NOVELTY - An in-line reactor (500) comprises series of chambers (501) between inlet (506) and outlet (507); a gap (502) formed between upper and lower bodies (503, 504) of chamber, where a ratio of a narrowest width to a narrowest height for each chamber is at least 15 and gap of each chamber is aligned with the gap of the other chambers in the series; a temperature controller; and a temperature controller; a mechanism to move substrate (505) from the inlet to the outlet through chambers; and gas inlet.    USE - For processing a substrate according to a predetermined temperature profile useful for forming compound layer of a Group IB, IIIA and VIA of the Periodic Table on a surface of flexible roll (claimed); for forming thin films of semiconductor films for radiation detector and photovoltaic applications e.g. solar cells.    ADVANTAGE - The process provides cell efficiency that is strong function of the molar ratio of Group IB/IIIA of Periodic Table. The mechanism allows relative motion and physical contact between precursor layer and soft high-temperature material, such as quartz wool. The relative motion between soft high-temperature material and precursor layer distribute the reactant more uniformly to yield better uniformity in reaction. The reactor provides continuous, in-line processing of substrates, which may be fed into reactor in continuous manner.    DETAILED DESCRIPTION - An in-line reactor (500) comprises inlet (506); outlet (507); series of chambers (501) between the inlet and the outlet, each including an upper body (503), a lower body (504), a gap (502) formed between the upper and lower bodies, where a ratio of a narrowest width to a narrowest height for each chamber is at least 15 and gap of each chamber is aligned with the gap of the other chambers in the series; a temperature controller that regulates the temperature within the gap based upon the predetermined temperature profile so that there is a different temperature within the gap of at least some of the chambers; a mechanism to move substrate (505) from the inlet to the outlet through each gap of the series of chambers; and at least one gas inlet to deliver a gas into the gap of a corresponding of the chambers. The chambers are separated by a buffer region. The mechanism includes supply and receiving spools those are used to supply and receive, respectively, a flexible foil substrate. An INDEPENDENT CLAIM is included for forming a compound layer of Group IB, IIIA and VIA of Periodic Table on a surface of a flexible roll involving depositing a precursor layer comprising at least one Group IB material and at least one Group IIIA material on the surface of the flexible role; and annealing the flexible roll using a series of process chambers.    DESCRIPTION OF DRAWING(S) - The figure shows the in-line reactor to form absorber layer of group IB, IIIA and VIA of Periodic Table.    in-line reactor (500)    chamber (501)    gap (502)    upper body (503)    lower body (504)    substrate (505)    inlet (506)    outlet. (507)