X. D. Gao

Mechanism of charge generation in p -type doped layer in the connection unit of tandem-type organic light-emitting devices

A p-type doped organic layer combined with a hole-blocking layer has been experimentally demonstrated to serve as the charge generation unit in tandem-type organic light-emitting devices. The p-type layer functions as the source of both holes and electrons. Charge separation is explained by the tunneling model that the hole-blocking layer reduces the energy barrier for the electrons generated in the p-type layer to tunnel through into one light-emitting unit, while the holes generated in the p-type layer can transport to the other light-emitting unit easily under operation voltage.

Delayed-switch-on effect in metal-insulator-metal organic memories

We report a delayed-switch-on effect in organic memories; i.e., the organic memory devices can automatically switch from off state to on state after a certain period of time when biased at voltages below the threshold voltage. Meanwhile, the lower the voltage is, the longer the switching time will be. The time scales from milliseconds to about 104s with decreasing voltage. Moreover, by applying a certain voltage between threshold voltage and Vmax, intermediate states are also obtained. The existence of filamentary microconducting channels in the organic layer is proposed to be responsible for the observed switching phenomenon.

Magnetic field modulated exciton generation in organic semiconductors: An intermolecular quantum correlated effect

Magnetoelectroluminescence (MEL) of organic semiconductor has been experimentally tuned by adopting blended emitting layer consisting of hole transporting material and electron transporting material. Theory based on Hubbard model fits experimental MEL well, which reveals two findings: (1) spin scattering and spin mixing, respectively, dominate MEL in low-field and high-field region. (2) Blended ratio, and thus the mobility, determines the value of the relative change in the EL in a given magnetic field. Finally successful prediction about the increase in singlet excitons in low field with little change in triplet exciton population further confirms the first finding.

Magnetic field effects on the electroluminescence of organic light emitting devices: A tool to indicate the carrier mobility

The magnetoelectroluminescence (MEL) of organic light emitting devices with a N,N′-bis(l-naphthyl)-N,N′-diphenyl-1,l′-biphentl-4,4′-diamine:tris-(8-hydroxyquinoline) aluminum (NPB:Alq3) mixed emission layer (EML) has been investigated. We find that MEL is maximized when the volume ratio of NPB of the mixed EML reaches 30% and the EML thickness is 40 nm. The features of MEL under various magnetic field strengths are insensitive to the change in EML thickness and mixing ratio. Meanwhile, MEL has a close relationship with the carrier mobility. We have conducted a theoretical study to further verify the relationship. Our experimental and theoretical results confirm that MEL can function as a tool to indicate the mobility.

Interfacial reactions at Al/LiF and LiF/Al

High-resolution synchrotron radiation photoemission spectroscopy was used to investigate the chemical properties of Al–LiF interfaces. An electronic state appeared at the Al/LiF interface with a binding energy 4.8 eV higher than that of the metallic Al 2p core level, but the state was hardly found to be present at the LiF/Al interface. This indicates that intensive chemical reaction could occur at the Al/LiF interface, while the reaction occurring at the LiF/Al interface would be weak. This result explains well the unsymmetrical electron injection from different sides of the symmetrical device of indium-tin-oxide\Al\LiF\tris(8-hydroxyquinoline) aluminum\LiF\Al showing unsymmetrical current-voltage characteristics.

Internal potential distribution in organic light emitting diodes measured by dc bridge

Internal potential distribution of organic light-emitting diodes (OLEDs) is an essential problem. By using dc bridge to eliminate errors due to high resistance of the devices at low bias, the potential distribution has been accurately measured for both double-layer and single-layer OLEDs. It is found that the electric field inside the device is not uniform, and the potential distribution changes with external bias. This phenomenon could be the effect of space charge originating from the unequal injections of holes and electrons, which is confirmed by the results of the device with modified work function of the anode.

Exchange bias of NiFe/FeMn nanocaps

NiFe/FeMn bilayers are fabricated on two-dimensional closely packed arrays of monodisperse polystyrene spheres. Although the exchange field is inversely proportional to the NiFe layer thickness in both NiFe/FeMn nanocaps and continuous films, the former series have a smaller exchange field than the latter ones. The exchange field and the coercivity both have different evolution trends with the FeMn layer thickness for the two series of samples. More remarkably, the asymmetry of hysteresis loops is more pronounced for the NiFe/FeMn nanocaps. These phenomena are ascribed to the spatial distributions of the constituent layer thickness and the local normal direction along the curved surface.

Monte Carlo simulation of charge transport in electrically doped organic solids

A model to study the charge transport in electrically doped organic solids via Monte Carlo simulation is proposed. The model includes the finite rate of all possible electron transfers between host and dopant molecules. It is found that for efficiently doped materials, the conductivity increases with dopant concentration, first sublinearly and then superlinearly. The conductivity of electrically doped films is also dependent on the dopant–host energy-level offset: it is high when the ionization of dopants is energetically favourable, but decreases exponentially with increasing energy barrier for dopants to ionize.

Electron conductance of N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine at low temperatures

The electron conductance of N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine was found to increase with decreasing temperature experimentally. This phenomenon is quite abnormal since for most organic materials the conductance increases with increasing temperature. A probable explanation was given according to a previous work about soliton diffusion in polyacetylene within the framework of SSH model.

Photoemission study of C60-induced barrier reduction for hole injection at N, N′-bis(naphthalene-1-y1)-N, N′-bis(phenyl) benzidine/Al

Synchrotron radiation photoemission study showed that the energy level alignment at the interface between N, N′-bis(naphthalene-1-y1)-N, N′-bis(phenyl) benzidine (NPB), a typical hole transport material, and Al could be adjusted by precovering a thin C60 layer on Al. The interface dipoles so formed could shift both the highest occupied molecular orbital level of NPB and the secondary electron cutoff measured at the early stage of the NPB deposition. The barrier height for hole injection from Al to NPB could thus be lowered by as much as 0.98 eV, and the optimal thickness of the inserted C60 layer was found to be 8–12 A.