This research used solar cell capacitance simulator (SCAPS-1D) and COMSOL Multiphysics software to theoretically explore the effects of SiO₂ nanoparticles on electrical performance of semi-transparent ultra-thin Cu(In,Ga)Se₂ (CIGSe) solar cells. The results show that a large amount of photogenerated carriers are generated near the CIGSe/transparent conductive oxide (TCO) back interface under rear illumination, leading to a significant effect of photogenerated electron back recombination on cell performance. At a high back recombination rate (S_b=1.0×10⁷ cm/s), the introduction of SiO₂ nanoparticles at the CIGSe/TCO back interface results in a more significant effect on the reduction of photogenerated carrier concentration than the movement of space charge region, resulting in an overall reduction in back recombination current. However, the performance improvement from hole transport is overshadowed by high back recombination current. Therefore, for semi-transparent ultra-thin CIGSe solar cells under rear illumination, a low back recombination rate (passivated back interface) can improve cell performance, which is completely opposite to the conclusion that high S_b enhances cell performance under front illumination. Meanwhile, reducing the consumption of light capture due to back recombination to increase short-circuit current density is an effective way to improve cell performance. These findings represent the latest advancements in the study of rear illumination mechanisms, offering new possibilities for enhancing the performance of semi-transparent ultra-thin CIGSe solar cells.