Genetic algorithm for design of reflective filters: Application to AlxGa1-xN based Bragg reflectors

A genetic algorithm (GA) with adaptive mutations has been employed for the design of Bragg reflectors. The algorithm enables three different design types a) composition and thickness of two layers are chosen and the pair is repeated b) two compositions are chosen for the two alternating materials, and thickness of each layer is optimized c) composition and thickness of each layer are optimized. In all cases, the wavelength and composition dependence of the index of refraction is taken into account. Also, it is possible to impose constraints on the composition difference of the neighbouring layers, either with a penalty function or with narrowing the boundaries for possible compositions. This feature is important because the large lattice mismatch between GaN and AlN can cause poor surface morphology, so measured reflectivity would be lower than the calculated one due to the surface roughness. The algorithm enables finding the optimal design for two chosen incident and final media, and it is capable of taking into account the existence of a finite, optically thick substrate. We have investigated two systems: air/sapphire/AlxGa1-xN reflector/GaN and GaN/AlxGa1-xN/air.published_or_final_versio

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. (C) 1999 American Institute of Physics. [S0021-8979(99)05807-7]

Modeling the optical constants of solids using acceptance-probability-controlled simulated annealing with an adaptive move generation procedure

The acceptance-probability-controlled simulated annealing with an adaptive move generation procedure, an optimization technique derived from the simulated annealing algorithm, is presented. The adaptive move generation procedure was compared against the random move generation procedure on seven multiminima test functions, as well as on the synthetic data, resembling the optical constants of a metal. In all cases the algorithm proved to have faster convergence and superior escaping from local minima. This algorithm was then applied to fit the model dielectric function to data for platinum and aluminum

Dielectric function of GaN: Model calculations

In this work we have modeled the optical functions of hexagonal GaN (corresponding to E⊥c) in the range from 1 eV to 10 eV using a modified critical points model. The difference between the model employed and the standard critical points model is that the exponent m is an adjustable parameter, and does not have fixed value depending on the type of critical point. Excellent agreement with the experimental data has been achieved over the entire investigated spectral range. Obtained relative rms errors equal 0.6% for the real part, and 2.0% for the imaginary part of the index of refraction.link_to_subscribed_fulltex

Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength on the viewing angle

Organic microcavity light-emitting diodes typically exhibit a blueshift of the emitting wavelength with increasing viewing angle. We have modeled the shift of the resonance wavelength for several metal mirrors. Eight metals (Al, Ag, Cr, Ti, Au, Ni, Pt, and Cu) have been considered as top or bottom mirrors, depending on their work functions. The model fully takes into account the dependence of the phase change that occurs on reflection on angle and wavelength for both s and p polarization, as well as on dispersion in the organic layers. Different contributions to the emission wavelength shift are discussed. The influence of the thickness of the bottom mirror and of the choice and thickness of the organic materials inside the cavity has been investigated. Based on the results obtained, guidelines for a choice of materials to reduce blueshift; are given. (C) 2002 Optical Society of America

Modeling the optical constants of wide bandgap materials

Calculations of the optical constants of hexagonal GaN (in the range 1-10 eV), InN (in the range 2-20 eV), AlN (in the range 6-20 eV) and 6H-SiC (in the range 1-30 eV) for the component perpendicular to the c axis are presented. The employed model is modified Adachi's model of the optical properties of semiconductors. In the employed model, damping constant Γ describing broadening phenomenon is replaced with the frequency dependent expression Γ(ω). In such a manner, type of broadening represents adjustable parameter of the model, allowing broadening to vary over a range of functions with similar kernels but different wings. Excellent agreement with experimental data is obtained for all investigated materials. Obtained relative rms errors for the real and imaginary parts of the index of refraction are equal to 3.5% and 5.2% for 6H-SiC in the 1-30 eV range, 1.7% and 4.1% for GaN in the 1.5-10 eV range, 1.2% and 2.5% for InN in the 2-10 eV range and 1.5% and 1.9% for AlN in the 6-20 eV range.link_to_subscribed_fulltex

Modeling the optical constants of AlN and 6H-SiC

Optical constants of hexagonal AlN in the range 6-20 eV and 6H-SiC in the range 1-30 eV for the component perpendicular to the c axis are modeled using modified Adachi's model of the optical properties of semiconductors. Model parameters are determined by acceptance-probability-controlled simulated annealing. Main distinguishing feature of the model employed here is the use of variable broadening instead of the conventional Lorentzian one. In such a manner, broadening function can vary over a range of functions with similar kernels but different wings. Therefore, excessive absorption inherent to Lorentzian broadening can be reduced so that better agreement with experimental data can be achieved. Relative rms errors for the real and imaginary parts of the index of refraction, respectively, equal 3.5% and 5.2% for 6H-SiC and 1.5% and 1.9% for AlN.link_to_subscribed_fulltex

Optical constants of InP and GaP

Calculation of the optical constants of InP and GaP is presented. The employed model is a modification of Adachi's model of the optical constants of semiconductors, which employs adjustable broadening instead of the conventional Lorentzian one. In this work we do not take into account excitonic effects at E1 and E1 + Δ1 critical points. In such a manner, fewer adjustable model parameters are required and the term with dubious physical interpretation describing excitons at E1 and E1 + Δ1 is left out. We obtain excellent agreement with experimental data over the entire 1-6 eV range, with relative rms error for the refractive index equal 1.0% for InP and 1.2% for GaP.link_to_subscribed_fulltex

Recent progress of inverted organic-inorganic halide perovskite solar cells

In recent years, inverted perovskite solar cells (IPSCs) have attracted significant attention due to their low-temperature and cost-effective fabrication processes, hysteresis-free properties, excellent stability, and wide application. The efficiency gap between IPSCs and regular structures has shrunk to less than 1%. Over the past few years, IPSC research has mainly focused on optimizing power conversion efficiency to accelerate the development of IPSCs. This review provides an overview of recent improvements in the efficiency of IPSCs, including interface engineering and novel film production techniques to overcome critical obstacles. Tandem and integrated applications of IPSCs are also summarized. Furthermore, prospects for further development of IPSCs are discussed, including the development of new materials, methods, and device structures for novel IPSCs to meet the requirements of commercialization