Liquid-Liquid Interface-Based Thiocyanate Surface Treatment for Bright and Stable CsPbBr₃ Nanocrystals

Enhancing the efficiency and stability of lead halide perovskite devices is crucial to their practical application. Previous treatments with thiocyanate (SCN^-) have demonstrated significant improvements in the photoluminescence quantum yield (PLQY) and stability of CsPbBr₃ nanocrystals (NCs), but the underlying mechanisms remain partially unresolved. Addressing the challenge of low SCN^- solubility in traditional nonpolar solvents, our study introduces a urea-ammonium thiocyanate (UAT)-based ionic liquid surface treatment. This method facilitates a higher SCN^- loading by creating a liquid-liquid interface that is compatible with the organic colloidal suspension, preventing NC degradation, and achieving near-unity PLQY. Utilizing transmission electron microscopy techniques, we present atomic resolution evidence that thiocyanate-treated surfaces are rich in sulfur and display structural dilation of the lattice spacing of 3%. This supports that thiocyanate acts as a pseudohalide and binds to Pb cations on the NC surfaces. As a result, the treated NCs show enhanced stability against ionic substitution while maintaining the perovskite structure intact. Our findings provide conclusive evidence that the primary mechanism of performance enhancement is the passivation of surface traps attributed to bromide vacancies rather than the scavenging of excess lead cation. This surface treatment method slows ion migration, a prominent challenge in photovoltaics, offering a significant advancement in the development of perovskite-based devices.