Abstract: Both the efficiency and long-term stability of printed mesoscopic perovskite solar cells (p-MPSCs) are important for their commercial value. However, perovskite films are often decomposed in the presence of moisture and oxygen, resulting in a reduced device efficiency, so it is necessary to introduce additives to further improve the stability of perovskite materials. In this work, tetra-n-butylammonium hexafluorophosphate (TP6) is used as an additive to prepare a 2D/3D mixed-phase perovskite active layer. The results show that the addition of the additive TP6 makes the chalcogenide crystals more uniform and denser, suppresses the non-radiative complexation of the chalcogenide film carriers and enhances the charge extraction efficiency, and the optimal device PCE with the addition of TP6 reaches 15.58%. In addition, the formation of a mixed 2D/3D chalcogenide phase also contributes to enhanced hydrophobicity and thermal stability of the chalcogenide films. Compared to the unencapsulated devices, the devices with TP6 addition maintained more than 93% of the original efficiency after 90 d of ageing and storage at 50 ± 5% air humidity and room temperature, showing excellent long-term stability.