Investigation of alternative window materials for GaAs solar cells

The optimum window material for surface passivation of GaAs solar cells is investigated using theoretical analysis of optical losses due to window bandgap energy and thickness. A simplified expression is developed to calculate the effective surface recombination velocity in terms of lattice mismatch between the window layer and GaAs, which suggests using a window material with and indirect bandgap energy greater than 2.0 eV, a thickness of less than 0.05 μm, and a lattice mismatch of less than 0.05%. Experimental GaAs solar cells were fabricated and quantum efficiency measurements were made using no window (bare GaAs), Al_0.7Ga_0.3As, Na₂S, and ZnSe/Na₂S windows. The Al_0.7Ga_0.3As and Na₂S windows are shown to passivate the GaAs surface and reduce the surface recombination velocity to less than 10⁵ cm/s, while the ZnSe encapsulating layer was used to permanently maintain the temporary surface passivation effects from Na₂S