Organic–inorganic perovskite solar cells (PSCs), which have good environmental durability, are of great interest for practical applications. In this work, we show that a solution-processed MoO x layer acts as a buffer layer against high moisture stress to suppress defects in the perovskite and as a hole transport layer. The inversion of the photoinduced charge migration behaviors, that is, the electron preferentially moving toward the surface when MoO x is directly deposited onto the perovskite, is found to cause a significant loss in device functionality. The deposition of MoO x onto spiro-OMeTAD results in a lower photocurrent density–voltage (J–V) hysteresis behavior, a greatly enhanced electrical conductivity, and a significantly stabilized power conversion efficiency (PCE) when compared with those of devices without the MoO x layer. More importantly, the PCEs of the MoO x -based devices are retained at over 85% of their initial value, while only 75% is retained for a reference cell. This work highlights the facial fabrication approach of the solution-based MoO x layer and provides experimental evidence of the photogenerated charge migration behaviors on the perovskite/MoO x interface. This information would be beneficial for the further design and development of PSC technology.