Inorganic CsPbI3 perovskites have shown promising potential for achieving all-inorganic photovoltaic (PV) devices. However, the black perovskite polymorph (α-phase) of CsPbI3 easily converts into yellow colour (δ-phase) in an ambient environment and it is only stable at high temperature (above 320 °C), which limits its practical application. Here we tailor the three-dimensional CsPbI3 perovskite into quasi-two-dimension through adding a large radius cation phenylethylammonium (PEA+). The incorporation of PEA+ into the CsPbI3 perovskite significantly improves the film morphology as well as the phase stability. An optimal CsxPEA1-xPbI3 perovskite film remains stable in the α-phase from room temperature to 250 °C in air and yields a power conversion efficiency of 5.7% for its solar device. The concept of using large radius cations in the 3D perovskite system provides a new perspective to further enhance the phase stability while retaining the device performance.