Design, fabrication, characterization and improvement of Ge:Si solar cell below Si solar cell in a multi-junction solar cell system

Low band gap germanium:silicon (Ge:Si) solar cells for operation with a silicon solar cell in a multi-junction concentrator system was designed, fabricated, characterized and analyzed. First principle simulations show that an efficiency of 2.3% can be achieved for 88% Ge concentration Ge:Si solar cells below Si at 30 suns. Through solving critical shunting and open circuit voltage (Voc) problems, an efficiency of 0.79% with a Voc of 350 mV and a fill factor (FF) of 66% was achieved for our third generation Ge:Si solar cells below Si at 30 suns.