Y. Z. Wu

Electron blocking and hole injection: The role of N, N'-bis(naphthalen-1-y)-N, N'-bis(phenyl)benzidine in organic light-emitting devices

The current density-luminance-voltage characteristics of organic light-emitting devices (OLEDs) with N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium–tin–oxide/NPB/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) (30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM EL to the whole EL decreases with inc...

Role of hole playing in improving performance of organic light-emitting devices with an Al2O3 layer inserted at the cathode-organic interface

The role of hole playing in improving electron injection in the presence of an Al2O3 layer at the organic-cathode interface is discussed. It is deduced that, according to the model of tunneling barrier reduction and the carrier transporting mechanism in organic light-emitting devices, electron injection will be enhanced only if holes are injected and accumulated at the organic-buffer interface. The validity of this analysis is well confirmed by experimental results. The observed abnormal characteristic of operating voltage varying with the Al2O3 layer thickness and the efficiency improvement are also well explained by the model.

Mechanisms of injection enhancement in organic light-emitting diodes through insulating buffer

Three types of organic light-emitting diodes are fabricated. Tris-8-hydroxyquinoline aluminum (Alq3) is used as an electron-transporting layer (ETL) and sodium stearate (NaSt) as an electron-injecting buffer. The optimal thickness of NaSt for electron injection is different for cathodes of different metals, such as Mg, Al, and Ag. This is attributed to the different work functions of cathodes, which result in different initial barrier heights for electron injection from cathodes into ETL, and explained based on tunneling theory.

Metal-induced photoluminescence quenching of tri-(8-hydroxyquinoline) aluminum

Metal-induced photoluminescence (PL) quenching of organic thin film [tri-(8-hydroxyquinoline) aluminum (Alq)] has been investigated both experimentally and theoretically. By doing experiments in situ in high vacuum, we have measured the PL intensity of Alq film deposited on metal-doped Alq film or metal film as a function of its thickness. For the case of metal-doped Alq film, exciton diffusion length of Alq is derived as LD=8.6±0.1nm by analyzing experimental results and using a model based on diffusion and interface dissociation of excitons. For the case of metal film, another model considering exciton diffusion, interface dissociation, and nonradiative energy transfer to the metal is suggested to explain the experimental observation. Good agreement is achieved between theory and experiment.

Revealing the volume magnetic anisotropy of Fe films epitaxied on GaAs(001) surface

The in-plane magnetic anisotropy in Fe films grown on GaAs(001) was investigated quantitatively by the magneto-optic Kerr effect with a rotating magnetic field. The clear 1/dFe relation of the uniaxial magnetic anisotropy indicates a surprising volume contribution with easy axis along the GaAs [11¯0] direction. Such volume anisotropy was found to be sensitive to the growth temperature and also strongly correlate with the interface anisotropy. Our results may introduce a new aspect for further understanding the origin of uniaxial magnetic anisotropy in Fe/GaAs(001) system.

Control of carrier transport in organic semiconductors by aluminum doping

Control of carrier transport in organic semiconductors by aluminum doping is realized in organic light-emitting devices (OLEDs) for which electroluminescence can sensitively reflect the status of carrier transport. It is found that an Al-doped layer with proper thickness (∼1–10nm) may block hole transport completely and enhance electron transport to some extent regardless of its location in the organic carrier transport layers. Improvement in the efficiency of OLEDs with an aluminum cathode is achieved upon the introduction of a very thin (∼3nm) Al-doped region near the light-emitting area. The current efficiency obtained with such Al-doped devices is about 30% higher than that with undoped devices.

Exciton migration in organic thin films

Limitations of the analytical method for calculating the exciton distribution in organic thin films, attributed to the improper boundary conditions when the organic film approaches the exciton diffusion length, were analyzed by comparison with an exciton random walk simulation. The random walk simulation results are in better agreement with in situ photoluminescence measurements than predictions based on the one-dimensional (1D) diffusion equation, especially for thin films (<15nm). The three-dimensional exciton diffusion length in tris(8-hydroxyquinoline) aluminum is determined to be 26nm, equivalent to 15nm upon projection to 1D. The result is not sensitive to the molecular size, a parameter arbitrarily set in the simulation. In addition, the exciton distribution in operating organic light emitting devices was also simulated.

Different temperature scaling of strain-induced magneto-crystalline anisotropy and Gilbert damping in Co2FeAl film epitaxied on GaAs

The temperature dependence of the Gilbert damping and magnetic anisotropy are investigated in L21 Co2FeAl films epitaxially grown on GaAs (001) substrate by the time resolved magneto-optical Kerr effect. We found that the in-plane biaxial anisotropy increases by more than 90% with the temperature decreasing from 300 K to 80 K, which is mainly due to the strong variation of the magneto-elastic coefficients. In contrast, the intrinsic Gilbert damping rises only about 10%, which is mainly attributed to the reduction of the electron phonon scattering rate, independent of the strain-induced spin-orbit coupling energy.

Magnetic vortex generated by Dzyaloshinskii–Moriya interaction

We demonstrate that a magnetic vortex generated by the Dzyaloshinskii–Moriya interaction is stable state in a spatially confined system. The properties of the locally confined magnetic vortex structure are investigated by performing Monte-Carlo simulations and theoretical calculations. The results reveal the relationship between the confinement size and the magnetic vortex size. We obtain the structural size of the most stable magnetic vortex as well as the critical size above which the magnetic vortex becomes unstable. The field required to flip the vortex core was estimated theoretically and compared with simulation results. The thermal stability of the magnetic vortex is also discussed.

Excitation energy transfer between tris-(8-hydroxyquinoline) aluminum and a red dye

Excitation energy transfer between tris-(8-hydroxyquinoline) aluminum (Alq3) and a red dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) has been investigated by means of in situ measurements of photoluminescence (PL) of bilayer Alq3∕DCM films with various Alq3 thicknesses. It is found that exciton diffusion in Alq3 and Forster energy transfer from Alq3 to DCM are contributive to the PL spectra measured. A rate equation model based on these two processes is derived and used to fit experimental results. The Forster energy transfer radius obtained for the system is R0=3.3±0.1nm. Energy transfer in the blend system is also analyzed based on the model.