Phosphorus-doped nanocrystalline silicon as electron selective contact for epitaxial-free germanium thermophotovoltaic devices

Crystalline germanium (c-Ge) has emerged as a promising, cost-effective absorber material for thermophotovoltaic (TPV) cells. As with any other photovoltaic (PV) device, the development of high-quality selective contacts is crucial. Moreover, to maintain a low-cost strategy any epitaxially-grown layer should be avoided. In this work, we investigate the deposition of n-type nanocrystalline silicon (nc-Si(n)) onto p-type c-Ge substrates using Plasma-Enhanced Chemical Vapor Deposition to form nc-Si(n)/c-Ge(p) heterojunctions that act as electron-selective contact. The deposition parameters, SiH₄+PH₃ flow and RF power, are investigated and material characteristics are analyzed via Raman spectroscopy, Transfer Length Method and Hall effect measurement, confirming the nanocrystalline quality with high conductivity (42 Ω⁻¹ cm⁻¹) and low activation energy (0.013 eV) of the nc-Si(n) layer. The interface quality of the heterojunction is evaluated by measuring the effective carrier lifetime, revealing that introducing a thin intrinsic amorphous silicon interlayer significantly enhances passivation but degrades carrier transport through the heterojunction. The developed nc-Si(n) layers are deposited onto c-Ge substrates with doping concentrations of 2 × 10¹⁵ cm³ (LD) and 2 × 10¹⁶ cm³ (HD) to fabricate c-Ge TPV cells with full rear aluminum contact. The results indicate that HD devices exhibit three times lower series resistance than LD devices, primarily due to reduced rear contact resistivity. On the other hand, LD devices show ∼5 % higher IR reflectance, attributed to lower free carrier absorption. Modelling the HD and LD TPV devices predicts TPV efficiencies of ∼6.5 % and ∼2.9 %, respectively.