A process for high efficiency large area Cu(ln,Ga)(Se,S)(2) (CIGSSe) thin film solar modules is developed applying rapid thermal processing and controlled sodium doping. Continuous optimization aims at sufficient process maturity for a successful transfer into cost-effective mass production. The pilot line yields aperture area efficiencies of 11% (average) for 30 X 30 cm(2) modules and a certified champion efficiency of 13.1% for the first 60 X 90 cm(2) demonstrator module (unencapsulated). First Cd-free 30 X 30 cm(2) circuits yield up to 11.9% efficiency applying CBD-Zn (S,OH)-buffer layers. Investigations on the absorber formation by structural, compositional and electrical characterization will be given. Models for the Ga and S distribution will be discussed and experimentally verified. A favorable double band gap grading structure is accomplished by controlled sulfur profiling and by a process induced gallium accumulation towards the back electrode. The stability of the frameless low-cost packaged pilot line modules against humidity is confirmed externally by passing the damp heat test sequence according to IEC 61646. (C) 2003 Elsevier B.V. All rights reserved.