Characterization of Polymer Light Emitting Diodes Fabricated by Ionically Self-Assembled Monolayer Technique
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applications by Rubner and coworkers [8,9]. The technique allows for detailed structural control of materials at the molecular level combined with ease of fabrication and low cost. The ISAM method involves the alternate dipping of a charged substrate into an aqueous solution of a cation followed by dipping in an aqueous solution of an anion at room temperature. Since the adsorption is based on the electrostatic attraction of interlayer charges, each layer is self-limiting in thickness and uniform at the molecular level. This occurs because the film molecules are free to adjust their positions to improve the overall packing since they are not covalently bound to the substrate. Sequential layers are rapidly fixed by drying at room temperature and pressure. Multilayer films several microns in thickness are easily fabricated by repeating the dipping process with no limit to the number of layers that can be deposited. The resulting pliable films are mechanically very robust and are only removed by vigorous scraping. The advantages afforded by molecular control on the subnanometer level provide opportunities for dramatically enhanced photonic [10], electronic, and optoelectronic [11] devices. EXPERIMENTAL DETAILS The materials used for the fabrication of the ISAM films were cationic precursor poly(para-phenylene vinylene) (PPVpc), prepared by the classic route [8, 12] and poly(methacrylic acid) (PMA) purchased from Polysciences. The pH and the ionic strength were adjusted accordingly for the different studies as described below. The films were prepared at room temperature and atmosphere. After the deposition of the films on glass slides and indium tin oxide (ITO) coated glass slides, the films were thermally treated under dynamic vacuum at 300 ºC for approximately 8 hours. Absorption spectra were taken with a Jasco spectrophotometer and fluorescence spectra were taken with a Perkin Elmer fluorescence spectrometer. For the electroluminescence spectra we used an Ocean Optics spectrometer. The I-V curves were obtained using a Keithley unit source, and the luminance values were read with a Minolta luminance meter. All measurements were carried in room pressure and temperature. Evaporated aluminum was used as the top electrode of the devices. DISCUSSION The initial study was to optimize the parameters of the dipping solutions for the absorbance of the films. Two basic variables have been studied that control the amount of PPV precursor adsorbed into each monolayer: the presence of salt in the PPVpc solutions, and the pH of the PPVpc solution. The polyanion PMA dipping solution was adjusted to 10 mM concentration, pH=3.5 and salt concentration of 0.05 M. For the first series the precursor PPV was 1 mM, 0.05 M salt, and the pH values were 3.5, 6.5, and 8.0. The films were deposited on glass and we recorded the absorption spectra of the films for the three values of the pH, after the thermal conversion of the PPV. The best thermal conversion to the conjugated form of the PPV occurs for pH=3.5. The high pH values of the PPVpc sol
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