Triple‐junction solar cells from III–V compound semiconductors have thus far delivered the highest solar‐electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four‐junction solar cell architectures with optimum bandgap combination are found for lattice‐mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs‐based top tandem solar cell structure was bonded to an InP‐based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four‐junction solar cell with a new record efficiency of 44.7% at 297‐times concentration of the AM1.5d (ASTM G173‐03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi‐junction solar cells having four and in the future even more junctions. Copyright © 2014 John Wiley & Sons, Ltd.

In order to realize a clean energy society by using renewable energies, high-performance solar cells are a very attractive proposition. The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. As state-of-the-art of single-junction solar cells are approaching the Shockley–Queisser limit of 32%–33%, an important strategy to raise the efficiency of solar cells further is stacking solar cell materials with different bandgaps to absorb different colors of the solar spectrum. The III–V semiconductor materials provide a relatively convenient system for fabricating multi-junction solar cells providing semiconductor materials that effectively span the solar spectrum as demonstrated by world record efficiencies (39.2% under one-sun and 47.1% under concentration) for six-junction solar cells. This success has inspired attempts to achieve the same with other materials like perovskites for which lower manufacturing costs may be achieved. Recently, Si multi-junction solar cells such as III–V/Si, II–VI/Si, chalcopyrite/Si, and perovskite/Si have become popular and are getting closer to economic competitiveness. Here, we discuss the perspectives of multi-junction solar cells from the viewpoint of efficiency and low-cost potential based on scientific and technological arguments and possible market applications. In addition, this article provides a brief overview of recent developments with respect to III–V multi-junction solar cells, III–V/Si, II–VI/Si, perovskite/Si tandem solar cells, and some new ideas including so-called 3rd generation concepts.

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 % (V_oc = 3.248 V, j_sc = 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.