Heterojunctions of carbon nanotubes interfaced with silicon and doped with AuCl₃ can achieve attractive power conversion efficiencies when operated in the photovoltaic regime; however, the cost and long-term stability of such devices must be improved before they could become commercially viable. Here, we investigate the role of chemical treatment of the carbon nanotube/silicon interface with either SOCl₂ or HNO₃, prior to AuCl₃ doping, on the stability of the photovoltaic devices. We find that while both treatments initially lead to similar device performance, devices treated with HNO₃ are significantly more stable. Using X-ray photoemission spectroscopy, we demonstrate that pretreatment with the powerful organic oxidant SOCl₂ generates a variety of low-oxidation-state silicon species at the nanotube-silicon interface that are not generated by exposure to HNO₃. These species and their evolution over time are implicated in the reduced device stability, highlighting the importance of silicon oxidation states in determining the stability of carbon nanotube-silicon photovoltaic devices.