High performance anti-reflection coatings for broadband multi-junction solar cells Page: 1 of 17
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S/02o0 -04)91 -o
High Performance Anti-Reflection Coatings for
Broadband Multi-Junction Solar Cells
Author: Daniel J. Aiken
Affiliation: Sandia National Laboratories*, Albuquerque, NM, 87185
Phone: (505) 284-6607 Z0
Fax: (505) 844-6541
The success of bandgap engineering has made high efficiency broadband multi-junction solar cells
possible with photo-response out to the band edge of Ge. Modeling has been conducted which suggests that
current double layer anti-reflection coating technology is not adequate for these devices in certain cases.
Approaches for the development of higher performance anti-reflection coatings are examined. A new AR
coating structure based on the use of Herpin equivalent layers is presented. Optical modeling suggests a
decrease in the solar weighted reflectance of over 2.5% absolute as a result. This structure requires no
additional optical material development and characterization because no new optical materials are necessary.
Experimental results and a sensitivity analysis are presented.
Keywords: antireflection coating, multi-junction, solar weighted reflectance, Herpin equivalent layers
Group IlIl-V multi-junction solar cells have achieved world record efficiencies for monolithic, two
terminal devices . Dual junction InGaP/GaAs solar cells have reached an AMO efficiency of 26.9% (4 cm2)
. Triple junction InGaP/GaAs/Ge devices with an AMO efficiency of 26.7% (21.65 cm2) have also been
reported . These multi-junction solar cells are in production at three major space photovoltaics
manufacturing companies in the U.S. and are also appealing candidates for use in terrestrial concentrator
systems due to their high efficiencies. As a result of this success there is considerable interest in furthering
this concept with the development of a 1 eV material lattice matched to GaAs and Ge to make ultra-high
efficiency 4-junction solar cells possible [4,5].
Multiple bandgap solar cells convert a larger range of the solar spectrum to electric power as
compared to single bandgap solar cells, and have significantly higher limiting efficiencies as modeled by Henry
. The larger spectral range of these devices also requires a more broadband antireflection (AR) coating.
Reflection control for traditional single junction solar cells such as silicon, GaAs and InP, and even dual
junction solar cells such as AIGaAs/GaAs or InGaP/GaAs, has been adequately achieved using relatively
simple double layer interference coatings. This is possible due to the relatively narrow spectral range of these
solar cells. With the emergence of multi-junctions based on photovoltaically active Ge substrates, the spectral
range of multi-junctions has approximately doubled. Additionally, series interconnected multi-junctions place
greater demands on AR coating performance due to the need for current matching. This requires a re-
evaluation of what AR coating performance is necessary and what technologies will provide this level of
performance for these solar cells.
Several general approaches for improving the control of reflection at the front surface of a solar cell
are possible. Reflection control has been accomplished through the use of macroscopic and microscopic
surface texture [7-9], although mainly in silicon solar cell technology. Graded- or gradient-index concepts are
also appealing for potentially very high performance AR coatings and have been used mainly on glass
substrates thus far [10-12]. This paper focuses on the use of planar, homogeneous optical interference films
Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the U.S. Department of Energy under contract DE-AC04-94AL85000.
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AIKEN,DANIEL J. High performance anti-reflection coatings for broadband multi-junction solar cells, article, February 23, 2000; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc710323/m1/1/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.