Review of intrinsic vs. extrinsic recombination in germanium thermophotovoltaic converters

Germanium thermophotovoltaic converters (TPV) can be a cost-effective alternative for the introduction of thermal batteries in the electricity storage market. However, at the high irradiances associated with this application and for indirect band-gap semiconductors such as Ge, the impact of Auger recombination is usually presented as an unavoidable efficiency limiting factor. In this work, we challenge this idea by revisiting the relative weight of the different intrinsic and extrinsic recombination mechanisms in Ge. To this end we summarize the literature about reported lifetimes in Ge and analyze the impact of radiative, Auger, and Shockley-Read-Hall (SRH) recombination on them. SILVACO TCAD is used to simulate the contribution of each recombination mechanism (including surface recombination) on the dark saturation currents of three Ge-based TPV converters manufactured on substrates with different dopant concentrations (N_B = 3 × 10¹⁷ cm⁻³, N_B = 1 × 10¹⁶ cm⁻³, N_B = 1 × 10¹⁵ cm⁻³) under high irradiance conditions (J_SC∼5 A/cm²). This analysis shows that the role of recombination in the bulk of the semiconductor is limited in current Ge, even for devices with substrate dopant concentrations N_B = 3 × 10¹⁷ cm⁻³. This data is also used to estimate the impact of each recombination mechanism on experimental J-V curves previously reported for Ge-based TPV converters. This second analysis confirms that the cell performance barely increases after improving SRH bulk lifetimes. Instead, the impact of surface recombination is still critical, especially in medium-doped and lightly-doped devices, where an outstanding increase in open circuit voltage (V_OC) and fill factor (FF) are observed, leading to potential TPV efficiencies over 30 %.