The semi-conductive inorganic perovskite layer sandwiched between wide band-gap organic layers with low dielectric constant forms a quantum well-like structure in energies, and the introduced dielectric and quantum confinement effects also favor to enhance the exciton binding energy and boost the radiative recombination efficiency. [7][8][9] Although these outstanding optoelectronic properties have led to a big success in the quasi-2D perovskite LEDs, vacancy-induced defect states and insufficient energy transfer between the multiple quantum wells still impede the efficient radiative recombination in the quasi-2D perovskites. The abundant defects induced by uncoordinated lead (II) (Pb 2+ ) ions in the polycrystalline perovskite films would cause severe nonradiative recombination losses. [10][11][12] The heterogeneous QW width distribution of the quasi-2D perovskites would lead to an incomplete energy transfer from the wide-bandgap phase to the narrow-bandgap phase, which compromises the optical and electrical properties of the resultant material. [13][14][15] Functional additives have been widely used to regulate the quality of quasi-2D perovskite, such as the QW width distribution and the defect density, [16][17][18] which becomes a powerful tool for enhancing the performance of the PeLEDs. Previous studies have focused on the effects of synergistic interactions through additives and antisolvents towards regulating the crystal nucleation and growth, reducing the defects at grain boundaries, and reconfiguring the QW width distribution. [19][20][21][22] However, the additive engineering combined with antisolvent-assisted crystallization is extremely sensitive to the processing conditions and thus is not suitable for the batchto-batch large-scale manufacturing. The use of dual additives with C-O-C bonds achieved a significant progress in the fabrication of high-efficiency PeLEDs without anti-solvent treatment. The dual additives induced to form the perovskite nanocrystals with a defect-reduced quasi-core-shell structure by inhibiting the self-aggregation of organic ligands, and the resultant green PeLEDs achieved a maximum external quantum efficiency (EQE) of 28%. [23] Typically, the multifunctional additives would Quasi-2D perovskites are promising emitters in light-emitting diodes (LEDs) due to their natural quantum well (QW) structure and tunable exciton binding energy. Synergistic regulation of the defect passivation and QW width distribution of quasi-2D perovskites is crucial to achieve highperformance perovskite-based LEDs. Herein, a combination of two additives, i.e., 3-(diaminomethylidene)-1,1-dimethylguanidine (Metformin) and 1,4,7,10,13,16-hexaoxacyclooctadecane (Crown) having preferential interactions with different quasi-2D perovskite precursors is proposed to synergistically passivate the defects and regulate the QW width distribution of quasi-2D perovskites. Metformin additive interacts strongly with lead bromide, mainly passivating the defects. Crown additive has preferential interactions with organi...