Balanced Charge Injection and Singlet Exciton Quenching in Bilayer Organic Electroluminescent Devices
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Balanced Charge Injection and Singlet Exciton Quenching in Bilayer Organic Electroluminescent Devices Chimed Ganzorig and Masamichi Fujihira Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan ABSTRACT We demonstrate here balancing between hole and electron injection in bilayer organic tris(8hydroxyquinoline)aluminum (Alq3) based electroluminescent (EL) devices by measuring current-voltage (I-V) characteristics of single-carrier devices with different thickness and by comparing the device performance with various chemically modified ITO. We measure I-V characteristics due to holes in hole-only bilayer devices with various modified ITO anodes and those due to electrons in electron-only single-layer devices with C6H5COOLi(2 nm)/Al cathode. Here, hole-only devices are those, in which only holes are carriers and the current due to electrons is negligible. We found that the I-V characteristics observed for the hole-only bilayer devices with UV-ozone and ClC6H4COCl treated ITO were comparable to the I-V characteristics for the electron-only single-layer device of ITO(as-cleaned)/Alq3(150 nm)/C6H5COOLi(2 nm)/Al. In other words, hole and electron injections are well balanced when we use UV-ozone and ClC6H4COCl treated ITO anodes. EL characteristics have shown that luminance decreased clearly for bilayer organic EL devices with an Al cathode where electron injection was not enhanced significantly. The decrease in EL efficiency can be attributed to unbalanced hole and electron injection. The unbalanced injection results in excess of one carrier type that does not contribute to light emission, and results in deactivation of 1Alq3* at high currents due to . quenching 1Alq3* by large excess of Alq3+ in the emission zone.
INTRODUCTION Since the first report of a bilayer organic EL device in 1987 [1], intensive research in science and engineering technology has been carried out with the focus on its potential applications in full-color flat-panel displays. Several recent reviews [2-9] have dealt with some fundamental aspects of the basic device physics and related phenomena such as charge injection, transport and recombination relevant to molecular organic EL materials and devices. More recently, we have demonstrated that luminance increased more than linearly with an increase in current for Alq3based electroluminescent (EL) devices and the EL efficiency reached ~5 cd A-1 at 250 mA cm-2 when hole and electron injection was well balanced [8,10]. In particular, low-work-function cathodes and high-work-function anodes are needed in order to fabricate efficient EL devices. Fine tuning work function of electrodes by surface molecular design plays a crucial role in balancing the rates at which holes and electrons are injected into the organic two-layer thin film with a heterjunction [8,10-15]. In this paper, we demonstrate recent study of balancing between both carrier injections in our improved EL devices [10-15] by measuring I-V characteristics of
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