Improved methods and apparatus for forming thin film buffer layers of chalcogenide on a substrate web through the chemical combination of a metal and chalcogen in solution form. The web and/or the solutions may be heated by one or a plurality of heating elements that may be disposed out of physical contact with the web, allowing enhanced control over the reaction speed through fine temperature control. One or more properties of the chalcogenide layer may be measured, and the temperature of the system may be adjusted in response.