Cost management and toxic waste generation are two key issues that must be addressed before the commercialization of perovskite optoelectronic devices. We report a ground-breaking strategy for eco-friendly and cost-effective fabrication of highly efficient perovskitesolar cells. This strategy involves the usage of a high volatility co-solvent, which dilutesperovskite precursors to a lower concentration (<0.5 M) while retaining similarfilm qualityand device performance as a high concentration (>1.4 M) solution. More than 70% of toxicwaste and material cost can be reduced. Mechanistic insights reveal ultra-rapid evaporationof the co-solvent together with beneficial alteration of the precursor colloidal chemistryupon dilution with co-solvent, which in-situ studies and theoretical simulations confirm.The co-solvent tuned precursor colloidal properties also contribute to the enhancement ofthe stability of precursor solution, which extends its processing window thus minimizing thewaste. This strategy is universally successful across different perovskite compositions, andscales from small devices to large-scale modules using industrial spin-coating, potentiallyeasing the lab-to-fab translation of perovskite technologies.