Perovskite solar cells are approaching market readiness, yet large-scale commercialization demands precise control over film quality. A major source of uncertainty in processing arises from the elusive role of external influences and crystallization additives. While additive engineering has certainly improved reproducibility and performance, the underlying mechanisms are still debated, as common explanations using heterogeneous nucleation frequently fall short on explaining the observation from the lab. As a result, perovskite deposition often relies on heuristic approaches, guided by experience rather than by fundamental understanding, which can cause considerable frustration among researchers and engineers when small variations lead to unpredictable outcomes. 
After a year-long jorney and an unprecentably broad collaboration amongs various disciplins, we can now unravel that the decisive influence of typical crystallization additives lies not in the nucleation phase but in promoting grain coarsening through enhanced ion mobility across grain boundaries. In-situ and ex-situ spectroscopy, diffraction, and microscopy were combined with device performance analysis, phase-field simulations, and DFT calculations to build a coherent mechanistic picture that holds across various perovskite compositions and additive types. 
Our results indeed suggest that a plethora of polular crystallization additives interact with grain boundaries forming ion-conductive pathways during annealing – which in turn facilitates grain coarsening, that is the roop cause the outcome of successful additive engineering. Indeed it seems like most approaches of additive engineering follows identical mechanisms like several post-processing approaches, which is why we are now able to present a unified framework that seems to apply both for additive engineering and post-process grain engineering, which we believe will improve the comminates control over the perovskite formation.