Film treatment–driven evolution of caesium lead bromide nanocrystals into layered 3D/2D CsPbBr₃/CsPb₂Br₅ heterostructures

Abstract

Colloidal caesium lead bromide (CsPbBr3) nanocrystals (NCs) are attractive building blocks for optoelectronic devices due to their high optical quality, compositional tunability, and processing versatility. However, post-deposition treatments required to improve film connectivity and charge transport can induce phase instability, most notably the transformation of CsPbBr3 into the layered CsPb2Br5 phase. Here, we present a systematic investigation of how conventional thermal annealing, solvent washing, and ligand-exchange protocols applied to spin-coated CsPbBr3 nanocrystal films drive the formation of mixed-dimensional CsPbBr3/CsPb2Br5 heterostructures under ambient processing conditions. Using structural, morphological, and spectroscopic analyses, we show that solvent exposure—particularly when combined with mild thermal annealing—promotes the emergence of nanoscale CsPb2Br5 domains with treatment-dependent orientation and distribution. High-resolution electron microscopy confirms the intimate coexistence of the 3D and 2D phases within dense polycrystalline films. Steady-state and time-resolved optical spectroscopy, supported by femtosecond transient absorption measurements, reveal that controlled CsPb2Br5 formation can substantially enhance photoluminescence quantum yield and exciton lifetime by suppressing trap-assisted recombination, consistent with the formation of a type-I 3D/2D heterojunction. In contrast, treatments involving bifunctional additives may introduce additional trapping pathways, partially offsetting passivation benefits. Overall, this work clarifies the dual role of post-deposition processing in simultaneously enabling ligand removal and inducing phase transformation, demonstrating that CsPb2Br5 formation can be harnessed as a deliberate strategy to engineer interfacial passivation and improved carrier dynamics in CsPbBr3 nanocrystal thin films.