Tailoring Molecular Ordering and Energy Levels via Solvent Selection for Inverted QLEDs with Dual PVK Hole Transport Layers

In inverted quantum dot light-emitting diodes (QLEDs), the energy barrier for holes from the anode is significantly larger than that for electrons from the cathode. This barrier disparity is a major challenge, leading to low efficiency in inverted QLEDs. To address this issue, dual hole transport layers (HTLs) made of the same material, poly(N-vinyl carbazole) (PVK), but with different molecular assembly structures are introduced. These structures are achieved using two solvents with a large boiling-point gap: 1,4-dioxane (1,4-D) and gamma-valerolactone (GVL). The PVK film fabricated using GVL with a higher boiling point exhibits better-ordered and denser molecular assembly compared to that fabricated using 1,4-D. The highest occupied molecular orbital levels of the two PVK layers are stepwise, attributed to their distinct molecular assembly structures. Consequently, a device with dual HTLs demonstrates over 40% improvement in external quantum efficiency and power efficiency compared to a device with a single HTL. This result provides a novel approach to tuning the energy levels of functional layers in QLEDs, significantly enhancing device performance.