Alternating-Current Light-Emitting Diodes and their Transient Characteristics: Response Time and Carrier Transport
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ABSTRACT We report the fabrication and characterization of alternating current light-emitting diodes (LEDs) with quinquethiophene as the emitting material. We have obtained equal electroluminescence intensity in both bias sections. From the frequency response of the LEDs, we have estimated the device response times and compared them with the response times obtained from the transient response of dc LEDs. Langmuir-Blodgett film deposition technique has been employed to control the thickness of the emitting layer on the molecular scale. We have shown that the response times originate from the accumulation rather than the transit of charge carriers. We have compared the photo- and electroluminescence spectra of QT LEDs. INTRODUCTION Most of the organic light-emitting diodes (LEDs) can only operate under forward dc bias, because of the tunneling mechanism involved in these devices [1]. To study the response characteristics of LEDs basically two methods have been used. The transient electroluminescence (EL) characteristics of simple LED structures showed a time lag between the sharp rise of a square-wave voltage pulse and the first appearance of the EL signal [2-5]. This time lag can be considered as the response time of LEDs. Response times can also be estimated from alternating current (ac) LEDs. These have recently attracted much attention because of their unique mode of operation [6-8]. Symmetrically configured ac LEDs consist of an active electroluminescent layer sandwiched between insulating layers. In earlier reports [9,10), we have described the use of Langmuir-Blodgett (LB) films of the emitting polymer and the insulating materials to fabricate ac LEDs. From the frequency dependence of ac LEDs, we have estimated response times. The LB film deposition technique has the unique advantage of controlling the film thickness on the molecular scale. With this advantage, we have fabricated dc and ac LEDs with a precise thickness of the emitting material. This has enabled us to compare the response times obtained from both measurements. The role of interfaces has been emphasized and the operation mechanism of ac LEDs has been discussed. EXPERIMENT Symmetrically configured ac LEDs consist of an active EL layer sandwiched between two insulating layers. The active layer in the present work was quinquethiophene (QT), an oligomer of polythiophene. Polymethyl methacrylate (PMMA) was used for the insulators. QT was synthesized according to methods described elsewhere [11, 12]. The purity was checked by measuring the melting point (254 °C) and the IR, UV-Visible absorption, photoluminescence and 93 Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
excitation spectra [121. We have deposited LB films of QT mixed with arachidic acid (molar ratio 60%/40%) at a surface pressure of 30 mN/m. We have used a value of 2.5 nm as the average thickness of a monolayer [II]. Isotactic PMMA (molecular weight = 100,000) was purchased from Polyscience Inc. and used as received. PMMA LB films were deposited at a surface press
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