NOVELTY - Composition control of copper-indium-gallium diselenide solar cell includes performing co-evaporation process in process chamber, detecting where the transition from copper-rich to copper-deficient composition occurs by using physical parameter related to the same transition, detecting a shift of transition point using the physical parameter, and adjusting evaporant fluxes to bring the transition point back to reference transition point.    USE - For controlling composition of CIGS solar cell.    ADVANTAGE - The inventive method enables accurate control of co-deposited CIGS layer in an in-line system for fabrication of CIGS solar cells. It provides fast feedback loop to achieve good control dynamics.    DETAILED DESCRIPTION - Composition control of copper-indium-gallium diselenide (CIGS) solar cell includes performing co-evaporation process in a process chamber of an in-line, continuous substrate flow production system; detecting where the transition from copper-rich to copper-deficient composition occurs by using physical parameter related to the same transition, with the instant referred to as reference transition point; detecting a shift of the transition point using the physical parameter; and adjusting the evaporant fluxes to bring the transition point back to the reference transition point.    An INDEPENDENT CLAIM is also included for an in-line continuous substrate flow production apparatus for fabrication of CIGS solar cells, comprising a CIGS process chamber (7) in which substrates provided with a molybdenum back contact layer continuously move through a deposition zone. The process chamber comprises separated heating elements (10), and sensor(s) arranged in deposition zone and connected to a controller. The sensor measures physical parameter related to a transformation of the deposited CIGS film from a copper-excessive composition to a copper-deficient composition. The transformation takes place at a reference transition point in the process chamber as the substrate moves through the process chamber. The sensor is arranged to detect a shift of the actual transition point on the moving substrate from the reference transition point by measuring the physical parameter at the deposited CIGS film at the reference transition point. The controller receives as input the sensor output signal and delivers as output a corrective signal that adjusts the evaporant fluxes so that the actual transition point is brought back to the reference transition point.    DESCRIPTION OF DRAWING(S) - The figure is a side view of a process chamber.    Process chamber (7)    Outlet (9)    Heating elements (10)    Evaporation sources (11-13)    XRF device (20)    Selenium sources (26)