p-GaSb/n-GaAs heterojunction diodes for TPV and solar cell applications

A series of samples has been grown to investigate the effect of doping and GaSb epilayer thickness on dark and photo I-V curves, and on spectral response. The optimum GaSb thickness is a trade-off between the diffusion length and the thickness required to maximise light absorption. The highest open circuit voltage, V_oc~0.33 V, to date has been obtained from a thin (0.3 μm) GaSb layer doped at 6×10¹⁹ cm^-1, under illumination by a focused tungsten-halogen lamp (~1 W cm^-2). The short circuit current, I_oc, was ~13 mA/cm² with a corresponding fill factor of 0.63, and an external quantum efficiency of ~10%. The devices were not anti-reflection coated, and some parameters important for solar cells, such as contact form and series resistance, have not been optimised. The shape of the spectral response is strongly dependent on GaSb thickness. Increasing the thickness of the epilayer increases the external quantum efficiency above 870 nm (i.e. above the GaAs band edge) while the external quantum efficiency in the short wavelength range (below 870 nm) reduces