A method for controlling the deposition of vaporized organic material onto a substrate surface, includes providing a manifold having at least one aperture through which vaporized organic material passes for deposition onto the substrate surface; and providing a volume of organic material and maintaining the temperature of such organic material in a first condition so that its vapor pressure is below that needed to effectively form a layer on the substrate, and in a second condition heating a volume percentage of the initial volume of such organic material so that the vapor pressure of the heated organic material is sufficient to effectively form a layer.