Design criteria for HTL-blended emissive layers in red quantum dot light-emitting diodes

The use of hole transport layer (HTL)-blended emissive layers (EMLs) has been explored as an effective strategy for improving charge balance in quantum dot light-emitting diodes (QD-LEDs). However, their potential has not been fully realized because clear criteria for selecting suitable HTL materials are still lacking. Here, using a red-emitting CdZnSe core/shell QD/ZnMgO system, we identify two key criteria for HTLs in blended EMLs: efficient hole transport and tolerance to electron leakage at additional local HTL/electron transport layer (ETL) contacts that may form within the blended EML. Using three representative polymeric HTLs-PVK, TFB, and PF8Cz-we investigated how HTL-dependent charge transport and leakage susceptibility affect recombination behavior and device performance. For all three HTLs, hole-only devices (HODs) with blended EMLs exhibited higher current densities, indicating that HTL blending facilitated hole transport by providing additional hole-injection pathways throughout the EML. In contrast, voltage-evolution measurements of HTL/ZnMgO junction devices under constant-current operation revealed a strong dependence on the HTL material. The TFB-based junction device exhibited substantial voltage drift, while the corresponding TFB-blended QD-LED showed a distinct TFB-related shoulder emission, consistent with parasitic recombination. By contrast, the PVK- and PF8Cz-blended QD-LEDs exhibited no detectable HTL-related parasitic emission. Among the examined materials, the PF8Cz-blended device enabled lower-voltage operation, a broader near-unity S regime, and the largest relative enhancement in external quantum efficiency. These results establish hole-transport capability and tolerance to electron leakage at potential local HTL/ETL contacts as practical criteria for selecting HTLs for high-performance red QD-LEDs with blended EMLs.