Group IV Mid-Infrared Thermophotovoltaic Cells on Silicon

Abstract

Compound semiconductors have been the predominant building blocks for the current midinfrared thermophotovoltaic devices relevant to sub-$2000 \,\mathrm{K}$ heat conversion and power beaming. However, the prohibitively high cost associated with these technologies limits their broad adoption. Herein, to alleviate this challenge we introduce an all-group IV midinfrared cell consisting of GeSn alloy directly on a silicon wafer. This emerging class of semiconductors provides strain and composition as degrees of freedom to control the bandgap energy thus covering the entire midinfrared range. The proposed thermophotovoltaic device is composed of a fully relaxed Ge$_{0.83}$Sn$_{0.17}$ $p$-$i$-$n$ homojunction corresponding to a bandgap energy of $0.29 \,\mathrm{e\mathrm{V}}$. A theoretical framework is derived to evaluate cell performance under high injection. The black-body radiation absorption is investigated using the generalized transfer matrix method thereby considering the mixed coherent/incoherent layer stacking. Moreover, the intrinsic recombination mechanisms and their importance in a narrow bandgap semiconductor were also taken into account. In this regard, the parabolic band approximation and Fermi's golden rule were combined for an accurate estimation of the radiative recombination rate. Based on these analyses, power conversion efficiencies of up to 9% are predicted for Ge$_{0.83}$Sn$_{0.17}$ thermophotovoltaic cells under black-body radiation at temperatures in the 500–$1500 \,\mathrm{K}$ range. A slight improvement in the efficiency is observed under the frontside illumination but vanishes below $800 \,\mathrm{K}$, while the use of a backside reflector improves the efficiency across the investigated black-body temperature range. The effects of the heterostructure thickness, surface recombination velocity, and carrier lifetime are also elucidated and discussed.