Aleksandra B. Djurišić

OPTICAL PROPERTIES OF METALLIC FILMS FOR VERTICAL-CAVITY OPTOELECTRONIC DEVICES

We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Au/n-ZnO rectifying contact fabricated with hydrogen peroxide pretreatment

Au contacts were deposited on n-type ZnO single crystals with and without hydrogen peroxide pretreatment for the ZnO substrate. The Au/ZnO contacts fabricated on substrates without H2O2 pretreatment were Ohmic and those with H2O2 pretreatment were rectifying. With an aim of fabricating a good quality Schottky contact, the rectifying property of the Au/ZnO contact was systemically investigated by varying the treatment temperature and duration. The best performing Schottky contact was found to have an ideality factor of 1.15 and a leakage current of 10 �7 Ac m �2 . A multispectroscopic study, including scanning electron microscopy, positron annihilation spectroscopy, deep level transient spectroscopy, x-ray photoelectron spectroscopy, and photoluminescence, showed that the H2O2 treatment removed the OH impurity and created Zn-vacancy related defects hence decreasing the conductivity of the ZnO surface layer, a condition favorable for forming good Schottky contact. However, the H2O2 treatment also resulted in a deterioration of the surface morphology, leading to an increase in the Schottky contact ideality factor and leakage current in the case of nonoptimal treatment time and temperature.

Electrochemical Synthesis of ZnO Nanoporous Films at Low Temperature and Their Application in Dye-Sensitized Solar Cells

ZnO nanoporous structures were prepared on indium-tin-oxide substrates by cathodic electrodeposition at low temperature without using any surfactant. The morphology and structure of the films were studied, and the mechanisms for the formation of the porous structures were also discussed. Dye-sensitized solar cells based on ZnO nanoporous films were fabricated. The cell performance was strongly dependent on ZnO film morphology. For porous ZnO coated with TiO 2 , cells with 1% power conversion efficiency were obtained.

Modeling the optical constants of AlxGa1−xAs alloys

The extension of Adachis model with a Gaussian-like broadening function, in place of Lorentzian, is used to model the optical dielectric function of the alloy AlxGa1-xAs. Gaussian-like broadening is accomplished by replacing the damping constant in the Lorentzian line shape with a frequency dependent expression. In this way, the comparative simplicity of the analytic formulas of the model is preserved, while the accuracy becomes comparable to that of more intricate models, and/or models with significantly more parameters. The employed model accurately describes the optical dielectric function in the spectral range from 1.5 to 6.0 eV within the entire alloy composition range. The relative rms error obtained for the refractive index is below 2.2% for all compositions

Change of the emission spectra in organic light-emitting diodes by layer thickness modification

Electroluminescence and photoluminescence of organic light-emitting diodes consisting of an indium tin oxide anode, N,N8-di(naphthalene-1-yl)-N,N8-diphenyl-benzidine as a hole transport layer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured for different layer thickness values. It was found that, for a certain range of thickness values, multiple peak emission can be achieved. In addition, the emission spectra were dependent on the viewing angle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were 0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed.

Optically tunable intramolecular charge transfer dyes for vacuum deposited bulk heterojunction solar cells

We present the tunability of the photophysical and electrochemical properties of a series of intramolecular charge transfer compounds by facile molecular design and synthesis. The photovoltaic performances based on these sublimable materials and C(60) bulk heterojunction cells are compared and reported. The structural modification of the charge transfer dyes altered not only the electronic properties, but also the morphology of the bulk heterojunction thin films, as revealed by AFM and SEM studies. Addition of PEDOT:PSS between the ITO and the photoactive layer improved the hole injection from the photosensitizer into the anode, and the overall power conversion efficiency is also enhanced.

Modeling the optical constants of GaP, InP, and InAs

An extension of the Adachi model with the adjustable broadening function, instead of the Lorentzian one, is employed to model the optical constants of GaP, InP, and InAs. Adjustable broadening is modeled by replacing the damping constant with the frequency-dependent expression. The improved flexibility of the model enables achieving an excellent agreement with the experimental data. The relative rms errors obtained for the refractive index equal 1.2% for GaP, 1.0% for InP, and 1.6% for InAs.

Simulated-annealing-based genetic algorithm for modeling the optical constants of solids

We propose a simulated-annealing-based genetic algorithm for solving model parameter estimation problems. The algorithm incorporates advantages of both genetic algorithms and simulated annealing. Tests on computer-generated synthetic data that closely resemble optical constants of a metal were performed to compare the efficiency of plain genetic algorithms against the simulated-annealing-based genetic algorithms. These tests assess the ability of the algorithms to find the global minimum and the accuracy of values obtained for model parameters. Finally, the algorithm with the best performance is used to fit the model dielectric function to data for platinum and aluminum.

Device Optimization of Tris-Aluminum (Alq3) Based Bilayer Organic Light Emitting Diode Structures

In this work we present detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq3) based bilayer OLEDs as a function of: the choice of cathode, the thickness of organic layers, and the position of the hole transport layer/Alq3 interface. The calculations fully take into account dispersion in the glass substrate, the indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. Influence of the incoherent transparent substrate (1 mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N-diphenyl-N,N-(3-methylphenyl)-1,1-biphenyl-4,4-diamine (TPD) and N,N-di(naphthalene-1-yl)-N,N-diphenylbenzidine (NPB) were considered. As expected, emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in vicinity of the metal cathode, designs placing the emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, several different devices were fabricated and characterized and experimental and calculated emission spectra were compared.

Cavity design and optimization for organic microcavity OLEDs

We report on detailed simulations of the emission from microcavity OLEDs consisting of widely used organic materials, N,N-di(naphthalene-1-yl)-N,N-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq3) as emitting and electron transporting layer. The thick silver film was considered as a top mirror, while silver or copper films on quartz substrate were considered as bottom mirrors. The electroluminescence emission spectra, electric field distribution inside the device, carrier density and recombination rate were calculated as a function of the position of the emission layer, i.e. interface between NPB and Alq3. In order to achieve optimum emission from a microcavity OLED, it is necessary to align the position of the recombination region with the antinode of the standing wave inside the cavity. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The experimental results have been compared to the simulations and the influence of the emission region width and position on the performance of microcavity OLEDs was discussed.