Study Of Organic Light-Emitting Diodes Based On Radiative Coupling Of Spatially Separated Electron-Hole Pairs

At present,the normal device structure of OLEDs is anode/ hole transport layer(HTL)/ emission layer(EML)/ electron transport layer(ETL)/ cathode,the emission layer is achieved by doping organic emitters in host materials.And the luminescence mechanism is based on trap effect of guest or energy transfer from host to guest.As the guests,the conventional fluorescent molecules cannot utilize the triplet excitons for radiative transition and the maximum internal quantum efficiency is 25%.Although 100% internal quantum efficiency is achieved for phosphorescent emitters due to the strong spin orbital coupling effect between singlet and triplet excited states,non-renewable noble metal atoms are still indispensable for efficient phosphorescent emitters.It is meaningful to design pure organic emitters and new device structures.The devices based on organic plannar pn heterojunction consist of only two stacked organic functional layers,and the electroluminescence is achieved by interface exciplex.The high efficiency is achieved by thermally activated delayed fluorescent characteristic of exciplex.However,the narrow exciton distribution area generally causes poor luminance and severe efficiency roll-off.To solve these problems,a method by inserting an interlayer between HTL and ETL to get a long range exciplex was proposed.In chapter 2,we give a detailed discussion about the production process of long range exciplex and the behavior of carriers at the heterojunction.With inserting the interlayer mCP,the devices based on long range exciplex show much higher luminance and reduced efficiency roll-off than the devices based on TAPC/TmPyTZ interface exciplex.In chapter 3,a further discussion about the effect of different interlayers on the formation process of long range exicplex was given.Four materials with different HOMO levels: m-MTDATA,TAPC,DBCz-Ph and mCBP as interlayers and change the position of HTL and interlayers were selected to give a further discussion about the selective behavior of holes accumulated at the HTL/interlayer interface.In chapter 4,the long range exciplex was used as the exciton donor of conventional fluorescent emitters to achieve better color purity,and we got red OLEDs with maximum external quantum efficiency(EQE)value nearly 15%.Meanwhile,we give a detailed discussion about the energy transfer process of the devices based on normal host-guest structure,binary-mixed exciplex host,interface exciplex host and long range exciplex with conventional fluorescent emitters.In summary,we discussed the formation process,energy release process,dissociation process and energy transfer process of charge transfer excitons.Such device concept provides an effective strategy for radiative coupling of spatially separated electron-hole pairs towards structure-simplified and high performance fluorescent OLEDs.