Device modeling and simulation studies of a Cu(ln(1-x),Ga-x)Se-2 (CIGS) thin film solar cell have been carried out. A variety of graded band-gap structures, including space charge region (SCR) grading, back surface region grading, and double grading of the CIGS absorber layer, are examined. The device physics and performance parameters for different band-gap profiles were analyzed. Based on the simulation results, an optimal graded band-gap structure for the CIGS solar cell is proposed. The performance of the optimally graded band-gap cell is superior to that of the uniform bandgap cell. The SCR grading of the CIGS absorber layer improves the open-circuit voltage (V,,) without significantly sacrificing the short-circuit current density (J(sc)) compared to the uniform band-gap CIGS. The back surface grading enhances both V-oc and J(sc). An optimal graded band-gap profile, such as a double grading consisting of the SCR grading and back surface grading, improves significantly the efficiency up to 19.83% AM1.5G compared to the uniform bandgap profile with 15.42% efficiency. A comparison of the simulation results with published data for the CIGS cells shows an excellent agreement of photo-current density-voltage and quantum efficiency characteristics. (C) 2003 Elsevier Ltd. All rights reserved.