To address the carrier extraction mechanism that determines the fundamental characteristics, such as current density, open circuit voltage, and fill factor in nanostructure-based solar cells, we performed photoluminescence (PL) decay measurements of the Ge/Si quantum wells (QWs) in crystalline- silicon (c-Si) solar cells. We found that the PL decay time of Ge/Si QWs depends on the temperature and the applied electric field; this dependence reflects the carrier separation characteristics of electron-hole pairs in Ge/SiQWs. Above similar to 40 K, the electron-hole pairs are rapidly separated by the thermal excitation and the built-in electric field of c-Si solar cells. In contrast, at 20 K the PL decay time remains almost unchanged for an applied electric field of up to +/- 1 V. These results indicate that the electrons confined in the type-II band offsets could be thermally excited and then extracted by an applied electric field. (C) 2013 Elsevier B. V. All rights reserved.