Study In Device Efficiency And Efficiency Roll-off Behaviors In Phosphorescent OLEDs

This thesis puts emphasis up on the factors influencing device efficiency and the most crucial problem, efficiency roll-off, in phosphorescent OLEDs (PhOLEDs) and the mthods to improve device efficiency; meanwhile, the fundamental mechanisms of several important processes determining device efficiency behavior will also be investigated.In PhOLEDs, the emission layer (EML) is typically composite of a fluorescent host and a phosphorescent guest. This thesis firstly gets an insight into the emission processes (the exciton formation on host/guest, the carrier ransport, etc.) which play important roles in device efficiency behaviors of PhOLEDs with such co-doped EML. The delayed electroluminescence (EL) technique is introduced into PhOLEDs, and the dependences of exciton formation and emission mechanisms in prompt and delayed EL on guest doping concentration and host enengy levels are studied systemically; the difference beween these two different emission processes are also discussed. From this study, the delayed EL technique is proved to be an efficient method to study exciton formation and carrier transport processes in PhOLEDs based on co-doped EMLs. The phenomena of increased exciton diffusion and released trapped charges with increasing temperature are observed, and these phenomena cause different resultant changes in efficiency behaviors depending on the different emission mechanisms; these observations provide some reference for the thermal stability study of PhOLEDs. It has also been found that the carrier transport and exciton formation processes change with the EML thickness, leading to different changes in device efficiency behaviors. To explain these results, a noval carrier transport mechanism depending on the EML thickness is illustrated.The direct evidence based on real EL devices (not mathematic calculations or deductions from photoluminescence study as in previous reports) for the different factors induced efficiency roll-off in PhOLEDs is provided, and triplet-polaron quenching (TPQ) is proved to be the main factor; the evidence for the host-host triplet-triplet annihilation (TTA) is also provided here. The dopant induced charge trapping effect on device efficiency and efficiency roll-off behaviors is investigated, and improved device performance with higher efficiency are obtained with optimized EMLs; meanwhile, from the new standpoint of dopant induced charge trapping, a general direction for the EML structural optimization is bringed out. To reduce the TPQ effect in EML, a new conception, with introducing traps centers by other materials instead of partial phosphorescent dopant, is proposed and proved with experiment results.The charge accumulation and exciton diffusion at/to the EML interface are two important factors leading to the fast decrease of efficiency with increasing current density. Hence, the charge induced quenching at the interface has also been learnt, and its effective radius, which is usually ignored, is proved to be a long distance by inserting different thickness of a wide bandgap material at hole transport layer/EML interface to separate the accumulated holes and excitons. To reduce charge accumulation and enhance charge injection at the interface, two simple methods are provided, including C60 modified ITO anode for enhanced hole injection and mixed electron transport layer/hole blocking layer interface for improved electron injection. Furthermore, to reduce charge accumulation and utilize the diffused excitons compensating green emission from Ir(ppy)3 at the interface, the green and efficient fluorescent material, C545T, is firstly doped with carrier transport materials at the transport layer/EML interface. From the resultant efficiency performance, this method is proved to be efficient in enhancing device efficiency and reducing efficiency roll-off.An essential method to reduce the quenching processes and enhance device efficiency is to shorten the lifetime of triplet excitons. The change of triplet lifetime with proximity to metal cathode is observed, and this is proved to be a change in the radiative lifetime; the shortened radiative lifetime of triplet excitons increases the PL efficiency, i.e. the instrinsic quantum efficiency, of the phosphorescent emissive layer, leading to a corresponding increase in the EL efficiency. Furthermore, the efficiency roll-off has been reduced. These results shed the light on phenomena affecting the efficiency behavior of PhOLEDs, and provide new insights for device design that can help enhance efficiency performance.