Spray-driven halide exchange in solid-state CsPbX nanocrystal films.

CsPbI 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 CsPbI-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 CsPbBrI NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBrI 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 CsPbBr NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBrI 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 CsPbBrI NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr 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 CsPbBr thin films emitting at 520 nm to ∼84% for mixed halide CsPbBrI film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.