Interband casade thermophotovoltaic devices with Type-II superlattice absorbers of ∼0.4 eV bandgap

We present studies of two- and three-stage interband cascade thermophotovoltaic (TPV) devices grown by molecular beam epitaxy on GaSb substrates. The absorbers were composed of InAs-GaSb-Al_0.8In_0.2Sb-GaSb superlattices. The thin layers of AlInSb inserted into the superlattice enable the devices to have a relatively short cutoff wavelength of 3.0 μm at room temperature. In addition, the absorber lengths of the cascade stages were varied across the structure in order to achieve better photocurrent matching between stages. The devices´ photovoltaic properties were investigated with both a broadband blackbody source, and a mid-infrared laser with an emission wavelength within k_bT of the absorber bandgap. We observe that the three-stage devices can achieve higher values of output power than the two-stage devices, despite a certain mismatch of photocurrent between the stages. In addition, these three-stage devices are able to achieve values of open-circuit voltage comparable to widerbandgap GaSb-based TPV devices at significantly lower values of the short-circuit current density.