High-efficiency mechanically stacked bifacial III-V/HJT multijunction solar cell enabled by spectral albedo

Two-terminal III–V/Si tandems are often limited by current mismatch and parasitic optical coupling between sub-cells. We propose and experimentally validate a Back-to-Back Series Tandem that preserves a simple series electrical connection while optically decoupling the junctions: a monofacial III–V top cell operates under direct front illumination, and a bifacial HJT Si bottom cell harvests controlled rear-side albedo. By tuning the rear irradiance (∼0.3 Sun), the Si photocurrent is matched to the III–V current, minimizing mismatch losses without interlayer optics or tunnel junctions. Measurements (LIV and EQE) and PVsyst simulations based on extracted device parameters confirm stable, high-efficiency performance. The tandem achieves 35.18 % efficiency with V_oc = 3.41 V, J_sc = 12.54 mA cm⁻², and FF = 82.26 %. At the system level, a 1 MW array yields 1388 MWh year⁻¹ with PR ≈ 0.971, outperforming standalone III–V and HJT modules modeled under identical assumptions. The architecture is compatible with bifacial deployment and albedo-rich sites, aiding manufacturability and seasonal robustness. Additional gains are expected from enhancing the Si bottom cell via rear-side optical management, passivation and contact optimization, and reduced interfacial resistance. Overall, albedo-assisted, optically separated tandems offer a practical route to scalable, high-efficiency terrestrial photovoltaics.