Hydrothermal synthesis has emerged as a robust method for producing high-quality halide perovskite nanocrystals, particularly CsPbBr3, due to its ability to control crystallinity, morphology, and phase purity under elevated temperature and pressure conditions. This technique offers distinct advantages over room-temperature methods, including improved stability, reduced defect density, and enhanced optoelectronic properties. The process involves the reaction of precursor materials in a sealed autoclave, where the solvent system and reaction parameters are carefully optimized to achieve uniform nanocrystals with suppressed phase segregation.

The precursor chemistry for hydrothermal synthesis of CsPbBr3 typically involves cesium salts (e.g., CsBr), lead sources (e.g., PbBr2), and halide compounds dissolved in polar solvents. The choice of precursors is critical to avoid unwanted byproducts or incomplete reactions. For example, CsBr and PbBr2 are commonly used due to their high solubility in hydrothermal conditions, ensuring stoichiometric control. The solvent system often includes a mixture of water and organic solvents like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), which act as coordinating ligands to stabilize the nanocrystals during growth. Additives such as oleic acid and oleylamine are introduced to passivate surface defects and prevent aggregation.

Phase segregation is a significant challenge in halide perovskite nanocrystals, particularly in mixed-halide compositions, where halide ions migrate under external stimuli like light or electric fields. Hydrothermal synthesis addresses this issue through several strategies. First, the high-temperature environment promotes homogeneous nucleation, reducing the likelihood of halide-rich domains. Second, the use of stabilizing ligands and controlled cooling rates minimizes kinetic trapping of metastable phases. Third, encapsulation with inert matrices or surface passivation with long-chain ligands further suppresses ion migration. These measures collectively enhance the phase stability of CsPbBr3 nanocrystals, making them suitable for optoelectronic applications.

The hydrothermal method significantly improves the stability of perovskite nanocrystals compared to room-temperature synthesis. The high-pressure conditions facilitate the formation of dense, defect-free crystals with fewer surface traps, which are a primary source of degradation. Additionally, the in-situ passivation of surface defects during hydrothermal growth reduces susceptibility to moisture and oxygen, common degradation pathways for perovskites. Studies have shown that hydrothermally synthesized CsPbBr3 nanocrystals retain their photoluminescence quantum yield (PLQY) above 80% for weeks under ambient conditions, whereas room-temperature synthesized counterparts often degrade within days.

In optoelectronic applications, hydrothermally synthesized CsPbBr3 nanocrystals exhibit exceptional performance due to their high crystallinity and stability. In light-emitting diodes (LEDs), these nanocrystals achieve narrow emission spectra with full-width-at-half-maximum (FWHM) values below 20 nm, making them ideal for high-color-purity displays. Their high PLQY and charge transport properties contribute to efficient electroluminescence, with external quantum efficiencies (EQE) exceeding 15% in optimized devices. The suppressed phase segregation also ensures consistent emission characteristics under operational stresses, a critical requirement for LED longevity.

In photovoltaics, CsPbBr3 nanocrystals synthesized via hydrothermal methods demonstrate superior charge extraction and reduced recombination losses. The dense, defect-free crystals facilitate efficient carrier transport, leading to power conversion efficiencies (PCE) of over 12% in nanocrystal-based solar cells. The stability under continuous illumination and thermal stress further enhances their suitability for long-term photovoltaic applications. Unlike solution-processed perovskites, hydrothermally grown nanocrystals exhibit minimal hysteresis in current-voltage characteristics, indicating robust interfacial properties.

The hydrothermal technique also enables precise control over nanocrystal size and morphology, which are critical for tuning optoelectronic properties. By varying reaction time, temperature, and precursor concentrations, nanocrystals can be tailored from quantum dots (2-10 nm) to larger nanocrystals (20-50 nm), each with distinct bandgap energies. Smaller nanocrystals exhibit blue-shifted emission due to quantum confinement, while larger crystals emit in the green to red spectrum. This tunability is advantageous for applications requiring specific emission or absorption profiles, such as tandem solar cells or multi-color LEDs.

Despite these advantages, challenges remain in scaling up hydrothermal synthesis for industrial production. The requirement for high-pressure equipment and precise control over reaction parameters increases complexity and cost. However, ongoing research into continuous-flow hydrothermal systems and automated reaction monitoring may address these limitations, paving the way for large-scale adoption.

In summary, hydrothermal synthesis offers a reliable route to producing stable, high-performance CsPbBr3 nanocrystals with suppressed phase segregation and excellent optoelectronic properties. The method's ability to deliver defect-free crystals with tailored morphologies makes it a promising candidate for next-generation LEDs and photovoltaics. As research progresses, further optimization of precursor chemistry and reaction conditions will likely expand the scope of hydrothermally synthesized perovskite nanocrystals in advanced optoelectronic devices.