Enhanced Carrier Escape in MSQW Solar Cell and Its Impact on Photovoltaics Performance

A multiple stepped quantum wells (MSQWs) solar cell, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and improvements in short-circuit current and fill factor have been demonstrated. We have studied carrier recombination dynamics and carrier escape kinetics from the wells, comparing the MSQW with the normal multiple quantum well (MQW). Carrier recombination rate was examined by time-resolved photoluminescence (PL) and a longer carrier lifetime was observed for MSQWs than for MQWs. Carrier escape from the wells was also investigated in terms of temperature-dependent PL. Smaller thermal activation energy was observed for MSQWs than for MQWs. Carrier radiative recombination loss was investigated by bias-dependent PL, and it was found to be smaller for the MSQW than for the normal MQW. Such advantages of the MSQW allowed us to stack a sufficient number of quantum wells to increase short-circuit current without degrading fill factor. For normal MQW solar cells, degradation in fill factor is unavoidable.