Studies On The Singlet Exciton Fission And Exploring Methods Of Magnetic Field Effects In Organic Light-Emitting Diodes

Studies on organic light emitting devices and its magnetic effect have important scientific significance and broad application foreground due to their better electroluminescent performance, large emitting area, flexibility and low price. After years of development, the research about the field has made remarkable achievements. Different from traditional spin injection device, organic device is not contain any magnetic elements, so the microscopic explanation of magnetic field effect in none-magnetic organic device become one of pressing problem. Moreover, In the different structure of the device, the different magnetic field influence on the interaction dynamics between excitons (excited state) in device result to different micro-mechanisms of magnetic field effects(MFEs).Therefore, the exploring microscope mechanisms of MFEs has become the focus of attentionIn this M.S. Dissertation, first we study the MFEs based on the OLED existing singlet fission The experimental phenomenon was analyzed by research the magnetic field influence on interaction between excitons(excited state) in this device. In addition, we described the possible microscopic mechanism of MFEs based on the singlet fission theory. Finally, we change the structure of device by the doping, and qualitative research MFEs in two doped OLED, attempt to explore microscopic mechanism of MFEs through doping method. Our works in this Dissertation are follows:(1) First, we provided some basis knowledge about conventional OLED, such as hyperfine interaction, electron-hole pairs model, polaron model, bipolaron model, triplet-triplet annihilation singlet fission. The background of MFEs based on the OLED and research progress at present were also described briefly. The experimental preparing technology and measurement of OLED were introduced detailedly.(2) The OLEDs with structure of ITO/CuPc/NPB/Rubrene/BCP/LiF/Al were fabricated by using thermal evaporation technics. In addition, the control devices with structure of ITO/CuPc/NPB/rubrene/Alq3/LiF/Al and ITO/CuPc/NPB/Alq3/BCP/LiF/Al were prepared. The magneto-luminescence (MEL) in the devices was measured under different temperature and driving current. Results show that MEL can reach up to above23.5%and decrease with decreasing temperature. Considering the peculiarity of the energy of singlet and triplet excitons in rubrene, this can be attributed to the conversion between singlet fission and triplet fusion. Decreasing singlet fission due to external magnetic field at room temperature resulted to more singlet exciton, and hence large MEL. However, triplet-triplet annihilation is predominated in rubrene at low temperature, high field decline proven to be in consistent with experiment results that by studying the dependences of MEL on and temperature.(3) The doped OLEDs with structure of ITO/CuPc/NPB/mbrene:BCP/BCP/LiF/Al were fabricated by using co-evaporation technics. The MEL of devices with different doped concentration was measured. Results shows that MEL sharply decrease in high field (20mT<B<500mT) with decreasing concentration, whereas, sightly increase in low field (B<20mT). Singlet fission increase first and then decrease with enlarging the magnetic field strength. Therefore, singlet fission strength are weaken due to expansile molecular space by doping BCP into rubrene, and high field of MEL decrease with increasing doping concentration. Our analysis are consistent with experimental results.(4) LiF-doped OLEDs with structure of ITO/NPB/Alq3:LiF/BCP/LiF/Al were prepared in the fifth Chapter. MEL and MC were measured under different temperature and driving current. Results show that the magnetic field effects of LiF doped device have changed clearly, compared with the undoped device, whatever in magnitude or line-shape. The differences between the MFEs in doped and undoped devices can be attributed to the dielectric polarization of LiF particles in the device’s internal electric field, Special field effects were resulted by the reaction between polarized charge and excitons in emitting layer.