Two-terminal III-V//Si triple-junction solar cell with power conversion efficiency of 35.9 % at AM1.5g
III-V//Si multijunction solar cells offer a pathway to increase the power conversion efficiency beyond the fundamental Auger limit of silicon single-junctions. In this work, we demonstrate how the efficiency of a two-terminal wafer-bonded III-V//Si triple-junction solar cell is increased from 34.1 % to 35.9 % under an AM1.5g spectrum, by optimising the III-V top structure. This is the highest reported efficiency to date for silicon-based multijunction solar cell technologies. This improvement was accomplished by two main factors. First, the integration of a GaInAsP absorber in the middle cell increased the open-circuit voltage by 51 mV. Second, a better current matching of all subcells enhanced the short-circuit current by 0.7 mA/cm2. Two different growth directions, upright and inverted, were investigated. The highest cell efficiency of 35.9 % (Voc = 3.248 V, jsc = 13.1 mA/cm2, FF = 84.3 %) was achieved with an upright grown structure. Processing of upright structures requires additional bonding steps, which results in a reduced homogeneity of cell performance across the wafer. A detailed comparison with the currently best triple-junction solar cell reveals future improvement opportunities and limits, considering voltage and current, respectively.