Annealed Phosphorus-Doped Amorphous Silicon as Electron Selective Contact for Crystalline Germanium Thermophotovoltaic Cells

Thermophotovoltaic devices based on crystalline germanium (c-Ge) substrates that avoid the epitaxial growth of III-V compounds are a promising solution for reducing technological costs of such technology. Heterojunction based on n-type silicon layers deposited by PECVD on c-Ge are good candidates. However, deposited layers with an efficient electron extraction have proven to be difficult given the instability of c-Ge interface that leads to significant amount of recombination. Herein, we present a novel strategy consisting of a high-temperature anneal of a phosphorus-rich amorphous silicon layer deposited by PECVD. The annealing improves the conductivity and passivation of the structure by inducing a diffusion process of not only phosphorus but also silicon, as demonstrated by ToF-SIMS. We optimize the thermal process for high conductivity and surface passivation. The morphological changes are observed using Raman spectroscopy and X-Ray Diffraction, indicating a partial recrystallization of the amorphous silicon layer and confirming the formation of a SiGe alloy at the surface. Finally, the optimized structure is implemented in a device and tested under 1-sun illumination, yielding an efficiency of 4.53% and validating the feasibility of the approach.