Light-Emitting Diodes Using Semiconducting Oligothiophenes
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In this paper, we present the electrical and optical properties of the various types of organic LEDs using the alkyl-substituted oligothiophenes. Furthermore, the relationships between the layered structures and their LED properties are studied and reported. EXPERIMENTAL The alkyl-substituted oligothiophenes, i.e. dimethylquarterthiophene (DMQtT, tetramer), dimethylquinquethiophene (DMQqT, pentamer), and dimethylsexithiophene (DMSxT, hexamer), were synthesized and purified according to the methods described elsewhere1 3-15 . The deposition equipment consisted of two chambers; one was a deposition chamber and the other was a preparation chamber. The back pressure of the deposition chamber was kept at about 10-7Pa. The deposition was carried out using Knudsen cells (K-cells) and the deposition rate of all the oligomers was fixed at about I nm/min. The thickness of the film was controlled by the deposition time. For layered structure fabrication, the oligomers were deposited alternately from different K-cells. These oligomers were deposited on ITO-coated glass whose sheet resistance was about 50 f/o The device size was about 1mm x 1mm square. After deposition of the oligomers, aluminum electrode of 100nm thickness was vacuum-deposited on top of the oligomers. Electrical measurement was done using a Keithley 236 Source-Measure Unit. The output light power was measured using an Anritsu Optical Power Meter (ML910B) under a continuous device operation. RESULTS AND DISCUSSION Figure 1 shows a schematic diagram of the LED using DMSxT. This configuration is characterized as a Schottky diode and the DMSxT layer acts as not only an emitting layer but also a hole transport layer. As DMSxT functions as a p-type semiconductor and aluminum (Al) has a low work function (4.28eV) relative to DMSxT (4.9eV) 8 , the rectifying feature will be defined at the AI/DMSxT interface. Figure 2 shows the electrical properties of the LED using DMSxT. In this figure, ITO functioned as an anode. The good rectifying feature can be observed and its on-set voltage is about 4 V. Red-orange emission is also observed above 4V. The rectifying ratio at +IOV was about 1500, which was about 70 times larger than that of a device using vacuum-deposited polythiophene in a similar device structure 7 . 10 "1
Al electrode
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Fig. 1 Schematic diagram of the LED using DMSxT (hexamer) as an emitting layer.
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-5
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Voltage (V) Fig. 2 Current-voltage characteristics of the LED using DMSxT. Rectifying ratio was about 1500 at I0OV.
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Figure 3 displays output light power of the LED using DMSxT as a function of the current. The emission is increased with increasing current and the maximum output light power of about 52pW was achieved at 15mA. From Fig. 3, we estimate the quantum efficiency (photons emitted per carriers injected) according to Eq. 1, rlD=(q/hv )(AP/A1)(1) where 1iD is the differential quantum efficiency, q an elementary electric charge, hv a photon energy of the emitted l
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