Carrier localization at core-shell interfaces in semiconductor nanostructures plays a critical role in determining the optoelectronic properties of quantum-confined systems, particularly in applications such as high-efficiency light-emitting diodes (LEDs). Core-shell quantum dots (QDs), such as CdSe/ZnS, exhibit unique electronic and optical behaviors due to the spatial separation of charge carriers at the interface between the core and shell materials. This localization is instrumental in suppressing non-radiative Auger recombination, a major loss mechanism in QD-based LEDs.

The electronic structure of core-shell QDs is governed by the alignment of energy bands at the interface. In CdSe/ZnS, the conduction band offset is relatively small, while the valence band offset is significant, leading to strong hole confinement in the core. Electrons, however, experience a weaker confinement potential, allowing partial delocalization into the shell. This asymmetry in carrier localization influences recombination dynamics, as spatially separated electrons and holes reduce the likelihood of Auger processes, where an excited electron recombines non-radiatively by transferring energy to a third carrier.

Auger recombination is a three-particle interaction that becomes pronounced at high carrier densities, common in electroluminescent devices. In homogeneous QDs, Auger rates scale inversely with volume, making small nanostructures particularly susceptible. Core-shell architectures mitigate this by introducing a spatial separation between carriers. Studies on CdSe/ZnS QDs demonstrate that the Auger lifetime can be extended by an order of magnitude compared to bare CdSe cores. This suppression is attributed to the reduced overlap of electron and hole wavefunctions due to interfacial localization.

The thickness and composition of the shell further modulate carrier confinement. A thicker ZnS shell enhances electron delocalization while maintaining hole confinement, leading to improved Auger suppression. However, excessive shell growth can introduce strain and defects, degrading optical performance. Optimized shell thicknesses in the range of 3-5 monolayers have been shown to balance Auger suppression with structural integrity. Strain-induced defects at the core-shell interface can also localize carriers unintentionally, creating trap states that compete with radiative recombination.

Temperature-dependent photoluminescence studies reveal the role of interfacial localization in Auger suppression. At low temperatures, carriers are tightly confined, and Auger rates are minimized. As temperature increases, thermal activation allows carriers to overcome confinement barriers, increasing Auger recombination. In CdSe/ZnS QDs, the thermal activation energy for Auger processes is measurably higher than in homogeneous QDs, confirming the effectiveness of core-shell structures in suppressing non-radiative losses.

The impact of carrier localization extends to electroluminescent devices. QD-LEDs utilizing CdSe/ZnS cores exhibit higher external quantum efficiencies (EQEs) compared to those with homogeneous QDs. The suppression of Auger recombination reduces efficiency roll-off at high current densities, a common limitation in QD-LEDs. Device studies report EQEs exceeding 20% in optimized core-shell systems, with Auger-limited roll-off occurring at significantly higher current densities than in core-only counterparts.

Interfacial alloying between the core and shell can further tailor carrier localization. Graded interfaces, where the composition transitions smoothly from core to shell material, reduce strain and suppress defect formation while maintaining sufficient band offsets for carrier confinement. In CdSe/ZnS, controlled alloying at the interface has been shown to enhance photoluminescence quantum yield by reducing interfacial traps without compromising Auger suppression.

Surface passivation also plays a critical role in carrier localization. Unpassivated surface states can act as traps, localizing carriers and promoting non-radiative recombination. Effective shell growth encapsulates the core, eliminating surface traps and ensuring that localization is dominated by the core-shell band alignment rather than defects. ZnS shells are particularly effective due to their wide bandgap and chemical stability, which minimize surface-related losses.

Advanced spectroscopic techniques, such as transient absorption and time-resolved photoluminescence, provide insights into the dynamics of carrier localization. Measurements indicate that electron-hole separation in CdSe/ZnS QDs occurs on picosecond timescales, with Auger recombination rates significantly slower than in unpassivated cores. These findings underscore the importance of engineered interfaces in controlling carrier behavior for optoelectronic applications.

In summary, carrier localization at core-shell interfaces is a key factor in enhancing the performance of QD-based LEDs. By strategically designing the core-shell structure, Auger recombination can be suppressed, leading to higher efficiency and brightness in electroluminescent devices. The interplay between band alignment, shell thickness, interfacial strain, and surface passivation determines the extent of localization and its impact on device performance. Continued optimization of these parameters will further advance the development of high-performance QD-LEDs for displays, lighting, and other applications.

The following table summarizes key parameters influencing carrier localization and Auger suppression in CdSe/ZnS core-shell QDs:

Parameter Influence on Localization
Core size Stronger confinement for smaller cores
Shell thickness Optimal at 3-5 monolayers
Band offsets Valence band offset > conduction band offset
Interfacial alloying Reduces strain, maintains confinement
Temperature Higher temps reduce localization efficacy
Surface passivation Minimizes traps, enhances radiative efficiency

Understanding these factors enables precise control over carrier dynamics, paving the way for next-generation QD optoelectronics with superior efficiency and stability.