Growth optimization of quantum-well-enhanced multijunction photovoltaics

III-V materials enable the highest reported power conversion efficiency of any photovoltaic technology. The incorporation of high-quality nanostructures can tailor absorption to the available solar spectrum, allowing a further performance increase. Here we report a comprehensive study of the growth conditions of strain-balanced InGaAs quantum wells (QWs) incorporated into multijunction III-V photovoltaics by metalorganic vapor phase epitaxy (MOVPE). The fundamental growth mechanism leading to detrimental step-edge bunching in these devices is presented. Methods for mitigating step bunching through the composition of strain-balancing layers, growth temperature, and substrate offcut are shown. The addition of a distributed Bragg reflector, optically matched to the QW absorption region, extends the optical path of QWs, further increasing current generation to over 40 μA/cm2/QW. Results show a clear direction to mitigate voltage loss, enhance carrier collection, and lead to an impressive performance of 27.5% AM0 and 30.3% AM1.5G, two-junction QW solar cells.