In this study we perform an extensive campaign of numerical simulations of thin-film solar cell ;structures aimed at investigating how the conduction band offsets at buffer! window (Delta(BW)) and buffer/absorber (Delta(AB)) heterojunctions and the thickness and doping of the buffer layer combine to affect the performance parameters (J(sc), V-oc, FF and eta). For the two scenarios of ideal (i.e., without traps) and non-ideal (with traps) buffer/absorber interface, we vary Delta(AB) and Delta(BW) in the range -0.5 eV to 0.5 eV, and analyze, for each combination, the physical mechanisms limiting the cell performance and the way to optimize it by choosing optimal buffer doping and thickness. We show that assuming Delta(AB) as the main indicator for the potential performance of a cell can be misleading because Delta(BW) can heavily influence performance, even when Delta(AB) is positive (conduction band higher in the buffer than in the absorber) and near to its theoretical optimal value (0.3 eV). However, we also show that Delta(AB) < 0 (conduction band lower in the buffer than in the absorber) is usually coupled with an efficiency loss that is even worse if Delta(BW), > 0. We verify our findings by simulating several examples of CIGS-based solar cells with different buffer layers (CdS, Zn1-xMgxO, In2S3, Zn(O,S)) taken from the literature: these comparisons confirm the validity of our results and suggest that the combination of Delta(AB) and Delta(BW) is the predominant factor in the design of high-efficiency solar cells. We finally propose simple quantitative guidelines for thin-film solar cell design and optimization. (C) 2013 Elsevier B.V. All rights reserved.