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. A III‐V//Si triple‐junction solar cell with a record power conversion efficiency of 35.9 % under an AM1.5g spectrum is presented. The III–V top and the silicon bottom structures were monolithically connected by a direct wafer bond. The main improvements compared to the previous generation are the use of a GaInAsP absorber in the middle cell and better current match. This device achieves the highest conversion efficiency of all silicon‐based multijunction solar cells reported so far.