Optimization of multi-junction solar cell performance at infrared light by application of thin film Si:Ge solar cell

High Ge concentration Si:Ge solar cell based on low cost Si substrate fabricated by RTCVD can be applied in multi-junction solar cell system to absorb infrared light. First principle design shows that ideally Si:Ge solar cell with 90% Ge concentration can contribute 6.2% efficiency with 16.6mA/cm² J_sc, 472mV V_oc and 79.5% FF to a multi-junction solar cell system with a 300um silicon solar cell on its top under one sun. Under 50X suns, for the same multi-junction solar cell system, efficiency of bottom Si:Ge solar cell can reach to 7.8% with 574mV V_oc, 82.2% FF. Modeling results show that for 90% Ge concentration Si:Ge solar cell, if optical thickness which is 8 times of its physical thickness can be achieved, J_sc can be 11.3mA/cm², V_oc of 462mV,and FF of 79.2%. In this case, under 50X sun efficiency an reach to 5.2% with 565mV V_oc and 81.9% FF. Our initial Si:Ge solar cell without light trapping experimental results for 90%Ge with 5um absorber shows a J_sc of 5.76mA/cm² with Si filter on top under one sun. For the same cell, V_oc reaches to 205mV under 33X suns.