• • Nano-GT depth reduction from 15.3 nm to 4.4 nm via BAAc pre-incorporation directly correlates with elimination of grain-to-grain CL emission heterogeneity, establishing a nanoscale morphological threshold for cation homogenization.
• • Pristine FA-Cs perovskite films exhibit distinct CL emissions at grain boundaries, confirming that cation inhomogeneity is localized to GB grooves; shallowed nano-GTs yield uniform CL maps, validating cross-grain compositional uniformity.
• • Thermodynamics-driven GB grooving merges at triple junctions to form trap-like nanostructures that impede cation mixing; BAAc enhances heterointerface energy to suppress groove depth, offering a scalable chemical route for morphological control.
• • The 71% reduction in nano-GT depth (15.3 nm to 4.4 nm) provides a quantifiable process target for industrial perovskite deposition, potentially extending PSC operational stability by mitigating phase segregation pathways.