Silole Derivatives with a High and Non-dispersive Electron Mobility, and a 100 % Photoluminescence Quantum Efficiency
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Silole Derivatives with a High and Non-dispersive Electron Mobility, and a 100 % Photoluminescence Quantum Efficiency H. Murata1*, G. G. Malliaras2, M. Uchida3, Y. Shen2, and Z. H. Kafafi1 Optical Sciences Division, US Naval Research Laboratory, Washington, DC 20375, U.S.A. 2 Department of Material Science and Engineering, Cornell University, Ithaca, NY 14853, U.S.A. 3 Chisso Corporation, Yokohama, Kanagawa 236-8605, Japan 1
ABSTRACT Non-dispersive and fast electron transport was realized for an amorphous, vapor deposited film of 2,5-bis(2',2''-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene in ambient and inert atmospheres. An electron mobility of 2x10-4 cm2/Vs was measured by time-of-flight at an electric field of 6x105 V/cm. This mobility is more than two orders of magnitude larger than that of the most widely used electron transporter, tris(quinolin-8-olato) aluminum (III), in molecular organic light-emitting devices (MOLEDs). Another silole derivative, namely 1,2-bis (1-methyl2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane, exhibits bright fluorescent blue-green light with an absolute quantum yield close to 100 % in the solid state. MOLEDs composed of stacked neat films of these two silole derivatives and a hole transporter show a significantly low operating voltage and an external quantum efficiency of 4.8 %, close to the theoretical limit. INTRODUCTION Electron-transporting organic semiconductors are an important class of materials. They play an important role in electro-optic, electronic and optoelectronic devices such as organic light-emitting diodes, bipolar transistors and photovoltaic cells, and have a strong influence on their power consumption, efficiency and overall performance. Most of the organic semiconductors developed to date are hole conductors. In particular, very few organic compounds have substantially high electron mobilities, especially in air. Electron transport in disordered amorphous molecular solids occurs via a hopping mechanism. This may be viewed as a one-electron reduction process of a neutral molecule concomitant with the oxidation of its anion. A large electron affinity (Ea) is crucial in order to form a stable anion in the organic solid and reduce the trapping effects caused by oxygen. The attachment of strong electron withdrawing groups to extended π-electron conjugated systems is a well-established method for achieving a large Ea. Electron withdrawing groups such as nitro [1], carbonyl [2], and cyano [3] groups, or heterocyclic rings including metal complexes of quinoline [4], oxadiazole [5], and quinoxanline [6] have been reported. Unfortunately, the inclusion of strong electron withdrawing groups induces energetic disorder and often leads to a decrease in hole [7] and electron [8] mobilities. This effect has been attributed to a dipolar disorder, which is essentially a fluctuation of energy in electron hopping sites [9], and results in highly dispersive electron transport. In this study, we report a novel electron transport material with good air-stability and *
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