Effect of Ge/Si heterostructures on carrier extraction in Si solar cells with Ge quantum dots

Solar cells using quantum dots (QDs) have been extensively studied, in many of which the open circuit voltage (V_oc) always decreases. The mechanism of this V_oc reduction has not been understood well. We studied Si solar cells with multi-stacked Ge QDs (Ge/Si QD solar cells), in which the type-II Ge/Si band offsets are formed. From the current-voltage characteristics, the V_oc is found to increase at low temperatures, indicating that the V_oc reduction is due to thermal excitation of carriers. By comparing solar cells with different Ge thicknesses, we found that the V_oc reduction depends strongly on the Ge thickness.