Ahmad Puaad Othman

ELECTRICAL CHARACTERIZATION OF P3HT/PCBM BULK HETEROJUNCTION ORGANIC SOLAR CELL

Photovoltaic performance of bulk heterojunction organic solar cell based on poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were investigated. The active layer is a spin coated organic blend of a p material (P3HT) and an n-material from the fullerene derivative PCBM; it is sandwiched between electrodes ITO-PEDOT/PSS and Al/LiF as back-contact. Modeling of organic bulk heterojunction solar cells is complicated because of various internal mechanisms involved. Two models have been suggested, namely an effective medium model and a network model. We applied an effective medium model where the main assumption is the p–n nanostructure is treated as one single effective semiconductor layer, and parameters in this configuration are fed into a standard solar cell device simulator, called SCAPS. In this model, other non-carrier related properties, such as the refractive index n, the dielectric constant e and the absorption constant α are influenced by both p–n materials and used as input parameters. The power conversion efficiency of 3.88% with short circuit current density of 20.61 mA/cm2, open circuit voltage of 0.39 V and fill factor of 48% were obtained. Finally, factors which could limit cell conversion efficiency are discussed.

Strain modification of band edge energies in a pyramidal InAs-GaAs quantum dot system

We present the calculated band edge energies altered by strain in a nanostructure system of a pyramidal InAs quantum dot buried in a GaAs substrate. Our zinc-blende supercell system of dimension 11.9 nm × 11.9 nm × 8.5 nm and 55119 atoms contains a pyramidal In770As886 quantum dot of 1656 atoms with height of 3.03 nm and square base of length 6.06 nm. The strain energy of this nanostructure system is minimized by employing the Keating formulation of interatomic potential and Monte Carlo relaxation method via the Metropolis algorithm. This relaxation is run for 20 million Monte Carlo steps at simulation temperature of 4.2 K. The calculated strain is then used to determine the conduction and valence band edge energies of the nanostructure. We find that along the [001] growth direction in the InAs quantum dot region, strain increases the conduction band edge energy by 0.6 eV and in the valence band strain results in relatively sharp wells at the dot base for heavy holes and at the dot tip for light holes. Thus, our calculation predicts that strain leads to increased band gap and spatial splitting of holes in this nanostructure system.

Discontinuity mass of finite difference calculation in InAs-GaAs quantum dots

Recently, theoretical analysis of the electronic properties of quantum dot has attracted a great attention when modern nanotechnology has made it possible to fabricate a realistic quantum dots in laboratory [. Quantum dot structures which provide electron confinement in three dimensions can be grown by the so called self-assembly effect or Stranski-Krastanov growth mode. Particular interest attracts ordering effects in StranskiKrastanow growth which proceeds on a lattice-mismatched substrate via formation of essentially three-dimensional islands. This is especially true for the InAs-GaAs system where the lattice mismatch is high and the nucleation process is rapid. Although, quantum dots have being studied experimentally but large amount of numerical studies of electron confined states also have been developed to simulate electronic and optical properties in quantum dots. The single band effective mass is one of the formalism of envelope function which has been widely used to solve quantum dot systems. However, the effective mass m* is usually position dependent in semiconductor heterostrutures. Consequently, the concerning about the form of the boundary conditions to impose on different material interface arisen [3]. According to the present works [2, , the position dependent Hamiltonian is given by: . where m = m (r) is the position dependent effective mass of an electron in conduction band. The constant α, β, and γ is arbitrary set to satisfy α + β + γ = -1. Various approximations regarding the actual constant of α, β, and γ in position dependent effective mass have been observed, example Gora & William (by putting α = -1 and β = γ = 0), Zhu & Kroemer (α = γ = -1/2 and β = 0), and BenDaniel-Duke (α = γ = 0 and β = -1). Among them, β = 1 (known as the Ben DanielDuke Hamiltonian [) is most popular method for solving mass continuity problem on the classic Hamiltonian [. Extensively, these interface condition was been used to solved most of the heterostructure problem such as quantum dots [. However, there is a qualitative argument based upon the Ben DanielDuke choice violates the Heisenberg uncertainty principle and the issue of the correct effective-mass equation was further questioned by Pistol, M. E. which he claims that all the possible equations lead to the same interfacial conditions on the envelope function [. In this paper, we will investigate the effect of discontinuity mass within interface of two semiconductor materials inside InAs-GaAs quantum dot by using the classic constant mass Hamiltonian (CH), position dependent effective mass Hamiltonian (PDH) and Ben Daniel and Duke Hamiltonian (BDH). The most common analytic methods are solving the transcendental equation obtained by matching the interface boundary condition on the envelope function. But this kind of method will suffer from complexity of model quantum dots that contain multiple layer or geometry that unable to derive into analytic formulation. Thus, this study will focus on comparison between difference finite difference formalism to illustrate the mass discontinuity effect on the numerical solution.

Calculation of Confined Electron and Hole States in a Strained InAs-GaAs Pyramidal Quantum Dot System Based on Effective Mass Approximation

Computational studies on zero dimensional semiconductor structure have been centred on typically produced quantum dot of various geometries namely pyramidal and lens with lateral sizes ranging from 10 nm to 24 nm. In the case of an epitaxially grown quantum dot, strain plays another essential role apart from its size and shape in determining its electronic properties [. Among the most studied strained structures is the self-assembled InAs quantum dot capped by a GaAs matrix. A study by [ on InAs pyramidal quantum dot predicted no confined electron states for quantum dot with base lengths 6 nm and below. Nevertheless, a calculation by [3] based on atomistic psedudopotential predicted at most two confined states for both electron and hole in a self-assembled InAs-GaAs quantum pyramid system of base length 6.06 nm.

Calculation of Electronic Properties of InAs/GaAs Cubic, Spherical and Pyramidal Quantum Dots with Finite Difference Method

We have calculated the properties of electron states in an InAs/GaAs quantum dot system based on the effective mass approximation of a one-band Hamiltonian model. This semiconductor nanostructure system consisted of an InAs quantum dot embedded in a GaAs substrate. In this paper, the Schrödinger equation of an ideal cubic quantum dot with infinite barrier was solved using a finite difference approach. The sparse matrix of N3 x N3 for the Hamiltonian was diagonalized to calculate the lowest states of electrons in this nanostructure system. The calculation was performed for different dot size and the obtained energy levels are comparable to those calculated analytically. The finite difference method was relatively faster and applicable to quantum dots of any geometry or potential profile. This was proven by applying the developed computational procedure to quantum dots of cubic, spherical and pyramidal geometries for the InAs/GaAs nanostructure system.

DFT applications in characterizing electronic properties of cadmium telluride with relativistic effects

A computational study using the density fuctional through linear augmented plane wave (LAPW) and gradient generalized approximation (GGA) methods on the electronic properties of cadmium telluride (CdTe) in two modes namely with relativistic effect and non-relativistic effect is presented. Two electronic properties were obtained and compared between the computation with and without the relativistic effects. Firstly, plots of density of states were produced which were for the total CdTe. The total DOS showed that the conduction band was dominated by the states of Te atom, whereas the valence band is dominated by the states of Cd atom. Secondly, the total band structure plot obtained showed that the direct energy band gap, Eg calculated value with relativistic effect was about 1.0 eV while the non-relativistic effect value was 1.8 eV.