The preferred orientation of CIGS is related to the crystal structure of the bottom MoSe2 layer. The lattice mismatch of chalcopyrite (220) CIGS to hexagonal (102) MoSe2 is lower than other orientations. The lattice mismatch depends on the Ga composition x, reaching a minimum at x=0.35. 2D dislocation and trap densities were calculated as a function of lattice mismatch for various compositions. In good agreement with experiment, the best performance CIGS solar cell was obtained at x=0.35 in this numerical device simulation. The lattice mismatch between CIGS and hexagonal CdS can also lead to 2D dislocations in CdS at the top CIGS interface. Interface traps saturate the open circuit voltage at high Ga composition by a thermionic emission transport mechanism. These models well explain observed CIGS solar cell performance over the full range of Ga compositions.