Systematic Investigation of Nanoscale Adsorbate Effects at Organic Light-Emitting Diode Interfaces. Interfacial Structure−Charge Injection−Luminance Relationships

Molecule-scale structure effects at indium tin oxide (ITO) anode−hole transport layer (HTL) interfaces in organic light-emitting diode (OLED) heterostructures are systematically probed via a self-assembly approach. A series of ITO anode-linked silyltriarylamine precursors differing in aryl group and linker density are synthesized for this purpose and used to probe the relationship between nanoscale interfacial chemical structure and charge-injection/electroluminescence properties. These precursors form conformal and largely pinhole-free self-assembled monolayers (SAMs) on the ITO anode surface with angstrom-level thickness control. Deposition of a HTL on top of the SAMs places the probe molecules precisely at the anode−HTL interface. OLEDs containing ITO/SAM/HTL configurations have dramatically varied hole-injection magnitudes and OLED responses. These can be correlated with the probe molecular structures and electrochemically derived heterogeneous electron-transfer rates for such triarylamine fragments. The large observed interfacial molecular structure effects offer an approach to tuning OLED hole-injection flux over 1−2 orders of magnitude, resulting in up to 3-fold variations in OLED brightness at identical bias and up to a 2 V driving voltage reduction at identical brightness. Very bright and efficient (∼70 000 cd/m 2, ∼2.5% forward external quantum efficiency, ∼11 lm/W power efficiency) Alq (tris(8-hydroxyquinolinato)aluminum(III))-based OLEDs can thereby be fabricated.