Narrow-bandgap semiconductors allow for the conversion of near-infrared (NIR) photons into electricity. These are typically used for thermophotovoltaic (TPV) cells that convert blackbody radiation from thermal sources to electric power as well as for photovoltaic subcells in mechanically stacked systems or in spectral splitting systems that are designed to absorb the NIR tail of the solar spectrum. Examples of such cells include GaSb, InGaAsSb/GaSb, Ge and InGaAs/InP based cells. There is additional benefit from being able to fabricate these cells as thin film cells with complete substrate removal because of the reduced weight of the cells and the possibility of photon recycling. There is the added benefit of better thermal management of such thin-film cells by bonding them to CVD diamond. The elimination of the substrate’s thermal resistance helps to remove the heat from the cells more efficiently.
In this presentation, we demonstrate the development of thin-film GaSb and InGaAs PV cells that are freestanding after completely removing the substrate. The only method for realizing large-area thin-film GaSb PVs is to grow the structure on GaAs substrates metamorphically. However, this results in performance degradation due to the presence of threading dislocations in the material. InGaAs on InP is however shown to be very favorable for fabrication of such thin-film structures with the final devices showing slight improvement in performance compared to devices that are processed on the substrate. We will present solar cell characterization data for the devices described including J-V and quantum efficiency plots. We will also include extensive material characterization data including transmission electron microscopy and high resolution X-Ray diffraction data.