Polarity dependence of III–V multijunction solar cells fabricated via wafer bonding technique

Abstract GaAs//InGaAs dual-junction solar cells with n-on-p (n-emitter/p-base) and p-on-n (p-emitter/n-base) configurations were fabricated using the surface-activated bonding (SAB) technique, targeting high-efficiency heterogeneous integration. Bonding interface properties consisting of GaAs//InP and GaAs//InGaAs pairs were systematically investigated for both polarities. The bonding strength of GaAs//InP was insufficient, resulting in wafer separation and fabrication failure. Owing to doping limitations, n-on-p devices achieved low sheet-resistance interfaces using p⁺GaAs//n⁺InGaAs (6.6 mΩ cm 2 ), while p-on-n devices initially showed higher resistance (182 mΩ cm 2 for n⁺GaAs//p⁺InGaAs), which was reduced to 110 mΩ cm 2 by inserting a heavily doped n⁺InGaAs layer at the bonding interface (n⁺GaAs//n⁺InGaAs/p⁺InGaAs). The resulting GaAs//InGaAs dual-junction cells exhibited short-circuit current densities and fill factors close to expectations; however, the open-circuit voltage loss was larger than anticipated, likely due to defects introduced during SAB, post annealing and fabrication processing. Importantly, the study demonstrates that the fabrication processes for both polarities are mutually compatible, enabling a unified and reproducible approach for high-performance multi-junction solar cells.