Low‐dimensional Cu(I)‐based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self‐trapped excitons (STEs). Among them, Cs5Cu3Cl6I2, which emits blue light, is a promising candidate for applications as a next‐generation blue‐emitting material. In this article, an optimized colloidal process to synthesize uniform Cs5Cu3Cl6I2 nanocrystals (NCs) with a superior quantum yield (QY) is proposed. In addition, precise control of the synthesis parameters, enabling anisotropic growth and emission wavelength shifting is demonstrated. The synthesized Cs5Cu3Cl6I2 NCs have an excellent photoluminescence (PL) retention rate, even at high temperature, and exhibit high stability over multiple heating–cooling cycles under ambient conditions. Moreover, under 850‐nm femtosecond laser irradiation, the NCs exhibit three‐photon absorption (3PA)‐induced PL, highlighting the possibility of utilizing their nonlinear optical properties. Such thermally stable and highly luminescent Cs5Cu3Cl6I2 NCs with nonlinear optical properties overcome the limitations of conventional blue‐emitting nanomaterials. These findings provide insights into the mechanism of the colloidal synthesis of Cs5Cu3Cl6I2 NCs and a foundation for further research.

Organic optoelectronic devices that can be fabricated at low cost have attracted considerable attention because they can absorb light over a wide frequency range and have high conversion efficiency, as well as being lightweight and flexible. Moreover, their performance can be significantly affected by the choice of the charge-selective interlayer material. Nonstoichiometric nickel oxide (NiO x ) is an excellent material for the hole-transporting layer (HTL) of organic optoelectronic devices because of the good alignment of its valence band position with the highest occupied molecular orbital level of many p-type polymers. Herein, we report a simple low-temperature process for the synthesis of NiO x nanoparticles (NPs) that can be well dispersed in solution for long-term storage and easily used to form thin NiO x NP layers. NiO x NP-based organic photodiode (OPD) devices demonstrated high specific detectivity (D*) values of 10 12 −10 13 jones under various light intensities and negative biases. The D* value of the NiO x NP-based OPD device was 4 times higher than that of a conventional poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based device, an enhancement that originated mainly from the 16 times decreased leakage current. The NiO x NP-based OPD device demonstrated better reliability over a wide range of light intensities and operational biases in comparison to a device with a conventional sol−gel-processed NiO x film. More importantly, the NiO x NP-based OPD showed long-term device stability superior to those of the PEDOT:PSS and sol−gel-processed NiO x -based devices. We highlight that our low-temperature solution-processable NiO x NP-based HTL could become a crucial component in the fabrication of stable high-performance OPDs.