Moving past 2.0eV: Engineered ZnSe-GaAs alloys for multijunction solar cells

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. A top subcell 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 (E_G = 1.89eV) and GaAs middle cells (one sun efficiency of 40.1% for current-matched conditions and 49.1% without current matching). We have simulated 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.