Moving Past 2.0eV: Engineered ZnSe-GaAs Alloys for Multijunction Solar Cells
Kyle H. Montgomery and Jerry M. Woodall
35th IEEE Photovoltaic Specialists Conference, Honolulu, HI, 2010.
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The heterovalent quaternary ZnSe-GaAs is proposed for use in next generation multijunction solar cells due to its wide range of direct band gaps (1.42 to 2.67eV) and lattice-match with GaAs. The need for a wide band gap top cell is pronounced in the literature, as well as in the calculations we show. A top cell with band gap around 2eV is needed for optimal 3-junction cells with a fixed GaAs middle cell (one sun efficiency of 44.8% for current matched conditions and 45.8% without current matching). In 4-junction designs, the top cell needs to have a band gap around 2.4eV with fixed GaInP (EG = 1.89eV) and GaAs middle cells (one sun efficiency of 40.1% for current-matched conditions and 49.1% without current matching). To circumvent difficulties in controllably doping the alloy n- or p-type, we can take whichever type nature gives us and use oppositely doped ZnSe (wider band gap) as the emitter. Using liquid phase epitaxy, as we discuss, one can use a horizontal slider with two melt zones for expedient investigation of this design. Further, we simulate a “practical” cell with a (ZnSe)x(GaAs)1-x base layer with a band gap of 2.33eV. Assuming short non-radiative lifetimes and relatively low mobilities, a single cell efficiency of 11.4% with open circuit voltage of 1.94V and short circuit current of 6.5mA/cm2 can be attained, which is >70% of the theoretical max.
solar, multijunction, heterovalent alloys, ZnSe-GaAs.
K. H. Montgomery and J. M. Woodall, “Moving past 2.0eV: Engineered ZnSe-GaAs alloys for multijunction solar cells,” presented at the 35th IEEE Photovoltaic Specialists Conference (PVSC), Honolulu, HI, 2010, pp. 003369-003373.