CsPbI3 nanocrystals have been proposed as building blocks for photovoltaics and optoelectronics because they show a low bandgap and very high photoluminescent quantum yield (PLQY) near to the unit due to the extreme tolerance to surface defects. However, CsPbI3 shows an important drawback in terms of stability: α-phase can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment with moisture. One of the most successful approaches proposed to overcome this problem is to synthesize mixed halide CsPbIXBr3-X perovskites to improve the stability of the α-phase perovskite structure. This can be performed by a post-synthetic anion exchange reaction of halides in CsPbI3 nanocrystals because of the very high ion mobility of halides. Due to the labile nature of the α-phase CsPbI3 nanocrystals, a common strategy is to carry out the ligand exchange from CsPbBr3 as starting material because they show a higher phase stability and a high QY near to 90% in solution.

Although CsPbIXBr3-X nanocrystals can be successfully synthesized in solution with outstanding optical properties, the formation of high-quality thin films of perovskite with high QY is challenging owing to the degradation of their optical properties after deposition on a substrate by the effect of the moisture. Several approaches have been proposed to achieve solid-state anion exchange over CsPbBr3 thin films with discouraging results in terms of low PLQY, poor chemical stability and low film homogeneity. All these drawbacks show the necessity to develop new methods to obtain perovskite thin films with tunable optical properties and high QY.

In this work, we explore spray coating as a new route to carried out the solid-state anion exchange in thin films of CsPbBr3 nanocrystals at room conditions. Basically, spray coating consists in the formation of an aerosol from a solution precursor and its transport by a carrier gas to the CsPbBr3 thin film previously prepared. To the best of our knowledge, this is the first time that a solid-state anion exchange is carried out by spray-coating. As a I- precursor, best results are obtained with a solution of HI in methyl acetate, an antisolvent for CsPbBr3 that keeps unaltered the well-ordered microstructure of the NC thin films. We show how spraying provides a more intense anion exchange than immersion since fresh HI solution is added repeatedly. The light emission of thin films after our anion exchange procedure can be accurately tuned by spraying different HI solution volumes, obtaining a full gamut of emission wavelengths between 520 and 670 nm. However, the most impacting result is that QY improves after anion exchange, from QY=61% in the case of the original CsPbBr3 thin films emitting at 520 nm, to QY=80% in the case of the alloyed film CsPbBr3-xIx emitting at 640 nm after anion exchange. Morphological (TEM), structural (DXR) and optical characterization (UV-Vis spectroscopy, PL and TRPL measurements) were carried out. TEM images and DXR spectra show that the α-phase, cubic shape and the nanocrystal size is conserved after anion exchange. Finally, as a proof of concept, we demonstrated the production of stimulated emission and even random lasing in thin films after applying the anion exchange procedure. This observation is a further proof of the film quality (high emission QY) conserved after anion exchange.