Triplet-Triplet Annihilation And Singlet-Singlet Annihilation In Organic Light-Emmiting Diodes

The lack of efficient and stable blue emitter is a bottle-neck for the industry of organic light-emitting diodes(OLEDs).Because researchers are looking for new approaches to solve this major OLED bottle-neck and issue,blue OLEDs employing delayed fluorescence emitters have garnered much attention in these years.The triplet-triplet annihilation(TTA)and thermally activated delayed fluorescence(TADF)are the two main approaches to generate the non-luminous triplet excitons into emissive singlet excitons.In this thesis,the possibility to realize efficient and stable blue OLEDs through TTA and TADF processes are investigated.1.The TTA process has been demonstrated in the solution of 2-methyl-9,10-di(naphthalen-2-yl)anthracene(MADN).Blue fluorescent OLEDs using MADN as the host are fabricated and compared with the controlled devices using traditional host materials.All devices show a high external quantum efficiency(EQE)of nearly 5%at low current density,while the EQEs at high current density are found to be mainly dependent on the Ti energy levels of the hosts.The TTA process cannot be confirmed by the transient photoluminescence and electroluminescence measurement.The small roll-off of MADN based OLEDs are then contributed to the low Ti energy of MADN,which quenches the triplet excitons and prevents the excited state saturation in the emitting layer.2.0n the basis of the fluorescence dynamics measurement for a blue TADF emitter,DMAC-DPS,in organic films,the singlet-singlet annihilation(SSA)rather than TTA or TPA(triplet-polaron annihilation)process is found to dominate the degradation rate of the blue TADF OLEDs.This means we should shorten the time of an exciton in its Si state rather than the total excited-state lifetime to avoid the bi-excitonic annihilation process in OLEDs.Accordingly,we deduced and finally demonstrated by newly-designed blue TADF materials that the half-lifetime(LT50)of blue devices can be significantly improved by increasing the fluorescence rate(kF)of the TADF emitter(LT50 ? kF2).Our study provides new insight into the degradation mechanism of blue OLEDs and a practicable route for the design of efficient and stable blue TADF materials.