Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes

Abstract

We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A(-1), whereas maximum luminance values are as high as 8000 and 3000 cd m(-2) for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime.

Highly Improved Efficiency of Deep-Blue Fluorescent Polymer Light-Emitting Device Based on a Novel Hole Interface Modifier with 1,3,5-Triazine Core

“…The electrochemical properties; 1 H-NMR; 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 …”

“…12 Subsequently, the most important factor restricting the blue-light device performance is the limited hole injection from anode to blue emitter which possesses very deep highest occupied molecular orbital (HOMO) energy level. 3,13 For solving this problem, various hole injection layers (HILs) are inserted between tin-doped indium oxide (ITO) and blue emitter, such as poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonic acid) (PEDOT:PSS)/poly (vinylcarbazole) (PVK), [14][15][16] MoO 3 ,17 etc. However, there still exists large injection barrier to approach ohmic contact.…”

“…18 The work function of ITO is only about 4.7 eV, while the HOMO energy levels of the blue emitters are typically located from 5.5 eV to 6.3 eV, leading to the discontinuous laminated ohmic contact between ITO and deep-blue emitter with injection barrier higher than 0.3 eV. 2,13,19 Although multiple HILs are applied to match the energy level of ITO and deep-blue emitters to improve the hole injection, the big HOMO energy level gap among HILs, e.g., PEDOT:PSS and PVK, is still difficult to overcome to maximize the performance of deep-blue emitters. 20 The electron-deficient 1,3,5-triazine derivatives have been widely used as electron transport materials, [21][22][23] hole transport materials 24 and bipolar transport materials.…”

Spectral characterizations and photophysical properties of one-step synthesized blue fluorescent 4′-aryl substituted 2,2′:6′,2′′-terpyridine for OLEDs application

“…OLED is a solid state device composed of thin films of organic materials that generates emission of light with the application of potential, which is an emerging technology for both lighting and flat panel display appliances because of its distinctive advantages such as ultra-thin thickness, low power consumption, ability to emit light without any external backlight sources and wide viewing angle [4][5][6][7]. There are two main classes of electroluminescent materials used to design efficient OLEDs: one is based on small molecules and the other by utilizing polymers [8,9].…”

Ether solvent treatment to improve the device performance of the organic light emitting diodes with aluminum cathode

“…Since most organic light-emitting materials have low LUMO (lowest unoccupied molecular orbital) level, the injection of electrons from the cathode is extremely critical to balance the charges. Reducing the electron injection barrier by modifying the organic/metal interface, such as the deposition of low work-function metals [4,5], alkaline halides and alkali-earth halides [6], metallic compounds [7,8], water/alcohol soluble polymers like conjugated polyelectrolytes or their neutral precursor [9][10][11][12], non-conjugated interlayer materials like polyethylene imine (PEI) [13,14], and even solvents [15,16], has been proved to be an effective approach.…”