Ag/CIGS/n-Si/Al solar cells which were formed basing ultrathin film thickness by Pulse Laser Deposition (PLD) method, to be used as a sub-solar cell in tandem solar cells. The thickness of CIGS-ultrathin films enhancement with increasing the laser ablation pulse number. Crystallinity of CIGS-ultrathin films have improved and CIGS-ultrathin films have more photon absorption. Indium and Gallium ratios decreased and Copper and Selenium ratios increased in CIGS-ultrathin film as laser pulse number increased. The band gap, dielectric constant, Urbach energy and refractive index of CIGS ultrathin films were calculated. The rectification rate and photo current of CIGS/n-Si hetero-junctions were calculated based on CIGS-ultrathin film thickness under the illumination. Using SCAPS-1D program, the efficiencies of CIGS/n-Si heterojunction solar cells were calculated depend Auger electron/hole recombination coefficient, the interface defect density and the hole carrier density. Also, Jsc, FF, Voc, and efficiencies of CIGS solar cells were calculated based on contact surface recombination velocity and operation temperature. To room temperature, the calculated PV parameters were founded to be very compatible by the experimental parameters. CIGS-ultrathin film thicknesses indicating the highest efficiencies have been determined.