Ground-State Charge Transfer in MgF2-Organic Composites: A Universal Mechanism for a High-Performance Hole Injection Layer in OLEDs

Abstract

While magnesium fluoride (MgF2)-organic composite hole injection layers (HILs) are commercially employed in organic light-emitting diodes (OLEDs), their fundamental working principle remains elusive, hindering rational optimization. In this study, the underlying mechanisms of this comprehensive enhancement are systematically elucidated. It is demonstrated that an optimized MgF2-organic HIL dramatically reduces the driving voltage and extends operational lifetime by over an order of magnitude. Structural analysis reveals that MgF2 forms an intimate amorphous solid solution with the organic host, suppressing its own crystallization. This unique structure facilitates a powerful, synergistic dual-enhancement: i) In situ photoelectron spectroscopy confirms the suppression of interfacial band bending for barrier-free hole transport. ii) Concurrently, a quantitative analysis demonstrates a substantial, over 50-fold increase in hole carrier density via p-doping. Definitive spectroscopic and theoretical evidence proves that these benefits originate from a ground-state charge transfer from the organic host to MgF2, unequivocally establishing MgF2 as an effective p-dopant. Crucially, it is demonstrated that this entire mechanistic framework is a general and robust principle, universally applicable across chemically distinct hole transport materials. This work therefore resolves a long-standing ambiguity and establishes a foundational design strategy for next-generation OLEDs with superior efficiency and operational stability.