demonstrating a great potential in nextgeneration photovoltaic technology, lightemitting devices, and photodetectors. [1][2][3][4] Especially, their flourishing application in all-inorganic perovskite solar cell (PSC) with the recorded power conversion efficiency (PCE) over 11% provides a chance to balance the incompatible efficiency and stability of the organicinorganic hybrid PSCs. [5][6][7][8][9][10] Comparing to state-of-the-art PSC, the ultrahigh voltage output and commercializationavailable stability of CsPbBr 3 PSCs are the unique advantages for self-driving high-voltage-required electronic components. However, there is always a reversible phase transition between 3D CsPbBr 3 , 2D CsPb 2 Br 5 , and 0D Cs 4 PbBr 6 under external stimuli such as, hightemperature annealing treatment and solvent deprivation, [11,12] resulting in the inevitable co-formation of the congruent phases in the final product, which disorders the efficiency enhancement mechanism, as well as, the photoluminescence (PL) origin between CsPbBr 3 and Cs 4 PbBr 6 . [13] As well known, the electronic properties of these Cs-Pb-Br derivatives are changeable because the dimensionality of corner-sharing [PbBr 6 ] 4− octahedra strongly determines the exciton dissociation and recombination processes. The disconnected [PbBr 6 ] 4− units from 3D to 2D and then 0D phase will gradually minimize the adjacent electronic overlap, leading to a localized state and increased quantum confinement effect for detrimental charge recombination. [14,15] Therefore, the precise control on the phase composition in CsPbBr 3 film is important to enhance the optoelectronic performance.Because of the lower solubility of CsBr than PbBr 2 in N,N-dimethyl formamide (DMF) and/or dimethylsulfoxide solvent, high-quality CsPbBr 3 films for solar cells are generally fabricated by two-step or multi-step method, which involves first spin-coated PbBr 2 and subsequent reaction with CsBr. [16][17][18] Although vapor growth method and modified multistep spin-coating strategy have been proposed, [6,19] there is still a great challenge to accurately control the stoichiometric ratio and to hinder the formation of low-dimensional impurity phases. Especially, for traditional multi-step solution-processable technology, aiming to obtain idea perovskite film, there isThe precise phase control of Cs-Pb-Br derivatives from 3D CsPbBr 3 to 0D Cs 4 PbBr 6 highly determines the photovoltaic performance of all-inorganic CsPbBr 3 perovskite solar cells (PSCs). Herein, the preferred phase conversion from precursor to Cs-Pb-Br derivatives is revealed by theoretically calculating the Gibbs free energies (∆G) of various phase conversion processes, allowing for a simplified multi-step solution-processable spin-coating method to hinder the formation of detrimental 0D Cs 4 PbBr 6 phase and enhance the photovoltaic performance of a PSC because of its large exciton binding energy, which is regarded as a recombination center. By further accelerating the interfacial charge extraction with a novel 2D transition m...