Magnetic Field Effects And Optoelectronic Properties Of OLEDs

Organic light-emitting diodes（OLEDs）have shown great potential in the fields of solid-state lighting and display,which attract much attention by researchers.At present,OLEDs based on traditional fluorescence materials can only utilize 25%singlet excitons to emit light,which largely blocks their practical applications and development.Therefore,to further improve the electroluminescence（EL）performance of OLEDs,developing novel material systems and strategically designing device structures to fully utilize excitons are still necessary,and it is also important to deeply study the exciton dynamics and working mechanism of OLEDs,which can theoretically help to design the emitting materials and device structures of OLEDs,so as to improve the device performance.Up to now,most of the studies on OLEDs are still focused on the traditional electric and optical characterizations.In order to deeply understand the exciton dynamic processes in organic materials or devices,some ultra-fast or sophisticated equipment must be used.In recent years,the studies of organic magnetic field effects（MFEs）have been paid much attention.It is demonstrated that the measurements of MFEs can directly reveal the internal spin-related process of excitons in OLEDs.Moreover,in some special organic systems,the MFEs show fingerprint property for some certain internal dynamic processes,which is suitable for exploring the working mechanism of OLEDs.It is reported that the spin-related exchange conversion of excitons is meaningful for the manipulation of singlet-triplet ratio and thus the device efficiency,where the study of MFE responses can directly reflect these spin-related processes.Therefore,in this thesis,we selected the aggregation induced emission（AIE）,phosphorescence doped exciplex,and organic room-temperature phosphorescence（ORTP）materials,and deeply studied the physical processes of intramolecular spin conversion,intermolecular spin conversion and interfacial spin conversion.By measuring the magneto-electroluminescence（MEL）,magneto-photoluminescence（MPL）,transient EL（TREL）and PL（TRPL）,time-resolved EL and PL spectra,temperature-dependent MEL,temperature-dependent TREL and TRPL,and excited-light-dependent TRPL,the origins of different types of spin-exchange conversions and their impacts on device efficiency were well unraveled.Finally,we prepared high-performance OLEDs by optimizing and designing the device structures.1.By selecting the AIE molecule of TPB-AC that has almost 100%of photoluminescence quantum yield（PLQY）,we studied the intramolecular spin conversion process and its effect on device efficiency.The results indicate that the high-efficiency EL property and the special“M”shape of MFEs in TPB-AC-based devices are related to the existence of spin-exchange process between T₂and S₁,which leads to efficient excitons utilization.Accordingly,through adding thin phosphorescent layer to fully utilize the unused triplet excitons,the resulting TPB-AC-based blue OLEDs shows the maximum external quantum efficiency（EQE）of 7.93%.2.Furthermore,by comparing two other AIE-type molecules of CN-TPB-TPA and TPBCz C1,we further studied effect of the intramolecular spin conversion on device efficiency.We found that the position of the high-lying triplet states（T_n）has important impact on the device performance,where n<4 is crucial for the efficient T_n-S₁spin conversion because of the competitive relationship between the spin conversion of T_n-S₁and the internal crossing process of T_n-T_n-1.Finally,for CN-TPB-TPA device,we introduced m CBP:DMPPP layer with TTA up-conversion property to enhance the exciton utilization,the resulting CN-TPB-TPA-based blue OLEDs exhibit the maximum EQE of 10.2%,breaking the efficiency limit in AIE-based blue OLEDs.3.By choosing the phosphorescence doped exciplex,we systematically explored the intermolecular spin conversion process and its effect on device efficiency.It is demonstrated that the change of device efficiency and MFE line-shapes in m CBP:PO-T2T:Ir（ppy）₂（acac）devices with different doping concentrations of Ir（ppy）₂（acac）is attributed to the heavy-atom induced intermolecular spin-orbital coupling（SOC）,which will affect the singlet-triplet spin conversion on exciplex host.By optimizing the doping concentrations of Ir（ppy）₂（acac）in m CBP:PO-T2T,we could efficiently tune the singlet-triplet spin conversion on m CBP:PO-T2T exciplex host,increase the energy transfer efficiency and improve the utilization of excitons.In final,we fabricated the phosphorescent OLEDs with maximum EQE of 23%by only doping 2 wt%Ir（ppy）₂（acac）.4.By studying the ORTP molecule of PTZ-CN with AIE property,we confirmed the important role of interfacial spin-conversion process in device efficiency.It is clarified that the high-lying singlet fission（HLSF）process of PTZ-CN molecule will bring strong interfacial spin conversion between PTZ-CN and transport layer.It is shown that HLSF will increase the fluorescence lifetime and the intersystem crossing efficiency in PTZ-CN,thus enhancing the triplet exciton utilization.We finally prepared the non-doped pure ORTP-based OLEDs with maximum EQE of 16%by selecting m CP as the hole-transport layer.