| OLED has become the focus of attention of the research community because of its unique advantages, such as small size, light weight, high brightness, low cost, wide viewing angle, and flexible. However,the efficiency is still a key issue that is needed to resolve. In this work, we demonstrated enhanced efficiency of OLEDs by utilizing magnetic materils and photonic crystal structure into OLEDs, respectively.It is known that the EL efficiency of fluorescent OLEDs is determined by exciton formation rate, singlet exciton fraction, and quantum yield of radiant emission of singlet exciton. We want to increase quantum efficiency by increase singlet exciton fraction. We studied the green light emitting devices with the structure of ITO/PVK:Fe3O4(100:x)/Alq(50nm)/LiF(0.5nm)/Al. We measured the efficiency of devices with and without magnetic field, and found that the enhancement of the efficiency is related to the concentration of the doping Fe3O4 magnetic nano-particles. We found the devices in the doping concentration of 0.3% of the efficiency of the device with the largest improvement in the level of 11%. There are two possible mechanisms, namely, magnetic field effect or spin injection effect, which can be responsible for the increased singlet exciton fraction. We first rule out that the observed EL enhancement is caused by the applied magnetic field although a low external magnetic field could affect hyperfine interaction related singlet-triplet intersystem crossing, increasing singlet / triplet exciton ratio in organic materials. This is because the low magnetic field causes little change in the EL when the Fe3O4 is absent in the device. Therefore, we conclude that the enhancement of the efficiency refers to the Fe3O4 magnetic nano-particles which has high spin polarization resulting in the spin-polarized holes. The spin-polarized holes have a certain diffusion length, so they can be inject in Alq. The spin injection increases the singlet exciton fraction by 1/3 according to our analysis, which leads to the enhancement of the efficiency.The external quantum efficiency of the device is ratio of the photons coupled out of OLEDs to the total injected electrons. At present,the external quantum efficiency is very low Although internal quantum efficiency is increased through various means. This is due to the proportion of the coupled-out photons is too low. In the organic light-emitting devices, the produced photons will be absorbed,reflected when they go through all the organic layers, the substrate and electrodes. Lastly, only 17% of photons can be seen, the majority of the photons are lost through the total reflection of substrate and waveguide mode. The photonic crystal has periodic band-gap structure, so it can control the transmission of photons. We introduced photonic crystal structure namely the grating structure to the substrate of a simple device, and the out-coupling efficiency was increased through the interaction of photons with the grating. We can control resonant mode of the photonic crystal through designing the structure of grating, as well as its refractive index. Thereby the photons come out the device avoiding the total reflection of substrate and waveguide mode. We studied the nomal plan green light emitting device without grating, and the device with grating and with different period: ITO/PEDOT:photoresist/m-TDATA/NPB/Alq/LiF/Al. It is found that the 400nm period grating device has the highest enhancement of the emitting efficiency. The enhancement could be 136%.In conclusion, we increased the internal efficiency by doping Fe3O4 magnetic nano-particles in the OLED which increased the ratio of single exciton, and increased the external quantum efficiency by introducing grating structure. These works are important in improving the efficiency of organic light emitting devices. |
PDF Full Text Request