The InGaN ternary alloy has the potentiality to achieve high efficiency solar cells: tunable bandgap in the whole solar spectrum, high absorption coefficient, high stability and radiation tolerance. These very promising characteristics make InGaN potentially ideal for designing and developing next-generation high-efficiency thin films solar cells. However, challenging issues remain to address: (i) the difficulty to elaborate sufficiently thick monocrystalline InGaN layers with a high Indium content; (a) the high defects density and the spontaneous and piezoelectric polarizations; (iii) the p-doping which remains difficult to master. In this report, we use rigorous optimization approach based on state-of-the-art optimization algorithms to investigate the effect of defects and polarization (spontaneous and piezoelectric) on a double junction InGaN solar cell. A better understanding of the mechanisms involved in the heterostructure has a crucial impact on the design and elaboration of high efficiency InGaN thin films solar cells which require, in particular, a precise control of the Tunnel Junction elaboration which is still very challenging.