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Title: Multijunction Solar Cells With Graded Buffer Bragg Reflectors

Abstract

Metamorphic solar cells can have optimal bandgap combinations through the use of compositionally graded buffers, where the lattice constant is slowly varied over several microns of growth. Bragg reflectors consist of several microns of alternating layers with refractive index contrast and provide a useful internal reflection to multijunction solar cells with optically thin subcells. In this work, we implement distributed Bragg reflectors within the compositionally graded buffers of inverted metamorphic solar cells to add functionality to the buffer. The reflectance of this AlGaInAs 'graded buffer Bragg reflector' is very similar to the reflectance of a similar AlGaAs Bragg reflector external to a buffer as well as the reflectance predicted by the transfer matrix model, indicating that the roughness of the buffer does not drastically reduce the reflection. Reflectance of 72%, 91%, and 98% is achieved in 2, 4, and 8 um buffers using AlGaInAs layers that alternate between 30% and 70% aluminum content. Using a 2 um graded buffer Bragg reflector, the 1.0-eV mismatched subcell of a GaAs/GaInAs tandem has a minor increase in threading dislocation density compared to a standard graded buffer and a small, 20 mV, loss in voltage. As the buffer is thickened, the voltage loss ismore » recuperated and excellent subcell voltages are achieved, indicating that the Bragg reflector is not severely hindering dislocation glide. We demonstrate that the benefits of the graded buffer Bragg reflector for optically thin subcells and subcells containing quantum wells, and conclude that Bragg reflectors can effectively be implemented within graded buffers, adding functionality without adding cost.« less

Authors:
 [1];  [2];  [3];  [1];  [1];  [1]
  1. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  2. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  3. Univ. of New South Wales, Sydney, NSW (Australia)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S)
OSTI Identifier:
1478616
Report Number(s):
NREL/JA-5900-71716
Journal ID: ISSN 2156-3381
Grant/Contract Number:  
AC36-08GO28308
Resource Type:
Accepted Manuscript
Journal Name:
IEEE Journal of Photovoltaics
Additional Journal Information:
Journal Volume: 6; Journal Issue: 8; Journal ID: ISSN 2156-3381
Publisher:
IEEE
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; Bragg reflector; metamorphic; multijunction solar cells; quantum wells

Citation Formats

France, Ryan M., Espinet-Gonzalez, Pilar, Ekins-Daukes, Nicholas J., Guthrey, Harvey, Steiner, Myles A., and Geisz, John F. Multijunction Solar Cells With Graded Buffer Bragg Reflectors. United States: N. p., 2018. Web. doi:10.1109/JPHOTOV.2018.2869550.
France, Ryan M., Espinet-Gonzalez, Pilar, Ekins-Daukes, Nicholas J., Guthrey, Harvey, Steiner, Myles A., & Geisz, John F. Multijunction Solar Cells With Graded Buffer Bragg Reflectors. United States. doi:10.1109/JPHOTOV.2018.2869550.
France, Ryan M., Espinet-Gonzalez, Pilar, Ekins-Daukes, Nicholas J., Guthrey, Harvey, Steiner, Myles A., and Geisz, John F. Mon . "Multijunction Solar Cells With Graded Buffer Bragg Reflectors". United States. doi:10.1109/JPHOTOV.2018.2869550. https://www.osti.gov/servlets/purl/1478616.
@article{osti_1478616,
title = {Multijunction Solar Cells With Graded Buffer Bragg Reflectors},
author = {France, Ryan M. and Espinet-Gonzalez, Pilar and Ekins-Daukes, Nicholas J. and Guthrey, Harvey and Steiner, Myles A. and Geisz, John F.},
abstractNote = {Metamorphic solar cells can have optimal bandgap combinations through the use of compositionally graded buffers, where the lattice constant is slowly varied over several microns of growth. Bragg reflectors consist of several microns of alternating layers with refractive index contrast and provide a useful internal reflection to multijunction solar cells with optically thin subcells. In this work, we implement distributed Bragg reflectors within the compositionally graded buffers of inverted metamorphic solar cells to add functionality to the buffer. The reflectance of this AlGaInAs 'graded buffer Bragg reflector' is very similar to the reflectance of a similar AlGaAs Bragg reflector external to a buffer as well as the reflectance predicted by the transfer matrix model, indicating that the roughness of the buffer does not drastically reduce the reflection. Reflectance of 72%, 91%, and 98% is achieved in 2, 4, and 8 um buffers using AlGaInAs layers that alternate between 30% and 70% aluminum content. Using a 2 um graded buffer Bragg reflector, the 1.0-eV mismatched subcell of a GaAs/GaInAs tandem has a minor increase in threading dislocation density compared to a standard graded buffer and a small, 20 mV, loss in voltage. As the buffer is thickened, the voltage loss is recuperated and excellent subcell voltages are achieved, indicating that the Bragg reflector is not severely hindering dislocation glide. We demonstrate that the benefits of the graded buffer Bragg reflector for optically thin subcells and subcells containing quantum wells, and conclude that Bragg reflectors can effectively be implemented within graded buffers, adding functionality without adding cost.},
doi = {10.1109/JPHOTOV.2018.2869550},
journal = {IEEE Journal of Photovoltaics},
number = 8,
volume = 6,
place = {United States},
year = {2018},
month = {10}
}

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