Interfacial structural analysis with X-ray reflectivity for elucidation of alkyl-chain effects on hole injection in quantum-dot light-emitting diodes†

Stable quantum-dot light-emitting diodes (QD-LEDs) require sufficient hole injection, which requires understanding and controlling the hole transporting (HT) polymer/QD interface structure. We successfully illustrated and quantified the interface structure by applying X-ray reflectivity (XRR) analysis and clarified the dominant factors of hole injection. We developed HT polymers with various alkyl chains to investigate the relationship between the interface structure and hole injection. We revealed that longer alkyl chains interact more strongly with QDs, causing QD sinking (= interfacial mixing) and the formation of an interface favorable for hole injection. Thus, interfacial structure analysis with XRR is useful for designing HT polymers with enhanced hole injection.