Z. H. Xiong

ENHANCED HOLE INJECTION IN A BILAYER VACUUM-DEPOSITED ORGANIC LIGHT-EMITTING DEVICE USING A P-TYPE DOPED SILICON ANODE

We report the fabrication of a vacuum-deposited light-emitting device which emits light from its top surface through an Al cathode using p-type doped silicon as the anode material. Enhanced hole injection is clearly demonstrated from the p-Si anode as compared to the indium–tin–oxide (ITO) anode. The mechanisms of hole injection from both the p-Si and ITO anodes into the organic layer are investigated and a possible model based on anode surface band bending is proposed. During the operation of the organic light-emitting device, the surface band bending of the anode plays a very important role in modifying the interfacial barrier height between the anode and the organic layer.

Buffer-layer-induced barrier reduction: Role of tunneling in organic light-emitting devices

Based on the WKB approximation of the tunneling model, we calculate the J–V characteristics of organic light-emitting devices (OLEDs) having buffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.

Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode

A LiF-buffered silver cathode has been used in organic light-emitting devices (OLEDs) with structure indium–tin–oxide/N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (50 nm)/Alq3 (100 nm)/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent (EL) efficiency, a LiF layer of 3.0 nm significantly enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m2 and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model.

In situ photoluminescence investigation of doped Alq

We report the photoluminescence (PL) properties measured in situ from vacuum-deposited organic films of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM), where the red emission from the guest molecules is due to Forster energy transfer of excited state energy from host to guest. Both bare DCM-doped Alq (Alq:DCM) and bilayer Alq/Alq:DCM films have been studied, with the thickness of the Alq overlayer continuously varied in the latter case. The PL spectra from the bilayer structure contain no Alq contribution when its thickness is below 2.4 nm. Taking the value as the maximum distance for which the Alq exciton can travel in the film and still transfer its energy to a DCM molecule, the minimum DCM concentration in Alq:DCM necessary to produce red emission only can be estimated at 0.31 wt %. The most efficient red emission appears at the DCM concentration of about 1.7 wt %, at which more than 90% Alq-originated excitons are involved in t...

Sodium stearate, an effective amphiphilic molecule buffer material between organic and metal layers in organic light-emitting devices

Tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting devices using an amphiphilic molecule sodium stearate (NaSt) layer between aluminum (Al) cathode and Alq3 have been fabricated. By comparing the devices with those containing a LiF buffer layer, the results demonstrate that both have almost the same high electroluminescent (EL) brightness but the former is more stable. The amphiphilic property of NaSt is considered as the main reason for this enhancement.

Comparison of optical absorption in Si nanowire and nanoporous Si structures for photovoltaic applications

We numerically study the optical absorption in Si nanowire and nanoporous Si structures that have potential applications in solar cells. It is found that for the same thickness and filling ratio of Si, thin nanoporous structures can have much higher absorption than thin Si nanowire arrays. Above a critical filling ratio of Si (0.25), the nanoporous structures can have higher absorption even than thin films with the same thickness. For solar cells based on thin nanoporous Si structures, the maximal ultimate efficiency occurs when the filling ratio is around 0.3.

Bubble formation in organic light-emitting diodes

Bubbles in organic light-emitting diodes can be formed from gas release due to Joule heating effect at localized electrical shorts during operation, which could be simulated by a rapid thermal annealing. The gases in the bubbles consist of not only adsorbed moistures but also the decomposed organic species, which are detected in situ in an ultrahigh vacuum chamber. In the device of Al/tris-(8-hydroxyquinoline) aluminum (Alq/N,N′-diphenyl-N.N′-bis-{3-methylphenyl}{1,1′biphenyl}-4,4′-diamine/indium tin oxide (ITO), the gases released from ITO surface were mainly of adsorbed moistures, while those released from the organic layers were of both the decomposed products from Alq and the trapped moistures. The decomposition of Alq could not be easily avoided if there were severe localized electrical shorts in the devices.

Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq

The morphologic and luminescent behaviors of various 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) containing films have been investigated. This involves pure DCM layers deposited on top of a tris-(8-hydroxyquinoline) aluminum (Alq) layer or positioned between two Alq layer, DCM-doped Alq films, and periodically arranged Alq/DCM multilayer structures. The occurrence over a period of time of aggregation and permeation of DCM molecules at room temperature and at temperatures as low as ∼260 K is found in all the cases studied. Such a phenomenon will result in degradation of related organic light-emitting devices and is closely related to the electric polarity of the DCM molecule.

Effects of O, H and N passivation on photoluminescence from porous silicon

Abstract A simple but effective passivation method for porous silicon (PS) has been developed. Immersion of as-etched PS in dilute (NH4)2S/C2H5OH solution followed by ultraviolet light irradiation in air can lead to an enhancement of photoluminescence (PL) up to more than 20 times. Infrared absorption and Auger electron spectroscopic measurements show that the formation of SiH(O3), SiOSi and SiN bonds are formed during the post-treatment process. However, the PL intensity cannot be enhanced if the solution-treated sample is exposed to the laser beam in vacuum. It is thus concluded, that the PL enhancement can be attributed to the presence of compact passivation films consisting of the oxides and the nitride on both external and internal surfaces of the sponge-like PS samples.

Optical resonances in tubular microcavities with subwavelength wall thicknesses

Based on the Mie scattering theory, we study optical resonances with whispering gallery modes (WGMs) in tubular microcavities. Rigorous formulas are present to obtain resonant wavelengths and Q factors for the WGM resonances. It is found that the Q factors of microtubes can be dramatically increased by increasing the dielectric constants in tube walls. For common SiO/SiO2 based microtubes, Q factors can be improved by one order when the microtubes are coated with thin high-index HfO2 layers (n = 1.95, thickness = 10 nm). The results could be useful for designing better optical devices based on tubular microcavities.