Electrical energy has become an essential part of our life. Therefore, its supply must be sustainable, economical, and environment-friendly. The conversion of sunlight into electricity is made possible through the solar cell, a semiconductor device, however, the conversion efficiency of these cells is low which can be further improved. This research work presents the design and performance analysis of silicon-germanium (Si-Ge) solar cells. Amorphous silicon/crystalline silicon Heterojunction (a-Si/c-Si HIT) solar, Ge, Si-Ge alloy with 25% Si concentration solar cells are designed in Afors-Het software. An improved conversion efficiency (?) of 25.23%, 5.125%, and 11.53%, respectively is achieved.