Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy

Abstract

We have investigated ∼2.0 eV (AlxGa1−x)0.51In0.49P and ∼1.9 eV Ga0.51In0.49P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (AlxGa1−x)0.51In0.49P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (Voc) ranging from 1.29 to 1.30 V for Ga0.51In0.49P cells, and 1.35–1.37 V for (AlxGa1−x)0.51In0.49P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (Woc = Eg/q − Voc) of Ga0.51In0.49P cells to decrease from ∼575 mV to ∼565 mV, while that of (AlxGa1−x)0.51In0.49P cells remained nearly constant at 620 mV. The constant Woc as a function of substrate offcut for (AlxGa1−x)0.51In0.49P implies greater losses from non-radiative recombination compared with the Ga0.51In0.49P devices. In addition to larger Woc values, the (AlxGa1−x)0.51In0.49P cells exhibited significantly lower internal quantum efficiency (IQE) values than Ga0.51In0.49P cells due to recombination at the emitter/window layer interface. A thin emitter design is experimentally shown to be highly effective in improving IQE, particularly at short wavelengths. Our work shows that with further optimization of both cell structure and growth conditions, MBE-grown (AlxGa1−x)0.51In0.49P will be a promising wide-bandgap candidate material for high-efficiency, lattice-matched multi-junction solar cells.