The single highest-leverage knob in the entire factory. i-layer below ~4 nm triggers epitaxial growth → D_it spike; above ~9 nm, parasitic absorption eats J_sc. Calibrated to Kaneka / LONGi public iV_oc 745 mV @ nominal.
MPC controller M63 holds i-layer to ±0.05 nm by closing on plasma OES (M23) and downstream iV_oc soft-sensor (M28).
Epitaxial cliff visible as the steep iV_oc drop < 4 nm. Safe band 5–8 nm shaded.