Optimization of the hole-injection layer for quantum dot light-emitting diodes.

Quantum dot light-emitting diodes (QLEDs) are regarded as cornerstones of next-generation display technologies owing to their broad spectral tunability, high color purity, and exceptional efficiency. However, the deep valence band energy levels of quantum dots (QDs) result in a high hole-injection barrier, leading to a charge-injection imbalance and limiting the device performance. This review systematically summarizes the optimization strategies for the hole-injection layer (HIL) in QLEDs, focusing on the design and application of organic single-layer HILs (e.g., PEDOT : PSS), inorganic single-layer HILs (e.g., MoO3, NiOx, and V2O5), dual HIL structures (e.g., PEDOT : PSS/metal oxide), and doped HILs (e.g., metal-ion doping and organic-inorganic hybridization). Studies have demonstrated that dual HILs reduce the hole-injection barrier through stepped energy levels, doping strategies enhance the carrier mobility and interfacial stability, and metal oxide HILs exhibit superior thermal stability and environmental adaptability. Additionally, post-treatment processes such as rapid thermal annealing (RTA) can further optimize the interfacial properties. Although QLEDs possess immense potential in display and lighting applications, challenges remain in addressing the insufficient efficiency of cadmium-free blue QLEDs and the interfacial strain mismatch in flexible devices. This review provides a comprehensive reference for the rational design of HILs and outlines future directions for developing high-efficiency, stable, and scalable QLEDs.