Silicon heterojunction (SHJ) solar cells, the transparent conductive oxide (TCO) layers used are designed to balance resistive losses and parasitic light absorption. This paper aims to determine the rear TCO optimization that can be made in the case of bifacial SHJ cells, taking into account both optical and electrical aspects. First, internal reflectance in bifacial cells is simulated and characterized for more or less transparent TCO layers, and compared to monofacial cells, in order to establish if the use of a more transparent TCO layer stays beneficial for bifacial cells as well. Then, the internal reflectance is further optimized by tuning the TCO thickness. In this work, we thus show that using a more transparent ITO layer remains an important path of improvement for the internal reflectance in bifacial cells, and that TCO thickness should be optimized differently than for monofacial cells. These results have been confirmed at cell level with a gain in short-circuit current without significant resistive losses observed, resulting in a final efficiency gain. Moreover, this optimization is proved to be also beneficial for the cell’s bifaciality behavior.