Parameter fitting based on the classical equivalent circuit model does not always reach reasonable solutions. It often gave negative series resistance and exaggerated diode ideality factor. Three problems were identified. The first problem was illuminated I-V is not a simple voltage shift by lumped series resistance. The second problem was rounded I-V curves in concentrator cells. The third problem was statistical instability in data fitting algorithm. A new model was proposed to describe the second problem. It was also effective to partly solve the first and the third problems. (C) 2002 Elsevier Science B.V. All rights reserved.