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Multijunction Solar Cells With Graded Buffer Bragg Reflectors

France, Ryan M. and Espinet-González, Pilar and Ekins-Daukes, Nicholas J. and Guthrey, Harvey and Steiner, Myles A. and Geisz, John F. (2018) Multijunction Solar Cells With Graded Buffer Bragg Reflectors. IEEE Journal of Photovoltaics, 8 (6). pp. 1608-1615. ISSN 2156-3381.

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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 μ m buffers using AlGaInAs layers that alternate between 30% and 70% aluminum content. Using a 2 μ m 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
France, Ryan M.0000-0002-2040-4809
Espinet-González, Pilar0000-0002-7656-0077
Ekins-Daukes, Nicholas J.0000-0003-1875-9739
Guthrey, Harvey0000-0003-1574-3379
Steiner, Myles A.0000-0003-1643-9766
Geisz, John F.0000-0003-4818-653X
Additional Information:© 2018 IEEE. Manuscript received June 26, 2018; revised August 3, 2018; accepted September 5, 2018. Date of publication October 1, 2018; date of current version October 26, 2018. This work was supported in part by the U.S. Department of Energy under Contract no. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory, provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office (SETO) agreement number 30293. The authors thank J. Buencuerpo, M. Steger, and K. Schulte for valuable discussions, and W. Olavarria and M. Young for growth and processing.
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC36-08GO28308
Subject Keywords:Bragg reflector, metamorphic, multijunction solar cells, quantum wells
Issue or Number:6
Record Number:CaltechAUTHORS:20181008-154750869
Persistent URL:
Official Citation:R. M. France, P. Espinet-González, N. J. Ekins-Daukes, H. Guthrey, M. A. Steiner and J. F. Geisz, "Multijunction Solar Cells With Graded Buffer Bragg Reflectors," in IEEE Journal of Photovoltaics, vol. 8, no. 6, pp. 1608-1615, Nov. 2018. doi: 10.1109/JPHOTOV.2018.2869550
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90166
Deposited By: Tony Diaz
Deposited On:08 Oct 2018 23:01
Last Modified:26 Nov 2019 19:40

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