Quantum dots (QDs) offer size-dependent tunability across the infrared to ultraviolet range with narrow emission linewidths and high color purity, making them highly attractive for next-generation light-emitting devices. Quantum dot lightemitting diodes (QLEDs) further combine precise spectral control with scalable, low-cost solution processing, positioning them as strong candidates for wearable, stretchable, and AR/VR display technologies. However, conventional single-emission QLEDs suffer from charge imbalance, efficiency roll-off, and limited operational lifetime, necessitating new device architectures. Tandem QLEDs, which vertically stack multiple emissive layers (EMLs) connected by charge generation layers (CGLs), provide a compelling solution by enabling higher luminance, improved charge balance, and longer lifetime at equivalent current density. The CGL serves as the interfacial region mediating charge injection and generation between adjacent EMLs, directly determining device efficiency and stability. This review highlights recent progress in CGL engineering, categorizing representative designs into planar heterojunction, inorganic-based, and dipole-based configurations. Comparative analysis of their formation mechanisms, material systems, and process compatibilities reveals evolving charge-control strategies that extend beyond material selection. These insights establish design principles for next-generation tandem QLEDs with enhanced efficiency, durability, and manufacturability.

A poly[bis(4-butypheny)-bis(phenyl)benzidine] (poly-TPD) and poly(9-vinylcarbazole) (PVK) bilayer was employed as a hole transport layer (HTL) in solution-processed CdSe/ZnS quantum dot light-emitting diodes (QLEDs). The thickness of the PVK layer spin-coated onto the poly-TPD layer, whose thickness was fixed to 40 nm, was varied, with PVK layer thicknesses of 0 nm, 35 nm, 45 nm, and 55 nm. Because the thickness of the PVK can determine the hole transport properties of the HTL, a PVK thickness that maximizes the performance of the HTL for the QLEDs was investigated. By employing the optimized PVK thickness of 45 nm, the current efficiency of the QLED exhibited a 1.74 times improvement when compared with that of the QLED with poly-TPD based HTL without PVK. This was mainly attributed to the decrease in the energy barrier between the HTL and the quantum dot (QD) emitting layer (EML).