Mid-infrared emissivity of crystalline silicon solar cells

A. Riverola¹, A. Mellor², D. Alonso Alvarez², L. Ferre Llin³, I. Guarracino⁴, C. N. Markides⁴, D. J. Paul³, D. Chemisana¹, N. Ekins-Daukes^2
1Applied Physics Section of the Environmental Science Department, University of Lleida, 25001, Lleida, Spain
²Department of Physics, Imperial College London, London SW7 2AZ, UK
³School of Engineering, University of Glasgow, Glasgow G12 8LT, UK
⁴Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
The following article appeared in Solar Energy Materials and Solar Cells, 174, 607 (2017)

Abstract
The thermal emissivity of crystalline silicon photovoltaic (PV) solar cells plays a role in determining the operating temperature of a solar cell. To elucidate the physical origin of thermal emissivity we have made an experimental measurement of the full radiative spectrum of the crystalline silicon (c-Si) solar cell, which includes both absorption in the ultraviolet to near-infrared range and emission in the mid-infrared. Using optical modelling, we have identified the origin of radiative emissivity in both encapsulated and unencapsulated solar cells. We find that both encapsulated and unencapsulated c-Si solar cells are good radiative emitters but achieve this through different effects. The emissivity of an unencapsulated c-Si solar cell is determined to be 75% in the MIR range, and is dominated by free-carrier emission in the highly doped emitter and back surface field layers; both effects are greatly augmented through the enhanced optical outcoupling arising from the front surface texture. An encapsulated glass-covered cell has an average emissivity around 90% on the MIR, and dips to 70% at 10 µm and is dominated by the emissivity of the cover glass. These findings serve to illustrate the opportunity for optimising the emissivity of c-Si based collectors, either in conventional c-Si PV modules where high emissivity and low-temperature operation is desirable, or in hybrid PV-thermal collectors where low emissivity enables a higher thermal output to be achieved.

Effect of Sb in thick InGaAsSbN layers grown by liquid phase epitaxy

V. Donchev¹, M. Milanova², I. Asenova¹ , N. Shtinkov³, D. Alonso-Álvarez⁴, A. Mellor⁴, J. Karmakov¹ , S. Georgiev¹ and N. Ekins-Daukes^4
1Faculty of Physics, Sofia University, 5, J.Bourchier blvd., Sofia-1164, Bulgaria
²Central Laboratory of Applied Physics, 59 St. Petersburg blvd, 4000 Plovdiv, Bulgaria
³510-67 Cartier St., Ottawa, ON, K2P 1J6 Canada
⁴Department of Physics, Imperial College London, London, UK

Paper submitted to Journal of Crystal Growth

Abstract

Dilute nitride InGaAsSbN layers grown by low-temperature liquid phase epitaxy are studied in comparison with quaternary InGaAsN layers grown at the same growth conditions to understand the effect of Sb in the compound. The lattice mismatch to the GaAs substrate is found to be slightly larger for the InGaAsNSb layers, which is explained by the large atomic radius of Sb. A reduction of the band gap energy with respect to InGaAsN is demonstrated by means of photoluminescence (PL), surface photovoltage (SPV) spectroscopy and tight-binding calculations. The band-gap energies determined from PL and ellipsometry measurements are in good agreement, while SPV spectroscopy and the tight-binding calculations provide lower values. Possible reasons for these discrepancies are discussed. The PL spectra reveal localized electronic states in the band gap near the conduction band edge, which is confirmed by SPV spectroscopy. The analysis of the power dependence of the integrated PL has allowed determining the dominant recombination mechanisms in the layers.  The values of the refraction index in the 450 nm – 1300 nm range are found to be higher for the Sb containing layers.

Preprint available here

Solcore: A multi-scale python-based library for the modelling of solar cells and semiconductor materials

D. Alonso-Álvarez¹, T. Wilson¹, P. Pearce¹ and N. Ekins-Daukes-Daukes^{1, 2
1}Department of Physics, Imperial College London, London, United Kingdom
²School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, Australia

Paper submitted to Computer Physics Communications

Abstract

Solcore was born as a modular set of tools, written entirely in Python 3, to address some of the task we had to solve more often, such as fitting dark IV curves or luminescence decays. With time, however, it has evolved as a complete semiconductor solver able of modelling the optical and electrical properties of a wide range of solar cells, from quantum well devices to multi-junction solar cells. At present, it is a multi-scale simulation framework, with applicability from the nanoscale and the quantum properties of semiconductors nanostructures to assessing the performance of solar arrays or the modelling of the spectral irradiance based on atmospheric conditions. In this article we summarize its main characteristics as well as the physical insight and mathematical formulation behind the software with the purpose of serving both as a research and teaching tool.

Preprint available in arXiv:1709.06741

The Impact of Luminescent Down Shifting on the Performance of CdTe Photovoltaics: Impact of the Module Vintage

D. Ross, D. Alonso-Álvarez, E. Klampaftis and B. S. Richards
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, U.K.

J. Fritsche and M. Bauer
Calyxo GmbH, Bitterfeld-Wolfen 06766, Germany

M. G. Debije and R. M. Fifiel
Eindhoven University of Technology, Eindhoven 5600MB, The Netherlands

The following article appeared in IEEE Journal of Photovoltaics, 4, 457 (2013)

Abstract

Luminescent down-shifting (LDS) performance is presented for different production-line cadmium telluride (CdTe) mini-modules manufactured in 2009 (CX1) and 2012 (CX3). The 10 cm × 10 cm mini-modules were laminated with LDS layers that consist of luminescent organic dye-doped ethylene vinyl acetate (EVA) and a fluorinated ethylene propylene (FEP) cover sheet. The external quantum efficiency of the devices was measured before and after the encapsulation. All LDS layers improved the short-circuit current. Lumogen Yellow 083 (Y083) and 170 (Y170) dyes showed the greatest relative improvement in the short-circuit current for the CX1 devices (9.6% and 9.7%, respectively), while Lumogen Yellow 083 showed the largest improvement for the CX3 (5.3%). The different vintages of modules also enabled an investigation of how the enhancement possible with LDS relates to other technological efficiency improvements for CdTe cells and indicates the level of future improvements possible. Finally, initial results of the same EVA–FEP LDS layers that contain the V570, Y083, and mixture of the two dyes are also presented for full-size (120 cm × 60 cm) CX3 production line modules (2012). The full-size module results indicate that scale up of the method is feasible with the Y083 layer, which improves short-circuit current by 4.3%.

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Copyright © 2014, IEEE

Effect of Sb incorporation on the electronic structure of InAs quantum dots

A. G. Taboada, J. M. Llorens, D. Alonso-Álvarez, B. Alén, A. Rivera, Y. González and  J. M. Ripalda
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

The following article appeared in Physical Review B, 88, 085303 (2013)

Abstract

On the basis of optical characterization experiments and an eight band kp model, we have studied the effect of Sb incorporation on the electronic structure of InAs quantum dots (QDs). We have found that Sb incorporation in InAs QDs shifts the hole wave function to the center of the QD from the edges of the QD where it is otherwise pinned down by the effects of shear stress. The observed changes in the ground-state energy cannot merely be explained by a composition change upon Sb exposure but can be accounted for when the change in lateral size is taken into consideration. The Sb distribution inside the QDs produces distinctive changes in the density of states, particularly, in the separation between excitation shells. We find a 50% increase in the thermal escape activation energy compared with reference InAs quantum dots as well as an increment of the fundamental transition decay time with Sb incorporation. Furthermore, we find that Sb incorporation into quantum dots is strongly nonlinear with coverage, saturating at low doses. This suggests the existence of a solubility limit of the Sb incorporation into the quantum dots during growth.

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© 2013 The American Physical Society.

Strain driven migration of In during the growth of InAs/GaAs quantum posts

D. Alonso-Álvarez, B. Alén, J. M. Ripalda, A. Rivera, A. G. Taboada, J. M. Llorens, Y. González, L. González, F. Briones
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

The following article appeared in APL Materials, 1, 022112 (2013)

Abstract

Using the mechano-optical stress sensor technique, we observe a counter-intuitive reduction of the compressive stress when InAs is deposited on GaAs (001) during the growth of quantum posts. Through modelling of the strain fields, we find that such anomalous behaviour can be related to the strain-driven detachment of In atoms from the crystal and their surface diffusion towards the self-assembled nanostructures.

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Copyright 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

A. G. Taboada, D. Alonso-Álvarez, B. Alén, A. Rivera, J. M. Ripalda, J. M. Llorens, J. Martín-Sánchez, Y. González and J. M. García
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

A. M. Sánchez
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom

A. M. Beltrán and S. I. Molina
Departamento de Ciencia de los Materiales e I. M. y Q. I., Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro s/n, Puerto Real, Cádiz 11510, Spain

M. Bozkurt, J. M. Ulloa and P. M. Koenraad
COBRA Inter-University Research Institute, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands

The following article appeared in Physical Review B, 82, 235316 (2010)

Abstract

We present experimental evidence of Sb incorporation inside InAs/GaAs(001) quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to 1.36 μm accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

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© 2010 The American Physical Society.

Analysis of the 3D distribution of stacked self-assembled quantum dots by electron tomography

J. Hernández-Saz M. Herrera and S.I. Molina
INNANOMAT Group, Departamento de Ciencia de los Materiales e I.M. y Q.I., Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, s/n, Puerto Real, Cadiz, 11510, Spain

D. Alonso-Álvarez
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

The following article appeared in Nanoscale Research Letters, 7, 681 (2012)

Abstract

The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the highest performance in a variety of optoelectronic devices. In this work, we have measured this distribution in 3D using a combined procedure of needle-shaped specimen preparation and electron tomography. We show that conventional 2D measurements of the distribution of QDs are not reliable, and only 3D analysis allows an accurate correlation between the growth design and the structural characteristics.

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Copyright©2012 Hernández-Saz et al.; licensee Springer.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Enhancement of the room temperature luminescence of InAs quantum dots by GaSb capping

J. M. Ripalda, D. Alonso-Álvarez, B. Alén, A. G. Taboada, J. M. García, Y. González and L. González
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

The following article appeared in Applied Physics Letters, 91, 012111 (2007)

Abstract

The authors have studied the use of antimony for the optimization of the InAs/GaAs(001)self-assembled quantum dot (QD) luminescence characteristics in the 1.3 μm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at1.3 μm emission wavelength.

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Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

Epitaxial quantum dots for sunlight harvesting

D. Alonso-Álvarez, B. Alén and J. M. Ripalda
IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Spain

The following article appeared in Acta Futura, 4, 69-80 (2011)

Abstract

As the photovoltaic market grows, there is an increasing necessity for improving solar cell technologies beyond their current limitations. One of the suggested ways to achieve that improvement is through the use of epitaxial quantum dots and the intermediate band solar cell concept. In this work, we present an overview of this technology, from the fundamentals of the concept to its practical implementation with quantum dots (QD) including its current status of development.

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