High‐Efficiency Flexible GaAs/InGaAs Dual‐Junction Solar Cells Fabricated by Metallic Nanoparticle‐Based Wafer Bonding

Multijunction solar cells made from highly lattice‐mismatched (LMM) material systems offer an optimal bandgap combination for the ultrahigh conversion of solar energy to electricity. Conventional fabrication techniques for multijunction cells, such as metamorphic epitaxy, direct wafer bonding, and adhesive wafer bonding, are still expensive and produce low yields owing to the use of complex process steps and sophisticated equipment. Herein, flexible GaAs/In 0.53 Ga 0.47 As dual‐junction solar cells with a large lattice mismatch of 3.7% between the subcells are fabricated using an indium‐tin‐oxide (ITO) nanoparticle‐based wafer bonding process. The ITO bonding layer electrically and optically connecting the GaAs top and InGaAs bottom cells shows a low series resistance of 5.7 × 10 −2 Ω cm −2 and a high optical transmission of 90% in the infrared range of 870–1800 nm. The dark current characteristic of the ITO‐bonded dual‐junction cell exhibits a good rectifying behavior with a high on–off ratio of ≈10 5 at ±2 V. Under 1 sun AM 1.5G illumination, a high power conversion efficiency of 28.5% is achieved for the two‐terminal series‐connected dual‐junction cell, with an average external radiative efficiency of 2.6%. In these findings, it is suggested that ITO nanoparticle‐based wafer bonding may be a facile and cost‐effective route for fabricating LMM solar cells, potentially overcoming the Shockley–Queisser efficiency limit.