Interfacial chemistry of Alq3 and LiF with reactive metals

Abstract

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.

Enhanced Emission Using Thin Li-Halide Cathodic Interlayers for Improved Injection into Poly(p-phenylene vinylene) Derivative PLEDs

“…13 Furthermore, photoemission spectroscopy supports the claim that enhanced electron injection by Li metal doping occurs, suggesting that Li metal lowers the barrier to electron injection at the polymer/cathode interface by introducing gap states in the forbidden gap of the polymer semiconductor. 13,14 Similar behavior was also observed in the case of Cs dissociating from CsF upon deposition of Al and diffusing into and doping the underlying polymer layer. 15 Generally, the lattice energy of ionic solids can be defined as the energy per ion required to separate completely into ions in a crystal lattice at a temperature of absolute zero.…”

First-principles molecular dynamics study of Al/Alq3interfaces

“…Tris (8-hydroxyquinoline) aluminum ðAlq 3 Þ is the most famous electron transport and emission materials of OLEDs [1][2][3] and the Alq 3 /metal interfaces have been intensively studied experimentally [4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretically [18][19][20][21][22][23][24]. By using ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES), interfacial gap states were observed at Al=Alq 3 interfaces [10,15].…”

Effects of Low Workfunction Metal Acetate Layers on the Electroluminescent Characteristics of Organic Light-Emitting Diodes

“…LiF가 전자주입 및 소자 특성에 미치는 메커니즘에 대해서는 tunneling barrier reduction [4], chemical reaction [5][6][7], dipole formation [8] 등 여러 가지 모델로 설명하고 있으며 최근까지도 LiF 박막의 OLED 소자특성 향상 메커니즘에 관한 연구결과가 발표되고 있다 [9]. 이 밖에도 Al 전극과 유기층의 계면에 metal fluorides(LiF [3], NaF [10], CsF [11]), metal oxides(MgO [12], Al 2 O 3 [13]), organic metal complexes (Ca(acac) 2 [14], NaSt [15] …”