Y. C. Zhou

Zn vacancy induced room-temperature ferromagnetism in Mn-doped ZnO

X-ray absorption fine structure (XAFS) and first-principles calculations were employed to study the structure and ferromagnetism origin of Zn0.97Mn0.03O thin film grown by metal organic chemical vapor deposition. The magnetization measurements indicate that this sample is ferromagnetic at room temperature. The Mn ions are located at the substitutional Zn sites as revealed by the Mn K-edge XAFS spectroscopy. Moreover, the O K-edge XAFS analysis indicated the existence of numerous Zn vacancies. Based on first-principles calculations, the authors propose that the Zn vacancy can induce the room-temperature ferromagnetism in Mn-doped ZnO.

Wide-band “black silicon” based on porous silicon

Solar cells and optical detection devices often incorporate antireflective surfaces to reduce undesired reflection and enhance optical absorption. This letter reports a “black silicon” structure of antireflective porous silicon fabricated by using electrochemical etching. The sample has a gradient-index multilayer structure, i.e., the refraction indices of the structure increase from the top (near the air) to the bottom (near the Si substrate). Reflectance below 5% is obtained over a broad wave number range (3000–28000cm−1) and the depression mechanism of the optical reflectance is analyzed by simulating the structure with the transfer matrix method. The simulated result fits the measured spectra well.

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.

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.

High contrast organic light-emitting devices with improved electrical characteristics

High contrast organic light-emitting devices with low-reflection cathodes are fabricated. The cathode consists of a semitransparent metal layer, a phase-changing (PC) layer, and a reflective metal layer. With Al doped tris(8-hydroxyquinoline) aluminum as PC layer, devices exhibit the average reflectivity of the ambient light as low as about 13%. And its electrical characteristics are almost identical to that of a conventional device, although the thickness is increased by 70%. The improvement in conductivity could be attributed to the conductive Al cluster distributed in the organic matrix.

Structure of grain boundaries in nanostructured ZnO

The grain boundary (GB) of nanocrystalline ZnO films is investigated using the x-ray absorption fine structure technique. With the advantage of the dominant GB volume fraction in our samples, the GB structure is found to be neither simply “gas-like” nor “similar to that of coarse-grained phase,” but experiences a transition from the modestly ordered innermost coordination shell around centered atoms to partly disordered second coordination shell and then to completely disordered higher coordination shells.

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.