Nanometer-Scale Dielectric Self-assembly Process for Anode Modification in Organic Light-Emitting Diodes. Consequences for Charge Injection and Enhanced Luminous Efficiency

Abstract

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).

High‐Brightness Blue Light‐Emitting Polymer Diodes via Anode Modification Using a Self‐Assembled Monolayer

“…Note however from Figure 2C, that from 5±10 V the PEDOT-based device supports greater total current than the SAM-modified PLED. Within the PLED model of Murata et al [7,23] in which the barrier to hole injection at the ITO/PEDOT anode is overcome to various extents by electron density build-up near the barrier, the present results suggest that the nature of the PEDOT±PSS and TPD-Si 2 barriers are rather different, with the effect in the former being larger at lower voltages and the PLED properties more dependent on electron density build-up. The enhanced hole injection by TPD-Si 2 SAM modification may also be related to higher interfacial affinity between the SAM-modified ITO and PFO.…”

σ-π molecular dielectric multilayers for low-voltage organic thin-film transistors

“…3) with Cap forming a SiO x network (d cap ϭ 0.83 Ϯ 0.1 nm by x-ray reflectivity) (26), then k eff of I and II can be approximated by multilayers composed of Alk and Stb monolayers and oxide layer (k ox Ϸ 3.9) (10).…”

Modification of the Surface Properties of Indium Tin Oxide with Benzylphosphonic Acids: A Joint Experimental and Theoretical Study

“…[1,8,40] By attaching a dipolar organic molecule (with negative pole directed outwards) to the surface, the electrostatic potential energy (V vac ) in the vacuum region can be raised, resulting in an increase in the local work function. [1,8,10,17,20,27,[40][41][42][43][44] The opposite is true if the negative pole points toward the surface. Organic electronic devices must have excellent long-term stability for wide-spread acceptance, and it has been proposed that one contributor to device instability is unmatched surface energies between materials at their interfaces, leading to pinholes and possible delamination.…”