Spray-driven halide exchange in solid-state CsPbX₃ nanocrystal films

CsPbI₃ perovskite nanocrystals (NCs) are promising building blocks for photovoltaics and optoelectronics. However, they exhibit an essential drawback in the form of phase stability: α-phase, with a ∼1.80 eV bandgap, can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment. This leads to the need to carry out the CsPbI₃-based device fabrication in an inert atmosphere, which is technologically inconvenient and expensive. One of the most successful approaches proposed to overcome this problem is synthesizing mixed halide CsPbBr_3-I NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBr_3-I NCs with high PLQY is challenging owing to the degradation of their optical properties after deposition on a substrate. This work presents spray coating to carry out a solid-state anion exchange in CsPbBr₃ NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBr_3-I NC thin films with high optical quality and record high photoluminescence quantum yields (PLQY) based on spray-driven halide (Br^- to I^-) anion exchange in a solid-state phase. Besides, tunable emission wavelengths between 520 and 670 nm can be obtained from CsPbBr_3-I NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr₃ film to provide the halide exchange. The optical quality of the halide-exchanged PNCs films remains practically identical to that of initial Br-containing layers, with a remarkable PLQY enhancement after anion exchange, from ∼61% for CsPbBr₃ thin films emitting at 520 nm to ∼84% for mixed halide CsPbBr_3-I film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.