Organic light-emitting diodes (OLEDs) are widely recognized as the forefront technology for displays and lighting technology. Now, the global OLED market is nearly mature, driven by the rising demand for superior displays in smartphones. In recent years, numerous strategies have been introduced and demonstrated to optimize the hole injection layer to further enhance the efficiency of OLEDs. In this paper, different methods of optimizing the hole injection layer were elucidated, including using a suitable hole injection material to minimize the hole injection barrier and match the energy level with the emission layer, exploring new preparation methods to optimize the structure of hole injection layer, and so on. Meanwhile, this article can help people to understand the current research progress and the challenges still faced in relation to the hole injection layer in OLEDs, providing future research directions to enhance the properties of OLEDs.

Flexible organic light-emitting diodes (FOLEDs) have promising potential for future wearable applications because of their exceptional mechanical flexibility. Silver nanowire (Ag NW) networks are the most promising candidates to replace indium tin oxide (ITO), which is limited by its poor bendability. In this study, three different methods including methanol impregnation, argon plasma treatment, and ultraviolet radiation were used to reduce the junction resistance of Ag NWs to optimize the flexible transparent electrodes (FTEs); which were prepared using Ag NWs and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS). Then, the optoelectronic properties of the FTEs were further improved by using a co-doped system of silver nanowires and silver nanoparticles (Ag NPs), the structure of which consisted of PET/Ag NWs: Ag NPs/PEDOT: PSS/DMSO. The largest FOM value of 1.42 × 10⁻² ohm⁻¹ and a low sheet resistance value of 13.86 ohm/sq were obtained using the optimized FTEs. The prepared FOLED based on the optimized FTEs had a luminous efficiency of 6.04 cd/A and a maximum EQE of 1.92%, and exhibited no observed decline in efficiency when reaching maximum luminance. After 500 bending tests, the luminance still reached 82% of the original value. It is demonstrated that the FTEs prepared via the co-doped system have excellent optoelectronic properties as well as high mechanical stability.

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, NiO_x, 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.

The enhancement of organic light-emitting diode (OLED) device performance has been a key area of research in organic optoelectronic devices. Optimizing carrier mobility within OLED devices is a crucial strategy. In this study, the hole transport layer was optimized using a self-assembled monolayer (SAM) subjected to different annealing temperatures. Through submitting the SAM to an annealing temperature of 100 °C, a maximum luminous intensity of 32,290 cd/m² and a maximum EQE of 1.77 were achieved, the latter being more than two-fold higher than that without the SAM. As the SAM annealing temperature increased from 80 °C to 120 °C, both the vertical orientation of molecules in the hole transport layer and the hole mobility in hole-only devices (HODs) improved. This vertical orientation is beneficial to enhancing hole mobility. Electrochemical impedance spectroscopy and surface morphology analysis revealed that the introduction of a SAM leads to the formation of interface resistance. The synergy effect between the variation in the molecular transition dipole moment and the interface morphology of the hole transport layer optimizes the hole mobility of HODs and leads to the enhancement of OLED performance.