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

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.