Electrically Switchable Single-Photon Emission from Carbon Defects in hBN via a Ferroelectric In ₂ Se ₃ Substrate

van der Waals heterostructures, formed by stacking 2D materials, offer a route to programmable quantum light sources. Using first-principles calculations, we show that a ferroelectric In2Se3 substrate enables large, reversible tuning of absorption and zero-phonon lines (ZPLs) of carbon-related single-photon emitters (CBCN, C2CN, and VNCB) embedded in hBN by stacking and polarization switching. Polarization reversal induces ZPL shifts up to 0.3 eV and, for VNCB, switches the charge state (q = 0 → +1). These effects arise from strong, defect-site-dependent electrostatics at the hBN/In2Se3 interface and are robust to stacking variations. Our results establish ferroelectric substrates as an electrical knob for engineering spectrally tunable and spin-configurable 2D quantum emitters, easing the way for ultrathin, integrable single-photon sources.