Quantum dot light-emitting diodes (QLEDs) represent a significant advancement in display technology, leveraging the unique optoelectronic properties of semiconductor nanocrystals to achieve high color purity, tunable emission, and energy efficiency. The core of QLED performance lies in the careful design of device architecture, optimization of charge injection, and engineering of the emissive layer. Recent progress in red, green, and blue QLEDs has pushed the boundaries of efficiency and operational stability, though challenges such as efficiency roll-off and environmental sensitivity remain critical hurdles.

The typical QLED structure consists of multiple functional layers: a transparent anode, a hole injection layer (HIL), a hole transport layer (HTL), an emissive quantum dot (QD) layer, an electron transport layer (ETL), and a cathode. Indium tin oxide (ITO) is commonly used as the anode due to its high transparency and conductivity. The HIL, often composed of conductive polymers or metal oxides like MoO3, ensures efficient hole injection from the anode into the HTL. The HTL, typically made of organic materials such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or inorganic materials like nickel oxide (NiOx), facilitates hole transport to the QD layer. The QD emissive layer is the heart of the device, where excitons recombine to emit light. The ETL, usually an inorganic material such as zinc oxide (ZnO), transports electrons from the cathode to the QDs. Finally, the cathode, often a thin layer of aluminum or silver, completes the electrical circuit.

Charge injection balance is crucial for high-performance QLEDs. Imbalanced charge injection leads to excess electrons or holes, which can cause non-radiative recombination or charge-induced quenching. To mitigate this, researchers optimize the energy level alignment between layers. For example, introducing a thin insulating layer between the QDs and the ETL can reduce electron leakage and improve charge balance. Additionally, doping the HTL or ETL can fine-tune carrier mobility, ensuring that holes and electrons reach the QDs at comparable rates. Recent studies have demonstrated that graded heterojunctions, where the composition of transport layers gradually changes, can further enhance charge injection efficiency.

Emissive layer engineering focuses on improving the photoluminescence quantum yield (PLQY) and stability of QDs. Core-shell QD structures, such as CdSe/ZnS, are widely used to confine excitons within the core while passivating surface defects with the shell. Advances in shell engineering, including alloyed shells and multi-shell structures, have pushed PLQY values above 90% for red and green QDs. Blue QDs, however, lag due to their wider bandgaps and higher susceptibility to surface defects. Recent developments in heavy-metal-free QDs, such as InP-based nanocrystals, have shown promise for achieving high PLQY while addressing environmental concerns. Surface ligand management is another critical factor; long-chain ligands provide stability but hinder charge transport, whereas short-chain ligands improve conductivity at the expense of increased defect states. Hybrid ligand systems that combine long and short chains offer a compromise, enhancing both stability and charge injection.

Red QLEDs have achieved the highest efficiencies, with external quantum efficiencies (EQEs) exceeding 20% and lifetimes surpassing 100,000 hours at practical brightness levels. These devices benefit from mature synthesis techniques for Cd-based QDs and well-matched charge transport materials. Green QLEDs follow closely, with EQEs above 18%, though their operational stability remains slightly inferior to red devices. Blue QLEDs are the most challenging due to the difficulty in synthesizing high-quality wide-bandgap QDs. Current state-of-the-art blue QLEDs achieve EQEs of around 12-15%, with lifetimes significantly shorter than their red and green counterparts. Efforts to improve blue QLEDs include the development of ZnSe-based QDs and advanced shell passivation strategies.

Efficiency roll-off, the decline in EQE at high current densities, is a persistent issue in QLEDs. This phenomenon arises from Auger recombination, where excess charges non-radiatively recombine, generating heat instead of light. Mitigating Auger recombination requires reducing the QD charging probability through careful device design. For instance, using thinner QD layers or introducing exciton quenching layers can suppress charge accumulation. Another approach involves engineering QDs with suppressed Auger recombination rates, such as giant-shell QDs or compositionally graded structures.

Environmental degradation, particularly from oxygen and moisture, poses another challenge. QDs are susceptible to oxidation, which quenches their luminescence and degrades device performance. Encapsulation techniques, such as atomic layer deposition (ALD) of Al2O3 barriers, have proven effective in extending QLED lifetimes. Additionally, developing inherently stable QD materials, like those with robust inorganic shells or alloyed compositions, can reduce environmental sensitivity.

Recent breakthroughs in QLED technology include the demonstration of solution-processed, flexible QLEDs with performance comparable to rigid devices. These advances rely on optimizing the mechanical properties of each layer while maintaining electrical and optical performance. Another notable development is the integration of QLEDs with thin-film transistor backplanes for active-matrix displays, enabling high-resolution and energy-efficient panels.

In summary, QLEDs have made remarkable progress in efficiency and stability, particularly for red and green emission. Blue QLEDs remain an area of active research, with ongoing efforts to improve their performance and longevity. Addressing efficiency roll-off and environmental degradation will be key to unlocking the full potential of QLEDs for next-generation displays. Continued innovation in QD synthesis, device architecture, and charge management will drive further advancements in this field.