In this study, we performed numerical simulations using SILVACO-TICAD considering microelectronic and photonic structures to determine the feasibility and design the device structure to optimize the performance of a vertical multijunctions thin films solar cell based CIGS under air mass 1.5 global (AM1.5G) spectrums. The vertical tandem structure consists of a three junctions. The main purpose of this work is to show how conversion efficiency depends on thickness of the CIGS absorbing layer for the proposed (VMJ), our calculations product optimal photovoltaic parameters, namely the short-circuit current density (Jsc), open circuit voltage (Voc), are Jsc = 32.42 mA/cm 2, Voc = 2.52 V respectively with a conversion efficiency equal 34.68%.