Development and Characterization of Transparent Hydrogen-Doped Indium Oxide Layers for Use in Selective Contacts of Germanium Thermophotovoltaic Cells

Crystalline-germanium photovoltaic cells are promising candidates for thermophotovoltaics (TPV), but their potential is hindered by the difficulty in optimizing the out-of-band infrared reflectance of the back contacts. A proposed solution is to use reflective hole-selective back-contacts based on transition metal oxides (TMOs), contacted with indium tin oxide (ITO) and a silver mirror. We propose to substitute the ITO with a Hydrogen-doped Indium Oxide (IOH) layer to improve the optical response of such contacts. To study the possibility of using IOH in TPV applications, we RF-sputter IOH films and characterize them morphologically, electrically and optically, with a focus on their infrared (IR) optical properties, also including the complex refractive index of one IOH film from 350 nm to 16 μm. We obtain films with resistivities as low as 0.39 mΩ·cm and IR absorptions, from 2.5 to 16 μm and weighted with a blackbody spectrum at 1200 °C, as low as 6.1%; by comparison the absorption of ITO was 15.9%. Furthermore, we perform optical simulations of germanium wafers with IOH or ITO, followed by a silver mirror, to show the potential gain in the reflected power. These results are made possible by the high carrier mobilities, up to 130 cm2 V–1 s–1, that counterbalance the effect of low carrier concentrations on the electrical properties.