Light-Emitting Diodes with Voltage-Switchable Colors from Semiconducting Polymer/Polymer Heterojunctions
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charge transport materials in LEDs suffer from their low glass transition temperature or propensity to crystallize. Conjugated rigid-rod polymers, however, have the advantages of high temperature stability, ease of fabrication into large area thin film devices by solution spin-coating, and morphological stability. In this paper, we report semiconducting polymer heterojunction light-emitting diodes with voltage tunable colors using PPV as the p-type layer and polybenzobisthiazoles (PBTPV and PPyBT) as the n-type layer. Poly(benzobisthiazole1,4-phenylenebisvinylene) (PBTPV) and poly(2,5-pyridylene benzobisthiazole) (PPyBT) are two examples of n-type semiconducting polymers synthesized in our laboratory. Their synthesis, characterization, optical, electrochemical, and charge photogeneration properties have been reported elsewhere [8]. Their n-type characteristics were revealed by electrochemical studies (cyclic voltammetry) [8]. The chemical structures of PPV, PBTPV, and PPyBT are shown in Figure 1.
nn-Type •-•••I
PPV
Polymer
PPV
n
PBTPV S
ITO
Glass
PPyBT Figure 1. Chemical structures of PPV, PBTPV, and PpyBT, and the schematic of the heterojunction light-emitting diode. EXPERIMENTS
The semiconducting polymer heterojunction LEDs were prepared and investigated as sandwich structures between aluminum and indium-tin-oxide (ITO) electrodes as shown in Figure 1. The PPV thin films were deposited onto ITO coated glass by spin coating of the sulfonium precursor from methanol solution followed by thermal conversion in vacuum (250 'C for 1.5 hr). Thin films of PPyBT and PBTPV were spin coated on the PPV layer from their reversibly soluble Lewis acid (GaCI3) coordination complexes in nitromethane[9]. The film thickness was measured by an Alpha-step profilometer (Tencor Northern) with an accuracy of ±1 nm and confirmed by an optical absorption coefficient technique. Finally, 100-130 nm aluminum electrodes were vacuum (< 10-5 torr) evaporated onto the resulting polymer bilayers. Electroluminescence spectra were obtained by using a calibrated Photo Research Model PR-60 photo-colorimeter or using a Spex Fluorolog-2 spectrofluorimeter. Currentvoltage-luminance curves were recorded simultaneously by hooking up an HP4155A semiconductor parameter analyzer together with a Grasby S370 optometer equipped with a calibrated luminance sensor head. The EL quantum efficiency of the LEDs was estimated by using procedures similar to that previously reported [10]. All the fabrication and measurements were done under ambient laboratory conditions.
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RESULTS Figure 2 shows the optical absorption and photoluminescence spectra of PBTPV and PPyBT and the absorption spectrum of PPV. PBTPV has absorption peaks at 475 nm and 508 nm and absorption band edge at 2.1 eV. The emission peak of PBTPV is 630 nm. PPyBT has absorption peaks at 440 and 470 nm and absorption band edge at 2.48 eV. The emission peak of PPyBT is 560 nm. The absorption band edge of PPV is well known to be 2.4 eV [1]. Clearly, there is little overlap between the absorption
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