Bifacial thermophotovoltaics: Design and modeling of c-Ge devices

Thermophotovoltaic devices are among the most efficient technologies to convert heat into electricity by means of IR photons emitted from a hot body. Recently, bifacial thermophotovoltaic devices which receive photons on both surfaces have been proposed with potential advantages of higher power density and easier photon recycling. The latter is based on the device transparency to out-of-band photons ( E ph < E g ) that can go through it reducing thermal losses on the opposite thermal emitter. In this work, we report on Silvaco ATLAS 3D simulations of bifacial TPV devices based on p-type c-Ge absorbers exploring two substrate resistivities (1.2 and 0.13 Ω cm). We study the effect of the distance between metal fingers on both surfaces as a critical parameter leading to maximum electrical power densities for ∼100 μm. Interestingly, ohmic losses are relaxed by the increase in substrate conductivity due to photogenerated carriers. Bifaciality factor is close to one due to the highly symmetrical design of the device. Finally, we experimentally measure the absorption of out-of-band photons for both substrates. For 1.2 Ω cm substrates, the absorption is negligible up to λ = 10 μm permitting better photon recycling and higher efficiencies up to 21.8 %, compared to 0.13 Ω cm substrates that lead to 19.6 % despite its reduced ohmic losses. • 3D Silvaco ATLAS simulations of proposed structure for bifacial TPV devices based on p-type c-Ge substrates. • We explore two substrate resistivities (0.13 and 1.2 Ω cm) which show reasonable transparency for out-of-band photons. • Limited ohmic losses in the low doped material due to conductivity modulation effect. • Extremely high bifacial factor (98.2–100 %) because of the high symmetry of the proposed device structure. • Simulations show potential efficiencies in the range of 19–22 %.