Hole Selective Contacts Based on Transition Metal Oxides for C-Ge Thermophotovoltaic Devices

Thermophotovoltaics has become a very attractive solution for heat-to-electricity conversion due to its excellent conversion efficiencies. However, further research is needed to reduce the device cost which is typically based on III-V semiconductors. To tackle this limitation, crystalline germanium (c-Ge) has been proposed as an excellent substrate for low-cost devices. One of the key advances behind high system efficiencies is the excellent reflectance of the out-of-band photons at the rear surface of the photovoltaic device. These photons with lower energy than the absorber bandgap are reflected back to the thermal emitter reducing its thermal losses. In this work, we explore the performance of hole selective contacts based on evaporated transition metal oxides (MoOx, VOx, WOx) to be introduced at the rear surface of c-Ge devices. Regarding electrical properties, we characterize the selectivity of the contact by measuring effective surface recombination velocity (Seff) and contact resistivity (ρC). Best results are obtained with MoOx contacted by Ag/ITO with Seff = 588 cm/s and ρC = 55.6 mΩ cm2 which can be improved by using gold as a metal contact leading to Seff = 156 cm/s and ρC = 60.9 mΩ cm2. Regarding out-of-band reflectance, it is better for the case of Ag/ITO/MoOx contact with 87.5% compared to 78.9% for Au/MoOx when a 1473 K black body spectrum is used. Device simulations show potential system efficiencies in the range of 18–19% which are comparable to the best reported efficiencies using c-Ge thermophotovoltaic devices.