Anisotropic electron minibands and AM1.5G spectral usability windows in tensile-strained Si/relaxed Si1-xGex quantum-dot superlattices

Intermediate-band photovoltaics (PV) rely on electronic states that are bound and shielded from thermal effects, yet still allow enough coupling between dots and proper energy separation. We analyse 495 tensile-strained Si/relaxed Si 1-x Ge x quantum-dot superlattices using a 3D BenDaniel–Duke Hamiltonian, incorporating a Δ 2 -valley electron mass tensor of diag(0.19, 0.19, 0.916) m 0 and applying Bloch-periodic boundary conditions. The design variables are the cubic-dot side a = 3.0–8.0 nm, barrier thickness b = 1.0–5.0 nm, and Ge fraction x = 0.20–0.38. The screen retains 171 core geometries and 170 strict geometries. At b ≤ 1.5 nm, all 110 geometries fail the 5 meV isolation threshold; 22 also lose full binding. The retained region shifts towards x = 0.35–0.38. Heavy-hole levels are combined with the electron minibands to define the VB → IB, IB → CB, and direct VB → CB thresholds. The ASTM G173-03 (2020) AM1.5G reference spectrum is partitioned into non-overlapping photon-energy windows at the bottom, centre, and top of each finite-width miniband while enforcing E 1 + E 2 = E g eff . For the 170 strict geometries, the median matched two-step photon fluxes are 2.910, 2.786, and 2.681 × 10 2 ⁰ m −2 s −1 at the respective placements, with 31 geometries showing variations exceeding 20% between them. Given that E 2 < ΔEc ≤ 0.2432 eV, the valence-band-to-intermediate-band transition window is inherently narrow, and the ASTM spectral integration confirms this pathway as the limiting factor for two-step photon absorption across all strict configurations. These values represent upper limits based on incident light, not predictions of device efficiency or carrier escape rates.