All MOVPE Grown Quadruple Junction InGaP/InGaAs/Ge/Ge Solar Cell

Most commercially available InGaP/InGaAs/Ge triple-junction solar cells suffer from current mismatch due to the excess current generated by the Ge sub-cell. Combining epitaxial germanium with III–V materials would enable the realization of lattice-matched four- or five-junction solar cells, where the near-infrared spectrum could be split between two Ge sub-cells instead of one, thereby eliminating current mismatch in these devices and achieving higher conversion efficiency. In this work, we present the first demonstration of a quadruple-junction (4J) InGaP/InGaAs/Ge/Ge device, with all layers sequentially deposited in the same MOVPE growth chamber. The 4J device also features a novel architecture that exploits the “transistor effect” between the two Ge junctions to eliminate the current mismatch in the upper 3J InGaP/GaAs/Ge part. We describe the growth and the cell structure realization strategy developed to overcome—and, where beneficial, to exploit—the cross-contamination between III–V and group IV elements, thus avoiding the need for two separate deposition systems. The structural and electrical characterizations performed to ascertain the 4J device quality are presented. This result represents a key step toward the realization of highly efficient, all-MOVPE-grown, lattice-matched MJ solar structures that combine III–V and group IV alloys.