Investigation of the cause of reduced open circuit voltage in Ge/Si quantum dot solar cells

Quantum dots (QDs) have attracted much attention for use in photovoltaic applications because of their potential for overcoming the limits of conventional single-junction devices. We studied the current-voltage characteristics in Si solar cells with multi-stacked Ge QDs (Ge/Si QD solar cells) in the wide temperature range (100-300 K). From temperature dependence of the open-circuit voltage (V_oc), we found that the V_oc depends strongly on the bandgap energy of Ge QDs. From the extrapolation of the temperature dependence of V_oc, we found that the V_oc at the zero temperature limit corresponds to the bandgap energy of Ge QDs, rather than that of host Si crystals. These findings indicates that breaking of quasi-Fermi energy separation between the QD states and the host Si solar cells causes V_oc reduction in Ge/Si QD solar cells.