Alexandre W. Walker

Alexandre W. Walker
National Research Council Canada | NRC · Advanced Electronics and Photonics Research Centre

PhD

About

69
Publications
17,896
Reads
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915
Citations
Additional affiliations
August 2016 - present
National Research Council Canada
Position
  • Research Officer
October 2013 - June 2016
Fraunhofer Institute for Solar Energy Systems
Position
  • Researcher
January 2009 - August 2013
University of Ottawa
Position
  • PhD Student

Publications

Publications (69)
Article
Full-text available
Radiatively dominated III–V semiconductor solar cells are strongly influenced by the effects of photon recycling. From a modeling standpoint, the semiconductor transport equations must account for this to predict accurate open-circuit voltages , even for cells on substrate. Using Shockley–Read–Hall (SRH) lifetimes based on internal quantum efficien...
Article
Full-text available
A power-dependent relative photoluminescence measurement method is developed for double-heterostructures composed of III-V semiconductors. Analyzing the data yields insight into the radiative efficiency of the absorbing layer as a function of laser intensity. Four GaAs samples of different thicknesses are characterized, and the measured data are co...
Article
Full-text available
Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that...
Article
Full-text available
This work presents a novel InGaAs/InP SPAD structure fabricated using a selective area growth (SAG) method. The surface topography of the selectively grown film deposited within the 70 $\mu$ m diffusion apertures is used to engineer the Zn diffusion profile to suppress premature edge breakdown. The device achieves a highly uniform active area wit...
Article
Full-text available
Numerical simulation of the electric field distribution and photocurrent response of a planar InP/InGaAs avalanche photodiode is presented for small variations of the multiplication width. The Zn dopant diffusion front is obtained by numerically simulating the diffusion process. The simulation results indicate that while a local peak value of the e...
Conference Paper
Photonic power converters (PPCs) are photovoltaic cells that convert monochromatic light into electric power. The impact of luminescent coupling (LC) on InGaAs-based PPCs is studied. Multi-junction PPCs are simulated using an experimentally validated drift-diffusion model, and the contribution of LC is quantified. Up to 85% of the photons emitted a...
Conference Paper
We have developed a machine learning empowered computational framework to facilitate design space exploration for optoelectronic devices. In this work, we apply dimensionality reduction and clustering machine learning algorithms to identify optimal ten-junction C-band photonic power converter (PPC) designs. We outline our framework, design optimiza...
Article
In this work, the authors evaluated the access resistance of InP high electron mobility transistors (HEMTs) and their degradation during wafer processing. The transfer resistance, which was rarely separated from other components of the total access resistance of InP HEMTs in the literature, was found to be the dominant component of the access resis...
Article
Full-text available
InGaAs/InP double-heterostructure P–i–N diodes of various junction areas are characterized at room temperature and modeled using TCAD. Biasing the guard ring allows one to quantify the dark current contributions to the central diode from Shockley–Read–Hall (SRH) generation in the depletion region as well as the junction perimeter leakage. The latte...
Article
Full-text available
The bulk minority carrier lifetime and interface recombination velocity (IRV) in GaInP double-heterostructures (DHs) lattice matched to GaAs are extracted using time-resolved photoluminescence (PL) measured between 300 and 500 K. Effective lifetimes show a strong dependence on temperature for samples with insufficiently strong confinement potential...
Conference Paper
We show that areas of high edge gain in avalanche photodiodes are associated with locally enhanced diffusion depth by employing a novel method using scanning electron microscopy of selectively etched Zn diffused structures.
Article
Full-text available
The combination of time-resolved (TR) and power-dependent relative (PDR) photoluminescence (PL) measurements reveals the possibility of separating the radiative and non-radiative minority carrier lifetimes and measuring the sample-dependent effective radiative recombination coefficient in direct bandgap semiconductors. To demonstrate the method, me...
Article
Conventional CuηIn; GaSe2 (CIGS) solar cells suffer poor response in the shortwavelength region (280 to 520 nm) due to the parasitic absorption in the ZnO transparent conductive oxide and inefficient collection of generated electron-hole pairs in the CdS layer. The short-wavelength response can be enhanced using a downshifting (DS) layer mounted on...
Article
A novel method of extracting the nonradiative lifetime of Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.49</sub> P lattice matched to GaAs by exploiting luminescence coupling is discussed...
Preprint
Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due...
Article
Full-text available
Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due...
Article
Minority carrier diffusion lengths and lifetimes were determined for p-type Ga(1-x)InxAs with an In-content of 0 ≤ x ≤ 0.53 by cathodoluminescence and time-resolved photoluminescence measurements respectively under low injection conditions; the resulting minority carrier mobilities are also reported. Highly p-doped samples (3 × 10¹⁸ cm⁻³) demonstra...
Article
The radiation hardness of AlGaAs single-junction solar cells is investigated for various n-i-p solar cell designs. The material composition in both the n-p regions is varied between 3.5% and 16% Al-content, whereas the intrinsic region has a higher Al-content between 3.5% and 23%. The beginning-of-life and end-of-life quantum efficiency and current...
Article
Full-text available
The hole diffusion length in n-InGaAs is extracted for two samples of different doping concentrations using a set of long and thin diffused junction diodes separated by various distances on the order of the diffusion length. The methodology is described, including the ensuing analysis which yields diffusion lengths between 70 - 85 um at room temper...
Article
Full-text available
The down-shifting (DS) process is a purely optical approach used to improve the short-wavelength response of a solar cell by shifting high-energy photons to the visible range, which can be more efficiently absorbed by the solar cell. In addition to the DS effect, coupling a DS layer to the top surface of a solar cell results in a change in surface...
Article
Full-text available
Four-junction solar cells are being developed for space applications as they promise higher efficiencies compared to the present GaInP/GaInAs/Ge triple-junction industry standard. There are multiple technological routes to achieve four-junction cells with the ideal bandgap combination of 1.9 eV, 1.4 eV 1.05 eV and 0.7 eV. This includes metamorphic...
Article
Full-text available
The minority carrier lifetime of high-quality ordered GaInP lattice matched to GaAs and the surface recombination velocity at its interface to AlInP were measured using time-resolved photo-luminescence in the temperature range of 77 – 500 K. The surface recombination velocity was found to be relatively low (under 500 cm / s) over the measured tempe...
Conference Paper
Full-text available
Very reflective back side reflectors directly below the active region of a solar cell alter the photon absorption and carrier collection dynamics in a manner that can be exploited for enhanced efficiency. The enlarged optical path increases the short circuit current, and the confinement and (re)absorption of the photoluminescence leads to a higher...
Article
Full-text available
A temperature dependent modeling study is conducted on a GaAs laser power converter to identify the optimal incident laser wavelength for optical power transmission. Furthermore, the respective temperature dependent maximal conversion efficiencies in the radiative limit as well as in a practically achievable limit are presented. The model is based...
Article
Full-text available
Modeling single junction solar cells composed of III-V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon re...
Conference Paper
Full-text available
Single junction photovoltaic devices composed of direct bandgap III-V semiconductors such as GaAs can exploit the effects of photon recycling to achieve record-high open circuit voltages. Modeling such devices yields insight into the design and material criteria required to achieve high efficiencies. For a GaAs cell to reach 28 % efficiency without...
Article
Full-text available
The design of a triple junction solar cell’s front contact grid can significantly affect cell conversion efficiency under high concentration. We consider one aspect of grid design, choosing a linear grid within a distributed resistance cell model to optimize finger spacings at concentrations between 500 and 2500 suns under uniform and nonuniform il...
Article
Full-text available
A new triple-junction solar cell (3J) design exploiting the highly absorptive I–III–VI chalcopyrite CuInSe2 material is proposed as an alternative to III–V semiconductor 3J solar cells. The proposed structure consists of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/CuInSe2 (1 eV) which can be grown on a GaAs substrate in an inverted manner using epitaxial lift...
Article
The key performance metrics of quantum-dot (QD)- lattice-matched multijunction solar cells (MJSCs) composed of InGaP/(In)GaAs/Ge with InAs/GaAs QDs are explored under high-concentration illumination with a focus on the carrier dynamics in the QD layers of the middle subcell. An effective medium approach is used to describe generation and recombinat...
Article
The positioning of InAs quantum dot (QD) layers in a triple-junction GaInP/Ga(In)As/Ge solar cell is studied using numerical modeling techniques. An effective medium is used to describe the absorption characteristics and carrier dynamics in each QD layer. The effects of incorporating 110 layers in the emitter, base layers, as well as between these...
Article
The performance improvements of adding InAs quantum dots (QDs) in the middle subcell of a lattice matched triple-junction InGaP/InGaAs/Ge photovoltaic device are studied using the simulated external quantum efficiency, photocurrent, open circuit voltage, fill factor, and efficiency under standard testing conditions. The QDs and wetting layer are mo...
Article
Intermediate band (IB) photovoltaics have the potential to be highly efficient and cost effective solar cells. When the IB concept was proposed in 1997, there were no known intermediate band materials. In recent years, great progress has been made in developing materials with intermediate bands, though power conversion efficiencies have remained lo...
Article
Full-text available
A polycrystalline Cu(In,Ga)Se2 (CIGS) single junction solar cell model is developed with dependencies on the molar fraction of In and Ga with a 0.8 eV Shockley-Read-Hall (SRH) trap level above the valence band. The simulated performance of this solar cell over molar fraction compares well to data published in the literature using the SRH minority c...
Article
Full-text available
Concentrator photovoltaic (CPV) technology has come a long way, with multi-junction solar cell efficiencies now reaching up to 44.4%. Front contact grid design, crucial for improving efficiency, is typically performed for uniform illumination, but this does not account for the real world conditions of non-homogeneous irradiance distributions. In th...
Article
Full-text available
AlGaAs/GaAs tunnel junctions for use in high concentration multijunction solar cells were designed and grown by chemical beam epitaxy (CBE) using trimethyl aluminium (TMA) as the p-dopant source for the AlGaAs active layer. Controlled hole concentration up to 4.1020 cm-3 was achieved through variation in growth parameters. Fabricated tunnel junctio...
Article
The simulation of tunnel junctions is performed by using nonlocal band-to-band and trap assisted tunneling models that are capable of reproducing the experimental current-voltage characteristics of p++AlGaAs/ n++AlGaAs and p++AlGaAs/ n++GaAs based devices. These simulated characteristics are then implemented within a lattice matched InGaP/(In)GaAs/...
Article
Full-text available
The key characteristics of quantum dot (QD)-enhanced multijunction solar cells (MJSC) are explored theoretically by focusing on the generation and recombination rates throughout the QD layers in the middle subcell. The quantum dots are modeled using an effective medium to describe light absorption, confinement, and recombination properties. We repo...
Conference Paper
Full-text available
The optimization of quadruple junction solar cell designs in the detailed balance limit via an equivalent circuit model for each sub-cell is explored using spectral sharing from three perspectives: (i) current matching at short circuit current (as per etaOpt software), (ii) corrected current matching at short circuit current and (iii) unconstrained...
Conference Paper
Full-text available
The performance effects of silicon nanocrystals (SiNC) embedded in a silicon dioxide matrix to act as a downshifting (DS) layer mounted on the top surface of a polycrystalline Cu(In, Ga)Se2 solar cell are explored numerically. The DS layers are modeled by modifying the incident AM1.5G spectrum based on the absorption and emission properties of the...
Conference Paper
Solar cells with an n-p structure consisting of an InGaAsN dilute nitride emitter with n-type background doping and a p-type doped InGaAsN base are numerically simulated on GaAs and Ge substrates. The InGaAsN material parameters are chosen based on of structures reported in the literature. The short-circuit current of cells on GaAs substrates is 11...
Article
Full-text available
The key effects of adding quantum dots in the middle sub-cell of a lattice matched triple junction solar cell are studied as a function of concentration in an advanced numerical simulation environment. The quantum dots are modeled using an effective medium which contains a quantum mechanical absorption coefficient, quantum dot to bulk carrier dyna...
Article
Full-text available
A temperature calibrated equivalent circuit model of a high efficiency CPV solar cell is used to simulate a measured six-cell module J-V curve to estimate its average operating temperature. The simulation is based on a two diode equivalent circuit model for each subcell of a representative triple junction cell. Module J-V curves in a real CPV syste...
Article
Full-text available
The current-voltage characteristics of AlGaAs/AlGaAs tunnel junctions for use in multi-junction solar cells are studied experimentally, where tunneling current peaks of 1100 A=cm 2 and specific contact resistivities of 0.3x10-4 Ohm-cm2 at 7 A/cm 2 (typical concentrated photo-voltaic operating current) are measured. This represents an ideal tunnel j...
Conference Paper
Full-text available
The efficiency improvements achieved by adding idealized, top-mounted, down-conversion (DC) and luminescent down-shifting (LDS) layers to a commercial grade silicon solar cell are studied. A comparison is then made to silicon nanocrystals (Si-NC) LDS layer coupled to a silicon solar cell, where the optical properties of the Si-NC are based on measu...
Conference Paper
Full-text available
Silicon nanocrystal (Si-NC) luminescent down-shifting materials for photovoltaic (PV) applications were fabricated by ion implantation and plasma-enhanced chemical vapor deposition (PECVD). The absolute optical conversion efficiency of the Si-NC-emitted photoluminescence was measured using conventional methods, and an optical set-up involving an in...
Article
Full-text available
Quantum dot (QD) enhanced GaInP/InGaAs/Ge solar cells are presented and characterized under flash and continuous solar simulators. InAs QD within the middle sub‐cell increase the carrier generation due to absorption in the range 900–940 nm. These QD‐enhanced solar cells routinely achieve production efficiencies of ∼40%, and this set of research s...
Article
The external quantum efficiency of a commercial quantum dot enhanced multi-junction solar cell is measured over a range of temperatures (15 °C to 75 °C). A complete numerical model of the cell is built and calibrated based on the experimental data. The short circuit current density is calculated over different temperatures under standard AM1.5D ill...
Article
The temperature dependence of GaInP/GaAs/Ge multi-junction solar cells are numerically modeled. The temperature dependence of the solar cell dark current and the spectral sensitivity of the solar cell are demonstrated.
Article
The external quantum efficiency (EQE) of a high efficiency lattice matched multi-junction solar cell (MJSC) and a quantum dot enhanced MJSC are numerically simulated. An effective medium is developed and integrated into the model to simulate the absorption characteristics of the quantum dots in the latter device. A calibration of the model is carri...
Article
Full-text available
Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time-dependent and time-average methods are used to exp...
Article
Full-text available
The following four TJ designs, AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs are studied to determine minimum doping concentration to achieve a resistance of < 10(-4) Omega.cm(2) and a peak tunneling current suitable for MJ solar cells up to 1500-suns concentration (operating current of 21 A/cm(2)). Experimentally calibrated numerical mode...
Article
Simulations of AlxGa1-xAs/GaAs (x = 0.3) and AlxGa1-xAs/AlxGa1-xAs (x < 0.2) tunnel junction J-V characteristics are studied for integration into a 2D metamorphic multi-junction solar cell model composed of GaInP/GaAs/InGaAs. A comparison of the simulated solar cell J-V characteristics under AM1.5D spectrum is discussed in terms of short circuit cu...
Article
The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction is determined by taking a time-averaged measurement across the device. A tunnelling peak of ~950A/cm2 is recorded by this method. Measurements of the tunnelling peak and valley currents by the time averaging method are obscured due to the unstable nature of the negative...
Article
InAs quantum dots in a GaAs matrix are studied. Those quantum dots are used in applications to enhance the overall efficiency of multi-junction solar cells beyond 40%. Photoluminescence measurements at 77 K using a 532 nm laser have been performed on an epitaxially grown structure of self-assembled InAs quantum dots in a GaAs matrix upon a Ge subst...
Article
Full-text available
Multi-junction solar cells are devices composed of many layers of diverse materials with varying physical properties. Understanding the operation and design of such devices is challenging because of this diversity. To support these efforts, the computer modeling and simulation study is an essential tool. The principles of two main steps in a study,...
Conference Paper
Full-text available
AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems...

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