Impact of the growth temperature on the performance of 1.70-eV Al_0.22Ga_0.78As solar cells grown by MBE

Growth of high material quality Aluminum Gallium Arsenide (Al_xGa_1-xAs) is known to be challenging, in particular with an Al content x above 20%. As a result, the use of Al_xGa_1-xAs in devices requiring high minority carrier lifetimes, such as solar cells, has been limited. Nonetheless, it has long been established that the substrate temperature is a key parameter in improving Al_xGa_1-xAs material quality. In order to optimize the growth temperature of 1.70-eV Al_0.22Ga_0.78As solar cells, five samples have been grown by Solid-Source Molecular Beam Epitaxy (SSMBE) at 580 °C, 600 °C, 620 °C, 640 °C, and 660 °C, respectively. A strong improvement in performance is observed with increasing the growth temperature from 580 °C to 620 °C. An open-circuit voltage above 1.21 V has in particular been demonstrated on the sample grown at 620 °C, translating into a bandgap-voltage offset W_oc below 0.5 V. Above 620 °C, performances – in particular the short-circuit current density – moderately decrease. This trend is confirmed by photoluminescence, current density versus voltage characterization under illumination, and external quantum efficiency measurements.