Mid-Infrared Thermal Radiation Harvesting using Uncooled Narrow Bandgap GeSn Thermophotovoltaic cell

Abstract

Thermophotovoltaic (TPV) cells are increasingly attractive for applications in industrial waste heat harvesting, aerospace energy management, and compact power generation. Deploying midwave-infrared (MWIR) TPV in practical applications requires narrow-bandgap semiconductors that not only absorb low-energy photons but also integrate with scalable, low-cost platforms. Although high-performance TPV devices have been demonstrated using III-V materials such as InAs, GaSb, and InGaAs(P), their use remains limited by cost and substrate size. With this perspective, narrow bandgap GeSn alloys are a promising alternative that extend group-IV absorption into the MWIR while being silicon-compatible. Although the potential of GeSn TPV cells has been predicted, no experimental demonstration has been reported. Here, proof-of-concept Ge$_{0.91}$Sn$_{0.09}$ p-i-n TPV diodes (1 mm diameter) grown on silicon were fabricated and their performance was benchmarked against commercial InAs and extended-InGaAs devices. Measurements at 300 K under 2.33 $μ$m laser and $\sim$1500 K SiC Globar illumination revealed peak responsivity of $\sim$ 0.2 A/W at $\sim$ 1.7 $μ$m, and an output power of $\sim$ 0.41 mW/cm$^2$. These devices show trends comparable to those of the InAs diode under identical conditions, although at reduced absolute levels. To assess the intrinsic performance potential, Poisson-drift-diffusion modeling incorporating experimentally calibrated emitter emissivity predicts power densities exceeding 1 W/cm$^2$ under moderate MWIR thermal illumination, indicating that the present devices operate far below their fundamental limits and are primarily constrained by defect-assisted recombination and transport losses. These results establish GeSn as a scalable, silicon-compatible MWIR TPV platform and highlight a larger performance potential achievable through material and device optimization.

Figures (4)

• Figure 1: Ge$_{0.94}$Sn$_{0.06}$/Ge$_{0.91}$Sn$_{0.09}$/Ge$_{0.95}$Sn$_{0.05}$$p$-$i$-$n$ device structure and SiC-based illumination setup. (a) Schematic and optical micrograph of a representative $p$-$i$-$n$TPV device. The glossy-like material covering the device represents the SiO$_2$ passivation layer, whereas the gold-like structure on top on both the $p$- and $n$- GeSn layers are the metallic contacts. (b) Schematic of the measurement configuration for SiC-based TPV illumination, including beam delivery and device mounting stages.
• Figure 2: Broadband source characterization and dark I-V device characteristics. (a) Emission spectrum of the SiC heating element at ∼ 1500, highlighting its broadband gray-body distribution extending across the mid-infrared thorlabsSiC. (b) I-V characteristics under dark condition for devices with 500 and 1 active area diameter.
• Figure 3: (a) Spectral responsivity at 300 of GeSn, InAs, and extended-InGaAs photodiodes with identical active area diameters (1), measured, at 0, using FTIR spectroscopy and calibrated against InAs. The responsivities were normalized to highlight the trends and the cutoff wavelengths. (b) Dark current-voltage (I-V) characteristics of the same devices at 300, showing comparable current levels for GeSn and InAs but lower rectification ratios compared to extended-InGaAs.
• Figure 4: (a) I-V response of GeSn, InAs, and extended-InGaAs photodiodes under 2.3 laser illumination, demonstrating photoresponse and output power density differences among the devices. (b) I-V characteristics under broadband mid-infrared radiation from a ∼ 1500 SiC emitter positioned 3 above the detectors, reproducing trends observed under laser illumination.