Abstract: Process and apparatus are disclosed for forming excellent quality, large area thin films essentially without discontinuities and inhomogenuities, particularly for photovoltaic solar cells. The reactants are conducted past a heated substrate in a relatively thin gap, e.g., 0.030 inch, in a turbulent flow. In a specific embodiment, the reactants are contained in nebuli of an atomized solution, and the nebuli are sorted, preferably by gravity, prior to introduction thereof into the gap to prevent introduction of larger size nebuli into the reaction gap.