  In spite of having higher carrier mobilities and absorption coefficients of
germanium (Ge) than those of silicon (Si), there has been less focus on
Ge-based solar cells due to the low bandgap and high-cost of Ge wafer as well
as requirement of its high-purity level. Currently, availability of high-purity
Ge (HPGe), the low-cost wafer slicing method and proper design guidelines make
it possible to design HPGe-based solar cells. Accordingly, in this article, we
have designed and simulated a novel n-CdS/p-HPGe/p+-BaSi2 based npp+
double-heterojunction solar cell, where HPGe, cadmium sulfide (CdS) and
orthorhombic barium disilicide (beta-BaSi2) have been used as the absorber,
window and back-surface field (BSF) layers, respectively. Using the solar cell
capacitance simulator (SCAPS-1D), the effects of different physical parameters
such as the thickness, doping and defect densities, band offsets and
temperature on the photovoltaic (PV) parameters of the designed solar cells
have been investigated systematically. This article renders the optimized PV
parameters to improve the device performance with the highest power conversion
efficiency (PCE) of ~45.65% with a high open-circuit voltage of 1.16 V owing to
the high built-in voltage of 1.7 V for the n-CdS/p-HPGe/p+-BaSi2 solar cells.
This efficiency is almost consistent with the detailed-balance limit for double
heterojunction solar cell.
