Enhanced carrier extraction under strong light irradiation in Ge/Si type-II quantum dot solar cells

Ge quantum dot (QD) layers inserted in an intrinsic region of a Si p-i-n diode cause additional photon absorption at longer wavelengths of the solar spectrum. We studied the mechanism of carrier extraction in Ge/Si QD solar cells using photocurrent measurements. Under strong light irradiation, the photocurrent increased superlinearly with irradiation light power, which was quite different from the Si p-i-n diode without Ge QDs. This indicates that the external quantum efficiency in the Ge/Si QD solar cells increased under strong light irradiation. These findings indicate that in addition to the thermal escape from the QDs, the nonlinear carrier extraction mechanism, such as the two-step carrier generation via intermediate states and hot carrier generation due to Auger recombination, appeared under strong light irradiation conditions.