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100-period, 1.23-eV bandgap InGaAs/GaAsP quantum wells for high-efficiency GaAs solar cells: toward current-matched Ge-based tandem cells



Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain-balancing requirement, growing high-quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade-off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e-1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23-eV bandgap quantum wells, a cell with 100-period In0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti-reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut-off using a 665-nm long-pass filter, thus indicating the strong potential of MQW cells in Ge-based 3-J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.