-doped PhOLEDs and the dependences of exciton formation and emission mechanisms on the number of non-doped emission layer (EML) are studied systemically. From this study, the delayed EL technique is proved to be an efficient method to study the exciton formation and carrier transport processes in PhOLEDs based on non-doped EMLs.Firstly, the properties of the PhOLEDs with the non-doped EMLs of [CBP/Ir(ppy)3]n/CBP are studied. The impacting of the number of non-doped EMLs and guest layer thickness on the device performance has been analyzed. Experimental results show that using of appropriate number of non-doped EMLs and guest layer thickness can improve the brightness and efficiency of the devices.Then the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in PhOLEDs with non-doped EMLs. The delay EL results show that changing the number of the non-doped EML leads to marked changes in charge-trapping and host-host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal was detected in the delay EL measurement. It is concluded that the main electroluminescence mechanism in these device with less non-doped EML is the direct carrier recombination in Ir(ppy)3. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host-host TTA signal are detected. The trapped carrier increases when the non-doped EML increases. The host-host TTA signal shows that the carrier recombination and exciton creation on the host is not negligible and hence takes a role of host-guest energy transfer. All these results give the evidence that both the carrier recombination directly in Ir(ppy)3 and the energy transfer from CBP contribute to the emission of Ir(ppy)3. And the carrier recombination directly in Ir(ppy)3 is the main mechanism.