Computational modelling of the refractive index and reflectivity of a quantum well solar cell

In this paper the modelling of the refractive index and reflectivity of a quantum well solar cell (QWSC) is theoretically developed and computationally analysed. The model is based on the Modified Single Effective Oscillator model combined with the Fresnel equation. The model takes into consideration the effects of the design parameters including concentration levels, structural properties of the device (well length, etc.), operating temperature and electric field effects (due to doping concentration). Prior to analysis the authors show the model to have a solid foundation given it generates accurate results which match with experimental data for an AlGaAs/GaAs triple heterojunction cell. This justifies and promotes further work where the theoretical and computational analysis is repeated for a QWSC structure. In the case of the QWSC the results generated are for a bare AlGaAs/GaAs cell and the same cell with a ZnS anti-reflection coating. The results generated show promise for the new model however experimental verification is required. The analysis is performed for AM 1.5G spectrum. The model is intended to be an aid to QWSC designers.     

Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices

The intermixing characteristics of three widely used combinations of InP-based quantum wells (QW) are investigated using the impurity-free vacancy disordering (IFVD) technique. We demonstrate that the bandgap energy shift is highly dependent on the concentration gradient of the as-grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. In the case of the InGaAs/InGaAsP system, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs layer over the InP cladding is also investigated, which leads to a simple way to obtain three different bandgaps in a single intermixing step.     

无规则非均匀折射率场描述

在大气或流动介质中的折射率分布是无规则非均匀的,由于流动和温度变化的影响,无法用通常的梯度折射率公式来描述。提出采用自适应网格方法来描述无规则非均匀折射率场,将数据以动态八叉树结构存放在内存中,给出了网格划分的折射率判据和折射率梯度判据,并采用插值法来计算光线传播路径上的折射率和折射率梯度。以一种可以用公式描述的梯度折射率棒为例,在折射率阈值为0.005、折射率梯度阈值为0.000 5的条件下,建立了自适应网格并求出了一些位置处的折射率及其梯度,计算结果的均方根误差小于7×10－5,可以用于光线追迹。     

A revised ideal model for AlGaAs/GaAs quantum well solar cells

Quantum well solar cells (QWSCs) are heterostructure devices intended to achieve higher efficiencies than conventional cells. This paper extends a previous model for QWSC current-voltage characteristics by revising the equations for the absorbed flux and by introducing expressions to calculate radiative recombination coefficients and well effective densities-of-states. This revised model is in agreement with previous experimental results for AlGaAs/GaAs. Since the revised model incorporates detailed balance calculations, its predictions are consistent with the efficiency restrictions of this theory. The revised model, however, does predict efficiency enhancements for QWSCs in some configurations if non-radiative recombination is dominant, even in such a poor QWSC material as AlGaAs/GaAs.     

Photoresponse study of normal incidence detection in p-type GaAs/AIGaAs multiple quantum wells

We studied p-type GaAs/AIGaAs multiple quantum well (MQW) materials as a possible alternative to the current n-type GaAs/AIGaAs MQWs for infrared detection. The advantage of p-type MQWs is that absorption of infrared radiation at normal incidence is not selection rule forbidden as it is for the n-type. We have verified that significant photoresponse occurs at normal incidence in p-type MQWs. We studied changes in the photoresponse spectrum as a function of well width and temperature. The MQW heterostructures were designed to use bound to continuum intersubband absorption in the GaAs valence band and to have a peak photoresponse near 8 μm. The photoresponse spectrum was compared to the first theoretical model of the bound to continuum absorption in p-type GaAs/ AlGaAs MQWs. The theoretical absorption curve was found to be in good qualitative agreement with the experimental results.     

Well-width dependence of the phase coherent photorefractive effect in ZnSe quantum wells

We report on the well-width dependence of the phase coherent photorefractive (PCP) effect in ZnSe/ZnMgSe single quantum wells (QWs) using 90 fs light pulses. The experiments are performed in a four-wave-mixing configuration at temperatures between 25 and 65 K. The ZnMgSe/ZnSe QWs with 10, 5 and 3 nm well width were grown on GaAs substrate using molecular beam epitaxy. With decreasing QW thickness we observe a reduction of the PCP diffraction efficiency. This is attributed to the higher electron energy in small QWs due to the quantum size effect, which leads to a reduced equilibrium density of captured electrons in the QW. With increasing temperature the PCP signal further decreases which is attributed to thermal activation of the QW electrons back to the GaAs substrate.     

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

Design and analysis of a refractive index sensor based on dual-core large-mode-area fiber

We present a novel co-axial dual core large-mode-area (LMA) fiber design for refractive index sensing. In a dual-core fiber there is resonant coupling between the two cores, which is strongly affected by the refractive index (RI) of the outermost region. The transmittance of the fiber, therefore, varies sharply with the refractive index of surrounding medium. This characteristic of the proposed structure has been utilized to design a RI sensor. We have analyzed the structure by using the transfer matrix method. Our numerical results show that the proposed sensor is highly sensitive with the resolution of 2.0 × 10⁻⁶ around n_ex = 1.44376. Effect of design parameters on sensitivity of the proposed sensor has also been investigated.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

Modeling of two core photonic crystal fiber modal interferometer for refractive index measurement by equalization wavelength

We propose a modification structure model of in-fiber sensor based on intermodal interference in two core photonic crystal fiber for external refractive index measuring. Essential characteristics and influences of the structure are investigated. Formation of an extreme (equalization wavelength) in phase constant spectral dependence is presented and its using for simple external refractive index determination.     

Band-filling effect on the light emission spectra of InGaN/GaN quantum wells with highly doped barriers

We investigate spectra of InGaN/GaN quantum well (QW) light-emitting diode (LED) structures with heavily doped barriers at different excitation levels. We model the spectral shape and energy position in frames of dominating mechanism of free electron recombination accounting for the influence on the potential width of the QW of the random impurity potential penetrating from the doped barriers. The blue shift at high excitation is supposed to be due to the filling of the conduction band with degenerate 2D non-equilibrium electrons. A structure in the emission bands is observed and it is assumed to be a result from step-like 2D density of states in the QW. A good accordance is obtained between the calculated and experimental spectra assuming that the barriers are graded.     

Effects of boron incorporation on the strain and photoluminescence properties of GaAsSb/GaAs quantum wells

Boron-containing GaAsSb/GaAs quantum wells (QWs) with different antimony (Sb) mole fractions were grown by low-pressure metal–organic chemical vapor deposition for the first time. The effects of boron incorporation on the performance of GaAsSb/GaAs QWs are discussed. For samples with low compressive strain, injection of triethylboron can enhance the Sb content and increase the compressive strain, although boron incorporation can lead to a reduction in strain. This effect was less for strained GaAsSb/GaAs QWs, so the compressive strain of these QWs did not vary. Room-temperature photoluminescence emission at 1116 nm with a full-width at half-maximum (FWHM) value of 56 meV was obtained for strained BGaAsSb/GaAs QWs.     

一种新型太阳能电池减反射膜的制备

提高太阳电池的光电转换效率的方法有很多,该文提出了一种折射率不能变化的减反射膜,具有更好的减反射效果,增加了光生载流子的生成数量,从而提高晶硅太阳电池的转换效率。通过硅烷和氨气流量的连续变化,从而使减反射膜的折射率由上至下等速率地增大,可提高电池转换效率0.13%。     

Donor impurity-related optical absorption coefficients and refractive index changes in a rectangular GaAs quantum dot in the presence of electric field

Within the quasi-one-dimensional effective potential model and effective mass approximation, we obtain the wavefunctions and energy eigenvalues of the ground (j=1) and first 2 excited states (j=2 and 3) of a donor impurity in a rectangular GaAs quantum dot in the presence of electric field. The donor impurity-related linear and nonlinear optical absorption as well as refractive index changes for the transitions j=1-2 and j=2-3 are investigated. The results show that the impurity position, incident optical intensity and electric field play important roles in the optical absorption coefficients and refractive index changes. We find that the impurity effect induces the blueshift for j=1-2 and redshift for j=3-2 in the absence of the electric field, but it leads to redshift for j=1-2 and blueshift for j=3-2 in the existence of the field. Also, the optical coefficient for the higher energy transitions j=2-3 is insensitive to variation of impurity positions, while that for the low energy transition j=1-2 depends significantly on the positions of impurity. In addition, the saturation and splitting phenomenon of the optical absorption are observed as the incident optical intensity increases.     

改进长周期波导光栅折射率传感特性的研究

提出了一种附加高折射率覆盖层的长周期波导光栅折射率传感器结构.通过模拟外折射率变化所引起的高阶模式等效折射率的改变以及该高阶模式与基模相耦合的谐振波长的漂移,研究了长周期波导光栅对外界环境折射率的传感特性.模拟表明,高折射率外覆盖层的加入会使得原有的覆盖层高阶模式发生重组,高阶模式等效折射率和模式耦合的谐振波长随之发生跳变.此时,长周期波导光栅外折射率传感器的敏感度和工作范围将极大地提高.     

ZnSe quantum dots sensitized electrospun ZnO nanofibers as an efficient photoanode for improved performance of QDSSC

In the present study, zinc selenide (ZnSe) quantum dots (QDs) with an average size of ~2.6 nm were prepared by hot injection method and used as a sensitizer onto the electrospun ZnO nanofibers using 3-mercaptapropionic acid as a linker agent. The optical absorption, photoluminescence and time-resolved photoluminescence (TRPL) studies for ZnSe sensitized ZnO NFs were performed to give insight about the improvement in optical properties. The performances of fabricated QDSSCs was examined in detail using cobalt sulfide (CoS) as a counter electrode and polysulfide redox couple (S²⁻/S_x²⁻) as an electrolyte. The ZnSe QDs sensitized ZnO nanofibers showed an appreciable improvement in short circuit current density (6.60 mA/cm²) with a maximum power conversion efficiency of 1.24% under 1 sun illumination of 100 mW/cm². This enhancement is mainly due to better light harvesting ability of ZnSe QDs and ZnO NFs, and lower recombination of photoinjected electrons with the polysulfide electrolyte. The improvement in power conversion efficiency (PCE) and reduction in back electrons recombination are supported by photovoltaic and electrochemical impedance studies. Finally, stability test was carried out over a span of 30 days (720 h) under one sun illumination to know about the practical applicability of the resultant QDSSC.     

Structural and optical properties of AlxGa1-xN/AlyGa1-yN multiple quantum wells for deep ultraviolet emission

The AlxGa1-xN/AlyGa1-yN multiple quantum well (MQW) structure for deep ultraviolet emission has been grown on sapphire by metal organic chemical vapor deposition (MOCVD).High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and cath-odoluminescence (CL) are used to characterize the structural and optical properties of MQWs, respectively.Clear step flows can be observed in the AFM image indicating a two-dimensional growth model.There are many cracks on the surface of the MQW structure because of the high tensile stress.HRXRD shows multiple satellite peaks to the 2rid order.The HRXRD simulation shows that the MQW period is about 11.5 nm.The emission peak of AlxGa1-xN/AlyGa1-yN MQWs is about 295 nm in the deep ultraviolet region from the CL spectra.     

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.     

Investigation of scalability of In0.7Ga0.3As quantum well field effect transistor (QWFET) architecture for logic applications

In this paper, the scalability of In_0.7Ga_0.3As QWFET is investigated using two-dimensional numerical drift-diffusion simulation. Numerical drift-diffusion simulations were calibrated using experimental results on short-channel In_0.7Ga_0.3As QWFETs [7] to include the effects of velocity overshoot. Logic figures of merit (sub-threshold slope, saturated threshold voltage, drain induced barrier lowering, I_ON/I_OFF ratio over a specified gate swing, effective injection velocity and intrinsic switching delay) extracted from the numerical simulations are in excellent agreement with the experimental data. Three alternate QWFET device architectures are proposed and thoroughly investigated for 15 nm node and beyond logic applications. Amongst them, double-gate In_0.7Ga_0.3As QWFET shows the best scalability in terms of logic figures of merit, thus making it an ideal candidate for the design and demonstration of the ultimate scaled transistor.     

基于Matlab的太阳能电池数学模型及输出特性分析

该文主要讨论了太阳能电池的数学模型、温度及光照变化时4个典型参数Voc、Isc、Vm、Im的计算方法及仿真结果。以无锡尚德3909电池在25°C 1000W和4209电池在30°C、的测试数据为参考数据,分别给出了温度为25°C时800W、200W和光照在1 000W/m2时40和60°C两种情况下基于数学模型的Voc、Isc、Vm、Im的计算结果和基于Matlab的仿真图形,分析了输出特性曲线随温度和光照变化的变化规律,结果表明计算及仿真结果和实际测试数据基本相符。