NOVELTY - Formation of thin films on flexible substrate surfaces comprises processing a vertically traveling continuous flexible workpiece; remaining the workpiece and the solutions at a constant enhanced reaction temperature by preheating all reactant solutions, deionized (DI) water, and air; mixing chemical bath deposition (CBD) solution; spraying the clean DI water, freshly mixed and gradually aged reaction solutions; spraying the freshly mixed reaction solution continuously on the surface of a vertically traveling flexible substrate; and washing the residue solution away from the deposited surface.    USE - Method for the formation of thin films on flexible substrate surfaces used for fabrication of thin film solar cells (claimed).    ADVANTAGE - The method is simple and inexpensive.    DETAILED DESCRIPTION - Formation of thin films on flexible substrate surfaces comprises:    (A) processing a vertically traveling continuous flexible workpiece with its surface to be deposited wetted by spray nozzles (105) but the backside remaining dry with wind knives (102A, 102B), and with its surface to be deposited firstly washed with clean DI water, then sprayed by a freshly mixed reaction solution, followed by a number of spray steps with a series of gradually aged reaction solutions, washed, rinsed, and dried;    (B) remaining the workpiece and the solutions at a constant enhanced reaction temperature by preheating all of the reactant solutions, DI water, and the air before they enter a well isolated reactor, and heating the aging solution underneath a solution groove (108);    (C) mixing chemical bath deposition (CBD) solution in seconds from two or more preheated reactant solutions plus preheated DI water inside a rounded or pear shaped device with a small outlet;    (D) spraying the clean DI water, freshly mixed and gradually aged reaction solutions with a series of combinations composed of solution pipes, spray nozzles, and pumps or compressed air nozzles;    (E) spraying the freshly mixed reaction solution continuously on the surface of a vertically traveling flexible substrate, collecting the sprayed and gradually aged solution and spraying it back on the traveling substrate surface continuously until the predicted film thickness is obtained; and    (F) washing the residue solution away from the deposited surface using minimum rinsed water to reduce volumes of the waste to be treated and the DI rinsing water.    The continuous flexible workpiece coated with a solar cell absorber layer is loaded, and a buffer layer material such as cadmium sulfide, indium oxide, or zinc sulfide is deposited from the CBD reactor to fabricate thin film solar cells. An INDEPENDENT CLAIM is included for a system or apparatus for forming thin films on flexible substrate surfaces by CBD.    DESCRIPTION OF DRAWING(S) - The drawing is a top view of the apparatus for CBD.    Rollers (101)    Wind knives (102A, 102B)    Spray nozzles (105)    Pump (106)    Solution groove (108)