RESEARCH ARTICLE Two-terminal metal-inter-connected multijunction III–V solar cells Chieh-Ting Lin 1 * , William E. McMahon 2 , James S. Ward 2 , John F. Geisz 2 , Mark W. Wanlass 2 , Jeffrey J. Carapella 2 , Waldo Olavarria 2 , Emmett E. Perl 1 , Michelle Young 2 , Myles A. Steiner 2 , Ryan M. France 2 , Alan E. Kibbler 2 , Anna Duda 2 , Tom E. Moriarty 2 , Daniel J. Friedman 2 and John E. Bowers 1 1 Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA, USA 2 National Renewable Energy Laboratory, Golden, CO, USA ABSTRACT A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar-array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice-matched two- terminal, four-junction device that consists of an inverted top two-junction (2J) cell with 1.85 eV GaInP/1.42 eV GaAs, and an upright lower 2J cell with ~1 eV GaInAsP/0.74 eV GaInAs aimed for concentrator applications. The fabrication pro- cess and simulation of the metal topology are discussed along with the results of GaAs/GaInAs 2J and (GaInP + GaAs)/ GaInAs three-junction bonded cells. Bonding-related issues are also addressed along with optical coupling across the bonding interface. Copyright © 2014 John Wiley & Sons, Ltd. KEYWORDS III–V semiconductor; photovoltaic cells; multijunction; concentrator photovoltaic; device bonding; thermal compression bond *Correspondence Chieh-Ting Lin, Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA, USA. E-mail: clin01@umail.ucsb.edu Received 5 February 2013; Revised 26 October 2013; Accepted 10 December 2013 1. INTRODUCTION Multijunction photovoltaics have made significant progress recently because of an increase in the number of junctions and advancements in epitaxial growth technology. Researchers are now able to grow three-junction (3J) cells with low defect density and high efficiency with methods such as inverted metamorphic growth, which allows integra- tion of lattice-mismatched materials with improved bandgap selection. State-of-the-art 3J cells have achieved efficiencies greater than 40% using epitaxial methods [1–3], and further increases in efficiency are expected with the use of addi- tional lattice-mismatched junctions. Lattice-mismatched growth and the utilization of novel materials allow for better bandgap selection, but maintaining low defect densi- ties due to lattice constraints can still be an issue as addi- tional junctions are added to achieve higher efficiencies. Wafer bonding offers a method for combining materials with dissimilar lattice constants, with each subcell grown on an appropriate lattice-matched substrate. Therefore, a wafer-bonded four-junction (4J) cell could be fabricated from nearly defect-free subcells and thereby achieve higher efficiencies than other approaches. Soitec recently reported a direct-bonded 4J cell that achieved 44.7% efficiency at 297× concentration, which is a world record at the time of this writing [4]. Furthermore, wafer bonding widens the de- sign space for future devices and would allow for new con- cepts such as the integration of InGaN-based wide bandgap material for 5+ junction devices. Simulations for this type of device show that these devices can attain efficiencies greater than 50% at 1000× concentration under the ASTM G173 direct spectrum if losses can be kept to levels typical of current commercial multijunction cell structures [5–8]. 2. BACKGROUND A wafer bonding process applicable to photovoltaic devices must simultaneously satisfy strict requirements for electri- cal, optical, thermal, and mechanical coupling. Further- more, the bonding process cannot severely degrade the device performance of either tandem. While it is possible PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2468 Copyright © 2014 John Wiley & Sons, Ltd.