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.

Monolithic perovskite-based triple junction solar cells have the potential to surpass the power conversion efficiency (PCE) limits of single and dual-junction solar cells.

In this work, we present the fabrication and analysis of a wafer‐bonded GaInP/GaInAsP//Si triple‐junction solar cell with 36.1% conversion efficiency under AM1.5g spectral illumination. The new cell design presents an improvement over previous III‐V//Si triple‐junction cells by the implementation of a rear‐heterojunction for the middle cell. Furthermore, an advanced metallodielectric rear‐side grating was used for light trapping enhancement in the silicon bottom cell that increased the silicon subcell current by 1.4 mA/cm². The external radiative efficiency was quantified to be 1.5 times higher compared to a reference device with a GaInAsP homojunction middle cell. A luminescent coupling factor of 0.46 between the middle and bottom subcell was determined. The share of recombination in the space‐charge region was experimentally shown to be insignificant as intended by the rear‐heterojunction design. Overall, the open‐circuit voltage of the middle cell increased by 61 mV compared to the previous generation. Given the established long‐term stability of III‐V and silicon‐based solar cells, these results are promising steps towards the future employment of III‐V/Si tandem solar cells.

In this paper a method is presented to accurately and quickly interpolate a dataset of the complex refractive index of arbitrary compound semiconductors. The method is based on a parameter morphing algorithm which maps critical points of two endpoint materials with known optical parameter sets onto each other. Every desired intermediate material composition can be interpolated if the composition dependence of the band gap is known for the given material system. The accuracy and stability of the proposed procedure is validated experimentally using spectral ellipsometry and reflection measurements. Test samples of two III–V semiconductor material systems, AlGaAs and GaInAsP, with various compositions were grown using metalorganic vapour phase epitaxy and the morphed parameter sets are compared to corresponding measurement results. Modelling the absorption of a solar cell device and comparing it to the external quantum efficiency is presented as an application example of this method. The interpolation method is demonstrated to be a powerful tool for optical and electro-optical modelling of semiconductor structures if parameters of the complex refractive index are not available for the exact material composition.

Multijunction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting a broader range of the solar spectrum more efficiently. However, Si-based multijunction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range as Si subcells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multijunction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply scattered light into a range of diffraction angles. We minimize reflection to the zeroth order and parasitic plasmonic absorption in silver by engineering destructive interference in the patterned back-contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency over a planar back-reflector (+1.52 mA/cm²). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%_abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multijunction solar cells, paving the way for more efficient and sustainable solar energy technologies.