Window optimization enabling broadband double-layer antireflection coating for GaAsP/SiGe tandem on silicon Brianna Conrad a,⇑ , Xin Zhao a , Dun Li a , Li Wang a , Martin Diaz a , Anastasia Soeriyadi a , Anthony Lochtefeld b , Andrew Gerger c , Allen Barnett a , Ivan Perez-Wurfl a a School of Photovoltaic and Renewable Energy Engineering, UNSW Australia, Sydney 2052, Australia b AmberWave Inc., Salem, NH 02079, USA c SolAero Technologies, Albuquerque, NM 87123, USA Received 19 October 2015; received in revised form 31 December 2015; accepted 12 January 2016 Communicated by: Associate Editor Takhir M. Razykov Abstract A high-performance broadband double-layer antireflection coating (DARC) is designed for GaAsP/SiGe tandem solar cells on a sil- icon substrate. Optimization of the window layer as part of the ARC design allows lower reflection to be achieved across the full range of the device’s spectral response. Previous work has rarely fully optimized the window in DARC designs. An estimated non-zero collection profile from the window layer is used in optimization. Variation of the exact collection profile has minimal impact on the ideal design. DARC designs which result in currents within 1% of similarly optimized triple-layer designs, without the need for increased complexity in fabrication, are achieved. The benefit of an optimized window used with DARC is shown to apply also to 4-junction cells. Ó 2016 Elsevier Ltd. All rights reserved. Keywords: Antireflection coating; Multijunction solar cell; Optical design; Tandem solar cell on Si; Window layer 1. Introduction High efficiency solar cells, including multijunctions, require a well-designed antireflection coating (ARC) to maximize current. One common type of ARC for high effi- ciency devices is the double-layer ARC (DARC). Multijunction solar cells often require a low reflection across a wider spectral range than a standard single- junction cell. This means that a DARC may not be effective at reducing the reflection to levels required for high efficien- cies, and other solutions, such as the addition of a third ARC layer, may be required (Aiken, 2000). If the subcells in a multijunction device are not current-matched, the ARC need only minimize the reflection over the more nar- row spectral range of the limiting subcells, making a DARC sufficient (Homier et al., 2012; Valdivia et al., 2008). In this work, it is shown that the optimization of the thin (10–50 nm), passivating window layer present in these devices along with the DARC design allows DARC performance to approach that of a 3-layer ARC (3LARC). This is true even when current-matched subcells require low reflection across the entire multijunction range, up to 1800 nm for 4 J cells. The window layer in a III–V device affects performance due to both reflection and absorption losses. Therefore, when designing the ARC for a device with a window layer, this layer is usually included in the optical model (Al-Bustani and Feteha, 1995; Homier et al., 2012), although in some cases it has been ignored (Sueda et al., http://dx.doi.org/10.1016/j.solener.2016.01.019 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: bsmc@alum.mit.edu (B. Conrad). www.elsevier.com/locate/solener Available online at www.sciencedirect.com ScienceDirect Solar Energy 127 (2016) 216–222