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There has been extensive investigation into phase orientation and phase distribution for improving charge transfer in 2D perovskites-based PSCs, but very few reports have focused on the enhancement of inter-phase charge transfer by passivation. In this work, a synergetic strategy is developed by introducing hydrochloric acid and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) as additives to modulate phase distribution and passivate defects in (BA) 2 (FA) n−1 Pb n I 3n+1 (n = 9) quasi-2D perovskite films. In this process, perovskite films with suppressed phase segregation and PCBM molecules coated along the grain boundaries are obtained. PCBM is found to reduce the trap density by compensating defects, which accelerate inter-phase charge transfer and restrain nonradiative recombination, resulting in improved performance of quasi-2D PSCs. As a consequence, a high PCE of 15.01% with negligible hysteresis for quasi-2D PSCs is achieved. A synergetic strategy of phase distribution control and grain boundary passivation is developed for quasi-2D perovskite solar cells. The incorporation of PCBM into quasi-2D perovskite films with uniform phase distribution improves carrier lifetime and accelerates carrier extraction, resulting in an enhanced PCE. [Display omitted] • PCBM additive modifies the anti-solvent-assisted one-step process to fabricate the quasi-2D PSCs. • Inter-phase charge transfer is improved by the cooperation of optimised concentration of PCBM. • PCE is enhanced and hysteresis is eliminated for the devices with optimised PCBM incorporation. [ABSTRACT FROM AUTHOR]