Germanium Thermophotovoltaic Devices Achieving 7.3% Efficiency Under High-Temperature Emission by Empirical Calorimetry

We report the first empirical efficiency measurement of germanium-based thermophotovoltaic devices under high-temperature, high-irradiance conditions using a high view-factor calorimetric setup. Two TPV cell architectures were fabricated on p-type, highly doped (10^17 cm-3) Ge substrates, differing only in rear contact configuration. A standard device with a gold rear contact achieves a peak efficiency of 7.3 % and a power density of 1.77 W/cm2 at an emitter temperature of 1480 C, while a PERC-type device reaches 6.3 % efficiency and 1.22 W/cm2 at 1426 C. The superior performance of the standard device is attributed to lower series resistance, whereas the PERC design exhibits slightly higher efficiency at lower emitter temperatures (4.0 % vs. 3.8 % at 1150 C) due to enhanced rear-surface reflectivity. A detailed TPV model has been developed and validated across both device architectures. The model identifies out-of-band optical losses as the dominant efficiency-limiting mechanism, primarily caused by strong free-carrier absorption in the highly doped Ge substrate. Using this model, we predict device performance under idealized spectral conditions commonly assumed in prior literature. For a simulated AlN/W spectrally selective emitter, efficiencies as high as 22.3 % at 1800 C are obtained, consistent with previous semi-empirical predictions. In contrast, when previously reported Ge devices are modeled under the realistic graphite emitter spectrum used here, projected efficiencies decrease to as low as 8.1 % at 1480 C. These results show that earlier projections remain valid but idealized and underscore the importance of emitter spectral engineering and substrate optimization. Finally, we present the first direct comparison of Ge and InGaAs TPV devices under identical conditions, demonstrating the superior performance of InGaAs while confirming the cost-driven competitiveness of Ge.

💡 Research Summary

This paper presents the first fully empirical determination of the conversion efficiency of germanium‑based thermophotovoltaic (TPV) devices under realistic high‑temperature, high‑irradiance conditions. Two cell architectures were fabricated on heavily doped (10¹⁷ cm⁻³) p‑type Ge substrates: a conventional (CONV) device with a gold rear mirror directly deposited on the Ge wafer, and a PERC‑type device that incorporates a multilayer dielectric passivation stack (a‑SiC:H/Al₂O₃/a‑SiC) and laser‑fired point contacts (LFC) to achieve low‑resistance, highly reflective rear surfaces. Both devices share identical front‑side structures (n‑GaAs/GaInP epitaxial layers, AuGe/Ni/Au front contacts) and a mesa‑isolated active area of ≈1 cm².

The authors employed a high‑view‑factor calorimetric test bench equipped with a graphite emitter capable of reaching temperatures up to 1480 °C. By simultaneously measuring the electrical power generated by the cell and the heat absorbed from the emitter, they obtained true TPV efficiencies without relying on spectral assumptions. The CONV cell achieved a peak efficiency of 7.3 % and a power density of 1.77 W cm⁻² at 1480 °C, while the PERC cell reached 6.3 % efficiency and 1.22 W cm⁻² at a slightly lower emitter temperature of 1426 °C. At lower emitter temperatures (≈1150 °C) the PERC device showed a modest advantage (4.0 % vs 3.8 %) due to its higher rear‑surface reflectivity, but its series resistance was higher, limiting performance at the highest temperatures.

A detailed physics‑based TPV model was developed and calibrated against the experimental data. The model includes band‑to‑band absorption, free‑carrier absorption (FCA) in the heavily doped Ge bulk, carrier transport, series and shunt resistances, and thermal recycling of unabsorbed photons by the rear mirror. Sensitivity analysis identified out‑of‑band (sub‑bandgap) photon absorption via FCA as the dominant loss mechanism, accounting for roughly one‑third of the incident radiant power being converted to heat rather than electricity.

Using the calibrated model, the authors explored idealized spectral conditions. When a spectrally selective AlN/W emitter—transparent below the Ge bandgap and highly reflective above it—is assumed, the model predicts efficiencies as high as 22.3 % at 1800 °C, consistent with earlier semi‑empirical reports. In contrast, applying the realistic graphite emitter spectrum used in the experiments reduces the projected efficiency to ≤8 % at 1480 °C, demonstrating how optimistic literature values rely on idealized emitter spectra.

The paper also provides the first direct comparison of Ge and InGaAs TPV cells measured under identical calorimetric conditions. InGaAs devices exhibited higher open‑circuit voltages and fill factors, delivering superior electrical performance, yet Ge devices remain attractive because of their lower material cost, mature wafer‑scale processing, and compatibility with existing semiconductor infrastructure.

Overall, this work validates a rigorous experimental methodology for TPV efficiency measurement, quantifies the impact of free‑carrier absorption in heavily doped Ge, underscores the critical role of emitter spectral engineering, and offers a clear pathway for improving Ge‑based TPV systems through substrate doping optimization, rear‑surface passivation, and selective emitter design.