| Organic light-emitting devices (OLEDs) has drawn much attention due to its advantages such as light weight, fast response, large viewing angle, active luminescence without background light, easy to realize flexible display and three-dimensional display. In recent years, significant progress has been achieved in OLEDs, however, stability and efficiency is still key issue for its potential applications in flat-panel display and solid-state lighting. In this thesis, Fe3O4 has been applied into OLEDs as electrode modification layer and p type dopant to effectively reduce the driving voltage and power consumption and enhance the luminance of OLEDs. Moreover, we have studied the magnetic field effect of the OLEDs with Fe3O4 to increase the proportion of the singlet excitons, and finally enhance the internal quantum efficiency of the devices.(1) Enhanced hole injection for the bottom-emitting OLEDs (BOLEDs) with the transitional metal oxides Fe3O4 as indium-tin oxide (ITO) anodic buffer. The turn-on voltage of the OLEDs with the anodic buffer is reduced from 4 V to 2.5 V, and the brightness is increased from 9040 cd/m2 to 27540 cd/m2 at 12 V. The x-ray photoemission spectroscopy (XPS) and UV photoemission spectroscopy (UPS) measurements were performed to determine the interfacial energy level. The XPS results showed that the electrons transferred from ITO to Fe3O4 at the interface. The electron transfer across the interface results in a formation of a dipole layer at the interface, leading to an abrupt shift in the potential across the dipole. The core-level shift shows a 0.3 eV up-shift in the vacuum level after depositing 1 nm Fe3O4, which results in a reduced energy barrier at the ITO/Fe3O4/NPB interface and accordingly reduced driving voltage. The UPS spectra showed that the hole-injection barrier at the ITO/NPB interface is reduced by 0.22 eV when the Fe3O4 buffer layer is inserted between them. In addition to the energetics, the morphology of the interface can play a role in determining the injection efficiency. The effect of the Fe3O4 on the interfacial morphology between the ITO anode and the deposited NPB films was investigated by AFM. The AFM images revealed that the Fe3O4 capped ITO surface displays improved smoothness with a root-mean-square (rms) roughness of 0.72 nm compared to the bare ITO with a rms roughness of 1.04 nm. The above results indicates that Fe3O4 is a practical anodic buffer layer to improve the performance of the OLEDs by enhancing the hole injection.(2) Enhanced hole injection for the top-emitting OLEDs (TOLEDs) with the transitional metal oxides Fe3O4 as the silver (Ag) anodic buffer. The turn-on voltage of the Fe3O4 buffered TOLEDs was reduced from 5 V to 2.5 V, and the maximum brightness reached to 108297 cd/m2, which is eight times of that device without buffer layer. In order to study the relative effectiveness of the anodic modification of the Fe3O4, we have fabricated the devices with MoO3 as the buffer layer for comparison, which is one of the most effective anodic buffer materials for the Ag anode. The results indicated that Fe3O4 has comparable and even appreciably superior effect in modifying the Ag anode and improving the properties of the TOLEDs to the MoO3. The XPS and UPS measurements indicated that the introduction of the thin film Fe3O4 can greatly reduce the hole-injection barrier, enhance the hole injection ability and consequently improve the device performances.(3) Fe3O4 as the p-dopant to improve the hole injection and transport of the OLEDs. The mobility of the organic semiconductor is very low, while the p/n doping technology is a powerful solution to improve the charge conductivity, and the carrier injection ability could be improved simultaneously. In our work, the hole injection and transport ability were evidently enhanced by doping the p dopant Fe3O4 into different host materials 4,4',4"-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) and N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), respectively. The experiment results have demonstrated that the luminance, the current density, and the power efficiency of the OLEDs with Fe3O4 doped in two different hosts of m-MTDATA and NPB have been remarkably enhanced compared to those of the undoped devices. The brightness were 29360 cd/m2 and 6005 cd/m2 at 8 V for the devices with doped m-MTDATA and undoped m-MTDATA respectively, and 30590 cd/m2 and 1680 cd/m2 at 10 V for the devices with the doped NPB and undoped NPB, respectively. The turn-on voltage obtained the luminance of 1 cd/m2 was found to be greatly decreased from 3 to 2.4 V and from 5 to 2.5 V, respectively, for the devices with doped m-MTDATA and NPB. The role of the Fe3O4 as the p-dopant in different hosts has been studied. The Fe3O4-doped m-MTDATA layer in the OLEDs is more efficient in improving the hole transportation, while the Fe3O4-doped NPB layer is more efficient in lowering the hole-injection barrier.(4) Magnetic field effect on the OLEDs with the Fe3O4 as the magnetic buffer and dopant. Although 100% internal quantum efficiency can theoretically be achieved by introducing triplet emitter,25% singlet exciton formation ratio has been a bottleneck in improving the efficiency of the conventional fluorescent OLEDs. We chose magnetite Fe3O4 as the magnetic buffer and dopant to improve the singlet exciton formation ratio under an applied magnetic field. The efficiency with the presence of the magnetic field was enhanced by 10.5% compared to that of without the magnetic field for the Fe3O4 buffered OLEDs, this was because that the increase of the singlet exciton fraction due to the hole spin polarization injection. In order to increase the ratio of hole spin polarization, OLEDs with a magnetic dopant of Fe3O4 in a hole-transport layer (HTL) were fabricated and characterized. Magnetic field-dependent electroluminescence (EL) was observed and large enhancement of 24% for the current efficiency was obtained from the magnetic doped devices. Obviously, the efficiency enhancement for the OLEDs with the magnetic dopant was higher than that of the device based on Fe3O4 as the anodic buffer, which is attributed to the increased contact between the holes and magnetic material Fe3O4. We can come to a conclusion that magnetic field effect on the OLEDs employing the Fe3O4 presents an efficient pathway to enhance the EL efficiency of the fluorescent OLEDs.In summary, the application of the Fe3O4 in OLEDs has been systematically investigated. The Fe3O4 has been demonstrated as an effective anodic buffer and p dopant to improve the hole injection and transport. The magnetic field effect on the OLEDs with Fe3O4 as the magnetic buffer and dopant has been explored and enhanced singlet formation ratio has been obtained. Therefore, the use of Fe3O4 in OLEDs presents an efficient pathway to enhance the performance of the OLEDs, and the observational fact we attained may open a new avenue to broad application of Fe3O4, an environmentally benign, inexhaustible, and cheap material in OLEDs.
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