In this paper, we use numerical simulations to investigate ultrathin Cu (In1-xGax) Se2 solar cells. In the first part, we focus on the cell configuration in which the PV parameters fit and match the fabricated cell characteristics. Our goal is to investigate the impact of different loss mechanisms such as interface trap density (Dit) and absorber trap density (Nt) in different cell pitch sizes on cell performance. Dit defines the amount of carrier's traps at CIGS/Al2O3 interface to recombine

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... h photogenerated carriers. Nt defines the amount of carrier traps in the absorber layer. It has been found that the recombination via traps is the major loss mechanism in the investigated cell. Further numerical investigations quantify significant improvements in cell performance for different cell pitch sizes, absorber doping densities, Ga content, and graded bandgap at a fixed opening width in the Al2O3 layer. Consequently, for tandem configuration, the optimized single u-CIGS cell has been used as a top cell with a PERT silicon cell which aroused this recent decade as a promising strategy to achieve maximum efficiencies. The results from these simulations provide insights for ultrathin film CIGS solar cell optimization.