Methanol-induced fast CsBr release results in phase-pure CsPbBr perovskite nanoplatelets.

Formation of a non-emissive wide bandgap CsPbBr component often accompanies the synthesis of CsPbBr perovskites, introducing undesired energy states and impeding the charge transport. Here, we demonstrate that a small amount of a methanol additive can promote the CsBr release rate, facilitating CsPbBr formation and suppressing CsPbBr formation. Some of the methanol ionizes into CHO and CHO, which act as surface ligands and change the crystallization environment, inducing shape evolution from spherical nanocrystals to rectangular nanoplatelets (NPLs), leading to monodispersed and phase-pure 8 unit-cell-thick CsPbBr NPLs. Meanwhile, nonradiative recombination processes are inhibited as a result of NPL surface passivation. Bright CsPbBr NPLs with a photoluminescence quantum yield reaching 90% were employed as emitters for electroluminescent light-emitting devices.