Nanometer-scale dielectric self-assembly process for anode modification in organic light-emitting diodes. Consequences for charge injection and enhanced luminous efficiency

Layer-by-layer, self-limiting chemisorptive siloxane self-assembly using Si₃O₂Cl₈ as the precursor affords thin, conformal, relatively dense, largely pinhole-free dielectric films that can be deposited on oxide surfaces with sub-nanometer control of film thickness (8.3(1) Å/layer). Deposition chemistry, microstructure, and hole injection/work function modification properties of these (SiO₂)_x-like films on single-crystal Si(111) and polycrystalline indium tin oxide (ITO) substrates have been characterized by synchrotron specular X-ray reflectivity, cyclic voltammetry, X-ray and UV photoelectron spectroscopy, and atomic force microscopy. Chemisorption of these (SiO₂)_x films onto the ITO anodes of three-layer, vapor-deposited organic electroluminescent devices (ITO/(SiO₂)_x/TPD/Alq/Al) nearly triples the external quantum and luminous efficiencies. The efficiency enhancement is attributed to hole and electron injection fluence balance caused by modification of the effective voltage profile brought about by the assembly of well-ordered siloxane layers. Interestingly, as a function of increasing (SiO₂)_x layer thickness, device turn-on voltage first increases (x = 0 → 1), progressively decreases (x = 1 → 2 → 3), and then increases (x = 3 → 4). A theoretical model based upon computation at the ab initio level is proposed in which the self-assembled dielectric layers induce an additional, thickness-dependent "built-in" electric field across the organic transport layers, thereby simultaneously enhancing electron injection from the cathode (increasing luminescence efficiency) and decreasing the efficiency of hole injection (changing the turn-on voltage).