Recently, we have developed an improved three-stage growth method by simultaneously using a pyrometer and a thermocouple, to accurately control thickness and composition during growth of CuInGaSe2 (CIGS). As a result, we have obtained solar cells that show preliminary efficiencies up to 16.4% without anti-reflection coating. Using the technique, we have investigated possible mechanism behind non-optimal cell efficiency, focusing on the effects of the molybdenum contact layer. Some additional advantages of our pyrometer monitoring technique are described. Optimizing the properties of Mo layer leads to suppression of Na segregation and distinctive damage to the CIGS surface, improving both cell performance and reproducibility. (C) 2003 Elsevier Science B.V. All rights reserved.