Over 30% efficiency triple-junction GainP/GaAs/Ge quantum well solar cells

Increasing the conversion efficiency of a triple-junction GalnP/GaAs/Ge solar cell has been a challenge for a long time. One of the difficulties is the mismatched currents among three subcells because the middle cell has the lowest photocurrent while the bottom cell has the highest photocurrent. Some efforts have been taken to improve the problem, such as the growth of metamorphic layers, four junctions, increasing the bandgap of the top cell and reducing the thickness of the top cells. This paper presents a new effort to increase the photocurrent of the middle cell by introducing multi-quantum wells (MQWs) into its intrinsic region because the multi-quantum wells can lower the bandgap of the middle cell and keep good material quality by reaching balanced strain in epitaxial layers. Experimental results have shown that the triple junction solar cells with the multi-quantum wells can reach the conversion efficiency of 30.89% (AM0), higher than that of control cells. Including the multi-quantum wells can increase the short-circuit currents and decrease the open-circuit voltage. The influences of the multi-quantum wells on the solar cells are also discussed.