All-inorganic perovskite films hold promise for improving stability of perovskite solar cells. However, the desirable narrow-bandgap inorganic perovskites (such as CsPbI3 and CsPbI2Br) are suffering from the thermodynamic phase instability, as the photoactive black phase (α, β, or γ phase) spontaneously transforms into a more thermodynamically stable yellow phase (ÎŽ-phase) in ambient condition, which is limiting the further development of high-performance inorganic PSCs. The stabilization of black phase and the formation of high-quality perovskite films are promoting the remarkable progress for the performance and stability of inorganic perovskite solar cells. Firstly, we show that europium doping of CsPbI2Br stabilizes the black phase of narrow band gap inorganic perovskite at room temperature. We rationalize it by using solid-state nuclear magnetic resonance and high-angle annular dark-field scanning transmission electron microscopy, which show that europium is incorporated into the perovskite lattice. We demonstrate a power-conversion efficiency (PCE) of 13.71% for an inorganic PSC with the composition CsPb0.95Eu0.05I2Br perovskite. Stability tests show that the devices retain 93% of the initial efficiency after 370 hours under maximum power point tracking measurement. Secondly, we demonstrate a function of dopants by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared to the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high PCE of 14.0% with a high open-circuit voltage (Voc) of 1.33 V. Then we propose an intermediate-phase engineering strategy to improve the inorganic perovskite/metal oxide interface by utilizing volatile salts. The introduction of organic cations (such as methylammonium and formamidinium), which can be doped into the perovskite lattice, leads to the formation of an organic-inorganic hybrid perovskite intermediate phase, promoting a robust interfacial contact through hydrogen bonding. A champion CsPb(I0.75Br0.25)3-based device with a PCE of 17.0% and a Voc of 1.34 V was realized. Last, the intermediate phase engineering strategy is reported to promote the pure γ-phase CsPbI3 perovskite film formation. According to the investigation of the phase transformation processes of pure CsPbI3 and that with different additives, including NH4I, BAI, MAI, FAI and DMAI, we found that the formation of a 3D organic-inorganic hybrid perovskite intermediate phase is essential to avoid the generation of undesired yellow phase and promote the formation of pure black-phase CsPbI3 film. A champion PCE of 17.70% with a stabilized power output of 17.58% based on the optimized CsPbI3-DMAI device is achieved. Overall, we investigate two possible scenarios for element doping: (1) the dopants are incorporated into the crystal lattice for the improvement of stability and efficiency of inorganic PSCs; (2) the dopants form separate phases to induce the halide phase segregation and suppress non-radiative recombination of perovskite films. Besides, based on the formation of 3D organic-inorganic perovskite intermediate phase, we not only promote the formation of pure black phase CsPbI3, but also promotes a robust interfacial contact of inorganic perovskite and metal oxide transport layers.