In this paper, we report the evolution of silicon heterojunction solar cell properties focusing, in particular, on the indium tin oxide (ITO) layers upon consecutive thermal annealing. We find that the charge carrier density $N_{\rm e}$ of the ITO increases with higher thermal budget, while the carrier mobility remains constant. For the solar cells, their series resistance at the maximum power point $R_{\rm S}^{\rm MPP}$ first decreases due to the reduction of the ITO’s sheet resistance. With further annealing, $R_{\rm S}^{\rm MPP}$ increases again. As all monitored $R_{\rm S}$ components decrease, we attribute this to an increase of the contact resistance. The implied $V_{\text{OC}}$ and the implied fill factor both slightly degrade for annealing temperatures above 190 °C for our layers. This, as well as the change in $N_{\rm e}$ of the ITO, must be carefully considered when optimizing the thermal budget needed, e.g., for sputter damage or screen-printing paste curing.