Using Dilute Nitrides to Achieve Record Solar Cell Efficiencies
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Using Dilute Nitrides to Achieve Record Solar Cell Efficiencies Rebecca Jones-Albertus, Emily Becker, Robert Bergner, Taner Bilir, Daniel Derkacs, Onur Fidaner, David Jory, Ting Liu, Ewelina Lucow, Pranob Misra, Evan Pickett, Ferran Suarez, Arsen Sukiasyan, Ted Sun, Lan Zhang, Vijit Sabnis, Mike Wiemer, and Homan Yuen Solar Junction, 401 Charcot Avenue, San Jose, CA 95131, U.S.A. ABSTRACT High quality dilute nitride subcells for multijunction solar cells are achieved using GaInNAsSb. The effects on device performance of Sb composition, strain and purity of the GaInNAsSb material are discussed. New world records in efficiency have been set with latticematched InGaP/GaAs/GaInNAsSb triple junction solar cells and a roadmap to 50% efficiency with lattice-matched multijunction solar cells using GaInNAsSb is shown. INTRODUCTION Today’s highest efficiency solar cells are III-V multijunction solar cells. For a single junction solar cell, the theoretical efficiency is a function of its band gap [1], and is maximized by optimizing the trade-off between higher current and higher voltage. Lower band gap material converts more photons to electrons but at a lower voltage, with much of the photon energy lost to heat as photo-excited carriers relax to the semiconductor band edges, while higher band gap material produces a higher voltage but converts fewer photons to electrons. Multijunction solar cells allow for higher efficiencies by stacking individual subcells of increasing band gap such that the highest energy light is absorbed by the highest band gap subcell, with lower energy light absorbed by one or more lower subcells. In this way, a larger fraction of the solar spectrum can be absorbed and less of the photon energy is converted into heat. Accordingly, the theoretical efficiency of a multijunction solar cell increases as the number of subcells increases [2]. However, the practical efficiency depends on additional factors, including the band gaps of the subcells, the solar cell design and the material quality. Historically, the lattice-matched InGaP / (In)GaAs / Ge multijunction solar cell structure produced the highest efficiencies. It was favored because its three subcells are lattice-matched, enabling high material quality and reliability; however, its combination of band gaps is not optimal. This motivated research into new materials and structures with more optimal band gaps and higher potential efficiencies. There was significant interest in the dilute nitrides, specifically GaInNAs alloys, which have a band gap in the range near 1 eV that could not be reached by the more traditional phosphide and arsenide materials when lattice-matched to the standard GaAs and Ge substrates [3,4]. Dilute nitride subcells had the potential to improve multijunction solar cell efficiency by replacing the bottom Ge subcell or becoming a fourth subcell inserted above the Ge subcell [5]. However, previous research found that the minority carrier transport properties in dilute nitride subcells were too poor to improve multijunction solar cel
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